| Title of Invention | "A METHOD OF MAKING NANOWIRES WITH GROUP III NITRIDE SEMICONDUCTOR SUBSTRATE MATERIALS AND NANOWIRE ARRAY DEVICE FORMED THEREOF" |
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| Abstract | devices including high-quaiity (i.e., defect free) group III-N nanowires and uniform group III-N nanowire arrays as well as their scalable processes for manufacturing, where the position, orientation, cross-sectional features, length and the crystallinity of each nanowire can be precisely controlled. A pulsed growth mode can be used to fabricate the disclosed group III-N nanowires and/or nanowire arrays providing a uniform length of about 10 nm to about 1000 microns with constant cross-sectional features including an exemplary diameter of about 10- 1000 nm. In addition, high-quality GaN substrate structures can be formed by coalescing the plurality of GaN nanowires and/or nanowire arrays to facilitate the fabrication of visible LEDs and lasers. Furthermore, core-shell nanowire/MQW active structures can be formed by a core-shell growth on the nonpolar sidewalls of each nanowire. |
| Full Text | PULSED GROWTH OF GAN NANOWIRES AND APPLICATION IN GROUP III NITRIDE SEMICONDUCTOR SUBSTRATE MATERIALS AND DEVICES DESCRIPTION OF THE INVENTION Government Rights [0001] Tnis invention was made witn government support under Contract No. HR0011-C5-1-0006 awarded by the Defense Advanced Research Projects Agency/Army Research Office, and Contract No. F49620-C3-1-0013 and Contract No. FA8550-06-1 -0001 awarded by the Air Force Office of Scientific Research The government may have certain rights in the invention. Related Apptications [0002] This appiication claims priority from U.S. Provisional Patent Applications Ser. No. 60/780,833, filed March 10, 2006, Ser. No. 60/798,337, filed May 8, 2006, Ser. No. 60/808,153, filed May 25, 2006, and Ser. No. 60/889,363, filed February 12,2007, which are hereby incorporated by reference in their entirety. Field of the Invention [0003] This invention relates generally to semiconductor materials, devices, and methods for their manufacture and, more particularly, relates to semiconductor nanowires and senriiconductor nanowire active devices. Background of the invention [0004] Nanowires composed of group li!-N alloys {e.g., GaN) provide the potential for new semiconductor device configurations such as nanoscaie optoelectronic devices. For example, GaN nanowires can provide large bandgap, high melting point, and chemical stability that is useful for devices operating in corrosive or high-temperature environments, The larger bandgap of GaN and its related alloys also allows the fabrication of light sources in the visible range that are useful for displays and lighting applications. In addition, the unique geometry of each nanowire offers the potential to explore new device paradigms in photonics and in transport devices. To fuiiy realize this potential, . scalable process is needed for mailing high-oL-ality group lll-N nanowires and/or nanowire arrays with precise and uniforrrs control of the geometry, position and crystaliinity of each nanowire. [00005] Conventional nanowire fabrication is based on a vapor-liquid-soiid (VLS) growth mechanism, and involves the use of catalysts such 3S Au, N!, Fe, or in. Problems arise, however, because these convsniional catalytic processes cannot control the position and unifomity of the resulting nanowires. A further problem with conventional catalytic processes is that the catalyst is inevitably incorporated into the nanowires. This degrades the crystalline quality' of the resulting nanostruotures, which limits their applications. [0006] Thus, there is a need to overcome these and other problems of the prior art and to provide high-quality nanowires and/or nanowire arrays, and scalable methods for their manufacturing. It is further desirable to provide nanowire photoelectronic devices and their manufactunng based on tine high-quality nanowires and/or nanowire an-ays. SUMMARY OF THE INVENTION [0007] According to various embodiments, the present teachings include a method of making nanowires. In the method, a selective growth masl can be formed over a substrate. The selective growth mask can include a plurality of patterned apertures that expose a plurality of portions of the substrate. A semiconductor material can then be grown on each of the plurality of portions of the substrate exposed in each of the patterned apertures using a selective non-puised growth mode. The growth mode can be transitioned from the non-pulsed growth mods to a puissd growth mode. By continuing the puised grawth mode of tae semiconductor material, a piuralit' of ssmiconducior nanowlres can be formed. [00008] According to various embodiment. the present teachings also inciude a group III-N nanowire array, which can inciuds a selecnve growth masi; disposed over a substrate. The seiective growth mas!', can inciuds a plurality of patterned apertures that expose a plurality of portions of the substrate. A group fll-N nanowire can be connected to and extend from the exposed plurality of portions of the substrate and extend over a top of the selective growth mask. The group |[|-N nanowire can be oriented along a single direction and can maintain a oross-sectlona! feature of one of the piurality of selected surface regions. [0009] According to various embodiments, the present teachings further Inciude a GaN substrate structure. The GaN substrate structure can be a GaN film coalesced from a plurality of GaN nanowlres, which is defect free. The GaN film can have a defect density of about 10' cm" or lower. [0010] Additional objects and advantages of the invention wilt be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the Invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims. [0011] It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed. BRIEF DESCRlPTlON OF THE DRAWINES [00012] The accompanying drawing, which are incorDorated in and constitute a part of this spscifioatiou, illustrate ssveral embodiments of the invention snd logsthsr with the description, serve to explain the principles of the invention. [0013] FIGS. 1A-1C depict cross-sectional views of an exempiary semiconductor nanowire device at various stages of fabrication in accordance with the present teachings. [0014] FIG. 2 depicts a second exemplars semiconductor nanowire aevice in accordance with the present teachings. [0015] FIG. 3 depicts an exemplary process for forming a plurality of nanowires and/or nanowire arrays using a two-phase growth mode in accordance with the present teachings. [0016] FIGS. 4A-4C depict a third exemplary semiconductor nanowire device in accordance with the present teachings. |0017| FiG. 5 depicts a forth exemplary semiconductor nanowire device in accordance with the present teachings. [00181 FIGS. 6A-6D depict exempiary results for a plurality of ordered GaN nanowire arrays grown by the two-phase growrth mode without use of a catalyst In accordance with the present teachings. [0019] FIGS. 7A-7D depict four exemplary variants of semiconductor devices including GaN substrate structures fomned from the plurality of nanowires and/or nanowire arrays shown in FIGS. 1-6 in accordance with the present teachings, [0020] FIG, 8 depicts an exemplary core-shell nanowire/MQW (multiple quantum well) active structure device in accordance with the present teachings. 10021] F!G. 9 deplete another exemoiary cors-sheli nanowtre/iViCJW sctivt structure device in accordance with the present teachings. I0C522J FIGS. 10A -1OC depict an exempian; nanowire LED device formec' using the core-shell nanowlre/MQW active struciure described in FIGS. 8-9 in accordance with the present teaclnings. |0023| FIG. 11 depicts an exemplary nanowire laser device using the core- shell nanowlre/MQW active structure described In FIGS. 6-9 in accordance with tie present teachings. [00243 FIG. 12 depicts another exempian? nanowire laser device using the core-shell nanowlre/MQW active structure described in FIGS. 6-9 in accordance with the present teachings. DESCRIPTION OF THE EMBOPIMEMTS [0025] Reference will now be made in detail to exemplary embodiments of the invention, examples of which are Illustrated in ttie accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. In the following description, reference is made to the accompanying drawings that fonn a part thereof, and in which is shown byway of illustration specific exemplary embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention and it is to be understood that other embodiments may be utilized and that changes may be made without departing from the scope of the invention. The following description is, therefore, merely exemplary. [00261 While the invention has been illustrated with respect to one or more impiemeniations, alterations and/or modifications can be made to the illustrated examples without departing from the spirit and scope of the appended claims. In addition, while a particular feature of the inventicin may have been disclosed wltn respect to only one of several impiementations, such feature maj' be combined; with one or more other features of the othe- implementations as may be desired and, advantageous for any given or oarticular function. Furthermore, 13 the s):tent that tne terms "including", "includes", "having", "has", "with", or variante thereof are used in either the detaiied description and the claims, such terms are intended to be inciusive in a manner similar to the term "comprising," The term "at least one of is used to mean one or more of the listed items can be selected. J0027] Notwithstanding that the numerical ranges and parameters setting forth the broad scope of the invention are approximations, the numerical values set forth in the specific examples are reported as precisely as possible. Any numerical value, however, inherently contains certain errors necessarily resulting from the standard deviation found in their respective testing measurements. Moreover, all ranges disclosed herein are to be understood to encomipass any and all sub-ranges subsumed therein. For