Hisao Yoshimura

Optical absorption and carrier‐induced bleaching effect in quantum wire and quantum box structures

The optical absorption spectra of quantum wires and boxes are studied theoretically to evaluate the carrier‐induced bleaching effect. It is found that the absorption coefficients of these highly confined systems (HCSs) and their changes with the introduction of carriers can be far greater than those of less‐confined systems (LCSs) such as quantum wells (films) and bulk semiconductors. In addition, the carrier‐induced bleaching effect of HCS is found to remain very high at room temperature while that of LCS is substantially weakened by the thermal distribution of carriers.

Field‐induced decoupling of quantized levels and blue shift of absorption edge in a potential inserted quantum well structure

The electric field induced localization and decoupling of quantized levels (FILD) in double quantum wells is studied both theoretically and experimentally and is shown to result in a novel blue shift of optical absorption peaks. The absorption edge of doubly coupled 54 A GaAs wells separated by a 5.7 A AlAs inserted barrier is found to shift by 8 meV toward the higher energy under a moderate field (>30 kV/cm). Considering the field‐induced variation of quantized levels and excitonic effect, the observed shifts of most peaks are well explained by the simple effective mass theory. Device applications of this novel electro‐optic effect (FILD) are also discussed.

Carrier Concentration Dependent Absorption Spectra of Modulation Doped n-AlGaAs/GaAs Quantum Wells and Performance Analysis of Optical Modulators and Switches Using Carrier Induced Bleaching (CIB) and Refractive Index Change (CIRIC)

Optical absorption spectra of modulation doped quantum wells were studied systematically at 77 K and 300 K as functions of the two dimensional electron concentration Ns. It has been found that the absorption at the wavelength of exciton peak is bleached strongly from 2.1×104 cm-1 to 800 cm-1 at 77 K as Ns is raised from zero to 1×1012/cm2; this is due to the combined effect of exciton quenching and Burstein Moss shift, which override the band gap reduction caused mainly by the many body effect. Performance analysis of quantum well optical modulators and switches using this carrier induced bleaching (CIB) and refractive index change (CIRIC) is presented to show the picosecond switching capability with low drive voltages.

Increase of parasitic resistance in shallow p/sup +/ extension by SiN sidewall process and its improvement by Ge preamorphization for sub-0.25-/spl mu/m pMOSFET's

Anomalously high parasitic resistance is observed when SiN gate sidewall spacer is incorporated into sub-0.25-/spl mu/m pMOSFETs. The parasitic resistance in p/sup +/ S/D extension region increases remarkably by decreasing BF/sub 2/ ion implantation energy to lower than 10 keV. It is confirmed that low activation efficiency of boron in p/sup +/ extension is the reason for such high parasitic resistance. The reduction of activation efficiency of boron may result from hydrogen passivation of boron acceptor; Fourier transform infrared absorption (FT-IR) measurement suggests that diffused hydrogen from SIN into p/sup +/ extension region forms the silicon-hydrogen-boron complex. It is also found that the activation efficiency of boron correlates well both with implantation energy of BF/sub 2/ and the amorphization rate of substrate. Therefore, in sub-0.25-/spl mu/m era, the extra amorphization step is essential not only to form a shallow junction but also to enhance boron activation. Germanium preamorphization implantation (Ge PAI) is hence applied to p/sup +/ extension of 0.15 /spl mu/m pMOSFETs. It is finally demonstrated that this Ge PAI process reduces the total parasitic resistance to improve the drain saturation current by up to 10%.

Improved Ti Self-Aligned Silicide Technology Using High Dose Ge Pre-Amorphization for 0.10 µm CMOS and Beyond

Improved Ti self-aligned silicide (SALICIDE) technology for 0.1 µm complimentary metal-oxide-semiconductor (CMOS) using high dose pre-amorphization implantation (PAI) is developed. High dose PAI with As and Ge promotes the growth rate of silicidation on polycrystalline silicon gate even when its length is reduced to 0.1 µm. Thus it achieves low sheet resistivity at narrow lines. In addition, the advantage of Ge over As as PAI species is confirmed. Ge PAI does not affect the parasitic resistance increase of p channel metal-oxide-semiconductor field effect transistor (pMOSFET) or junction leakage characteristics because of its electrical neutrality and high solubility in silicon.

Transversal Time and Charge Accumulation in Double-Barrier Resonant Tunneling Structures

The tunnel escape time or the state lifetime of electrons that determines the transversal time in GaAs/AlAs double-barrier tunneling structures has been directly determined by measuring and analysing the photoluminescence (PL) decay trate. The measured values τ were found to agree very well with the theoretical value 2ħ/ΔE determined from the full width ΔE of resonant transmission peak. This agreement is maintained even when τ falls in the time range of 10~100 ps, suggesting that the escape rate is scarcely affected by the carrier scattering. In addition, the accumulation of electron wave during the resonant tunneling process has been studied directly by measuring the PL and PL excitation spectra as functions of bias voltage. The measured density of accumulated charge is found to follow closely with the resonant current and determined to be 5×1011/cm2 at the resonant voltage.

Magneto-luminescence study of carrier-induced excitonic to free-carrier transition of radiative recombination in a semiconductor quantum well

Abstract We have investigated the magnetic field dependence of the photoluminescence of a modulation-doped n-AlGaAs/GaAs single quantum well structure while changing the electron density in the well from zero to 7 × 1011 cm−2 via gate electric fields. A clear transition from excitonic to free-carrier recombination was observed. At intermediate electron density, the existence of magneto-excitons is suggested.