Process for growing nanowires or nanopyramids on graphitic substrates

Abstract

A process for growing nanowires or nanopyramids comprising: (I) providing a graphitic substrate and depositing AlGaN, InGaN, AlN or AlGa(In)N on said graphitic substrate at an elevated temperature to form a buffer layer or nanoscale nucleation islands of said compounds; (II) growing a plurality of semiconducting group III-V nanowires or nanopyramids, preferably III-nitride nanowires or nanopyramids, on the said buffer layer or nucleation islands on the graphitic substrate, preferably via MOVPE or MBE.

Claims

1. A process for growing nanowires or nanopyramids comprising: (I) providing a graphitic substrate and treating said graphitic substrate with a plasma formed from nitrogen gas (N.sub.2) at a temperature of at least 100° C. to form atomic steps/ledges; and (II) growing a plurality of semiconducting group III-V nanowires or nanopyramids on the treated graphitic substrate.

2. The process as claimed in claim 1, wherein the plurality of semiconducting group III-V nanowires or nanopyramids are grown epitaxially from the treated graphitic substrate.

3. The process as claimed in claim 1, wherein said graphitic substrate has a thickness of up to 20 nm.

4. The process as claimed in claim 1, wherein the plurality of semiconducting group III-V nanowires or nanopyramids are doped.

5. The process as claimed in claim 1, wherein the plurality of semiconducting group III-V nanowires or nanopyramids are core-shell nanowires or nanopyramids.

6. The process as claimed in claim 1, wherein a graphitic top contact layer is present on top of the plurality of semiconducting group III-V nanowires or nanopyramids.

7. The process as claimed in claim 1, wherein the plurality of semiconducting group III-V nanowires or nanopyramids are grown with or without the presence of a catalyst.

8. The process as claimed in claim 1, wherein the plurality of semiconducting group III-V nanowires or nanopyramids are GaN, AlGaN, AlN or InGaN.

9. The process as claimed in claim 1, wherein the plurality of semiconducting group III-V nanowires are grown in the [111] (for cubic crystal structure) or (for hexagonal crystal structure) direction.

10. The process as claimed in claim 1, wherein the plurality of semiconducting group III-V nanowires or nanopyramids comprise a tunnel junction.

11. The process as claimed in claim 1, wherein the plurality of semiconducting group III-V nanowires or nanopyramids comprise an (Al)GaN/Al(Ga)N superlattice.

12. The process as claimed in claim 1, wherein the plurality of semiconducting group III-V nanowires or nanopyramids comprise AlGaN with an increasing or decreasing concentration of Al along an axial or radial direction in the nanowire or nanopyramid.

13. The process as claimed in claim 1, wherein the plurality of semiconducting group III-V nanowires or nanopyramids are grown axially and are therefore formed from a first section and a second section, wherein the first section and the second section are each doped and the first section is doped differently than the second section to generate a p-n junction or p-i-n junction.

14. The process as claimed in claim 1, wherein the plurality of semiconducting group III-V nanowires or nanopyramids comprise AlGaN, InGaN, or AlGaInN.

15. The process as claimed in claim 1, wherein the temperature is from 100° C. to 755° C.

16. The process as claimed in claim 1, wherein the temperature is at least 755° C.

17. A process for growing nanowires or nanopyramids comprising: (I) providing a graphitic substrate and depositing on said graphitic substrate Al to form an Al layer or nanoscale Al islands; (II) exposing said Al layer or nanoscale Al islands to a flux of at least one group V species, thereby forming a buffer layer or nanoscale islands of Al-group V compound(s); (III) growing a plurality of semiconducting group III-V nanowires or nanopyramids on said buffer layer or nanoscale islands on the graphitic substrate.

18. The process as claimed in claim 17, wherein the group V element is not N.

19. A process for growing nanowires or nanopyramids comprising: (I) providing a graphitic substrate and treating said graphitic substrate with oxygen plasma or with ozone to form atomic steps/ledges on the graphitic substrate surface and/or so as to form graphene oxide with epoxide groups (C—O) on its surface; (II) annealing the treated substrate of step (I) in the presence of hydrogen to convert at least a portion of said epoxide groups (C—O) to C—H bonds; (III) growing a plurality of semiconducting group III-V nanowires or nanopyramids on the annealed surface of step (II).

20. The process as claimed in claim 19, wherein the plurality of semiconducting group III-V nanowires or nanopyramids are grown epitaxially from the annealed surface of step (II).

