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This study investigates the optical anisotropy spectrum in the R-plane (i.e., the [ 10 1 ¯ 2 ] -oriented layer plane) of GaN/Al0.2Ga0.8N quantum wells of different widths. 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 [ 10 1 ¯ 2 ] -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.

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.

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

The application of a low-frequency oscillating magnetic field perpendicular to a magnetic fluid thin film leads to the separation of a phase that is concentrated in particles from a dilute phase. The concentrated phase forms cylindrical columns that construct two-dimensional lattices. The ordered structure of magnetic fluid thin films is the basis for potential optical applications. We investigate the dynamical ordering formation of columnar lattices in magnetic fluid thin films subjected to oscillating perpendicular magnetic fields.

For the no-common-atom superlattices, the new phenomenon of zero-field spin splitting is predicted by the modified bond orbital model. Apart from the (001)- and (111)-oriented superlattices, this spin splitting appears along the growth directions of superlattices. Moreover, it originates from the existence of heterobonds inside the interfacial unit cells. This phenomenon due to the microscopic symmetry reduction is similar to the concept of the Dresselhaus effect and thus termed as the Dresselhaus-like spin splitting.

The zero-field spin splitting and in-plane anisotropy in InGaAs/InP quantum wells, which originate from the inversion asymmetry between two interfaces, are discussed by a modified bond orbital model. To include the microscopic interface effect, the potential term of the bond orbital model is modified by the concept of the heuristic Hbf model. The proposed model allows flexible and convenient calculations for various growth directions. It is found that the inversion asymmetric effect strongly depends on the growth direction. The origins of the anisotropy and spin splitting on the (001)-, (111)-, and (110)-oriented quantum wells will be discussed in detail.

The interface perturbations at the atomic scale are neglected in the bond orbital model (BOM) due to the unit-cell-scale orbitals. This makes the breakdown of the higher symmetry Hamiltonian of BOM used for calculating the microscopic interface phenomena. To improve this problem, we introduce the intracell effects into the BOM by expanding the BOM basis in terms of the tetrahedral (anti)bonding orbitals and using the potential operator instead of the scalar potential for the extraction of the microscopic information. The modified BOM method provides the direct insight into the microscopic symmetry of the crystal chemical bonds in the vicinity of the heterostructure interfaces. It indicates more physical meanings and easier manipulations than the other methods in the mathematical calculations to discuss the symmetry breaking phenomena. The InAs–GaSb superlattice as an example can support the proposed model well.

The optical polarization of GaN/AlGaN wurtzite quantum wells in various orientations is studied using an arbitrarily-oriented [ h k i l ] Hamiltonian potential matrix. The optical matrix elements in the wurtzite quantum wells are calculated using the k ⋅ p finite difference scheme. The results reveal the presence of giant in-plane optical anisotropy (polarized normal to [0001]) in the M-plane (i.e., the ( 10 1 ¯ 0 ) -oriented layer plane) GaN/Al0.2Ga0.8N quantum well, due to the positive crystal-field split energy effect ( Δ CR > 0 ). The present theoretical results are consistent with the photoluminescence measurements presented in the literature [B. Rau, et al., Appl. Phys. Lett. 77 (2000) 3343].

A simple theoretical six-band $\mathbf{k}\ensuremath{\cdot}\mathbf{p}$ finite difference method is developed and applied to calculate the electronic band structures and electronic Landau-level structures of the symmetric and asymmetric ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{S}\mathrm{b}/\mathrm{I}\mathrm{n}\mathrm{A}\mathrm{s}$ quantum wells (QW's). The QW may exhibit a semiconductor-semimetal transition by changing the Al composition in the ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{Sb}$ layer. The cyclotron-resonance splitting in the semiconducting structures is due to the large InAs conduction-band nonparabolicity and Zeeman effect. Broken-gap type-II semimetallic QW, in which the conduction-valence Landau-level mixing can yield a significant spin splitting for the InAs conduction-band Landau levels, produces a prominent electron double-line structure in the cyclotron-resonance spectra, whether the QW is symmetric or asymmetric. Strong oscillations in the electron cyclotron-resonance mass, amplitude, and linewidth are evident. The abnormal cyclotron-resonance mass jumps, amplitude minima, and linewidth maxima occurring near the even filling factors are due to the conduction-valence Landau-level mixing effect. These results are in good agreement with the experimental results.

