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Dilute nitride bismide GaNBiAs is a potential semiconductor alloy for near- and mid-infrared applications, particularly in 1.55 µm optical communication systems. Incorporating dilute amounts of bismuth (Bi) into GaAs reduces the effective bandgap rapidly, while significantly increasing the spin-orbit-splitting energy. Additional incorporation of dilute amounts of nitrogen (N) helps to attain lattice matching with GaAs, while providing a route for flexible bandgap tuning. Here we present a study of the electronic bandstructure and optical gain of the lattice matched GaNx-BiyAs1-x-y/GaAs quaternary alloy quantum well (QW) based on the 16-band k⋅p model. We have taken into consideration the interactions between the N and Bi impurity states with the host material based on the band anticrossing and valence band anticrossing model. The optical gain calculation is based on the density matrix theory. We have considered different lattice matched GaNBiAs QW cases and studied their energy dispersion curves, optical gain spectrum, maximum optical gain, and differential gain and compared their performances based on these factors. The thickness and composition of these QWs were varied in order to keep the emission peak fixed at 1.55 µm. The well thickness has an effect on the spectral width of the gain curves. On the other hand, a variation in the injection carrier density has different effects on the maximum gain and differential gain of QWs of varying thicknesses. Among the cases studied, we found that the 6.3 nm thick GaN3Bi5.17As91.83 lattice matched QW was most suited for 1.55 µm (0.8 eV) GaAs-based photonic applications. [ABSTRACT FROM AUTHOR]

The effect of lateral size and vertical thickness of CdSe and CdS nanoplatelets (NPLs) on their electronic structure and optical properties are investigated using an effective-mass envelope function theory based on the 8-band k · p model with valence force field considerations. Volumetrically larger NPLs have lower photon emission energy due to limited quantum confinement, but a greater transition matrix element (TME) due to larger electron-hole wavefunction overlap. The optical gain characteristics depend on several factors such as TME, Fermi factor, carrier density, NPL dimensions, material composition, and dephasing rate. There is a red shift in the peak position, more so with an increase in thickness than lateral size. For an increasing carrier density, the gain spectrum undergoes a slight blue shift due to band filling effect. For a fixed carrier density, the Fermi factor is higher for volumetrically larger NPLs and so is the difference between the quasi-Fermi level separation and the effective bandgap. The transparency injection carrier density (and thus input current density threshold) is dimension dependent and falls for volumetrically larger NPLs, as they can attain the requisite exciton count for transparency with a relatively lower density. Between CdSe and CdS, CdSe has lower emission energy due to smaller bandgap, but a higher TME due to lower effective mass. CdS, however, has a higher so hole contribution due to a lower spin-orbit splitting energy. Both CdSe and CdS NPLs are suitable candidates for short-wavelength LEDs and lasers in the visible spectrum, but CdSe is expected to exhibit better optical performance. [ABSTRACT FROM AUTHOR]

The magnetization dynamics of disordered Fe0.5(Pd1-xPtx)0.5 alloy films was studied by time-resolved magneto-optical Kerr effect and ferromagnetic resonance. The intrinsic Gilbert damping parameter α0 and the resonance linewidth change linearly with the Pt atomic concentration. In particular, the induced in-plane uniaxial anisotropy constant KU also increases for x increasing from 0 to 1. All these results can be attributed to the tuning effect of the spin orbit coupling. For the disordered ternary alloys, an approach is proposed to control the induced in-plane uniaxial anisotropy, different from conventional thermal treat methods, which is helpful to design and fabrications of spintronic devices. [ABSTRACT FROM AUTHOR]

