WO2005110057A2

WO2005110057A2 - Crystallographic alignment of high-density nanowire arrays

Info

Publication number: WO2005110057A2
Application number: PCT/US2005/000568
Authority: WO
Inventors: Peidong Yang; Tevye Kuykendal; Peter Pauzauskie
Original assignee: The Regents Of The University Of California
Priority date: 2004-01-06
Filing date: 2005-01-06
Publication date: 2005-11-24
Also published as: WO2005110057A3

Abstract

A method for controlling the crystallographic growth direction and geometric and physical characteristics of nanowires using a metal-organic chemical vapor deposition and substrate selection. As an illustration of the method, epitaxial growth of wurtzite gallium nitride on (100) y -LiAIO2 and (111) MgO single crystal substrates resulted in the selective growth of nanowires in the orthogonal [110] and [001] directions, respectively. Triangular and hexagonal cross sections were observed as a result of substrate-induced constraints of lattice parameter matching and symmetry registry. These nanowire arrays exhibit a systematic difference in their temperature dependent band-edge emission resulting from the different size, shape, and anisotropic polarity of the nanostructures. Scaling of the synthetic process is entirely compatible with existing GaN thin-film technology and should enable the realization of a new generation of GaN nanowire devices and systems.

Description

CRYSTALLOGRAPHIC ALIGNMENT OF HIGH-DENSITY NANOWIRE ARRAYS

CROSS-REFERENCE TO RELATED APPLICATIONS [0001] This application claims priority from U.S. provisional application serial number 60/534,759 filed on January 6, 2004, incorporated herein by reference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0002] This invention was made with Government support under Contract No. DE-FG03-02ER46021 with the Department of Energy and Grant No. DMR- 0092086 awarded by the National Science Foundation. The Government has certain rights in this invention.

INCORPORATION-BY-REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISC [0003] Not Applicable BACKGROUND OF THE INVENTION 1. Field of the Invention [0004] This invention pertains generally to fabrication of nanostructures, and more particularly to crystallographic alignment of high-density semiconductor nanowire arrays. 2. Description of Related Art

[0005] Semiconductor nanowires of various compositions and properties are beginning to emerge as important building blocks for miniaturization to provide nanoscale electronic and optoelectronic devices. Carbon nanowires, for example, have been extensively studied for use in miniaturization. However, the chirality of the carbon nanowires may make the wires conductors, semiconductors or non-conductors and therefore other semiconductor materials such as Si, GaN and GaAs are preferred over carbon. Also, the size of individual semiconductor nanowires is suitable for waveguiding ultraviolet frequencies of light, analogous to infrared waveguiding in silica- based fiber optic cables. In comparison, individual carbon nanotube structures have radial dimensions that are too small to confine and waveguide ultraviolet and visible radiation. This makes these arrays prime candidates for the architectural foundation of integrated photonic sensors and information processors. [0006] Increasing attention has been devoted to the lll-V nitride semiconductors in recent years because of their chemical and physical properties. GaN, for example, is an ideal material for high-power/high temperature electronic applications because of superior thermal conductivity, large band gap and excellent electron transport properties. For example, the ability of GaN nanowires to be light sensitive has been found to be useful in the design and production of quantum devices such as light detectors and lasers as well as electronic components like diodes and transistors.

