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Negative photoconductivity of illuminated α-Se layers
Journal of Non-Crystalline Solids 311 (2002) 42–47 www.elsevier.com/locate/jnoncrysol
Negative photoconductivity of illuminated a-Se layers Br. Petr_etis *, M. Balci unien_e Material Structure Department, Institute of Physics, Savanoriuz 231, Vilnius 2028, Lithuania Received 30 May 2001; received in revised form 7 March 2002
Abstract The investigation results of the negative conductivity effect of a-Se layers illuminated by white light are presented in this paper. It is shown that with illumination of the a-Se layer the value of the current can decrease several times as compared with the dark value of the current. This effect is observed only in the case when the voltage–current characteristic changes into the function dependence (J vs V n ), the exponent index of which n ¼ 7:7. This effect is caused by recharge of deep states when the sample is illuminated. Ó 2002 Elsevier Science B.V. All rights reserved.
1. Introduction The distribution parameters of localized states in the band gap of amorphous materials and semiconductors are among the main material properties, which are required when different semiconductor devices are created. Many semiconductor and dielectric properties of the material, such as the carrier transfer, photoconductivity, the photovoltage effect, the relaxation of the current, etc. are affected by the concentration of localized states and their energy position in the band gap. On the other hand, the distribution of the localized states of amorphous and polymeric semiconductors can be investigated by measurement and analysis [1–4]. When the kinetics of photoexcited conductivity are analyzed by means of the Laplace transform, it is possible to determine the distribution of the lo*
Corresponding author. E-mail address: brpet@delfi.lt (Br. Petr_etis).
calized states in the band gap of a wide energy range [4–6]. Much information about parameters of the localized state distribution in semiconductors can be obtained, when the relaxation of the surface and space charge [3,7–10] or the dependence of the limited space charge current on the electric field strength, the sample thickness and other parameters [1,11] are investigated. Some regions with the different functional dependence of the current on the voltage can be observed in the voltage–current characteristic measurement (VCCh). The position of break points of VCCh is conditioned by the charge transfer mechanism, the concentration of deep states and their energy location in the band gap. The investigation of the limited space charge current shows that the deep trapping levels with the energy E in a-Se are distributed with an exponential law [12–14]: Nt ðEÞ ¼ N0 exp
0022-3093/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 0 2 ) 0 1 3 2 7 - 3
E ; kTt
ð1Þ
Br. Petr_etis, M. Balciunien_e / Journal of Non-Crystalline Solids 311 (2002) 42–47
where N0 is the concentration of the states at the top of the valence band (N0 ¼ 1020 cm3 eV1 ) and kTt is the parameter of exponential distribution (kTt ¼ e0 ¼ 0:067 eV). An analogous result is obtained when the relaxation of the surface and the space charge in the amorphous selenium layers are analyzed [3,7,14]. The energy distribution of the deep layers of amorphous selenium and their concentration depend on the admixtures and on the technological conditions of the layers production. It is determined [15,16] that the energy spectrum of the generation centers of a-Se doped with the admixtures of phosphorus moves to the lower energy, if the layer is photoexcited before measurement. The deep states, which exist at the Fermi level, recharge and relax to the equilibrium state, when excitation is broken off. An analogous effect is achieved in the layers of selenium which are not doped with admixtures [17], but it is less pronounced, because the concentration of the centers which take part in the recharge is lower. In the case of the space charge limited current, the deep centers of the charge transfer generation become deep trapping centers. So, when the energy spectrum of deep centers changes the positions of characteristic break points of the current dependence on the voltage must change, because these points are caused by the mechanism of the charge transfer. The purpose of this work is to investigate the conductivity dependence of amorphous selenium layers on the intensity of illumination in the case of the space charge limited current.
