T. Murotani

Growth temperature dependence in molecular beam epitaxy of gallium arsenide

Abstract Epitaxial layers of GaAs were grown on GaAs(100) at substrate temperatures ranging from 400° to 600°C by molecular beam epitaxy. Surface structures of the substrate and the epitaxial layers were investigated by means of low-energy electron diffraction. Two new structures of c(4 × 4) and c(8 × 8) were observed from layers grown at the low temperature of 400°C. The electrical and optical properties of layers doped with Si were investigated by measurement of Hall effect and photoluminescence as a function of growth temperature. It is found that a semi-insulating layer is grown below a critical temperature, and the layer is useful as a buffer layer for GaAs FETs. Variation of carrier concentration was observed near the interface between layers grown at different temperatures under a constant Sn beam flux. The effect is attributed to defect-induced segregation of Sn.

Molecular beam epitaxy of CdTe and Hg1-xCdxTe ON GaAs (100)

Using the molecular beam epitaxial (MBE) technique, CdTe and Hg1-xCdxTe have been grown on Cr-doped GaAs (100) sub-strates. A single effusion cell charged with polycrystal-line CdTe is used for the growth of CdTe films. The CdTe films grown at 200 °C with a growth rate of ~ 2 μm/hr show both streaked and “Kikuchi” patterns, indicating single crystalline CdTe films are smoothly grown on the GaAs sub-strates. A sharp emission peak is observed at near band-edge (7865 Å, 1.577 eV) in the photoluminescence spectrum at 77 K. For the growth of Hg1-xCdxTe films, separate sources of HgTe, Cd and Te are used. Hg0.6Cd0.4Te films are grown at 50 °C with a growth rate of 1.7 μm/hr. The surfaces are mirror-smooth and the interfaces between the films and the substrates are very flat and smooth. As-grown Hg0.6Cd0.4Te films are p-type and converted into n-type by annealing in Hg pressure. Carrier concentration and Hall mobility of an annealed Hg0.6Cd0.4Te film are 1 × 1017 cm−3 and 1000 cm2/V-sec at 77 K, respectively.

MOCVD GaAs growth on Ge (100) and Si (100) substrates

Abstract GaAs epitaxial layers are grown on exact (100) and slightly off-oriented (100) Ge and Si substrates by MOCVD. The etched patterns with molten KOH of the GaAs layers on exact (100) Ge and Si substrates show antiphase domain structure, but those on 2° off (100) toward [011] Ge and Si substrates reveal single domain. The single domain is supposed to arise from “phaselocking steps” existing on the surface of 2° off (100) Ge and Si substrates. These GaAs layers are characterized by X-ray rocking curves and 4 K photoluminescence spectra.

Reduction of Dislocation Density in GaAs/Si by Strained-Layer Superlattice of InxGa1-xAs-GaAsyP1-y

Dislocation density in GaAs layers grown on Si substrates has been investigated by measuring in-depth profile of the etch pit density (EPD) using a molten KOH. Reduction of EPD into 107 cm-2 level for the layer of 2-3 µm thickness is attained by the post annealing at 900°C for 30 min. Further reduction of dislocation has been achieved using InxGa1-xAs-GaAsyP1-y strained-layer superlattices (SLSs); step-like reduction of the dislocation at the SLSs and its continuous decrease even passing through the SLSs have been observed.

Improvement of InP crystal quality grown on GaAs substrates and device applications

We have investigated the effect of strained-layer superlattice (SLS) buffer layers on the crystal quality of InP grown on GaAs substrates. The various SLSs such as In0.63Ga0.37As/InP, In0.73Ga0.27As/InP, In0.9Ga0.1P/InP, and GaAs/InP were tested. It is found that the optical property of InP layers grown on In0.63Ga0.37As/InP SLSs was dramatically improved. An InGaAsP/InP double heterostructure laser diode and an InGaAs PIN photodiode (PD) on InP/GaAs with an In0.63Ga0.37As/InP SLS have been fabricated. The CW threshold current of the laser at room temperature was as low as 31 mA, and the dark current of the PD was 30 nA at - 10 V.

