Shinji Koh

Ultrahigh room-temperature hole Hall and effective mobility in Si0.3Ge0.7/Ge/Si0.3Ge0.7 heterostructures

We have obtained ultrahigh room-temperature (RT) hole Hall and effective mobility in Si0.3Ge0.7/Ge/Si0.3Ge0.7 heterostructures with very small parallel conduction. Reducing parallel conduction was achieved by employing Sb doping in Si0.3Ge0.7 buffer layers, which drastically increased RT hole Hall mobility up to 2100 cm2/V s in the strained Ge channel modulation-doped structures and improved device characteristics of the p-type metal–oxide–semiconductor field-effect transistors with the strained Ge channel. The peak effective mobility reached to 2700 cm2/V s at RT, which was much higher than the bulk Ge drift mobility.

Room temperature circularly polarized lasing in an optically spin injected vertical-cavity surface-emitting laser with (110) GaAs quantum wells

We fabricated a (110)-oriented vertical-cavity surface-emitting laser (VCSEL) with GaAs/AlGaAs quantum wells (QWs) and characterized the lasing properties of the VCSEL under optical spin injection. We demonstrated circularly polarized lasing at a high degree of circular polarization of 0.96 for our VCSEL at room temperature that originated from a long electron spin relaxation time of 0.7 ns in the (110) GaAs QWs despite a really small initial electron spin polarization of 0.04, which was well reproduced by using a rate equation analysis.

In-plane strain fluctuation in strained-Si/SiGe heterostructures

In-plane strain fluctuation in the strained-Si/relaxed-SiGe heterostructure was studied by micro-Raman spectroscopy. It was found that misfit dislocation, which is necessarily induced by strain relaxation of SiGe buffer layers, caused micrometer-scale inhomogeneous strain field in the strained-Si layer as well as SiGe buffer, which may degrade device performance. After annealing, the fluctuation was found to be enhanced due to partial strain relaxation of strained Si, particularly in the region where tensile strain was relatively high before annealing. From homoepitaxial growth of SiGe on planarized SiGe buffer layers, it was confirmed that the growth rate also fluctuated laterally, in correspondence with the in-plane strain variation.

Extremely high room-temperature two-dimensional hole gas mobility in Ge/Si0.33Ge0.67/Si(001) p-type modulation-doped heterostructures

To extract the room-temperature drift mobility and sheet carrier density of two-dimensional hole gas (2DHG) that form in Ge strained channels of various thicknesses in Ge/Si0.33Ge0.67/Si(001) p-type modulation-doped heterostructures, the magnetic field dependences of the magnetoresistance and Hall resistance at temperature of 295 K were measured and the technique of maximum entropy mobility spectrum analysis was applied. This technique allows a unique determination of mobility and sheet carrier density of each group of carriers present in parallel conducting multilayers semiconductor heterostructures. Extremely high room-temperature drift mobility (at sheet carrier density) of 2DHG 2940 cm2 V–1 s–1 (5.11×1011 cm–2) was obtained in a sample with a 20 nm thick Ge strained channel.

GAAS/GE/GAAS SUBLATTICE REVERSAL EPITAXY ON GAAS (100) AND (111) SUBSTRATES FOR NONLINEAR OPTICAL DEVICES

Sublattice reversal epitaxy is demonstrated in lattice-matched GaAs/Ge/GaAs (100) and (111) systems using molecular beam epitaxy, and confirmed by reflection high energy electron diffraction and preferential etching. In the GaAs/Ge/GaAs (100) system, the sublattice reversal is assisted by self-annihilation of the antiphase domains generated at the GaAs/Ge interface. In the GaAs/Ge/GaAs (111) system, the sublattice reversal results from the unique structure of the As-terminated Ge (111) surfaces. The quality of the sublattice-reversed GaAs crystal is investigated using cross-sectional transmission electron microscopy. A method to fabricate a periodically domain-inverted structure using sublattice reversal epitaxy is demonstrated for the GaAs/Ge/GaAs (100) system.

Fabrication of high-quality strain-relaxed thin SiGe layers on ion-implanted Si substrates

We fabricated high-quality strain-relaxed thin SiGe layers by Ar ion implantation into Si substrates before epitaxial growth. The surface of 100-nm-thick Si0.8Ge0.2 layers, the relaxation ratio of which was more than 80%, was found to be very smooth, with a rms roughness of 0.34 nm. Cross-sectional transmission electron microscopy analysis confirmed that strain-relieving dislocations were effectively generated due to the ion-implantation-induced defects and confined in the vicinity of the heterointerface, resulting in a dislocation-free SiGe surface. Moreover, in-plane strain-field fluctuation was found to be largely reduced by this ion implantation method.

Circularly polarized lasing in a (110)-oriented quantum well vertical-cavity surface-emitting laser under optical spin injection

We fabricated and characterized a vertical-cavity surface-emitting laser (VCSEL) based on (110) InGaAs/GaAs multiple quantum wells (MQWs). Circularly polarized lasing in the (110) VCSEL by optical injection of spin-polarized electrons has been demonstrated at 77 K and room temperature. A high degree of circular polarization, 0.94, was observed at 77 K, reflecting the long electron spin relaxation time in the (110) MQWs.

Surface Planarization of Strain-Relaxed SiGe Buffer Layers by CMP and Post Cleaning

Surface planarization of strain-relaxed SiGe buffer layers by chemical mechanical polishing (CMP), particularly the influence of a post-CMP cleaning process which is indispensable after CMP, on the surface morphology of SiGe buffer layers was investigated. It was found that the cleaning tended to enhance the surface roughness due to the etching effect that increased with increasing cleaning temperature. The etching effect was suppressed by optimizing cleaning reagents, and the ultrasmooth surfaces of SiGe buffer layers with Ge contents of 30 to 70% were obtained, irrespective to growth methods. The root mean square roughness reached 0.4 to 0.6 nm, which was the lowest value that was ever obtained.

Hole density dependence of effective mass, mobility and transport time in strained Ge channel modulation-doped heterostructures

We performed systematic low-temperature (T = 350 mK–15 K) magnetotransport measurements on the two-dimensional hole gas with various sheet carrier densities Ps = (0.57–2.1)×1012 cm–2 formed in the strained Ge channel modulation-doped (MOD) SiGe heterostructures grown on Si substrates. It was found that the effective hole mass deduced by temperature dependent Shubnikov–de Hass oscillations increased monotonically from (0.087±0.05)m0 to (0.19±0.01)m0 with the increase of Ps, showing large band nonparabolicity in strained Ge. In contrast to this result, the increase of the mobility with increasing Ps (up to 29 000 cm2/V s) was observed, suggesting that Coulomb scattering played a dominant role in the transport of the Ge channel at low temperatures. In addition, the Dingle ratio of the transport time to the quantum lifetime was found to increase with increasing Ps, which was attributed to the increase of remote impurity scattering with the increase of the doping concentration in MOD SiGe layers.

Sublattice Reversal in GaAs/Si/GaAs (100) Heterostructures by Molecular Beam Epitaxy

Sublattice reversal in III-V compound semiconductors grown on group-IV epitaxial layers on III-V substrates has been proposed for fabricating nonlinear optical devices with domain-inverted compound semiconductor structures. Sublattice reversal epitaxy is demonstrated in the GaAs/Si/GaAs (100) system and confirmed by reflection high energy electron diffraction, cross-sectional transmission electron microscopy, anisotropic etching, and optical second-harmonic generation measurements. The present sublattice reversal seems to be assisted by self annihilation of antiphase domains generated at GaAs/Si interfaces.