COMPOSITION OF MATTER

Abstract

A composition of matter comprising: a plurality of group III-V nanowires or nanopyramids epitaxially grown on a polycrystalline or single-crystalline graphene layer, said graphene layer being directly supported on a crystalline substrate such as a group III-V semiconductor, sapphire, SiC or diamond substrate, wherein the epitaxy, crystal orientation and facet orientations of said nanowires or nanopyramids are directed by the crystalline substrate.

Claims

1. A composition of matter comprising: a plurality of group III-V nanowires or nanopyramids epitaxially grown on a polycrystalline or single-crystalline graphene layer, said graphene layer being directly supported on a crystalline substrate such as a group III-V semiconductor, sapphire, SiC, Si, Ga.sub.2O.sub.3, or diamond substrate, wherein the epitaxy, crystal orientation, and facet orientations of said nanowires or nanopyramids are directed by the crystalline substrate.

2. A process comprising: epitaxially growing group III-V nanowires or nanopyramids on a polycrystalline or single-crystalline graphene layer, wherein the polycrystalline or single-crystalline graphene layer is directly supported on a crystalline substrate such as a group III-V semiconductor, sapphire, SiC, Si, Ga.sub.2O.sub.3, or diamond substrate, wherein the epitaxy, crystal orientation, and facet orientations of said nanowires or nanopyramids are directed by the crystalline substrate; and optionally separating the crystalline substrate from the graphene layer with the grown III-V nanowires or nanopyramids.

3. A light-emitting diode or photodetector device comprising: a plurality of group III-V nanowires or nanopyramids epitaxially grown on a polycrystalline or single-crystalline graphene layer, said graphene layer being directly supported on a crystalline substrate such as a group III-V semiconductor, sapphire, SiC, Si, Ga.sub.2O.sub.3, or diamond substrate, wherein the epitaxy, crystal orientation, and facet orientations of said nanowires or nanopyramids are directed by the crystalline substrate; said nanowires or nanopyramids having a p-n or p-i-n junction; a first electrode in electrical contact with said graphene layer; a second electrode in contact with the top of at least a portion of said nanowires or nanopyramids, optionally in the form of a light-reflective layer; wherein said nanowires or nanopyramids comprise at least one group III-V compound semiconductor.

4. The device as claimed in claim 3, wherein said nanowires or nanopyramids are grown through the holes of a hole-patterned mask on said polycrystalline or single-crystalline graphene layer.

5. The device as claimed in claim 3, wherein the polycrystalline or single-crystalline graphene layer is 15 Angstroms or less in thickness.

6. The device as claimed in claim 3, wherein the nanowires or nanopyramids comprise GaN, AlGaN, InGaN, or AlInGaN.

7. The device as claimed in claim 3, wherein the nanowires or nanopyramids comprise a multiple quantum well, such as an Al(In)GaN MQW.

8. The device as claimed in claim 3, wherein the nanowires or nanopyramids contain an electron blocking layer, which could be either a single barrier or a multiquantum barrier.

9. The device as claimed in claim 3, wherein the device emits or absorbs in the UV spectrum.

10. The device as claimed in claim 3, wherein the p-n or p-i-n junction within a nanowire is axial or radial.

11. The device as claimed in claims 3, wherein the nanowires or nanopyramids comprise a tunnel junction with a GaN, AlN, AlGaN, or InGaN barrier layer.

12. The device as claimed in claims 3, wherein the nanowires or nanopyramids comprise an (Al)GaN/Al(Ga)N superlattice.

13. The device as claimed in claim 3, wherein the nanowires or nanopyramids comprise AlGaN with an increasing or decreasing concentration of Al along a direction, such as axially, in the nanowire or nanopyramid.

14. The device as claimed in claim 3, wherein the nanowires or nanopyramids are doped using Si, Mg, Zn, or Be.

15. The device as claimed in claim 3, wherein the space between the nanowires or nanopyramids is filled by a supporting and electrically isolating filler material transparent to the light emitted or absorbed in said device.

