LITHIUM PASSIVATION OF ZN AND CD ACCEPTORS IN P-TYPE GAAS

We report lithium passivation of the shallow acceptors Zn and Cd in p-type GaAs which we attribute to the formation of neutral Li-Zn and Li-Cd complexes. Similar to hydrogen, another group-I element, lithium strongly reduces the concentration of free holes when introduced into p-type GaAs. The passivation is inferred from an increase of both the hole mobility and the resisitivity throughout the bulk of the sample. It is observed most clearly for Li concentrations comparable to the shallow-acceptor concentration. In addition, compensation of shallow acceptors by randomly distributed donors is present in varying degree in the Li-diffused samples. Unlike hydrogenation of n-type GaAs, Li doping shows no evidence of neutralizing shallow donors in GaAs.

WANNIER-STARK LOCALIZATION IN INGAAS/GAAS SUPERLATTICES AND ITS APPLICATION TO ELECTROOPTICAL DEVICES

We have observed Wannier-Stark localization in strained In0.2Ga0.8As/GaAs superlattices by low- and room-temperature photocurrent spectra measurements. The experimental results are well in agreement with the theoretical predictions. A large field-induced modulation response of the absorption edge of the superlattices at room temperature suggests the possibilities of the application to the design of various kinds of electro-optical devices operating at a wavelength of 0.98 mum, based on Wannier-Stark localization effects.

WELL WIDTH DEPENDENCE OF THE EXCITON LIFETIME IN NARROW GAAS/GAALAS QUANTUM-WELLS

We have studied the radiative excitonic lifetime as a function of the well width in GaAs/GaAlAs quantum wells. An increasing lifetime with decreasing well width has been observed in very narrow and high quality GaAs/GaAlAs quantum wells, and attributed to the reduced overlap of the electron and hole wave functions and the increase of the exciton effective volume. This is the first observation of its kind in the conventional GaAs/GaAlAs quantum wells.

OPTICAL CHARACTERISTICS OF GAAS/ALGAAS RIDGE-QUANTUM-WELL-WIRES GROWN BY MBE ON NONPLANAR SUBSTRATES

With conventional photolithography and wet chemical etching, we have realized GaAs/AlGaAs buried ridge-quantum-well-wires (RQWWs) with vertically stacked wires in lateral arrays promising for device application, which were grown in situ by a single-step molecular beam epitaxy growth and formed at the ridge tops of mesas on nonplanar substrates. Confocal photoluminescence (CPL) and polarization-dependent photoreflectance (PR) are applied to study optical characteristics of RQWWs. Lateral bandgap modulation due to lateral variation of QW layer thickness is demonstrated not only by CPL but also by PR. As one evidence for RQWWs, a large blue shift is observed at the energy level positions for electronic transitions corresponding to quantum wells (QWs) at the ridge tops of mesas compared with those corresponding to QWs on nonpatterned areas of the same sample. The blue shift is in contradiction with the fact that the GaAs QW layers at the tops of the mesas are thicker than those on nonpatterned areas. The other evidence for RQWWs, optical anisotropy is provided by the polarization-dependent PR, which results from lateral quantum size effect existing at the tops of the mesas.

ELECTRON-TRANSPORT THROUGH GAALAS BARRIERS IN GAAS

We have analyzed electronic transport through a single, 200-angstrom-thick, Ga0.74Al0.36As barrier embedded in GaAs. At low temperatures and high electric field, the Fowler-Nordheim regime is observed, indicating that the barrier acts as insulating layers. At higher temperatures the thermionic regime provides an apparent barrier height, decreasing with the field, which is equal to the expected band offset when extrapolated to zero field. However, for some samples, the current is dominated by the presence of electron traps located in the barrier. A careful analysis of the temperature and field behavior of this current allows to deduce that the mechanism involved is field-enhanced emission from electron traps. The defects responsible are tentatively identified as DX centers, resulting from the contamination of the barrier by donor impurities.