Band structures of tensile strained and n+ doped Ge/GeSi quantum wells (QWs) are calculated by multiple-band k·p method. The energy dispersion curves of the Γ and L conduction subbands are obtained. The effects of tensile strain and n+ doping in Ge on direct bandgap optical gain and spontaneous radiative recombination rate spectra are investigated including the electron leakage from Γ to L conduction subbands. Our results show that the optical gain and spontaneous radiative recombination rate can be significantly increased with the tensile strain, n-type doping concentration, and injection carrier density in the Ge QW. The free carrier absorption is calculated and cannot be ignored because of the heavily doped Ge. The pure TM mode polarized net optical gain up to 1153 cm-1 can be achieved for the Ge/Ge0.986Si0.014 QW with tensile strain of 1.61% and n-type doping concentration of 30 × 1018 cm-3. [ABSTRACT FROM AUTHOR]

A ten-band k·p Hamiltonian for III-V-N dilute nitride semiconductor quantum wells (QWs) grown on the (11N)-oriented substrates is presented. The energy dispersion curves, optical transition matrix elements, internal piezoelectric field, and optical gain of InGaAsN/GaAs on the (110), (111), (113), and (11∞)-oriented substrates are investigated including band-anti-crossing, strain, and piezoelectric field effects. The band structures and optical gain are sensitive to the substrate orientation. The fundamental transition energy is the largest for the (111)-oriented QW and the smallest for (11∞)-oriented QW. The absolute values of internal piezoelectric field in the well and barrier layers reach the maximum for the (111)-QW, and zero for the (110) and (11∞)-oriented QWs. There exists an injection current density turning point. When the injection current density is below the turning point, the (111)-oriented QW has the largest peak gain. At the larger injection current density, the (11∞)-oriented QW has the largest peak gain. [ABSTRACT FROM AUTHOR]

Microphotoluminescence (μ-PL) investigation has been performed at room temperature on InAs quantum dot (QD) vertical cavity surface emitting laser (VCSEL) structure in order to characterize the QD epitaxial structure which was designed for 1.3 μm wave band emission. Actual and precise QD emission spectra including distinct ground state (GS) and excited state (ES) transition peaks are obtained by an edge-excitation and edge-emission (EEEE) μ-PL configuration. Conventional photoluminescence methods for QD-VCSELs structure analysis are compared and discussed, which indicate the EEEE μ-PL is a useful tool to determine the optical features of the QD active region in an as-grown VCSEL structure. Some experimental results have been compared with simulation results obtained with the aid of the plane-wave admittance method. After adjustment of epitaxial growth according to EEEE μ-PL measurement results, QD-VCSEL structure wafer with QD GS transition wavelength of 1300 nm and lasing wavelength of 1301 nm was obtained. [ABSTRACT FROM AUTHOR]

The self-heating effect in 1.3 μm p-doped InAs/GaAs quantum dot (QD) vertical cavity surface emitting lasers (VCSELs) has been investigated using a self-consistent theoretical model. Good agreement is obtained between theoretical analysis and experimental results under pulsed operation. The results show that in p-doped QD VCSELs, the output power is significantly influenced by self-heating. About 60% of output power is limited by self-heating in a device with oxide aperture of 5×6 μm2. This value reduces to 55% and 48%, respectively, as the oxide aperture increases to 7×8 and 15×15 μm2. The temperature increase in the active region and injection efficiency of the QDs are calculated and discussed based on the different oxide aperture areas and duty cycle. [ABSTRACT FROM AUTHOR]

The electronic band structures and optical gains of InAs1-xNx/GaAs pyramid quantum dots (QDs) are calculated using the ten-band k·p model and the valence force field method. The optical gains are calculated using the zero-dimensional optical gain formula with taking into consideration of both homogeneous and inhomogeneous broadenings due to the size fluctuation of quantum dots which follows a normal distribution. With the variation of QD sizes and nitrogen composition, it can be shown that the nitrogen composition and the strains can significantly affect the energy levels especially the conduction band which has repulsion interaction with nitrogen resonant state due to the band anticrossing interaction. It facilitates to achieve emission of longer wavelength (1.33 or 1.55 μm) lasers for optical fiber communication system. For QD with higher nitrogen composition, it has longer emission wavelength and less detrimental effect of higher excited state transition, but nitrogen composition can affect the maximum gain depending on the factors of transition matrix element and the Fermi–Dirac distributions for electrons in the conduction bands and holes in the valence bands respectively. For larger QD, its maximum optical gain is greater at lower carrier density, but it is slowly surpassed by smaller QD as carrier concentration increases. Larger QD can reach its saturation gain faster, but this saturation gain is smaller than that of smaller QD. So the trade-off between longer wavelength, maximum optical, saturation gain, and differential gain must be considered to select the appropriate QD size according to the specific application requirement. [ABSTRACT FROM AUTHOR]

