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Resonant Raman scattering in amorphous bulk selenium
Solid State Communications. Vol. 33, pp. 1143—1145. Pergamon Press Ltd. 1980. Printed in Great Britain. RESONANT RAMAN SCATTERING IN AMORPHOUS BULK SELENIUM G. Carini, M. Cutroni, MY. Fontana, G. Gall and P. Migliardo Istituto di Fisica dell’Universitá di Messina and Gruppo Nazionale di Struttura della Materia del CNR, 98100 Messina, Italy (Received 9 September 1979 by R. Fieschi) The resonant Raman spectrum of a-Se bulk has been observed in the low frequency region and in the range of the wavelength 5145—6328 A. The maximum of the resonance is centered at 5998 A and shows a sharp peak at 3 1 cm~Stokes shift, whose intensity varies with wavelength. The experimental data were interpreted, and related to the behaviour of the derivative of the dielectric function in the same material.
THE STRUCTURE and vibrational dynamics of crystalline selenium in both its trigonal (D3) and mono-
We have studied the RRS behaviour in bulk a-Se
with particular attention to the low frequency part of clinic (D4d) form have been extensively studied and are the spectrum where the ring-related vibrations in the reasonably well understood [i}; however there is still monocinic structure give their contribution. Furthersome uncertainty on the structure of bulk amorphous more the dependence of the RRS cross-section on Se and its associated vibrational and electronic states, excitation light frequency was investigated with Amorphous selenium is assumed to be structured particular care in the spectral region where most as a mixture of the Se8 rings and helicoidal chains differences exist in the optical behaviour of chains and characteristic of the ~-monoclinic and trigonal crystal ring-related structures respectively, i.e. the region 6300— phases respectively. What is the respective proportion of 5800 A In fact RRS can be interpreted as a form of these two components and what is the associated microscopic modulation spectroscopy [81.Given some vibrational dynamics is not yet clear. It is certainly kind of elementary excitation of frequency ~ which dependent on the thermal history of the material [21. couples to the electronic states of the material, their Raman spectroscopy has been used to try to determine contribution to the frequency dependence of the the chain-to-ring ratio in a-Se by analyzing the shape of dielectric susceptibility can be singled out by studying the doublet band in the 250 cm’ spectral region and the behaviour of the RRS cross-section as a function of comparing it with the crystalline spectra in the monoexcitation light frequency w~.In a simple classical model clinic and trigonal phases [3]. It has been noted [4] approximation the RRS intensity at a given Raman however that such a procedure may not be entirely frequency is proportional to the square of the derivative justified since the immediate local environment of Se of the dielectric susceptibility x(w,, w) with respect to atoms is the same in both ring and chain structures, and the normal mode at that frequency. thus the shape change of the 250 cm’ band doublet Thus RRS singles out structures in x(w1, w) which alone cannot yield the needed information about are connected with a specific vibration that modulates a chain-to-ring statistics, specific electronic state such as, for instance, the localAdditional information can be obtained by a careful ized electronic states eventually connected with the study of the temperature dependence of the 250 cm~ presence of ring or chain structures in a-Se. Raman band through the glass transition temperature Our samples are prepared from Se powder (purity Tg. We have performed such a study and the results will 5—9) sealed in quartz ampoules under l0~torrresidual be published elsewhere [5]. pressure. The ampoules were heated at 300°Cfor 24hr In this paper we wish to report a study of Resonant and were air-quenched. They were then kept in storage Raman Scattering (RRS) in bulk amorphous selenium, in the dark for at least three months. X-ray diffraction The only study of this kind we know [6] was conpatterns were taken before each measurement