A. Aydinli

H. Özkan

N. M. Gasanly

English

Crossref

Cryst. Res. Technol. 36 2001 12 1393–1398 
 
© WILEY-VCH Verlag Berlin GmbH, 13086 Berlin, 2001 0232-1300/01/1212-1393 $ 17.50+.50/0 
Raman spectra of GaSe layered crystal have been measured using a He-Ne laser and temperature tuning 
the free-to-bound gap in the range 10-290 K. Resonance enhancement of E’’ (2) mode has been observed 
for both incident and scattered photon energies equal to the free-to-bound transition energy. 
 
Keywords: resonant Raman scattering, semiconductor compounds, layered crystals, GaSe 
 
(Received June 18, 2001; Accepted July 17, 2001) 
1. Introduction 
Resonant Raman scattering (RRS) around excitonic transitions in layered crystals has been a 
subject of extensive investigation in recent years (ASHOKAN et al.; BALTOG et al; JULIEN et 
al.; NAKAOKA et al.; SCAMARCIO et al.). Among various semiconductors, the layered 
compound GaSe seems to be particularly worth studying. In GaSe, the conduction band has 
an absolute minimum at k ≠ 0 (indirect gap) and a relative minimum at k = 0 (direct gap).  
As-grown ε-GaSe is an p-type semiconductor having an indirect and direct band gaps Egi = 
2.103 and Egd = 2.128 eV at 4.2 K . It has direct as well as indirect exciton states within 55 
meV below the direct gap energy (VOITCHOVSKY, MERCIER). 
Observation of one-phonon RRS in ε-GaSe crystal as the exciting laser frequency varies 
around direct and indirect free exciton transitions has already been reported by several 
authors (BALKANSKI et al; CAMASSEL et al.; CHIANG et al.; HOFF,  IRWIN;  REYDELLET, 
BESSON). Because of strong Fröhlich interaction only the ''(4)2 ( )LOA  and 
'(4)( )LOE  modes at 
247.0 and 253.2 cm-1, respectively, show strong resonant enhancement, whereas '(4)1A (134.1 
cm-1) and '(2)1A  (307.8 cm-1) modes show first an anti-resonant and then resonant behavior as 
the exciting laser energy approaches the direct free exciton energy (HOFF, IRWIN). 
Resonance in the Raman cross-section for '(4)1A , 
''(4)
2 ( )LOA  and 
'(4)( )LOE  modes of GaSe at 
80 K has been observed for both incident and scattered photon energies equal to the direct 
free exciton energy (REYDELLET,  BESSON). Double resonance in Raman scattering on 
'(4)( )LOE  mode of GaSe at 77 K has also been studied (CHIANG et al.). It was shown that 
the dispersion of RRS is dominated by resonances with the direct and indirect free exciton 
states. 
Typically, resonant enhancement of phonon Raman scattering takes place when the 
incoming and/or outgoing photon energy is resonant with a singularity in the energy band 
structure of the solid. Resonance is most often encountered through free and bound excitons. 
N. M. GASANLY, A. AYDINLI*, H. ÖZKAN 
Department of Physics, Middle East Technical University, Ankara, Turkey 
*Department of Physics, Bilkent University, Ankara, Turkey 
Resonant Raman Scattering near the Free-to-Bound 
Transition in Undoped p-GaSe 
1394 N. M. GASANLY et al.: Resonant Raman Scattering in GaSe 
To our knowledge there is only one paper in literature (HENNING, KESSENER), in which the 
authors reveal a resonant enhancement of phonon Raman scattering in GaAs when the 
energy of the incoming light is resonant with a nonexcitonic (donor-to-acceptor) transition. 
In this paper we report an observation of RRS when the energy of the incoming and 
outgoing light is resonant with free-to-bound transition in layered ε-GaSe crystal. 
 
 
Fig. 1: Temperature dependence of 
free-to-bound transition energy. 
 
 
 
 
2. Experimental 
Gallium selenide polycrystals were synthesized from high-purity elements (at least 99.999%) 
taken in stoichiometric proportions. ε-GaSe single crystals were grown by the modified 
Bridgman method. The analysis of X-ray diffraction data showed that they crystallize in 
hexagonal unit cell with parameters: a = 0.3745 and c = 1.5921 nm. Sample suitable for 
measurements was obtained by easy cleavage perpendicular to optical c-axis, which is 
parallel to z-axis of the crystal and perpendicular to the layer plane. Alongside with two 
natural cleavage planes, the sample had also two ground and polished surfaces perpendicular 
to them and forming a right angle between themselves. Such a treatment of the specimen 
made it possible to carry out the polarization measurements at any scattering geometries. 
Raman scattering measurements in GaSe layered crystal were performed in the right-angle 
scattering geometry in the frequency range 10 to 320 cm-1. 30 mW He -Ne laser (632.8 nm) 
was used as the exciting light source. The scattered light was analyzed using a U-1000 
“Jobin Yvon” double grating spectrometer and a cooled GaAs photomultiplier supplied with 
necessary photon counting electronics. The Raman line positions were determined within an 
accuracy of ± 0.1 cm-1. A “ CTI-Cryogenics M-22 ” closed-cycle helium cryostat was used to 
Cryst. Res. Technol. 36 (2001) 12  1395 
cool the crystals from room temperature down to 10 K. The temperature was controlled 
within an accuracy of ± 0.5K. 
3. Results and discussion 
In order to study the resonance, the Raman spectrum of ε-GaSe was recorded using a He-Ne 
laser and temperature tuning the gap. By varying the temperature between 10 and 290 K, it 
was possible to tune the free-to-bound gap from 2.027 to 1.923 eV, through the He-Ne laser 
line (hνL = 1.959 eV ) (Fig. 1). The effect of changing the temperature is equivalent to 
changing the photon energy of the laser by an amount equal in magnitude and of opposite 
sign to the corresponding change in the free-to-bound gap. Thus, it was possible to measure 
the Raman spectrum for a fine mesh of scattering photon energies (or their temperature-tuned 
equivalent) covering the region from (Egi – EA ) − hνL = + 0.068 to (Egi – EA ) − hνL = − 0.036 
eV. In our previous study the presence of acceptor level located at EA = 0.076 eV above the 
top of the valence band in p-GaSe crystal was revealed by photoluminescence measurements 
in the temperature range of 10-290 K (AYDINLI et al.). This shallow acceptor level is 
associated with stacking faults, which is easy to produce in this crystals due to weak van der 
Waals forces between the layers. 
 