Effectiveness of AlGaAs/GaAs superlattices in reducing dislocation density in GaAs on Si

Abstract The effectiveness in reducing the dislocation density in a GaAs layer on a Si substrate (GaAs-on-Si) is investigated by using AlGaAs/GaAs superlattices grown by metalorganic vapor phase epitaxy (MOVPE). The degree of dislocation density reduction is compared among several types of AlGaAs/GaAs superlattices with various layer thicknesses, various numbers of layers and various AlAs content of AlGaAs layer. The thicker each layer is, the larger the number of layers, or the larger the AlAs content, the larger is the degree of dislocation density reduction. As a result, the dislocation density is reduced to 1×10 6 cm −2 by using five periods of (20 nm Al 0.85 Ga 0.15 As)/(100 nm GaAs superlattice). The reduction of the dislocation density and the degree of this effect can be explained by the crystal hardening of AlGaAs and the bending of dislocations at the AlGaAs/GaAs superlattice.

High-efficient operation of large-area (100 cm2) thin film polycrystalline silicon solar cell based on SOI structure

Abstract High-efficient operation of a large-area thin film polycrystalline Si solar cell with a novel structure based on a silicon on insulator (SOI) structure prepared by zone-melting recrystallization (ZMR) is reported. The (100) crystal orientation area over 90% has successfully been obtained by controlling the ZMR conditions, which allowed to form a uniform random pyramidal structure at the cell surface. The effect of hydrogen passivation has also been investigated for further improvement of the cell characteristics. By employing a light trapping structure (textured surface) and hydrogen passivation, an efficiency of 14.22% was obtained for a practical 100 cm 2 size.

High efficiency thin film silicon solar cells prepared by zone-melting recrystallization

A new type of silicon solar cell is demonstrated using chemical vapor deposited silicon thin films on a silicon dioxide layer. In order to improve the crystal quality of the thin films, zone‐melting recrystallization (ZMR) is applied and grain boundaries of polycrystalline Si films are passivated with H+. It is found that H+ passivation is quite effective for thin film Si solar cells and ZMR conditions to provide dominant (100) orientation is essential for achieving higher conversion efficiency. This (100) nature is also favorable for making effective light confinement scheme with pyramidal textured surface using anisotropic chemical etching. The conversion efficiency as high as 14.2% for a practical size of 10×10 cm2 is achieved. This is the highest for large area thin film polycrystalline Si solar cells so far.

Effect of Employing Positions of Thermal Cyclic Annealing and Strained-Layer Superlattice on Defect Reduction in GaAs-on-Si

This paper describes the dependence of the dislocation density reduction effect on the employing position of either thermal cyclic annealing (TCA) or InGaAs-GaAs strained-layer superlattice (SLS) in GaAs-on-Si grown by metalorganic chemical vapor deposition (MOCVD). The dislocation density is reduced to one twenty-fifth of that in as-grown sample by the TCA as the position of TCA becomes farther than about 1.5 µm from the Si surface. The dislocation density is additionally reduced to one third by the SLS as the position of SLS becomes farther than about 2.0 µm. As a result, the dislocation density is reduced to 1.5 × 106 cm-2 by the combined use of TCA and SLS. The dislocation density reduction effect of TCA is determined mainly by the degree of residual stress. That effect of SLS is determined mainly by the degree of additional stress generated by SLS.

Radiation Annealing of Si- and S-Implanted GaAs

Radiation annealing of Si- and S-implanted GaAs using tungsten lamps was studied. In capless radiation annealing, electrical activations were 70–80%, which were 20–30% higher than that in conventional furnace annealing for a low dose below 2×1013 cm-2. Effects of encapsulation on electrical activation were also investigated. It was found that electrical activations of high dose samples were improved by encapsulating the surface with SiO2 film for Si-implanted samples and with Si3N4 film for S-implanted samples.