Raman and Photo-Luminescence Studies on Intrinsic and Cr-Doped Znse Single Crystals
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365 Mat. Res. Soc. Symp. Proc. Vol. 484 01998 Materials Research Society
obtained from the Coherent Innova 99 cw Ar+ laser. Details for recording the spectra and band analysis have been reported elsewhere. RESULTS AND DISCUSSIONS Phonon-Plasmon Coupling Typical spectra for ZnSe single crystals and three Cr-doped ZnSe at room temperature are shown in Fig.1. In each case, the 514.5 nm Ar-ion laser line was used for excitation. The (111) surface of ZnSe was kept perpendicular to the incident and scattered lights. While, in the case of ZnSe:Cr samples, the orientation of the surface was not defined. Both, intrinsic and doped, samples of ZnSe show essentially the same features in their Raman spectra. The two sharp and intense lines at -250 and -205 cm' have been identified as LO and TO phonon modes of ZnSe 10.The weak and broad structure at -150 cm" has been reported as 2TA, the first overtone of transverse-acoustical phonon." In addition, a weak and broad combination mode 2TA+TO and the first overtone of LO phonon is observed in all the spectra at -270 and -500 cm', respectively. " Fig. 1 Room temperature Raman
10800
d)Cr-doped ZnSe single crystals with Cr concentration of 1.0, 3 1.8, 10.2 (x 1019 cm- ) respectively, using an excitation
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spectra of (a)intrinsic and (b,c,
TO
1
e
line of 514.5 nm. A Peak fit
r 5400
analysis of spectrum (d) shows
E
additional four weaker bands,
which are assigned to LO and TO modes of two types of 2 Chromium ions Cr +and Cr+. 9
LO
7200
3800
2TA
d
2TA TO
1.00 b 0 kb_
100
' A
200
"
-
300
400
500
Raman Shift (cm"1)
The peak position of the
LO mode shows a downward shift as the doping concentration is increased; the peak position of the TO mode, however, remains unchanged. Effect of temperature on the peak positions of the LO and TO modes for the three samples is shown in the Fig. 2. A consistent downward shift of the LO mode with increased doping concentration is noticeable, at each temperature. The upward shift of the Raman modes with decreasing temperatures is a manifestation of anharmonicity and has long been studied in various semiconductors, including ZnSe .2Our data for the frequency shift in the temperature range of 70-290K show an excellent agreement with the theoretical calculations based on cubic anharmonicity to second order. However, for a wider range of temperature (70 to 550K), the cubic harmonic terms to second order are insufficient to fit the data and the inclusion of higherorder terms involving cubic and quartic anharmonicity in necessitated for a satisfactory fitting . Two types of interactions of optical-phonons with free-carriers in the semiconductors have been known to occur" : (i) interaction of single-particle electron (hole) excitations with TO phonons through the deformation potential mechanism. (ii) interaction of LO phonons with collective excitations, i.e. plasmons, of electron (hole). Owing to the associated longitudinal electric fields, only LO phonons couple with the plasmons. The shifts in the peak position of LO phonons, in
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