example, a range of "ie.ss than 10" can include any and a!! sub-ranges between (and including) the minimum value of zero and the maximum value of 10, that is, any and all sub-ranges having a minimum value of equal to or greater than zero and a maximum value of equal to or less than 10, e.g., 1 to 5. [0Q28| Exemplary embodiments provide semiconductor devices including high-quality (i,e., defect free) group Ili-H nanowlres and unifomn group Ill-N nanowire arrays as well as scalable processes for their manufacturing, where the position, orientation, cross-sectional features, length and/or Ifie crystallinity of each nanowire can fas precisely controlled. Specifically, a plurality of nanowlres and/or nanowire an-ays can be fomned using a selective growth mode followed by a growth-mode-transition from the selective growth mode to a puised growth mode. The cross- sestional features, for example, the oross-sectlona! dimensions (e.g., diamsier or width), and the oross-sectJonal shapsE. of each nanowirB obtained from tm ssisotive pro>v!h mods can be maintainsd by continuing the growth using tiie puissd growth mods, tn this manner, nanovwres with a high aspect ratio can be formed, in ars sxempiary embodiment the length of sach nanowirs can oe, for example, aoout 10 nm to about 1000 microns, for example about 10 nm to about 100 microns. [QQ29I in addition, high-quality group itI-N films, for example, high-quality GaK Aims, can be formed by terminating and coalescing the pluralitr of nanowires and/or nanowire arrays. These GaN films can be used as GaN substrate structures to facilitate the fabrication of GaN-based devices such as visible LEDs and lasers for the emerging soiid-state lighting and UV sensor industries. [0030] Furthermore, because each of the pulsed-grown nanowires and/or nanowire arrays can provide nonpolar sidewalls, there are advantages in using a core-shell growth to build an MQW active shell structure on the sidewalls of each nanowire. Such core-shell nanowire/MQW active structures can be used in nanoscale photoeiectronic devices having high efficiencies, such as, for example, nanowire LEDs and/or nanowire lasers. [0031] As used herein, the term "nanowire" generally refers to any elongated conductive or semiconductive material that includes at least one minor dimension, for example, one of the cross-sectional dimensions such as width or diameter, of less than or equal to about 1Q00 nm. In various embodiments, the minor dimension can be less than about 100 nm. In various other embodiments, the minor dimension can be less than about 10 nm. The nanowires can have an aspect ratio (e.g., length: width and/or major dimension: minor dimension) of about 100 or greater. In various embodiments, the aspect ratio can be about 200 or greater. In various other embodiments, the aspect ratio can be about 2000 or greater, in art exemplar' ambodinnent, tiie cross-section of the nanowire can be highly asymmetric such tnsi in one direction of the cross-sectlonE! dimension can be much less than 1000 nm and In an orthogonal direction the dimension can be substaniialiy greater than 1000 nm, [0Q32] !t is also intended that the term "nanowires" aiso encompass other elongated structures of like dimensions inciuding, but not limited to, nanoshafts, nanopiiiars, nanoneedles, nanorods, and nanotubes (e.g., single wall nanotubss, or multiwall nanotubes), and their various functionalized and dehvatized fibril forms, such as nanofibers in the form of thread, yarn, fabrics, etc. [00331 The nanowires can have various cross-sectional shapes, such as, for example, rectangular, polygonal, square, oval, or circular shape. Accordingly, the nanowires can have cylindrical and/or cone-like three dimensional (3-D) shapes. In various embodiments, a plurality of nanowires can be, for example, substantially parallel, arcuate, sinusoida!, etc., with respect to each other. [0034] The nanowires can be formed on/from a support, which can include selected surface regions where the nanowires can be connected to and extend {e.g., be grown) from, The support of the nanowires can also include a substrate formed from a variety of materials including Si, SIC, sapphire, lll-V semiconductor compounds such as GaN or GaAs, metals, ceramics or glass. The support of the nanowires can also include a selective growth masl various embodiments, the support of the nanowires can further include a buffer layer disposed between the selective growth mask and the substrate. [0035J In various embodiments, nanowire active devices, for example, nanowire LEDs or nanowire lasers, can be formed using the nanowires and/or nanowtre arrays, io various embodimente, the nanowiras and/or nanowirs arrays and the nanowtre active devices can be formed using a IIS-V oompounc' semiconducror materials system, for eKampse.; the group \\\-H compound materials system. Examples of the group III elements can include Ga, In, or At, which can bs formed from exemplary group lii precursors, such as trimethylgallium (TMGa) ar triethylgaiiium (TEGa), trimethylindium (TMln) or trimethylaluminum (TMAt). Exempiary N precursors can be, for example, ammonia (NH3). Other group V elements can also be used, for example, P or As, with exemplarf group V precursors, such as tertiarybutylphoshine (TBP), or arsine (.AsHs). [00361 In the following description, group lll-N semiconductor alloy compositions can be described by the combination of group lll-N elements, such as, for example, GaH, AIN, InN, InGaN, AIGaN, or AllnGaN. Generally, the elements in a composition can be combined with various molar fractions. For example, the semiconductor alloy composition InGaN can stand for InxGaofN, where the moiar fraction, x, can be any number less than 1.00. in addition, depending on the molar fraction value, various active devices can be made by similar compositions. For example, an lno.3Gao.7N (where x is about 0.3) can be used in the MQW active region of LEDs for a blue light emission, while an Ina-aGaas/N (where x is about 0.43) can be used in the MQW active region of LEDs for a green light emission. [0037] In various embodiments, the nanowires, nanowire arrays, and/or the nanowire active devices can include a dopant from a group consisting of. a p-type dopant from Group II of the periodic table, for example, Mg, Zn, Cd and Hg; a p-type dopant from Group IV of the periodic table, for example, C; or an n-type dopant selected from a group consisting of: Si, Ge, Sn, S, Se and Te. 10038] In various smbodimsnts, the nanowirss and/or nanowire arrays as well as ths nanowire active devices can have high-qua!itv' heterogeneous structures gnc b-': formsc' by various crystai growth techniques inclLiding, but not iimitee to. meta!-orgEnic chemica! vapor deoosition (MOCVD) (also known as organometalilc vapor phase epitaxy (OMVPE)), moiecuiar-beam epitaxy (MBE), gas source IVIBE (3SW1BE), metal-organic MBE (MOMBE), atomic layer epitaxy (ALE), or hydride vapor phase epitax/ (HVPE), [0039] In various embodiments, a multiple-phase growth mode, for example, a two-phase growth mods, can be used for the high-quality crystal growth of nanowires and/or nanowire arrays as well as nanowire active devices. For example, a first phase growth mode such as a selective growth mode can be used to provide a condition for growth selectivity and nudeation of the nanowires and/or nanowire arrays. In the selective growth mode, standard crystai growth methods, for example, standard MOCVD, can be used to nucleate the growth of the nanowires with a desired thickness of, for example, about 10 nm or more. [0040] The second phase growth mode can create a process to continue the growth of each nanowire and maintain its cross-sectional features from the first growth mode, and also provide an arbitrary desired length. The second phase growth mode can be applied by a growth-mode-transition, which can tenninate the first phase growth mode, in the second phase grovirth mode, a pulsed growth mode, for example, a pulsed MOCVD growth, can be used. [0041] As used herein, the terni "pulsed growth mode" refers to a process in which the group 111 and group V precursor gases are introduced alternately in a crystal growth reactor with a designed sequence. For example, TMGa and NH3 can be used as tjie precursors for an exemplary formation of GaN nanowires and/or nanowire arrays and/or GaN nanowire active devices, in the puised growth mods, TMGa and NHa can be introduced alternately in a sequencs that introduces TM3&. wliTi E designed fiovv rate (s.g., aoout 1C; seem) for a certain Deriod of time (E.g., about 20 seconds) foliowed by introducing NHa with a designed fiow rate (e.g., about 1500 seem) for a time period (e.g., about 30 seconds). In various embodiments, one cr more sequence loops can be conducted (e.g., repeated) tor a designed iengtli of each nanowire. in various embodiments, the growth rate of each nano>wire can be orientation dependent. [0042] In various embodiments, dielectric materials can be involved in formation of the disclosed nanowires, nanowire arrays, and/or nanowire active devices. For example, Uie selective growth mask can be made of dielectric materials during the formation of the plurality of nano\wires and/or nanowire arrays, in another example, dielectric materials can be used for electrical isolation for active devices such as nanowire LEDs and/or nanowire lasers. As used herein, the dielectric materials can include, but are not limited to, silicon dioKide (SiOz), silicon nitride (Si3N4), silicon oxynitride (SiON), fiuorinated silicon dioxide (SiOF), silicon oxycarbide (SiOC), hafnium oxide (HfOz), hafnium-silicate (HfSiO), nitride hafnium-silicate (HfSiON), zirconium oxide (ZrOa), aluminum oxide (AlaOg), barium strontium titanate (BST), lead zirconate titanate (PZT), zirconium silicate (ZrSiOa), tantalum oxide (Ta02) or other insulating materials. According to various other embodiments, a conducting metal growth mask, such as, for example, tungsten can be used for selective growth of the disclosed nanowires. [00431 Exemplary embodiments for semiconductor devices of nanowires and/or nanowire