2. Experimental method The samples were obtained by thermal evaporation of a selenium layer (2) in a vacuum of 103 Pa on an aluminum substrate (1) covered with a thin (1 lm thickness) As2 Se3 layer (4) or without the covering layer (Fig. 1). The aluminum substrate before evaporation was polished, deprived of fat and cleaned off with ethyl alcohol. The temperature of the substrate during the evaporation was 328–333 K, the growth rate of the layer
43
Fig. 1. The scheme of the sample electrodes: (a) the system of Al–a-Se–Au; (b) the system of Al–As2 Se3 –a-Se–Au; (c) the ionic contact of Al–a-Se; (1) the Al substrate; (2) the a-Se layer; (3) a semi-clear Au electrode; (4) a barrage layer of As2 Se3 ; (5) the ionic beam; (6) a screen.
was 50–60 nm/s and the duration of evaporation was 15–17 min. The charge carriers were injected into the layer by means of an ionic electrode (5) or by the semi-clear Au layer (3) in the sandwich-type system. The layer of As2 Se3 was in the part of a barrage layer. The aurum electrode on the layer of selenium and the barrage layer of As2 Se3 on the substrate of Al were also evaporated in a vacuum. The shape of an aurum electrode was a circle, the diameter of which was about 10–12 mm and the cross-section area of the ionic stream was from 100 to 500 mm2 . The current and photocurrent were measured with the electrometer connected to the resistivity, the nominal value of which was chosen to be about 1000 times less than the resistivity of the measured layer in the range of the measured voltage. The selenium layer was excited by exposing it to light of the incandescent filament in a standard regime. The power density of the light stream was changed by neutral optic filters. All measurements of the current were carried out under normal conditions at room temperature, when T ¼ 293 K. The random errors of the current and voltage measurements did not exceed 1%. In the VCCh system with the ionic contact, the random errors could reach up to 3–4%. From the qualitative point of view, these results, however, coincide with the measurements in the system with an aurum electrode. The intensity of the illumination was measured and controlled with the photodetector, the measurement error of which was <5%.
44
Br. Petr_etis, M. Balciunien_e / Journal of Non-Crystalline Solids 311 (2002) 42–47
3. Results It is determined that the current in the system Al–a-Se–Au (Fig. 1(a)) decreases when the semiclear gold electrode is illuminated, i.e., the effect of the negative photoconductivity is formed (Fig. 2). When the electric circuit is switched on in the dark, the kinetics of the current make up about 5 s (Fig. 2(a)). When the sample is illuminated, the current during some seconds decreases several times. When the voltage is switched off, the current relaxes to the position of zero during 3–5 s. An analogous effect of the light influence, i.e., when the current flowing through the illuminated sample is less than the flowing current when the sample is in the dark, is observed in the case when the sample at first is illuminated and the voltage is switched on later (Fig. 2(b)). In all cases the duration of the current kinetics is approximately the
Fig. 2. The kinetics of the current density of the amorphous selenium layers when the intensity of illumination of the sample and the voltage are switched on and switched off. The sample is in the dark (a) and illuminated (b, c) when the voltage is switched on. The arrows indicate the moments when the voltage is switched on (1) and switched off (2). the sample is in the the sample is illuminated. dark.
same and is equal to 3–5 s. Analogous changes of the current also occur when the potential is less, but the value of the current in this case is considerably less (Fig. 2(c)). It is established that the effect of the negative photoconductivity does not appear, if the barrage As2 Se3 layer is inserted between aluminum and selenium layers (Fig. 1(b)). Besides, the effect of the negative photoconductivity is observed in the Al–a-Se–Au system only in the case, when the negative pole of the electric field is formed at an aurum electrode, i.e., when the holes are injected into the layer of selenium. The negative photoconductivity effect appears in the Al–a-Se system too, if the positive charge carriers are injected from the ionic stream (Fig. 1(c)). The investigations show that in the amorphous layers of selenium the negative photoconductivity effect manifests itself only when a certain strength of the electric field and the illumination intensity are used [1,11]. The Ohmo law is valid in the region of small electric fields in the dark. When the strength of the electric field is increased, the current dependence becomes quadric (I vs V 2 ). In the region of a strong field, near the breakdown, the current dependence on the voltage is described by a function (I vs V n ), the exponent index of which n ¼ 7:7 (Fig. 3, curve 1). The break points exist between separate parts of VCCh. When the sample is illuminated, analogous motion of VCCh is observed too (Fig. 3, curves 2–5). It is determined that the value of the break point voltage UB depends on the intensity of illumination of the sample. With its increase, UB increases as well. When the intensity is increased 250 times, the value of the limited voltage UB increases from 200 to 620 V. At a certain point, VCCh of the illuminated sample crosses the dark VCCh and changes from the positive photoconductivity, which is usual for semiconducting materials, to the negative photoconductivity region. The analysis of VCCh of the illuminated sample shows that in the region of the voltages less than UB the positive photoconductivity is observed, which can be characterized by the relation K¼
rL ; rD
Br. Petr_etis, M. Balciunien_e / Journal of Non-Crystalline Solids 311 (2002) 42–47
Fig. 3. The voltage–current characteristic in the dark (1) and in the region with the different intensity of illumination (2–5). The intensity of illumination: I2 =I0 ¼ 1 (2); I3 =I0 ¼ 5 (3); I4 =I0 ¼ 25 (4) and I5 =I0 ¼ 250 (5).