The dependences of intersubband transitions on well width and nitrogen (N) content in n-type In0.23Ga0.77As1-xNx/GaAs quantum wells (QWs) are investigated using a ten-band k·p model. The absorption peak energy is found to increase first with the well width starting from 2 nm. It becomes insensitive from about 2.5 to 4.5 nm although the absorption intensity increases and bandwidth decreases monotonically, and then keeps decreasing with the well width beyond 4.5 nm. The peak energy is much larger than that of the N-free structure for narrower wells, but the difference decreases quickly with increasing well width. In the case of wider wells, the absorption peak energy shows relatively slow monotonic increase with increasing N content up to 3% because of the N-band and conduction-band coupling. In the nearly lattice-matched GaAsN/AlGaAs QWs the absorption peak energy shows a redshift with increasing N content from 0% to 0.4% and then increases gradually. The theoretical results are consistent with the reported experimental data. [ABSTRACT FROM AUTHOR]

The electronic structure, Zeeman splitting, and Stark shift of In1-yMnyAs1-xNx oblate quantum dots are studied using the ten-band k·p model including the sp-d exchange interaction between the carriers and the magnetic ion. The Zeeman splitting of the electron ground states is almost isotropic. The Zeeman splitting of the hole ground states is highly anisotropic, with an anisotropy factor of 918 at B=0.1 T. The Zeeman splittings of some of the electron and hole excited states are also highly anisotropic. It is because of the spin-orbit coupling which couples the spin states with the anisotropic space-wave functions due to the anisotropic shape. It is found that when the magnetic quantum number of total orbital angular momentum is nearly zero, the spin states couple with the space-wave functions very little, and the Zeeman splitting is isotropic. Conversely, if the magnetic quantum number of total orbital angular momentum is not zero, the space-wave functions in the degenerate states are different, and the Zeeman splitting is highly anisotropic. The electron and hole Stark shifts of oblate quantum dots are also highly anisotropic. The decrease of band gap with increasing nitrogen composition is much more obvious in the smaller radius case because the lowest conduction level is increased by the quantum confinement effect and is closer to the nitrogen level. [ABSTRACT FROM AUTHOR]

Interdiffusion in In0.32Ga0.68As0.984N0.016/GaAs multiple quantum wells with well widths of 2 and 4 nm, respectively, was investigated both experimentally and theoretically. Maximum blueshifts of 206 and 264 meV in the photoluminescence spectra were observed. Secondary ion mass spectrometry showed that both In–Ga and N–As interdiffusions played key roles for the large blueshifts. The significant In–Ga interdiffusion occurred at 650 °C while the N diffusion occurred at a temperature above 700 °C. The theoretical results are in good agreement with the experimental observations. [ABSTRACT FROM AUTHOR]

We report the design of an active region of InxGa1-xAs1-yNy/AlAs quantum cascade laser structure emitting in the near infrared wavelength range based on an eight-band k·p model. The InxGa1-xAs1-yNy/AlAs heterostructure system is of significant interest for the development of short wavelength quantum cascade lasers due to its large conduction band discontinuity (∼1.5 eV) and compatibility with the mature GaAs fabrication process. A detailed analysis of the intersubband transition energy within the conduction band as a function of layer thickness, composition, electric field, and temperature has been carried out. Finally, an optimized combination of In0.2Ga0.8As0.97N0.03/AlAs three-coupled-well structure has been obtained. Under an applied field of 100 kV/cm and at room temperature, a shortest wavelength of 3.4 μm has been achieved by making use of this system without introducing an upper lasing level beyond the X minima of the AlAs barrier. [ABSTRACT FROM AUTHOR]