21. A process for growing nanowires or nanopyramids comprising: (I) providing a graphitic substrate and depositing on said graphitic substrate an Al layer; (II) oxidizing at least the top part of said Al layer to form an oxidized Al layer; (III) depositing on said oxidized Al layer an amorphous Si layer; (IV) heating in order to cause an interchange of the Al layer and amorphous Si layer, and metal-induced-crystallization (MIC) of the amorphous Si to form a crystallized Si layer; (V) removing the Al layer and the oxidized Al layer; (VI) growing a plurality of semiconducting group III-V nanowires or nanopyramids on the subsequent crystallized Si layer.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1A shows a schematic representation of deposition of buffer layer on graphitic substrate, followed by the nanowire growth.

(2) FIG. 1B shows a schematic representation of deposition of nucleation islands on graphitic substrate, followed by the nanowire growth.

(3) FIG. 2A and FIG. 2B show representative results of the formation of nucleation island and nanowire growth scheme. FIG. 2A is an SEM image of AlGaN nucleation islands grown on graphene by MOVPE. FIG. 2B is an SEM image of GaN nanowires grown on the AlGaN nucleation islands on graphene by MOVPE. Inset: SEM image of GaN growth without AlGaN nucleation islands on graphene, where no growth of perpendicular GaN nanowires can be seen.

(4) FIG. 3A is a cross-sectional high-resolution scanning transmission electron microscope (STEM) image of a GaN nanowire grown on graphene using AlGaN nucleation islands. FIG. 3B is a high-angle annular dark-field STEM image of the same nanowire in FIG. 3A, showing the AlGaN nucleation island.

(5) FIG. 4A is a SEM image of (Al)GaN nanopyramids of the invention grown in a regular array. After growing the AlGaN nucleation islands, AlGaN with 3% Al in gas phase was grown for 150 s. FIG. 4B is a closer image of said nanopyramids.

(6) FIG. 5A is a schematic diagram showing the growth of nanowires on graphite flake and the top and bottom contacts to the nanowires. FIG. 5B is a tilted view SEM image of selectively grown GaN nanowires on multi-layer graphene flakes by MBE. FIG. 5C is a high-resolution SEM image of selectively grown GaN nanowires on multi-layer graphene flakes by MBE.

(7) FIG. 6 shows an SEM image of self-catalyzed GaAsSb nanowires grown by MBE using AlAsSb nanoscale islands for enhanced nucleation on pristine graphitic substrate. Inset: Magnified view of the perpendicular GaAsSb nanowires.

(8) FIG. 7A shows atomic force microscopy (AFM) topography image after the treatment of graphite with UV-ozone.

(9) FIG. 7B shows atomic force microscopy (AFM) topography image after the treatment of graphite with UV-ozone and H.sub.2 annealing in Ar atmosphere.

(10) FIG. 8A shows the AFM height profile along the solid line in FIG. 6A of the graphite surface after the treatment with UV-ozone.

(11) FIG. 8B shows the AFM height profile along the dashed line in FIG. 6B of the graphite surface after the UV-ozone treatment and the following H.sub.2 annealing in Ar atmosphere, showing the formation of atomic steps and ledges. (Nanowires or nanopyramids are then grown on the treated graphitic substrate.)

(12) FIG. 9A shows an SEM image of GaAsSb nanowires grown on untreated pristine graphitic surface. FIG. 9B shows an SEM image of GaAsSb nanowires grown on UV-ozone treated and H.sub.2 annealed graphitic surface. Improved density of perpendicular nanowires can be seen in FIG. 9B as compared to FIG. 9A.

(13) FIG. 10 shows the main process steps of aluminium-induced crystallization (MIC) of silicon on graphene layer 1, where amorphous silicon (a-Si) layer 3 diffuses through an aluminium metal layer 2 by thermal activation. At the graphene-Al interface the silicon rearranges into a polycrystalline structure (p-Si) with [111]-orientation. The aluminium metal layer and the oxide layer above the p-Si structure may be etched using HCl and HF, respectively. (Nanowires or nanopyramids are then grown on the MIC silicon on graphene.)

(14) FIG. 11 shows an SEM image of self-catalyzed GaAs nanowires grown by MBE on amorphous (SiO.sub.2) substrate covered with MIC silicon.