[0007] However, the controlled, directional growth of nanowires of GaN and other materials at predetermined sites has not been accomplished to date. For example, semiconducting nanowires of Gallium nitride (GaN) are typically formed by the reaction of pure gallium with ammonia at temperatures between 850 °C and 900 °C. Nanowires produced by this process in the art have been seen to have inconsistent structural characteristics, with some wires being curvy and random and others being straight and uniform. [0008] Control over the direction of nanowire growth is important because such growth may determine the anisotropic parameters of the nanowire such as thermal and electrical conductivity, index of refraction, piezoelectric polarization and band gap. There is a need for a process that will permit the fabrication of nanowires with uniform characteristics and precise control over the direction and characteristics of the nanowire. [0009] The present invention satisfies that need by providing techniques to control the crystallographic growth direction as well size, aspect ratio, position and composition of the nanowires that are compatible with existing chemical vapor deposition techniques. BRIEF SUMMARY OF THE INVENTION [0010] Control over nanowire growth direction may be used to tune the physical properties of nanowires made from a given material. The disclosed method will greatly facilitate the realization of nanowire materials-by-design. Gallium nitride and zinc oxide nanowires are used to illustrate the process. However, the disclosed methods will work for lll-V binary compositions and their ternary and quaternary alloys as well as other materials capable of crystallization into nanowires or nanostructures. [0011] By way of example, and not of limitation, the preferred method for producing nanowires is a metal-organic chemical vapor deposition (MOCVD) process that takes advantage of a vapor-liquid-solid growth mechanism using Au, Co, Fe, In, or Ni nanoparticles as catalysts. Existing synthesis schemes for GaN based materials have employed laser ablation, chemical vapor transport, or hydride vapor epitaxy etc. Most of these processes use Ga metal as a high temperature vapor source. Metals like Ni, Fe, and Au have been used as initiators for vapor-liquid-solid (VLS) nanowire growth, although it is also possible to generate nanowires in this system via self-catalytic VLS or a direct vapor-solid process. [0012] The present invention generally provides a method for controlling the growth and structure of a nanowire, in part, by matching a substrate with the material forming the nanowire of interest. One factor in the selection of the substrate is the similarity of the substrate to one or more lattice parameters of the material forming the crystalline nanostructure. Another factor is matching the symmetry between the substrate crystal structures and the nanowire. [0013] Substrates of lithium aluminum oxide and magnesium oxide were used with GaN and ZnO. Epitaxial growth of wurtzite gallium nitride on (100) γ - LiAIO₂ and (111 ) MgO single crystal substrates resulted in the selective growth of nanowires in the orthogonal [110] and [001] directions, and produced distinct triangular and hexagonal cross sections. [0014] The crystals of both substrate materials are geometrically compatible with gallium nitride crystals, but the lithium aluminum oxide features a two-fold symmetry that matches the symmetry along one plane of the gallium nitride crystals, whereas the magnesium oxide has a three-fold symmetry that matches gallium nitride symmetry along a different plane. Consequently, when the GaN crystal precipitates from a saturated metal catalyst droplet, the resulting nanowires grow epitaxially to the substrate but aligned in a unique direction, perpendicular to each substrate. Because of the different growth directions, cross sections of the gallium nitride nanowires grown on lithium aluminum oxide form an isosceles triangle, while the cross-sections of those grown on magnesium oxide are hexagonal. [0015] It was also shown that the nanowires made from the same GaN material, but grown on different substrates, produced different light emissions blue-shifted by approximately 100 milli-electron volts. Systematic differences in the optical properties of the wires were observed to depend on the crystallographic orientation of the primary growth axis. [0016] In accordance with one aspect of the invention, a method for deterministic growth of high-density arrays of nanowires, comprises matching symmetry and at least one lattice parameter of a substrate with a nanowire material, and forming nanowires on said substrate. According to another embodiment, a method for fabricating single crystalline, one dimensional semiconductor nanostructures, comprises selecting a semiconducting base material, selecting a substrate that has matching symmetry and at least one matching lattice constant parameter with said semiconducting base material, applying a catalyst onto the substrate, and forming said nanowires on said substrate. According to a further aspect of the invention, a method for fabricating GaN nanowires, comprises selecting a substrate, applying a catalyst onto the substrate, and forming GaN nanowires on said substrate. These nanowires can be formed using processes such as vapor-liquid-solid growth, chemical vapor deposition, metal-organic chemical vapor deposition, chemical vapor transport, hydride vapor epitaxy, laser ablation and molecular beam epitaxy. [0017] Another aspect of the present invention is to provide a method for the deterministic growth of high-density arrays of vertically aligned nanowires of gallium nitride or other material using a metal-organic chemical vapor deposition process. [0018] Another aspect of the invention is to provide a method for deterministic growth of high-density arrays of vertically aligned gallium nitride nanowires using a metal-organic chemical vapor deposition process. In one mode, gallium nitride nanowires are grown on a ^ -LiAIO₂ substrate. In another mode, gallium nitride nanowires are grown on a MgO substrate. In still another mode, nanowires are selectively grown in the orthogonal [1 Ϊ0] and [001] directions by epitaxial growth of wurtzite gallium nitride on (100) ^-LiAIO₂ and (111 ) MgO single crystal substrates. [0019] Another aspect of the invention is a method for selective growth of high-density gallium nitride arrays with specific crystallographic growth directions by selection of substrate symmetry and lattice constant parameters. In one mode, the substrate comprises MgO. In another mode, the substrate comprises ^-LiAIO₂. [0020] Another aspect of the invention is a method for deterministic growth of high-density arrays of vertically oriented zinc oxide nanowires using a metal- organic chemical vapor deposition process. In one mode, the nanowires are selectively grown by epitaxial growth of zinc oxide on LiAIO₂ substrate. [0021] Another aspect of the invention is a method for fabricating GaN nanowires, comprising selecting (100) ^ -LiAIO₂ and (111 ) MgO substrates, thermally evaporating a thin film of Ni, Fe, or Au catalyst onto the substrates, using a shadow mask for patterned nanowire growth, and growing said nanowires using vapor-liquid-solid growth on said substrates. [0022] Another aspect of the invention is to provide a method for controlling crystallographic growing directions in arrays of nanostructures using metal- organic chemical vapor deposition and substrate selection. [0023] Another aspect of the invention to form nanostructures, aligned nanostructure arrays, aligned nanowire arrays, nanowire devices, nanowire systems using controlled crystallographic growth according to any of the methods herein. [0024] Further aspects of the invention will be brought out in the following portions of the specification, wherein the detailed description is for the purpose of fully disclosing preferred embodiments of the invention without placing limitations thereon. BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS [0025] The invention will be more fully understood by reference to the following drawings which are for illustrative purposes only: [0026] FIG. 1 A-C are transmission electron microscopy images of the GaN nanowire grown on a (100) y-LiAIO₂ substrate in the orthogonal [001] growth direction showing wires with a triangular cross-section produced according to the invention. [0027] FIG. 2A-B are transmission electron microscopy images of the GaN nanowire grown on a (111 ) MgO substrate in the orthogonal [110] growth direction showing wires with a hexagonal cross-section. [0028] FIG. 3A-B are X-ray diffraction patterns recorded on wires grown on the (100) y -UAIO₂ substrate and the (111 ) MgO substrate respectively. The gray shapes indicate the respective orientation of the GaN crystal faces at the interface.

