- Email: [email protected]
Influence of light-soaking and annealing on electron and hole mobility–lifetime products in a-Si:H
Journal of Non-Crystalline Solids 338–340 (2004) 386–389 www.elsevier.com/locate/jnoncrysol
Influence of light-soaking and annealing on electron and hole mobility–lifetime products in a-Si:H E. Morgado
*
Centro de Fısica Molecular, Instituto Superior Tecnico, Technical University of Lisbon, Av. Rovisco Pais, 1049-001 Lisbon, Portugal Available online 6 May 2004
Abstract Light-soaking and annealing effects on majority and minority carrier properties in undoped a-Si:H and simultaneous changes in subgap optical absorption are investigated. Subgap absorption was obtained by constant photocurrent method (CPM). Electron mobility–lifetime product was deduced from photoconductivity. Hole mobility–lifetime product was estimated from measurements of ambipolar diffusion length by steady state photocarrier grating technique (SSPG). Hysteresis-like behaviour is found in the relationship between mobility–lifetime products of electrons and holes during photodegradation and isothermal annealing which is a signature of distinct evolutions of the gap-states density along the different sections of the metastability cycle. Analysis of changes of the sub-gap absorption coefficient at different photon energies and numerical simulations with a recombination model suggest the consideration of two species of metastable states with different sensitivities to light-exposure and annealing. Ó 2004 Elsevier B.V. All rights reserved. PACS: 73.50.Gr; 73.50.Pz; 72.20.J; 73.61.Jc
1. Introduction Most studies on the Staebler–Wronsky effect [1] concern majority carrier dominated photoconductivity with few reports on minority carrier properties, especially for the annealing step. There are indications that besides silicon dangling-bonds other states are involved in light-induced/annealing photoconductivity changes [2–5]. Investigating simultaneously the changes of both the majority and the minority carrier lifetimes can help in determining the role of different kinds of gap states in the metastable process. Recently, a hysteresis-like dependence of the hole mobility–lifetime product on the defect density, along the light-soaking/annealing cycle, was found in a-Si:H [6] which is further support for different competitive mechanisms in the process. In the present paper we investigate the relationship between changes of the mobility–lifetime product of electrons and holes and changes of the subgap absorption coefficient at different photon energies, during lightexposure and isothermal annealing, in undoped a-Si:H.
*
Fax: +351-21 846 4455. E-mail address: [email protected] (E. Morgado).
0022-3093/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2004.03.004
By using a numerically solved recombination model involving different kinds of gap states, we tentatively attribute the observed behaviour of the mobility–lifetime products to the sequence of gap-state metastable events that yields simulated results qualitatively consistent with the data.
2. Experimental The sample is an undoped a-Si:H thin film, 1 lm thick, deposited on glass at a substrate temperature of 250 °C by plasma enhanced chemical vapor deposition (PECVD). Photodegradation was performed with light from a tungsten–halogen lamp at an incident power density of 100 mW cm2 . Isothermal annealing was performed at 140 °C in the dark. Optical absorption spectra were obtained by the constant photocurrent method (CPM) for different light-exposure and annealing times. The CPM-derived spectra were calibrated by matching to the optical absorption coefficient obtained from optical transmission spectroscopy for the band-gap energy of a-Si:H around 1.7 eV. The ambipolar diffusion length was measured by means of the steady state photocarrier grating technique (SSPG) using a
E. Morgado / Journal of Non-Crystalline Solids 338–340 (2004) 386–389
-6
10
µeτe (cm2V-1)
2 mW He–Ne laser (632.8 nm). In order to minimise photodegradation effects during the CPM and SSPG experiments, the pre-annealed sample was exposed to the lamp light over 10 min before the first measurements. All measurements were performed at room temperature.
387
10
-7
3. Results 3.1. Experimental results
light soaking annealing
-8
10
10-9
µhτh (cm2V-1)
Fig. 1. Relationship between mobility–lifetime products of electrons le se and holes lh sh during photodegradation and annealing. Lines are guides to the eye.
