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GaAs/Ge/GaAs and AlAs/Ge/AlAs heterostructures were grown both on (113)B and (113)A GaAs substrates by molecular beam epitaxy. Sublattice reversal in these heterostructures were identified by comparing the anisotropic etching profile of the epitaxy sample with that for reference (113)B and (113)A GaAs substrates. Sublattice reversal in GaAs/Ge/GaAs heterostructures was achieved on (113)B GaAs substrate. On the other hand, sublattice reversal on (113)A GaAs substrate was obtained by using AlAs/Ge/AlAs heterostructures.

We have investigated the effects of Sb-soak on InAs quantum dots (QDs) grown on (001) and (113)B GaAs substrates by molecular beam epitaxy. Surface morphologies of the QDs were characterized by atomic force microscopy. The optical properties of buried QDs were investigated by photoluminescence (PL). We showed that effects of Sb-soak on density and PL of (001) and (113)B QDs were quite different. The increased density and blue-shift of (001) QDs can be explained by the surfactant effect of Sb atoms which increase the areal density of the kinks for nucleation. On the other hand, for (113)B QDs, the incorporation effect should be responsible for the red-shift because the Sb atoms may be diffused into QDs.

Two-color surface-emitting lasers were fabricated using a GaAs-based coupled multilayer cavity structure grown by molecular beam epitaxy. InGaAs/GaAs multiple quantum wells were introduced only in the upper cavity for two-mode emission in the near-infrared region. Two-color lasing of the device was successfully demonstrated under pulsed current operations at room temperature. We also observed good temporal coherence of the two-color laser light using a Michelson interferometer. A coherent terahertz source is expected when a wafer-bonded coupled cavity consisting of (001) and non-(001) epitaxial films is used for the two-color laser device, in which the difference-frequency generation can be enabled by the second-order nonlinear response in the lower cavity.

Self-assembled InAs quantum dots (QDs), without and with an AlAs cap, were grown on (311)B GaAs substrates by molecular beam epitaxy. Surface morphologies of QDs were characterized by atomic force microscopy. Photoluminescence (PL) was performed in the 4–300 K temperature range. For QDs without AlAs cap, sharp and intense PL emitted from wetting layer was observed. PL from QDs was relatively weak at 4 K. The PL intensity of QDs decreased as measurement temperature increased and was not observed at 300 K. For QDs with AlAs cap, PL from WL vanished while PL from QDs were substantially enhanced at 300 K. Suppression of PL from WL indicates that the thickness of WL was reduced due to phase separation result from AlAs cap, which is an effective way to improve the PL of InAs QDs grown on (311)B GaAs substrate.

InAs quantum dots (QDs) were introduced as efficient nonlinear materials into a GaAs/AlAs coupled multilayer cavity, which was recently demonstrated as a novel THz emission device based on difference-frequency generation (DFG) of the two cavity modes. The couple multilayer cavity containing QDs was grown by molecular beam epitaxy and the THz-DFG was measured using femtosecond laser pulses.

Time-resolved transmission change of Er-doped InAs quantum dots (QDs) embedded in strain-relaxed In0.45Ga0.55As barriers has been studied using different excitation wavelengths (1.4–1.55μm) to understand the QD-size dependence of the photocarrier relaxation. Each measured temporal profile was well reproduced by a sum of three exponential decays. Ultrafast (∼1.6ps) and fast (∼6–9ps) components are dominant in the initial stage of decay, and then the slow (∼70–130ps) component due to the radiative recombination in the QDs appears to be seen. The first two components come from photocarrier relaxation into the nonradiative centers related to Er dopants or the lattice relaxation. The difference is considered to result from whether the nonradiative centers are positioned inside or outside the QDs. The decay time for the outside case becomes shorter as the excitation wavelength is shortened, which might be due to the weaker confinement of photocarriers in the smaller-size QDs. Suppression of the QDs contributing to the radiative recombination is also more significant for the smaller-size QD excitation.

We investigated the contact resistance of a non-alloyed Ti/Pt/Au ohmic electrode to obtain thermally stable source–drain resistance of cryogenically cooled In 0.75 Ga 0.25 As / In 0.52 Al 0.48 As high electron mobility transistors (HEMTs) fabricated on (4 1 1)A-oriented InP substrates by molecular beam epitaxy. A contact resistance of 0.26 Ω mm , which strongly depended on InAlAs spacer and barrier layers, was achieved at 16 K (25% below the value at 300 K) for the non-alloyed Ti/Pt/Au ohmic electrode formed on the In 0.75 Ga 0.25 As / In 0.52 Al 0.48 As HEMT structure with 3-nm-thick InAlAs spacer and 10-nm-thick InAlAs barrier layers. For a 195-nm-gate HEMT, we achieved a maximum transconductance ( g m ) of 2.25 S / mm at 16 K (26% above the value at 300 K), which, to our knowledge, is one of the highest values for HEMTs ever reported. This extremely high g m was attributed to not only 33% lower source–drain resistance ( 0.18 Ω mm at 16 K) because of thermally stable and low contact resistance but also 19% lower transit time under the gate (0.39 ps at 16 K) because of phonon scattering suppression compared with the 300 K values.