[0029] FIG. 4A-B are transmission electron microscopy images of the GaN nanowire grown on (A-C) (100) y-LiAIO₂ substrate. The inset in FIG. 4A is an electron diffraction recorded along [001] zone axis. [0030] FIG. 4D-E are transmission electron microscopy images of the GaN nanowire grown on (111 ) MgO substrate. The inset in FIG. 4D shows the hexagonal cross-section of the wire and an electron diffraction pattern recorded along [1 Ϊ0] zone axis. [0031] FIG. 4C and FIG. 4F show space-filling structural models for the nanowires with triangular and hexagonal cross sections. [0032] FIG. 5 is a graph of temperature-dependent photoluminescence collected on the two sets of GaN nanowires of FIG. 1 and FIG. 2 with different growth directions. The emission energy is plotted as a function of the temperature and the ♦ symbol represents the hexagonal [001] wires, and the ▲ symbol represents the triangular [110] nanowires. [0033] FIG. 6A is a graph of X-ray absorption spectra for triangular [1 Ϊ0] nanowires showing the N K-edge absorption as the array sample rotates with respect to polarization. [0034] FIG. 6B is a graph of X-ray absorption spectra for triangular [001] nanowires showing the N K-edge absorption as the array sample rotates with respect to polarization. [0035] FIG. 7A is a scanning electron microscopy image of ZnO nanowire arrays grown on sapphire substrates showing hexagonal cross-section and [001] growth direction. [0036] FIG. 7B is a scanning electron microscopy image of ZnO nanoribbon arrays grown on LiAIO₂ substrate showing ribbon geometry and [1 Ϊ0] growth direction.

[0037] FIG. 8A-B are X-Ray diffraction (XRD) diffraction patterns collected on ZnO nanowire arrays grown on sapphire substrates showing [001] growth direction in FIG. 8A and ZnO nanoribbon arrays grown on LiAIO₂ substrate showing [1 Ϊ0] growth direction in FIG. 8B. DETAILED DESCRIPTION OF THE INVENTION

[0038] Nanowires of various compositions are important building blocks for circuit miniaturization and allow complex nanoscale electronic and optoelectronic devices. Nanowires can be fabricated through the use of a number of known techniques that are generally classified as either "top down," including electron beam nanolithography, or "bottom up," techniques that include direct chemical reactions. However, there are limited techniques that allow one to control the location and size of the nanowires. Control over the direction of nanowire growth is important because such growth may determine the anisotropic parameters of the nanowire such as thermal and electrical conductivity, index of refraction, piezoelectric polarization and band gap.

[0039] Techniques to control the crystallographic growth direction as well size, aspect ratio, position and composition of the nanowires are provided that are compatible with existing chemical vapor deposition hardware and techniques. Although, the production of gallium nitride and zinc oxide nanowires with varying characteristics are used to illustrate the methods, it will be understood that the materials, substrates and methods are not limited to the illustrations.

[0040] The following examples illustrate the control that may be exerted over the location and structure of nanowires as well as their corresponding physical characteristics. It will be seen that the three dimensional structure of the nanowire can be determined by the selection of the appropriate substrate for the particular crystalline material that is chosen for the nanowire.