We have studied the effect of well thickness on the two-dimensional electron-hole system in semimetallic ${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$Sb/InAs quantum wells by Shubnikov--de Haas (SdH) measurements. The number of hole carriers in the ${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$Sb barriers was changed by a negative persistent photoconductivity effect. From the amplitude of SdH oscillation for the sample with the thinnest well, we found that the interface roughness scattering dominated when the number of hole carriers is small. After the number of holes is increased by the negative persistent photoconductivity effect, the electron-hole scattering becomes more important, and results in a reduction of the electron quantum lifetime. The competition between electron-hole scattering and interface roughness scattering depends on the amount of holes in the barriers.

Applying an oscillating magnetic field perpendicularly on the high-quality magnetic fluid thin film, the phase separation of particles in the liquid matrix will occur. The concentrated phase makes up the cylindrical columns that can form two-dimensional lattices. The ordered structure of magnetic fluid thin films is the basis for the potential optical application. We explore the dynamical ordering formation of columnar lattices in magnetic fluid thin films subjected to oscillating perpendicular magnetic fields in this study.

We develop a 16-band atomic bond-orbital model (16ABOM) to compute the spin splitting induced by bulk inversion asymmetry in zincblende materials. This model is derived from the linear combination of atomic-orbital (LCAO) scheme such that the characteristics of the real atomic orbitals can be preserved to calculate the spin splitting. The Hamiltonian of 16ABOM is based on a similarity transformation performed on the nearest-neighbor LCAO Hamiltonian with a second-order Taylor expansion k at the Γ point. The spin-splitting energies in bulk zincblende semiconductors, GaAs and InSb, are calculated, and the results agree with the LCAO and first-principles calculations. However, we find that the spin-orbit coupling between bonding and antibonding p-like states, evaluated by the 16ABOM, dominates the spin splitting of the lowest conduction bands in the zincblende materials.

The k · p finite difference method is popular to calculate the band structure of semiconductors and their heterostructures. However, the effect of the step length in the differential process on the band structure is not clear yet. A comparative study between the k · p finite difference method and the bond orbital model is then made. With the optimum step length, one monolayer spacing along the growth direction, the calculated band structure of the k · p finite difference method is almost as accurate as that of the bond orbital model.

The transmission spectra of the multivalley double-barrier structures for various quantum-well widths are investigated within an antibonding-orbital model. We found some interesting interference phenomena due to the intervalley mixing effect: For each quantum-well width, the transmission spectrum exhibits a pair of closely spaced resonance peaks; the energy separation between the two resonance peaks and the ratio of the two resonance widths are oscillatory functions of the quantum-well width; and the energy separation between the two resonance peaks may be changed significantly by applying a voltage difference across the double-barrier structure when the energy separation at zero bias is small. We also demonstrated that these interference phenomena can be observed by analyzing the current-voltage characteristics of the multivalley double-barrier structures.

We have developed a two-band effective-mass model to study the resonant-tunneling spectra for a structure in which the interaction or a heavy-mass state (nearly discrete) with a light-mass continuum leads to interesting interference effects. We round two types or interference effects: Type I exhibits a zero-transmission dip preceding a resonance-tunneling peak and type-II exhibits a zero-transmission dip following a resonance-tunneling peak. Our theory demonstrates that by analyzing the interference structure in the resonant-tunneling spectra one can obtain a direct measurement or the strength or band mixing in semiconductor heterostructures

P-wave-enhanced spin field-effect transistor made of AlGaN/GaN heterostructure was designed for the spintronic devices operated at high power and high temperature. The operation theory is based on the spin-polarized field-effect transistor designed by Datta and Das [Appl. Phys. Lett. 56, 665 (1990)]. The mechanism of the p-wave enhancement in AlGaN/GaN heterostructure was investigated. The recent development and related patents in the spin-polarized field-effect transistor were reviewed. In particular, we will focus on the recent patents which could enhance p-wave probability and control of spin precession of 2DEG in the AlGaN/GaN transistor structure.