to check cerned with thin film samples and looked at the high for “amorphousness”. Thermal and photoeffects due to frequency overtones of the 250 cm~band. Furthermore the incoming laser light have always been a major probno dependence of the RRS spectra on the excitation 1cm in the laser spectroscopy of selenium and similar light frequency was reported and some irreproducibility materials. We have taken particular care to avoid in the data, probably due to laser heating effects of the thermal and optical damage to our samples. In particular, samples, was also reported without discussion, excitation light was not focused and its intensity was 1143
1144
RESONANT RAMAN SCATTERING IN AMORPHOUS BULK SELENIUM
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Fig. 2. Raman intensity of the 31 cm~ peak vs excitation light. ~i~iI5
RAMAN SHlFT(cm~’)
Fig. 1. Normalized spectra for a-Se bulk at 6328 and 5998 A excitation light respectively, always kept below 5 mW. The samples have cylindrical shape and were rotated about their axes at a frequency of 50 Hz. Thus the effective power density at the samples was less than 20mW cm2. Details on sample handling, geometry and other experimental procedures are to be found in [5]. The experimental spectra taken under the aforementioned experimental conditions were found to be reproducible and time independent for all incoming light frequencies used. Furthermore the absence of laser induced structural changes was checked by taking X-ray diffraction patterns of the samples after each experimental run. Raman spectroscopy was performed with a SPEX Ramalog spectrometer. Excitation light was supplied by the 6328 A line of a He—Ne laser, the 5145 A line of the Ar laser and the continuum distribution in the range 5800—6200 A of a rhodamine 6G dye laser. The dye laser fluorescence background was filtered out by a 0.25 m Jarrel—Ash grating monochromator. The optical constants of our samples were determined by reflectance measurements and Kramers—Kronig analysis. Details on these measurements and their analysis will be published elsewhere [9]. In the spectral region investigated, the absorption coefficient is high and smoothly varying, ranging from 1.27 x 10~cm~at 6328 A to 7.45 x 10~cm~at 5145 A. Since our Raman spectra were taken in the back scattering geometry and the absorption coefficient was high, the corrections for optical absorption and reflection are simple: we have used the
formulae (4.20) reported for instance in Richter’s review article [10]. Finally the experimental spectra were normalized by the usual factor [n(w) + 1 ]/w, where w is the Raman frequency and n(w) the Bose—Einstein factor. All spectra were taken at room temperature. In Fig. 1 we show the normalized spectra obtained with 6328 and 5998 A excitation. The strikingly strong resonant enhancement of the Raman scattering in the low frequency range is evident. Note that the intensity of the peak at 31 cm’ in the 5998 A spectrum is approximately 30 times higher than that of the 250 cm’ band. In Fig. 2 we show the behaviour of the corrected spectral intensity of the 31 cm’ peak vs excitation light frequency. As a reference “standard” for comparing intensities at different excitation wavelength we have used the residual “background” in the region about 190 cm’, where no spectral contribution exists. Such background was verified to be independent of the excitation light wavelength. The results shown in Figs. 1 and 2 are striking. They show not only a considerable qualitative change of the low frequency spectra as the laser wavelength is decreased, but also a reasonably sharp resonance in the corresponding Raman cross-section. In Fig. 2 we also show for comparison the measured behaviour of Ide”IdwI2. Note that d”/dwI2 is peaked in the same spectral region as the RRS cross-section. In order to interpret our results we recall that RRS may single out a very specific contribution to the frequency spectrum of the dielectric susceptibility. Thus, although x(w) is smoothly varying, IdxIdwI2 is not, in fact it clearly features a resonance corresponding to a density of electronic states which are strongly coupled to the resonating vibrational mode.
