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Photoconductivity and optical absorption on the high energy side in an Se treated ZnSe monocrystal
J. Phys. Chem. Solids Vol. 52, No. 4, pp. 57S578, Printed in Great Britain.
1991
0022.3697/91 $3.00 + 0.00 Q 1991 Pergamon Fves plc
PHOTOCONDUCTIVrTY AND OPTICAL ABSORPTION ON THE HIGH ENERGY SIDE IN AN Se TREATED ZnSe MONOCRYSTAL F. RABACO,~ J. M. MARTIN,$ A. B. VINCENTJ: and N. V. JOSHIS tmstituto de Fisica, Universidad Autbnoma de San Luis Potosi, av. A. Obregen 64, 78000 San Luis Potosi, S.L.P., Mexico SCentro de Estudios Avanzados en Optica, Fact&ad de Ciencias, Universidad de 10s Andes, M&da, Venezuela (Received 15 June 1990; accepted 26 September 1990)
Abstract-Photoconductivity and optical absorption spectra of selenium treated ZnSe were examined. It was found that an excess of selenium increases the photoresponse substantially, particularly on the high energy side of the band gap. This is attributed to the transition from a high density of states below the top of the valence band to the bottom of the conduction band. The presence of this high density of states is also confirmed from an optical absorption study. ~ey~v~d~~ Zinc selenide, single crystal, p~otoconductivity,
INTRODUCTION Zinc selenide is a wide gap semiconductor with interesting optical properties. It is a good material for IR windows 111.Potential applications are increasing strongly as recently super-lattice and micro structures have been suc.cessfuIly obtained by forming alloys with manganese or cadmium metals. Electrical and optical properties of this material have been extensively examined, both in intrinsic form and in zinc or selenium treated samples. In a search for p type ZnSe, some authors reported n to p type conversion by treating the as grown material in selenium [2]. However, in Se treated ZnSe, photo~ndu~tivity, which is an important property from the photonic device point of view, has not been thoroughly examined. An excess of group VI elements in some insulators has two effects. It induces impurity states within the band gap and, as has recently been reported [3,4], it creates high energy states in the valence band (v.b.). It is necessary, therefore, to examine the photoconductivity spectrum on the high energy side of the band gap of the Se treated ZnSe single crystal, where the transition from the spin-orbit (s.o.) v.b. structure is also expected (A,, for cubic ZnSe has been calculated to be about 0.43 eV [S]). Such a study has not been reported so far for ZnSe. The purpose of the present work, therefore, is to examine the photoresponse of an Se treated ZnSe monocrystal from this point of view. Moreover, an increase of the photoresponse towards the UV region is a desirable feature for solid state photodet~tors, for military purposes and space research among many other applications. EXPERIMENTAL Crystals were grown by the chemical vapour transport (CVT) technique which produced good quality
optical absorption, valence band states.
ZnSe crystal boules [6]. The selenium-rich samples were prepared by annealing them at 800°C for 24 h in a selenium atmosphere (3.5 atm of Se pressure). For this purpose, selenium pellets (99.999%) obtained from Aifa Products were used. Optical absorption and phot~onductivity spectra were measured in a conventional manner. Usually the investigation of the high density states within the v.b., by optical absorption spectroscopy, is a rather difficult task as the band edge masks the possible structures located on the high energy side and the absorption coefficient is very high. However, in the present case, these structures were revealed using a computer controlled spectrometer (Spex 1702, with a Datamate console model DM-I). Each spectrum was scanned 10 times and examined by running average procedures. This reduces the noise dramatically and renders the peaks visible. The present experimental technique, and the use of a high sensitive GaAs thermoelectrically cooled photomultiplier, permitted one to resolve the spectrum on the high energy side of the band gap even though the thickness of the sample was about 1 mm. The photoconductivity spectrum was recorded by a computer-controlled apparatus, the details of which are given in a previous publication [7,8]. RESULTS AND DISCUSSION Figure 1 shows the normalized phot~ondu~~vity spectrum of the Se treated ZnSe crystal. The peak photoresponse was of the order of I-2nA per PW x cmm2 of light intensity. Since the band gap of ZnSe at R.T. is 2.7 eV (460 nm), clearly the weak peak B corresponds to the band gap transition. It is worth mentioning that in addition to this fundamental transition, three peaks are clearly visible. A high
F. h~~cio et al.
576
WAVELENGTH
Fig.