arrays and their scaieable processes for growth are shown In FIGS. 1A-1C, FIGS. 2-3, FIGS. 4.AC, FIG. 5, and FIGS. 6A-BD. |0044] FiQS. 1A-1C aepici cross-sectiDnai views of an exemptary semiconductor nanowire dsviGS 100 at varicus atagss of fabrication in accoraance with ins prssefiit teactiings. It snould be readily apparent to one of crdinary skiii sr the art that ths nanowire device 100 depicted in FIGS. 1A-1C represents a generalized scliematic illusxration and that other layers/nanowires may be added or existing iayers/nanowlres may be removed or modified. [0045] As shown in FIG. 1A, the nanowire device 100 can include a substrate 110, a selective growth mask 135, and a plurality of patterned apertures 138. The selective growth mask 135 and the plurality' of patterned apertures 138 can be disposed over the substrate 110, wherein ths plurality of patterned apertures 138 can be interspersed through the selective growth mask 135. [0046] The substrate 110 can be any substrate on which a group III-N material can be grown. In various embodiments, the substrate 110 can inciude, but is not limited to, sapphire, silicon carbide, silicon, silicon-on-insuiator (SOI), lll-V semiconductor compounds such as GaN orGaAs, metals, ceramics or glass. [0047] The selective growth mask 135 can be formed by patterning and etching a dielectric layer (not shown) formed over the substrate 110. In various embodiments, the dielectric layer can be made of any dielectric material and formed using techniques known to one of ordinary skill in the art. The dielectric layer can then be patterned using one or more of interi'erometric lithography (IL) including immersion interferometric lithography and nonlinear interi'erometric lithography, nanoimprint lithography (NL), and e-beam lithography, which can produce nanostmctures or patterns of nanostructures over wide and macroscopic areas. After the patterning, an etching process, for example, a reactive Ion etching, can be used to fonn the plurality of patterned aperi:ures 138. The etching process can be stopped at the surface of the unasrlying iaysr. I.e., tne substrate 110, and sxposing a pluraito' of surface portions 13e of the substrate 110, In various embodinientfc, tne ssiectivs growth mesk 135 aan oe a aonducnng metal grovi'th. masK made of, for eKamole, tungsteri, to provids selective growth as desired for puised nanowire growtii. [00481 The plurality of patterned apertures 138 can have a tiiickness the same as the selective growth mask 135, for example, about 30 nm or less, and a cross-sectional dimension, such as a diameter, of about 10 nm to about 1000 nm. As an additional example, the diameter can be about 10 nm to about 100 nm. in an exemplary embodiment, the plurality of patterned apertures 138 can have a hexagonal array with a pitch (i.e., center-to-center spacing between any two adjacent patterned apertures) ranging from about 50 nm to about 10 nm. in various embodiments, arrays of the plurality of patterned apertures 138 can be formed. Thereafter, the nanoscale features of the plurality of the patterned apertures 138 can be transfen"ed to the subsequent processes for the formation of nanowires and/or nanowire arrays. 10049] in various embodiments, various cleaning procedures can be conducted on the device 100 shown In FIG. 1A prior to the subsequent growth of the nanowires and/or nanowire arrays. For exampis, the cleaning processes can include an ex-situ cieaning (i.e., the cleaning is conducted outside the growth reactor) followed by an in-situ cleaning (i.e., the cleaning is conducted within the growth reactor). Depending on materials used for e selective growth mask 135, various cieaning methods can be used. In an exemplary embodiment, a silicon nitride selective growrth mask can be cleaned by a standard ex-situ cieaning followed by an in-situ cleaning by loading the device 100 into an exemplary MOCVD reactor and heating the device 100 tc about 95D °C fo" approximately £ minutes under flowing hydrogen. This hydrogen-reducing-atmosphere can remove undesirable native oxiaes on tiie surfaces of tiis device 10G. Depending on tiie material combination of tiie substrate 110 and selective growtii mask 13£\ one of ordinary sitill in ttie an wtO understand tiiat alternative cleaning procedures can be used. [06501 in FIG. 1B, a plurality of nanostructure nuclei 140 can be selectively grown from the exposed plurality of surface portions 139 of the substrate 110 to fill each of the plurality of patterned apertures 138, which can be defined by the selective growth mask 135. The selective growth mask 135 can serve as a selective growth mold to negatively replicate its nanopatterns from the plurality' of patterned apertures 138 to the plurality of nanostructure nuclei 140. In this manner, the position and the cross-seciional features, such as the shape and dimensions, of each of the plurality of nanostructure nuclei 140 can be determined by that of each patterned aperture of the plurality of patterned apertures 138. For example, the plurality of patterned apertures 138 can include a hexagonal array with a dimension of about 250 nm. The hexagonal array can then be transferred to the growth of the plurality of nanostructure nuclei 140 with a similar or smaller dimension of about 250 nm or less, in another example, if the one or more apertures of the plurality of patterned apertures 138 are approximately circular with an exemplary diameter of about 100 nm, one or more nuclei of the plurality of nanostructure nuclei 140 can be grown in the circular apertures with a similar diameter of about 100 nm or less. Thus, the plurality of nanostructure nuclei 140 can be positioned in a well-defined location and shaped con-espondingly to the plurality of the patterned apertures 138 defined by the selective growth mask 135. in various embodiments, the plurality of nanosrructure nuciei 140 can be. formed by, for sxampis, & standard iVIOCVD process- FOCfgl] in this manner, the device 100 shown in FIG. 1B csn be ussd as £ support for nanowirss and/cr nanowire arrays, which can include a piurallty of selected surface regions (i.e., each surface of the plurality of nanostructure nuoiei 140) A pluraiit\' of nanowlres and/or nanowire arrays can then be grown from the plurality of selected surface regions. In various embodiments, the seSective grovrth mask 135 can be removed by a suitable etching process to expose the plurality of nanostructure nuclei 140 after the formation of the plurality of the nanowlres. [0OS2I in FIG. 1C, a plurality of nanowlres 145 can be formed by continuing the growth of the plurality of nanostructure nuclei 140 by, for example, terminating the selective growth mode and applying a pulsed growth mode, before the plurality' of nanostructure nuclel 140 protrudes from a top of the selective growth masic 135. The plurality of nanowlres 145 can be formed of the same material of the nanostructure nuclei 140, for example, GaN, AIN, InN, InGaN, AllnGaN, or AlGaN, In various embodiments, heterostructures can be formed fomi each of the plurality of nanowlres 145. In various embodiments, n-type and/or p-type dopants can be incorporated Into the plurality of nanovwres 145 depending on the desired application. [00531 By transitioning to the pulsed growth mode before growth of the piurallty of nanostructure nuciei 140 protrudes from the top of the selective growth mask 135, features such as cross-sectional shape and dimensions of each of the piurallty of nanowlres 145 can be preserved until a desired length is reached. In other words, the cross-sectional features of the nanowlres 145, such as shape and/or dimension, can remain substantially constant, the same or similar as that of the aperfe-irss 138. \n various emDodiments, the length of each nanowire can be on an order of micromsters, for examois, about 2C am o- more. fS0g4j in various embodiments, a buffer aver can be formed in the nanowire dgvicBE. FIG. 2 depicts a second SKemplary semiconductor nanowire device 200 including a buffer layer in accordance witli trie pressnt teachings. As shown, the nanowire device 200 can include a buffer layer 220 disDosed between a substrate such as the substrate 110 and a seiective growth masK such as the selective growth mask 135 (see FIGS. 1A-1C). in various embodiments, the buffer layer 220 can be a planar semiconductor film formed of, for example, Gals!, AIN, SnN, inGaN, AllnGaN or AlGaN, by, for example, standard MOCVD. In various embodiments, the thickness of the buffer layer 220 can be, for example, about 100 nm to about 10 um. In various embodiments, the buffer layer 220 can be doped with either an n-type or a p-type dopant in order to provide an electrical connection to the lower end of each nanowire of the plurality of nanowires 140. Various dopants known to one of ordinary? slill in the art can be used. [0055] In various embodiments, the orientation of the plurality of nanostaicture nude! 140 can be controlled along a single direction, which can in turn be controlled by intentionally orienting the plurality of patterned apertures 138 along the single crystal direction. For example, the plurality of patterned aperi:ures 138 can be intentionally oriented along a single direction of the buffer layer 220 as shown in FIG. 2. in an exemplary embodiment during IL patterning, the apertures in the selective growth mask 135 can be intentionally oriented along the direction of a GaN buffer layer. In another exemplary embodiment when the GaN buffer layer is grown on a sapphire substrate, there can be a 30° rotation about the c axis between the GaN buffer layer and the sapphire unit cells. |O053| FIG. 3 dsDtcts. an exemplar/ process for forming s; piuralify cf nano'!Mire& and/or nanowire arrays using tm two-phase cpwtn rnode in accordance vifith tns present teachings. Spscificaliy, FIG. 3 illustrates precursor aas flow curves (including a first gas flow curve 302 and a ssconci gas flow curve 306) during a selective growtii 310 and a subsequent pulsed growin 320 for the formation of, for example, tiie plurality of nanowires 145 as described in FIGS. 1-2. As