Vol. 33, No. 11
RESONANT RAMAN SCATTERING IN AMORPHOUS BULK SELENIUM
The region where the resonance at 3 1 Cm” and the other low frequency peaks takes place happens to be that in which most changes occur in the optical absorption of crystalline Se in passing from the trigonal to the monoclinic structure. Also the 31 cm~and the other resonance-enhanced peaks are all in the spectral region of vibrations characteristic of the monoclinic ring structure. The trigonal chain structure has no vibrational mode in this region. Thus it is safe to conclude that the resonating electronic states couple with vibrational modes connected with the existence of Se8 rings in the amorphous structure. Furthermore the resonance peak about 6000 A is connected with a sharply peaked density of electronic states belonging to the Se atoms located on the ring structures themselves, Another interesting result concerns the behaviour of the 250cm”’ band. We observe that its intensity decreases steadily upon increasing incoming light frequencies. In particular for 5145 A excitation, under our experimental conditions, we were not able to detect the 250cm’ band (or for that matter any other part of the spectrum). We could however record the normal, out-ofresonance Raman spectrum upon increasing power level. In this case the spectra were not entirely reproducible and were time-dependent. Thus in order to make any statements about resonance effects such as those made in [6], extreme care must be taken to verify the total absence of effects due to laser heating and damage. As far as our data are concerned, and at the sensitivity level of our measurements, the 250 cm” band seems to show a kind of anti-resonant behaviour, i.e. it steadily decreases as the excitation light probes the higher ftcquency region of the dielectric susceptibility. In conclusion, we have shown that RRS can
1145
discriminate between chain and ring structures in a-Se and therefore be used to probe the chain-to-ring statistics in this material as a function of temperature, thermal treatment, etc. Furthermore we believe that our data show for the first time the existence of a reasonably sharp resonance in the frequency dependence of the dielectric susceptibility of amorphous materials, as probed by RRS spectroscopy. REFERENCES Chizhikow & V.P. Shchastlivyi, Selenium and Selenides. London and Wellingborough (1968). 2. R.B. Stephens,J. AppI. Phys. 49, 5856 (1978) and references cited therein. 3. G. Lucovsky, A. Mooradian, W. Taylor, G.B. Wright & R.C. State Commun. 5, 113 (1967); M.Keezer, GarmanSolid & S.A. Solin, Solid State Commun. 18, 1401 (1976); A. Mooradian & G.B. Wright, Physics ofSelenium and Tellurium (Edited by W.C. Cooper), p. 269. Pergamon Press, Oxford (1969). 4. M.H. Brodsky, Light Scattering in Solids (Edited by M. (1975). Cardona), p. 204. Springer-Verlag, New York 5. G. Carini, M. Cutrom, G. Galli, P. Migliardo & F. Wanoleringh, (in press). 6. N. Ohta, W. Scheuermarn et aL, Solid State 1325 (1978). 7. Commun. E.A. Davis,27, Electronic and Structural Properties ofAmorphous Semiconductors (Edited by P.G. Le Comber and J. Mort), p.452. Academic Press, New York (1973). 8. M. Cardona, Modulation Spectroscopy Solid State Phys., Section 11. Academic Press, New York 9. (190?). G. Galli er aL (to be published). 10. W. Richter, Solid State Physics (Edited by G. Hohier) Springer-Verlag, New York. 1.
THE STRUCTURE and vibrational dynamics of crystalline selenium in both its trigonal (D3) and mono-
We have studied the RRS behaviour in bulk a-Se
with particular attention to the low frequency part of clinic (D4d) form have been extensively studied and are the spectrum where the ring-related vibrations in the reasonably well understood [i}; however there is still monocinic structure give their contribution. Furthersome uncertainty on the structure of bulk amorphous more the dependence of the RRS cross-section on Se and its associated vibrational and electronic states, excitation light frequency was investigated with Amorphous selenium is assumed to be structured particular care in the spectral region where most as a mixture of the Se8 rings and helicoidal chains differences exist in the optical behaviour of chains and characteristic of the ~-monoclinic and trigonal crystal ring-related structures respectively, i.e. the region 6300— phases respectively. What is the respective proportion of 5800 A In fact RRS can be interpreted as a form of these two components and what is the associated microscopic modulation spectroscopy [81.Given some vibrational dynamics is not yet clear. It is certainly kind of elementary excitation of frequency ~ which dependent on the thermal history of the material [21. couples to the electronic states of the material, their Raman spectroscopy has been used to try to determine contribution to the frequency dependence of the the chain-to-ring ratio in a-Se by analyzing the shape of dielectric susceptibility can be singled out by studying the doublet band in the 250 cm’ spectral region and the behaviour of the RRS cross-section as a function of comparing it with the crystalline spectra in the monoexcitation light frequency w~.In a simple classical model clinic and trigonal phases [3]. It has been noted [4] approximation the RRS intensity at a given Raman however that such a procedure may not be entirely frequency is proportional to the square of the derivative justified since the immediate local environment of Se of the dielectric susceptibility x(w,, w) with respect to atoms is the same in both ring and