where rL is the conductivity of the illuminated sample and rD is the conductivity in the dark. When the voltage is increased, K decreases. When the voltage is close to UB , the layer becomes insensitive to light, and the current in the dark and in the light becomes equal ðK ¼ 1Þ. The effect of the negative photoconductivity ðK < 1Þ occurs only when the voltage is close to or larger than UB . It is ascertained that in a-Se layers, when the voltage is constant, the photoconductivity decreases lineary, when the intensity of illumination is decreased (Fig. 4). However, this dependence is valid in the region of the large intensity of illumination up to a certain intensity IB , which depends on the value of the voltage. When the voltage increases, the limited value of illumination IB also increases. For example, when the voltage increases from 200 to 450 V, the limited value of the intensity IB increases about 20 times.
4. Discussion The investigations show that VCCh of the Al– a-Se–Au system in the dark is characteristic of the
45
Fig. 4. The dependence of the conductivity ratio ðrL =rD Þ of the selenium layer on the intensity of illumination (I). rD ––conductivity in the dark, rL ––conductivity of the illuminated layer. The voltage in the sample: (1) 200 V; (2) 310 V and (3) 460 V. I – the region of the positive conductivity, II – the region of the negative photoconductivity.
dependence of the space charge limited current on the voltage [1,11]. The Ohmo law is valid in the region of the low voltage (J vs V). When the strength of the electric field is increased, the current dependence on the voltage becomes quadric (J vs V 2 ). In the region of a strong field, close to the breakdown, it is described by a function (J vs V n ), the exponent index of which n > 2. It is proved by different methods that in amorphous selenium layers the deep localized states are distributed exponentially [3,7–9,12] Nt ðEÞ ¼ N0 exp
E EV ; e0
ð2Þ
where e0 is a parameter of the exponential distribution of the states, EV is the energy of the top of valency band. It is determined that e0 depends on the conditions of the layer production [16] and on the
46
Br. Petr_etis, M. Balciunien_e / Journal of Non-Crystalline Solids 311 (2002) 42–47
external influence, i.e., illumination [15]. As the energetic positions of deep states with respect to the valency band are in the region from 0.7 to 1.0 eV, so their recharge duration in the dark in certain cases can be 10 h [15]. So the recharge duration of deep localized states as compared with the duration of the measurement is very long and these kinetics may be not observed. Experiments, carried out in this work, confirm that in the system Al–a-Se–Au the dependence of VCCh on the external field in the dark as well as in the light is caused by space charge limited currents. It must be noted that measurements of the current were carried out in the sandwich-type samples and only the region of the injected electrode was illuminated, but not the whole sample. So the pairs of charge carriers, excited by light, form the reservoir contact in the sample. When the voltage is increased, the region of the VCCh quadric dependence changes to the region of the function dependence, the exponent index of which n > 2. The region of the function dependence is conditioned by the recharge of states in the electric field [13]. The break point of the VCCh dependence is caused by the ratio of the concentration of light injected carriers to the concentration of thermally generated carriers [1]. It is defined that at the break point of the dependence the value of the current JBL , when the sample is illuminated, is less than the dark current JBD with the same voltage UB (Fig. 2). Thus, when the voltage is close to UB , the negative effect of the photocurrent is obtained. When the intensity of illumination increases, the value of the current excited by light and, accordingly, the value of the break point UB increase as well. However, a negative effect of the photocurrent remains (Fig. 2, curves 2–5). When the value of the voltage reaches the break point voltage of the VCCh dependence UB , the deep localized states recharge under the influence of the electric field and the quasi-Fermi level moves to a new energetic position. With the increase of illumination, the concentration of injected carriers increases, and the influence of recharge of deep levels manifests itself only when the external field is stronger. It should be pointed out that the light in the region of the electrode
forms a reservoir not only of main carriers (holes) of the current but of the electrons as well. Since the electrons in amorphous selenium are less mobile as compared with holes, they are trapped into deep states and have an influence on the recharge of the states and can cause inhomogeneous distribution of the field in the area near the electrodes.
