- Email: [email protected]
Photodarkening of amorphous Se and Se95Te5 films under μs light pulses
Optics Communications 283 (2010) 1366–1369
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Photodarkening of amorphous Se and Se95Te5 films under ls light pulses E. Flaxer a,*, M. Klebanov b, V. Lyubin b, D. Abrahamoff c a
Afeka-Tel-Aviv Academic College of Engineering, 69107 Tel-Aviv, Israel Department of Physics, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel c Dynamic Media Solutions Laboratory, 69512 Tel-Aviv, Israel b
a r t i c l e
i n f o
Article history: Received 31 August 2009 Accepted 1 December 2009
Keywords: Thin amorphous films Photodarkening Pulse photorecording
a b s t r a c t Photodarkening phenomena in the amorphous Se and Se95Te5 films at the ls light pulses irradiation are investigated. The process of photodarkening during each of the individual 1, 10 or 100 ls pulses and also the processes proceeding between successive pulses are recorded and analyzed. The darkened film was shown to restore initial optical properties after some stay in darkness without any additional irradiation or heating. Photodarkening is very strongly dependent on the light pulse fluence. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Photodarkening (PD) effect in the films of binary chalcogenide glasses (e.g. As2Se3, As2S3, GeSe2) has been studied by many researchers [1,2 and references therein]. The PD effect in the films of amorphous selenium (a-Se) that is a simplest chalcogenide glass, has previously only been observed under cooling and was not found at room temperature [3–5]. Later, by application of the two beam technique, it was shown that PD in a-Se films can also be induced at room temperature [6,7]. Recently, there has been a growing interest in photoinduced processes in a-Se, due to the expanding field of the application of these films in the very sensitive vidicon camera tubes for television broadcasting as well as for medical X-ray imaging applications [8]. In all previous works the PD in a-Se films was only studied under CW light irradiation and there are no data concerning the behavior of these films under short light pulses. Recently, pulse excitation of binary chalcogenide films showed many peculiarities in the PD process [9,10], therefore it was interesting to study processes of short pulse excitation of PD in the a-Se films. In this letter, we introduce the first results of investigation of PD in a-Se films under the action of ls light pulses.
cleaned Corning glass substrates, from quarts crucibles, in a vacuum of 2 10 6 Torr. The films had a thickness of 0.5–2.0 lm. PD phenomena in both types of films, under CW and pulse irradiation, were practically similar although Se95Te5 films were more stable and therefore, in many cases, we preferred to study these films only. For excitation, the pulse laser (Stocker Yale Canada Inc), working on the wavelength 650 nm was used. The laser could generate 1.0–100 ls pulses with a frequency of up to 100 Hz. Maximum light intensity was 16 kW/cm2 at the diameter of the beam of 7.0 lm. We observed the shape and amplitude of light pulses transmitted through the studied film on the screen of digital storage oscilloscope (Keithley – PCI-433) and recorded them to the personal computer for analysis. The sampling interval during the measurement was 10 ns. The measurement of the electrical signal value during the pulse was carried out with an accuracy of not less than 0.1%. Magnitude of transmission T was determined as a ratio of fluencies of transmitted and falling pulses. Such a system allowed the observation of the shape and amplitude of each chosen pulse. All experiments were carried out at room temperature.
3. Experimental results 2. Experimental We investigated amorphous Se and Se95Te5 films, fabricated by thermal evaporation of glassy Se and Se95Te5 powders on carefully * Corresponding author. Tel.: +972 3 6449344; fax: +972 3 6480944. E-mail address: fl[email protected] (E. Flaxer). 0030-4018/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2009.12.007
Fig. 1 shows several pulses from a typical series of 1 ls successive pulses of light, transmitted through the Se95Te5 film having a thickness of 1 lm. Energy fluence of each exciting light pulse is 16 kW/cm2, whilst the rate of the pulses is 1 Hz. We demonstrate here the 1st, 10th, 50th and 200th pulse. During each pulse we did not observe any visible darkening, however, there is a very
1367
E. Flaxer et al. / Optics Communications 283 (2010) 1366–1369
0.8 1
0.7
Transmission
0.6
10
0.5
50
0.4
200
0.3 0.2 0.1 0 0
0.5
1
1.5
2
Time (μs) Fig. 1. Series of successive 1 ls pulses of light with intensity of 16 kW/cm2 transmitted through the Se95Te5 film with a thickness of 1 lm. Ordinal numbers of pulses are shown near each pulse.