1.4 eV 1.3 eV 1.2 eV 1.1 eV 1.0 eV 0.9 eV
5
4
α (cm-1)
From the CPM spectra, estimates of deep defect density around 1016 –1017 cm3 and Urbach slope of 52 ± 5 meV were obtained. The mobility–lifetime products of electrons and holes both decrease during light exposure and increase along the subsequent annealing [6]. The mobility–lifetime product of electrons, ðlsÞe was deduced from the photon flux at the gap energy 1.7 eV in the CPM measurements, as follows. Assuming uniform illumination through the sample the secondary photocurrent, Iph is given by Iph eAUag½ðlsÞe þ ðlsÞh , where e is the electron charge, A accounts for reflection at interfaces and geometry, U is the photon flux, a is the optical absorption coefficient, g is the quantum efficiency, and ðlsÞe , ðlsÞh are the mobility–lifetime products of electrons and holes, respectively. Since all the CPM spectra were taken for the same constant photocurrent value and were matched at the a-Si:H gap energy, one has ðlsÞe / 1=Uð1:7 eVÞ, under the common assumption that ðlsÞe ðlsÞh . The mobility–lifetime productpofffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi holes, ðlsÞh was deduced from the relation Lamb ð2kT =eÞðlsÞh , where Lamb is the ambipolar diffusion length obtained from the SSPG measurements and kT is the thermal energy [7]. The position of the Fermi level relative to the conduction band edge, EC EF was estimated from the measurement of the dark conductivity at room temperature by using the conductivity prefactor r0 ¼ 200 X1 cm1 . Fermi level approaches mid gap during lightsoaking and shows the opposite trend during annealing, varying in the range 0.7–0.8 eV [6]. In a previous paper [6] we have reported a hysteresislike dependence of the mobility–lifetime product of both carriers on the CPM-derived optical absorption coefficient at 1.2 eV, considered often as a measure of the deep defect density. Hysteresis-like behaviour also appears when the electron and hole mobility–lifetime products, measured at each step of the light-soaking/ annealing process, are plotted one as a function of the other as is shown in Fig. 1. The plot in Fig. 1 has the advantage of being independent of any measure of the defect density in revealing the hysteresis effect. In Fig. 2 it is shown the evolution of the CPM-derived absorption, during photodegradation (Fig. 2(a)) and subsequent isothermal annealing (Fig. 2(b)). Dif-
(b)
(a)
3
2
1
10
100
1000 10000
t exposure(min)
100
1000
t
10000
(s)
annealing
Fig. 2. CPM-deduced optical absorption coefficient at different hm as a function of (a) light exposure time, texposure , and (b) annealing time, tannealing .
ferent sub-gap absorption kinetics are found for different photon energy ranges. Combining the data of Fig. 2(a) and (b) by elimination of the explicit dependence on time, anti-clockwise hysteresis is observed in the relationship between high-energy absorption and low-energy absorption in the CPM subgap spectra. This is illustrated in Fig. 3 for the case of the absorption coefficient at 1.0 eV and at 1.4 eV, a1:0 and a1:4 , respectively. 3.2. Simulation analysis The data in Figs. 1 and 2 suggest distinct evolutions of the gap-states density along the photodegradation and annealing portions of the metastability cycle. The simplest, commonly accepted, gap-states description for
388
E. Morgado / Journal of Non-Crystalline Solids 338–340 (2004) 386–389
1
-7
10
(4)
(3)
60
4 -8
10
3
2
Ann.
E0V (meV)
4
τe(s)
α1.4 (cm-1)
5
L.S.
55
light soaking annealing
(1) 15
10
3 0.2
0.4
0.6
0.8
1.0
1.2
α1.0 (cm-1) Fig. 3. High-energy absorption a1:4 versus low-energy absorption a1:0 along the light-soaking/annealing cycle. Lines are guides to the eye.
a-Si:H is the standard dangling bond model with exponential band-tails. We have performed simulation studies with a numerically solved recombination model involving these two kinds of states: dangling bonds with density ND represented by correlated discrete levels Dþ , D0 , D [8], and conduction and valence band-tail exponential distributions with characteristic energies Eoc and Eov , respectively. Basic equations of the model are the equilibrium between photo-generation rate and total recombination rates, and charge neutrality condition [9]. Assuming that light-soaking, as well as annealing, are two step processes, changes in the dangling-bond density and in the tail states were introduced in the model in different sequences. We were able to qualitatively reproduce the hysteresis pattern in Fig. 1 described by the two carrier lifetimes, as well as the Fermi level movement, by considering the following sequence of events: light-soaking is represented by two steps, step 1 where ND increases (with Eov constant) and step 2 where ND as well as Eov both increase; reversibly, annealing is made up of step 3 where ND decreases (Eov constant), and step 4 that closes the cycle by decreasing ND and Eov . Results of the calculations are shown in Fig. 4 where the electron-, se , and hole-, sh , lifetimes were calculated for the values of ND and Eov indicated in the insert. Parameter values are: band tail slopes Eoc ¼ 0:025 eV, Eov ¼ 0:055–0:060 eV, 0 ED ¼ EC 1 eV, correlation energy EU ¼ 0:3 eV, capture cross sections of 1016 cm2 for band-tails, 1015 cm2 for neutral D0 states, 1014 cm2 for charged Dþ and D states, and photogeneration rate G ¼ 1018 cm3 s1 . The position of the equilibrium Fermi level relative to the conduction band edge, EC EF , was calculated from charge neutrality condition for each pair of values (ND ; Eov ) considered in the simulation: EC EF ¼ 0:792 ! 0:811 ! 0:815 eV during simulated light-soaking; EC EF ¼ 0:815 ! 0:796 ! 0:792 eV along the annealing steps.