1.
(nm)
Normalized photoconductivity spectrum of an Se treated ZnSe single crystal recorded at room temperature.
intensity peak A is located at 415 nm (2.98 eV), i.e. on the high energy side, separated by 0.31 eV above the band gap value. The so. splitting for this material should give a peak at 3.13 eV (396 nm); it is clear that in the present case, peak A cannot be attributed to the S.O. splitting. Moreover, this peak is present only in selenium treated samples, hence, its origin is associated with an excess of selenium atoms. These atoms will occupy substitutional sites; however, because of their high diffusion coefficient, they migrate very easily and a considerable number of them are located at interstitial positions. Theoretically [9] it has been found that, like sulphur in CdInzS, [3], group VI atoms create additional states below the top of the v.b. and their origin is attributed to the presence of upper s states of elements of group VI; in the present case, 4s states of selenium. Peak A, therefore, is associated with transitions from these lower v.b. levels to the bottom of the conduction band. Relative intensities of peaks A and B show that the higher contribution comes from the transition located on the high energy side of the band gap. This confirms the importance of excess selenium for photosensitivity improvement processes in ZnSe. It is known that ZnSe (stoichiometric and impurity free) is a poor photoconductor. We have tried to carry out photoconductivity measurements on a grown sample [6], but no photoresponse could be obtained. Only after selenium annealing was it possible to carry out the photoconductivity measurements. Towards the low energy side, a weak peak, C, is observed at about 485 nm (2.55 eV). This peak has been reported only in Se treated ZnSe and an exten-
sive study of it has been carried out by Kishida et al. [lo]. Photoluminescence analyses, including variation of the peak position with respect to the intensity of the excitation source along with time resolved studies, show that this band does not originate from donor-acceptor pair recombinations, but is due to transitions from the selenium-induced states within the band gap. There is a broad band D centred at about 570 nm (2.1 ev). In the last two decades, much work has been carried out in order to investigate the origin for this band, but, in spite of this, its origin remains an open issue. Very recently, Kishida et al. [ll] have also observed the same peak in zinc-treated samples and concluded that the origin could be attributed to selenium vacancies. In the present case, we have noticed this peak even in a selenium-rich sample, suggesting that the origin must be different. The appearance of this structure in the photoconductivity spectrum and its very broad nature, suggest that there could be a semi-continuum set of levels formed by the native defect states which take part in the photoconduction and photoluminescence processes. Earlier experimental works on optical absorption and photoconductivity confirm that annealing creates high density of states, but their origin is not clear. It has already been inferred that ZnSe crystals may have a continuous distribution of trapping levels below the bottom of the conduction band between 0.2 and 0.9 eV with increased concentrations at particular values [12]. We therefore tentatively associate this broad D band with transitions from an equivalent semicontinuum set of levels within the band gap
Photoconductivity
410
and optical absorption on a ZnSe monocrystal
420
577
440
430 WAVELENQTH
hm)
Fig. 2. Optical absorption of the high energy side of the spectrum of an Se treated ZnSe single crystal
recorded at room temperature.
to the conduction band; this set would then be situated in between 0.4 and 0.7 eV below the bottom of the conduction band, very close to the well-known copper sensitizing centres. However, precise location of these states with respect to the v.b. is not possible with the present experimental data. In order to obtain additional experimental confirmation for the density of states located below the top of the v.b., an experimental investigation, on the high energy side, of the optical absorption coefficient of the same sample, was carried out, and a selected portion of the corresponding spectrum is shown in Fig. 2. The absorption coefficient is expressed in arbitrary units as we are only interested in locating the positions of the peaks and hence, the energy states. No structures were observed above the band gap in the original as grown material for which optical absorption was reported in the visible [13] and in the IR [l]. Peak A (at 422 nm), which is very intense, is associated with the high density of states caused by the excess of selenium 4s electrons and is located very close to the split so. structure of the v.b. The E peak, located at 449 nm (2.76 eV), may be associated with a rlsv -*rlc transition [14]. A detailed analysis of this structure is not pertinent here.
the high energy side of the spectrum. This is attributed to the high density of states created in the lower part of the valence band due to the selenium 4 s states.
