T. Kita

Development of Integration Process of InGaAs/InP Heterojunction Bipolar Transistors with InP-Passivated InGaAs pin Photodiodes

We have successfully developed a fabrication process for OEICs in which each device structure and performance can be optimized. We integrated InGaAs/InP HBTs and InP-passivated InGaAs pin PDs on a semi-insulating InP substrate using multi-epi growth process. For the PDs with a 60 mum mesa diameter, the capacitance of 0.2 pF and the dark current of 0.1 nA at a reverse bias voltage of 5 V were obtained. For the HBTs with a 5.6-mum2 emitter area, the current gain of 44, the turn-on voltage of 0.77 V, the cutoff frequency of 122 GHz, and the maximum oscillation frequency of 165 GHz at a collector current density of 1 mA/mum2 were obtained. The HBTs show good uniformity and HBTs near the pin PDs show the same characteristics as those of remote HBTs. The result promises that this process can be used in fabrication of high-speed OEICs for practical applications.

Emission-wavelength extension of InAs/GaAs quantum dots by controlling lattice-mismatch strain

Nitrided InAs quantum dots (QDs) have been shown to suppress In-segregation in QDs and achieve emission at 1.3 mum. Effects of strain on structural and optical properties of QDs have been demonstrated through transmission electron microscope and photoluminescence analyses

Order‐Parameter Dependence of Spontaneous Electron Accumulation at Ga0.5In0.5P/GaAs Studied by Raman‐Scattering and Photoluminescence Measurements

We have investigated spontaneously accumulated two‐dimensional electrons at the long‐range ordered Ga0.5In0.5P/GaAs interface as a function of the order parameter. Raman‐scattering and photoluminescence measurements reveal that the accumulated electron density and interface built‐in electric field depend systematically on the order parameter. These results agree well with self‐consistent calculations taking into account electronic band structure with the two‐dimensional electrons.

High-density electron gas induced by atomic ordering in undoped Ga/sub 0.5/In/sub 0.5/P/GaAs heterostructure

We investigated two-dimensional properties of electrons accumulated at a long-range ordered Ga/sub 0.5/In/sub 0.5/P/GaAs heterointerface by magneto-photoluminescence (PL) measurements. The disordered Ga/sub 0.5/In/sub 0.5/P/GaAs sample shows a typical GaAs-PL spectrum for the bulk GaAs grown by metalorganic vapor-phase epitaxy. On the other hand, the GaAs-PL spectrum for the ordered sample with order parameter /spl eta/ of 0.30 shows transitions related to a quantized electron state in a triangular potential buried at the heterointerface and emission of the Fermi-edge singularity. The observed PL spectrum for the ordered sample demonstrates that a two-dimensional electron gas is induced at the Ga/sub 0.5/In/sub 0.5/P/GaAs heterointerface. In magneto-PL measurements, anisotropy of the reduced mass between parallel and perpendicular to the heterointerface was confirmed. Furthermore, the clear optical Shubnikov-de Haas oscillation was found in PL transition energy under the perpendicular magnetic field. The sheet-carrier density of 2DEG is deduced from the period of the observed Shubnikov-de Haas oscillation to be /spl sim/1.20/spl times/10/sup 12/ cm/sup -2/.

Internal electric field effects at ordered Ga/sub 0.5/In/sub 0.5/P/GaAs heterointerface investigated by photoreflectance spectroscopy

We performed photoreflectance (PR) and Raman-scattering measurements of long-range ordered Ga/sub 0.5/In/sub 0.5/P/GaAs heterointerfaces to investigate effects of the internal electric field on the carrier-modulation mechanism. The PR spectrum of an ordered Ga/sub 0.5/In/sub 0.5/P/GaAs heterointerface shows the Franz-Keldysh oscillation due to a strong internal electric field. However, the PR-signal amplitude for the ordered sample is /spl sim/1/20 of that for a disordered sample. Raman-scattering results of the ordered Ga/sub 0.5/In/sub 0.5/P/GaAs heterointerfaces reveal plasmon-phonon coupled modes due to the spontaneous electron accumulation. We suggest that the electron accumulation reduces the mean electric field for the PR modulation.

