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GePbS vitreous semiconductors with bipolar photoconductivity
Journal of Non-Crystalline Solids 63 (1984) 415-418 North-Holland, Amsterdam
415
Letter to the Editor G e - P b - S VITREOUS SEMICONDUCTORS WITH BIPOLAR PHOTOCONDUCTIVITY G.A. BORDOVSKY 1, L.P. KAZAKOVA 2, E.A. LEBEDEV 2, V.M. LYUBIN and N.A. SAVINOVA 1 l A.L Herzen Pedagogical Institute, Leningrad, 191186, USSR 2 A.F. loffe Physico-Technical Institute, Leningrad, 194021, USSR Received 9 September 1983
The photoelectric phenomena in As-based chalcogenide vitreous semiconductors (ChVS) have been carefully studied in the past few years [1]. On the other hand, non-As ChVS, in particular the G e - P b - S system materials have been investigated substantially less [2,3]. This paper deals with the drift mobility results on photoconductivity and non-equilibrium charge carriers in some ChVS of the G e - P b - S system, which demonstrate many of the essential characteristics that make these materials differ from the As-based ChVS. The Ge(43.s_x)PbxSsr.5(x = 15-20) materials were under investigation. Thin films ranging in thickness from 0.5 to 3 # m were prepared by the flash evaporation technique, which ensured the constancy of the composition throughout the film thickness. The samples were provided with Al-electrodes. The specific dark conductivity at room temperature was about 10 -13 0 -1 c m -1
High photoconductivity was a characteristic of all the samples. The light to dark conductivity ratio was found to be about 101-103 at a light intensity of 3 × 103 lux. The photoconductivity spectral distribution maximum was established to be in the 550-630 nm range for various film thicknesses and poorly dependent on the material compositions. As a result of the investigation many interesting features of photoconductivity in the longitudinal regime were obtained. Lux-ampere characteristics (LAC) had two parts: linear at weak light intensities and sublinear with the power n = 0.5-0.6 at higher light intensities. The fact that the linear part gradually disappeared (fig. 1) with the temperature rise in contrast to the As-based ChVS where the linear part of LAC predominated at high temperature [1], presented some special interest. Investigations of kinetic characteristics have shown a strong dependence of the photoresponse time on light intensity % - I " with the power n = - (0.5-0.7). This is the result of increasing the fast component of photoresponse with the light intensity rise. The conclusion may be drawn from L u x - a m p e r e and
G.A. Bordousky et al. / G e - P b - S uitreous conductors
416
l
-6
"L i...-.i -9
$-75C,~,-926.
-iO -il
t
-6
I
-5
'
-~
-k
-2
-}
t
0
I ,.~.. nnt
Fig. 1. L u x - a m p e r e characteristics at various temperatures. The s a m p l e used was Ge2s sPb18S56.5 L = 2 ~m; E = 2×10 4 V/cm. Fig. 2. Spectral d i s t r i b u t i o n of l o n g i t u d i n a l p h o t o c o n d u c t i v i t y at tWO voltage polarities. The s a m p l e used was Gezs.sPblsS56.s; L = 2 bLm; E = 2 x 10 4 V / c m .
kinetic photocurrent characteristics that the bimolecular recombination predominates in the ChVS under consideration. By contrast, the monomolecular recombination prevails in As-based ChVS [1]. The steady state photoconductivity increased exponentially with the temperature rise. The values of 0.3-0.4 eV were found for the activation energy of this process. The power dependence of photoconductivity versus electric field strength with the power n = 0.5-1.0 for various samples has been acquired in that range of fields where the dark conductivity remained constant [3]. Spectral characteristics of photoconductivity were found to be independent of voltage polarity on the illuminated electrode in the longitudinal photoconductivity regime (fig. 2), the polarity dependence of LAC was absent as well. The properties described, greatly differ from those of As-based ChVS, where the change of the illuminated electrode polarity distinctly modified the spectral and Lux-ampere characteristic forms [1,4], probably due to the photoconducY 2
-4
-X'
,~(t,,l--
Fig. 3. Transient photocurrent at light pulse excitation. The sample used was Ge25.sPblsSs6.s; L = 2 vm; E = 2 × 1 0 5 V / c m .