We have investigated substrate off-angle dependence of surface segregation of In atoms during molecular beam epitaxy (MBE) of pseudomorphic In 0.08 Ga 0.92 As/GaAs superlattices (SLs) on (n 1 1)A-oriented GaAs substrates (n = 3-5). Surface segregation length of the In atoms (λ: 1/e decay length of the In content profile along the growth direction) was characterized by high resolution X-ray diffraction (HRXRD) and low-temperature (11 K) photoluminescence (PL) measurements. λ obtained for the (n 1 1)A SLs were 1.2-1.4 times longer than that (λ = 1.57nm) for the simultaneously grown (100) SL, and the (41 1)A SL showed the longest 2 of 2.16 nm. The obtained substrate off-angle dependence of the In segregation length is quite similar to the incorporation life-time (τ c ) of Ga adatoms reported for MBE growth of GaAs on channeled (100) substrates, indicating that larger surface migration of Ga adatoms during MBE growth on (n 1 1)A substrates causes enhanced surface segregation of In atoms in the (n 1 1)A InGaAs system.

The surface migration length of As 4 molecules on (411)A surfaces during molecular beam epitaxy has been determined from lateral profiles of As content (x) in GaAs x , P 2 , layers grown on (41 1)A GaAs substrates with a (100) side-slope using As 4 and P 2 molecular beams. The migration length of As 4 molecules on (411)A surfaces at a substrate temperature of 535°C was found to steeply increase from 4 to 10 μm and saturate to 10 μm with increasing V/III flux ratio. This /VIII ratio dependence of migration length of As 4 is understood by a proposed model where an As 4 molecule is dissociated and incorporated into the crystal phase when an As 4 molecule encounters a Ga adatom on the (411)A surface. Quantitative analysis is made using rate equations based on the model and the experimental results are reproduced with reasonable fitting parameter.

In order to characterize interface roughness scattering, two-dimensional electron gas (2DEG) mobility in (411)A and (100) selectively-doped In 0.53 Ga 0.47 As/In0 .52 Al 0.48 As quantum well (QW) HEMT structures, which were grown on InP substrates by molecular beam epitaxy, with a gate contact was measured at 20K as a function of sheet electron concentration (Ns) by changing gate bias. Thickness of the QW was designed to be rather small (6 nm) for enhancing the interface roughness scattering for both samples. 2 DEG mobilities of the (411)A sample were 21,000-51,400 cm 2 /Vs in the range of Ns = 0.7-1.7 x10 12 cm -2 , which are about more than 2 times higher than the mobilities (8,700-25, 400cm 2 / Vs) of the conventional (100) sample. 2DEG mobility was calculated by taking into account interface roughness scattering and ionized remote impurity scattering. By fitting the calculated results to the observed ones, lateral size (A) and height (A) of the interface roughness of the (411)A sample were determined to be 3.5 and 0.23 nm, respectively, which are 30% and 50% smaller than the corresponding values (Λ = 5.0 nm and Δ = 0.43 nm) of the (100) sample.

Effects of erbium doping on photocarrier lifetime have been investigated for self-assembled InAs quantum dots (QDs) embedded in strain-relaxed In0.35Ga0.65As barriers grown on (1 0 0) GaAs substrates by molecular beam epitaxy. Time-resolved transmission change measurements were performed on 20-layer stacks of the Er-doped InAs QDs at room temperature using 0.1 ps laser pulses with the center wavelength of 1.5 μ m . Fast and slow decay components were observed in the temporal profile of each Er-doped InAs QD sample. The slow decay component attributed to the radiative recombination process in the QDs was well suppressed by increasing Er-doping density. Moreover, the decay time of the fast component due to the nonradiative process was markedly reduced by the incorporation of Er dopants during the QD formation. A decay time of 3.5 ps was obtained for the Er-doped InAs QD sample with a sheet density of incorporated Er dopants per QD layer of 2.7 × 10 13 cm − 2 .

