Optical Properties of Pseudomorphic Sn X Ge 1-x Alloys
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Mat. Res. Soc. Symp. Proc. Vol. 588 ©2000 Materials Research Society
high energy electron diffraction (RHEED) patterns show the SnxGe,_x epitaxial layers are single crystalline with atomically rough surfaces, consistent with postgrowth characterization by high resolution X-ray diffraction and atomic force microscopy. The rms surface roughness of 500 x 500 nm regions SnxGel_x epitaxial layer increases with Sn content, 1.1 nm for x < 0.06 and 1.5 nm for 0.06 < x < 0.115. Rutherford backscattering spectroscopy is used to determine the Sn composition of the SnxGeIx epitaxial layer. The Sn peaks are of constant height, thus, the composition is uniform and free of Sn surface segregation as seen in Fig. 1. Knowing the Sn composition, the relaxed lattice constant of SnxGeI_x is calculated using the virtual crystal approximation which has been shown to be experimentally accurate.1 The coherency strain parallel and perpendicular to the substrate surface are calculated as a function of the relaxed lattice constant (aj0):
e =eyy =
aGe - af, (x)
afo(x)
1+v ezz =exx + I
(1)
f (2)
wherefis the lattice mismatch given by: aGe - afo (x) aGe The angular displacement of the SnxGei_x (004) X-ray reflection with respect to Ge(004) is measured using an X-ray rocking curve diffractometer. The (004) angular displacement yields the lattice constant in the growth direction and an experimental value for e, In order to determine the strain in the plane of the substrate, a (422) asymmetric reflection is measured from which exx is determined. The experimental values of the strain components agree with the values derived from the theory using the virtual crystal approximation as illustrated in Fig.2. Ge
100
0.02
Sin
Experimental Data 0.01
80 60 o,,
40
z0
X=O0.115 X = 0.06 x = 0.035
20
-0.01
z,, , i ,~~~~~
A3
1.2
o0.00
1.3
1.4
1.5
1.6
1.7
A"•0
1.8
Energy (MeV)
0.02
0.04
0.06
0.08
0.10
0.12
X
Sn
Fig. 2: Comparison between theory (solid line) and experimental value of the strain parallel (squares) and perpendicular (triangles) to the substrate surface.
Fig. 1: RBS spectra of 100 nm SnxGelix on Ge(OO 1). The leading edge of Sn and Ge are represented by dashed lines.
200
60
04
Energy (eV)
06
0.5
07
08
0.9
04
50
05
Energy (eV)
06
0.7
08
09
58 5
40
54
52 550
30 _)
1 48 4-
.-
Ge sub Sn Ge 0056095
44
A
Sn
20
A 0.92
.
4000
5000
_ Ge Sn 0.035 0.965 -------Sn Ge
0.115
N 6000
0.94
Ge
Sn
10
40 3000
-
0.06
Ge 0.08
42
E
0.885
0 7000
8000
3000
Wavenumber (cm-1)
4000
5000
6000
7000
8000
Wavenumber (cm-1)
Fig. 3a: FT-IR spectra of Reflectance taken at 500 ofn-Ge(001) and Sn5 Gel-, on n-Ge(001) for x = 0.05 and 0.08.
Fig.3b: FT-IR spectra of Transmittance at normal incidence of Sn 5GelI, onpGe(001) for x = 0.035, 0.06, and 0.115.
Thus, HR-XRD of SnxGel-x alloys with 0.035 < x < 0.115 and film thickness of 100 nm confirms the epitaxial layers are coherent to the Ge(001) substrate. Fourier transform infrared transmission measurements in reflectance and transmittance m
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