ALBERT

Notes: In situ observations of As atoms at step sites of vicinal Si (100) surfaces have been performed by coaxial impact-collision ion scattering spectroscopy. It is found that some As atoms remain at Si step sites even at a high substrate temperature of 780 °C under an As residual pressure, in spite of evaporation of As atoms from terrace sites. This result indicates that As atoms at step sites are energetically more stable than the As dimers on the terrace. Moreover, the angular profiles of the scattering intensity from As atoms at step sites suggest that there is atomic displacement of As atoms towards the Si substrate at the step sites. An atomic model of the As/Si system is proposed from the results of computer simulation for the scattering intensity profiles.

Notes: We report the growth mechanism of GaAs on vicinal Si(110) substrates by molecular beam epitaxy, and investigate the effects of off-angle and -direction on the crystalline quality of grown films. The quality was improved with the increase of the off-angle and strongly influenced by the off-direction. Off-direction toward the [001] was better than that toward the [11¯0] for obtaining high-quality films. The cause is related to the structure and density of the steps. We infer from reflection high-energy electron diffraction results that GaAs films on vicinal Si(110) with off-angles above 2° toward the [001] direction grew up to a thickness of 3 nm in step-flow-like mode. The spotty reflection high-energy electron diffraction patterns characteristic of three-dimensional growth in GaAs/Si(100) were not observed at a thickness above 3 nm. © 1994 American Institute of Physics.

Notes: Solid-phase-epitaxial (SPE) regrowth from selectively As+-implanted amorphous Si is analyzed by cross-sectional transmission electron microscopy. SPE regrowth in the (100) wafers proceeds into both vertical and lateral directions simultaneously. For (111) samples, SPE regrowth in the diagonal direction, i.e., 〈100〉 direction near the mask edge, is dominant. In addition to the end of range defects and projected range defects, in the SPE process, defects of a third type are generated just beneath the implantation mask edge. Differences in mask material which control the applied stress at the mask edge have little influence on edge defect generation. Substrate orientation and mask pattern direction play important roles in the edge defect formation mechanism.

Notes: Annealing behavior of secondary defects in 2-MeV boron ion-implanted (100) silicon has been investigated mainly through cross-sectional TEM observations. The maximum defect density is located at a mean depth of 3.2 μm from the surface and the location is 0.3 μm deeper than that of the projected range of boron ions. This defect position in the crystal is constant under all annealing conditions (e.g., a temperature range of between 700 and 1000 °C, annealing time of up to 6780 min at 1000 °C), although the vertical distribution width of defects changes with both annealing temperature and time.

Notes: Threading dislocation morphologies and characteristics, as well as their generation conditions in InxGa1−xAs films grown by molecular beam epitaxy on GaAs (001) substrates have been investigated, mainly using cross-sectional transmission electron microscopy. The 3-μm-thick InxGa1−xAs films are mostly examined for x ranging over x=0.01, 0.05, 0.1, 0.15, 0.2, 0.3, and 0.5. Moreover, for x=0.2, epitaxial layers having film thicknesses of 0.5, 1, and 2 μm are investigated. The formation of a high density of threading dislocations which reach the film surface is detected in epilayers of x≥0.2 at a fixed film thickness of 3 μm and with film thicknesses greater than 2 μm at x=0.2. In layers of x≤0.15, such threading dislocations are rarely detected, although dislocation segments on the {111} planes threading into the upper regions from the interface are frequently observed. Most of the observed threading dislocations are 60° and pure-edge type dislocations along the 〈211(approximately-greater-than) and 〈110(approximately-greater-than), and [001] directions, respectively. The former type dislocations are mainly observed in layers of x≤0.15; the latter predominantly exist in layers of x≥0.3. In epilayers of x=0.2, the two types of threading dislocations mentioned above coexist. It is also discussed that the formation of the above-mentioned threading dislocations is strongly associated with misfit dislocations which are introduced in the InxGa1−xAs layers under the different growth modes, depending on x.

Notes: Heteroepitaxial layers of InP with thickness D ranging from 0.1 to 6.0 μm were grown by low-pressure metalorganic chemical vapor deposition on (001) surfaces of GaAs substrates. Their dislocation structure, induced strains, and nature of the radiative recombinations were investigated as a function of D with transmission electron microscopy, x-ray diffraction, and photoluminescence spectroscopy. For D〈2 μm, the films are highly dislocated with a tangle of interfacial and threading dislocations above the heterointerface. The spatial extent of the interfacial dislocations and the density of threading dislocations increase with increasing D. For D(approximately-greater-than)2 μm the portion of the layers away from the heterointerface by more than 1.5 μm shows a decrease in the density of threading dislocations and a dramatic improvement in the crystalline quality with increasing D.Typical dislocation densities in the neighborhood of the top surface are in the mid 107 cm−2 range when D surpasses 4.0 μm. Concomitant with the improved crystalline quality, the following observations are made. Firstly, the full width at half maximum of the x-ray rocking curves diminishes from values larger than 500 arcsec for D〈1.0 μm to about 200 arcsec for D(approximately-greater-than)4.0 μm. Secondly, the near-band-edge photoluminescence transitions, which for D〈2.0 μm are predominantly determined by defect-induced band tailing, display excitonic character. Thirdly, below-band-gap transitions due to interfacial defects decrease in intensity. Biaxial compressive strain is present in the layers because of lattice mismatch and differences in linear thermal expansion with the substrate.The strain removes the degeneracy between the light- and heavy-hole states at the top of the valence band, and consequently with increasing temperature above 12 K recombinations from the conduction to the split valence bands are observed in the photoluminescence spectra for all D. The identification of such transitions follows from their temperature dependence and the activation energy yield for the thermalization of the holes. The measured valence-band splitting decreases from 12.5 meV for D=0.3 μm to saturation values of 5.6 meV for D(approximately-greater-than)3.0 μm, indicating strain relaxation with D in qualitative agreement with x-ray determinations. Quantitative differences between both methods are realized and are attributed to a temperature dependence of the differential linear thermal expansion. The contribution to the strain from the lattice mismatch is much larger than expected from equilibrium models. The dislocation generation at different stages during the growth is inferred from the strain relaxation against D and the observed location of the dislocations throughout the layers.