essential accumulation of darkening at irradiation with the series of light pulses. This accumulation is especially strongly expressed during the first ten pulses, gradually decreasing later. Fig. 2 shows several similar successive pulses of transmitted light with a diminished fluence of each pulse to 12 kW/cm2, keeping the same pulse rate of 1 Hz. Here the 1st, 10th and 100th pulses (as well as all following pulses) are indistinguishable, meaning that even at so small a decrease of the light pulse fluence the PD is practically not observed. In Fig. 3 we demonstrate a typical PD of the a-Se film with a thickness of 1 lm after absorption of a series of 1 ls pulses with the pulses rate of 1 Hz. It is seen that the successive pulses cause gradual darkening. The light interruption after 200 and 250 pulses is accompanied by the increase of the film transparency (in the
Fig. 3 the light was interrupted for 1 min and a more prolonged interruption led to a stronger transparency increase, until the full bleaching of the film). Following pulse irradiation after the light interruption indicated continuation of PD. Unlike the situation with short light pulses, irradiation of films with longer light pulses in the ls range showed darkening during the pulse. Such an effect is illustrated in Fig. 4 for the case of a series of 10 ls pulses excited the Se95Te5 film having thickness of 0.7 lm. It is seen, that during each of the pulses, transparency of the film is essentially diminished. At the start of 50th and 300th pulses, the transparency of the film was aspiring to the initial transparency, while the PD of the film is gradually increased. The light interruption after 3 min, for 300th pulses, is accompanied by the practically complete restoration of initial film transparency.
0.8 0.7
1 10 100
Transmission
0.6 0.5 0.4 0.3 0.2 0.1 0 0
0.2
0.4
0.6
0.8
1 1.2 Time (µs)
1.4
1.6
1.8
2
Fig. 2. Series of successive 1 ls light pulses with intensity of 12 kW/cm2 transmitted through the Se95Te5 film with a thickness of 1 lm. Ordinal numbers of pulses are shown in the frame.
1368
E. Flaxer et al. / Optics Communications 283 (2010) 1366–1369
Relative Trasmission (T / To)
1 Δt = 1 min. 0.9
0.8
0.7
0
40
80
120
160
200
240
280
320
Pulses Number (n) Fig. 3. Photodarkening of the a-Se film with a thickness of 1 lm after absorbing a series of 1 ls pulses with the pulses rate 1 Hz. After 200 and 250 pulses the exciting light was interrupted for 1 min.
0.5 Δt = 3 min
Transmission
0.4
Δt = 3 min
1
1
1
0.3 50
50
0.2 300
300
300
0.1
0
0
5
10
15 20 Time (μs)
25
30
35
Fig. 4. Series of successive 10 ls pulses of light with intensity of 16 kW/cm2 transmitted through the Se95Te5 film with a thickness of 0.7 lm. Ordinal numbers of pulses are shown near each pulse.
Fig. 5 shows the PD of the a-Se film having a thickness of 1 lm during one 100 ls pulse. It is seen that the transparency of the film is almost doubly diminished. The light interruption in this case also led to the gradual restoration of the initial film transparency. 4. Discussion In these experiments we could study the process of PD during each of the individual 1, 10 or 100 ls pulses and could also reach the conclusion regarding the processes proceeding between successive pulses. We demonstrated that irradiation of the amorphous
Se and Se95Te5 films, with 1 ls light pulses, does not lead to any darkening during the pulse, but at the same time we observed a very essential accumulation of darkening as a result of irradiation with a series of light pulses. The results demonstrated in Fig. 2 shows that the PD process is very strongly dependent on the light pulse fluence and the PD decreases sharply with the diminishing of the pulse fluence. Single 10 and 100 ls light pulses are able to essentially darken the film. It is extremely important that, in all considered cases, the photodarkened films can restore initial optical properties after being in darkness for some time, without any additional irradiation or heating, i.e. display a self-bleaching effect.
1369
E. Flaxer et al. / Optics Communications 283 (2010) 1366–1369
0.8 0.7
Transmission
0.6 0.5 0.4 0.3 0.2 0.1
0 0
20
40
60 Time (μs)
80
100
120
Fig. 5. Photodarkening of the a-Se film with a thickness of 1 lm during one 100 ls light pulse with intensity of 16 kW/cm2.
