H. Arimoto

Magneto-optical study of semiconductor nanostructures in high magnetic fields

A review is presented on the recent advances in magneto-optical studies of semiconductor nanostructure devices in pulsed high magnetic fields up to several hundred teslas, produced by three different techniques: electromagnetic flux compression (up to 500 T), the single-turn coil technique (up to 150 T), and non-destructive long-pulse magnets (up to 50 T). We discuss magneto-optical spectra of excitons in quantum wells, quantum wires, quantum dots, and short-period superlattices. We also discuss infrared cyclotron resonance in quantum wells under magnetic fields tilted away from the growth direction.

CYCLOTRON RESONANCE IN CD1-XFEXS AND GA1-XMNXAS AT MEGAGAUSS MAGNETIC FIELDS

Abstract We have performed infrared and far-infrared magneto-transmission experiments for Cd1–xFexS (x=0.05) and Ga1–xMnxAs (x=0.00004, 0.053) at very high magnetic fields up to 500 T. The cyclotron mass of electrons in Cd1–xFexS (x=0.05) was found to be larger than that in CdS when we applied magnetic fields parallel to the c-axis of the crystal; the relative increase of the cyclotron mass shows anomalous temperature dependence at 85–300 K. Hole cyclotron resonance was observed in Ga1–xMnxAs (x=0.00004) at 119 μm and 70.5 μm. In Ga1–xMnxAs (x=0.053) far-infrared transmission does not show any resonance but a rapid decrease of transmission with magnetic field; the magneto-transmission spectrum depends strongly on temperature.

Study of band nonparabolicity using electron–cyclotron resonance of InGaAs/InAlAs quantum wells below

AbstractNonparabolic tendency ofconduction subbands in InGaAs=InAlAs quantum wells (QWs), lattice-matched to InP, werestudiedusingcyclotronresonance(CR)becauseitsconnementpotentialwasstrongerthaninInGaAs =InPandGaAs=GaAlAsQWs. Thickness ofInGaAs well was 5–10 nm. Barrier thickness was more than 10 nm. Field-scanned pulse CR andwavelength-scanned CR were observed. CR energy changed sublinearly with magnetic eld. This curve agreed with ourcalculation, based on Kane’s three-level band theory and Landau quantization. ? 2002 Elsevier Science B.V. All rightsreserved. PACS: 71.20.Nr; 76.40.+b; 78.67.DeKeywords: Nonparabolicity; Cyclotron resonance; Quantum wells 1. IntroductionBand structure in a low-dimensional system isessential for physics and application of dot-sizecrystals. In InGaAs=InP or GaAs=GaAlAs quantumwell (QW) system, penetration ofwave function intoInPorGaAlAsbarrieroccurs,wherethebarrierheightis as small as 0.22 or 0:32 eV. In GaAs=GaAlAs QWsystem, two theoretical masses in directions normaland parallel to the QW plane increased dierentlywith energy. Evidence ofsuch band anisotropy in

Low-dimensional systems in ultra-high magnetic fields: magnetic-field-induced type I to type II transitions in short-period semiconductor superlattices

We present a review on the recent study of the type I to type II transition in short-period superlattices of GaAs/AlAs by means of cyclotron resonance and interband magneto-optical spectroscopy in pulsed high magnetic fields up to 500 T. In the magneto-photoluminescence spectra of excitons in , the magnetic-field-induced type I to type II transition was observed with and without the simultaneous application of high pressure. The behaviour of the transition varies depending on the thickness of the AlAs layers. In cyclotron resonance of , the resonance peak at the X minima was observed in the type II regime for n smaller than 14, whereas the resonance at the point was observed for n>15. It was found that the angular dependence of the peak position does not obey the simple cosine dependence due to the subband mixing in high magnetic fields. From the angular dependence, the effective masses at the X point were determined. In high-field cyclotron resonance measurements at 129 meV up to 400 T for n = 16 (type I), the resonance of the X minima expected at around 260 T was indiscernible, despite the fact that the transition should have occurred at lower fields.

Angular dependent cyclotron resonance in short period (GaAs)n/(AlAs)n superlattices

Abstract Cyclotron resonance (CR) measurements have been carried out to evaluate the effective masses of electrons at the AlAsX point of the Brillouin zone in short period (GaAs)n/(AlAs)n superlattices (SLs) with n = 8–14. The longitudinal effective mass was deduced to be m1 = 1.04m0 by analyzing a deviation of CR position from the cos φ-dependence for tilted magnetic fields in (GaAs)14/(AlAs)14 SL.