chain structures, and the normal mode at that frequency. thus the shape change of the 250 cm’ band doublet Thus RRS singles out structures in x(w1, w) which alone cannot yield the needed information about are connected with a specific vibration that modulates a chain-to-ring statistics, specific electronic state such as, for instance, the localAdditional information can be obtained by a careful ized electronic states eventually connected with the study of the temperature dependence of the 250 cm~ presence of ring or chain structures in a-Se. Raman band through the glass transition temperature Our samples are prepared from Se powder (purity Tg. We have performed such a study and the results will 5—9) sealed in quartz ampoules under l0~torrresidual be published elsewhere [5]. pressure. The ampoules were heated at 300°Cfor 24hr In this paper we wish to report a study of Resonant and were air-quenched. They were then kept in storage Raman Scattering (RRS) in bulk amorphous selenium, in the dark for at least three months. X-ray diffraction The only study of this kind we know [6] was conpatterns were taken before each measurement to check cerned with thin film samples and looked at the high for “amorphousness”. Thermal and photoeffects due to frequency overtones of the 250 cm~band. Furthermore the incoming laser light have always been a major probno dependence of the RRS spectra on the excitation 1cm in the laser spectroscopy of selenium and similar light frequency was reported and some irreproducibility materials. We have taken particular care to avoid in the data, probably due to laser heating effects of the thermal and optical damage to our samples. In particular, samples, was also reported without discussion, excitation light was not focused and its intensity was 1143
1144
RESONANT RAMAN SCATTERING IN AMORPHOUS BULK SELENIUM
1\ ---~e~
-
A59S8A
—
BOL
Vol. 33, No. 11
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I
Ill
//
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/
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//
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Fig. 2. Raman intensity of the 31 cm~ peak vs excitation light. ~i~iI5
RAMAN SHlFT(cm~’)
Fig. 1. Normalized spectra for a-Se bulk at 6328 and 5998 A excitation light respectively, always kept below 5 mW. The samples have cylindrical shape and were rotated about their axes at a frequency of 50 Hz. Thus the effective power density at the samples was less than 20mW cm2. Details on sample handling, geometry and other experimental procedures are to be found in [5]. The experimental spectra taken under the aforementioned experimental conditions were found to be reproducible and time independent for all incoming light frequencies used. Furthermore the absence of laser induced structural changes was checked by taking X-ray diffraction patterns of the samples after each experimental run. Raman spectroscopy was performed with a SPEX Ramalog spectrometer. Excitation light was supplied by the 6328 A line of a He—Ne laser, the 5145 A line of the Ar laser and the continuum distribution in the range 5800—6200 A of a rhodamine 6G dye laser. The dye laser fluorescence background was filtered out by a 0.25 m Jarrel—Ash grating monochromator. The optical constants of our samples were determined by reflectance measurements and Kramers—Kronig analysis. Details on these measurements and their analysis will be published elsewhere [9]. In the spectral region investigated, the absorption coefficient is high and smoothly varying, ranging from 1.27 x 10~cm~at 6328 A to 7.45 x 10~cm~at 5145 A. Since our Raman spectra were taken in the back scattering geometry and the absorption coefficient was high, the corrections for optical absorption and reflection are simple: we have used the
formulae (4.20) reported for instance in Richter’s review article [10]. Finally the experimental spectra were normalized by the usual factor [n(w) + 1 ]/w, where w is the Raman frequency and n(w) the Bose—Einstein factor. All spectra were taken at room temperature. In Fig. 1 we show the normalized spectra obtained with 6328 and 5998 A excitation. The strikingly strong resonant enhancement of the Raman scattering in the low frequency range is evident. Note that the intensity of the peak at 31 cm’ in the 5998 A spectrum is approximately 30 times higher than that of the 250 cm’ band. In Fig. 2 we show the behaviour of the corrected spectral intensity of the 31 cm’ peak vs excitation light frequency. As a reference “standard” for comparing intensities at different excitation wavelength we have used the residual “background” in the region about 190 cm’, where no spectral contribution exists. Such background was verified to be independent of the excitation light wavelength. The results shown in Figs. 1 and 2 are striking. They show not only a considerable qualitative change of the low frequency spectra as the laser wavelength is decreased, but also a reasonably sharp resonance in the corresponding Raman cross-section. In Fig. 2 we also show for comparison the measured behaviour of Ide”IdwI2. Note that d”/dwI2 is peaked in the same spectral region as the RRS cross-section. In order to interpret our results we recall that RRS may single out a very specific contribution to the frequency spectrum of the dielectric susceptibility. Thus, although x(w) is smoothly varying, IdxIdwI2 is not, in fact it clearly features a resonance corresponding to a density of electronic states which are strongly coupled to the resonating vibrational mode.