CONTROLLING OF SCHOTTKY-BARRIER HEIGHTS FOR AU/N-GAAS AND TI/N-GAAS WITH HYDROGEN INTRODUCED AFTER METAL-DEPOSITION BY BIAS ANNEALING

Up to now, in most of the research work done on the effect of hydrogen on a Schottky barrier, the hydrogen was introduced into the semiconductor before metal deposition. This letter reports that hydrogen can be effectively introduced into the Schottky barriers (SBs) of Au/n-GaAs and Ti/n-GaAs by plasma hydrogen treatment (PHT) after metal deposition on [100] oriented n-GaAs substrates. The Schottky barrier height (SBH) of a SB containing hydrogen shows the zero/reverse bias annealing (ZBA/RBA) effect. ZBA makes the SBH decrease and RBA makes it increase. The variations in the SBHs are reversible. In order to obtain obvious ZBA/RBA effects, selection of the temperature for plasma hydrogen treatment is important, and it is indicated that 100-degrees-C for Au/n-GaAs and 150-degrees-C for Ti/n-GaAs are suitable temperatures. It is concluded from the analysis of experimental results that only the hydrogen located at or near the metal-semiconductor interface, rather than the hydrogen in the bulk of either the semiconductor or the metal, is responsible for the ZBA/RBA effect on SBH.

SHIFTING PHOTOLUMINESCENCE BANDS IN HIGH-RESISTIVITY LI-COMPENSATED GAAS

It is shown that Li diffusion of GaAs can give rise to semi-insulating samples with electrical resistivity as high as 10(7) OMEGAcm in undoped, n-type, and p-type starting materials. The optical properties of the compensated samples are correlated with the depletion of free carriers caused by the Li diffusion. The radiative recombination of the Li-compensated samples is dominated by emissions with excitation-dependent peak positions that shift to lower energies with increasing compensation. The photoluminescence properties are characteristic of fluctuations of the electrostatic potential in strongly doped, compensated crystals.

NEAR-RESONANT RAMAN-SCATTERING FROM GAAS/ALAS SUPERLATTICES

This article presents the results of near-resonant Raman scattering measurements on GaAs/AlAs superlattices at room temperature. A strong enhancement of GaAs LO phonon-even modes resulted owing to a dipole-allowed Frohlich interaction in superlattices. Similar to the previous results, the LO phonon-even modes in a polarized configuration are observed. In contrast to previous work, however, what we observed in depolarized configurations is the LO phonon-odd modes instead of even modes. It is confirmed that the selection rules for near-resonant Raman scattering from LO phonons in this kind of superlattices are the same as those for off-resonant scattering. From the second-order Raman scattering, it is confirmed that polarized second-order Raman scattering spectra consist of overtones and combinations of two even modes, and depolarized second-order Raman scattering spectra consist of combinations of an even mode and an odd mode. Our experimental results coincide with the predictions using the recently developed Huang-Zhu model. A brief discussion on interface modes and their combination with confined modes is also presented.

PHOTOREFLECTANCE SPECTROSCOPY OF SEMIINSULATING GAAS

Clear observations of photoreflectance (PR) spectra due to excitonic transitions in semi-insulating GaAs bulk materials are reported. The modulation mechanism is attributed to the electromodulation induced by the Dember effect. This study indicates that the PR spectroscopy provides an important method for characterizing the crystal quality of high-resistivity GaAs.

WANNIER LOCALIZATION IN GAAS/GAALAS SUPERLATTICES UNDER ELECTRIC-FIELD

We have studied the Wannier-Stark effect in GaAs/GaAlAs short-period superlattices under applied electric field perpendicular to the layers by room- and low-temperature photocurrent measurements. The changes in the transition intensities with biasing are well fitted to a theoretical calculation based on the finite Kronig-Penney model on which the potential of an applied electric field is superposed. With increasing electric field, the 0h peak grows to a maximum while the -1h and +1h peaks monotonousely decrease. By a comparison of the spectra measured at different temperatures, the two peaks in the room temperature photocurrent spectra at relatively low electric field (1.0 X 10(4) V/cm) are identified to be caused by the Wannier localization effect instead of saddle-point excitons.

OXYGEN IN INXGA1-XASYP1-Y GROWN ON GAAS

The measurement of DLTS on the alloy InxGa1-xAsyP1-y (0 less-than-or-equal-to y less-than-or-equal-to 0.3; 0.5 greater-than-or-equal-to x greater-than-or-equal-to 0.35) shows a new signal, labeled as E2, with an activation energy of E(c) - 0.61 eV and the SIMS signals show a large number of oxygen. To clarify is further, the energy of the deep level E2 is quantitatively calculated by using Vogl's tight-binding theory and Hjalmarson's deep level theory. As a result, the deep A1-symmetric level associated with an oxygen on the anion site of InxGa1-xAsyP1-y locates deeply in the band gap. Thus, the level E2 is considered to be induced by the oxygen impurity.