EXAMPLE 1

(15) Experimental Procedure of Growing GaN Nanowires on Graphitic Surface Using AlGaN Nucleation Islands:

(16) Commercial CVD-grown graphene on Cu foil, transferred on Si(001), Si(111), and sapphire supporting substrates, were used for this experiment. The growth of GaN nanowires was carried out in a horizontal flow MOVPE reactor (Aixtron 200RF). After loading the sample, the reactor was evacuated and purged with N.sub.2 to remove oxygen and water in the reactor. The reactor pressure was set to 75 Torr and H.sub.2 was used as the carrier gas for the growth. Subsequently, the substrate was thermally cleaned under H.sub.2 atmosphere at a substrate temperature of ˜1200° C. for 5 min. After that a nitridation step was carried out using NH.sub.3 flow of 600 sccm for 10 min. Subsequently, TMGa and TMAl was introduced for 40 s with a flow of 44.8 and 26.3 μmol/min, respectively, to grow AlGaN nucleation islands, followed by a 2 min nitridation step.

(17) For the growth of GaN nanowires, the substrate temperature was lowered to ˜1150° C. and the NH.sub.3 flow was set to 25 sccm. When the temperature became stable, Si-doped GaN nanowire growth was carried out for 3.5 min by introducing TMGa and Silane with a flow of 44.8 and 0.03 μmol/min, respectively. After the growth, the sample was cooled down under NH.sub.3 flow of 25 sccm until the temperature dropped below 500° C.

EXAMPLE 2

(18) Experimental Procedure for Nanowires Growth on Nitrogen Plasma Treated Graphitic Surface:

(19) For this experiment multi-layer graphene was mechanically exfoliated from Kish graphite flakes and then indium-bonded to a SiO.sub.2/Si supporting substrate. A mask material such as Al.sub.2O.sub.3 and SiO.sub.2 can be optionally deposited on the graphite flake. A big hole of 10 μm in diameter is etched in the mask material using photolithography such that the graphite surface is exposed in the hole. Optionally, several periodically spaced small holes of diameter ˜100 nm can be etched using e-beam lithography, such that the nanowires selectively grow on the graphitic surface exposed in the hole. The nitrogen plasma treatment and the nanowire growth were carried out in a Veeco Gen 930 MBE system equipped with a nitrogen plasma source, a Ga dual filament cell, and an Al double-crucible cell.

(20) The above samples are then loaded into the MBE system for sample outgassing and nanowire growth. The samples are annealed at a substrate temperature of 550° C. for a duration of 30 min to get rid of any oxide residues and any other contaminants on the substrate. The substrate temperature is then increased to a temperature suitable for GaN nanowire growth: i.e. typically 755° C.

(21) The temperatures of the Ga and Al effusion cell is preset to yield nominal planar growth rate of 0.3 and 0.2 μm per hour, respectively. The nitrogen plasma is generated using a RF generator power of 450 W and nitrogen gas flow of 2.8 sccm. After the sample temperature reaches the growth temperature, the gate valve and the shutter in front of the nitrogen source was opened for 1 min, such that the nitrogen plasma is directed on to the sample. The sample can then either be subjected to the nanowire growth by MBE or taken out of the MBE growth chamber for the nanowire or nanopyramid growth by MOCVD. In the case of nanowire growth by MBE, an Al flux was supplied for 6 seconds or longer and then an Al flux and a nitrogen plasma was supplied for 1 minute or longer. It was followed by the opening of the shutter in front of the Ga and nitrogen source to supply Ga flux and nitrogen plasma simultaneously, to initiate the growth of intrinsic (intentionally undoped) GaN nanowires. Si dopant was supplied to obtain n-type GaN nanowires and either Be or Mg dopant was supplied to obtain p-type GaN nanowires. After the growth, all the shutters are closed and simultaneously the substrate temperate is ramped down.

EXAMPLE 3

(22) Experimental Procedure for the MBE Growth of High-Yield Perpendicular GaAsSb Nanowires on Graphitic Surface Via AlIAsSb Nanoscale Islands for Nucleation:

(23) Nanowires are grown in a Varian Gen II Modular MBE system equipped with a Ga dual filament cell, an Al double-crucible cell, an As valved cracker cell, and an Sb valved cracker cell. The cracker cells allow to fix the proportion of monomers, dimers and tetramers. In this example, the major species of arsenic and antimony are As.sub.2 and Sb.sub.2, respectively.

