Tokuo Yodo

Buffer-enhanced room-temperature growth and characterization of epitaxial ZnO thin films

The room-temperature epitaxial growth of ZnO thin films on NiO buffered sapphire (0001) substrate was achieved by using the laser molecular-beam-epitaxy method. The obtained ZnO films had the ultrasmooth surface reflecting the nanostepped structure of the sapphire substrate. The crystal structure at the surface was investigated in situ by means of coaxial impact-collision ion scattering spectroscopy. It was proved that the buffer-enhanced epitaxial ZnO thin films grown at room temperature had +c polarity, while the polarity of high-temperature grown ZnO thin films on the sapphire was −c. Photoluminescence spectra at room temperature were measured for the epitaxial ZnO films, showing only the strong ultraviolet emission near 380nm.

Strong band edge luminescence from InN films grown on Si substrates by electron cyclotron resonance-assisted molecular beam epitaxy

We observed strong band edge luminescence at 8.5–200 K from 200–880 nm thick InN films grown on 10 nm thick InN buffer layers on Si(001) and Si(111) substrates by electron cyclotron resonance-assisted molecular beam epitaxy. The InN film on the Si(001) substrate exhibited strong band edge photoluminescence (PL) emission at 1.814 eV at 8.5 K, tentatively assigned as donor to acceptor pair [DAP (α-InN)] emission from wurtzite-InN (α-InN) crystal grains, while those on Si(111) showed other stronger band edge PL emissions at 1.880, 2.081 and 2.156 eV, tentatively assigned as donor bound exciton [D0X(α-InN)] from α-InN grains, DAP (β-InN) and D0X (β-InN) emissions from zinc blende-InN (β-InN) grains, respectively.

Heteroepitaxy on high-quality GaAs on Si for optical interconnection on Si chip

Recent progress concerning GaAs on Si technology is discussed from the viewpoint of how to suppress the threading dislocation density in GaAs layers to the level of 104cm2 on the basis of recently obtained results. In particular, we consider the effects of new approaches for realizing two-dimensional growth, new materials of buffer layers and insertion layers, post-growth annealing, high energy ion implantation and the confinement of growth areas on the reduction of threading dislocation generation and propagation.

Growth and Characterization of InN Heteroepitaxial Layers Grown on Si Substrates by ECR‐Assisted MBE

For the first time, we observed strong band-edge photoluminescence at 1.814 eV, and two stronger emissions at 1.880 and 2.081 eV at 8.5 K from the respective 880 nm thick InN heteroepitaxial layers (heteroepilayers) with 10 nm thick InN buffer layers grown on Si(001) and Si(111) substrates by electron cyclotron resonance-assisted molecular beam epitaxy. The former was probably assigned as donor-to-acceptor pair (DAP(a-InN)) emission from wurtzite-InN (a-InN) crystal grains, the latter were assigned as donor bound exciton (D°X(a-InN)) emission, and D 0 X(β-InN) or DAP(β-InN) emission from zincblende-InN (β-InN) crystal grains, respectively. Substrate annealing before growth and the introduction of a buffer layer had strong influences on the crystal structure and crystalline quality of the initial InN heteroepilayers.

Damage Due to Nitrogen Molecular Ions of GaN Heteroepitaxial Layers Grown on Si(001) Substrates by Molecular Beam Epitaxy Assisted by Electron Cyclotron Resonance

We have observed that the intensity of plasma emission at 391 nm from nitrogen molecular ions in nitrogen plasma is closely related to the crystalline quality and the surface morphology of GaN heteroepitaxial layers grown on Si(001). When plasma emission intensity is increased, the surface morphology is degraded, the photoluminescence (PL) intensities of two donor bound exciton (D0X) emissions from mixed crystal grains of wurtzite-GaN (α-GaN) and zincblende-GaN (β-GaN) and of yellow emissions are abruptly decreased, and the full-width at half maximum of the D0X is broadened. These reflect the influences of damage due to nitrogen molecular ions. The damage generates nonradiative centers. A small number of (001)- and (111)-oriented β-GaN crystal grains exist in the layers, together with a large number of (0001)-oriented GaN. PL efficiency from β-GaN is markedly higher than that from α-GaN, probably because the majority of the carriers accumulate in the β-GaN side at the interface between α- and β-GaN. The broad PL emissions at 3.10 and 3.29 eV with weak intensities are not changed by the damage. The peak energy position of the 3.29 eV emission almost coincides with that of D0X(β-GaN). The damage is not easily eliminated even by high-temperature growth at 900°C.