Acknowledgements-Financial support by both C.D.C.H.T.U.L.A. and Conicit-Venezuela is greatly appreciated. This research project was carried out under a cooperation agreement between Conacyt-Mexico and ConicitVenezuela. One of us, F.R., is grateful to J. J. Giambiagi president of the Centro Latinoamericano de Fisica (C.L.A.F.).
REFERENCES 1. Rabago F., Vincent A. B. and Joshi N. V., Mat. Lett. 9. 480 (1990). Yu P. %. and Park Y. S., Appl. Phys. L&t. 22, 345 (1973).
6. 7.
CONCLUSIONS
8.
The photoresponse of ZnSe increases substantially when treated with selenium and shifts towards
9.
Vincent A. B., Rodriguez C. E. and Joshi N. V., Can. J. Phys. 62, 883 (1984). Joshi N. V., Photoconductivity: Art, Science & Technology, p. 276. Marcel Dekker, New York (1990). Segall B. and Marple D. T. F. in Physics and Chemistry of II-VI Compounds (Edited by M. Aven and J. S. Prener), p. 345. N.H.P.C., Amsterdam (1976). Triboulet R., Rdbago F., Legros R., Lozykowski H. and Didier G., J. Crys. Growth 59, 172 (1982). Barreto D., Luengo J., De Vita Y. and Joshi N. V., Appl. Opt. 26, 5280 (1987). Joshi N. V., Photoconductivity: Art, Science & Technology, p. 50. Marcel Dekker, New York (1990). Cartling B. G., J. Phys. C 8, 3183 (1975).
578
F.
RABAGO
10. Kishida S., Matsuura K., Mori H., Yanagawa T., Tsurumi I. and Hamaguchi C., Phys. Stat. Sol. (a) 106, 283 (1988). 11. Kishida S., Matsuura K., Nagase H. and Tsurumi I., Phys. Stat. Sol. (a) 103, 613 (1987). 12. Vincent A. B., Electrical properties of zinc selenide, Durham University Thesis (U.K.), p. 98 (1980).
et
al.
13. Conde-Gallardo A., Hemandez-Calderbn 1. and Rabago F., Proceedings of the 9th National Meeting of Surface Science, Mexico, 48 (1989). 14. Herman F., Kortum R. L., Kuglin C. D. and Shay J. L., in II-VI Semiconducting Compounds (Edited by D. G. Thomas), p. 503. Benjamin, New York (1967).
1991
0022.3697/91 $3.00 + 0.00 Q 1991 Pergamon Fves plc
PHOTOCONDUCTIVrTY AND OPTICAL ABSORPTION ON THE HIGH ENERGY SIDE IN AN Se TREATED ZnSe MONOCRYSTAL F. RABACO,~ J. M. MARTIN,$ A. B. VINCENTJ: and N. V. JOSHIS tmstituto de Fisica, Universidad Autbnoma de San Luis Potosi, av. A. Obregen 64, 78000 San Luis Potosi, S.L.P., Mexico SCentro de Estudios Avanzados en Optica, Fact&ad de Ciencias, Universidad de 10s Andes, M&da, Venezuela (Received 15 June 1990; accepted 26 September 1990)
Abstract-Photoconductivity and optical absorption spectra of selenium treated ZnSe were examined. It was found that an excess of selenium increases the photoresponse substantially, particularly on the high energy side of the band gap. This is attributed to the transition from a high density of states below the top of the valence band to the bottom of the conduction band. The presence of this high density of states is also confirmed from an optical absorption study. ~ey~v~d~~ Zinc selenide, single crystal, p~otoconductivity,
INTRODUCTION Zinc selenide is a wide gap semiconductor with interesting optical properties. It is a good material for IR windows 111.Potential applications are increasing strongly as recently super-lattice and micro structures have been suc.cessfuIly obtained by forming alloys with manganese or cadmium metals. Electrical and optical properties of this material have been extensively examined, both in intrinsic form and in zinc or selenium treated samples. In a search for p type ZnSe, some authors reported n to p type conversion by treating the as grown material in selenium [2]. However, in Se treated ZnSe, photo~ndu~tivity, which is an important property from the photonic device point of view, has not been thoroughly examined. An excess of group VI elements in some insulators has two effects. It induces impurity states within the band gap and, as has recently been reported [3,4], it creates high energy states in the valence band (v.b.). It is necessary, therefore, to examine the photoconductivity spectrum on the high energy side of the band gap of the Se treated ZnSe single crystal, where the transition from the spin-orbit (s.o.) v.b. structure is also expected (A,, for cubic ZnSe has been calculated to be about 0.43 eV [S]). Such a study has not been reported so far for ZnSe. The purpose of the present work, therefore, is to examine the photoresponse of an Se treated ZnSe monocrystal from this point of view. Moreover, an increase of the photoresponse towards the UV region is a desirable feature for solid state photodet~tors, for military purposes and space research among many other applications. EXPERIMENTAL Crystals were grown by the chemical vapour transport (CVT) technique which produced good quality
optical absorption, valence band states.
