Hitoshi Kawanami

RHEED studies of Si(100) surface structures induced by Ga evaporation

Abstract Si(100) surface structures induced by Ga molecular beam deposition in an ultra-high vacuum have been investigated using a reflection high-energy electron diffraction system (RHEED). It has been found that the Ga evaporation of submonolayer thickness on a clean Si(100) 2 × 1 surface produces surface structures of Si(100) 3 ×2, 5 × 2, 2 ×2 and 8 × 1 sequentially in the temperature range from 350 to 680° C. The RHEED patterns and a two-dimensional phase diagram including five superstructures are presented.

Si epitaxy by molecular beam method

Abstract The epitaxial growth of Si on a Si substrate using the molecular beam method in ultrahigh vacuum has been studied. We have used RHEED in the deposition system and differential interference microscopy for investigation of the surface structure and morphology of the films. It has been found that clean and smooth surfaces are obtained by preheating the Si substrate with a thin SiO2 film at a high temperature (1240°C) for a short period (2 min). A Si(100) (2 × 2) surface structure was obtained on the clean surface after preheating. A Si(100) c(4 × 4) surface structure appeared on the epitaxial films for growth temperatures in the range between 700 and 800°C. All of the epitaxial films (450–1100°C) were free of stacking faults.

Band Discontinuities in Gallium Phosphide/Crystalline Silicon Heterojunctions Studied by Internal Photoemission

We measured the band lineup of gallium phosphide (GaP) on crystalline silicon (c-Si) heterojunctions (HJs) by using internal photoemission (IPE), where the heterojunctions were prepared by using solid-source molecular beam epitaxy. It was found that the conduction-band and valence-band discontinuities, denoted by ΔEc and ΔEv, are 0.09±0.01 and 1.05±0.01 eV, respectively. By performing measurements on samples with different GaP layer thicknesses, we clarified that ΔEv of the present GaP-on-Si HJs is not affected by strain normal to the growth direction. The values of ΔEc and ΔEv obtained for the GaP-on-Si HJs are significantly different from those reported for thin Si-on-GaP HJs, and the implications of this discrepancy are briefly discussed.

The Growth of Single Domain GaAs Films on Double Domain Si(001) Substrates by Molecular Beam Epitaxy

Single domain GaAs layers were successfully grown on double domain Si(001) substrates by molecular beam epitaxy (MBE). During the growth, reflection high energy electron diffraction (RHEED) patterns of GaAs films grown on the Si substrates were monitored in situ. They changed gradually from initial spotty unreconstructed patterns to As stabilized (001)2×4 patterns. The morphologies of as grown surfaces and molten KOH etched surfaces of the films were observed by Nomarski optical microscope and also indicate that the films were single domain.

LOCAL STRUCTURES OF ISOVALENT AND HETEROVALENT DILUTE IMPURITIES IN SI CRYSTAL PROBED BY FLUORESCENCE X-RAY ABSORPTION FINE STRUCTURE

Local structures of dilute isovalent and heterovalent impurity atoms in Si crystal (Si:X, X=Ga, Ge, As) have been studied by fluorescence x-ray absorption fine structure. The distortion of local lattice around the impurity atoms was evaluated from the Si–X bond length determined by extended x-ray absorption fine structure. The results demonstrate that the local lattice deformation is strongly dependent on the electronic configuration of impurity atoms, i.e., we find an anomalous expansion (0.09±0.01 A) along the [111] direction for donor (As) atoms but much smaller magnitude (0.03±0.01 A) for isovalent (Ge) atoms and acceptor (Ga) atoms. The results suggest that the local lattice distortions are strongly affected by the Coulomb interactions between the localized charge, which piles up to screen the ion core and the bond charge, and the ion-core repulsion. Absence of anomaly in case of negatively charged Ga atoms suggests that the former mechanism is a dominant factor for anomalous lattice expansion.

Lattice strain relaxation at the initial stages of heteroepitaxy of GaAs on (100)Si by molecular beam epitaxy

Abstract The relaxation of lattice strain at the initial stages of heteroepitaxy of GaAs on (100)Si substrates grown either by molecular beam epitaxy or by migration enhanced epitaxy is studied using dynamic reflection high energy electron diffraction technique. The low temperature growth of GaAs layers was performed after depositing As prelayers on Si substrates at low temperature. The evolution of RHEED patterns, the variation of surface lattice parameter, and the variations of the intensity and half width of diffraction spots during growth are investigated. The relaxation of lattice occurs before the completion of one monolayer growth of GaAs. The critical thickness for the strain relaxation is much smaller than the calculated one. It is speculated that the misfit stress is accommodated by a faulted Si-As interface.

Antiphase boundary of GaAs films grown on Si(001) substrates by molecular beam epitaxy

Single and double domain GaAs film growth on double domain Si(001) substrates by molecular beam epitaxy were observed. The domain structure of the film did not succeed to the domain structure of the substrate surface but was decided by a direction of slight misorientation of the substrate. To explain this, it has been proposed that the antiphase boundary (APB) of the film is dominantly non-stoichiometric,i.e., the APB is composed of the same polar {11n} or {nn1} (n = 1, 2, ….) planes of the adjacent domains. Growth simulation based on this model about the APB has explained well the experimental results that a double domain film can grow on a Si(001) surface which is exactly oriented or is misoriented towards a 〈100〉 direction.

Fabrication of hydrogenated amorphous Si/crystalline Si1−xGex (x ≤ 0.84) heterojunction solar cells grown by solid source molecular beam epitaxy

We characterized hydrogenated amorphous Si/single-crystalline Si1−xGex heterojunction solar cells grown on Si substrates with Ge content (x) between 0.49 and 0.84 in an effort to develop materials with a bandgap range of 0.9–1.0 eV for solar applications. We showed that 3-µm-thick Si0.16Ge0.84 films grown by molecular-beam epitaxy on a virtual substrate composed of buffer layers with stepwise gradation of their composition have an almost fully strain-relaxed condition and a low dislocation density, less than 105 cm−2. An absorption edge extending up to 1300 nm and an increased quantum efficiency were observed in Si1−xGex cells with x = 0.49, 0.70, and 0.84. Consequently, the short-circuit current density increased non-linearly with Ge content, being 14.0 and 24.0 mA/cm2 for 3-µm-thick Si and Si0.16Ge0.84 cells, respectively.

Molecular beam epitaxy of single domain InP films directly grown on Si(001) substrates

Abstract MBE growth of single domain InP films directly on Si(001) substrates was successfully achieved for the first time. Domain structures of the substrates and films were observed by RHEED during growth. The surface of the substrate showed a Si(001) 2×1 two domain structure. Initial thin ( ∽ 300A) InP film grown on the substrate showed a double domain structure. With increasing film thickness, the domain structure of the film became single domain.

Formation of Zn-O complexes during the molecular beam epitaxial growth of GaP on Si

Heteroepitaxial GaP films were grown on (100) oriented Si substrates by molecular beam epitaxy. During the growth, the substrates were irradiated with a Zn molecular beam. At growth temperatures lower than 450°C, a broad band luminescence due to Zn–O complexes was observed at room temperature.