[0041] In the examples shown below, substrates of lithium aluminum oxide and magnesium oxide were used in the production of gallium nitride and zinc oxide nanowires. The crystals of both substrate materials are geometrically compatible with gallium nitride crystals, but the lithium aluminum oxide features a two-fold symmetry that matches the symmetry along one plane of the gallium nitride crystals, whereas the magnesium oxide has a three-fold symmetry that matches gallium nitride symmetry along a different plane. [0042] A metal initiated, metal-organic chemical vapor deposition (MOCVD) process (see, T. Kuykendall et al., Nano. Lett. 3, 867 (2003), incorporated by reference herein in its entirety) is preferably used to produce nanowires with distinct geometric and physical properties. In the example of gallium nitride nanowires, trimethylgallium (TMG) and ammonia source materials were used as Ga and N precursors, which are fully compatible with the current GaN thin film technology. [0043] In this process, when a vapor of gallium nitride condenses on either of the lithium aluminum oxide or magnesium oxide substrates, the resulting nanowires grow perpendicular to the substrate but aligned in a direction unique to each substrate. Because of the different growth directions, cross sections of the gallium nitride nanowires grown on lithium aluminum oxide form an isosceles triangle, while the cross sections of those grown on magnesium oxide are hexagonal. In one embodiment, epitaxial growth of wurtzite gallium nitride on (100) γ-LiAIO₂ and (111 ) MgO single crystal substrates results in the selective growth of nanowires in the orthogonal [110] and [001] directions, respectively. [0044] The selection of single crystal substrates is critical for achieving deterministic growth direction control. A close match of both symmetry and lattice constant between the substrate and nanowire material composition is essential for successful heteroepitaxy and strongly influences the nanowire growth direction. It will be apparent to one skilled in the art that essentially any selected nanowire material can be matched with a suitable substrate to provide a desired nanowire structure and size. [0045] In the case of the gallium nitride example, the oxygen sublattice in the (100) plane of -LiAIO₂ has two-fold symmetry, which matches well with the two-fold symmetry of the (100) plane of wurtzite GaN. The lattice constants a=5.17 A and c=6.28 A of y-LiAIO₂ match closely with the lattice constants c=5.19 A and two times a=3.19 A of GaN respectively. In contrast, the (111 ) plane of MgO has three-fold symmetry and an inter-atomic separation of 2.98 A for atoms in the (111 ) plane. This matches well with the 3-fold symmetry of the (001 ) plane of GaN and the lattice constant a=3.19 A. As a result, we chose these two substrates with the expectation to selectively grow GaN nanowire with [1 Ϊ0] and [001] directions, respectively. [0046] Nanowires with triangular, hexagonal and rectangular cross sections were observed as a result of the substrate induced constraints of lattice parameter matching and symmetry registry. The illustrative nanowire arrays exhibit systematic differences in their temperature dependent band-edge emissions as a consequence of the different size, shape and anisotropic polarity of the nanostructures, even though they are compositionally identical.

[0047] Accordingly, one embodiment of the process for controlling the growth parameters of nanowires comprises the steps: 1 ) selecting a nanowire material that is capable of single crystal growth and is an electrical conductor or semi-conductor; 2) selecting a substrate by matching lattice parameters and symmetry; 3) providing an initiator; and 4) growing the nanowire under a suitable conditions. [0048] There are a number of considerations in the selection of a nanowire material. However, the primary consideration is whether the material has the physical capability of forming a nanowire and is a conductor or semiconductor or some other desirable physical property. Presently, nanowire materials of interest include, single elements such as silicon (Si) and germanium (Ge) as well as Group lll-V binary compounds such as gallium nitride (GaN), aluminum nitride (AIN), gallium phosphide (GaP), indium phosphide (InP), gallium arsinide (GaAs), indium antimonide (InSb), gallium antimonide (GaSb). Other materials such as zinc oxide (ZnO) and Group IV compounds such as silicon germanium (Si-Ge) and silicon nitride (SiN) are also suitable. Ternary alloys and some quaternary alloys may also be suitable for use with the methods. For example, Ga-AI-N, In-Ga-N, AI-ln-N, In-Ga-As, Ga-AI-Sb as well as In-Ga-As-P, In-Ga-As-Sb and the like may also be used. Although these materials are suitable, other group-IIIA-VA compositions, nitrides, oxides and semiconductor materials may also be suitable for nanowire material. [0049] Selection of a substrate is determined in part by the chemical identity of the nanowire material and the type crystalline structure that can be formed by the process. Substrate suitability concerns matching closely the lattice constants and symmetry of the proposed substrate with the nanowire material. The lattice constants and symmetry between the substrate and the nanowire materials do not have to be identical for the substrate candidate to be suitable. It can also be seen that one substrate may be suitable for use with a number of different nanowire materials. [0050] Nanowire initiators may be used to initiate or catalyze the growth of the individual nanowire and are preferably uniform nanoparticles of metal. Particulates of metals such as gold (Au), nickel (Ni), indium (In), cobalt (Co) or iron (Fe) having approximately the same diameter are preferred. The initiators can be applied through the use of patterned films, paints or other methods of dispersion.

[0051] The preferred method of nanowire growth is a metal initiated metal organic chemical vapor deposition (MOCVD). However, it will be understood that other known methods of nanowire growth can also be used. The MOCVD process is entirely compatible with the current GaN thin film technology and should lead to easy scale up and device integration. [0052] The present invention may be more particularly described in the following examples that are intended for illustrative purposes only, since numerous modifications, adaptations and variations to the apparatus and methods will be apparent to those skilled in the art. [0053] Example 1