It is interesting to compare the action of short light pulses on amorphous Se and Se95Te5 films with those on the binary chalcogenide glassy films, initially the As50Se50 films studied in [9–11]. In the As50Se50 films irradiation by the 5 ns light pulses did not lead to essential darkening during the pulse, the darkening only appearing later, following several microseconds [9,11]. Irradiation by the 1 ls light pulses showed both small PD during each pulse and a strong accumulation of darkening during the following pulses [10]. Such behavior of As50Se50 films was explained in the following manner: initial 5 ns pulse created free electrons and holes which did not seriously change the optical properties of the films but the following strong (accumulated) darkening on the ls time scale was due to structural transformations, induced by the generated electron–hole pairs causing the breaking of inter-atomic bonds [9,11]. The accumulated level of darkening was very stable. We can assume a similar explanation for the PD processes in the Se and Se95Te5 films. In this case, 1 ls light pulse generates electron–hole pairs which cause the following structural transformations accompanied by the change of optical properties, both the increase of absorption coefficient, as it is shown in this paper, and the increase of refractive index, as was recently discovered [12]. There are two differences between photoinduced effects in both types of disordered films: the quantitative difference in the time scales and the appearance of self-bleaching in the Se and Se95Te5 films instead of stable darkening in the As50Se50 films. These diversities are caused by different inter-atomic bonding in both kinds of film (a homobonding in the Se and Se95Te5 films and a heterobonding in the As50Se50 films) displayed in different kinds of photoinduced defects. As it is known, the creation of these defects is the basis of the PD phenomena [1,13,14]. 5. Conclusion In this letter we introduced the first results of the investigation of PD in amorphous Se and Se95Te5 films under the action of ls
light pulses. This study revealed many common effects in the short light pulses induced PD, observed in different amorphous materials. Unlike PD in most amorphous and glassy films, photodarkened Se and Se95Te5 films restore initial optical properties after a period in darkness, without any additional irradiation or heating. The revealed distinctions are assumed to be caused by different inter-atomic bonding in the studied films and in the films of binary chalcogenide glasses. In conclusion, we can mention that the photosensitive amorphous Se and Se95Te5 films, due to their selfbleaching effect, could be interesting for real time optical recording and for dynamical holography. References [1] K. Shimakawa, A. Kolobov, S.R. Elliott, Adv. Phys. 44 (1995) 475. [2] M. Frumar, B. Frumarova, T. Wagner, P. Nemec, in: A.V. Kolobov (Ed.), Photoinduced Metastability in Amorphous Semiconductors, Wiley-VCH, New York, 2003, p. 23. [3] V.L. Averianov, A.V. Kolobov, B.T. Kolomiets, V.M. Lyubin, Phys. Status Solidi A 57 (1980) 81. [4] K. Tanaka, A. Odajima, Solid State Commun. 43 (1982) 962. [5] R.T. Phillips, J. Non-Cryst. Solids 70 (1985) 359. [6] A. Ganjoo, K. Shimakawa, K. Kitano, E.A. Davis, J. Non-Cryst. Solids 299–302 (2002) 917. [7] A. Reznik, B.J.M. Lui, V. Lyubin, M. Klebanov, K. Tanioka, M. Kubota, K. Miyakawa, Y. Ohkawa, J.A. Rowlands, J. Non-Cryst. Solids 352 (2006) 1595. [8] W. Zhao, D. Li, A. Reznik, B.J.M. Lui, D.C. Hunt, Y. Ohkawa, K. Tanioka, J.A. Rowlands, Med. Phys. 32 (2005) 2954. [9] G. Rosenblum, B.G. Sfez, Z. Kotler, V. Lyubin, M. Klebanov, Appl. Phys. Lett. 75 (1999) 3249. [10] E. Flaxer, M. Klebanov, D. Abrahamoff, S. Noah, V. Lyubin, Opt. Mater. 31 (2009) 688. [11] B.G. Sfez, G. Rosenblum, Z. Kotler, V. Lyubin, M. Klebanov, Mater. Sci. Semicond. Process. 3 (2000) 499. [12] A. Reznik, M. Klebanov, V. Lyubin, J. Appl. Phys. 105 (2009) 013518. [13] H. Fritzzshe, Phys. Rev. B 52 (1995) 15854. [14] A.V. Kolobov, Ka. Tanaka, in: H.S. Nalwa (Ed.), Handbook of Advanced Electronic and Photonic Materials and Devices, vol. 5, Academic Press, San Diego, 2001, p. 47.
