We demonstrate that fluorescence EXAFS (Extended X-ray Absorption Fine Structure) using synchrotron radiation is now a powerful technique to analyze local structures around impurity atoms of the order of 10^<18> cm^<-3> which is a typical concentration used in semiconductors. Taking the Er atoms in InP as an example, growth condition dependences of light emission from Er and of local structures are investigated. A drastic change of PL intensity at 1.54 pm from the Er intra-4f transition by the growth temperature is successfully elucidated by the analysis of the Er local structures with the fluorescence EXAFS. At the growth temperatures lower than 550℃, the Er atoms form the zincblende structure with the nearest neighbor atoms (P) and at the higher temperatures they form the rocksalt structure. The relationship between the local structure and the PL intensity is discussed. The local structure in the Er and O co-doped GaAs is also investigated and a clear relationship between the local structure and the PL characteristics is obtained.
Understanding essential mechanisms of Ge growth on Si is crucial for fabricating high-quality Ge/Si and SiGe/ Si heterostructures applied to the electronic and optoelectronic devices, such as field-effect transistors with high electron mobilities, photodetectors in Si-based integrated circuits, and devices consisted of self-organized quantum dot structures with novel optical and electronic transport properties. In this paper, I report on recent progress in studies of Ge growth on Si (001) substrates based on our experimental results. Three topics are presented: Ge island formation in Stranski-Krastanov mode, surfactant epitaxy with atomic hydrogen, and a novel technique for growing fully strain-relaxed Ge thin films with flat surfaces. Here we discuss the issues of surface morphological evolution during the growth, defect generation, and strain relaxation in the film, all of which are closely related each other.
Two mismatches are involved in III-V/Si. One is the large lattice mismatch between a III-V and Si crystal. The other is the large difference in the thermal expansion coefficients between them, in other words, the thermal mismatch. We would like to discuss the dislocation reactions in such mismatched III-V /Si systems from the viewpoint of the lattice mismatch and of the thermal mismatch. With regard to the lattice mismatch, we show the dislocationdensity dependence on III-V film thickness, we show that the dislocation density can be reduced by thermal cyclic annealing and by using strained-layer super-lattices, and we show 2-3 dimensional growth modes in the initial stage of III-V /Si heteroepitaxy. With regard to the thermal mismatch, we show the dislocation generation in the cooling stage from growth temperature to room temperature and the reduction of dislocation generation by slow cooling. The results show that the dislocation density is reduced by controlling the dislocation reactions under stress.
We found that two types of self-organized InGaAS quantum dots (QDs) appear on GaAs (001) substrates by controlling the growth temperature and the cycle number in alternate source supply metalorganic vapor phase epitaxy (MOVPE). The two types of QDs are well distinguished each other by photoluminescence (PL). One type of QD (Type A) exhibits broad PL around 1.2μm. The other type of QD (Type B) exhibits sharp PL around 1.3μm. At low temperature, the source supply first forms Type A QDs and the following source supply transfers Type A QDs to Type B QDs whose size is much larger than that of Type A QDs. :When the temperature is sufficiently low, the transition induces formation of stacking faults. We also investigated the multistacking growth of these QDS and found a significant increase in the Type B QD density in the bottom layer compared to the singlestacking growth. These results suggest that the three-dimensional condensation of atoms in wetting layers occurs during the formation of Type B QDs.
Semiconductingβ-FeSi_2 having a direct band gap of about 0.85 eV at room temperature has attracted considerable attention because it can be grown epitaxially on Si substrates. In this article, we present firstly the epitaxial growth ofβ-FeSi_2 layers on Si (001) substrates by reactive deposition epitaxy (RDE; deposition of Fe on a hot Si substrate). Then, aggregation of monocrystallineβ-FeSi_2 by annealing and by Si overlayer growth is presented. A β-FeSi_2 film on Si (001) aggregated into islands after annealing at 850℃ for 1 hour in ultrahigh vacuum (UHV). The β-FeSi_2 islands aggregated further into a spherical shape in Si crystals when a 1-μm-thick Si overlayer was grown epitaxially at 750℃ by molecular beam epitaxy (MBE). Cross-sectional transimission microscope (XTEM) observation revealed that the epitaxial relationship between the two materials and monocrystalline nature were revealed after the annealing and the Si overgrowth.
A comparison has been made of the surface morphology of thin InAs films grown on GaAs (001) and (111)A substrates by molecular beam epitaxy using in-situ reflection high energy electron diffraction and ex-situ atomic force microscopy. InAs growth on (001) surface proceeds via the Stranski-Krastanov mechanism, with three dimensional island formation beginning at the critical layer thickness. In contrast, InAs on (111)A surface showed a two-dimensional growth mode, independent of detailed growth conditions. The strain relaxation in the InAs/GaAs (111) A heteroepitaxy has been studied on an atomic scale by scanning tunneling microscopy. The coalescence of small islands and the formation of a dislocation network were identified. The atomic displacement around the threading segments and the strain fields induced by the misfit dislocations were both identified . We also compare the electrical properties of InAs thin films embedded in GaAs layers grown on (111)A and (001) substrates. A major improvement in Hall mobility through the use of (111)A substrates was confirmed. Self-consistent calculation assuming interface Fermi level pinning produces resulted in a good agreement with the experimental results. The use of a novel index substrate like GaAs (111)A plane provides the opportunity of fabricating a wide range of high quality heterostructures.