Interdiffusion effects at long-range ordered Ga/sub 0.5/In/sub 0.5/P and GaAs heterointerfaces

We measured photoluminescence and Raman-scattering spectra of a long-range ordered Ga/sub 0.5/In/sub 0.5/P/GaAs heterointerface. With increasing order parameter, the GaAs-PL spectrum is gradually changed from that of the bulk GaAs to an anomalous spectrum with two broad peaks. In the PL-excitation measurements, we found that these broad peaks are related to localized states in GaAs and Ga/sub 0.5/In/sub 0.5/P layers. On the other hand, in the Raman-scattering measurements, the Ga-As LO-phonon mode shows a red shift in contrast to that of the bulk GaAs, and the Ga-As TO mode appears in spite of a forbidden transition. These results suggest interdiffusion of P atoms, which induces non-crystallization of the GaAs. We consider that GaAsP alloys are constituted at the ordered Ga/sub 0.5/In/sub 0.5/P/GaAs heterointerface.

Cooling Process of Hot Excitons in Ordered Ga0.5In0.5P

Time-resolved photoluminescence (PL) spectroscopy has been performed to investigate hot-exciton relaxation process in long-range ordered Ga0.5 In0.5 P. The cooling process of excitons shows strong order parameter dependence. The rise time of the exciton PL is getting faster with increasing degree of ordering. This indicates that optically created excitons rapidly relax in a fluctuated potential of the partially ordered epitaxial film. The exciton-PL lifetime becomes small with increasing order parameter. Also, the PL of ordered Ga0.5 In0.5 P exhibits a localized exciton feature with an much slower decay. These findings suggest that excitons tend to localize in ordered domain structure and can relax into localized deep centers via the nonradiative recombination pass in the ordered domain ensemble.

Energy relaxation by multiphonon processes in partially ordered (Al/sub 0.5/Ga/sub 0.5/)/sub 0.5/In/sub 0.5/P

We demonstrate inelastic phonon scattering to be the dominant intradomain carrier-relaxation mechanism in partially ordered (Al/sub 0.5/Ga/sub 0.5/)/sub 0.5/In/sub 0.5/P. Carrier relaxation and recombination in partially ordered (Al/sub 0.5/Ga/sub 0.5/)/sub 0.5/In/sub 0.5/P have been studied by selectively excited photoluminescence (PL) spectroscopy. The partial ordering of the column-III sublattices causes a multidomain structure in the epitaxial film. The strongly modulated near-resonant excitation efficiency enables selective excitation of the ordered domains with a ground-state transition energy below the excitation energy, In the selectively excited PL spectra, we observed LA-phonon resonances as well as LO-phonon resonances. The LA-phonon resonances are new phonon modes caused by zone-folding effects and alloy disordering effects in the ordered alloy. The observation of the phonon resonances in the PL spectra is discussed in analogy to hot exciton relaxation in higher dimensional semiconductor system and proposed to be intricately bound to the inhomogeneity of the ordered domain ensemble.

Spin-polarized excitons in long-range ordered Ga/sub 0.5/In/sub 0.5/P

Circularly polarized excitation light produces spin-polarized excitons in long-range ordered Ga/sub 0.5/In/sub 0.5/P, because of a splitting at the valence-band maximum. We observed the spin relaxation process in ordered Ga/sub 0.5/In/sub 0.5/P under resonant excitation of heavy-hole excitons. The circularly polarized exciton luminescence shows a maximum anisotropy of about 35%. The decay profile of the depolarized luminescence is described by two components; the rapid decay by exciton-relaxation processes and the slower exciton recombination. An excitonic spin-relaxation process occurrs during the initial rapid decay.

Anisotropic photocurrent in long-range ordered Ga/sub 0.5/In/sub 0.5/P

In this study, we focus our attention on PC responses in the monolayer superlattice of long-range ordered Ga/sub 0.5/In/sub 0.5/P alloys. Based on our analysis of measured PC spectra by theoretical calculations, the properties of the anisotropic PC as well as a statistical distribution of partially ordered domains and the relationship between the oscillator strength and order parameter are discussed.