G.A. Bordovsky et al. / G e - P b - S
oitreous conductors
417
tivity hole component dominance (over that of electron). The noted independence of the photoconductivity characteristics on voltage polarity in the longitudinal photoconductivity regime for the G e - P b - S system is likely to testify to either a very small drift length of nonequilibrium charge carriers or the equality between hole and electron components of photoconductivity. To clear the matter up, the drift mobilities of nonequilibrium charge carriers were examined by means of the time-of-flight method [5]. The process of pulling the carriers generated by the strongly absorbed light impulse (t - 10- 8 s) through the sample caused the measured transient photocurrent. Fig. 3 shows the obtained dependences I ( t ) in a lg-lg plot, which demonstrate the identity of the transient photocurrent forms for electrons and holes. Two parts are distinguished on the curves I(t): that of slow and fast photocurrent decay. The time-of-flight value was determined from the moment of the transition from the first part to the second (t v is marked by an arrow in fig. 3). The values of drift mobilities were calculated using the formula: # = L / E t T, where L is the film thickness, E is the electric field strength. The drift mobilities of holes and electrons are shown to be approximately equal and have the values of (3-9) × 10 - 7 cm2/V s at E = 105 V / c m and T = 300 K. It was observed that the field dependence of hole and electron mobilities in the range of the fields examined followed the power law /~e,h- E", where n = 1.2. The temperature dependences of hole and electron mobilities have exponential character with the activation energy of 0.45-0.65 eV. As a result of the transient photocurrent form analysis the conclusion is made that general dispersive characteristics of charge carriers in the G e - P b - S system are similar to those of As-based ChVS [6]. Therefore, the charge carrier drift and photoconductivity investigations made it possible to conclude that in the ChVS of the G e - P b - S system the carriers of both signs are mobile. It is the bipolar photoconductivity that makes these materials different from As-based ChVS, which are characterized by the monopolar (hole) drift and photoconductivity. Such a discrepancy seems to be due to the different structure of the allowed bands in these two groups of disordered materials. The top of the valance band is known to be formed by the chalcogene lone pair electrons in As-based ChVS. The essential contribution to the valence band formation in the materials of the G e - P b - S system is evidently supplied by the p-electrons of Ge and Pb. The parallel investigations of stationary photoconductivity and nonequilibrium charge carrier drift have proved, to some extent, that temperature and electric field dependences of photoconductivity may be determined to a certain degree by the corresponding dependences of nonequilibrium charge carrier mobility. Some quantitative difference between the parameters of photoconductivity and drift mobility (activation energies, powers in field dependences) may be caused by the corresponding dependences of other parameters, which determine the photoconductivity and, principally, the lifetime of nonequilibrium charge carriers.
418
G.A, Bordovsky et al, / G e - P b - S vitreous conductors
The high photosensitivity a n d dark resistance of the G e - P b - S materials m a k e them suitable for utilizing in vidicon type television camera tubes, xerographic plates a n d drums, p h o t o t h e r m o p l a s t i c light recording media. The b i p o l a r p h o t o c o n d u c t i v i t y is especially i m p o r t a n t , insofar as it makes it possible to create different types of a m o r p h o u s film heterostructures a n d two layer xerographic a n d p h o t o t h e r m o p l a s t i c plates which m a y work at any c o r o n a polarity.
References [1] B.T. Kolomiets and V.M. Lyubin, Phys. Stat. Sol. (a)17 (1973) 11. [2] G.A. Bordovsky, V.I. Bogoslovsky, A.I. Beschlebny, V.A. Izvozchikov and V.M. Lyubin, in Proc. Conf. Amorphous semiconductors - 80, Kishinev (1980) p. 170. [3] G.A. Bordovsky,V.T. Avanesyan,N.A. Savinovaand V.V. Stepanov, in Proc. Conf. Amorphous semiconductors - 82, Bucharest (1982) p. 238. [4] B.T. Kolomiets and V.M. Lyubin, Dokl. Akad. Nauk SSSR 129 (1959) 789. [5] W.E. Spear, J. Non-Crystalline Solids 1 (1969) 197. [6] L. Toot, E.A. Lebedev and L.P. Kazakova, in Proc. Conf. Amorphous semiconductors - 80, Kishinev (1980) p. B64.