Surface migration lengths of As adatoms on (4 1 1)A GaAs surfaces during molecular beam epitaxy were determined for the first time from lateral profiles of the arsenic content ( x ) in the GaAs x P 1− x layers grown on GaAs channeled substrates (CSs) using As 4 and P 2 beams. The x on the (4 1 1)A side-slope region near the edge of the (1 0 0) region increased from that on the flat (4 1 1)A GaAs substrate, indicating that As adatoms flow from the (1 0 0) region to the (4 1 1)A side-slope region. The observed surface migration length of As adatoms was 15 ±2 μ m on the (4 1 1)A GaAs surface at 535°C. The surface migration length of As adatoms on (4 1 1)A GaAs surface slightly increases to 20 ±2 μ m with increase of the substrate temperature to 605°C . The substrate temperature (T s ) dependence of surface migration length of As adatoms on the (4 1 1)A GaAs surface is discussed.

High-optical quality lattice-matched InxGa1−xAs/InxAl1−xAs quantum wells (QWs) with indium contents of x=0.18–0.19 have been successfully grown on (4 1 1)A- and (1 0 0)-oriented InGaAs ternary substrates by molecular beam epitaxy. Strong photoluminescence (PL) with a narrow linewidth was observed from both (4 1 1)A and (1 0 0) QWs at 12 K. The peak energy of PL from QWs had a good correlation with the calculated exciton emission energy based on the band parameter of the InGaAs well layers and InAlAs barrier layers. The result indicates that the (4 1 1)A and (1 0 0) InGaAs/InAlAs QWs on InGaAs ternary substrates have 0.2 eV larger energy gap difference between their well and barrier materials than GaAs/AlAs QWs on GaAs binary substrates or InGaAs/InAlAs QWs on InP binary substrates. In addition, the (4 1 1)A In0.18Ga0.82As/In0.18Al0.82As QWs had a narrower PL linewidth than (1 0 0) QWs indicating that smoother interfaces were realized in the (4 1 1)A QWs.

We have investigated channel thickness (L w ) dependence of transport properties of two-dimensional-electron gas (2DEG) in pseudomorphic In 0.74 Ga 0.26 As/In 0.52 Al 0.48 As quantum well high electron mobility transistor (QW-HEMT) structures with extremely flat heterointerfaces [(411)A super-flat interfaces] grown on (411)A InP substrates by molecular beam epitaxy (MBE). The highest electron mobility of 90,500 cm 2 /Vs (77 K) with a sheet carrier concentration (N s ) of 3.1 × 10 12 cm - was observed for the (411)A QW-HEMT structure with L w = 8 nm. which is about 1.5 times larger than the best value (μ = 61.000 cm 2 /Vs at 77 K) of ever reported electron mobility with a similar N s of 3.0 x 10 12 cm 2 for the InGaAs/InAlAs QW-HEMT structure grown on a ( 100) InP substrate. This enhancement of the electron mobility of the (4 11)A QW-HEMT structure is mainly due to much improved flatness of the (411)A InGaAs InAlAs heterointerfaces compared with those of conventional (100) interfaces, which results in a large reduction of interface roughness scattering of 2DEG.

Lateral profiles of In content in a 1.5 μm thick In x Ga 1-x As (x ≃ 0.2) layer grown on GaAs channeled substrates (CSs) with (411)A side-slopes by molecular beam epitaxy (MBE) have been investigated with the use of energy dispersive X-ray spectroscopy (EDX). The observed profiles of the In content suggested that In atoms migrate preferentially in the [122] direction on the (411)A plane during MBE growth. This preferential migration of In atoms along [122] on the (411)A plane was confirmed by comparing observed lateral profiles of In content in InGaAs layers grown on GaAs CSs and simulated In profiles which are calculated by taking into account of an additional one-way flow of In atoms along [122]

We observed the initial stages of Al growth on Si(001) using scanning tunneling microscopy. The Al films were deposited under various substrate conditions; on a clean surface at room temperature (RT) and 500°C, and on a hydrogen-terminated surface at RT. At RT on a clean surface, rectangular patches of Al elongating in the [110] direction were formed on 2 × 2 array terraces above the first 0.5 monolayers, in addition to three dimensional island formation. On the other hand, at 500°C, Al or AlSi clusters, well ordered one-dimensionally in the [110] or [110] direction, appeared. Arrangement of the clusters exhibited a variety of local symmetries. Also, we deposited Al on a H-terminated Si(001)-3 × 1 surface. The scanning tunneling microscopy (STM) image showed a characteristic growth pattern of the Al film and higher density of nuclei than that on a clean surface.