Rearrangement of misfit dislocations in GaAs on Si by post-growth annealing

Abstract The effects of post-growth annealing have been investigated on the movement and rearrangement of two different groups of misfit dislocations in molecular beam epitaxially grown GaAs on tilted (3° toward [110]) Si(001) substrates by using transmission electron microscope. The group I misfit dislocations show a cross-grid structure running along two 〈110〉 directions at the interface and are already formed in small islands of 20–30 nm in size. The spacing of ∼ 9.5 nm between the dislocations along each 〈110〉 is constant throughout all of the growth stages from 10 nm to 3 μm in GaAs thickness, although the cross grid pattern becomes more clear and regular due to annealing treatment. On the other hand, the group II misfit dislocations show short and segmented structures approximately along the 〈110〉 directions, which we consider to be related to threading dislocations. They are very mobile, and gradually become rearranged with the extension of their length along the 〈110〉 directions near to the interface, as the annealing temperature increases. The group I and II misfit dislocations are thought to be generated due to stresses caused by the lattice misfit and the thermal expansion misfit between GaAs and Si, respectively.

Growth and Characterization of GaAs/GaSe/Si Heterostructures

We have grown and characterized epitaxial GaAs grown on layered structure GaSe on As-passivated Si(111) for the purpose of using layered structure GaSe as a lattice mismatch/thermal expansion mismatch buffer layer in the GaAs on Si system. Films were grown on nominally (111) oriented Si substrates by Molecular Beam Epitaxy (MBE) and characterized by in-situ Reflection High Energy Electron Diffraction (RHEED), as well as ex-situ plan-view and cross-sectional Transmission Electron Microscopy (TEM). After GaSe was epitaxially grown at 500°C on As-passivated Si(111) substrates, GaAs growth was carried out at 500°C: As grown GaAs films (300 A) were highly twinned. In-situ annealing at 650°C for 10 minutes reduced the density of twins as observed by RHEED. In plan-view TEM, Moire fringes from both GaAs and GaSe are observed and showed conclusively that the GaAs grew epitaxially on the GaSe without contacting the Si substrate. Cross-sectional TEM showed the interface between the Si and GaSe was not smooth on the atomic scale. In spite of this, the GaSe becomes smooth within about 2 monolayers of growth and the GaAs/GaSe interface appeared to be very smooth. Despite the crystalline defects seen in cross section in the GaSe film, the GaAs film grows epitaxially.

Room-temperature growth of ultrasmooth AlN epitaxial thin films on sapphire with NiO buffer layer

Room-temperature epitaxy of AlN thin films on sapphire (0001) substrates was achieved by pulsed laser deposition using an epitaxial NiO ultrathin buffer layer (approximately 6 nm thick). Four-circle x-ray diffraction analysis indicates a double heteroepitaxial structure of AlN (0001)/NiO(111)/sapphire (0001) with the epitaxial relationship of AlN [10-10] ‖ NiO [11-2] ‖ sapphire [11-20]. The surface morphology of room-temperature grown AlN thin films was found to be atomically smooth and nanostepped, reflecting the surface of the ultrasmooth sapphire substrate with 0.2-nm-high steps.

Growth of GaSe on As-passivated Si(111) substrates

We have studied the growth of GaSe on As-passivated Si(111) substrates by molecular beam epitaxy (MBE) for a wide range of substrate temperatures (375–500°C) and SeGa beam equivalent pressure (BEP) ratios (∼ 30 to > 200). We see three distinct regions when our data are plotted as a phase diagram in SeGa BEP and growth temperature. For high SeGa BEP ratios and high substrate temperatures, continuous layered structure GaSe films are formed on the substrate. Reflection high energy electron diffraction (RHEED) indicates that these films are flat during growth, but after growth is stopped, the surface goes through a transformation and becomes rough on a microscopic scale. At low SeGa BEP ratios and high substrate temperatures, small Ga droplets form on the substrate surface, and no GaSe forms on the substrate. GaSe films with Ga droplets on the surface are formed when either the substrate temperatures or the SeGa BEP ratio is low. In this paper we will discuss possible mechanisms for growth (or the inhibition of growth) in the three regions of our phase diagram and compare our results with those already published for MBE growth of GaSe on Si(111)7 × 7 and GaAs(111).

GaAs on Si(111) with a layered structure GaSe buffer layer

Abstract In this paper, we report the growth and characterization of 1 μm thick GaAs(111) films on GaSe(0001) films grown on As:Si(111) (As-passivated Si(111)) substrates, and compare these films to 1 μm thick GaAs(111) films grown directly on As:Si(111) using the same GaAs growth conditions. We used secondary electron microscopy (SEM) to look at the surface morphology, and we used reflection high-energy electron diffraction (RHEED), transmission electron microscopy (TEM) and X-ray diffraction to characterize the crystal quality and determine the nature of the crystalline defects in the films. In general, we find that the use of a GaSe(0001) buffer layer causes only a modest decrease in the overall crystalline quality. However, using the GaSe buffer layer reduced the effectiveness of thermal annealing as a means of reducing the defect density. We suggest that due to its layered structure, the GaSe effectively absorbs the thermal expansion mismatch strain during annealing, which reduces the driving force for defect motion during thermal annealing.