Notes: The initial growth stages of the highly lattice-mismatched GaAs/InAs system was studied by coaxial impact collision ion scattering spectroscopy (CAICISS). The GaAs coverage on the InAs substrate during GaAs molecular beam epitaxial growth was monitored in situ by low incident angle CAICISS. The scattering intensity as a function of the deposited amount of GaAs can be divided into three characteristic regions. First, the scattering intensity from In decreases proportionally with the amount of deposited GaAs molecules. However, the intensity decrease stops abruptly before the surface is completely covered with a GaAs layer, and remains constant. Then, the intensity gradually decreases. This result shows that there exist three kinds of growth stages in the process of GaAs deposition on an InAs substrate. The mechanism of the growth mode transition, corresponding to the three kinds of growth stages is discussed from the viewpoint of a strain energy change on the surface.

Notes: We have grown and characterized epitaxial layered structure GaSe on As-passivated Si(111) and GaAs on GaSe on As-passivated Si(111) for the ultimate purpose of using the 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 and characterized by in situ reflection high energy electron diffraction, as well as by ex situ scanning electron microscopy and both plan-view and cross-sectional TEM (transmission electron microscopy). In this study, GaSe was grown epitaxially on As-passivated Si(111) substrates at 500 °C with Se/Ga BEP (beam equivalent pressure) ratios of ∼10 and ∼20. Small droplets were observed on the surface after GaSe growth. These are thought to be droplets of unreacted Ga. The density and size of the droplets decreases with the increasing Se/Ga BEP ratio. When the GaSe surface is exposed to As, the droplets become GaAs islands. Subsequent GaAs growth was carried out at 400 and 500 °C, giving the following results for 300-A(ring)-thick films: as grown GaAs films were highly twinned, and some polycrystalline GaAs was present in the film grown at 400 °C. In situ annealing at 650 °C for 10 min reduced the density of twins in both cases. In plan-view TEM, Moiré fringes from both GaAs and GaSe are observed and show conclusively that the GaAs grew epitaxially on the GaSe without contacting the Si substrate. Cross-sectional TEM shows the interface between the Si and GaSe is not smooth on the atomic scale. In spite of this, the GaSe becomes smooth with about 2 monolayers of growth and the GaAs/GaSe interface appears to be very smooth.

Notes: GaAs layers grown on (001) InP surfaces by low-pressure metalorganic chemical vapor deposition were investigated with photoluminescence spectroscopy, x-ray diffraction, Raman scattering, and transmission electron microscopy. Correlations between the optoelectronic properties, the strain relaxation, and the structural defects were established for layer thickness D ranging between 0.1 and 3.0 μm. A comparison with the case of InP layers grown on GaAs substrates is presented. Radiative recombinations to split light- and heavy-hole valence bands near the zone center are seen at 12 K in the photoluminescence spectra. The splitting is due to a biaxial tensile strain. With increasing temperature, the heavy-hole transitions gain intensity and at around 140 K they are the only features in the spectra. In the 12–50 K temperature range the intensity ratio between the heavy- and light-hole transitions also depends on laser power. The hole activation energy determined from the temperature dependence of the intensity ratio above 50 K agrees with the valence-band splitting. The strain for D(approximately-greater-than)0.3 μm arises from differences in linear thermal expansion and has contributions from the lattice mismatch in thinner layers. The strain values yielded by x-ray diffraction are smaller than those obtained from the valence-band splitting measured with photoluminescence. The difference is attributed to a temperature dependence of the linear thermal expansion, which was corroborated by the shifts of the longitudinal optical phonon frequencies measured with Raman spectroscopy at 300 and 12 K. A comparison is made of the absolute magnitude of the strain and the x-ray diffraction linewidths for heteroepitaxial GaAs and InP layers on InP and GaAs substrates, respectively. The contribution to the strain from the lattice mismatch relaxes in GaAs faster than in InP and the GaAs x-ray linewidths are narrower for D〈1.0 μm. These results are understood in terms of the growth habit and the behavior of threading and interfacial dislocations.