Katsuhiko Higuchi

High Gm In0.5Al0.5As/In0.5Ga0.5As high electron mobility transistors grown lattice-mismatched on GaAs substrates

Abstract We have fabricated high quality In 0.5 Al 0.5 As/In 0.5 Ga 0.5 As high electron mobility transistors (HEMTs) with thin buffer layers lattice-mismatched on GaAs substrates by molecular beam epitaxy (MBE). Our step-graded In y Al 1− y As buffer layers efficiently terminate lattice-misfit dislocations at each step interface according to cross-sectional transmission electron microscopy (TEM) and plan-view TEM observations. The buffer layers facilitate high mobility (approximate values of 10,000 cm 2 / V · s at room temperatures), even though the total buffer thickness is smaller than 600 nm. The mobility is comparable to that for HEMTs grown on InP substrates and better than that reported for HEMTs grown on GaAs with much thicker buffers. For a device with a 0.5 μm long, 20 μm wide gate and a 600 nm buffer layer, the peak extrinsic G m is 750 mS/mm, which is higher than that for previously reported HEMTs with the same gate length, including devices on InP substrates. In 0.5 Al 0.5 As/In 0.5 Ga 0.5 As single quantum wells (SQWs) grown on the step-graded In y Al 1− y As buffer layers show intense, sharp photoluminescence spectra, comparable to those of SQWs grown lattice-matched on InP. These results show that the tin step-graded buffer enabled growth of high-quality In 0.5 Al 0.5 As/In 0.5 Ga 0.5 As heterostructures on GaAs substrates.

Arbitrary choice of basic variables in density functional theory : Formalism

The Hohenberg-Kohn theorem of the density functional theory (DFT) is extended by modifying the Levy constrained-search formulation. The theorem allows us to choose arbitrary physical quantities as basic variables which determine the ground-state properties of the system. Moreover, the theorem establishes a minimum principle with respect to variations in chosen basic variables as well as with respect to variations in the density. By using this theorem, self-consistent single-particle equations are derived. N single-particle orbitals introduced reproduce not only the electron density but also arbitrary physical quantities which are chosen as basic variables. The validity of the theory is confirmed by examples where the spin density or paramagnetic current density is chosen as one of basic variables. The resulting single-particle equations coincide with the Kohn-Sham equations of the spin-density functional theory or current-density functional theory, respectively. By choosing basic variables appropriate to the system, the present theory can describe the ground-state properties more efficiently than the conventional DFT.

Arbitrary choice of basic variables in density functional theory. II. Illustrative applications

Our recent extended constrained-search theory [M. Higuchi and K. Higuchi, Phys. Rev. B 69, 035113 (2004)] enables us to choose arbitrary quantities as the basic variables of density functional theory. In this paper, we apply it to several cases. In the case in which the occupation matrix of localized orbitals is chosen as a basic variable, we can obtain the single-particle equation which is equivalent to that of the

HIGH-EFFICIENCY DELTA-DOPED AMORPHOUS-SILICON SOLAR-CELLS PREPARED BY PHOTOCHEMICAL VAPOR-DEPOSITION

\mathrm{LDA}+\mathrm{U}

Selective wet-etching of InGaAs on InAlAs using adipic acid and its application to InAlAs/InGaAs HEMTs

method. The theory also leads to the Hartree-Fock-Kohn-Sham equation by letting the exchange energy be a basic variable. Furthermore, if the quantity associated with the density of states near the Fermi level is chosen as a basic variable, the resulting single-particle equation includes the additional potential which could mainly modify the energy-band structures near the Fermi level.

Pair density-functional theory by means of the correlated wave function

Amorphous Si solar cells with delta-doped (δ-doped) p-layers were prepared by a multichamber photochemical vapor deposition (photo-CVD) system. By optimizing the structure of the δ-doped p-layer, a conversion efficiency of 12.3% (AM1) was obtained for small-area solar cells with a δ-doped p-layer using B2H6 as a dopant source. The δ-doped p-layers deposited with trimethylboron (TMB) and triethylboron (TEB) as new boron sources were characterized. It was found that the boron layers obtained with TMB by photo-CVD contain a large amount of carbon atoms which degrade the solar cell performance. Carbon contamination was suppressed both by the plasma CVD method with a gas mixture of TMB, H2 and He and by the photo-CVD method with TEB.

Optimum design and fabrication of InAlAs/InGaAs HEMT's on GaAs with both high breakdown voltage and high maximum frequency of oscillation

Highly selective wet-etching of InGaAs on InAlAs was demonstrated using pH-controlled adipic acid, and solutions. A maximum selectivity of 250 was obtained by controlling the InGaAs and InAlAs etching mechanisms. By identifying the rate-determining steps for the etching of InAlAs and InGaAs, we found that the high selectivity is due to the difference in solubility between the oxide of InAlAs and that of InGaAs in the adipic acid solution. InAlAs/InGaAs HEMTs fabricated in a diameter wafer by using this highly selective etching had a threshold voltage and a transconductance with standard deviations of 38 mV and , respectively.

First high performance InAlAs/InGaAs HEMTs on GaAs exceeding that on InP

We present a density functional scheme for calculating the pair density (PD) by means of the correlated wave function. This scheme is free from both of problems related to PD functional theory, i.e., (a) the need to constrain the variational principle to

High-Performance In0.5Al0.5As/In0.5Ga0.5As High Electron Mobility Transistors on GaAs

N

High Breakdown Voltage InAlAs/InGaAs High Electron Mobility Transistors on GaAs with Wide Recess Structure

-representable PDs and (b) the development of a kinetic energy functional. By using the correlated wave function, the searching region for the ground-state PD is substantially extended as compared with our previous theory[Physica B \textbf{372} (2007), in press]. The variational principle results in the simultaneous equations that yield the best PD beyond the previous theory, not to mention the Hartree-Fock approximation.