We have observed the $k$-dependent spin splitting in wurtzite ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}∕\mathrm{Ga}\mathrm{N}$ heterostructures. An anomalous beating pattern was observed in Shubnikov--de Haas measurements due to the interference of Rashba and Dresselhaus spin-orbit interactions. The dominant mechanism for the $k$-dependent spin splitting at high values of $k$ is attributed to Dresselhaus term which is enhanced by the ${\ensuremath{\Delta}}_{\mathrm{C}1}\text{\ensuremath{-}}{\ensuremath{\Delta}}_{\mathrm{C}3}$ coupling of wurtzite band folding effect. The interference of Rashba and Dresselhaus effects in the ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}∕\mathrm{Ga}\mathrm{N}$ heterostructure provides a potential candidate for the gate-controlled spin-polarized spintronic devices.

The electronic properties of Fe-doped ${\mathrm{Al}}_{0.31}{\mathrm{Ga}}_{0.69}\mathrm{N}∕\mathrm{GaN}$ heterostructures have been studied by Shubnikov-de Haas measurement. The lowest two subbands of the two-dimensional electron gas in the heterointerface were populated. After the low temperature illumination, the electron density increases from $11.99\ifmmode\times\else\texttimes\fi{}{10}^{12}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$ to $13.40\ifmmode\times\else\texttimes\fi{}{10}^{12}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$ for the first subband and from $0.66\ifmmode\times\else\texttimes\fi{}{10}^{12}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$ to $0.94\ifmmode\times\else\texttimes\fi{}{10}^{12}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$ for the second subband. The persistent photoconductivity effect ($\ensuremath{\sim}13%$ increase) is mostly attributed to the Fe-related deep-donor level in $\mathrm{GaN}$ layer. The second subband starts to populate when the first subband is filled at a density of $9.40\ifmmode\times\else\texttimes\fi{}{10}^{12}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$. We obtained the energy separation between the first and second subbands to be $105\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$.

The two-dimensional electron gas in δ-doped double quantum wells has been studied by using the Shubnikov–de Haas measurement. It was discovered that the δ-doping layer at the central barrier is able to prevent the electron coupling in the two wells and that the persistent photoconductivity can be reduced due to the thinner barrier. A δ-doped multiple quantum well structure for photodetector application was also proposed.

This letter reports the theoretical study of the current–voltage characteristics of the AlAs–GaAs–AlAs double-barrier structure with a thin GaAs well and thin AlAs barriers at room temperature, within a third-neighbor sp3 bond-orbital model. We demonstrate that the tunneling current caused by the L-valley electrons gives a significant contribution to the valley current at room temperature. Moreover, if the AlAs barriers become thick enough (e.g., >3 nm), the tunneling current at room temperature may be dominated by the L electrons instead of Γ electrons.

A different mechanism (${\ensuremath{\Delta}}_{C1}\text{\ensuremath{-}}{\ensuremath{\Delta}}_{C3}$ coupling) accounts for the spin splitting of wurtzite $\mathrm{GaN}$, that originated from the intrinsic wurtzite effects (band folding and structure inversion asymmetry). The band-folding effect generates two conduction bands (${\ensuremath{\Delta}}_{C1}$ and ${\ensuremath{\Delta}}_{C3}$), in which $p$-wave probability has tremendous change when ${k}_{\mathrm{z}}$ approaches the anticrossing zone. The spin-splitting energy induced by the ${\ensuremath{\Delta}}_{C1}\text{\ensuremath{-}}{\ensuremath{\Delta}}_{C3}$ coupling and wurtzite structure inversion asymmetry is much larger than that evaluated by traditional Rashba or Dresselhaus effects. When we apply the coupling to $\mathrm{GaN}∕\mathrm{AlN}$ quantum wells, we find that the spin-splitting energy is sensitively controllable by an electric field. Based on the mechanism, we proposed a $p$-wave-enhanced spin-polarized field effect transistor, made of ${\mathrm{In}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}∕{\mathrm{In}}_{y}{\mathrm{Al}}_{1\ensuremath{-}y}\mathrm{N}$, for spintronics application.