ZnSe crystal boules [6]. The selenium-rich samples were prepared by annealing them at 800°C for 24 h in a selenium atmosphere (3.5 atm of Se pressure). For this purpose, selenium pellets (99.999%) obtained from Aifa Products were used. Optical absorption and phot~onductivity spectra were measured in a conventional manner. Usually the investigation of the high density states within the v.b., by optical absorption spectroscopy, is a rather difficult task as the band edge masks the possible structures located on the high energy side and the absorption coefficient is very high. However, in the present case, these structures were revealed using a computer controlled spectrometer (Spex 1702, with a Datamate console model DM-I). Each spectrum was scanned 10 times and examined by running average procedures. This reduces the noise dramatically and renders the peaks visible. The present experimental technique, and the use of a high sensitive GaAs thermoelectrically cooled photomultiplier, permitted one to resolve the spectrum on the high energy side of the band gap even though the thickness of the sample was about 1 mm. The photoconductivity spectrum was recorded by a computer-controlled apparatus, the details of which are given in a previous publication [7,8]. RESULTS AND DISCUSSION Figure 1 shows the normalized phot~ondu~~vity spectrum of the Se treated ZnSe crystal. The peak photoresponse was of the order of I-2nA per PW x cmm2 of light intensity. Since the band gap of ZnSe at R.T. is 2.7 eV (460 nm), clearly the weak peak B corresponds to the band gap transition. It is worth mentioning that in addition to this fundamental transition, three peaks are clearly visible. A high
F. h~~cio et al.
576
WAVELENGTH
Fig.
1.
(nm)
Normalized photoconductivity spectrum of an Se treated ZnSe single crystal recorded at room temperature.
intensity peak A is located at 415 nm (2.98 eV), i.e. on the high energy side, separated by 0.31 eV above the band gap value. The so. splitting for this material should give a peak at 3.13 eV (396 nm); it is clear that in the present case, peak A cannot be attributed to the S.O. splitting. Moreover, this peak is present only in selenium treated samples, hence, its origin is associated with an excess of selenium atoms. These atoms will occupy substitutional sites; however, because of their high diffusion coefficient, they migrate very easily and a considerable number of them are located at interstitial positions. Theoretically [9] it has been found that, like sulphur in CdInzS, [3], group VI atoms create additional states below the top of the v.b. and their origin is attributed to the presence of upper s states of elements of group VI; in the present case, 4s states of selenium. Peak A, therefore, is associated with transitions from these lower v.b. levels to the bottom of the conduction band. Relative intensities of peaks A and B show that the higher contribution comes from the transition located on the high energy side of the band gap. This confirms the importance of excess selenium for photosensitivity improvement processes in ZnSe. It is known that ZnSe (stoichiometric and impurity free) is a poor photoconductor. We have tried to carry out photoconductivity measurements on a grown sample [6], but no photoresponse could be obtained. Only after selenium annealing was it possible to carry out the photoconductivity measurements. Towards the low energy side, a weak peak, C, is observed at about 485 nm (2.55 eV). This peak has been reported only in Se treated ZnSe and an exten-
sive study of it has been carried out by Kishida et al. [lo]. Photoluminescence analyses, including variation of the peak position with respect to the intensity of the excitation source along with time resolved studies, show that this band does not originate from donor-acceptor pair recombinations, but is due to transitions from the selenium-induced states within the band gap. There is a broad band D centred at about 570 nm (2.1 ev). In the last two decades, much work has been carried out in order to investigate the origin for this band, but, in spite of this, its origin remains an open issue. Very recently, Kishida et al. [ll] have also observed the same peak in zinc-treated samples and concluded that the origin could be attributed to selenium vacancies. In the present case, we have noticed this peak even in a selenium-rich sample, suggesting that the origin must be different. The appearance of this structure in the photoconductivity spectrum and its very broad nature, suggest that there could be a semi-continuum set of levels formed by the native defect states which take part in the photoconduction and photoluminescence processes. Earlier experimental works on optical absorption and photoconductivity confirm that annealing creates high density of states, but their origin is not clear. It has already been inferred that ZnSe crystals may have a continuous distribution of trapping levels below the bottom of the conduction band between 0.2 and 0.9 eV with increased concentrations at particular values [12]. We therefore tentatively associate this broad D band with transitions from an equivalent semicontinuum set of levels within the band gap
Photoconductivity
410
and optical absorption on a ZnSe monocrystal
420
577
440
430 WAVELENQTH
hm)
Fig. 2. Optical absorption of the high energy side of the spectrum of an Se treated ZnSe single crystal
recorded at room temperature.