16. The device as claimed in claim 3, wherein, in use, light is emitted or absorbed in a direction substantially parallel to but opposite from the growth direction of the nanowires.

17. The device as claimed in claim 3, wherein said graphene layer is a polycrystalline graphene layer.

18. The composition of matter of claim 1, wherein said nanowires or nanopyramids comprise an n-type doped region and a p-type doped region separated by an intrinsic region, said p-type doped region comprising an electron blocking layer.

19-42. (canceled)

Description

BRIEF DESCRIPTION OF THE FIGURES

[0269] FIG. 1 shows a composition of matter of the invention with nanowires grown on a thin polycrystalline or single-crystalline graphene layer directly supported on a crystalline substrate where the epitaxy is dictated by the crystalline substrate.

[0270] FIG. 2 shows a possible flip chip design. In use therefore, light is emitted through the top of the device (2) (marked hu). Crystalline substrate 1 is preferably formed from sapphire or AlN. The use of other crystalline transparent substrates is also possible. In use, the substrate, if still present, is positioned upper most in the device and hence it is important that the substrate is transparent to the emitted light and thus allows light out of the device.

[0271] Layer 3 is the polycrystalline or single-crystalline graphene layer which can be one atomic layer thick.

[0272] Nanowires 4 are grown from the polycrystalline or single-crystalline graphene layer 3 employing remote epitaxy. Ideally, the nanowires are formed from Al(In)GaN, AlN or GaN and are doped to create n-i-p or n-p junctions.

[0273] A filler 5 can be positioned between grown nanowires. A top electrode/light reflective layer 6 is positioned on top of nanowires 4. The light-reflective layer may also be provided with a p-electrode comprising Ni or Au. In use, this layer reflects any light emitted by the device to ensure that the light is emitted through the top of the device opposite the reflective layer. This is the so-called flip chip arrangement as the device is upside down compared to a conventional LED.

[0274] Electrode 10 is positioned on the polycrystalline or single-crystalline graphene layer 3. That electrode might comprise Ti, Al, Ni or/and Au. The graphene layer may be provided with a mask 7 to allow growth of the nanowires in definitive positions on the polycrystalline or single-crystalline graphene.

[0275] The whole device is soldered to conductive tracks/pads 13 on a submount 8 via solder layer 9.

[0276] When a forward current is passed across the device, visible or UV light, dependent on composition of matter, is generated in the nanowires and is emitted, possibly after reflecting off the reflective layer out the top of the device.

[0277] When a reverse current is passed across the device and when the device is exposed to visible or UV light, the nanowires absorb the visible or UV light, dependent on composition of matter, and converts it into current, working as a photodetector.

[0278] FIG. 3 shows a potential nanowire of the invention. The nanowire is provided with different components in an axial direction by variation of the elements being supplied during the growing phase. Initially, an n-type doped GaN, or preferably an n-type AlGaN, material is deposited, followed by n-AlGaN. In the central section of the nanowire as shown are a series of multiple quantum wells formed from (In)(Al)GaN. Then follows the p-type doped region based on AlGaN, and an electron blocking layer based on p-Al(Ga)N and finally a p-GaN layer.

[0279] FIG. 4 shows an alternative chip design in which the nanowires are grown also radially creating core-shell structures. In use therefore, light is emitted through the top of the device (marked hν). Crystalline substrate 1 is preferably formed from sapphire or a group III-V semiconductor. In use, the substrate, if still present, is positioned upper most in the device and hence it is important that the substrate is transparent to the emitted light and thus allows light out of the device.

[0280] Layer 3 is the polycrystalline or single-crystalline graphene layer which can be one atomic layer thick or thicker, such as one which is up to 5 nm in thickness.

[0281] Nanowires 4 are grown from layer 3 epitaxially to reflect the underlying crystalline substrate. Ideally, the nanowires are formed from Al(In)GaN, AlN or GaN and are doped to create n-i-p or n-p junctions. The polycrystalline or single-crystalline graphene can be provided with a mask layer 7.