The two-dimensional electron system in strongly coupled ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_{0.3}{\mathrm{Ga}}_{0.7}\mathrm{As}$ double quantum wells has been studied by Shubnikov--de Haas (SdH) measurements. The degenerate subbands of the double quantum wells are lifted into two subbands with a symmetric or antisymmetric wave function. We observed SdH oscillations due to the second subband of the antisymmetric wave function. The effective mass of the second subband increases with the magnetic field for a field range from 0.686 to 1.776 T. The magnetic-field-induced mass enhancement for double quantum wells with an equal well thickness is greater than that for those with an unequal one. This mass enhancement is attributed to the many-body effects of electron-electron interaction.

This letter reports the theoretical study of the current–voltage characteristics of the AlSb–InAs–AlSb double‐barrier structure with a thin InAs well at both 77 and 300 K, within a nearest‐neighbor sp3 bond‐orbital model in which the band‐bending effects are also taken into account. We demonstrate that the tunneling current due to the incident electrons derived from the valence bands gives a significant contribution to the valley and background currents; in addition, it gives the dominant contribution to the valley current when the temperature is low enough.

This letter reports the study of the resonant‐tunneling characteristics of the AlSb‐InAs‐AlSb double‐barrier structures within a second‐neighbor sp3 bond‐orbital model. The model employs one s‐like antibonding orbital and 3 p‐like bonding orbitals per unit site and is capable of describing the lowest conduction band accurately throughout the entire Brillouin zone. The current‐voltage curve thus obtained, compared to those obtained using the nearest‐neighbor sp3 bond‐orbital model or those obtained from the two‐band model, gives a much better agreement with the experiment.

We report the theoretical study of the current-voltage characteristics of an n-type Ga 1 - x Al x As/GaAs/Ga 1 - x Al x As double-barrier structure in a strong magnetic field perpendicular to the interface. Axial approximation is used. We demonstrate that the Landau index-conserving (Δn=0 and Δs=0) resonant tunneling leads to the main resonant peak, and the Landau index-nonconserving (An = I and Δs=-1) resonant tunneling yields the weak secondary peak beyond the main peak, due to the conservation of the Landau-level mixing index q. For conduction electrons, q is the sum of the Landau index n and the spin s.

The spin-splitting energies in biaxially strained bulk wurtzite material AlN are calculated using the linear combination of atomic orbital (LCAO) method, and the equi-spin-splitting distributions in k-space near the minimum-spin-splitting (MSS) surfaces are illustrated. These data are compared with those derived analytically by two-band k · p (2KP) model. It is found that the results from these two methods are in good agreement for small k. However, the ellipsoidal MSS surface under biaxial compressive strain does not exist in the 2KP model, because the data points are far from the Γ point. Instead, three basic shapes of the MSS surface occur in the wurtzite Brillouin zone: a hyperboloid of two sheets, a hexagonal cone, and a hyperboloid of one sheet, evaluated from the LCAO method across the range of biaxial strains from compressive to tensile.

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.

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.

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.

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.

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.

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.

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.

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.

The spin splitting of wurtzite GaN was calculated by 32×32 Hamiltonian with spin-orbital interaction. The band-folding effect generates two conduction bands ΔC1 and ΔC3 in which the p-wave probability shows a tremendous change when kz approaches the anticrossing zone. We found that a large spin plitting in GaN∕AlN quantum wells is produced due to ΔC1-ΔC3 coupling, and is effectively controlled by electric field. Based on the mechanism and gate-controllable spin splitting, we proposed a p-wave-enhanced quantum well, InxGa1-xN∕InyAl1−yN, for the application of the spin-polarized field effect transistor designed by Datta and Das [Appl. Phys. Lett. 56, 665 (1990)].