Masakatsu Suzuki

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 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.

First principles calculation of effective mass parameters of GaN

Abstract First principles electronic band calculations are performed for the wurtzite and zincblende GaN, by using a full-potential linearized augmented plane wave method. In order to clarify the difference of electronic properties between two kinds of crystal structures, and to give important information on the characteristic analysis of GaN-based quantum well devices, the Luttinger-like valence band parameters, as well as the electron and hole effective masses and splitting energies, are derived from the calculated band structures with the assistance of the k·p theory, considering the cubic and hexagonal symmetries. The small spin-orbit coupling of a nitrogen makes us use the 6 × 6 k·p Hamiltonians for both structures in the analysis of the valence band states.

A 1.4µm front-side illuminated image sensor with novel light guiding structure consisting of stacked lightpipes

A frontside illuminated image (FSI) sensor with novel light guiding structures consisting of stacked lightpipes was developed using 45nm Cu processing on 300 mm wafers. We demonstrated a high quantum efficiency (QE) of 75% and maximum incident angle of 40°, which exceeds the performance of backside illuminated image (BSI) sensors [1,2,3]

Electronic band structure of Li2CuO2

Abstract Electronic band calculations for non-magnetic and antiferromagnetic states of Li 2 CuO 2 are performed by using the FLAPW method. Cu-3d states are strongly hybridized with O-2p states. For the non-magnetic state, a narrow antibonding band is separated from broad dp mixing bands and half filled with an electron. For the antiferromagnetic state, the calculated magnetic moment is extended on the CuO 2 chain.

Interaction between N2 and stabilized ZnSe surface

Abstract The interaction between excited N 2 molecules and dimers at stabilized ZnSe (100) surface has been studied theoretically. The dimerization at the surface is treated as a one-dimensional lattice distortion, the so-called Peierls transition, and the stability of the dimerization is evaluated within the mean field approximation. When the orbital energy in the N 2 molecule gets close to the energy band of the one-dimensional electron gas, the dimerization is not a preferable state, and this result is consistent with the N 2 adsorption and dissociation mechanism on a ZnSe surface we had proposed using the cluster model in ref. [3].

Theoretical analysis of optical gain and exciton effect in GaN/AlGaN quantum wells

Optical gains of wurtzite GaN/AlGaN quantum wells have been studied by a first-principles calculation and the k.p method. Most of the parameters in the k.p method were determined by fitting the band structures to the first- principles calculation. Owing to the small spin-orbit splitting energies and the strong electronegativity, the large hole density of states causes the higher threshold current density of the wurtzite GaN/AlGaN quantum well lasers. Then, we have proposed a new mechanism of the optical gain for laser diodes, using a localized level and the excitonic effects. The excitonic effects enhance an oscillator strength of the optical transition. When a localized level is exist in the band-gap, the optical gain between the localized state and one of the band edge states is produced with the very small carrier density. This optical gain is enhanced by the excitonic effect a the band edges and it might show a possibility of very low threshold current density for wide-gap laser diodes.

Electronic Band Structure of Li_2CuO_2 and Application to Battery Electrode

The electronic band calculations of Li2CuO2 and Li-deintercalated CuO2 have been performed by the fullpotential linearized augmented plane wave (FLAPW) method within the local spin density approximation (LSDA). It is found that Cu 3d states are strongly hybridized with O 2p states and that Li atom is almost ionized. On the basis of the total energy calculations, we have studied the applicability of Li2CuO2 as a cathode material of rechargeable Li-battery. The average battery voltage in the chemical reaction, Li2CuCO2 → CuO2 + 2Li, is estimated. The structural stability is also discussed.