Noriaki Tsukada

Photocurrent spectroscopy in GaAs/AlGaAs multiple quantum wells under a high electric field perpendicular to the heterointerface

Photocurrent (PC) spectroscopy under high electric fields perpendicular to the heterointerface has been utilized to characterize subband structures in GaAs/AlGaAs multiple quantum wells (MQW’s). The PC spectra from MQW’s under high electric fields show clear exciton peaks corresponding to forbidden transitions between electron and hole subbands. From PC spectra, we have precisely determined the splitting energies of the two lowest electron and four highest hole subbands in 55‐, 82‐, and 105‐A‐thick wells. The experimental results indicate that the conduction‐band offset is 60±3% of the band‐gap discontinuity. Our data are also in excellent agreement with finite square well calculations using a heavy‐hole effective mass of 0.34 and a light‐hole effective mass of 0.12.

Spectroscopic Characterization of Low-Temperature Grown GaAs Epitaxial Films

Femtosecond time-resolved reflectance and Raman scattering studies have been made on GaAs epitaxial layers grown at temperatures between 200 and 300° C and subsequently annealed. A subpicosecond carrier lifetime (∼0.25 ps) has been measured for a sample grown at 250° C and annealed at 600° C. Raman measurements using a back scattering geometry show a strong TO phonon band for samples grown at 200° C and 250° C, while it is absent for samples grown at 275 and 300° C. The band width of the LO band increases with decreasing growth temperature. A phonon band corresponding to As precipitates is also observed at 200 cm-1 for samples grown at 200 and 250° C. A strong correlation is found between the measured carrier lifetime and Raman profile.

Emission spectra of single quantum well lasers with inhomogeneous current injection

Emission spectra of a tandem‐type GaAs single quantum well laser diode were investigated under pulsed operating conditions. By controlling the two injection current levels, we could force the device to operate not only at the lowest (n=1) quantized state transition but also at the second (n=2) transition. Anomalous pulse responses, moreover, were found for the two lasing modes which were simultaneously observed in time‐integrated spectra. The dynamic emission behavior was utilized to achieve a variety of intensity combinations of the two radiation modes at the widely different wavelengths.

Widely separate wavelength switching of single quantum well laser diode by injection-current control

It is demonstrated for the first time that by injection‐current control, the lasing wavelength was able to be switched from the lowest (n=1) quantized state transition to the second (n=2) transition for a single quantum well laser diode. The separation in energy was as wide as >50 meV. This novel function of a laser was realized as a result of appropriately increased threshold gain. This diode, moreover, acted as a multiple wavelength‐emitting laser.

Photoluminescence line shape of excitons in GaAs single‐quantum wells with and without heterointerface ordering

The photoluminescence line shape of excitons at low temperatures is investigated in GaAs single‐quantum wells grown by molecular‐beam epitaxy with and without intentional heterointerface ordering. From the study of the excitation density dependence in the range between 1015 and 1017 cm−3, it is found that the inhomogeneous linewidth is significantly affected by band‐filling effects of intrinsic defect states spatially localized in the wider wells. In the sample with heterointerface ordering the band‐filling effects are directly identified by observation of saturation of the localized excitonic emissions, which are split as a result of increased spatial coherence of the excitonic states.

Observation of free excitons in room temperature photoluminescence of GaAs/AlGaAs single quantum wells

Observation of free excitons is reported in room‐temperature photoluminescence of nominally undoped GaAs/AlGaAs single quantum wells grown by molecular beam epitaxy. A comparative study with parallel‐conduction photocurrent spectroscopy shows that a sharp luminescence peak with a linewidth of 14 meV observed for the 6.1‐nm‐wide well coincides within 2 meV with the n=1 heavy‐hole free‐exciton resonance line at excess carrier densities less than 1017 cm−3. Excitonic decay channels dominate the luminescence spectrum even at room temperature as a result of quantum confinement effects, although photogenerated carriers are also provided for photocurrents by exciton dissociations.

Distinct observation of interwell coupling effect on optical transitions in double quantum wells in an electric field

We present results of an experimental study of interwell coupling effects using asymmetric double quantum well structures under electric fields. Data were obtained by means of photocurrent spectroscopy. It was concluded that remarkable variations in both energy and intensity of excitonic transitions, which were observed for the strongly coupled quantum wells, originate from the intersection of two quantized levels in the conduction band.

Dual‐wavelength emission from a twin‐stripe single quantum well laser

We demonstrate a dual‐wavelength laser constructed from a single quantum well structure. The device includes twin‐stripe waveguides which differ in width. The two constituent emitters in the device of appropriate cavity lengths operate at widely different wavelengths, which are based on the lowest (n=1) and the second (n=2) quantized state transitions. Lasing behavior is interpreted in terms of the difference of the internal cavity loss of the waveguides.

Self‐electro‐optic effect based on anticrossing of excitonic transitions in a coupled quantum well structure

We demonstrate electrically controlled photocurrent (absorption) tristability in an asymmetric coupled quantum well p‐i‐n diode connected to a series resistive load. The phenomenon, which is explained in terms of a self‐electro‐optic effect, is brought about by making use of an anticrossing behavior of excitonic optical transitions instead of the usual quantum‐confined Stark effect.

Linear polarization effects on photoluminescence properties of GaAs/AlAs quantum well heterostructures

Abstract Investigations of linear polarization effects on both free-carrier and excitonic recombinations in coupled and uncoupled GaAs/AlAs quantum well heterostructures show firm evidence for the formation of localized states, the transitions of which follow heavy-hole like optical selection rules. However, for the predominant emission band for the coupled wells, the transition exhibits highly mixed heavy- and light-hole characters of the valence subbands. These results are explained in terms of the spatial modulation effects due to layer thickness fluctuations in the synthesized coupled well systems.