Takehiro Nishimura

Single-mode optical waveguide fabricated by oxidization of selectively doped titanium porous silicon

Metal-organic compound molecules were selectively doped into the larger pores of a two-layered porous silicon (PS) which had different pore sizes. PS with a graded pore-size distribution was area-selectively formed by utilizing a thin film mask. Titanium-organic compound molecules were doped into the larger pores and the PS was oxidized to form an optical waveguide. Single-mode transmission of 1.55 μm wavelength laser light was successfully observed.

Formation of electric-field domains in an asymmetric double-quantum-well GaAs/AlAs superlattice

We report the observation of electric-field domain formation in an asymmetric double-quantum-well GaAs/AlAs superlattice. Photoluminescence (PL) spectra exhibit several branches with increasing bias voltage. These steeply red-shifted PL branches can be attributed to Stark-ladder transitions between the heavy hole ground state in the narrow quantum well and the electron ground state in the wide quantum well in the high-field domain. According to our calculations, electrons and holes in the high-field domain are separated into different quantum wells, resulting in a large reduction of the radiative recombination energy with increasing electric field. Each PL branch most likely corresponds to the moving of the domain boundary.

High-Rate Deposition of Amorphous Silicon Films by Microwave-Excited High-Density Plasma

In this study, the deposition of amorphous silicon (a-Si) thin films using a microwave-excited high-density plasma system is described. We investigate the effects of plasma excitation gas species (argon or helium), total gas pressure, silane (SiH4) flow rate, and substrate stage temperature, estimating the resultant films from cross-sectional morphology, photoconductivity, and dark conductivity measured without light-induced degradation. It is confirmed that high-quality a-Si films (photosensitivity= 1.29 ×106) can be formed in the plasma excitation gas helium at a pressure of 13.3 Pa by relatively high rate (1.1 nm/s) deposition. At the same time, we measure the plasma emission derived from various radicals such as Si and SiH radicals in order to discuss the mechanism of radical generation in the plasma. The result of the measurement implies that when argon is used as plasma excitation gas, metastable states of argon markedly dissociate silane, which produces low-quality a-Si films. On the other hand, it seems that electrons dissociate silane mainly, which produces high-quality a-Si films, in helium.

Efficient short-wavelength light emission from asymmetric double quantum wells by using electron and hole collection into the same narrow quantum well

We propose an effective method for carrier injection into the high-energy Γ ground state in a narrow quantum well (QW) in asymmetric double QW systems using Γ–X–Γ electron transfer and simultaneous hole tunneling. Although the high-energy state is type-II band-aligned for the electrons, our method enables an efficient injection of both electrons and holes into the same narrow QW, and it thus demonstrates relatively strong light emission from the higher energy state.

Photoluminescence from high Γ-electron subbands and intersubband electroluminescence using X-Γ carrier injection in a simple GaAs/AlAs superlattice

We report the interband photoluminescence from high Γ-electron subbands and mid-infrared electroluminescence originating from an intersubband transition in a simple GaAs (15.3 nm)/AlAs (4.5 nm) superlattice embedded in a p–i–n structure. Interband photoluminescence properties under applied bias voltages provide conclusive evidence that electrons populate the fourth Γ (Γ4) electron subband in the GaAs layer. This electron population results from the carrier injection into the Γ4 subband from the adjacent X1 subband in the AlAs layer, which is initiated by the X1–Γ4 resonance. We calculate the overlap integral of the envelope functions for Γ-electron and heavy-hole subbands in order to discuss the carrier population in high Γ subbands based on the photoluminescence intensities. The results of analysis suggest that a population inversion can be obtained between the Γ4 and Γ3 subbands under the X1–Γ4 resonant condition. The energy of the intersubband electroluminescence, 100 meV, agrees with the energy spacing between the Γ4 and Γ3 subbands. This demonstrates that the carrier injection into the higher Γ subband using X–Γ scattering will be useful for designing of intersubband-emission devices.

Intersubband electroluminescence using X−Γ carrier injection in a GaAs/AlAs superlattice

We report midinfrared electroluminescence originated from an intersubband transition in a GaAs/AlAs superlattice consisting of asymmetric double quantum wells. It is confirmed from interband photoluminescence properties under applied bias voltages that electrons populate at the second Γ (Γ2) subband in the GaAs layer. The electron population results from the carrier injection into the Γ2 subband from the adjacent X1 subband in the AlAs layer, which is initiated by the X1–Γ2 resonance. The energy of the intersubband electroluminescence, 190 meV, agrees with the energy spacing between the Γ2 and Γ1 subbands. This demonstrates that the carrier injection into the higher Γ subband using X–Γ scattering is useful for the design of intersubband-emission devices.