[0054] Turning now to FIG. 1 A through FIG. 1 C, scanning electron microscopy images of GaN nanowires grown on a (100) -LiAIO₂ substrate are shown. Field emission scanning electron microscopy (FE-SEM) was used to investigate the nanowire length, shape and overall substrate coverage. [0055] The images shown in FIG. 2A-B are images of nanowires grown on a (111 ) MgO substrate and discussed in Example 2. In both examples, a 2-3 nm thick thin film of Ni, Fe, or Au catalyst was thermally evaporated onto the substrates. For patterned nanowire growth, a TEM grid is used as a shadow mask during metal deposition. Subsequent vapor-liquid-solid growth of GaN nanowires occurred at a substrate temperature of approximately 900°C. The reaction was carried out in an oxygen-free environment at atmospheric pressure. TMG was kept cool in a minus 10 °C temperature bath. Nitrogen, used as a carrier gas, was percolated through the TMG precursor and coupled with a second nitrogen line to give a total nitrogen flow rate of 250 seem. These were supplied through a 4 trim ID quartz tube. Hydrogen and ammonia sources were supplied through a 22 mm ID outer quartz tube at a total flow rate of -155 seem. The placement of the substrates relative to the organic precursor outlet was relatively unimportant, showing thick wire coverage on substrates placed from 1 to 10 cm away. The deposition generally took 5-30 minutes. [0056] There are several important features of the GaN nanowires that are shown in FIG. 1 A-C. First, the GaN nanowires grow perpendicular to the substrate. Such arrays have extremely low dislocation densities when compared to conventional thin films grown in the same direction and will contribute to future improvement of nanowire device performance.

[0057] Second, as a result of the VLS growth, nanowires grow only in areas with Au thin film coating, which readily yields patterned nanowires as seen in the figure. This selective growth capability would be very helpful for future nanowire device integration.

[0058] Third, all nanowires exhibit symmetry-matched isosceles cross- sections. The widths, measured along the base of the triangles, are 15-40 nm and the lengths measure from 1-5 μ m. Longer wires, with similar widths have also been achieved by using longer growth times. Significantly, the in- plane crystallographic alignment of these nanowires is apparent from the micrograph shown in FIG. 1 C. It can be seen that the in-plane C₂-rotation axis for the isosceles triangles (parallel with the c-axis of GaN) is always parallel with the in-plane [1 Ϊ0] direction for ^-LiAIO₂. This is because of the excellent symmetry and lattice match between the wire and substrate crystal structures.

[0059] As seen in FIG. 3A, X-ray diffraction results on the nanowire arrays grown on χ-LiAIO₂ shows almost exclusively (100) diffraction, a clear indication that these nanowires grow perfectly along the [1 Ϊ0] direction. [0060] These nanowires were dispersed on transmission electron microscopy (TEM) grids in order to carry out additional structural and compositional analysis. FIG. 4A shows a TEM image of an individual GaN nanowire, where a metal droplet can be clearly seen on its tip. The composition of the Au metal tip was confirmed with energy dispersive X-ray spectroscopy. Analysis of the electron diffraction pattern (inset) taken along the [001] zone axis indicates that the nanowire grows in the [1 Ϊ0] direction, perpendicular to (100) crystal plane. [0061] FIG. 4B shows a high resolution TEM image of the nanowire, showing exactly the (100) lattice plane perpendicular to the wire axis. No dislocations were observed in these wires. In addition, electron energy loss spectroscopy analysis clearly showed a nitrogen peak and the absence of an oxygen peak, confirming the compositional purity of the nanowires. Earlier TEM studies on [110] oriented GaN nanowires indicated that these triangular cross-sections are not equal-lateral, that is, the observation of these triangular cross-sections is not a result of viewing along the six-fold crystallographic symmetry axis (c- axis). Instead, the isosceles triangular cross-section is a manifestation of the 2-fold symmetry along the [1 Ϊ0] crystallographic direction, and the nanowires are enclosed by the (112), (ϊ 12) and (001) side planes (FIG. 4C).

[0062] Example 2

[0063] Interestingly, when (111 ) MgO was used as the substrate under nearly identical synthetic conditions, similar vertically aligned GaN nanowire arrays were obtained (FIG. 2A, 2B), but with the very different and orthogonal [001] growth direction. The nanowires have an overall similar dimension as those wires grown on LiAIO₂ substrates, except that every wire has a hexagonal cross-section. Again, all of these hexagonal wires exhibit excellent in-plane crystallographic alignment resulting from the excellent symmetry and lattice match at the interface.

[0064] The structural differences between the GaN nanowires prepared on two different substrates can be compared in FIG 4A-4F. Transmission electron microscopy (TEM) images of the GaN nanowire grown on (100) γ- LiAIO₂ and (111 ) MgO substrates are shown in FIG. 4A-B and FIG. 4D-E respectively. FIG. 4C and FIG. 4F show space-filling structural models for the nanowires with triangular and hexagonal cross sections. [0065] The inset in FIG. 4A is an electron diffraction recorded along [001] zone axis and the inset in FIG. 4D shows the hexagonal cross-section of the wire and an electron diffraction pattern recorded along [1 Ϊ0] zone axis. The (001 ) lattice fringe can be readily seen in FIG. 4E as well as a hexagonal cross-section in FIG. 4D inset. TEM analysis of these nanowires clearly indicates that they grow along [001] direction and are enclosed by {100} side planes as seen in FIG. 4F. [0066] In addition, FIG. 3A and FIG. 3B shows X-ray diffraction (XRD) patterns recorded on wires grown on two substrates showing different GaN orientation. FIG. 3A shows XRD patterns for GaN nanowire growth on (100) y-LiAIO₂ substrates and FIG. 3B shows XRD results for GaN nanowire growth on MgO substrates. The Insets show the crystal structure of the substrate at the interface between the substrate and the wire that dictate the selective GaN nanowire growth. The gray polygons indicate the respective orientation of the GaN crystal faces at the interface. The (100) peak corresponds to the orthogonal [1 Ϊ0] growth direction as seen in FIG. 3A, and the (002) peak corresponds to the orthogonal [001] growth direction as seen in FIG. 3B. In contrast to the wires grown on (100) -LiAIO₂ substrates, the XRD exhibits only (002) diffraction in FIG. 3B, indicating that the wires grow along the [001] direction. This is further confirmed by the TEM characterization shown in FIG. 4. [0067] It can be seen that the GaN semiconductor nanowires were produced with selected, vertically aligned, crystallographic growth directions (here, [001] and [1 Ϊ0]), through a simple selection of the substrates. A good lattice and symmetry match between the nanowires and the substrate is an important factor for this selectivity. [0068] Example 3

