Nanostructure

Abstract

A composition of matter comprising at least one nanostructure grown epitaxially on an optionally doped β-Ga.sub.2O.sub.3 substrate, wherein said nanostructure comprises at least one group III-V compound.

Claims

1. A composition of matter comprising at least one nanostructure grown epitaxially on a doped or undoped β-Ga.sub.2O.sub.3 substrate, wherein said nanostructure comprises at least one group III-V compound; wherein said nanostructure grows from a (−201) or (100) β-Ga.sub.2O.sub.3 substrate plane; and wherein said nanostructure is grown in the absence of a catalyst.

2. The composition of matter of claim 1, wherein said at least one nanostructure is p-type doped.

3. The composition of matter of claim 1, wherein said at least one nanostructure comprises a radial or axial heterostructure.

4. The composition of matter of claim 1, wherein: the at least one nanostructure comprises at least one core semiconductor nanostructure; a semiconductor shell surrounding said core nanostructure, said shell comprising at least one group III-V compound; wherein said core semiconductor nanostructure is doped forming an n-type or p-type semiconductor; and wherein said shell is doped forming a p-type semiconductor or an n-type semiconductor opposite to said core; and an outer conducting coating surrounding at least a part of said shell to form an electrode contact.

5. The composition of matter of claim 1, wherein said semiconductor nanostructure is doped forming axial n-type and p-type semiconductor regions.

6. The composition of matter of claim 1, wherein the nanostructure comprises a group III-N compound.

7. The composition of matter of claim 1, wherein the nanostructure comprises GaN, AlN, AlGaN, InGaN, or AlInGaN.

8. The composition of matter of claim 1, wherein said nanostructure is a nanowire or nanopyramid.

9. The composition of matter of claim 8, wherein said nanowire is no more than 400 nm in diameter and has a length of up to 5 microns.

10. The composition of matter of claim 1, wherein said substrate comprises a plurality of nanowires and wherein said plurality of nanowires are substantially parallel.

11. The composition of matter of claim 1, further comprising a doped or undoped group III-V buffer layer positioned between the substrate and the nanostructure.

12. The composition of matter of claim 11, wherein said buffer layer is doped or undoped GaN.

13. The composition of matter of claim 1, further comprising a hole patterned mask comprising a plurality of holes on the substrate and wherein said nanostructures grow through the plurality of holes of said mask.

14. The composition of matter of claim 1, further comprising a hole patterned mask comprising a plurality of holes on the substrate, a doped or undoped group III-V compound buffer layer present in the plurality of holes of said mask, and wherein said nanostructures grow from said buffer layer through the plurality of holes of said mask.

15. The composition of matter of claim 1, wherein the substrate comprises a doped or undoped group III-V compound buffer layer comprising a doped or undoped GaN buffer layer and a doped or undoped β-Ga.sub.2O.sub.3 layer.

16. The composition of matter of claim 1, wherein said substrate comprises a hole patterned mask layer comprising a plurality of holes through which said at least one nanostructure is grown, and wherein a bottom of the plurality of holes of the hole patterned mask layer adjacent to the substrate is coated in a doped or undoped group III-V compound buffer.

17. A device, comprising the composition of claim 1.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) The figures are not to scale. FIG. 1 shows a possible flip chip design. In use therefore, light is emitted through the top of the device (marked hu). Layer 1 is the β-Ga.sub.2O.sub.3 substrate.

(2) Nanowires 2 are grown from substrate 1 epitaxially. Ideally, the β-Ga.sub.2O.sub.3 substrate is n-type doped and the nanowires are formed from Al(In)GaN, AlN or GaN and are doped to create n-i-p or n-p junctions.

(3) A filler 3 can be positioned between grown nanowires. A top electrode/light reflecting layer 4 is positioned on top of nanowires 2. The light reflecting 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.

(4) The n-electrode 8 is positioned on the substrate 1. That electrode might comprise Ti, Al, Ni or/and Au. The substrate may be provided with a mask 5 to allow growth of the nanowires in definitive positions on the substrate.

(5) The whole device may be soldered to a submount 6 via solder layer 7.

(6) When a forward current is passed vertically 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.

(7) 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.

(8) FIG. 2 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 material is deposited on the n-type doped β-Ga.sub.2O.sub.3 substrate, followed by n-AlN or n-(Al)GaN. In the central section of the nanowire as shown are a series of multiple quantum wells formed from (In)(Al)GaN. There follows the p-type doped region based on GaN, AlGaN or (Al)GaN, and an electron blocking layer based on p-Al(Ga)N and finally a highly doped p-GaN layer for ohmic contacting to a p-electrode.

(9) FIG. 3 shows an alternative chip design in which the nanowires are grown radially creating core shell structures. In use therefore, light is emitted through the top of the device (marked hu).

(10) Layer 1 is the β-Ga.sub.2O.sub.3 substrate. Nanowires 2 are grown from substrate layer 1 epitaxially. Ideally, β-Ga.sub.2O.sub.3 substrate is n-type doped and the nanowires formed from Al(In)GaN, AlN or GaN and are doped to create n-i-p or n-p junctions. The substrate 1 can be provided with a mask layer 5.

(11) A filler 3 can be positioned between grown nanowires. A top electrode/light reflecting layer 4 is positioned on top of nanowires 2. The light reflecting layer may also be provided with a p-electrode comprising Ni or/and 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.

(12) An n-electrode 8 is positioned on the n-doped substrate 1. When a forward current is passed vertically 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.

