Epitaxial Growth of CuAu-Ordered CuInSe 2 Structural Polytypes by Migration Enhanced Epitaxy
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X-ray diffraction data was acquired with a Philips PW3710 diffractometer using a copper anode filtered to provide predominately K, radiation. Film compositions were measured with a JOEL electron microprobe using a 25keV beam accelerating voltage and calibrated to a stoichiometric single-crystal CuInSe 2 standard provided by NREL. The overall composition of the indiumrich sample whose XRD , AFM, SE-SEM, and Raman data are shown in figures 2, 4, 5, and 6 respectively, is [Cu]=24.9±0.6%at., [In]=25.2+0.7%at., and [Se]=49.9+0.3%at.; that of the sample used for the TEM shown in figure 3 is [Cu]=25.3+0.6%at., [In]=25.0+0.7%at., and [Se]=49.7±0.3%at. The copper-rich sample in figures 5 and 6 had an overall composition of [Cu]=26.7±0.6%at., [In]=25.3+0.7%at., and [Se]=48.0±0.3%at. RESULTS XRD
X-ray diffraction 0-20 scans exhibit an unusually prominent series of diffraction peaks at 0 {15.450, 30.93-, 47.19°, 64.17-1 that we tentatively assign to the (002), (004), (006), and (008) reflections of the CA structure. Only the (008) peak has been previously reported [13] and the other reflections are not symmetry-allowed for the chalcopyrite structure. Our modeling of the theoretical XRD spectrum for the CA structure predicts the presence of each of these peaks. 1.E+06 66.11
26 31.69 66.11 15.45 26.67 30.93
1.E+05 31.69 1.E+04
64.17
30.93 144.29
47.19
47.19 64.17
15.45
Assignments (200)GaAs (400)GaAs (002)ca -CulnSe2 (112)ch -CulnSe2 (004)ca -CulnSe2 (220)ch -CuInSe2 (006)ca -CuInSe2 (008)ca -CulnSe2
X-ray source Cu K B echoes: (200)GaAs 28.61
1.E+03 -59.09
59.09
26.67 ý' 1.
(400)GaAs
44.2J9
1.E+02 10
Figure 2
20
30 40 2 0 [0] - {-
50
60
70
11(110))
XRD spectrum of indium-rich cIs grown epitaxially on (001) GaAs by MEE.
TEM
Figure 3 displays (from right to left) theoretical dynamical electron diffraction patterns along the [010] direction of the CH and CA structures, the experimental transmission electron diffraction (TED) pattern, and experimental TEM dark-field cross-sectional images taken using either the (001) diffraction spot (top) or (101) diffraction spot (bottom). The latter two images display the spatial distribution of the two phases, since the theoretical diffraction pattern modeling shows only a very weak (001) spot for the CH structure (due to double-diffraction) and no intensity for
the CA structure in the (101) position. It is apparent from these images that the epilayer nucleates initially in the CH structure but converts to the CA structure as growth continues. The presence of
the underlying CH-CuInSe 2 also explains the weak peaks from this structure evident in the XRD spectrum shown in the preceding section.
197
F125-7 CIS/GaAs
f
A I J01
Dxkt Iidd: Chto~p~ritc
Figure 3
.hffrm t Irn,
lt
A
c
TEM-DF cross-sections, TED pattern along [010], and theoretical TED patterns.
Raman Raman spectra exhibit heretofore-unreported peaks at 52, 186, and 462 cm-', in addition to the A, mode at 175 cm-1 and the LO modes at 232 cm-1 sometimes observed in CH-CuInSe 2. The 52 and 186 cm
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