[0069] In order to demonstrate an orientation-induced effect of dimension and crystallographic orientation on the optical and electrical properties of the nanowire, temperature-dependent photoluminescence studies were carried out and emissions from the two types of nanowire samples described in Example 1 and Example 2 were compared. Referring now to FIG. 5, the temperature-dependent photoluminescence data collected on the two sets of GaN nanowires with different growth directions were gathered with the ♦ representing the [001] wires and the A symbol representing the [110] nanowires. The band-edge emission energy is plotted as a function of the temperature. Photoluminescence (PL) of the nanowires was collected at different temperatures within an optical cryostat. The sample was excited with the 325 nm line of a He-Cd CW laser. The nanowire emission was collected, dispersed with a 0.3 m monochromator (1200 grooves/mm grating) and detected by an intensified CCD detector. [0070] FIG. 5 shows a clear difference in band-edge emission from these two different GaN arrays at different temperatures from 5 K to 285 K. In general, the emission for the [110] wires is blue-shifted by -100-200 meV from that of [001] wires. As these two sets of wires have similar diameter distribution, it appears that the emission difference is a clear manifestation of the different wurtzite growth directions. In particular, the [001] and [1 Ϊ0] directions represent two orthogonal crystallographic orientations within the wurtzite GaN crystal structure, of which, one is polar and the other non-polar, respectively. It has been shown that the presence of spontaneous, piezoelectric polarization in GaN has a drastic impact on electron-hole overlap, radiative lifetimes, and subsequent emission wavelength and quantum efficiencies for gallium nitride. Films grown in the [1 Ϊ0] direction were shown to have superior optical qualities due to the absence of intrinsic electrostatic polarity. [0071] In addition, the unique isosceles triangle cross-section of the [110] wires might also lead to interesting carrier confinement effects at the triangle vertices, which could contribute to the blue shifting of photoluminescence. Furthermore, significant anisotropy in the valence band is expected for wurtzite nanostructures, especially for the anisotropic nanostructures shown in FIG. 4. The nitrogen partial density of states (N-PDOS) in the conduction band in the two sets of samples was probed with polarized x-ray absorption spectroscopy (XAS) with undulator beamline 8.0 at the Advanced Light Source (ALS) of the Lawrence Berkeley National Laboratory. Linearly polarized soft x-rays were tuned to the nitrogen K-edge ranging between 395 and 430 eV, with a spot size of roughly 0.1 x 1.0 mm and resolution =0.2 eV. Total fluorescence yield absorption was measured using the signal from a channeltron, biased for detection of emitted photons, recorded as function of photon energy and incidence angle. [0072] The polarized X-ray absorption studies are orbital symmetry specific, orientation dependent, and capable of projecting conduction states separately along specific crystallographic directions. In the XAS experiments, sample rotation allowed the incident beam angle to be varied from near-normal incidence (10°) to near-grazing (75°), thus varying the polarization e-vector from perpendicular to near-parallel to the nanowire axis, respectively.

[0073] Referring particularly to FIG. 6A, XAS spectra for [110] wires showing the nitrogen K-edge absorption as the array sample rotates with respect to the polarization. The inset of FIG. 6A shows the sample rotation configuration with the c-axis in the plane of the incident photon polarization. Similarly, FIG. 6B shows the XAS spectra for [001] wires showing the nitrogen K-edge absorption as the array sample rotates with respect to the polarization. The numbers "10, 45, and 75" in the figures refer to the sample rotation angle. [0074] The angular dependence observed in XAS experiments can be explained by the large degree of electronic anisotropy in wurtzite's noncentrosymmetric crystal structure. This has been demonstrated previously with polarized synchrotron radiation through analysis of cubic and hexagonal GaN thin films, and of polycrystalline GaN powders. Photon absorption depends on the matrix element:

where "e" is the polarization vector of the incoming photon, "r" is the position vector of the electron relative to the nitrogen nucleus, and "fa_sf are the initial and final state wavefunctions of the nitrogen core electron. In the wurtzite lattice, X-ray absorption spectra depend on the excitation of the two distinct "π" and "σ" modes of bonding, with symmetrically distinct r position vectors. The π-mode occurs along the c-axis (having C₃ symmetry) and contains a large amount of nitrogen-p_z character. The σ-mode makes up the three remaining low-symmetry bonds of the GaN pseudo-tetrahedron, forming a σ-plane with a large amount of nitrogen-p_xy character. Transition of the 1s electron to the final p_z state is seen as peaks I and III in FIG. 6 A and 6B. This transition is not permitted if the polarization of the photon is perpendicular to the c-axis, whereas transition to the final p_xy state, seen as peak II in FIG. 6A-6B, is not permitted if the polarization is parallel to the c-axis. [0075] As seen in FIG. 6A, absorption of the [1 Ϊ0] nanowire arrays showed a strong dependence on the angle of incidence with the c-axis near-normal to e at 75° and near-parallel at 10°. Likewise, the [001] array absorption spectra in FIG. 6B showed similar strong angular dependence with the c-axis near- normal to e at 10° and near-parallel at 75°, only with the intensity trend reversed as the result of the two orthogonal crystallographic directions in the samples. The dependence of the x-ray absorption spectra on array orientation agrees strongly with the XRD and SEM data in the prior examples, unambiguously showing that these arrays exhibit excellent crystallographic alignment as well as significant anisotropy in morphology and electronic structure. [0076] Example 4

[0077] In order to show the broad application of the process, similar crystallographic control was demonstrated by the production of zinc oxide (ZnO) nanostructures. In addition to demonstrating the controlled growth of vertically aligned GaN nanowires in the [001] and [110] orientations, similar control over the orientation of ZnO nanowires was also achieved.

[0078] It had been previously shown with the use of an a-plane sapphire as the substrate, that vertical hexagonal ZnO nanowire arrays could be produced (see, M. Huang et al., Science, 292 1897 (2001 ), for example). Due to the close match between lattice parameters of GaN and ZnO, similar results to those achieved for GaN were expected. In fact, when LiAIO₂ was used as the substrate for ZnO growth, ZnO nanoribbon arrays with excellent [1 Ϊ0] vertical alignment have been achieved. Rectangular, as opposed to triangular, cross sections are observed for the ZnO arrays, likely due to the different polarity and surface energies for GaN and ZnO crystal structure. [0079] Turning now to FIG. 7A, ZnO nanowires were produced using the synthetic conditions were essentially the same as those reported by M. Huang et al., Science, 292 1897 (2001 ), incorporated herein by reference in its entirety. The nanowires produced are seen in the scanning electron microscopy (SEM) images of ZnO nanowire arrays grown on sapphire substrates showing hexagonal cross-section and [001] growth direction. By comparison, FIG. 7B is a SEM image of ZnO nanoribbon arrays grown on LiAIO₂ substrate showing ribbon geometry and [110] growth direction according to the invention. [0080] FIG. 8A shows XRD diffraction patterns collected on ZnO nanowire arrays grown on sapphire substrates showing [001] growth direction. Diffraction patterns of ZnO nanoribbon arrays grown on LiAIO₂ substrate showing [110] growth direction are shown on FIG. 8B. Rectangular, as opposed to triangular, cross sections are observed for the ZnO arrays, likely due to the different polarity and surface energies for GaN and ZnO crystals. [0081] Being able to control the nanowire growth direction represents one significant step towards tuning material properties through rational nanostructure synthesis as the optical and electrical properties of the anisotropic nanostructures often depend not only on their dimensions, but on their crystallographic orientations as well. It has been demonstrated that rational selection of substrate symmetry and lattice constant parameters can allow for the selective growth of high-density nanowire arrays with specific crystallographic growth directions. In addition, the MOCVD process is entirely compatible with the current GaN thin film technology, which would lead to easy scale-up and device integration. [0082] Accordingly, it is anticipated that deterministic growth control over crystallographically aligned, dislocation-free GaN nanowires will have significant implications for the design of technologically relevant to high electron-mobility transistors, white light-emitting diodes, and UV laser diodes. Likewise, the pairing of substrates with specific nanowire materials to permit control over the dimension, growth direction and orientation of nanowire crystals will provide the desired control over the optical and electrical properties of the nanowires. [0083] Although the description above contains many details, these should not be construed as limiting the scope of the invention but as merely providing illustrations of some of the presently preferred embodiments of this invention. Therefore, it will be appreciated that the scope of the present invention fully encompasses other embodiments that may become obvious to those skilled in the art. In the appended claims, reference to an element in the singular is not intended to mean "one and only one" unless explicitly so stated, but rather "one or more." All structural, chemical, and functional equivalents to the elements of the above-described preferred embodiment that are known to those of ordinary skill in the art are expressly incorporated herein by reference and are intended to be encompassed by the present claims. Moreover, it is not necessary for a device or method to address each and every problem sought to be solved by the present invention, for it to be encompassed by the present claims. Furthermore, no element, component, or method step in the present disclosure is intended to be dedicated to the public regardless of whether the element, component, or method step is explicitly recited in the claims. No claim element herein is to be construed under the provisions of 35 U.S.C. 112, sixth paragraph, unless the element is expressly recited using the phrase "means for."