Fig. 2: Resonant Raman 
spectra of GaSe at different 
temperatures. 
 
 
 
 
 
1396 N. M. GASANLY et al.: Resonant Raman Scattering in GaSe 
Figure 2 presents the Raman spectra of ε-GaSe crystal in y(xx)z  scattering geometry at 
different temperatures. A typical nonresonant Raman spectrum is shown in Fig. 2a. In y(xx)z 
oriented spectra the compressional intralayer '(4)1A (134.1 cm-1) and
'(2)
1A  (307.8 cm-1) modes 
and shear interlayer '(2)E  (19.7 cm-1) and intralayer '(4)E (213.5 cm-1) modes are allowed. 
The shear nonpolar modes ''(2)E  (59.4 cm-1) and ''(4)E (209.7 cm-1) are Raman-active in the 
(yz or zx) geometry. The scattering geometry (xx) was chosen so that the normal off 
resonance ''E modes would be forbidden. It is known that the resonant enhancement of the 
modes in the allowed scattering configuration is weaker than those in the forbidden 
geometries. Furthermore, the enhancement is always strongest when both the incident and 
scattering radiations were polarized parallel to each other irrespective of the symmetry of the 
crystal (YU, CARDONA). 
 
 
 
Fig. 3: Resonant behavior of the 
''(2)E mode. The broken curves 
drawn through the data points to 
guide the eye. 
 
 
 
 
 
 
The shear mode ''(2)E  (59.4 cm-1) exhibit resonance maxima for incident and scattered photon 
energies equal to that of free-to-bound transition (Egi – EA ) (Fig. 2). The second 
''(4)E shear 
Cryst. Res. Technol. 36 (2001) 12  1397 
mode (209.7 cm-1) shows single resonance (not shown) at 210 K indicating that, this 
resonance is mediated through free-to-bound transitions as well. However, resonance of 
''(4)E shear mode is much weaker and it is observed only with the incident light. In this 
report, we concentrate on the ''(2)E mode, which shows stronger resonance. The laser photon 
hνL excites the electron by means of indirect free-to-bound transition. Although momentum 
conservation forbids the direct excitation of indirect free-to-bound transition, stacking faults 
in the layered crystals can easily induce such a process (CHIANG et al.). The resonant 
enhancement of the mode arises through a vanishing denominator in the expression for the 
scattering cross section. The vanishing factor may be either (Egi – EA ) − hνL or (Egi – EA ) − 
hνS (incoming or outgoing resonance (YU,  CARDONA)). Here (Egi – EA ) is the resonant 
intermediate state energy. The difference between hνL and hνS is equal to the 
''(2)E mode 
energy (7.4 meV). 
Figure 3 shows the resonant behavior of ''(2)E mode of ε-GaSe in the region of the free-
to-bound gap. With decreasing temperature down to 230 K, Raman intensity ratios 
''(2) '(4)
1) / ( )( II E A  and 
''(2) '(2)
1) / ( )( II E A  increase, then decrease reaching the minimum value 
at 220 K and again increase having a second maximum value at 210 K. Below this 
temperature the intensity ratios decrease with a slower rate down to 10 K.  
The energy separation obtained between the position of two maxima is equal to 8.8 meV, 
which is in good agreement with the energy value of 7.4 meV ( ''(2)E mode energy). This 
agreement supports our assignment of these two maxima to double resonance, which occurs 
due to the resonance of incident and scattered photons with free-to-bound transition in GaSe. 
In conclusion, we have presented direct evidence of free-to-bound transition resonance in 
phonon Raman scattering of GaSe. Resonant enhancement of ''(2)E mode was observed in 
GaSe when the energies of the exciting and scattered light coincide with free-to-bound 
transition energy. For the second shear mode, ''(4)E , we observe a weak single resonance. 
 
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Contact information: 
 
Prof. Dr. N. M. GASANLY*,  
Prof. Dr. H. OZKAN 
Department of Physics, Middle East Technical Universty 
06531 Ankara 
Turkey 
 
Prof. Dr. A. AYDINLI 
Department of Physics 
Bilkent University 
06533 Ankara 
Turkey 
 
*corresponding author 
e-mail: nizami@metu.edu.tr 
