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The surface structure of Si(111) post-annealed at 980 °C after nitrogen ion induced nitridation has been investigated by using a scanning tunneling microscope (STM) and low energy electron diffraction (LEED). The LEED and STM results indicated the formation of ordered domain of quadruplet structure in the silicon nitride layer. The LEED pattern taken from the nitrated Si(111) surface showed a coexistence of 7×7 domain with quadruplet one. In the STM image taken from the same surface, a three directional periodicity with a periodic arrangement of white protrusions was observed in the local area of silicon nitride island and its symmetry directions were rotated about 10° with respect to those of Si(111) surface. In addition to the quadruplet structure of the silicon nitride island, meta-stable structures such as 9×9, c(4×2), and 2×2 as well as 7×7 phase boundaries were observed to have been formed on the Si(111) surface during the rapid cooling of nitrated surface from the post-annealing temperature of 980 °C. The investigation of the surface structure of nitrated Si(111) showed that the surface nitrated at high temperature had better epitaxial silicon nitride layer than that post-annealed after nitridation at room temperature.
The initial oxidation of Si(111)-7×7 surface has been investigated by taking the STM images of samples dosed with oxygen at room temperature and high temperatures between 500°C and 750 °C. In particular, different site selectivities between two oxygen-induced features, bright and dark sites, were observed and explained in terms of the difference in potential energy curves. In addition to such a strong site selectivity under low oxygen partial pressure (l×10-9 torr), heavy surface etching by oxygen was observed at higher O2 partial pressures and temperatures resulting in the high density of monolayer-deep etch marks on terraces.
The correlation of surface morphology with strain relaxation in the In0.15Ga0.85As epilayer on GaAs(100) grown by chemical beam epitaxy using unprecracked monoethylarsine has been investigated. The surface morphology of InGaAs was analyzed by atomic force microscopy as the epilayer thickness was increased from 0.025 to 1.668 μm. The changes in the surface morphology indicated that surface roughening is related to the process of strain relaxation in the film. The strain-induced shifts in the GaAs-like longitudinal optical phonon in the Raman spectrum also indicated that the strains in the InGaAs epilayer relax via step-wise process with increasing the film thickness beyond the critical thickness, which agrees well with the changes of surface mophology.
We have grown GaAs epilayers by ultrahigh vacuum chemical vapor deposition(UHVCVD) using adsorbed hydrides and metalorganic compounds via a surface decomposition process. This result indicates that unprecracked arsine(AsH3) can be used in chemical beam epitaxy(CBE) and that a new hydride source with a low decomposition temperature, monoethylarsine(MEAs) can replace the precracked AsH3 source in CBE. The impurity concentrations in GaAs grown with trimethylgallium(TMG) and triethylgallium(TEG) were found to be very sensitve to growth temperature. It was also found that the uptake of carbon impurity is significantly reduced when TMG is replaced with TEG. The carbon concentrations in epilayers grown using TMG and TEG with unprecracked AsH3 and MEAs were reduced by 2-3 orders of magnitude compared to those by CBE process employing TMG and arsenics from precracked hydrides. We have also found that the hydrogen atoms play an important role in the reduction of carbon content in GaAs epilayer. Intermediates like dihydrides from MEAs decomposed on the surface are considered to supply hydrogen atoms and hydrides during growth, which may remove other carbon containing species.
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