The n-type InGaAs/AlxGa1-xAs quantum well infrared photodetector (QWIP) with asymmetric graded barriers for broadband detection has been investigated theoretically based on the eight-band k·p model. It is found that the intersubband transitions from the ground state to all bound and continuum excited states contribute to the overall absorption and the bound-to-continuum (B-to-C) transitions dominate. The superposition of the bound-to-bound and B-to-C transitions results in a broad detection bandwidth, and both the detected wavelength and bandwidth can be tuned by the applied voltage. The analysis method is also applicable to the GaAs/InxGa1-xAs/InyGa1-yAs QWIP with step quantum wells. The calculated results are consistent with the reported experimental observations. [ABSTRACT FROM AUTHOR]

Deep-level transient spectra (DLTS) and photoresponsivity were measured for Ga0.90In0.10N0.033As0.967/GaAs and Ga0.96In0.04N0.028As0.967Sb0.005/GaAs p-i-n photodetector structures. The GaInNAs and GaInNAsSb layers were grown closely lattice matched to GaAs substrate at 460 °C using molecular beam epitaxy. Two hole-trap levels were observed in the DLTS spectra of the GaInNAs sample with activation energies of 0.152 and 0.400 eV (labeled as H-1 and H-2 peak, respectively). The lower activation energy is believed to be associated with nitrogen-related defects and the higher activation energy is associated with arsenic antisite defects (AsGa). Following the incorporation of Sb into GaInNAs, the H-1 peak vanished from the DLTS spectra of the GaInNAsSb sample, and the AsGa defect-related DLTS signal was significantly reduced. Analysis of the DLTS data also showed that the trap concentration related to AsGa was reduced from 2.15×1015 to 2.58×1014 cm-3. The DLTS results are in good agreement with the photoresponsivity results, in which the GaInNAsSb sample showed 10× higher photoresponse compared to the GaInNAs sample. This indicates the incorporation of Sb into GaInNAs has effectively improved the p-i-n photodetector device performance. [ABSTRACT FROM AUTHOR]

Low-temperature (10 K) photoluminescence measurements of GaAs1-xNx epitaxial layers grown on GaAs reveal an anomalous second peak in solid-source molecular beam epitaxy. Rapid thermal annealing (RTA) of a specific GaAsN sample reveals a lower energy peak (γ) which redshifts and a higher energy peak (α) which blueshifts under increasing annealing temperature. The band-anticrossing model is used to identify the origins of the two peaks and we propose a model to explain the RTA observations by the concept of increased confinement in areas of higher N concentrations by trapped N localized states. The γ peak is due to the accumulation of N content near the GaAs/GaAsN interface. Hence, this abnormal annealing behavior occurs in layers with nonuniform N concentration at the GaAsN/GaAs interface. [ABSTRACT FROM AUTHOR]

We report observation of transverse electric (TE) dominant intersubband absorption in Si-doped GaInAsN/GaAs multiple-quantum-well structures. The TE dominant absorption is believed to be caused by the incorporation of nitrogen and the associated nitrogen state. When the confinement is strong in narrow quantum wells, the ground state is pushed up, which enhances the interaction with nitrogen state and significantly changes the nature of the state. [ABSTRACT FROM AUTHOR]

The band structures of wurtzite ZnO are calculated by the empirical pseudopotential method (EPM). The eight parameters of the Zn and O atomic pseudopotential form factors are obtained with the formula of Schluter et al.[Phys. Rev. B 12, 4200 (1975)]. The band parameters are extracted by using a k·p Hamiltonian to fit the EPM results. The calculated band-edge energies (Eg, EA, EB, and EC) at the Γ point are in good agreement with the experimental results. Based on the band parameters obtained, valence subbands of wurtzite ZnO/MgxZn1-xO tensile-strained quantum wells with different well widths and Mg compositions are calculated by the six-band k·p method. [ABSTRACT FROM AUTHOR]