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Photodarkening of amorphous Se and Se95Te5 films under ls light pulses E. Flaxer a,*, M. Klebanov b, V. Lyubin b, D. Abrahamoff c a
Afeka-Tel-Aviv Academic College of Engineering, 69107 Tel-Aviv, Israel Department of Physics, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel c Dynamic Media Solutions Laboratory, 69512 Tel-Aviv, Israel b
a r t i c l e
i n f o
Article history: Received 31 August 2009 Accepted 1 December 2009
Keywords: Thin amorphous films Photodarkening Pulse photorecording
a b s t r a c t Photodarkening phenomena in the amorphous Se and Se95Te5 films at the ls light pulses irradiation are investigated. The process of photodarkening during each of the individual 1, 10 or 100 ls pulses and also the processes proceeding between successive pulses are recorded and analyzed. The darkened film was shown to restore initial optical properties after some stay in darkness without any additional irradiation or heating. Photodarkening is very strongly dependent on the light pulse fluence. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Photodarkening (PD) effect in the films of binary chalcogenide glasses (e.g. As2Se3, As2S3, GeSe2) has been studied by many researchers [1,2 and references therein]. The PD effect in the films of amorphous selenium (a-Se) that is a simplest chalcogenide glass, has previously only been observed under cooling and was not found at room temperature [3–5]. Later, by application of the two beam technique, it was shown that PD in a-Se films can also be induced at room temperature [6,7]. Recently, there has been a growing interest in photoinduced processes in a-Se, due to the expanding field of the application of these films in the very sensitive vidicon camera tubes for television broadcasting as well as for medical X-ray imaging applications [8]. In all previous works the PD in a-Se films was only studied under CW light irradiation and there are no data concerning the behavior of these films under short light pulses. Recently, pulse excitation of binary chalcogenide films showed many peculiarities in the PD process [9,10], therefore it was interesting to study processes of short pulse excitation of PD in the a-Se films. In this letter, we introduce the first results of investigation of PD in a-Se films under the action of ls light pulses.
cleaned Corning glass substrates, from quarts crucibles, in a vacuum of 2 10 6 Torr. The films had a thickness of 0.5–2.0 lm. PD phenomena in both types of films, under CW and pulse irradiation, were practically similar although Se95Te5 films were more stable and therefore, in many cases, we preferred to study these films only. For excitation, the pulse laser (Stocker Yale Canada Inc), working on the wavelength 650 nm was used. The laser could generate 1.0–100 ls pulses with a frequency of up to 100 Hz. Maximum light intensity was 16 kW/cm2 at the diameter of the beam of 7.0 lm. We observed the shape and amplitude of light pulses transmitted through the studied film on the screen of digital storage oscilloscope (Keithley – PCI-433) and recorded them to the personal computer for analysis. The sampling interval during the measurement was 10 ns. The measurement of the electrical signal value during the pulse was carried out with an accuracy of not less than 0.1%. Magnitude of transmission T was determined as a ratio of fluencies of transmitted and falling pulses. Such a system allowed the observation of the shape and amplitude of each chosen pulse. All experiments were carried out at room temperature.
3. Experimental results 2. Experimental We investigated amorphous Se and Se95Te5 films, fabricated by thermal evaporation of glassy Se and Se95Te5 powders on carefully * Corresponding author. Tel.: +972 3 6449344; fax: +972 3 6480944. E-mail address: fl[email protected] (E. Flaxer). 0030-4018/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2009.12.007
Fig. 1 shows several pulses from a typical series of 1 ls successive pulses of light, transmitted through the Se95Te5 film having a thickness of 1 lm. Energy fluence of each exciting light pulse is 16 kW/cm2, whilst the rate of the pulses is 1 Hz. We demonstrate here the 1st, 10th, 50th and 200th pulse. During each pulse we did not observe any visible darkening, however, there is a very
1367
E. Flaxer et al. / Optics Communications 283 (2010) 1366–1369
0.8 1
0.7
Transmission
0.6
10
0.5
50
0.4
200
0.3 0.2 0.1 0 0
0.5
1
1.5
2
Time (μs) Fig. 1. Series of successive 1 ls pulses of light with intensity of 16 kW/cm2 transmitted through the Se95Te5 film with a thickness of 1 lm. Ordinal numbers of pulses are shown near each pulse.
