Photoreflectance Study of MBE Grown Te-doped GaSb at the E 0 +A 0 Transition
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EXPEREMENT The samples used in this study were MBE grown GaSb at a growth temperature of 350 'C on semi-insulating GaAs substrate. The epilayer thicknesses ranged from 4 to 5 gtm. The Tedoped sample concentration was estimated to be 4-5x10 17 cm"3 at 300 K. A standard PR system was setup. A probe beam from a 100 W tungsten halogen lamp source dispersed through a 0.25 m monochromator was used to scan the samples. Modulation was achieved by using a 10 mW He-Ne laser operating at 632.8 nm, chopped at 250 Hz. The laser intensities were varied between 0.08 to 0.8 mW/mm 2 by placing neutral density filters in front of the laser output to investigate the dependence of PR line shape and signal amplitude on laser beam intensity. Sample cooling for low-temperature measurements was provided by a Joule-Thompson cryogenic refrigerator system. 57 Mat. Res. Soc. Symp. Proc. Vol. 484 01998 Materials Research Society
RESULTS Figure 1 shows PR signal at 4 K of a Te-doped GaSb sample. The line shape exhibits no Franz-Keldysh oscillations and is fitted very well using the first derivative of a Lorentzian functional form3 which is appropriate for an excitonic transition:
ARR"=Re[Ce'i (E-Ecp+i')"2]
(I)
where E =h(o is the energy of the probe beam, C and 0 are an amplitude and phase factor that slowly vary with E, Ep and F are the critical-point energy and the broadening parameter, respectively.
r 4
0.20 0.15 0.10 0.05 0.00 -0.05 -0.10 -0.15 -0.20 -0.25 -0.30 1.35
1.40
1.45
1.50
1.55
1.60
1.65
1.70
1.75
1.80
Energy (eV)
Figure 1. PR signal of MBE sample at 4 K
The PR line shapes measured in the 4 to 250 K range were found to be independent of the modulating pump intensity, as illustrated in Fig. 2 in the limited intensity range investigated, while
the amplitude of the PR signal varied logarithmically with the intensity. These observations are indicative of the PR in the low-field regime.
58
0.05
C U
-0.25 ' 1.35
1.40
1.45
1.50
1.55
1.60
1.65
1.70
1.75
1 1.80
Energy (eV)
Figure 2. PR amplitude as a function of modulating laser intensity, 2 top curve: 0.08, middle: 0.4, and bottom: 0.8 mW/mm Figure 3 shows the theoretical fit to the experimental data using Eq. (1). The best fit results obtained correspond to Eo+A 0= 1.614 eV and r= 53 meV. 0.15-
0.100.05 0.00-o S C -0.05-
-0.10 9 -0.15 C
'0.20 '0.25 -0.30 -
1.35
1.40
1.45
1.50
1.55
1.60
1.65
1.70
1.75
1.60
Energy (eV)
Figure 3. Theoretical simulation of the PR of MBE sample at 4 K. Open circle represents experimental data and solid line represents theoretical simulation. The temperature dependence of the Eo+Ao transition energy (see Fig. 4) has been investigated using the semi-empirical Varshni's 4 relation: E(T)=E(0)- a T2/ (3+T)
(2) 59
where E(O) is the energy at 0 K, and a and f3are the empirical parameters. The amplitude of the E0 +A0 transition decreases and the spectra widens with increasing temperature as expected. The values of the parameters that describe the temperature dependence were found to be E(0)=1l.615 eV, ota 4.8x10 4
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