5. Conclusions 1. In the layers of amorphous selenium, the effect of the negative photoconductivity is observed only under these conditions: (a) one electrode must inject the main charge carriers (holes) into a-Se; (b) the pole of the positive electric field must be formed at the injecting contact. 2. In VCCh the limited value of the voltage UB exists, and the positive photoconductivity is observed up to this value. When the voltage is higher UB; the negative photoconductivity takes place in VCCh. 3. When the intensity of the sample illumination increases, the limited voltage UB increases as well.
References [1] M.A. Lampert, P. Mark, Current Injection in Solids, Academic Press, New York, 1970. [2] T. Koslowski, J. Phys.: Condens. Matter 9 (1997) 613. [3] A. Kubilius, Br. Petr_etis, V.I. Archipov, A.I. Rudenko, J. Phys. D 18 (1985) 901. [4] T. Nagase, K. Kishimoto, H. Naito, J. Appl. Phys. 86 (1999) 5026. [5] H. Naito, T. Nagase, T. Ishii, M. Okada, T. Kawaguchi, S. Maruno, J. Non-Cryst. Solids 198–200 (1986) 363. [6] C. Eiche, D. Mayer, D. Sinerius, J. Weese, K.B. Benz, J. Honerkamp, J. Appl. Phys. 4 (1993) 6667. [7] A. Kubilius, Br. Petr_etis, V.I. Arkchipov, A.I. Rudenko, J. Non-Cryst. Solids 87 (1986) 290. [8] M. Abkowitz, J.M. Markovks, Philos. Mag. B 49 (1984) L31. [9] M. Abkowitz, F. Jansen, A.R. Melnyk, Philos. Mag. B 51 (1985) 405. [10] H. Naito, K. Matomuro, M. Okudaad, Jpn. J. Appl. Phys. 22 (1983) L531.
Br. Petr_etis, M. Balciunien_e / Journal of Non-Crystalline Solids 311 (2002) 42–47 [11] [12] [13] [14]
H. Lanyon, Phys. Rev. 130 (1962) 134. J.L. Hartke, Phys. Rev. 125 (1962) 1177. J. Dresner, J. Chem. Phys. 35 (1961) 1628. A. Kubilius, Br. Petr_etis, V.I. Arkchipov, A.I. Rudenko, Sov. Phys. Semicond. (Translation of Fizika I Technika Poluprovodnikov) 20 (1987) 2183.
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[15] Br. Petr_etis, M. Balci unien_e, J. Non-Cryst. Solids 226 (1998) 294. [16] A. Kubilius, Br. Petr_etis, Lithuan. Phys. Coll. (Translation of Lietuvos Fizikos Rinkinys) 25 (1985) 75. [17] M. Abkowitz, R.C. Enck, Phys. Rev. B 25 (1982) 2567.
Negative photoconductivity of illuminated a-Se layers Br. Petr_etis *, M. Balci unien_e Material Structure Department, Institute of Physics, Savanoriuz 231, Vilnius 2028, Lithuania Received 30 May 2001; received in revised form 7 March 2002
Abstract The investigation results of the negative conductivity effect of a-Se layers illuminated by white light are presented in this paper. It is shown that with illumination of the a-Se layer the value of the current can decrease several times as compared with the dark value of the current. This effect is observed only in the case when the voltage–current characteristic changes into the function dependence (J vs V n ), the exponent index of which n ¼ 7:7. This effect is caused by recharge of deep states when the sample is illuminated. Ó 2002 Elsevier Science B.V. All rights reserved.