10 -9
10
(2)
1016 ND(cm-3) -8
τh(s) Fig. 4. Calculated evolution of electron se and hole sh lifetimes along the metastability cycle for the sequence (ND ; Eov ) in the insert. (1-2-3) corresponds to photodegradation. (3-4-1) corresponds to annealing. See details in the text.
4. Discussion The hysteresis patterns in Figs. 1 and 3 suggest that different kinds of states with different kinetics are involved in the light-soaking/annealing process. CPM measures only transitions contributing to the photocurrent, i.e., leading to the excitation of carriers to the extended states, and is dominated by the carrier with the higher mobility–lifetime product. In undoped aSi:H, CPM-derived optical absorption at photon energy hm, ahm , essentially accounts for transitions between electron occupied gap-states with energy E P EC hm, where EC is the conduction band mobility edge, and conduction band extended states. We may assign the absorption coefficient in the range 0.9–1.1 eV, to occupied dangling bond states near the middle of the gap. For the discussion, we take the absorption coefficient at 1.0 eV, a1:0 , as a measure of the deep dangling-bond density. In the higher subgap energy range, the 1.3–1.4 eV absorption curves in Fig. 2 show a steeper increase when compared to the lower energy curves, which may be attributed to other kind of metastable states. The absorption at 1.4 eV includes contributions from all electron occupied states distant from EC by less or equal than 1.4 eV, and therefore include the contribution of dangling bond states. However, since a1:4 a1:0 , we may consider that changes in a1:4 mainly reflect changes in the states located around this energy, close to the exponential region of the absorption spectrum. Under these assignments, the hysteresis-like dependence of the absorption coefficients in Fig. 3 may be described as follows. In the beginning of light soaking the increase in dangling-bond density has some preference and is followed by the increase in both deep dangling-bonds and states located around EC 1:4 eV; reversibly, metasta-
E. Morgado / Journal of Non-Crystalline Solids 338–340 (2004) 386–389
ble dangling-bonds are more sensible to annealing before both species of states are reduced. The described gap-states evolution corresponds, in a schematic way, to the sequence of events used in the simulation study and represented in Fig. 4, where changes in the states around EC 1:4 eV are simulated by changes in the characteristic energy of the valence band tail, Eov ¼ 0:055–0:060 eV. It should be noted that the assumed change in the magnitude of Eov stays within the experimental error in the estimation of the Urbach slope from the CPM spectra. The results of the calculations depicted in Fig. 4 are qualitatively consistent with the data in Fig. 1 as far as a similar clock-wise hysteresis cycle in the (sh ; se ) space is obtained. The associated movement of the Fermi level also agrees with the data trend. Hysteresis effect has been commonly found in the dependence of electron dominated photoconductivity on defect density along the light-soaking/annealing process [10–12]. It has been related to a kinetics, based on a distribution of defect annealing energies, where the defects close to midgap are annealed out preferentially [10], in similarity with our above analysis. It has also been proposed that native defects and light-induced defects have different capture cross sections or different correlation energies [11]. Recently, it is becoming apparent that light-soaking and annealing are two step processes determined by the competition between bondbreaking and network disorder [13,14]. The essential feature of our tentative approach was the consideration of two species of gap states, deep defects and disorder related valence-band-tail-like centers, with different sensitivities to light-exposure and annealing. The interesting qualitative parallelism between the simulation results and data cannot discard other alternative interpretations. Including minority carrier lifetime data in the analysis of the SW effect shall always be helpful.
affect preferentially those states controlling the electron lifetime before creation of hole recombination centers becomes dominant. Subsequent isothermal annealing begins to reduce preferentially the electron recombination states before recovery of both carrier lifetimes occurs. This can be considered as a clear signature, either of distinct evolutions of different kinds of gap-states, or of changes in the electronic parameters of the defects, along the metastability cycle. Simultaneously, hysteresis was also found in the relationship between high-energy and low-energy sub-gap absorption in the CPM spectra. A simulation study predicts a similar hysteresis behaviour of the carrier lifetimes if one attributes to deep dangling-bond and shallow valence band tail states different sensitivities to light-exposure and annealing.