essential accumulation of darkening at irradiation with the series of light pulses. This accumulation is especially strongly expressed during the first ten pulses, gradually decreasing later. Fig. 2 shows several similar successive pulses of transmitted light with a diminished fluence of each pulse to 12 kW/cm2, keeping the same pulse rate of 1 Hz. Here the 1st, 10th and 100th pulses (as well as all following pulses) are indistinguishable, meaning that even at so small a decrease of the light pulse fluence the PD is practically not observed. In Fig. 3 we demonstrate a typical PD of the a-Se film with a thickness of 1 lm after absorption of a series of 1 ls pulses with the pulses rate of 1 Hz. It is seen that the successive pulses cause gradual darkening. The light interruption after 200 and 250 pulses is accompanied by the increase of the film transparency (in the
Fig. 3 the light was interrupted for 1 min and a more prolonged interruption led to a stronger transparency increase, until the full bleaching of the film). Following pulse irradiation after the light interruption indicated continuation of PD. Unlike the situation with short light pulses, irradiation of films with longer light pulses in the ls range showed darkening during the pulse. Such an effect is illustrated in Fig. 4 for the case of a series of 10 ls pulses excited the Se95Te5 film having thickness of 0.7 lm. It is seen, that during each of the pulses, transparency of the film is essentially diminished. At the start of 50th and 300th pulses, the transparency of the film was aspiring to the initial transparency, while the PD of the film is gradually increased. The light interruption after 3 min, for 300th pulses, is accompanied by the practically complete restoration of initial film transparency.
0.8 0.7
1 10 100
Transmission
0.6 0.5 0.4 0.3 0.2 0.1 0 0
0.2
0.4
0.6
0.8
1 1.2 Time (µs)
1.4
1.6
1.8
2
Fig. 2. Series of successive 1 ls light pulses with intensity of 12 kW/cm2 transmitted through the Se95Te5 film with a thickness of 1 lm. Ordinal numbers of pulses are shown in the frame.
1368
E. Flaxer et al. / Optics Communications 283 (2010) 1366–1369
Relative Trasmission (T / To)
1 Δt = 1 min. 0.9
0.8
0.7
0
40
80
120
160
200
240
280
320
Pulses Number (n) Fig. 3. Photodarkening of the a-Se film with a thickness of 1 lm after absorbing a series of 1 ls pulses with the pulses rate 1 Hz. After 200 and 250 pulses the exciting light was interrupted for 1 min.
0.5 Δt = 3 min
Transmission
0.4
Δt = 3 min
1
1
1
0.3 50
50
0.2 300
300
300
0.1
0
0
5
10
15 20 Time (μs)
25
30
35
Fig. 4. Series of successive 10 ls pulses of light with intensity of 16 kW/cm2 transmitted through the Se95Te5 film with a thickness of 0.7 lm. Ordinal numbers of pulses are shown near each pulse.
Fig. 5 shows the PD of the a-Se film having a thickness of 1 lm during one 100 ls pulse. It is seen that the transparency of the film is almost doubly diminished. The light interruption in this case also led to the gradual restoration of the initial film transparency. 4. Discussion In these experiments we could study the process of PD during each of the individual 1, 10 or 100 ls pulses and could also reach the conclusion regarding the processes proceeding between successive pulses. We demonstrated that irradiation of the amorphous
Se and Se95Te5 films, with 1 ls light pulses, does not lead to any darkening during the pulse, but at the same time we observed a very essential accumulation of darkening as a result of irradiation with a series of light pulses. The results demonstrated in Fig. 2 shows that the PD process is very strongly dependent on the light pulse fluence and the PD decreases sharply with the diminishing of the pulse fluence. Single 10 and 100 ls light pulses are able to essentially darken the film. It is extremely important that, in all considered cases, the photodarkened films can restore initial optical properties after being in darkness for some time, without any additional irradiation or heating, i.e. display a self-bleaching effect.
1369
E. Flaxer et al. / Optics Communications 283 (2010) 1366–1369
0.8 0.7
Transmission
0.6 0.5 0.4 0.3 0.2 0.1
0 0
20
40
60 Time (μs)
80
100
120
Fig. 5. Photodarkening of the a-Se film with a thickness of 1 lm during one 100 ls light pulse with intensity of 16 kW/cm2.