Vol. 33, No. 11
RESONANT RAMAN SCATTERING IN AMORPHOUS BULK SELENIUM
The region where the resonance at 3 1 Cm” and the other low frequency peaks takes place happens to be that in which most changes occur in the optical absorption of crystalline Se in passing from the trigonal to the monoclinic structure. Also the 31 cm~and the other resonance-enhanced peaks are all in the spectral region of vibrations characteristic of the monoclinic ring structure. The trigonal chain structure has no vibrational mode in this region. Thus it is safe to conclude that the resonating electronic states couple with vibrational modes connected with the existence of Se8 rings in the amorphous structure. Furthermore the resonance peak about 6000 A is connected with a sharply peaked density of electronic states belonging to the Se atoms located on the ring structures themselves, Another interesting result concerns the behaviour of the 250cm”’ band. We observe that its intensity decreases steadily upon increasing incoming light frequencies. In particular for 5145 A excitation, under our experimental conditions, we were not able to detect the 250cm’ band (or for that matter any other part of the spectrum). We could however record the normal, out-ofresonance Raman spectrum upon increasing power level. In this case the spectra were not entirely reproducible and were time-dependent. Thus in order to make any statements about resonance effects such as those made in [6], extreme care must be taken to verify the total absence of effects due to laser heating and damage. As far as our data are concerned, and at the sensitivity level of our measurements, the 250 cm” band seems to show a kind of anti-resonant behaviour, i.e. it steadily decreases as the excitation light probes the higher ftcquency region of the dielectric susceptibility. In conclusion, we have shown that RRS can
1145
discriminate between chain and ring structures in a-Se and therefore be used to probe the chain-to-ring statistics in this material as a function of temperature, thermal treatment, etc. Furthermore we believe that our data show for the first time the existence of a reasonably sharp resonance in the frequency dependence of the dielectric susceptibility of amorphous materials, as probed by RRS spectroscopy. REFERENCES Chizhikow & V.P. Shchastlivyi, Selenium and Selenides. London and Wellingborough (1968). 2. R.B. Stephens,J. AppI. Phys. 49, 5856 (1978) and references cited therein. 3. G. Lucovsky, A. Mooradian, W. Taylor, G.B. Wright & R.C. State Commun. 5, 113 (1967); M.Keezer, GarmanSolid & S.A. Solin, Solid State Commun. 18, 1401 (1976); A. Mooradian & G.B. Wright, Physics ofSelenium and Tellurium (Edited by W.C. Cooper), p. 269. Pergamon Press, Oxford (1969). 4. M.H. Brodsky, Light Scattering in Solids (Edited by M. (1975). Cardona), p. 204. Springer-Verlag, New York 5. G. Carini, M. Cutrom, G. Galli, P. Migliardo & F. Wanoleringh, (in press). 6. N. Ohta, W. Scheuermarn et aL, Solid State 1325 (1978). 7. Commun. E.A. Davis,27, Electronic and Structural Properties ofAmorphous Semiconductors (Edited by P.G. Le Comber and J. Mort), p.452. Academic Press, New York (1973). 8. M. Cardona, Modulation Spectroscopy Solid State Phys., Section 11. Academic Press, New York 9. (190?). G. Galli er aL (to be published). 10. W. Richter, Solid State Physics (Edited by G. Hohier) Springer-Verlag, New York. 1.