Takeshi Uenoyama

Biaxial Strain Effect on Wurtzite GaN/AlGaN Quantum Well Lasers

Subband structures and optical gains of both unstrained and biaxial strained wurtzite GaN/AlGaN quantum well (QW) laser diodes (LDs) are theoretically investigated by the 8×8 k\cdotpp theory, with the assistance of the first-principles calculations in the derivation of the required parameters such as deformation potentials. The strong electron affinity and the small spin-orbit coupling of a nitrogen yield much heavier effective masses even in the QWs. It plays an essential role in causing a higher threshold current density for any well length than GaAs/AlGaAs QW LDs. Considering a biaxial strain induced by the lattice mismatch, the optical gain property qualitatively improves for any well length. However, the effect on the reduction of the threshold current density is quantitatively not so effective as GaAs/AlGaAs QW LDs.

Strain effect on electronic and optical properties of GaN/AlGaN quantum‐well lasers

In order to clarify the strain effect on the GaN‐based lasers and to give the important guideline on their device design, the subband structure and the optical gains of strained wurtzite GaN/AlGaN quantum wells are theoretically investigated on the basis of k⋅p theory. First‐principles band calculations are used for deriving the unknown physical parameters. It is found that neither compressive nor tensile biaxial strains in the c plane are so effective on the reduction of the threshold carrier density as conventional zinc‐blende lasers and that the uniaxial strain in the c plane is very useful for reducing it. The relation between the uniaxial strain’s direction and the optical polarization is also clarified. As a result, we suggest that the uniaxial strain in the c plane is one of the preferable approaches for the efficient improvement of the GaN‐based lasers performance.

Valence subband structures of wurtzite GaN/AlGaN quantum wells

Valence subband structures of wurtzite GaN/AlGaN quantum wells have been studied, using the k⋅p method in which the parameters are derived by first‐principles calculations. Since wurtzite GaN and AlN have the small spin‐orbit splitting energies (≤20 meV), the mixing among the six bands including the spin‐orbit split band should be considered to obtain the subband states correctly. According to our parameters for the GaN/AlGaN quantum well, the hole carrier confinement in the two‐dimensional systems does not lead to a significant reduction of hole masses, and so the threshold current density of wurtzite GaN/AlGaN quantum well laser diodes is not decreased.

Optical gain and crystal symmetry in III–V nitride lasers

Optical gains of bulk GaN and GaN/AlGaN quantum wells (QWs) are calculated in wurtzite (WZ) and zinc‐blende (ZB) structures. The physical parameters are extracted by reproducing the band‐edge electronic structures from the first‐principles band calculations. It is found that the lower crystal symmetry, that is the WZ structure, is preferable for the lower transparent current density in bulk GaN. Although the introduction of QW structures leads to symmetry lowering only in the ZB structure, we cannot find a significant benefit of the ZB QW structure, owing to the weak spin‐orbit coupling of the nitrogen atom.

Optical Gain Calculation of Wurtzite GaN/AlGaN Quantum Well Laser.

Optical gain properties of wurtzite GaN/Al0.2Ga0.8N quantum well lasers are theoretically analyzed using physical parameters from ab initio calculations for the first time. The valence band of wurtzite GaN exhibits strong non-paraboticity, and the hole density of states is significantly large in comparison with the conventional zincblende crystals. This valence band feature causes high transparency cartier density of 7.5×1018 cm-3 in the 50 A thick GaN quantum well. This result predicts that the threshold current of wurtzite GaN/AlGaN quantum well laser is higher than the conventional lasers with zincblende crystals.

Reduction of Threshold Current Density of Wurtzite GaN/AlGaN Quantum Well Lasers by Uniaxial Strain in (0001) Plane

The uniaxial strain effect in the (0001) plane on the electronic and optical gain properties of wurtzite GaN/AlGaN quantum well lasers is investigated on the basis of kp theory. In order to obtain the required physical parameters, the first-principles band calculations are used. It is found that the uniaxial strain in the (0001) plane causes much lower threshold current density than the biaxial strain does. The relation between the uniaxial strains direction and the optical polarization is clarified as well. As a result, we predict that the uniaxial strain in the (0001) plane is one of the preferable approaches for the efficient improvement of the GaN-based lasers performance.

First-Principles Calculation of Effective Mass Parameters of Gallium Nitride

The electronic band structure calculation is carried out for wurtzite-type GaN by using a full-potential linearized augmented plane wave (FLAPW) method. In order to give useful information on the material and device designs for short-wavelength optical devices, the first-principles calculation is connected with the effective mass approximation for the wurtzite structure. The effective mass parameters, such as electron effective mass, Luttinger-like parameters, crystal field splitting and spin-orbit splitting, are derived for the first time from reproducing the calculated band structure near the Γ point. The obtained value of the electron effective mass is in good agreement with the observed values. It is also found that the cubic approximation is available to analyze the valence band structure of the wurtzite-type nitrides.

Analysis of GaInP/AlGaInP compressive strained multiple-quantum-well laser

We have analyzed GaInP/AlGaInP compressive strained MQW lasers, with theoretical calculation and experimental results. Our calculations of TE polarized gain, where the valence subband mixing and the heterobarrier leakage current are taken into account, are in good agreement with the experimental results. When a compressive strain of up to 0.5% is induced in the quantum wells, the density of states near the valence band edge is decreased, due to the reduction of heavy-hole and light-hole subband mixing. At the threshold condition, the compressive strain reduces not only the radiative recombination current, but also the hetero-barrier leakage current. Therefore, the threshold current is reduced, and its temperature dependence is found to be small, In the analysis, we also show that when larger compressive strain of more than 0.5% is induced in the 40-/spl Aring/-thick quantum wells, the threshold characteristics are degraded. >

Theoretical Study of Momentum Matrix Elements of GaN

The interaction between the conduction band and valence bands in wurtzite GaN is theoretically investigated by means of the first-principles calculation and the k\cdotpp theory. The momentum matrix elements are derived from the first-principles electronic band structure, calculated by a full-potential linearized augmented plane wave method. We also show analytical expressions of the momentum matrix elements, based on the 8×8 k\cdotpp theory for the wurtzite structure. The values estimated from the analytical expressions are in good agreement with those obtained from the first-principles calculation.

Electronic Properties of Nitrogen Delta-Doped Silicon Carbide Layers

Nitrogen delta-doped silicon carbide (SiC) layers were grown by a new pulse doping method in a chemical vapor deposition. Doping distribution with high peak concentration (1.×10 18 cm −3 ) and narrow distribution width (12 nm) was fabricated in the nitrogen delta-doped structure of SiC. Mobility enhancement due to spatial separation of electrons and their ionized parent donors was observed for the delta-doped structure. Metal-semiconductor field-effect transistors with a nitrogen delta-doped channel and a recess gate structure were fabricated. The devices had large source-drain breakdown voltages, high drain current capability and easy control of the threshold voltage with a good pinch-off characteristics.