415
Letter to the Editor G e - P b - S VITREOUS SEMICONDUCTORS WITH BIPOLAR PHOTOCONDUCTIVITY G.A. BORDOVSKY 1, L.P. KAZAKOVA 2, E.A. LEBEDEV 2, V.M. LYUBIN and N.A. SAVINOVA 1 l A.L Herzen Pedagogical Institute, Leningrad, 191186, USSR 2 A.F. loffe Physico-Technical Institute, Leningrad, 194021, USSR Received 9 September 1983
The photoelectric phenomena in As-based chalcogenide vitreous semiconductors (ChVS) have been carefully studied in the past few years [1]. On the other hand, non-As ChVS, in particular the G e - P b - S system materials have been investigated substantially less [2,3]. This paper deals with the drift mobility results on photoconductivity and non-equilibrium charge carriers in some ChVS of the G e - P b - S system, which demonstrate many of the essential characteristics that make these materials differ from the As-based ChVS. The Ge(43.s_x)PbxSsr.5(x = 15-20) materials were under investigation. Thin films ranging in thickness from 0.5 to 3 # m were prepared by the flash evaporation technique, which ensured the constancy of the composition throughout the film thickness. The samples were provided with Al-electrodes. The specific dark conductivity at room temperature was about 10 -13 0 -1 c m -1
High photoconductivity was a characteristic of all the samples. The light to dark conductivity ratio was found to be about 101-103 at a light intensity of 3 × 103 lux. The photoconductivity spectral distribution maximum was established to be in the 550-630 nm range for various film thicknesses and poorly dependent on the material compositions. As a result of the investigation many interesting features of photoconductivity in the longitudinal regime were obtained. Lux-ampere characteristics (LAC) had two parts: linear at weak light intensities and sublinear with the power n = 0.5-0.6 at higher light intensities. The fact that the linear part gradually disappeared (fig. 1) with the temperature rise in contrast to the As-based ChVS where the linear part of LAC predominated at high temperature [1], presented some special interest. Investigations of kinetic characteristics have shown a strong dependence of the photoresponse time on light intensity % - I " with the power n = - (0.5-0.7). This is the result of increasing the fast component of photoresponse with the light intensity rise. The conclusion may be drawn from L u x - a m p e r e and
G.A. Bordousky et al. / G e - P b - S uitreous conductors
416
l
-6
"L i...-.i -9
$-75C,~,-926.
-iO -il
t
-6
I
-5
'
-~
-k
-2
-}
t
0
I ,.~.. nnt
Fig. 1. L u x - a m p e r e characteristics at various temperatures. The s a m p l e used was Ge2s sPb18S56.5 L = 2 ~m; E = 2×10 4 V/cm. Fig. 2. Spectral d i s t r i b u t i o n of l o n g i t u d i n a l p h o t o c o n d u c t i v i t y at tWO voltage polarities. The s a m p l e used was Gezs.sPblsS56.s; L = 2 bLm; E = 2 x 10 4 V / c m .
kinetic photocurrent characteristics that the bimolecular recombination predominates in the ChVS under consideration. By contrast, the monomolecular recombination prevails in As-based ChVS [1]. The steady state photoconductivity increased exponentially with the temperature rise. The values of 0.3-0.4 eV were found for the activation energy of this process. The power dependence of photoconductivity versus electric field strength with the power n = 0.5-1.0 for various samples has been acquired in that range of fields where the dark conductivity remained constant [3]. Spectral characteristics of photoconductivity were found to be independent of voltage polarity on the illuminated electrode in the longitudinal photoconductivity regime (fig. 2), the polarity dependence of LAC was absent as well. The properties described, greatly differ from those of As-based ChVS, where the change of the illuminated electrode polarity distinctly modified the spectral and Lux-ampere characteristic forms [1,4], probably due to the photoconducY 2
-4
-X'
,~(t,,l--
Fig. 3. Transient photocurrent at light pulse excitation. The sample used was Ge25.sPblsSs6.s; L = 2 vm; E = 2 × 1 0 5 V / c m .