to the conduction band; this set would then be situated in between 0.4 and 0.7 eV below the bottom of the conduction band, very close to the well-known copper sensitizing centres. However, precise location of these states with respect to the v.b. is not possible with the present experimental data. In order to obtain additional experimental confirmation for the density of states located below the top of the v.b., an experimental investigation, on the high energy side, of the optical absorption coefficient of the same sample, was carried out, and a selected portion of the corresponding spectrum is shown in Fig. 2. The absorption coefficient is expressed in arbitrary units as we are only interested in locating the positions of the peaks and hence, the energy states. No structures were observed above the band gap in the original as grown material for which optical absorption was reported in the visible [13] and in the IR [l]. Peak A (at 422 nm), which is very intense, is associated with the high density of states caused by the excess of selenium 4s electrons and is located very close to the split so. structure of the v.b. The E peak, located at 449 nm (2.76 eV), may be associated with a rlsv -*rlc transition [14]. A detailed analysis of this structure is not pertinent here.
the high energy side of the spectrum. This is attributed to the high density of states created in the lower part of the valence band due to the selenium 4 s states.
Acknowledgements-Financial support by both C.D.C.H.T.U.L.A. and Conicit-Venezuela is greatly appreciated. This research project was carried out under a cooperation agreement between Conacyt-Mexico and ConicitVenezuela. One of us, F.R., is grateful to J. J. Giambiagi president of the Centro Latinoamericano de Fisica (C.L.A.F.).
REFERENCES 1. Rabago F., Vincent A. B. and Joshi N. V., Mat. Lett. 9. 480 (1990). Yu P. %. and Park Y. S., Appl. Phys. L&t. 22, 345 (1973).
6. 7.
CONCLUSIONS
8.
The photoresponse of ZnSe increases substantially when treated with selenium and shifts towards
9.
Vincent A. B., Rodriguez C. E. and Joshi N. V., Can. J. Phys. 62, 883 (1984). Joshi N. V., Photoconductivity: Art, Science & Technology, p. 276. Marcel Dekker, New York (1990). Segall B. and Marple D. T. F. in Physics and Chemistry of II-VI Compounds (Edited by M. Aven and J. S. Prener), p. 345. N.H.P.C., Amsterdam (1976). Triboulet R., Rdbago F., Legros R., Lozykowski H. and Didier G., J. Crys. Growth 59, 172 (1982). Barreto D., Luengo J., De Vita Y. and Joshi N. V., Appl. Opt. 26, 5280 (1987). Joshi N. V., Photoconductivity: Art, Science & Technology, p. 50. Marcel Dekker, New York (1990). Cartling B. G., J. Phys. C 8, 3183 (1975).
578
F.
RABAGO
10. Kishida S., Matsuura K., Mori H., Yanagawa T., Tsurumi I. and Hamaguchi C., Phys. Stat. Sol. (a) 106, 283 (1988). 11. Kishida S., Matsuura K., Nagase H. and Tsurumi I., Phys. Stat. Sol. (a) 103, 613 (1987). 12. Vincent A. B., Electrical properties of zinc selenide, Durham University Thesis (U.K.), p. 98 (1980).
et
al.
13. Conde-Gallardo A., Hemandez-Calderbn 1. and Rabago F., Proceedings of the 9th National Meeting of Surface Science, Mexico, 48 (1989). 14. Herman F., Kortum R. L., Kuglin C. D. and Shay J. L., in II-VI Semiconducting Compounds (Edited by D. G. Thomas), p. 503. Benjamin, New York (1967).