1. Introduction The distribution parameters of localized states in the band gap of amorphous materials and semiconductors are among the main material properties, which are required when different semiconductor devices are created. Many semiconductor and dielectric properties of the material, such as the carrier transfer, photoconductivity, the photovoltage effect, the relaxation of the current, etc. are affected by the concentration of localized states and their energy position in the band gap. On the other hand, the distribution of the localized states of amorphous and polymeric semiconductors can be investigated by measurement and analysis [1–4]. When the kinetics of photoexcited conductivity are analyzed by means of the Laplace transform, it is possible to determine the distribution of the lo*
Corresponding author. E-mail address: brpet@delfi.lt (Br. Petr_etis).
calized states in the band gap of a wide energy range [4–6]. Much information about parameters of the localized state distribution in semiconductors can be obtained, when the relaxation of the surface and space charge [3,7–10] or the dependence of the limited space charge current on the electric field strength, the sample thickness and other parameters [1,11] are investigated. Some regions with the different functional dependence of the current on the voltage can be observed in the voltage–current characteristic measurement (VCCh). The position of break points of VCCh is conditioned by the charge transfer mechanism, the concentration of deep states and their energy location in the band gap. The investigation of the limited space charge current shows that the deep trapping levels with the energy E in a-Se are distributed with an exponential law [12–14]: Nt ðEÞ ¼ N0 exp
0022-3093/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 0 2 ) 0 1 3 2 7 - 3
E ; kTt
ð1Þ
Br. Petr_etis, M. Balciunien_e / Journal of Non-Crystalline Solids 311 (2002) 42–47
where N0 is the concentration of the states at the top of the valence band (N0 ¼ 1020 cm3 eV1 ) and kTt is the parameter of exponential distribution (kTt ¼ e0 ¼ 0:067 eV). An analogous result is obtained when the relaxation of the surface and the space charge in the amorphous selenium layers are analyzed [3,7,14]. The energy distribution of the deep layers of amorphous selenium and their concentration depend on the admixtures and on the technological conditions of the layers production. It is determined [15,16] that the energy spectrum of the generation centers of a-Se doped with the admixtures of phosphorus moves to the lower energy, if the layer is photoexcited before measurement. The deep states, which exist at the Fermi level, recharge and relax to the equilibrium state, when excitation is broken off. An analogous effect is achieved in the layers of selenium which are not doped with admixtures [17], but it is less pronounced, because the concentration of the centers which take part in the recharge is lower. In the case of the space charge limited current, the deep centers of the charge transfer generation become deep trapping centers. So, when the energy spectrum of deep centers changes the positions of characteristic break points of the current dependence on the voltage must change, because these points are caused by the mechanism of the charge transfer. The purpose of this work is to investigate the conductivity dependence of amorphous selenium layers on the intensity of illumination in the case of the space charge limited current.
2. Experimental method The samples were obtained by thermal evaporation of a selenium layer (2) in a vacuum of 103 Pa on an aluminum substrate (1) covered with a thin (1 lm thickness) As2 Se3 layer (4) or without the covering layer (Fig. 1). The aluminum substrate before evaporation was polished, deprived of fat and cleaned off with ethyl alcohol. The temperature of the substrate during the evaporation was 328–333 K, the growth rate of the layer
43
Fig. 1. The scheme of the sample electrodes: (a) the system of Al–a-Se–Au; (b) the system of Al–As2 Se3 –a-Se–Au; (c) the ionic contact of Al–a-Se; (1) the Al substrate; (2) the a-Se layer; (3) a semi-clear Au electrode; (4) a barrage layer of As2 Se3 ; (5) the ionic beam; (6) a screen.
was 50–60 nm/s and the duration of evaporation was 15–17 min. The charge carriers were injected into the layer by means of an ionic electrode (5) or by the semi-clear Au layer (3) in the sandwich-type system. The layer of As2 Se3 was in the part of a barrage layer. The aurum electrode on the layer of selenium and the barrage layer of As2 Se3 on the substrate of Al were also evaporated in a vacuum. The shape of an aurum electrode was a circle, the diameter of which was about 10–12 mm and the cross-section area of the ionic stream was from 100 to 500 mm2 . The current and photocurrent were measured with the electrometer connected to the resistivity, the nominal value of which was chosen to be about 1000 times less than the resistivity of the measured layer in the range of the measured voltage. The selenium layer was excited by exposing it to light of the incandescent filament in a standard regime. The power density of the light stream was changed by neutral optic filters. All measurements of the current were carried out under normal conditions at room temperature, when T ¼ 293 K. The random errors of the current and voltage measurements did not exceed 1%. In the VCCh system with the ionic contact, the random errors could reach up to 3–4%. From the qualitative point of view, these results, however, coincide with the measurements in the system with an aurum electrode. The intensity of the illumination was measured and controlled with the photodetector, the measurement error of which was <5%.