(13) The whole device may be soldered to a submount 6 via solder layer 7.

(14) 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.

(15) FIG. 4 shows a nanowire grown radially but having the same components as those of FIG. 2 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 GaN core material is deposited, followed by n-AlN or n-(Al)GaN. In the central shell of the nanowire as shown are a series of multiple quantum wells formed from (In)(Al)GaN. There follows the p-type doped region based on Al(Ga)N, and an electron blocking shell based on p-Al(Ga)N and finally a highly doped p-GaN shell for ohmic contacting of the p-electrode.

(16) FIG. 5 shows the β-Ga.sub.2O.sub.3 unit cell with the (100), (−201), and (010), crystallographic planes shown in red, blue, and green, respectively. Oxygen atoms are shown in red and Ga atoms in green.

(17) FIG. 6 show the β-Ga.sub.2O.sub.3 unit cell with the (−201) and (010) crystallographic planes as well as some physical properties of the β-Ga.sub.2O.sub.3 substrate. Oxygen atoms are shown in red and Ga atoms in green.

(18) FIG. 7 shows the examples of (Al, In)GaN nanowires grown in the [0001] direction on a β-Ga.sub.2O.sub.3 substrate with (−201) surface orientation. Due to the very small lattice mismatch between β-Ga.sub.2O.sub.3 and (Al)GaN, i.e. [−201]/[0001], high-quality vertical (Al, In)GaN nanowires can be epitaxially grown on the (−201) surface of the β-Ga.sub.2O.sub.3 substrate. For specific applications, solar-blind photodetectors can be made from p-type (Al, In)GaN nanowires/n-type β-Ga.sub.2O.sub.3 substrate and LEDs can be fabricated from n-i-p (Al, In)GaN nanowires on top of the n-type β-Ga.sub.2O.sub.3 substrate.

(19) The (Al, In)GaN nanowires can be directly grown on top of β-Ga.sub.2O.sub.3 with (−201) surface orientation without (FIG. 7(a)) or with (FIG. 7(b)) a thin epitaxial III-V buffer layer. The buffer layer can be made of, e.g. (Al, In)GaN, as shown in FIG. 7 (b).

(20) FIG. 7(c), shows the scanning electron microscopy (SEM) image of vertical n-type doped GaN nanowires grown on top of a (−201) β-Ga.sub.2O.sub.3 substrate with an n-type doped GaN buffer layer. The GaN nanowires and GaN buffer layer were grown with nitrogen plasma-assisted MBE as described in the Experimental Procedure section.

(21) FIGS. 8(a) and (c), show p-type (Al, In)GaN nanowires epitaxially grown on an n-type β-Ga.sub.2O.sub.3 substrate without (a) and with (c) a p-type epitaxial (Al, In)GaN buffer layer, respectively. This forms p-n junctions between the n-doped Ga.sub.2O.sub.3 substrate and p-type (Al, In)GaN nanowires, which can be used as material for solar-blind photodiode detector.

(22) In FIGS. 8(b) and (d), n-i-p doped (Al, In)GaN nanowires are epitaxially grown on n-type β-Ga.sub.2O.sub.3 substrates without (b) and with (d) an epitaxial n-type (Al, In)GaN buffer layer, respectively. The Ga.sub.2O.sub.3 substrate can here e.g. act as a transparent (for photons with an energy up to the Ga.sub.2O.sub.3 bandgap ˜4.8 eV) and conductive electrode for vertical current injection (Al, In)GaN nanowire LEDs.

EXAMPLE 1

(23) N-type doped GaN nanowires have been grown on the (−201) plane of an n-doped β-Ga.sub.2O.sub.3 substrate under N-rich conditions in a Veeco GEN 930 molecular beam epitaxy system with a radio-frequency nitrogen plasma source (PA-MBE, equipped with an isolation gate valve). Prior to loading to the growth chamber, the substrate is thermally cleaned at 500° C. for 1 hour in a preparation chamber. An n-type doped GaN buffer layer was grown at 550° C. with Ga flux of 0.1 ML/s and N.sub.2 flow of 2.5 sccm at 495 W for 60 min. The buffer growth was initiated by opening the Ga shutter and N.sub.2 gate valve and shutter simultaneously. After the successful growth of the buffer layer, the substrate temperature was ramped up to 765° C. The nanowire growth was then initiated by opening the Ga shutter and N.sub.2 gate valve and shutter simultaneously, and nanowire growth proceeded under a Ga flux of 0.5 ML/s and N.sub.2 flow of 2.5 sccm at 495 W for 4 hours. The GaN buffer layer and nanowires are n-type doped with Si using a Si cell temperature of 1100° C.

EXAMPLE 2

(24) N-doped GaN nanowires are grown on the (−201) plane of an n-doped β-Ga.sub.2O.sub.3 substrate using PA-MBE under N-rich conditions. A standard Knudsen effusion cell is used to supply Ga and Si atoms, while atomic nitrogen is generated from a radio-frequency plasma source that operates at 450 W. Prior to loading to the growth chamber, the substrate is thermally cleaned at 350° C. for 1 hour in a preparation chamber. Catalyst-free, self-assembled n-doped GaN nanowires are then grown directly on the n-doped β-Ga.sub.2O.sub.3 substrate, without any intermediate buffer layer. The growth process is initiated by opening the Ga and N.sub.2 shutters simultaneously, i.e. no intentional nitridation takes place on the surface of the substrate. The GaN nanowires are n-type doped with Si using a cell temperature of 800° C. and a growth time of 90 minutes.