Masashi Mizuta

Low Temperature Growth of GaN and AlN on GaAs Utilizing Metalorganics and Hydrazine

The growth and characterization of AlN and GaN on GaAs are presented. Trimethylgallium (TMG) and trimethylaluminum (TMA) were used as group III sources and hydrazine as a nitrogen source. It was found for both nitrides that mass-transport-limited growth took place at high temperature, and that at low temperature the surface catalyzed decomposition of respective metalorganics manifested itself. Deposition of AlN film was observed at as low as 220°C and that of GaN at 450°C both on GaAs substrates. A distinct proof of cubic-form GaN grown on GaAs is obtained from results of the X-ray precession measurement. The direction of GaN on (100) GaAs, however, is slightly tilted from that of the substrate. It is speculated that this tilt results from the very large lattice-mismatch existing between GaN and GaAs.

Coherent Growth of ZnSe on GaAs by MOCVD

The lattice parameters of ZnSe have been studied for epitaxial layers of different thicknesses grown on GaAs (100) substrates by metalorganic chemical vapor deposition (MOCVD) using dimethylzinc and diethylselenide as source materials. The results indicate that layers thinner than 0.15 µm can grow coherently on GaAs involving strains due to the lattice mismatch between ZnSe and GaAs, while thicker layers contain misfit dislocations introduced by the relaxation of strain.

Room-Temperature Continuous-Wave Operation of InGaN Multi-Quantum-Well Laser Diodes Grown on an n-GaN Substrate with a Backside n-Contact

Continuous-wave operation at room-temperature has been demonstrated for InGaN multi-quantum-well (MQW) laser diodes (LDs) grown on low-dislocation-density n-GaN substrates with a backside n-contact. The current, current density and voltage at the lasing threshold were 144 mA, 10.9 kA/cm2 and 10.5 V, respectively, for a 3 µm wide ridge-geometry diode with high-reflection dielectric coated mirrors. Single-transverse-mode emission was observed in the far-field pattern of the LDs and the beam full width at half power in the parallel and perpendicular directions was 6° and 25°, respectively.

The Local-Environment-Dependent DX Centers: Evidence for the Single Energy Level with a Specified Configuration

The energy-level structure of the DX centers corresponding to specific local environments was investigated under hydrostatic pressure. Several discrete DX levels, each of which was a well-defined single level, were clearly resolved. The energy level of the Si DX center surrounded only with Ga was determined to be 295 meV above the ?-conduction band edge of GaAs. When Al is coordinated as the 2nd-nearest neighbor, the energy level is lowered by as much as 120 meV. The energy level is sensitive not only to the number of Al 2nd-nearest neighbors, but also to the atomic configuration itself.

Alloy Fluctuation in Mixed Compound Semiconductors as Studied by Deep Level Transient Spectroscopy

Deep level transient spectroscopy (DLTS) has been utilized to detect alloy fluctuation around defects in several kinds of alloy semiconductors. An increased half-width in the DLTS spectrum has been observed in the ternary and quaternary systems of InGaP and InGaAsP whereas the binary systems of GaP and InP have not shown such broadening. The broadening can be interpreted as a fluctuation of defect properties due to the increased freedom of atom arrangements around the relevant defect in the alloy systems.

Novel Ridge-Type InGaN Multiple-Quantum-Well Laser Diodes Fabricated by Selective Area Re-Growth on n-GaN Substrates

A novel ridge structure fabricated by selective-area epitaxial re-growth is proposed for InGaN multiple-quantum-well (MQW) laser diodes (LDs). This technique is capable of precisely controlling the active ridge width and height, thus enabling stable single tran sverse-mode operation. Together with a backside n-contact on a low-dislocation-density GaN substrate, this structure provides high productivity and performance for GaN-based blue-violet LDs. A stable fundamental transverse mode up to 40 mW was demonstrated for the certain range of ridge dimensions. The minimum aspect ratio of the far-field patterns (FFPs) was about 2.1 in the fabricated ridge-type InGaN MQW LDs.

A Capacitance Investigation of InGaAs/InP Isotype Heterojunction

We have measured deep levels near the InGaAs/InP heterointerface by DLTS and C-V method. Three deep levels, E1, E2 and E3, have been found near the heterointerface, whose activation energies are 0.17 eV, 0.37 eV and 0.54 eV, respectively. The concentrations of E2 and E3 rapidly decrease when approaching the heterointerface from the InP, indicating that the two levels are located only in the InP substrate. The E1 level, on the other hand, can be found only near the heterointerface. The density of the E1 level is well correlated with the interface charge density which is determined by the C-V analysis. Both of the densities are dependent on the degree of the lattice mismatch between the InGaAs epitaxial layer and the InP substrate.

Surface charge limit in NEA superlattice photocathodes of polarized electron source

The “surface charge limit (SCL)” phenomenon in negative electron affinity (NEA) photocathodes with GaAs–AlGaAs superlattice and InGaAs–AlGaAs strained-layer superlattice structures has been investigated systematically using a 70 keV polarized electron gun and a nanosecond multi-bunch laser. The space-charge-limited beam with multi-bunch structure (1.6 A peak current, 12 ns bunch width and 15 or 25 ns bunch separation) could be produced from the superlattice photocathodes without suffering the SCL phenomenon. From the experimental results, it has been confirmed that the SCL phenomenon is governed by two physical mechanisms at the NEA surface region, the tunneling of conduction electrons against the surface potential barrier (escaping process) and that of valence holes against the surface band bending barrier (recombination process); these effects can be enhanced using the superlattice structure and heavy p-doping at the surface, respectively. We conclude that a superlattice with heavily p-doped surface is the best photocathode for producing the multi-bunch electron beam required for future linear colliders.

DX Deep Centers in AlxGa1-xAs Grown by Liquid-Phase Epitaxy

Deep levels, the so-called DX centers, in the AlxGa1-xAs alloy system grown by liquid-phase epitaxy (LPE) were investigated by junction-capacitance spectroscopy. The dependence of the activation energy of the DX center in Sn-doped AlxGa1-xAs on the alloy composition was determined by DLTS. This dependence seems to reflect the change in the nature of the conduction bands in this alloy system. The other donor species studied: Si, Te and Se, were also found to form DX centers. An anomalously high concentration of DX centers was determined using low-temperature C-V techniques.

Electron accumulation in AlGaSb/InAs/AlGaSb quantum well system

The mechanism of the electron accumulation in the AlGaSb/InAs/AlGaSb QW system with Al composition higher than 0.5 is discussed. In this QW system, it is found that the dominant electron source is donor levels in AlGaSb rather than interface levels which have, so far, been proposed as the electron source. We have found two contradictory results, that undoped AlGaSb shows p‐type conduction and Be‐doping is compensated electrically, are satisfactorily explained by the model of coexistence of donors and a larger concentration of deep acceptors. Electron accumulation in InAs QW is shown to be controllable by shifting the quantum level with respect to the deep acceptor level.