[0282] A filler 5 can be positioned between grown nanowires. A top electrode/light-reflective layer 6 is positioned on top of nanowires 4. The light-reflective layer may also be provided with a p-electrode comprising Ni or/and Au or may itself be an electrode. In use, this layer reflects any light emitted by the device to ensure that the light is emitted through the top of the device opposite the reflective layer. This is the so-called flip chip arrangement as the device is upside down compared to a conventional LED.

[0283] Electrode 10 is positioned on the polycrystalline or single-crystalline graphene layer 3. When a forward current is passed across the device, visible or UV light, dependent on composition of matter, is generated in the nanowires and is emitted, possibly after reflecting off the reflective layer out the top of the device.

[0284] The whole device is soldered to conductive tracks/pads 13 on a submount 8 via solder layer 9.

[0285] When a reverse current is passed across the device and when the device is exposed to visible or UV light, the nanowires absorb the visible or UV light, dependent on composition of matter, and converts it into current, working as a photodetector.

[0286] FIG. 5 shows a nanowire grown radially having the same components as those of FIG. 3 in a shell arrangement. The nanowire is provided with different components in a radial direction by variation of the elements being supplied during the growing phase. Initially, an n-type doped (Al)GaN material is deposited, followed by n-AlGaN. In the central shell of the nanowire as shown are a series of multiple quantum wells formed from (In)(Al)GaN. Then follows the p-type doped region based on AlGaN, and an electron blocking shell based on p-Al(Ga)N and finally a p-GaN shell.

[0287] FIG. 6 shows a photodetector. In use therefore, light (2) (marked hu)is accepted through the top of the device. Crystalline substrate 1 is preferably formed from sapphire or AlN. In use, the substrate, if still present, is positioned upper most in the device and hence it is important that the support is transparent to the accepted light and thus allows light in to the device.

[0288] Layer 3 is the polycrystalline or single-crystalline graphene layer which can be one atomic layer thick.

[0289] Nanowires 4 are grown from crystalline substrate layer 3 epitaxially. Ideally, the nanowires are formed from Al(In)GaN, AlN or GaN and are doped to create n-i-p or n-p junctions.

[0290] A filler 5 can be positioned between grown nanowires. A top electrode layer 11 is positioned on top of nanowires 4. This electrode is ideally a p-electrode comprising Ni or Au.

[0291] Electrode 10 is positioned on the polycrystalline or single-crystalline graphene layer 3. The graphene layer may be provided with a mask 7 to allow growth of the nanowires in definitive positions on the polycrystalline or single-crystalline graphene.

[0292] The whole device is soldered to conductive tracks/pads 13 on a submount 8 via solder layer 9.

[0293] When a reverse current is passed across the device and when the device is exposed to visible or UV light, the nanowires absorb the visible or UV light, dependent on composition of matter, and converts it into current, working as a photodetector.

[0294] FIG. 7a is a theoretical top-view cross section of a regular hexagonal array of nanowires on polycrystalline graphene where the crystal structure of the crystalline substrate determines the nanowire orientation through remote epitaxy (nanowires have parallel facets with each other).

[0295] FIG. 7b is a theoretical top-view cross section of hexagonal nanowires grown on polycrystalline graphene where the crystalline substrate is unable to influence nanowire orientation. Nanowires are grown in two different domains/grains and have different facet orientations to each other. In this example, the nanowires grow epitaxially on each graphene domain/grain (nanowires on each domain/grain have the same facet orientations).

[0296] FIGS. 8-15 concern positioned nanowires/nanopyramids using graphene as a hole mask on a crystalline substrate/intermediate-layer and experimental results of LEDs fabricated using this method.

