Shigenori Takagishi

Selective epitaxial growth of GaAs by low-pressure MOVPE

Abstract We have studied selective growth of GaAs by low-pressure MOVPE at 10 Torr on a (001) GaAs substrate partially masked with a SiN x film. A GaAs epitaxial layer is selectively grown on the unmasked area, while no deposition of polycrystalline GaAs occurs on the SiN x film. No ridge growth takes place on the GaAs substrate near the SiN x film edge, so that the epitaxial layer is uniform throughout in thickness. The side surfaces of selectively grown layers are bounded by the { 111 }B planes when the stripe is formed in the [110] direction, and by the {111}A and the { 111 }B planes when the stripe is formed in the [ 1 10] direction. The mechanism of selective growth is discussed briefly.

High-radiation-resistant InGaP, InGaAsP, and InGaAs solar cells for multijuction solar cells

The radiation response of 3 MeV proton-irradiated InGaP, InGaAsP and InGaAs solar cells was measured and analyzed in comparison with those of InP and GaAs. The degradation of the minority-carrier diffusion length was estimated from the spectral response data. The damage coefficient KL for the 3 MeV proton-irradiated InGaP, InGaAsP and InGaAs was also determined. The radiation resistance increases with an increase in the fraction of In–P bonds in InGaP, InGaAsP and InGaAs. Differences in the radiation resistance of InGaP, InGaAs and InGaAs materials are discussed. Minority-carrier injection under forward bias is found to cause partial recovery of the degradation on irradiated InGaP and InGaAsP cells.

Near-field photoluminescence study of GaNAs alloy epilayer at room and cryogenic temperature

We have measured the spatial distribution of the optical properties of a GaNAs (N∼0.8%) epilayer to investigate the carrier recombination mechanism at both room temperature and cryogenic temperature using a near-field scanning optical microscope. A difference between the macro and near-field photoluminescence (PL) spectra at room temperature was not observed. At low temperature, we found spatial inhomogeneity of the optical properties and sharp features in the near-field PL spectrum. These findings indicate that the dominant emission mechanism changes from recombination of delocalized carriers at room temperature to recombination of localized carriers (excitons) trapped in the local potential minimum due to compositional fluctuation at low temperature.

Metalorganic Vapor Phase Epitaxial Growth of GaNAs Using Tertiarybutylarsine (TBA) and Dimethylhydrazine (DMHy).

GaNAs alloys were successfully grown on GaAs substrates by low-pressure metalorganic vapor phase epitaxy (MOVPE) with all organometallic sources of triethylgallium (TEG), tertiarybutylarsine (TBA), and dimethylhydrazine (DMHy). For nitrogen, the desorption coefficient of 30 kcal/mol was derived from the nitrogen incorporation dependence on growth temperature. Since the nitrogen concentration above 3% was easily achieved by our growth technique, the combination of TBA-DMHy as V precursors is a candidate for the growth of other III-V alloys containing nitrogen. We observed a decrease in PL intensity with enhancing nitrogen incorporation into solids. In order to recover from degradation of optical properties, rapid thermal annealing (RTA) was demonstrated and found to be effective. Therefore MOVPE using TBA-DMHy combined with postgrowth annealing is expected to obtain GaNAs alloys with high nitrogen concentration as well as excellent optical properties.

Effect of Operating Pressure on the Properties of GaAs Grown by Low-Pressure MOCVD

Undoped GaAs epitaxial layers were grown by low-pressure MOCVD in a wide range of operating pressure (3×10-3 to 75 Torr). These epitaxial layers had smooth surfaces. Conductivity of the epitaxial layers grown under the same [AsH3]/[TMG] ratio (=75) changed from n-type to p-type at about 5×10-1 Torr as operating pressure was reduced. Low temperature photoluminescence spectra indicate that carbon is the dominant acceptor and its concentration increases as operating pressure is reduced.