44
Br. Petr_etis, M. Balciunien_e / Journal of Non-Crystalline Solids 311 (2002) 42–47
3. Results It is determined that the current in the system Al–a-Se–Au (Fig. 1(a)) decreases when the semiclear gold electrode is illuminated, i.e., the effect of the negative photoconductivity is formed (Fig. 2). When the electric circuit is switched on in the dark, the kinetics of the current make up about 5 s (Fig. 2(a)). When the sample is illuminated, the current during some seconds decreases several times. When the voltage is switched off, the current relaxes to the position of zero during 3–5 s. An analogous effect of the light influence, i.e., when the current flowing through the illuminated sample is less than the flowing current when the sample is in the dark, is observed in the case when the sample at first is illuminated and the voltage is switched on later (Fig. 2(b)). In all cases the duration of the current kinetics is approximately the
Fig. 2. The kinetics of the current density of the amorphous selenium layers when the intensity of illumination of the sample and the voltage are switched on and switched off. The sample is in the dark (a) and illuminated (b, c) when the voltage is switched on. The arrows indicate the moments when the voltage is switched on (1) and switched off (2). the sample is in the the sample is illuminated. dark.
same and is equal to 3–5 s. Analogous changes of the current also occur when the potential is less, but the value of the current in this case is considerably less (Fig. 2(c)). It is established that the effect of the negative photoconductivity does not appear, if the barrage As2 Se3 layer is inserted between aluminum and selenium layers (Fig. 1(b)). Besides, the effect of the negative photoconductivity is observed in the Al–a-Se–Au system only in the case, when the negative pole of the electric field is formed at an aurum electrode, i.e., when the holes are injected into the layer of selenium. The negative photoconductivity effect appears in the Al–a-Se system too, if the positive charge carriers are injected from the ionic stream (Fig. 1(c)). The investigations show that in the amorphous layers of selenium the negative photoconductivity effect manifests itself only when a certain strength of the electric field and the illumination intensity are used [1,11]. The Ohmo law is valid in the region of small electric fields in the dark. When the strength of the electric field is increased, the current dependence becomes quadric (I vs V 2 ). In the region of a strong field, near the breakdown, the current dependence on the voltage is described by a function (I vs V n ), the exponent index of which n ¼ 7:7 (Fig. 3, curve 1). The break points exist between separate parts of VCCh. When the sample is illuminated, analogous motion of VCCh is observed too (Fig. 3, curves 2–5). It is determined that the value of the break point voltage UB depends on the intensity of illumination of the sample. With its increase, UB increases as well. When the intensity is increased 250 times, the value of the limited voltage UB increases from 200 to 620 V. At a certain point, VCCh of the illuminated sample crosses the dark VCCh and changes from the positive photoconductivity, which is usual for semiconducting materials, to the negative photoconductivity region. The analysis of VCCh of the illuminated sample shows that in the region of the voltages less than UB the positive photoconductivity is observed, which can be characterized by the relation K¼
rL ; rD
Br. Petr_etis, M. Balciunien_e / Journal of Non-Crystalline Solids 311 (2002) 42–47
Fig. 3. The voltage–current characteristic in the dark (1) and in the region with the different intensity of illumination (2–5). The intensity of illumination: I2 =I0 ¼ 1 (2); I3 =I0 ¼ 5 (3); I4 =I0 ¼ 25 (4) and I5 =I0 ¼ 250 (5).
where rL is the conductivity of the illuminated sample and rD is the conductivity in the dark. When the voltage is increased, K decreases. When the voltage is close to UB , the layer becomes insensitive to light, and the current in the dark and in the light becomes equal ðK ¼ 1Þ. The effect of the negative photoconductivity ðK < 1Þ occurs only when the voltage is close to or larger than UB . It is ascertained that in a-Se layers, when the voltage is constant, the photoconductivity decreases lineary, when the intensity of illumination is decreased (Fig. 4). However, this dependence is valid in the region of the large intensity of illumination up to a certain intensity IB , which depends on the value of the voltage. When the voltage increases, the limited value of illumination IB also increases. For example, when the voltage increases from 200 to 450 V, the limited value of the intensity IB increases about 20 times.