G.A. Bordovsky et al. / G e - P b - S
oitreous conductors
417
tivity hole component dominance (over that of electron). The noted independence of the photoconductivity characteristics on voltage polarity in the longitudinal photoconductivity regime for the G e - P b - S system is likely to testify to either a very small drift length of nonequilibrium charge carriers or the equality between hole and electron components of photoconductivity. To clear the matter up, the drift mobilities of nonequilibrium charge carriers were examined by means of the time-of-flight method [5]. The process of pulling the carriers generated by the strongly absorbed light impulse (t - 10- 8 s) through the sample caused the measured transient photocurrent. Fig. 3 shows the obtained dependences I ( t ) in a lg-lg plot, which demonstrate the identity of the transient photocurrent forms for electrons and holes. Two parts are distinguished on the curves I(t): that of slow and fast photocurrent decay. The time-of-flight value was determined from the moment of the transition from the first part to the second (t v is marked by an arrow in fig. 3). The values of drift mobilities were calculated using the formula: # = L / E t T, where L is the film thickness, E is the electric field strength. The drift mobilities of holes and electrons are shown to be approximately equal and have the values of (3-9) × 10 - 7 cm2/V s at E = 105 V / c m and T = 300 K. It was observed that the field dependence of hole and electron mobilities in the range of the fields examined followed the power law /~e,h- E", where n = 1.2. The temperature dependences of hole and electron mobilities have exponential character with the activation energy of 0.45-0.65 eV. As a result of the transient photocurrent form analysis the conclusion is made that general dispersive characteristics of charge carriers in the G e - P b - S system are similar to those of As-based ChVS [6]. Therefore, the charge carrier drift and photoconductivity investigations made it possible to conclude that in the ChVS of the G e - P b - S system the carriers of both signs are mobile. It is the bipolar photoconductivity that makes these materials different from As-based ChVS, which are characterized by the monopolar (hole) drift and photoconductivity. Such a discrepancy seems to be due to the different structure of the allowed bands in these two groups of disordered materials. The top of the valance band is known to be formed by the chalcogene lone pair electrons in As-based ChVS. The essential contribution to the valence band formation in the materials of the G e - P b - S system is evidently supplied by the p-electrons of Ge and Pb. The parallel investigations of stationary photoconductivity and nonequilibrium charge carrier drift have proved, to some extent, that temperature and electric field dependences of photoconductivity may be determined to a certain degree by the corresponding dependences of nonequilibrium charge carrier mobility. Some quantitative difference between the parameters of photoconductivity and drift mobility (activation energies, powers in field dependences) may be caused by the corresponding dependences of other parameters, which determine the photoconductivity and, principally, the lifetime of nonequilibrium charge carriers.
418
G.A, Bordovsky et al, / G e - P b - S vitreous conductors
The high photosensitivity a n d dark resistance of the G e - P b - S materials m a k e them suitable for utilizing in vidicon type television camera tubes, xerographic plates a n d drums, p h o t o t h e r m o p l a s t i c light recording media. The b i p o l a r p h o t o c o n d u c t i v i t y is especially i m p o r t a n t , insofar as it makes it possible to create different types of a m o r p h o u s film heterostructures a n d two layer xerographic a n d p h o t o t h e r m o p l a s t i c plates which m a y work at any c o r o n a polarity.
References [1] B.T. Kolomiets and V.M. Lyubin, Phys. Stat. Sol. (a)17 (1973) 11. [2] G.A. Bordovsky, V.I. Bogoslovsky, A.I. Beschlebny, V.A. Izvozchikov and V.M. Lyubin, in Proc. Conf. Amorphous semiconductors - 80, Kishinev (1980) p. 170. [3] G.A. Bordovsky,V.T. Avanesyan,N.A. Savinovaand V.V. Stepanov, in Proc. Conf. Amorphous semiconductors - 82, Bucharest (1982) p. 238. [4] B.T. Kolomiets and V.M. Lyubin, Dokl. Akad. Nauk SSSR 129 (1959) 789. [5] W.E. Spear, J. Non-Crystalline Solids 1 (1969) 197. [6] L. Toot, E.A. Lebedev and L.P. Kazakova, in Proc. Conf. Amorphous semiconductors - 80, Kishinev (1980) p. B64.