Observation of nonparabolic conduction subbands in InGaAs/InAlAs multi-quantum wells by very-high-field pulse cyclotron resonance

Abstract Observation of nonparabolic conduction band is rather difficult because electrons normally occupy near the bottom of conduction band. Use of two-dimensional electron gas and Landau quantization made it possible to scan equivalent electron energy by a magnetic field. We report a clear evidence of a non-parabolic conduction subband in 5–10 nm wide InGaAs quantum wells in InGaAs/InAlAs multi-quantum well structures. Infrared optical transitions due to cyclotron resonance take place under pulsed high magnetic fields up to 100 T. In experiments, electron absorption is discriminated by a circular polarized light of a carbon dioxide laser. Water and methanol vapor lasers were also used to measure the mass of low-energy electrons. Measurements were made near 30 K.

Nonparabolic effective masses of conduction subbands in InGaAs/InAlAs quantum wells in normal and parallel directions

Nonparabolic effective masses of conduction electrons were comprehensively studied in two-dimensional InGaAs quantum wells (QWs) deeply confined within InAlAs barriers of the 0.52-eV band offset. Cause of nonparabolicity was attributed not to the penetration of wavefunctions into barriers but to the InGaAs bulk band structure of bandgap energy of 0.74 eV. Band calculations by a Kanes three-level model for narrow-gap semiconductors and a Zawadzkis model under Landau quantization modified for QW confinement were fairly compared with in-plane apparent cyclotron masses of electrons measured in a 10-nm-wide InGaAs QW. Simulation of optical transmittance through complex epitaxial wafer structures fit quite well with cyclotron resonance experiments. Masses normal to the QW plane were also determined from a series of eigen-energies observed in 20-nm-wide InGaAs QW. In-plane nonparabolicity was found to be several times larger than normal nonparabolicity.

Cyclotron resonance in (GaAs)n/(AlAs)n superlattices under ultra-high magnetic fields

Abstract We present a review on recent study of the type I to type II transition in short-period superlattices (SLs) of GaAs/AlAs by means of cyclotron resonance (CR) in pulsed high magnetic fields. The behavior of CR varies depending on the thickness of the GaAs and AlAs layers. In CR of (GaAs) n /(AlAs) n , the resonance peak at the X minima was observed in the type II regime for n smaller than 14, whereas the resonance at the Γ point was observed for n >15. We estimated electron masses on X and Γ point in the SLs by using the empirical sp 3 tight-binding method including second-nearest-neighbor interaction. These calculations have shown good agreement with the experimental results. Moreover, it was found that the angular dependence of the CR peak position does not obey the simple cosine dependence due to the subband mixing in high magnetic fields. From the angular dependence in the SLs, the longitudinal and transverses electron masses of AlAs at the X point were deduced to be m t =0.21 m 0 and m l =1.04 m 0 , respectively.

Impurity cyclotron resonance in type-I (GaAs)n/(AlAs)n superlattices

We have carried out measurements of cyclotron resonance (CR) in type-I (GaAs)n/(AlAs)n superlattices (SLs) applying pulsed high magnetic fields up to 150 T. Two types of peaks have been observed in CR signals; one originates from a transition between Landau levels of free electrons, and the other from transition between impurity levels. The free-electron CR signals are dominant at room temperature, and the impurity CR signals become large as temperature decreases. From the peak position of CR spectra, at room temperature, the effective mass of the electron in (GaAs)n/(AlAs)n SL is deduced. To analyze the impurity transition in high magnetic fields, we have calculated the impurity levels in the SLs using the variational method assuming a single quantum well for simplicity. In spite of a very simple model, the calculated results are in good agreement with the CR results of type-I (GaAs)n/(AlAs)n SLs.

Characterization of heavy masses of two-dimensional conduction subband in InGaAs/InAlAs MQW structures by pulsed cyclotron resonance technology

Conduction-band effective masses in a direction parallel to the quantum well plane were investigated in n-type- modulation-doped InGaAs/InAlAs multi-quantum well system. Thicknesses of well and barrier were 5 and 10 nm. Three highly-doped specimens having about 1 X 1012 cm-2 per one quantum well were prepared by MBE. Double-crystal x-ray diffraction was used to check the crystal quality. Heavy electron effective masses, almost 50 percent bigger than the band edger mass of 0.041m0, were measured by far-IR and IR cyclotron resonances under pulse high magnetic fields up to 100 T. Nonparabolicity of this subband was less than 12 percent by comparing the two cyclotron resonances. Observed 2D subband structure was quite different from conduction band in a direction perpendicular to the same quantum well and from that of GaAs/GaAlAs quantum well system.