siiown, tlie selective growth 310 can be terminated by starting a pulsed growth 320 (I.E., growlh- mode-transition) at a transition time ti. Tiie pulsed growth 320 can further include a number of pulsed sequences, for example, a first sequence loop 324, a second sequence loop 328 and/or additional sequence loops, In various embodiments, the first sequence loop 324 can be repeated as the second sequence loop 328. [00571 In an exemplary embodiment for the formation of GaN nanowires and/or nanowre an-ays, the first gas flow curve 302 can be plotted for a first precursor gas such as trimethylgallium (TMGa), and the second gas flow curve 305 can be plotted for a second precursor gas such as ammonia (NH3). During the selective growth 310, the exempiary GaN nanowires and/or nanowire arrays can be formed in a MOCVD reactor including the first precursor gas TMGa with a constant flow rate of about 10 seem, and the second precursor gas NH3 with a constant flow rate of about 1500 seem. That means, during the selective growth 310, the precursor gases (i.e., TWlGa and NHg) can be flowed continuously, not pulsed (i.e., both Group III and Group V precursor gases are provided to the substrate together in a continuous, non-pulsed growth mode), Moreover, the group V precursor gas (e.g., TMGa) and group III precursor gas (e.g. NH3) can be introduced simultaneously and the group V/ group III ratio can be maintained, for example, at about 100 to about 500. In an exemplary embodiment, the group V/ group 111 ratio can be maintained at about 150. Further, other reEotor conditions for tins seisctive growth 310 can includs, forexampls, an initiai reaction temperature of about 1015 °C to S'DDU: IQS-C °C, a rsastor pressure cf about 100 Torr, and a hydrogsn/nitrogen earner gas- mixture having a laminar flow of about 4000 seem. Any suitaols WIOCVC; raactor may be used, such as the Vssco TurboDisk model P75 MOCVD reactor in which the substrE'css are rotated at a higl"; speed during deposition. [0058| During pulsed growth 320, the first precursor csas such as TMGa and the second precursor gas such as NHs can be introduced alternately into the growth reactor in a designed sequence, for example, shown as the first sequence loop 324. In various embodiments, the duration of each alternating step within the puised sequence can affect the growth of the nanowires and/or nanowire arrays, which can further be optimized tor specific reactor geometries. For example, in the first pulsed sequence loop 324, TMGa can be introduced with a flow rate of about 10 seem for a certain period of time such as about 20 seconds (not illustrated) followed by, for example, a 10 second carrier-gas purge (e.g., a mixture of hydrogen/nitrogen ) during which no precursor gases are introduced, and followed by introducing NH3 with a flow rate of about 1500 seem for a time period such as about 30 seconds (not illustrated) followed by, for example, a 10 second carrier-gas purge (e.g., a mixture of hydrogen/nitrogen) with no precursor gases involved. Other pulse durations may also be used depending on the reactor configurations, such as for example 15-40 seconds for the Group 111 reactant, 15-40 seconds for the Group V reactant and 5-15 seconds for the purge gases between each reactant introduction step. In various embodiments, the puised sequence such as the first sequence loop 324 can be repeated until a certain length of the GaN nanowires is reached. For example, the sequence loop 324 can be repeated as the second sequence loop 328, the third sequence loop (not iliustrated) and so on. In each sequence loop, the group V precursor gas (e.g., TMGa) and grout' Iv: orecLTsor ga? (e.g. NH3) can have an effective V/lll ratio in a range ov. fc;- exampie, from about 6C to about 300. in various. embodiments, ths temperature, reactor pressure, and carrier gas fiow for the pulsed growth 320 can remain at their same settings m tor the selective growth 310. One of ordinar\f skill in the art will understand that the disclosed growth parameters are exemplar)' and can var' depending on the specific reactor used. [0059] in various embodiments, the transition time (ti) can be determined by the duration of the selective growth 310. The transition time (ti) can be dependent on the growth rate inside each aperture, for example, each of the plurality of patterned apertures 138 shown in FIGS. 1-2. The growth rate inside each aperture can in turn depend on the gas flows (e.g., shown as gas flow curves 302 and 304) of each precursor gas and the geometry of each aperture of the plurality of patterned apertures 138. This geometrical dependence can occur because the growth nutrients, for example, from TMGa and/or NH3, can be deposited both on the selective growth masic and in the open apertures. During selective growth 310, the nutrient that deposits on the selective growth mask can have a high surface mobility and can either leave the mask surface or, if it is close enough to an open aperture, diffuse to that aperture and contribute to the growth rate in that aperture. This additional growth rate contribution can therefore vary based on the size of the apertures and the distance between frie apertures. In an exemplary embodiment for forming a plurality of GaN nanowires and/or nanowire an-ays, the growth-mode-transition can occur after a 1 minute duration of the selective growth (i.e., ti=1 minute), which can be experimentally determined by the GaN growth rate inside the patterned apertures. For example, the GaN growth rate can be about 0.6 nm/hr and the psttemed apertures can bs In the form of a hexagonsi srray naving g dismeter of about 200 nm and s pitch of about 1 urn. fOOSOJ in various smbodimente, the growth of tn; amralit)' of nsnowires and/or nanowire arrays can be affected b\' whsn ths growth-moas-transition is applied. For example, ths growtii-mode-transition can be applied after growth of the plurality of nanostructure nuclei 140 protrude overfrie top of the selective growth masf {such as 135 seen in FIGS. 1-2). in various embodiments, different configurations/dimensions can be obtained for the nanowires and/or nanowire arrays, depending on whether the growth-mode-transitlon is applied "before" (e.g., as shown in FIGS. 1-2) or "after" the nanowire nuclei have grown to protrude over the top of the selective growth mask. [00611 FIGS. 4A-4C depict a third exemplary semiconductor nanowire device 400 formed by having a growth-mode-transition "after" the nanowire nuclei have grown to protrude over the top of the selective growth mask. It should be readily apparent to one of ordinary skil! in the art that ths nanowire device 400 depicted in FIGS. 4A-4C represents a generalized schematic illustration and that other layers/nanowires can be added or existing iayers/nanowires can be removed or modified. [0062] In FIG. 4A, the device 400 can include a similar structure and be formed by a similar fabrication process as described in FIG. 1C for the device 100. As shown, yie device 400 can include a substrate 410, a selective growth mask 435 and a plurality of nanostructure nucEei 440.. The selective growth mask 435 and the plurality of nanostructure nuclei 440 can be formed over the substrate 410, wherein tiie plurality of nanostructure nuclei 440 can be interspersed through the selective growth mask 435. [00631 The substrate 410 can be an\' substrate similar to tne substrate 110 of the dsvics 100, on which a group IW-H msteria! can be growr;. The substrgte- 41D can b£, fcr sxsmpie, sapphire, silicon carb:3s, or siiicon. Likewise, the pluraiiti' c--nanostruciure nuctei 440 can bs formed simiiarh' to that of the piuraiity of nanostructure nuclei 140 of the device 100 shown in FIG. 1B. For sxannpie, the piuraiity of nanostructurs nuclei 440 can be fomned by first forming a plurality' of patterned apertures (not shown) defined by the ssiective growth masi' 435 over the substrate 410. Each of the piuraiity of patterned apertures can then be filled by growing a semiconductor material (e.g., GaN) therein using, for example, standard MOCVD. The plurality of nanostructure nuclei 440 can have a thickness of the selective growth mask 435, for example, about 30 nm, and a cross-sectional dimension, such as a width or a diameter, of, for example, about 10 nm to about 200 nm. And as an additional example, the width or diameter of the cross-seotiona! dimension can be about 10 nm to about 100 nm, [00S4] in FIG. 4B, the device 400 can include a piuraiity/ of nanostructures 442 grown laterally as well as vertically from the plurality of nanostructure nuciei 440, when the growth-mode-transition occurs "after" the plurality of nanostructure nuclei 440 proimdes over the top of the selective growth mask 435. For example, each of the plurality of nanostructures 442 can be grown laterally, spreading sideways, and partially on the surface of the selective growth mask 435. In various embodiments, the plurality of nanostructures 442 can include a pyramid-shaped structure providing a top crystal facet. For example, a plurality of GaN pyramid-shaped nanostructures can include a (0001) top facet and the dimensions of this top facet can be controlled by the extent of the growth of each nanostructure. Specifically, at the early stage of the growth, when the plurality of nanostructures 442 is growing laterally and partially on the surface of ths selective growth mask 435, the toD racst dimensions can be increased and be broader than the cros&-aecBQnal dimension? of ihe Diuraiits' of nanostructure nuclei ML. When the growth is continued, the rap facsi dimensions can De aecreased such that a point of the top facet dimensions can be smaller than tiiat of the plurality of nanostructure nuciel 44D. Therefore, the dimensions of each pyramid top facet can be controiled oy, for example, a terminatian of ths seiective growth mode (i.e., to apply the growth-mode-transition) to stop the growth of tns plurality of pyramid-shaped nanostructures. In various embodiments, ths exemplary pyramid-shaped top facets can be truncated and the dimension of each truncated top facet can then be maintained for the subsequent growth of the nanowires and/or nanowire arrays using the pulsed growth mode. In various embodiments, the truncated top facet diameter of each of the plurality of nanostructures 442 can be controlled to be smaller than that of each of the plurality of the nanostructure nuclei 440. In various embodiments, the top facet of each of the pluraliti' of nanostructures 442 can have an exemplary cross-sectional shape of, for example, a square, a polygon, a rectangle, an oval, and a circle. [00651 The device 400 shown in FIG. 4B can be used as a support of nanowires and/or nanowire arrays, which can aiso include a plurality of selected surface regions (i.e., the surface of each top facet of the plurality of nanostructures 442). A plurality of nanowires and/or nanowire an-ays can then be grown from the plurality of selected surface regions and maintain the cross-sectional features (e.g., dimensions and shapes) of each of the plurality of selected surface regions. 