Jih-Chen Chiang

Dresselhaus effect in bulk wurtzite materials

The spin-splitting energies of the conduction band for ideal wurtzite materials are calculated within the nearest-neighbor tight-binding method. It is found that ideal wurtzite bulk inversion asymmetry yields not only a spin-degenerate line (along the kz axis) but also a minimum-spin-splitting surface, which can be regarded as a spin-degenerate surface in the form of bkz2−k‖2=0 (b≈4) near the Γ point. This phenomenon is referred to as the Dresselhaus effect (defined as the cubic-in-k term) in bulk wurtzite materials because it generates a term γwz(bkz2−k‖2)(σxky−σykx) in the two-band k∙p Hamiltonian.

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.

Optical anisotropy in [hkil]-oriented wurtzite semiconductor quantum wells

An 8×8 band edge potential matrix of the [hkil]-oriented wurtzite Hamiltonian is developed and applied to explore optical anisotropy in [hkil]-oriented wurtzite semiconductor quantum wells. The wave-vector-dependent optical matrix elements are expressed entirely in terms of Hamiltonian matrix elements, thus avoiding the requirement to introduce any additional optical parameters. To accommodate the noncubic symmetry of the wurtzite lattice, spinor rotation is taken into account when performing the calculations for different crystal orientations. The optical matrix elements are formulated and calculated for both the real finite-barrier-height case and the approximate infinite-barrier-height case. It is found that giant anisotropy of the optical matrix elements appears in the [101¯0]- and [101¯2]-oriented well planes.

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.

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.

Application of block diagonal technique to Hamiltonian matrix in performing spin-splitting calculations for GaAs zincblende bulk and quantum wells

The 2×2 conduction band, 4×4 hole band, and 2×2 spin-orbit split-off band matrices of zincblende semiconductors are obtained by using a block diagonal technique. Importantly, the block diagonal matrices incorporate not only the interband coupling effect but also the bulk inversion asymmetry effect. Analytical expressions for the conduction band spin-splitting energies of GaAs zincblende bulk and quantum wells grown on [001]-, [111]-, and [110]-oriented substrates are formulated by solving the block diagonal matrices. The results show that odd-in-k terms exist in both the bulk and the quantum well expressions due to the bulk inversion asymmetry effect. The presence of these terms is shown to induce the spin-splitting phenomenon.

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.

Effect of bulk inversion asymmetry on optical transitions of zinc blende semiconductor quantum wells

The influence of bulk inversion asymmetry (tetrahedral) on the optical transitions in zinc blende quantum wells is analyzed using an enhanced k∙p optical calculation framework which takes intracell interactions into account. It is shown that inversion asymmetry results in a marked variation of the optical transition strength. Significantly, this effect cannot be revealed by the conventional k∙p optical transition formalism, which considers intercell interactions only.

Effects of Giant Optical Anisotropy in R-plane GaN/AlGaN Quantum Wells by Valence Band Mixing

Investigation of optical anisotropy spectra in the R-plane (i. e., the [1012]-oriented layer plane) of GaN/Al0.2Ga0.8N quantum wells with different widths is studied. The optical matrix elements in the wurtzite quantum wells are calculated using the k·p finite difference scheme. The calculations show that the valence band mixing effect produces giant in-plane optical anisotropy in [1012]-oriented GaN/Al0.2Ga0.8N quantum wells with a narrow width. The nature of the in-plane optical anisotropy is found to be dependent on the well width. Specifically, it is found that the anisotropy changes from x′-polarization to y′-polarization as the well width increases. DOI: 10.2529/PIERS060801054904

Magnetotransport study on the defect levels of delta-doped In0.22Ga0.78As∕GaAs quantum wells

We have studied the electronic properties of delta-doped In0.22Ga0.78As∕GaAs quantum wells (QWs) by van der Pauw Hall measurements and Shubnikov–de Haas measurements. From the temperature-dependent van der Pauw Hall measurements, we observed two kinds of donors, which have binding energies of 104±7 and 9.6±0.1meV. After inserting In0.1Ga0.9As layers between the In0.22Ga0.78As and GaAs layers, a single donor with binding energy of 50±2meV was observed. The carrier concentration determined by SdH measurements did not change after the QWs were illuminated at low temperature, which indicates that these deep donors could not produce a persistent photoconductivity in delta-doped In0.22Ga0.78As∕GaAs QWs.