Mineo Washima

Single and Double δ-Doped Al0.25Ga0.75As/In0.25Ga0.75As Pseudomorphic Heterostructures Grown by Molecular-Beam Epitaxy

Single and double δ-doped Al0.25Ga0.75As/In0.25Ga0.75As pseudomorphic heterostructures grown by molecular-beam epitaxy are examined to improve the performance of high-electron-mobility transistors (HEMTs). Double δ-doped layers, which are inserted in uniformly-doped AlGaAs carrier supply layers, can reduce the distance between the surface and the channel down to 19 nm without degrading the electrical properties. The mobility and sheet electron concentration at room temperature are 6050 cm2/(Vs) and 2.32×1012 cm-2, respectively ( 16270 cm2/(Vs) and 2.30×1012 cm-2 at 77 K). Shubnikov-de Haas oscillations confirm that the parallel conduction in the carrier supply layer is negligible. This work indicates that δ-doping in uniformly-doped carrier supply layers can be a key factor in improving the performance of HEMTs.

Mechanism of Carrier Accumulation at the Hetero Interface between InSb and GaAs

Various surface orientations of GaAs were used as substrates for InSb epitaxial growth, and the dependence of the interface carrier density on substrate surface indexes was discovered. The interface carrier density increased in proportion to the dangling bond density at the hetero epitaxial interface, and carrier accumulation was significantly suppressed using a (111) substrate which had the least dangling bond density among all the surface indexes of a zinc-blende structure. A carrier accumulation model based on the estimated band diagrams at the hetero junction was proposed and well explained these experimental results.

Highly strained InGaAs layers on GaAs grown by molecular beam epitaxy for high electron mobility transistors

Abstract We have grown highly strained In x Ga 1− x As layers on GaAs, by molecular beam epitaxy, showing excellent electrical and optical quality for high electron mobility transistors (HEMTs). Over a very narrow range of substrate temperature from 400 to 420°C, the mobility of 6 nm thick In 0.35 Ga 0.65 As channel HEMTs at room temperature exceeds 6200 cm 2 /V · s. Furthermore, the In composition can be increased up to 0.42 without generating misfit dislocations. The peak energy and full width at half maximum of a 77 K photoluminescence spectrum from an In 0.42 Ga 0.58 As/GaAs (well width = 6 nm) strained single quantum well are as small as 1.082 eV and 9.8 meV, respectively. The highly strained In x Ga 1− x As channel HEMTs (0.3 ≤ x ≤ 0.4) show superior transconductance over conventional In 0.25 Ga 0.75 As channel HEMTs.

Electrical and optical properties of selectively doped Al0.25Ga0.75As/InyGa1−yAs (0.25≤y≤0.45) pseudomorphic heterostructures grown by molecular‐beam epitaxy

Electrical and optical properties of highly strained selectively doped Al0.25Ga0.75As/InyGa1−yAs (0.25≤y≤0.45) pseudomorphic heterostructures grown by molecular‐beam epitaxy are investigated. At a low growth temperature of 400 °C, the In mole fraction exceeds than 0.30 without degrading crystalline quality. The maximum mobility and sheet electron concentration at room temperature reach 6560 cm2/(V s) (y=0.34) and 2.94×1012 cm−2 (y=0.425), respectively. Photoluminescence measurements confirm the In mole fraction. This work suggests that InyGa1−yAs channels (y≳0.3) grown at 400 °C improve the performance of high‐electron‐mobility transistors.

Evidence of nonparabolicity and size of wave function confined in In0.53Ga0.47As/In0.52Al0.48As multi quantum wells

Nonparabolicity of the conduction band in In0.53Ga0.47As was analyzed using nanoscale InGaAs/InAlAs multi quantum well structures. The nonparabolic effective mass was determined from a set of eigen-energies using a single finite-square-well model. Conduction-band eigen-energies were obtained by analyzing the interband optical transition spectra of photocurrent and photoreflectance. Regarding the conduction band effective mass, Kane’s theory was applicable over the wide range from 0 to 0.5 eV. Based on the theory, the size of the electron wave function was determined with the resolution of 0.1 nm. This method provides a direct means to determine the quantum well thickness.

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.

Thickness evaluation of InGaAs/InAlAs quantum wells

This work proposes a new optoelectronic measurement of quantum well (QW) thickness and applies it to doped and undoped In0.53Ga0.47As/In0.52Al0.48As multiple-QW structures. Near-infrared spectroscopic identification of the interband optical transition at 100–300 K gave the eigenenergies of the conduction band in the QW. Evaluation of the QW thickness involved analysis of the effective mass at the corresponding eigenenergy. QW thicknesses in the range of 5.45–20.8 nm were determined in six different wafers. These thicknesses agreed well with the QW thicknesses estimated by double-crystal x-ray diffraction within almost two monolayers. This measurement was used to determine the distance of potential boundaries confining the electron wave functions.

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.

Spectrum analysis of interband optical transmissions and quantitative model of eigen-energies and absorptions in In0.53Ga0.47As/In0.52Al0.48As multi-quantum well structures

Interband transmission spectra were measured for three In0.53Ga0.47As/In0.52Al10.48As multi-quantum well specimens having different carrier concentrations by modulation-doping. Spectral shapes of transmissions were clear steplike structures but exciton peaks of first order transitions were masked with the carrier concentrations. The spectral shapes changed hardly between 100 and 310 K. Using our parameters of quantum wells, calculated eigen-energies for three specimens agreed with experiments and absorption coefficients reproduced experimental transmission spectra at any temperature.