100683 In F'G. 4C, a plurality of nanowires 445 can be fomied by continuing the growth of the semiconductor material (e.g., GaN) from the plurality of selected surface regions of the device 400 (i.e., from each top facet of the plurality of nanostruotures 442) using the pulssc growth mode-. As s. result, the piurslity of nanowtres 445 can be reguiariy spaced and have an sxempian' diamets- rEnglng trom about 20 to about 500 nrn. snd an exemotary cross-secttonai shaoe o*i. far exampie, a square, a poiygon, a rectangle, an ovai, and a circte. |00S7] By using the pulsed growth mode -after" tne ssmiconductor material is grown to protrude over the top of the selective grovdh masi 435, the pluralitj' of nanowires 445 can be formed on the top facets of the exemplary pyrsmid-shaped structures of the plurality of nanostruotures 442. Features such as cross-sectional shapes and dimensions of each of the plurality of nanowires 445 can remain constant with that of the truncated top facets until a desired length is reacheci. in various embodiments, the length of each nanowire can be controlled on an order of micrometers, such as, for example, about 20 im or higher. [0068] FIG. 5 depicts another exemplary semiconductor nanowire device 500 including a buffer layer In accordance with the present teachings. As shown, the nanowire device 500 can include a buffer layer 520 disposed between a substrate, such as the substrate 410, and a selective growth mask, such as the selective growth mask 435. The buffer layer 520 can be a similar layer to the buffer layer 220 shown in FIG. 2. The buffer layer 520 can be a planar film formed of, for example, GaN, AIN, InN or AIGaN, using, for example, standard MOCVD. In various embodiments, the thickness of the buffer layer 520 can be about 100 nm to about 10 nm. In various embodiments, the buffer layer 520 can be doped with either an n-type or a p-type dopant in order to provide an electrical connection to the lower end of each nanowire. JOOSS] FIGS, 6A-6D depict exemplary results for a plurality of ordered GaN nanowires and/or nanowire arrays grown by the multiple-phase growth mode without use of a cataiyst in accordance with tlis present teachings {both the nanostmcture riijciel 140.440' and the nanowirss 145, 445 are grown without the use of £ mstal catalyst dsoositsd on ths substrate). As shown in FIGS. 5A-6D, the plurality of GaN nanowires SIC- can grow with large scale uniformtt\/ of position, orientaijon length, cross-sectional features (e.g., the dimensions and/or shapes), and cri'stalhnlly. As described herein, in some embodiments, the position and dimensions of each nanowirs can correspond with that of each aperture of the plurality/ of patterned apertures 138 shown in FIGS. 1-2. In other embodiments, the position and dimensions of each nanowire can con-espond with that of each top facet of the plurality of nanostructures 442 shown in FIGS. 4-5. [0070] FIG, 6A shows a ciose-up scanning electron micrograph (SEM) result for the exemplary GaN nanowires 610, while FIG. 6B shows a SEWI result with iong-range order for the GaN nanowires 610. in various embodiments, each GaN nanowire can have a single crystal nature, [0071] F!G. 6C shows that the orientation of the GaN nanowires 610 can be along a single crystal direction, for example, along the (0001) crystallographic direction of tiie exemplary GaN nanowires 510. Additionally, the small central (0001) top facet of each nanowire can be bounded by inclined (1102} facets on top of each nanowire. [00721 PI2. 6D Is a plan view of the exemplary GaN nanowires 610 showing the hexagonal symmetry of the sidewall facets of each GaN nanowire, The sidewall facets can be perpendicular to the direction of the selective growth mask 620 having tie sidewall facets of the {1100} family. In various embodiments, the diameter of the exemplary GaN nanowires 610 can be about 1000 nm or less. 100731 The Invariancs of ths lateral nanowlre geometn/ (e.g., tne cross- sectiona! features) shown \u F!G£, 6A-6D indicatss that the GaN groMt! rate car. only occur in the vertioal direction, that is, on the (0001) and {1102} top facets, =or example, the vertioal growth rates for the plurality' of GaN nanowires 610 of the puised growth can be, for sxampie, about 2 um/nr or higher. On the other hand, the GaN grow'th rate on the {1100} sidewal! facets (i.e., lateral direction) can be essential negligible in spite of their much larger area, in an exemplan/ embodiment, the GaN nanowires 610 can be grown having a uniform length of about 20 \xm or higher and maintain a uniform diameter of about 250 nm or less, when a 30-nm-selective- growth-masl carrier gas mixture can be used to control ths nanowlre geometry. E0074J In addition, the exemplary unitonn GaN nanowires 610 shown in FIGS. 6A-6D can be of high-qualiti', that is, with essentially no threading dislocations (TD). For example, there can be no threading dislocations observed with the GaN nanowires 145 and/or 445 shown in FIG. 2 and FIG. 5, even if the threading dislocations can be observed in the GaN buffer layer 220 and/or 520 underlying the selective growth mask 135 and/or 435, since it is believed that these dislocations bend away from the nanowires and terminate at a surface beneath the growth mask. Furthermore, the defect-free GaN nanowires 610 can be grown on various substrates, such as, for example, sapphire, silicon carbide such as 6H-SiC, or silicon such as Si (111). [0075] In various embodiments, the uniform and high-quality GaN nanowires and/or nanowlre arrays can be used for fabrication of high-quality GaN substrate structures. Commercially viable GaN substrates are desired because GaN substrates can greatly facilitate the fabrication of visible LEDs and lasers for the emerging soiid-state iigniinc and UV sensor industries. Ucreove:, GaN substrates can also be used in other related applications, sucii as hi-power RF circuits and devices. [0076] In various srr.bodimentt; GaN substrate structures osii be formed by ternifnating and coalescing tiie plurality of GaN nanowires sucn as tnose described in FIGS. 1-6 using Techniques such as nanoiieteroepitaxy. FIGS. 7A-7D aepictfour exempiarv semiconductor devices including GaM substrate structures 712, 714,715, and 717 formed from the plurality of GaN nanowires of the devscs 100 (see FIG. 1C), the device 200 (see FIG. 2), the device 400 (see FIG. 4C), and the device 500 (see FIG. 5), respectively. 10077] For example, the GaN growth conditions can be modified to allow coalescence of the fomied plurality of nanowires (e.g., 145 or 445) after they have grown to a suitable height, and then formation of a GaN substrate structure (e.g., the substrate 712, 714, 715, or 717). The GaN substrate structure can be a continuous, epitaxial, and fully coalesced planar film. The "suitable heighf can be detenmined for each nanowire (e.g., GaN) and substrate (e.g., SiC or Si) combination and can be a height that allows a significant reduction in defect density in the upper coalesced GaN film (i.e., the GaN substrate structure). In addition, the "suitable height" can be a height that can maintain a mechanically-robust structure for the resulting semiconductor devices, for example, those shown in FIGS. 7A-7D. In various embodiments, because threading defects are not present in the plurality of GaN nanowires (e.g., 145 or 445), the coalescence of the GaN substrate structure (e.g., the substrate 712, 714,715, or 717) on top of these pluralities of nanowires can then occur and provide the GaN substrate structure containing an extremely low defect density', such as, for example, about 10' cm" or lower. [0078] According to various smDodiments of the nanowire formation process, ths process stsps, (e.g., ths depositior, patterning and stching of the selective growth mask, the ssiective growth of nanowirs nucieL tne DUised growth of nanowires, and ths formation o" the exsmpjary GaN aubsirate struciurss) can be scaieabis to large substrate areas. They can also oe readiiy sxtendad to manufacturing requirements including automatic wafer handling and extended to larger size wafers for establishing efficacy of photonic crystals for light extractson from visible and near-UV LEDs. [0079] FIGS, 8-12 depict exemplary embodiments for nanowire active aevices including nanowire LEDs and nanowire lasers, and their scalable processes for manufacturing, in various embodiments, the disclosed group Ill-N nanowires and nanowire arrays such as GaN nanowires and/or nanowire arrays can provide their active devices with unique properties. This is because each pufsed-grown GaN nanowire can have sidewalls of {1100} family and the normal to each of these side planes can be a nonpolar direction tor group Ui-N materials. High-quality quantum group lll-N wells such as quantum InGaN/GaN wells, quantum AIGaN/GaN wells or other quantum Ili-N wells, can therefore be fonmed on these side facets of each GaN nanowire. [00801 Forexample, the nanowire growth behavior can be changed significantly when other precursor gases such as trimethylaiuminum (Al) or trimethylindium (In) are added to the exemplary MOCVD gas