Wen-Yuan Pang

Line defects of M-plane GaN grown on γ-LiAlO2 by plasma-assisted molecular beam epitaxy

The edge and threading dislocations of M-plane GaN epilayers grown on γ-LiAlO2 have been studied by high-resolution transmission electron microscope. We found that edge dislocations were grown in [11¯00] direction while threading dislocations were generated along a1 or −a2 axes. We also observed a single stacking fault in the M-plane GaN epilayer.

Self-assembled GaN hexagonal micropyramid and microdisk

The self-assembled GaN hexagonal micropyramid and microdisk were grown on LiAlO2 by plasma-assisted molecular-beam epitaxy. It was found that the (0001¯) disk was established with the capture of N atoms by most-outside Ga atoms as the (1×1) surface was constructing, while the pyramid was obtained due to the missing of most-outside N atoms. The intensity of cathode luminescence excited from the microdisk was one order of amplitude greater than that from M-plane GaN.

Self-Assembled c-Plane GaN Nanopillars on γ-LiAlO2 Substrate Grown by Plasma-Assisted Molecular-Beam Epitaxy

We have grown M-plane GaN films with self-assembled C-plane GaN nanopillars on a γ-LiAlO2 substrate by plasma-assisted molecular-beam epitaxy. The diameters of the basal plane of the nanopillars are about 200 to 900 nm and the height is up to 600 nm. The formation of self-assembled c-plane GaN nanopillars is through nucleation on hexagonal anionic bases of γ-LiAlO2. Dislocation defects were observed and analyzed by transmission electron microscopy. From the experimental results, we developed a mechanism underlying the simultaneous growth of three-dimensional c-plane nanopillars and two-dimensional M-plane films on a γ-LiAlO2 substrate.

InGaN/GaN single-quantum-well microdisks

We have grown In{sub x}Ga{sub 1-x}N/GaN quantum wells atop GaN microdisk with {gamma}-LiAlO{sub 2} substrate by using plasma-assisted molecular beam epitaxy. The structural and optical properties of the samples were analyzed by transmission electron microscopy, x-ray diffraction, cathodoluminescence, and photoluminescence measurements. Based on the measured results, we obtained the indium concentration of the In{sub x}Ga{sub 1-x}N/GaN single quantum well to be x = 0.25 with a band-gap energy of 2.31 eV, which is consistent with the bowing effect of bulk In{sub x}Ga{sub 1-x}N: E{sub g}(x) = [3.42 - x * 2.65 - x * (1 - x) * 2.4] eV.

Spin splitting in AlxGa1―xN/GaN quasiballistic quantum wires

We have observed beating Shubnikov–de Haas oscillations in Al0.18Ga0.82N/GaN [112¯0]-direction quantum wires grown on (0001) sapphire. The spin-splitting energy, (2.4±0.3) meV for 200 nm wire, was suppressed to (1.2±0.3) meV for 100 nm wire and smeared by the scattering from edge states and intersubbands. The spin splitting of Rashba effect can be used to control the differential phase shift of spin-polarized electrons when a gate bias is applied to a nanometer arm of quantum ring. Based on the results of spin-splitting for the [112¯0]-direction AlxGa1−xN/GaN nanowire, the spin splitting of one-dimensional electron system in AlGaN/GaN nanowire can be applied to a low-power consuming quantum-ring interferometer.

Green light emission by InGaN/GaN multiple-quantum-well microdisks

The high-quality In{sub x}Ga{sub 1−x}N/GaN multiple quantum wells were grown on GaN microdisks with γ-LiAlO{sub 2} substrate by using low-temperature two-step technique of plasma-assisted molecular beam epitaxy. We demonstrated that the hexagonal GaN microdisk can be used as a strain-free substrate to grow the advanced In{sub x}Ga{sub 1−x}N/GaN quantum wells for the optoelectronic applications. We showed that the green light of 566-nm wavelength (2.192 eV) emitted from the In{sub x}Ga{sub 1−x}N/GaN quantum wells was tremendously enhanced in an order of amplitude higher than the UV light of 367-nm wavelength (3.383 eV) from GaN.

Spin-splitting in an AlxGa1−xN/GaN nanowire for a quantum-ring interferometer

An Al0.18Ga0.82N/GaN heterostructure was used to fabricate a ballistic nanowire with a wire width of 200 nm by focused ion beam. We observed the beating Shubnikov–de Haas oscillations in the nanowire with a spin-splitting energy of (2.4±0.3) meV. Based on the results, we proposed a spin-Hall quantum-ring interferometer made of AlxGa1−xN/GaN nanowires for spintronic applications.

Spin-degenerate surface and the resonant spin lifetime transistor in wurtzite structures

Spin-splitting energies of wurtzite AlN and InN are calculated using the linear combination of atomic orbital method, and the data are analyzed utilizing the two-band k⋅p model. It is found that in the k⋅p scheme, a spin-degenerate surface exists in the wurtzite Brillouin zone. Consequently, the D’yakonov-Perel’ spin relaxation mechanism can be effectively suppressed for all spin components in the [001]-grown wurtzite quantum wells (QWs) at a resonant condition through application of appropriate strain or a suitable gate bias. Therefore, wurtzite QWs (e.g., InGaN/AlGaN and GaN/AlGaN) are potential structures for spintronic devices such as the resonant spin lifetime transistor.

Self-confined GaN heterophased quantum wells

Wurtzite/zinc-blende/wurtzite GaN heterophased quantum wells (QWs) were grown by plasma-assisted molecular beam epitaxy. A self-assembling mechanism was used to simulate the heterophased QW, in which a wurtzite/zinc-blende phase transition was created by rotating the threefold symmetric N-Ga vertical bond 60°. The GaN heterophased QW was attested by transmission electron microscopy, selective area electron diffraction and cathodoluminescence measurements.

Electrical contact for wurtzite GaN microdisks

We developed a back processing to fabricate an electrical contact of wurtzite GaN microdisk on transparent p-type GaN template. The interface welding between the GaN microdisk and p-type GaN template produced a very solid and secure epi-film contact for the electrical current passing through, with a resistance of 45.0 KΩ and threshold voltage of 5.9 V. The back processing can resolve the obstacle of electrical contacts for self-assembled wurtzite nano-devices.