It is interesting to compare the action of short light pulses on amorphous Se and Se95Te5 films with those on the binary chalcogenide glassy films, initially the As50Se50 films studied in [9–11]. In the As50Se50 films irradiation by the 5 ns light pulses did not lead to essential darkening during the pulse, the darkening only appearing later, following several microseconds [9,11]. Irradiation by the 1 ls light pulses showed both small PD during each pulse and a strong accumulation of darkening during the following pulses [10]. Such behavior of As50Se50 films was explained in the following manner: initial 5 ns pulse created free electrons and holes which did not seriously change the optical properties of the films but the following strong (accumulated) darkening on the ls time scale was due to structural transformations, induced by the generated electron–hole pairs causing the breaking of inter-atomic bonds [9,11]. The accumulated level of darkening was very stable. We can assume a similar explanation for the PD processes in the Se and Se95Te5 films. In this case, 1 ls light pulse generates electron–hole pairs which cause the following structural transformations accompanied by the change of optical properties, both the increase of absorption coefficient, as it is shown in this paper, and the increase of refractive index, as was recently discovered [12]. There are two differences between photoinduced effects in both types of disordered films: the quantitative difference in the time scales and the appearance of self-bleaching in the Se and Se95Te5 films instead of stable darkening in the As50Se50 films. These diversities are caused by different inter-atomic bonding in both kinds of film (a homobonding in the Se and Se95Te5 films and a heterobonding in the As50Se50 films) displayed in different kinds of photoinduced defects. As it is known, the creation of these defects is the basis of the PD phenomena [1,13,14]. 5. Conclusion In this letter we introduced the first results of the investigation of PD in amorphous Se and Se95Te5 films under the action of ls
light pulses. This study revealed many common effects in the short light pulses induced PD, observed in different amorphous materials. Unlike PD in most amorphous and glassy films, photodarkened Se and Se95Te5 films restore initial optical properties after a period in darkness, without any additional irradiation or heating. The revealed distinctions are assumed to be caused by different inter-atomic bonding in the studied films and in the films of binary chalcogenide glasses. In conclusion, we can mention that the photosensitive amorphous Se and Se95Te5 films, due to their selfbleaching effect, could be interesting for real time optical recording and for dynamical holography. References [1] K. Shimakawa, A. Kolobov, S.R. Elliott, Adv. Phys. 44 (1995) 475. [2] M. Frumar, B. Frumarova, T. Wagner, P. Nemec, in: A.V. Kolobov (Ed.), Photoinduced Metastability in Amorphous Semiconductors, Wiley-VCH, New York, 2003, p. 23. [3] V.L. Averianov, A.V. Kolobov, B.T. Kolomiets, V.M. Lyubin, Phys. Status Solidi A 57 (1980) 81. [4] K. Tanaka, A. Odajima, Solid State Commun. 43 (1982) 962. [5] R.T. Phillips, J. Non-Cryst. Solids 70 (1985) 359. [6] A. Ganjoo, K. Shimakawa, K. Kitano, E.A. Davis, J. Non-Cryst. Solids 299–302 (2002) 917. [7] A. Reznik, B.J.M. Lui, V. Lyubin, M. Klebanov, K. Tanioka, M. Kubota, K. Miyakawa, Y. Ohkawa, J.A. Rowlands, J. Non-Cryst. Solids 352 (2006) 1595. [8] W. Zhao, D. Li, A. Reznik, B.J.M. Lui, D.C. Hunt, Y. Ohkawa, K. Tanioka, J.A. Rowlands, Med. Phys. 32 (2005) 2954. [9] G. Rosenblum, B.G. Sfez, Z. Kotler, V. Lyubin, M. Klebanov, Appl. Phys. Lett. 75 (1999) 3249. [10] E. Flaxer, M. Klebanov, D. Abrahamoff, S. Noah, V. Lyubin, Opt. Mater. 31 (2009) 688. [11] B.G. Sfez, G. Rosenblum, Z. Kotler, V. Lyubin, M. Klebanov, Mater. Sci. Semicond. Process. 3 (2000) 499. [12] A. Reznik, M. Klebanov, V. Lyubin, J. Appl. Phys. 105 (2009) 013518. [13] H. Fritzzshe, Phys. Rev. B 52 (1995) 15854. [14] A.V. Kolobov, Ka. Tanaka, in: H.S. Nalwa (Ed.), Handbook of Advanced Electronic and Photonic Materials and Devices, vol. 5, Academic Press, San Diego, 2001, p. 47.