phase during the puised growth mode. In this case, even a smali molecular fraction (e.g., about 1%) of Al or In added to the GaN nanowires and/or nanowire arrays can result in each GaN nanowire growing laterally with its cross-sectional dimensions (e.g., width or diameter) increasing over time. This lateral growth behavior can allow creation of B core-shell heterostructurs, that is, cuantum welis including exempiaQf maieriais of sucr. as InQaN and AIGEN. afioys can be grovs?n on snc envelop each GaN nanowire core. As a result, the core-shall growth can create a core-shell nanowire/MQW active structure for light emitting devices, [0081] In various embodiments, an additional third growth condition can be established to growths core-shell of the exemplary InGaN and AlGaN alloys, after the GaN nanowire has been grown using the disclosed two-phase growtn mode. This third growth mode can be a continuous growth similar to that used in the selective growth mode, for example, as shown at 310 in Fig. 3. In various other embodiments, a pulsed growth mode can be used for the third growth condition. [0082] In various embodiments, the core-shell nanowire/MQW active structure can be used to provide high efficiency nanoscale optoelectronic devices, such as, for example, nanowire LEDs and/or nanowire lasers. For example, the resulting core-sheli nanowire/MQW active structure (I.e., having the MQW active sheli on sidewalts of each nanowire core) can be free from piezoelectric fields, and also free from the associated quantum-confined Stark effect (QC5E) because each nanowire core has non-polar sidewalls. The elimination of frie QCSE can increase the radiative recombination efficiency in the active region to improve the performance of the LEDs and lasers. Additionally, the absence of QCSE can allow wider quantum welis to be used, which can improve the overlap integral and cavity gain of the nanowire based lasers. A further exemplary efficiency benefit of using the core-shell nanowire/MQW active structure is that the active region area can be significant increased because of the unique core-shell structure. [0083] FIG. 8 depicts a cross-sectional layered structure of an exemplary core-shell nanowire/MQW active structure device 800 In accordance with the present leacnings. it snouia be rsadiiy apparent to one of orciinan' skill in ths art that the device 800 depicted in FIG. B reprssents a generalized schemaib iliustratlor. anc that other matsrials/layers/shells can be added or existing' materiais/iayers/shelis can be removed or modified. [0084] As shown, the device 800 can inciude E substrate 810, a doped buffer iaysr 820, a selective growth nnask 825, a doped nanowire core 83C. and a shell structure 635 including a first doped shell 840, a MQW shell staicture 850, a second doped shell 860, and a third doped shell 870. [0085| The selective grov\4h nnasK 825 can be formed over the doped buffer layer 820 over the substrate 810, The doped nanowire core 830 can be connected to and extend from the doped buffer layer 820 through the selective grovvth maslv 825, wherein the doped nanowire core 830 can be isolated by the selective growth mask 825, The shell structure 835 can be fonned to "shell" the doped nanowire core 830 having a core-shell active structure, and the shell structure 835 can also be situated on the selective growth mask 825. In addition, the she!! structure 835 can be fonned by depositing the third doped shell 870 over the second doped shell 860, which can be formed over the MQW shell structure 850 over a first doped shell 840, [00863 The substrate 810 can be a substrate similar to the substrates 110 and 410 (see FIGS. 1-2 and FIGS. 4-5) including, but not limited to, sapphire, silicon carbide, silicon and Ill-V substrates such as GaAs, or GaN. [0087] The doped buffer layer 820 can be fomned over the substrate 810. The doped buffer layer 820 can be similar to the buffer layers 220 and/or 520 {see FIG. 2 and FIG. 5). The doped buffer layer 820 can be fonned of, for example, GaN, AIN, InN, AIGaN, InGaN or AllnGaM, by various crystal grovirth methods known to one of ordinary skill in the art. In various embodiments, frie doped buffer layer 820 can be doped with g conductiviri' lype similar to the doped nanowire core 830. in some embodiments, the doped buffer layer 820 can be removed from the device BOO. [008S| The selective growth masic 825 can be a selective growth mast; similar to tlie ssEsctivs growtn masks 136 and/or 435 (see FIGS. 1-2 and FIGS. 4-5) formed on the buffer layer 820. In various embodiments, the selective growth masi; 825 can bs formeci dlreotiy on the substrate 610. The selective growth mask 825 can define ihe selective growth of the plurality of nanowires and/or nanowire arrays. The selective growth masl [0089] The doped nanowire core 830 can use any nanowire of the plurality of nanowires shown in FIGS. 1-2 and FIGS. 4-7 formed using the two-phase grovrth mode. The doped nanowire core 830 can be formed of, for example, GaN, AIM, SnN, AIGaN, InGaN or AIlnGaN, which can be made an n-type by doping with various impurities such as silicon, germanium, selenium, sulfur and tellurium. In various embodiments, the doped nanowire core 830 can be made p-type by introducing beryllium, strontium, barium, zinc, or magnesium. Other dopants lnown to one of ordinary skill in the art can be used. In various embodiments, the height of the doped nanowire core 830 can define the approximate height of the active structure device 800. For example, the doped nanowire core 830 can have a height of about 1 m to about 1000 urn. [0090] The doped nanowire core 830 can have non-polar sidewall facets of {1100} family (i.e., "m"-plane facets), when the material GaN is used for the doped nanowire core 830. The shell stnjcture 835 including the WIQW shell structure 850 can be grown by core-shell growth on these facets and the device 800 can therefore be free from piezoeieatric fields, and free from the associated quantum-confined Stark effect (QCSEl [C0S1] The first doped shell 84D can D& formed from and coated on the non- polar sidewall facets of the dooed nanowire core 830 by an exempian/ core-snel! growth, when the pulsed growth mode is used. For eKample, the first doped snell B40 can be formed by adding a small amount of A! during the pulsed growth of the doped nanowire core 830 forming a cors-shell heterostructurs. The conductivity type of the first doped shell 840 and the doped nanowire core B30 can be made similar, for example, n-type, in various embodiments, the first doped shell 840 can include a material of Al;;Gai.xN, where x can be any number less than 1,00 such as 0.05 or 0.10. [0092] The MQW sheli structure 850 can be formed on the first doped shell 840 by the exemplary core-shell growth, when the pulsed growth mode is used. Specifically, the MQW shell structure B50 can be formed by adding a small amount of Al and/or In during the pulsed growth of the first doped shell 840 to continue the formation of the core-shell heterostructure. In various embodiments, the MQW shell structure 850 can include, for example, alternating layers of AlxGai.xN and GaN where x can be, for example, 0.05 or any other number less than 1.00. The MQW shell structure 850 can also include alternating layers of, for example, InGai-xN and GaN, where x can be any number less than 1,00, for example, any number in a range from about 0.20 to about 0.45. [0093] The second doped shell 860 can be formed on the MQW shell structure 850. The second doped shell 860 can be used as a barrier layer for the MQW shell structure 850 with a sufficient thickness of, such as, for example, about 500 nm to about 2000 nm. The second doped shell 860 can be formed of, for example, Alj.G&v/N, where x can be an}' number less Inan 1.00 such as C.20' orG.30. The second aoped shell 860 can be GODSCI with a coflduafivttv' lype simila-io the third doped sheS! 870 [0©94] Tna third doped she!! 870 csn DS formed by continuing the core-sheli groPifth irom the second aopea shell 860 to cap the active structure device BOL. The third doped shel! 670 can be formed of, for sxampie, GaFvJ and dooed to DE an n-type or a p-type. In various embodsments, if the first doped shell 830 is an n-type shell, the second doped shell 860 and/or the third doped shell 870 can be a p-type shell and vice versa. In various embodiments, the third doped shell 570 can have a thicl shown in FIG. 8 can be electrically isolated from each other, when a number of devices 800 are included in a large area such as a wafer. FIG. 9 depicts an active structure device 900 including a dielectric material 910 deposited to isolate each core-shell nanowire/MQW active structure shown in FIG. 8 in accordance with the present teachings. [00961 As shown in FIG. 9, the dielectric material 910 can be deposited on the selective growth mask 825 and laterally connected with the sidewalls of the shell stmcture 835, more specifically, the sidewalls of the third doped shell 870. In various embodiments, the dielectric materia! 