[0297] FIG. 8 (case 1.1) shows positioned flat-tip nanowires grown epitaxially on a crystalline substrate/intermediate-layer carrying a graphene mask layer through which holes have been etched. The nanowires first nucleate on the substrate/intermediate-layer epitaxially through the holes in the graphene. As the nanowires continue to grow both axially and radially, they also grow on top of the graphene layer maintaining the epitaxial relationship with the substrate/intermediate-layer. The graphene layer forms electrical contact with the nanowires both by nanowire contact with the graphene surface as well as contact with the edges of the graphene holes. Hence the graphene layer forms a conductive transparent electrode. The nanowires can be grown with either an axial or radial heterostructure in order to fabricate axial or radial n-i-p/p-i-n junction nanowire device structures, respectively. In the case of the radial n-i-p/p-i-n junction nanowire device structure, growth of the p/n nanowire shell layer on graphene must be avoided (gaps needed) to avoid shortening between n/p nanowire core and p/n nanowire shell.

[0298] FIG. 9 (case 1.2) is analogous to FIG. 8, with the only difference being that the nanowires have a pyramidal tip. FIG. 9 shows positioned pyramid-tip nanowires grown epitaxially on a crystalline substrate/intermediate-layer carrying a graphene mask layer through which holes have been etched.

[0299] FIG. 10 (case 1.3) is analogous to the axial n-i-p junction device of FIG. 9, but the nanowires in FIG. 10 are completely coalesced as a result of the growth of an additional n-AlGaN nanowire shell layer. FIG. 10 therefore shows positioned pyramid-tip nanowires grown epitaxially on a crystalline substrate/intermediate-layer carrying a graphene mask layer through which holes have been etched, but the nanowires are completely coalesced as a result of the growth of an additional n-AlGaN nanowire shell layer.

[0300] FIG. 11 (case 1.4) is analogous to FIG. 10, but with coalesced nanopyramids instead of coalesced nanowires. FIG. 11 therefore shows positioned nanopyramids grown epitaxially on a crystalline substrate/intermediate-layer carrying a graphene mask layer through which holes have been etched, and the nanopyramids are completely coalesced as a result of the growth of an additional n-AlGaN nanowire shell layer.

[0301] FIG. 12 depicts nanopyramid growth on a graphene hole mask layer on a sapphire (0001) substrate. The grown structure is a coalesced axial n-n-i-p junction GaN/AlGaN nanopyramid light emitting diode (LED) structure (as schematically described in FIG. 11 above). FIG. 12a is a top-view SEM image taken after the initial growth of n-AlGaN nanopyramids and FIG. 12b is a top-view SEM image taken after the complete growth of the n-AlGaN/n-AlGaN/i-GaN/p-AlGaN nanopyramid LED structure.

[0302] FIG. 13 demonstrates device characteristics of the sample shown in FIG. 12b processed into a flip-chip LED with a size of 50 μm×50 μm. (a) Current-voltage curve and (b) electroluminescence (EL) spectrum of the corresponding LED showing emission at 360 nm.

[0303] FIG. 14. Schematic illustration of growing nanopyramids (e.g. AlGaN NW) in the holes directly from the intermediate layer (e.g. AlN), and nanoislands/thin film (e.g. AlGaN) on graphene via remote epitaxy. Due to remote epitaxy, the growth on graphene will also have good crystalline quality. Subsequently, a device structure with n-i-p junction be obtained.

[0304] FIG. 15 depicts nanopyramid growth on a graphene hole mask layer on an AlN/sapphire (0001) substrate. The grown coalesced structure is an axial n-n-i-p junction GaN/AlGaN nanopyramid light emitting diode (LED) structure (as schematically described in FIG. 11 above). FIG. 15a is a top-view SEM image taken after the initial growth of n-GaN nanopyramids and FIG. 15b is a top-view SEM image taken after the complete growth of the n-GaN/n-AlGaN/i-GaN/p-AlGaN nanopyramid LED structure. FIG. 15c shows a top-view SEM image of seven positioned n-GaN nanopyramids showing one n-GaN triangular-based nanopyramid nucleated on the graphene mask by remote epitaxy. One can see that the nanoisland has nucleated with its three facets parallel to the facet orientation of three of the six facets of the hexagonal nanopyramid. FIG. 15d demonstrates the current-voltage curve of the sample shown in FIG. 15 processed into a flip-chip LED with a size of 50 μm×50 μm.