Toshio Matsusue

Interface roughness scattering in GaAs/AlAs quantum wells

We study experimentally and theoretically the influence of interface roughness on the mobility of two‐dimensional electrons in modulation‐doped AlAs/GaAs quantum wells. It is shown that interface roughness scattering is the dominant scattering mechanism in thin quantum wells with a well thickness Lw<60 A, where electron mobilities are proportional to L6w, reaching 2×103 cm2/V s at Lw∼55 A. From detailed comparison between theory and experiment, it is determined that the ‘‘GaAs‐on‐AlAs’’ interface grown by molecular beam epitaxy has a roughness with the height of 3–5 A and a lateral size of 50–70 A.

Radiative recombination coefficient of free carriers in GaAs‐AlGaAs quantum wells and its dependence on temperature

The radiative recombination coefficient B, defined by dN/dt=dP/dt≡−BNP, of two‐dimensional free carriers, N and P, in a selectively doped GaAs/n‐Al0.3Ga0.7As quantum well was determined in the temperature range T between 15 and 300 K by measuring the decay time τ of photoluminescence from the quantum well with electron density N0=1×1012/cm2. It is found that B(=1/τN0) is 1.1–1.9×10−4 cm2/s at 300 K and increases as T is lowered, reaching 2.3×10−3 cm2/s (τ∼0.43 ns) at 15 K. The initial increase of B is nearly proportional to 1/T, whereas B tends to saturate at low temperature. These experimental findings are shown to be well explained by the theory of band‐to‐band recombination of two‐dimensional carriers in quantum wells.

Resonant tunneling in quantum heterostructures: electron transport, dynamics, and device applications

The current understanding of the resonant tunneling process of electrons in double-barrier (DB) diodes and in coupled quantum well structures is described. The authors examine the validity of the simple Fabry-Perot model in describing the electron transport in actual DB diodes. They then describe a picosecond laser study to clarify the dynamics of resonant tunneling, including the intrinsic time delay associated with the multiple reflection of electron waves. Lastly, they discuss both the current state and prospects of device applications for high-speed electronics and optoelectronics. >

Photoluminescence characterization of GaInNAs/GaAs quantum well carrier dynamics

Variable temperature photoluminescence (PL) measurement of a thermal-annealed 6 nm GaInNAs/GaAs quantum well (QW) is carried out to understand its low-temperature carrier dynamic characteristics. It is found that the effect of carrier localization, which remained after a thermal anneal, is due possibly to a center characterized by a transition with activation energy of 13.7 meV below the e1 state. This result is deduced from fitting the integrated PL intensity versus the temperature data with a single-activation-energy model. A comparable value of 11 meV was also obtained between the low-energy (main localized state) and high-energy (e1 state) Gaussian functions used to fit the low-temperature PL spectrum. The localization effect in the thermal-annealed GaInNAs/GaAs QW is further confirmed by time-resolved PL measurements at 17 K, which showed emission-energy-dependent PL decay time characteristic for the low-energy regime (below 1.045 eV) and nearly constant decay time of about 0.14 ns at the high-energy...

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.

Detection of oxygen incorporated in molecular‐beam epitaxy grown GaAs‐on‐AlAs interfaces and AlAs layers by secondary ion mass spectrometry

By employing a long growth interruption (GI) in molecular beam epitaxy, we have successfully determined the concentration of oxygen incorporated in GaAs/AlAs interfaces and AlAs layers by secondary ion mass spectrometry. The concentration of oxygen atoms incorporated on AlAs surfaces during GI is found to be proportional to the period of GI when the incoming fluxes of residual oxygen‐related species reach steady‐state values. The net incorporation rate of oxygen on the AlAs surface is found to be constant for a wide range of substrate temperatures from 540 to 620 °C, indicating that the oxygen desorption is negligible.

MBE growth and properties of monolayer and submonolayer InAs layer embedded in GaAs/AlAs quantum wells

Abstract Novel GaAs/AlAs quantum wells (QWs) in which monolayer or submonolayer InAs is inserted in the center of the well are grown by MBE and alternate beam (AB) MBE to clarify both the segregation and the coalescence processes of InAs. The oscillation of specular beam intensity in reflection high energy electron diffraction has shown that growth dynamics of GaAs before and after the deposition of monolayer thick InAs are different from each other, possibly due to the influence of segregated In on the growth surface. Photoluminescence measurements were done to assess semi-quantitatively effects of In segregation under various growth conditions and the way to minimize the segregation was shown. In addition, a GaAs QW in which a half-monolayer of InAs is embedded in its center was grown by AB-MBE and the electron mobility μ was measured and analyzed to determine the lateral size Λ of InAs islands. It was found that the InAs islands have a lateral size of 130 A and the effective scattering potential is estimated to be 6 meV, indicating the possible use of island-inserted QWs as novel quantum box structures.

Suppression of lateral diffusion of excitons in GaAs quantum wells under perpendicular magnetic fields

We have performed lateral diffusion measurements of photo‐excited electron‐hole pairs in a GaAs/AlAs multiple quantum well structure under magnetic fields applied perpendicular to the wells, by using an all‐optical time‐of‐flight technique with a single‐mode optical fiber probe. Remarkable suppression of exciton diffusion is observed, and is ascribed to the magnetic perturbation by which moving excitons are affected proportionally to the square of wave vectors.

Photo‐induced ferromagnetism in bulk‐Cd0.95Mn0.05Te via exciton magnetic polarons

Free exciton magnetic polarons in bulk CdMnTe with low Mn concentration have been investigated through the time‐resolved and spectral‐resolved photo‐induced Faraday rotation measurements. The time‐resolved photo‐induced Faraday rotation measurements have revealed the spin relaxation dynamics and show a finite rotation angle lasting longer than 13 ns. This previously unreported long decay is attributed to dark exciton magnetic polarons.

Measurement of third-order nonlinear susceptibility tensor in InP using extended Z-scan technique with elliptical polarization

The elliptical polarization dependence of the two-photon absorption coefficient β in InP has been measured by the extended Z-scan technique for thick materials in the wavelength range from 1640 to 1800 nm. The analytical formula of the Z-scan technique has been extended with consideration of multiple reflections. The Z-scan results have been fitted very well by the formula and β has been evaluated accurately. The three independent elements of the third-order nonlinear susceptibility tensor in InP have also been determined accurately from the elliptical polarization dependence of β.