910 can be any dielectric material for eiectrica! isolation, such as, for example, silicon oxide (SiOa), silicon nitride {Si3N4), silicon oxynltride (SiON), or other insulating materials. In some embodiments, the dielectric material 910 can be a curable dielectric. The dielectric material 910 can be formed by, for example, chemical vapor deposition (CVD) or spin-on techniques, with a desired height or thicl<:ness. in various embodiments ths heinht nf tho> uisieciriG maiensi s'lu can oe Tutiner aajusiea ay rsmovjng a ponion orine aieiscino material from the top of the deposited dtsiectrio materis: using, for example, etching cr lift-off prDcedurss known tc one of :::l!nanf skift lu thfr art. The thickness of the dieiectric material 910 can bs adjusted depending on specific appilcations wnere the core-sheii nanowire/MQW active structure is used, jiOOSTJ in various embodiments, various nanowire LEDs and nanowire iasers can be formed by the core-shsli growth described in FIGS. 8-9, because MQW active shell structures can be created on the nonpoiar sidewalls of the pulsed-grown nanowires, For example, if the nanowtres are arranged in a hexagonal array wth s pitch that is equal to 1.12, where X is the emission viavelength of the exemplary LED or laser, the array of nanowires can provide optical feedback to stimulate light-emitting action. FIGS. 10-12 depict exemplary nanoscale active devices formed based on the structures shown in FIGS. 8-9 in accordance with the present teachings. [0098] FI6S.10A-10C depict an exemplary nanowire LED device 1DDO using the core-shell nanowire/MQW active structure described In FIGS. B-9 in accordance with the present teachings. [0099] In various embodiments, the nanowire LED device 1000 can be fabricated including electrical contacts formed on, for example, the device 900. The electrical contacts can include conductive structures formed from metals such as titanium (Ti), aluminum (AI), platinum (Pt), nickel (Ni) or gold (Au) in a number of multi-layered combinations such as AinTt/Pt/Au, Ni/Au, Ti/AI, Ti/Au, Ti/AI/Ti/Au, Ti/AI/Au, AI or Au using techniques known to one of ordinary skill in the art, [00100] In FIG. 10A, the device 1000 can include a conductive structure 1040 formed on the surface of the device 900, i.e., on each surface of the dielectric materia! 910 and the third doped shell 870 of the shell structurs 835. The conduciivs structure 104C' can be a transparsnt layer used tor a pi-siectroae of the LED aevlce 1000 fabricated subsequently, in an exempisjy embodiment :ne conductive structure 1040 (or p-elecrrods) can be, rorsxampie, a layered meial combination of Ti/Au. 100101] in various embodiments, the device 1000 can further include a dieiectric layer 1010 having an adjusted thickness (or height). By adjusting the thickness of the dielectric iayer 1010, the extent (e.g., thiciness or height) of the conductive structure 1040 (or p-electrode) formed on and along the sidewall of the shell structure 835 can be adjusted according to the desired application of nanowire active device, For example, a thick layer of the dieiectric 1010 can confine the conductive staicture 1040 (or p-electrode) to the top of the core-shell structured active devices, for example, for nanowire LEDs and/or nanowire lasers. Alternatively, an adjusted thin dielectric layer 1010 can allow the conductive structure 1040 (or p-electrode) to have a greater thickness or height (i.e., an increased extent), which can reduce the resistance of the active devices. In various embodiments, the greater thlcl performance. In various embcdimenfe, the LED device 1000 can have E tofeJ nelghl of, forsKampie, about 10 ym. [001031 iri FIG. 10B, the device 1000 can further include s p-eisotrods 104£, e dieiscinc 1015. and g seisctive contact mas: 1025 having trenches 1036 stoned into the seisctive growth mask 825 (see FIG. 10A). [00104] The p-eisctroae 1045 and ths unaeriying dielectric 1015 csri oe formed by patterning and etching the conductive structure 1040 and the dieisctric layer 1010 (see FIG. 10A). As a result, portions (not shown) of surface, of the selective growth mask 835 can be exposed and separated by the dieiectric 1015 on both sides of each core-shell structure. After the patterning and etching processes, a selective contact mask 1025 can be formed by forming trenches 1035 through the exposed portions of surface of the selective growth masl [00105] In various embodiments, the thickness of the selective contact mask 1025 can be critical for the performance of the LED device 1000, For example, a silicon nitride selective growth maslc having a thicl region including the doped buffer layer 820 and the nanowirs core BSD. The centrai conductive region can be, far example, s heavily aoped rC GaN region. In vgrtous embodimenis, the r*-electrod9; 1080 csr. inctudf- conductFV& structures formed by depositing electrode materials onto each surface of the seisctive contact masSc 1026 and the bottoms of the trsncnes 1035, in an exempiar/ embodiment, the r-eiectrodes 1080 can oe formed ot for example, a layered mstai combination, such as Al/Ti/Pt/Au. [00107] At 1099, the resulting light of the nanowlre LED device 1000 in FIG. 10C can be extracted through the substrate 820, which can be transparent at green and blue \A/avelengths. In various embodiments, a more diffuse light output can occur on the top side of the device 1000 (not shown) since the nanowirs LED device 1000 can be small enough for sufficient diffraction. This diffuse light output can be advantageous in some solid-state lighting applications. [00108] In this manner, the disclosed nanowire LED device 1000 can provide unique properties as compared with traditional LED devices. First, it can have a higher brightness because the core-shell grown active region area {i.e., the IVIQW active shell area) can be increased, for example, by a factor of approximately 10 times compared to a conventional planar LED structure. Second, the light extraction can be improved to increase the output efficiency of the LED. This is because the LED device's geometry can make the most of the active region area oriented norma! to the wafer surface, i.e., the substrate surface. The confinement regions on either side of the MQW active region can tend to guide the LED light in the vertical direction. Third, because of the high precision of the position and diameter of each of the plurality of nanowires and/or nanowire arrays, the resulting arrays of the LED devices 1000 can also be configured as a photonic-crystal, which can further improve the iight output coupiing efficiency. Fourth, the nanowJre LED resisrancs can be significantly decreased because of the increase of the eisctricEi cor-tact srsE, for example, tie contact area of ins p-eiectrode 104£. Finally, sine© tne LED aevice 1000 can provide a specified light power with higner brightness, more devices can bs processed on a given wafer, which can decrease tne cost of production and also increase the manufacturing efficiency. For exampie, to allow for metai contacis, the LED device 1000 can include a pitch spacing (i.e., a ceraer-to-csnter spacing between any two adjacent nanowire devices) of, for example, about 100 jann. A 4-inch wafer can then include a number of nanowire LED devices 1000, for example, about 0.78 million devices or more, which can be manufactured simultaneously, in various embodiments, the pitch spacing can be reduced further to allow a single 4-inch wafer to contain, for example, more than one million LED devices 1000. [00109] FIGS. 11-12 depict exemplary nanowire laser devices using the core-shell grown nanowire/MQW active stmcture shown in FIGS. 8-10 in accordance with the present teachings, Because the sidewal! facets of the nanowires and/or nanowire arrays are exact {1100} facets with a flatness on the scale of an atomic monolayer, high quality IWQW active regions for laser devices can be formed on these superior fiat "sidewali substrates." in addition, the vertical orientation of the sidewail facets, and the uniform periodicity and length of tlie nanowires can provide a high-throughput method of etching or cleaving facets to form an optical cavity. The uniform periodicity can allow a photonic crystal optical cavity to be established sfraightforwardly. [001101 As shown in FIG. 11, the nanowire laser device 1100 can be fabricated from the processes described in FIGS. 8-10 using the core-sheli grown nanowire/MQW active structure as laser active structure. The nanowire laser device 1100 can include a poiished shell structure 1135, a polisned p-etectrode 1145, and a passivation iayer 1195, which can be formed on each surtace of the polisnsd snsi strucrurs ;".35 and the polished o-elecirode 1 'f45 K* cap the aser sctive struciure. [001111 The polished shell staicturr 1135 sns tne poiishad p-eiectrodfc 1145 can D3 formed by polishing (i.e., removing) on the top end (with respect to the substrate 810 as the bottom end) of the core-shell nanowire/WIQW active structure (i.e., laser active structure) such as tiiat shown in FtG. IOC: Various polishing processes, for example, a chemtcal-mechamcal polishing, can be used using the etched dielectric 1015 as a mechanical support. [00112] The polishing step can be used to polish a number of laser facets at the same time without diminishing the manufacturabillty of the nanowine laser devices 1100, For example, a number of nanowire laser devices 1100 such as about 0.78 million or more, can be fomied on a 4-inch wafer for a high manufacturing efficiency. In various embodiments, the pitch spacing can be reduced further to allow a single 4-inch wafer to contain, for example, more than one million laser devices 1100. [00113] In various embodiments, the extent (e.g., thickness or height) of the polished p- electrode 1145 fomried along the sidewalls of the polished shell structure 1135 can be adjusted by adjusting thickness of the underlying etched dielectric 1015 for optimum perfonnance of the laser device 1100. In various embodiments, the thickness of the polished p-eiectrode 1145 along the sidewall of the polished shell structure 1135 shown in FIG. 11 can range from about 1 p.m to about 5 m, when the overall height is about 10 fim. [00114] The passivation layer 1195 can be formed at the polished top end of each laser active structure, i.e., on each surface of the polished p-slectrode 1145 and the poirshed shell structure 1135. The passivation iaysr 1125 can be ccnfigursd to avoid undue non-radiative rscombination orjunotion isakage of the nsnolrs IRBST device HOC, in various embodiments, tne passivatiori layer 1195 can oe to-msd of, for example, any dielectno material known to one of ordinar\/ skill in the an with a thickness of about 10 to about 10D nm. [001151 In some embodiments, the composjtion and reiractivs index of the materials used for tine polished shell structure 1135 surrounding the nanowire cavity (i.e., the nanowire core 830) can affect the optical lasing process at 119B. For example, when the nanowires have an exemplary diameter of about 200 nm, some of the optical lasing mode can exist outside the cavity. The laser can therefore be more sensitive to the composition