Claims

CLAIMS What is claimed is:

1. A method for deterministic growth of high-density arrays of nanowires, comprising: matching symmetry and at least one lattice parameter of a substrate with a nanowire material; and forming nanowires on said substrate.

2. A method as recited in claim 1 , wherein said nanowire material comprises a semiconducting material.

3. A method as recited in claim 1 , wherein said nanowire material comprises an alloy of at least one group IMA element with a group VA element.

4. A method as recited in claim 1 , wherein said nanowire material comprises a material selected from the group of materials consisting essentially of gallium nitride, gallium arsenide, indium phosphide, silicon, zinc oxide and germanium.

5. A method as recited in claim 1 , wherein said nanowire material comprises a material selected from the group of materials consisting essentially of aluminum nitride (AIN), gallium phosphide (GaP), indium antimonide (InSb) and gallium antimonide (GaSb).

6. A method as recited in claim 1 , wherein said nanowire material comprises a material selected from the group of materials consisting essentially of Ga-AI-N, In-Ga-N, AI-ln-N, In-Ga-As, Ga-AI-Sb, In-Ga-As-P and In-Ga-As-Sb.

7. A method as recited in claim 1 , wherein said substrate comprises γ ^■

LiAIO₂.

8. A method as recited in claim 1 , wherein said substrate comprises MgO.

9. A method as recited in claim 1 , further comprising an initiator, wherein said initiator is a nucleation site for the growth of a nanowire.

10. A method as recited in claim 9, wherein said initiator is selected from the group of initiators consisting essentially of gold, cobalt, indium, iron and nickel.

11. A method as recited in claim 9, wherein said initiator comprises nanoparticles with approximately equal diameters.

12. A method as recited in claim 9, further comprising a carrier associated with said initiator, wherein said initiator can be applied to said substrate.

13. A method as recited in claim 1 , wherein said carrier is a thin film.

14. A method as recited in claim 1 , further comprising forming said nanowires on said substrate using a process selected from the group of processes consisting essentially of vapor-liquid-solid growth, chemical vapor deposition, metal- organic chemical vapor deposition, chemical vapor transport, hydride vapor epitaxy, laser ablation and molecular beam epitaxy.

15. A method for fabricating single crystalline, one dimensional semiconductor nanostructures, comprising: selecting a semiconducting base material; selecting a substrate that has matching symmetry and at least one matching lattice constant parameter with said semiconducting base material; applying a catalyst onto the substrate; and forming said nanowires on said substrate.

16. A method as recited in claim 15, wherein said catalyst is applied by thermally evaporating a thin film of catalyst onto the substrate.

17. A method as recited in claim 16, further comprising: using a shadow mask for patterned nanowire growth.

18. A method as recited in claim 15, wherein said base material comprises an alloy of at least one group IMA element with a group VA element.

19. A method as recited in claim 15, wherein said semiconducting base material comprises a material selected from the group of materials consisting essentially of gallium nitride, gallium arsenide, indium phosphide, silicon, zinc oxide and germanium.

20. A method as recited in claim 15, wherein said nanowire material comprises a material selected from the group of materials consisting essentially of aluminum nitride (AIN), gallium phosphide (GaP), indium antimonide (InSb) and gallium antimonide (GaSb).

21. A method as recited in claim 15, wherein said nanowire material comprises a material selected from the group of materials consisting essentially of Ga-AI-N, In-Ga-N, AI-ln-N, In-Ga-As, Ga-AI-Sb, In-Ga-As-P and In-Ga-As-Sb.

22. A method as recited in claim 15, wherein said substrate comprises (100) ^ -LiAIO₂.

23. A method as recited in claim 15, wherein said substrate comprises (111 ) MgO.

24. A method as recited in claim 15, wherein said catalyst is selected from the group of catalysts consisting essentially of gold, cobalt, indium, iron and nickel.

25. A method as recited in claim 15, wherein said vapor-liquid-solid growth process is selected from the group of processes consisting essentially of chemical vapor transport, hydride vapor epitaxy, laser ablation and molecular beam epitaxy.

26. A method as recited in claim 15, further comprising forming said nanowires on said substrate using a process selected from the group of processes consisting essentially of vapor-liquid-solid growth, chemical vapor deposition, metal- organic chemical vapor deposition, chemical vapor transport, hydride vapor epitaxy, laser ablation and molecular beam epitaxy.

27. A method for fabricating GaN nanowires, comprising: selecting a substrate; applying a catalyst onto the substrate; and forming GaN nanowires on said substrate.

28. A method as recited in claim 27, wherein said substrate comprises (100) ^ -LiAIO₂.

29. A method as recited in claim 27, wherein said substrate comprises (111 ) MgO.

30. A method as recited in claim 27, wherein said catalyst is selected from the group of catalysts consisting essentially of gold, cobalt, indium, iron and nickel.

31. A method as recited in claim 27, further comprising forming said nanowires on said substrate using a process selected from the group of processes consisting essentially of vapor-liquid-solid growth, chemical vapor deposition, metal- organic chemical vapor deposition, chemical vapor transport, hydride vapor epitaxy, laser ablation and molecular beam epitaxy.