Molecular Beam Epitaxy of InGaP Films Grown on Si(001) Substrates with Various Kinds of Initial Buffer Layers
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expansion coefficient of IlI-V being approximately twice that of Si. Usually, mismatched epitaxial growth is performed by inserting some kind of initial interfacial buffer layer between substrate and film. By the interfacial buffer layer, it is expected to reduce and relax the effects of abrupt change of lattice constant, polar / non polar problems and so on [6]. However, for the solar cell performance and the cost, the buffer layer should be as thin as possible and its band gap is wide enough not to cut the light incident into Si bottom cell. A few studies were reported on InGaP growth on Si(001) [7]. The effect of the compositional step-graded InGaP interfacial buffer layer on a thin initial GaP layer was reported [3]. Also, a few studies were reported on the MBE growth of phosphorous based compound semiconductors on Si because of the difficulty of vapor pressure control of solid phosphorous [8-10]. In spite of the above mentioned situations, the research by MBE is important to study the growth process by in situ monitoring methods such as RHEED, XPS, STM, and so on. This paper reports molecular beam epitaxial (MBE) growth of single domain InxGa(1-x) P (x=0.4) films with solid phosphorous source. Four kind of layers such as InGaP (i.e. direct growth without buffer layer), GaP, AIP, and GaAs are used for interfacial buffer layer. The effects of the buffer layer on the film were examined by the in situ monitoring by RHEED, by the observation of the surface morphology of the films and photoluminescence measurement. EXPERIMENT The MBE system used in these experiments was an ultra high vacuum (UHV) system consisting of a growth chamber, an analysis chamber and a sample entry chamber. Molecular beam sources and a substrate heater were surrounded by liquid nitrogen shrouds in the growth 147
Mat. Res. Soc. Symp. Proc. Vol. 399 01996 Materials Research Society
chamber. The phosphorus source had a hot cracking zone where P4 was cracked to P2. Surface of the substrate and the growth of the epitaxial layer were studied in situ by the RHEED system with a 20 KeV electron beam. Base pressure of the growth chamber was 5x10-1 0 Torr with an ion pump and a turbo molecular pump. Si(001) substrates cut from ordinary 6-inch Si wafers of 25 mm x 20 mm in size were used in the experiments. The wafers used in the experiments were Si(001) 4 degree off towards a [110]. It has already been indicated that single domain GaAs films grow on these substrates by MBE. The substrates were degreased and finally boiled in an HCI:H202:H20 (4:1:1) solution to form a protective thin oxide film. They were preheated at about 300 'C for 1 hr to remove water vapor and other gases in the analysis chamber of the MBE system. Then they were transferred to the growth chamber and heated at 1000 °C for 20 min to remove thin oxide films on the substrate surfaces and to rearrange surface Si atoms. After the surface cleaning, the substrate temperature was set to 300 °C. The growth of InGaP on Si was performed by the two step procedure same as reported by Nishi et
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