4. Discussion The investigations show that VCCh of the Al– a-Se–Au system in the dark is characteristic of the
45
Fig. 4. The dependence of the conductivity ratio ðrL =rD Þ of the selenium layer on the intensity of illumination (I). rD ––conductivity in the dark, rL ––conductivity of the illuminated layer. The voltage in the sample: (1) 200 V; (2) 310 V and (3) 460 V. I – the region of the positive conductivity, II – the region of the negative photoconductivity.
dependence of the space charge limited current on the voltage [1,11]. The Ohmo law is valid in the region of the low voltage (J vs V). When the strength of the electric field is increased, the current dependence on the voltage becomes quadric (J vs V 2 ). In the region of a strong field, close to the breakdown, it is described by a function (J vs V n ), the exponent index of which n > 2. It is proved by different methods that in amorphous selenium layers the deep localized states are distributed exponentially [3,7–9,12] Nt ðEÞ ¼ N0 exp
E EV ; e0
ð2Þ
where e0 is a parameter of the exponential distribution of the states, EV is the energy of the top of valency band. It is determined that e0 depends on the conditions of the layer production [16] and on the
46
Br. Petr_etis, M. Balciunien_e / Journal of Non-Crystalline Solids 311 (2002) 42–47
external influence, i.e., illumination [15]. As the energetic positions of deep states with respect to the valency band are in the region from 0.7 to 1.0 eV, so their recharge duration in the dark in certain cases can be 10 h [15]. So the recharge duration of deep localized states as compared with the duration of the measurement is very long and these kinetics may be not observed. Experiments, carried out in this work, confirm that in the system Al–a-Se–Au the dependence of VCCh on the external field in the dark as well as in the light is caused by space charge limited currents. It must be noted that measurements of the current were carried out in the sandwich-type samples and only the region of the injected electrode was illuminated, but not the whole sample. So the pairs of charge carriers, excited by light, form the reservoir contact in the sample. When the voltage is increased, the region of the VCCh quadric dependence changes to the region of the function dependence, the exponent index of which n > 2. The region of the function dependence is conditioned by the recharge of states in the electric field [13]. The break point of the VCCh dependence is caused by the ratio of the concentration of light injected carriers to the concentration of thermally generated carriers [1]. It is defined that at the break point of the dependence the value of the current JBL , when the sample is illuminated, is less than the dark current JBD with the same voltage UB (Fig. 2). Thus, when the voltage is close to UB , the negative effect of the photocurrent is obtained. When the intensity of illumination increases, the value of the current excited by light and, accordingly, the value of the break point UB increase as well. However, a negative effect of the photocurrent remains (Fig. 2, curves 2–5). When the value of the voltage reaches the break point voltage of the VCCh dependence UB , the deep localized states recharge under the influence of the electric field and the quasi-Fermi level moves to a new energetic position. With the increase of illumination, the concentration of injected carriers increases, and the influence of recharge of deep levels manifests itself only when the external field is stronger. It should be pointed out that the light in the region of the electrode
forms a reservoir not only of main carriers (holes) of the current but of the electrons as well. Since the electrons in amorphous selenium are less mobile as compared with holes, they are trapped into deep states and have an influence on the recharge of the states and can cause inhomogeneous distribution of the field in the area near the electrodes.
5. Conclusions 1. In the layers of amorphous selenium, the effect of the negative photoconductivity is observed only under these conditions: (a) one electrode must inject the main charge carriers (holes) into a-Se; (b) the pole of the positive electric field must be formed at the injecting contact. 2. In VCCh the limited value of the voltage UB exists, and the positive photoconductivity is observed up to this value. When the voltage is higher UB; the negative photoconductivity takes place in VCCh. 3. When the intensity of the sample illumination increases, the limited voltage UB increases as well.
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