Morihiko Kimata

Low Temperature Growth of Gallium Nitride

High-quality single crystal GaN films have been obtained by RMBE method using the reaction of Ga with NH3. Then, the addition of ionized N2 with decreasing NH3 improved the electrical properties of GaN. Residual H2 decomposed from NH3 has been disturbing the GaN growth and promoting generation of nitrogen vacancies and the complexes.

Molecular Beam Epitaxy of GaSb and GaSbxAs1-x

GaSb and GaSbxAs1-x single crystal thin films were successfully grown by molecular beam epitaxy. Undoped GaSb showed p-type conduction, and Te was found to be effective as a donor impurity for the MBE-grown GaSb. An in-depth profile of the electrical property of these films revealed that many defects are contained in the epitaxial layer near the interface between the grown layer and the substrate. GaSbxAs1-x films with entire composition were prepared, and the energy gap measured by photoabsorption shows a downward bowing as a function of the composition.

Molecular Beam Epitaxial Growth of InAs

Thin crystalline films of InAs have been heteroepitaxially grown on (100) surface of GaAs by molecular beam epitaxy. The films are evaluated by optical microscope, SEM, RHEED and electrical measurements. In spite of the existence of a large lattice mismatch between InAs and GaAs, specular InAs films are obtained. Undoped InAs films show the n-type conduction, and their electron concentrations and mobilities are varied depending on the substrate temperature (Ts) during growth. At Ts=480°C, single crystalline films with high crystallographic quality and flat surfaces are grown. Furthermore, Mg atoms are successfully doped into InAs during the MBE growth to obtain p-type films.

Molecular beam epitaxy of GaSb and InGaSb

Undoped GaSb and InxGa1−xSb (0 < x < 0.2) were grown on Te-doped GaSb(100) and Cr-doped GaAs(100) substrates. The quality of the films grown on GaSb was better than that of the films grown on the GaAs substrates. But the electrical isolation by the p-n junction between the grown films and the GaSb substrates was insufficient; therefore, GaAs substrates were subsequently used instead of the GaSb substrates to facilitate the determination of the electrical properties of the grown films. To relax the huge lattice mismatch between the films and the GaAs substrates and thereby improve the electrical properties, a buffer layer of AlSb and AllnSb was successfully grown and effectively used.

S-Doping of MBE-GaSb with H2S Gas

The possibility of S-doping MBE-GaSb using H2S gas has been demonstrated. The sulfer atoms are thermally dissociated from the H2S gas, and are adsorbed at Sb vacancy sites. The S sticking coefficient depends on the Sb coverage. The electron mobility of S-doped GaSb is relatively low. This may be caused by a high compensation ratio and lattice mismatch between the GaAs substrate and the GaSb film. From the experimental results, the activation energy of the S-donor is estimated to be about 75 meV.

Interdiffusion of In, Te at the interface of molecular beam epitaxial grown CdTeInSb heterostructures

Abstract One of the serious problems for the formation of good quality interfaces of InSb CdTe by molecular beam epitaxy (MBE) growth is the precipitation of InTe related compounds at the interface resulting from interdiffusion of In and Te. We have tried to insert an α-Sn layer at the interface to prevent the interdiffusion and the propagation of planar defects which may enhance the diffusion. In this study, proof that an α-Sn layer grown at 150°C prevents the interdiffusion is given by comparing Raman spectroscopy data on CdTe/α-Sn/InSb with those on α-Sn/CdTe/InSb and CdTe InSb structures. When the α-Sn layer was grown at 220°C, the Raman data showed that the prevention of interdiffusion became imperfect, although, the in-situ observed reflection high energy electron diffraction (RHEED) pattern indicated a high quality α-Sn layer. An insertion of one monolayer of Cd at the interface of α- Sn CdTe improved the quality of the α-Sn layer and an over pressure of Cd during the growth of CdTe improved the quality of the CdTe caplayer.

Growth and interdiffusion in CdTe/InSb multilayers

Abstract Molecular beam epitaxy of a CdTe/InSb multilayer sequence is reported for the first time. (110) orientated GaAs or InSb is used for the substrates. By reducing the growth rate of InSb to 0.1 μm/h at the total operating pressure with beams on of 10 -8 Pa, the substrate temperature could be reduced to 220–240°C which is in the optimum temperature range for the growth of CdTe. During the growth of the multilayer, direct change from streak to streak in a RHEED pattern was observed. The quality of the epilayers was examined with a double crystal X-ray system. The full width at half maximum of the InSb peak is 24 seconds of arc which is narrower than that of a GaAs substrate which is 30 seconds of arc. To determine the interdiffusion depth at the interface, sputter Auger electron spectroscopy was employed. An abrupt interface about 10A thick was obtained.

MOLECULAR BEAM EPITAXIAL GROWTH OF InP.

InP thin crystalline films were grown on (100)-oriented GaAs by molecular beam epitaxy (MBE) and evaluated by RHEED, SEM and IMA. Thin films with high crystalline quality were obtained when the substrate temperature was about 240°C and temperatures of In and P cells were 840~880°C and 370~400°C, respectively. Furthermore, Sn atoms were easily doped into InP during MBE growth, and the surface morphology of InP was greatly improved by Sn doping.

CdTe/InSb/α-Sn heterostructures grown by molecular beam epitaxy

Abstract MBE-grown CdTe/InSb/CdTe single quantum wells were characterized by TEM, FTIR and photoconduction. Even in the presence of In, Te related compounds and CdTe buffer-width dependent Fabry-Perot interference, a step-like quantum size effect was observed in the SQW FTIR data. The photocurrent peaks are also attributable to quantum well intersubband transitions due to higher subband energy levels. With the improvement of the CdTe/InSb interface in mind, we studied the characteristics of α-Sn layers grown on top of CdTe. TEM pictures show that the propagation of dislocation planes originating from the CdTe buffer layer is stopped at the α-Sn/CdTe heterointerface.

Analysis of GaSb/AlSb strained layers superlattices using small angle X-ray diffraction

Strained layer superlattices of GaAs and AlSb are analysed by augmenting (004) double crystal X-ray diffraction with small angle diffraction. The measurement of layer widths by the latter method will be shown to be more accurate since it does not involve any assumption on the state of strain. The results are confirmed by photoluminescence at 77 K. Actual strain is calculated from the average lattice parameter and the AlSb mole fraction obtained by (004) and small angle diffraction, respectively. The shift in the heavy hole band gaps of the layers due to the in-plane bi-axial stress is then discussed.