and refractive Index of the materials surrounding the cavity, that Is, materials used for each layer of the polished shell structure 1135. [001161 In other embodiments, because there is no physical lower facet on the laser optical cavity (i.e., the nanowire core 830), there can be a change of effective refractive index in the vicinity' of the selective growth mask 1025. This index change can in fact be helped (I.e., made larger) by the fact that some of the optical iasing mode can exist outside the cavity. In an exemplary embodiment, the nanowire laser device 11 GO (see Fig. 11) can be optically tuned by adjusting the thickness of the selective contact mask 1025 for a maximum reflectivity. For example, the optical thickness of the selective contact mask 1025 for the laser device 1100 can be in a range of about 220 nm to about 230 nm when the device is emitting blue light at 450 nm. [00117] FIG. 12 depicts another exemplary laser device 1200, in which a distributed Bragg reflector (DBR) mirror stack 1220 can be disposed between the layers of the substrate 810 and the selective growth masfc 1025, as opposed to the doped buffer layer 820 being disposed between these two iaysrs of the laser device 1100 shown in Fit. 11. [00113] The, DBR mirror stack 122G can bs an epitaxial DBF. mirror stack, The DBR mirror stack 1220 can include, for example, quarter-wavs aitemsting layers o;, for example, GaN and AIGaN, In various embodiments, the DBR mirror stack 1220 can bs tuned to improve refiectivity and to increase cavity Q of the laser 1299. [00119] In various embodiments, ali the nanowire active devices shown in FIGS. 10-12 can provide a iow device resistance because more resistive p-eiectrodes {e.g., the p-electrode 1045 and/or 1145) of the heterostructure can be located at the larger-area, which is outer periphery of each core-shell nanowire/MQW active structure. For example, for the LED device 1000 (shown In FIG. 10), the p-electrode 1045 can be patterned to completely cover the top of the device 1000 to further decrease the device resistance. [00120] Although a single nanowire is depicted In FIGS. B-12 for the purpose of description, one of ordinary skill in the art will understand that the core-shell growth processes on each nanowire of the plurality of nanowires and/or nanowire arrays (e.g., shown In FIGS. 1-6) for nanoscale active devices can be simultaneously conducted in a large area (e.g., a whole wafer). [00121] Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims. WHAT IS CLAIMED !S: 1. A method of making nanowires comprising: forming a seiectivs growth mask over a substrate, wherein the selactive growth mask comprises a plurality of patterned apertures that expose a plurality of portions of ths substrate; using a selective non-pulsed growth mode to grow a semiconductor materia! on each of the plurality of portions of the substrate exposed in each of the pattemgd apertures; performing a growth-mode transition from the non-pulsed growth mode to a pulsed growth mode; and forming a plurality of semiconductor nanowires by continuing the pulsed growth mode of the semiconductor material. 2. The method of claim 1, wherein the substrate comprises a buffer layer over a supporting substrate surface, and the semiconductor material is selectively grown through the plurality of patterned apertures on the buffer layer. 3. The method of claim 1 or 2, wherein the substrate comprises one or more materials selected from the group consisting of Si, SiC, sapphire, GaN and GaAs. 4. The method of claim 1 or 2, further comprising one or more cleaning processes prior to the selective non-pulsed growth of the semiconductor material. J 5. The methed of claim 1 or 2, wherein the plurslity of patterned aperturss forms a hexagonal array having a diameter of about 10 nm to about 1000 nm and a Pitch of about 50 nm to about 10um. 6. The method of claim 1 or 2, wherein a cross-sectional feature of each of the Dlurallty of semiconauctor nanowirss and each of the plurality of patterned apertures is substantially similar. 7. The method of claim 6, wherein the cross-sectional feature is a shape selected from the group consisting of a polygon, a rectangle, a square, an oval, and a circle. 8. The method of claim 6, wherein the step of performing a growth-mode transition from the non-pulsed growth mode to the pulsed growth mode occurs before growth of the semiconductor materia! protrudes over a top of the selective growth mask. 9. The method of claim 1 or 2, wherein the semiconductor material for the plurality of semiconductor nanowires comprises one or more materials selected from the group consisting of GaN, AIN, InN, InGaN, AllnGaN and AIGaN, 10. The method of claim 1 or 2, wherein the selective growth comprises Group Ill and Group V precursor gases having a lll/V ratio ranging from about 100 to about 500. 11. The method of claim 1 or 2, wherein the pulsed growth comprises alternately introducing Group III and Group V precursor gases of the semiconductor material in a growth rsactor with one or more sequence loops, wherein the precursor gases comprise s III/V ratio ranging from about 50 to about 300. 12.. The method of claim 1 or 2. wnsrein the pulsed growth comprises a vertical growth rate of about 2 um/hr or higher. 13. The method of claim 1 or 2, wherein each of the plurality of nanowires has a length of about 10 nm to about 100 um. 14. The method of claim 1 or 2, wherein: the step of transitioning from the non-pulsed growth mode to the pulsed growth mode occurs after growth of the semiconductor material protrudes over a top of the selective growth mask to form a plurality of truncated pyramid-shaped nanostructures partially disposed on a surface of the selective growth mask; and the step of forming the plurality of nanowires comprises forming a semiconductor nanowire on each of the plurality of pyramid-shaped nanostructures by continuing the pulsed growth of the semiconductor materia! such that a cross-sectiona! feature of the semiconductor nanowire and a top facet of each of the plurality of pyramid-shaped nanostructures is substantially similar. 15. The method of claim 14, wherein the semiconductor nanowire comprises a cross-sectional dimension smaller than that of each of the plurality of patterned apertures. 16. A group lll-N nanowire array formed by the method of claim 1 or 2, comprising: a, support comprising a plurality of seiected surface regions; and a group lll-N nanowire connected to and extending from each of the piuraliry of selected surface regions of the support, wherain the group lll-N nanowirs is oriented aiong a singie direction and maintains a cross-sectional feature of one of tne piuraiity of selected surfacs regions. 17. The nanowire array of claim 16, further comprising a GaN nanowire oriented along (0001) crystallographic direction. 18. The nanowire array of claim 16, wherein the group lll-N nanowire comprises one or more materials selected from the group consisting of GaN, AIN, inN, InGaN, AIGaN, and AllnGaN. 19. The nanowire array of claim 16, wherein the group lll-N nanowire comprises one or more cross-sectional shapes selected from the group consisting of s polygon, a rectangle, a square, an oval, and a circle. 20. The nanowire array of claim 16, wherein the group lll-N nanowire further comprises an aspect ratio of about 100 or higher and a cross-sectional dimension of about 250 nm or less. 21. The nanowire array of claim 16, wherein the support comprises a group III-N nanowire nucleus disposed on each of a plurality of portions of a substrate through a selective growth mask disposed on the substrate, wherein a surface of the group lll-N nanowire nucleus comprises one of the plurality of selected surface regions of the support. 22. The nanowire array of claim 21, wherein the support further comprises a pyramic-shaped group III-N nanostructure formed from the groupIII-N nanowire nucleus and partially disposed on the selective growth mask, wherein a top facet of the pyrarnid- shaped group Ill-N nanostructure comprises one of the plurality of ssiected surface regions of the support 23. A group lll-N nanowire array comprising: a substrate; a selective growth mask over the substrate, wherein the selective growth mask comprises a plurality of patterned apertures that expose a plurality of portions of the substrate; and a group lll-N nanowire connected to and extending from each of the plurality of portions of the substrate, wherein the group lll-N nanowire is oriented along a single direction and maintains a cross-sectional feature of one of the plurality of selected surface regions, and wherein the group Ill-N nanowire extends over a top of the selective growth mask. 24. A GaN substrate structure comprising: the nanowire array formed by the method of claim 1 or 2 comprising a plurality of GaN nanowires, wherein each of the plurality of GaN nanowlres is defect free; and a GaN film coalesced from the plurality of GaN nanowires, wherein the GaN film has a defect density of about 10 cm2 or lower. 25. A substrate comprising a plurality of nanowires formed by the method of claim 1 or 2. |
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| Patent Number | 279619 | ||||||||||||||||||||
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| Indian Patent Application Number | 4729/CHENP/2008 | ||||||||||||||||||||
| PG Journal Number | 05/2017 | ||||||||||||||||||||
| Publication Date | 03-Feb-2017 | ||||||||||||||||||||
| Grant Date | 27-Jan-2017 | ||||||||||||||||||||
| Date of Filing | 08-Sep-2008 | ||||||||||||||||||||
| Name of Patentee | STC.UNM | ||||||||||||||||||||
| Applicant Address | RESERARCH & TECHNOLOGY LAW (R&TL) UNIVERSITY BLVD. SE, SUITE 104, ALBUQUERQUE, NEW MEXICO 87106 , | ||||||||||||||||||||
Inventors:
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| PCT International Classification Number | H01L 21/20 | ||||||||||||||||||||
| PCT International Application Number | PCT/US07/36373 | ||||||||||||||||||||
| PCT International Filing date | 2007-03-09 | ||||||||||||||||||||
PCT Conventions:
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