Selective Embedded Growth of AlxGa1-xAs by Low-Pressure Organometallic Vapor Phase Epitaxy

Selective embedded growth of AlxGa1-xAs (x≤0.35) in grooves of patterned substrates has been achieved for the first time by low-pressure organometallic vapor phase epitaxy (LP-OMVPE). Epitaxial AlxGa1-xAs layers are embedded only in chemically etched grooves, while no polycrystalline deposition occurs on the masked area. The layers grown in grooves with the reverse-mesa cross sections, formed in the [110] direction, are uniform in thickness and show flat-smooth surfaces throughout. These results indicate that LP-OMVPE is a very promising technique for monolithic device integration.

GaInP/GaAs and mechanically stacked GaInAs solar cells grown by MOCVD using TBAs and TBP as V-precursors

We have applied metallorganic chemical vapor deposition (MOCVD) using less toxic group V-precursors to the fabrication of the monolithic dual-junction GaInP/GaAs and mechanically-stacked GaAs/GaInAs cells, targeting for the super-high-efficiency triple-junction GaInP/GaAs/GaInAs solar cells. The dual-junction GaInP/GaAs cell grown on an n-type GaAs substrate, which is suitable for higher optical transmittance to the bottom cell, showed a conversion efficiency of 25.9% at AM 1.5, 1-sun. Combined with an efficiency of 5.1% for GaInAs bottom cell grown on an InP substrate under the mechanically stacked GaAs top cell, it is possible to attain an efficiency of over 30% by the all organometallic-source MOCVD method.

Hydrogen and carbon incorporation in GaInNAs

We have investigated the dependence of impurity incorporation of GaInNAs on the In concentration, substrate orientation, and growth temperature by secondary ion mass spectroscopy (SIMS). GaInNAs films were grown by low-pressure metalorganic vapor-phase epitaxy (MOVPE) on GaAs substrates. It was found that H and C concentrations were strongly related to the N concentration in the solid and the dimethylhydrazine (DMHy) flux at the surface. Furthermore, intentional Si-doping study revealed that incorporated H was stable, which is most likely a consequence of neutral bonding N, although the activation energy of H was as large as 197 kcal/mol derived from the observed growth-temperature dependence.

Epitaxial Growth of High-Purity GaAs by Low-Pressure MOCVD

High-purity GaAs epitaxial layers were reproducibly obtained at operating pressures lower than conventional low-pressure MOCVD. The epitaxial layer grown at 17 Torr exhibited a high mobility of 105,000 cm2/Vsec at 77 K and a fine exciton structure in the low-temperature PL spectrum. The conductivity of epitaxial layers grown at 8 Torr changed from p-type to n-type as the [AsH3]/[TMG] ratio increased. PL spectra indicated that carbon is the dominant acceptor and that carbon concentration and fine exciton structure depend strongly upon the [AsH3]/[TMG] ratio.

1.3 µm Traveling-Wave GaInNAs Semiconductor Optical Amplifier

We fabricated a GaInNAs semiconductor optical amplifier (SOA) by applying a facet coating to a buried-ridge-stripe GaInNAs laser. Due to a low reflectivity (<0.1%) and a wide bandwidth (70 nm) coating, Fabry–Perot (FP) modes of the GaInNAs laser were suppressed sufficiently, and thus a 1.3 µm traveling-wave GaInNAs SOA was realized for the first time. Peak chip gains of more than 9.6 dB and a 3-dB-gain bandwidth above 49 nm (9 THz) were obtained simultaneously with a cavity length between 600 µm and 900 µm. In addition, on/off ratios between 20 and 30 dB were obtained by switching the current on and off, which seems sufficient for the SOA to work as a switching device. With the temperature characteristics, we found that the ASE intensity and the gain coefficient of the GaInNAs SOA were much less dependent on temperature than those of conventional InP-based SOAs. These results demonstrate the superior temperature characteristics of the GaInNAs SOA compared with conventional InP-based SOAs.