(24) Growth of NWs is performed either on a Kish graphite flake or on a graphene film grown on SiC substrates by using a high-temperature sublimation technique. The graphene film samples are purchased from external supplier. The Kish graphite samples are cleaned by isopropanol followed by a blow dry with nitrogen, then indium-bonded to a silicon wafer and finally cleaved to provide a fresh graphitic surface for growth of NWs. The graphene/SiC substrates are blow dried with nitrogen, and then indium-bonded to a silicon wafer.
The samples are then loaded into the MBE system for sample outgassing and nanowire growth. The samples are annealed at a substrate temperature of 550° C. for a duration of 30 min to get rid of any oxide residues on the substrate. The substrate temperature is then increased to a temperature suitable for GaAs or GaAsSb nanowire growth: i.e. 630° C.
The temperatures of the Al and Ga effusion cells are preset to yield nominal planar growth rates of 0.1 μm per hour and 0.7 μm per hour, respectively. To form the GaAs(Sb) nanowires, an As.sub.2 flux of 2.5×10.sup.−6 Torr is used, whereas the Sb.sub.2 flux is set to a value in the range 0-1×10.sup.−6 Torr (dependent on the intended GaAsSb composition), for example 6×10.sup.−7 Torr.
The Al flux is first supplied to the surface during a time interval of typically 1 s or longer, while the shutters/valves for the other sources are closed. The Al shutter is then closed and As and/or Sb flux are supplied to the surface for a time interval of typically 60 s to form AlAs(Sb) nanoscale islands on the graphitic surface. The group V shutters and valves are then closed and the Ga shutter opened, typically for 5 s, to supply Ga flux to the surface to initiate the formation of Ga droplets at the nanoscale islands. The relevant group V shutters and valves are then opened again to initiate the growth of nanowires. For example, in case of GaAs nanowire growth, only the As shutter and valve are opened at this point, whereas in case of GaAsSb nanowire growth, also the Sb shutter and valve are opened. The duration of the nanowire growth depends on the intended length of the nanowires. In case of the GaAsSb nanowires sample depicted in FIG. 6, the nanowire growth time was 5 min. The growth is stopped by closing all the shutters/valves, and simultaneously ramping down the substrate to room temperature.

EXAMPLE 4

(25) Experimental Procedure for UV-Ozone Treatment and H.sub.2 Annealing of the Graphitic Surface, and MBE Growth of Perpendicular GaAs(Sb) Nanowires:

(26) For this experiment Kish graphite flakes were used as the graphitic substrates. The Kish graphite samples are cleaned by isopropanol followed by a blow dry with nitrogen, then indium-bonded to a silicon wafer and finally cleaved to provide a fresh graphitic surface for growth of nanowires. The substrates were treated in UV-ozone at ˜150° C. for 6 min, followed by annealing in H.sub.2 at ˜300° C. for 45 min.

(27) Nanowires are grown in the same MBE system as described in Example 3. The major species of arsenic and antimony are As.sub.2 and Sb.sub.2, respectively.

(28) The samples are loaded into the MBE system and outgassed at ˜550° C. for a duration of 30 min to get rid of any oxide residues on the substrate. The substrate temperature is then increased to a temperature suitable for GaAs or GaAsSb nanowire growth: i.e. 630° C.

(29) The temperatures of the Ga effusion cell is preset to yield nominal planar growth rate of 0.7 μm per hour. To form Ga droplet, Ga flux was supplied for 10 s at a substrate temperature of ˜630° C. After that the temperature is reduced to ˜250° C. and an Sb.sub.2 flux of 8×10.sup.−7 Torr and As.sub.2 flux of 2.5×10.sup.−6 Torr are subsequently supplied for 50 s and 40 s, respectively. Then the substrate temperature is again increased to ˜630° C. To form the GaAs(Sb) nanowires, Ga flux was supplied for 10 min together with an As.sub.2 flux of 2.5×10.sup.−6 Torr, whereas the Sb.sub.2 flux is set to a value in the range 0-1×10.sup.−6 Torr (dependent on the intended GaAsSb composition), for example 8×10.sup.−7 Torr. After the growth, all the shutters are closed and simultaneously the substrate temperate is ramped down.

EXAMPLE 5

(30) Experimental Procedure for the Formation of Si(111) by Metal Induced Crystallization (MIC) on Graphene

(31) The MIC poly-Si(111) on graphene samples were of commercial chemical vapor deposition (CVD) grown monolayer graphene transferred onto Si(001). On these samples, 50 nm Al was deposited by e-beam evaporation at a rate of 1 Å/s and a pressure of ˜10.sup.−8 Torr. The samples were oxidized for 24 h in an ISO5 cleanroom atmosphere before depositing 50 nm amorphous Si (a-Si) by e-beam evaporation at a rate of 1 Å/s and a pressure of ˜10.sup.−8 Torr. All depositions were done at room temperature. The samples were annealed for 15 h at 500° C. in a nitrogen gas. After the layer exchange by annealing, the top layer of Al was removed by etching in a phosphoric acid mixture.