Strain-Induced Diffusion in Heteroepitaxially Grown CuInSe 2 on GaAs Substrates
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549 Mat. Res. Soc. Symp. Proc. Vol. 399 0 1996 Materials Research Society
was confirmed in-situ by RHEED. The films were grown under slightly Cu rich conditions. 3 Details of the growth procedure have been described elsewhere . The GaAs substrates were (001) LEC GaAs with no intentional miscut. The pseudo-substrates consisted of a linearly-graded InXGal-,As buffer layer with x varying from x=0 to 0.29 using a concentration gradient of -1% In/400A. On top of the linearly-graded buffer a -1 gm thick layer of InrGaj.As of constant x=0.29 composition was grown. The pseudo-substrates were grown in a separate chamber and an As2 cap was used to protect the surface during transport to the growth system. Triple-axis X-ray diffraction (XRD) measurements of these substrates indicated that the final layer was about 90% relaxed. Use of a linearly graded buffer to provide strain relaxation and a reduction in threading dislocation densities below 106 cm-2 has been demonstrated for large mismatch systems such as SiGe/Si4 and InGaP/GaAs5 . The room temperature in-plane lattice mismatch was Aa/a-2.2% and Aa/a-0.2% for the CuInSe 2 /GaAs and CuInSe 2 /In 029Ga 071As layers, respectively. During the initial states of growth, a sharp change in surface reconstruction from a streaky (1 X 2) reconstruction with respect to the [1 TO] and [110] directions to a (1 X 3) reconstruction occurred. The presence of a slight waviness along the [110] direction of the (1 X 3) surface was noted as well. This change occurred after approximately 300 A of CuInSe2 . was deposited. In part, to understand the origin of this change in surface reconstruction, a series of CuInSe 2 films of various thicknesses were grown on GaAs and In0 29Ga 071As pseudo-substrates. CuInSe2 films of thicknesses 42, 126, 400, 630, 1260, and 2700 A 'were grown. These films were then investigated by high-resolution x-ray diffraction (HRXRD) and transmission electron microscopy (TEM). Surface topology of several films was examined by atomic force microscopy (AFM) as well. HRXRD measurements were performed using a MAC Science 18,000 watt rotating anode Cu target. A Phillips MRD diffractometer with a 4-bounce Ge(220) monchrometer was used to select Cu Ka, radiation (X=1.540981 A). A 1.2 mm receiving slit was used in front of the detector. Transmission electron microscopy measurements were performed on a 400 keV JEOL 4000 FX microscope. The cross sections were prepared in the standard way using dimpling and Ar ion milling. RESULTS AND DISCUSSION A model of critical layer thickness above which film relaxation should occur has been calculated by People and Bean.6 Application of this model to the CuInSe 2 /GaAs system yields a critical layer thickness of 80 A. As the thermal expansion coefficient of CuInSe 2 is larger than that of GaAs by a factor of roughly two, at the growth temperature of the lattice mismatch is expected to be approximately 2.7% compressive. This should yield a even smaller value for critical layer thickness. Figure 1 shows a series of scans along the [001] directio
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