Acknowledgements We thank Professor Reinhard Schwarz for the a-Si:H sample produced at Technical University of Munich, Physics Department. The present work is supported by Fundacß~ao para a Ci^encia e Tecnologia.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
5. Conclusions Hysteresis-like behaviour was observed in the mutual dependence of the mobility–lifetime products of both carriers along the light-soaking/isothermal annealing process in undoped a-Si:H. Photodegradation begins to
389
[11] [12] [13] [14]
D.L. Staebler, C.R. Wronsky, Appl. Phys. Lett. 31 (1977) 292. D. Han, H. Fritzsche, J. Non-Cryst. Solids 59&60 (1983) 397. S. Guha, C.Y. Huang, S.J. Hudgens, Phys. Rev. B 29 (1984) 5995. G. Schumm, K. Nitsch, G.H. Bauer, Philos. Mag. B 58 (1988) 411. C. Longeaud, D. Roy, Z. Teukam Hangouan, Appl. Phys. Lett. 77 (2000) 3604. E. Morgado, J. Non-Cryst. Solids 299–302 (2002) 471. L. Yang, A. Catalano, R.R. Arya, I. Balberg, Appl. Phys. Lett. 57 (1990) 908. E. Morgado, Philos. Mag. B 63 (1991) 529. E. Morgado, Mater. Res. Soc. Symp. Proc. 336 (1994) 419; E. Morgado, Def. Diff. Forum 134&135 (1996) 39. K. Shepard, Z.E. Smith, S. Aljish, S. Wagner, Appl. Phys. Lett. 53 (1988) 1644. P. Stradins, H. Fritzsche, Philos. Mag. B 69 (1994) 121. T. Shimizu, J. Non-Cryst. Solids 164–166 (1993) 163. S. Gupta, R.S. Katiyar, S.Z. Weisz, I. Balberg, J. Non-Cryst. Solids 266–269 (2000) 496. P. Kounavis, E. Mytilineou, J. Phys.: Condens. Matter 11 (1999) 9105.
Influence of light-soaking and annealing on electron and hole mobility–lifetime products in a-Si:H E. Morgado
*
Centro de Fısica Molecular, Instituto Superior Tecnico, Technical University of Lisbon, Av. Rovisco Pais, 1049-001 Lisbon, Portugal Available online 6 May 2004
Abstract Light-soaking and annealing effects on majority and minority carrier properties in undoped a-Si:H and simultaneous changes in subgap optical absorption are investigated. Subgap absorption was obtained by constant photocurrent method (CPM). Electron mobility–lifetime product was deduced from photoconductivity. Hole mobility–lifetime product was estimated from measurements of ambipolar diffusion length by steady state photocarrier grating technique (SSPG). Hysteresis-like behaviour is found in the relationship between mobility–lifetime products of electrons and holes during photodegradation and isothermal annealing which is a signature of distinct evolutions of the gap-states density along the different sections of the metastability cycle. Analysis of changes of the sub-gap absorption coefficient at different photon energies and numerical simulations with a recombination model suggest the consideration of two species of metastable states with different sensitivities to light-exposure and annealing. Ó 2004 Elsevier B.V. All rights reserved. PACS: 73.50.Gr; 73.50.Pz; 72.20.J; 73.61.Jc
1. Introduction Most studies on the Staebler–Wronsky effect [1] concern majority carrier dominated photoconductivity with few reports on minority carrier properties, especially for the annealing step. There are indications that besides silicon dangling-bonds other states are involved in light-induced/annealing photoconductivity changes [2–5]. Investigating simultaneously the changes of both the majority and the minority carrier lifetimes can help in determining the role of different kinds of gap states in the metastable process. Recently, a hysteresis-like dependence of the hole mobility–lifetime product on the defect density, along the light-soaking/annealing cycle, was found in a-Si:H [6] which is further support for different competitive mechanisms in the process. In the present paper we investigate the relationship between changes of the mobility–lifetime product of electrons and holes and changes of the subgap absorption coefficient at different photon energies, during lightexposure and isothermal annealing, in undoped a-Si:H.
*
Fax: +351-21 846 4455. E-mail address: [email protected] (E. Morgado).
0022-3093/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2004.03.004
By using a numerically solved recombination model involving different kinds of gap states, we tentatively attribute the observed behaviour of the mobility–lifetime products to the sequence of gap-state metastable events that yields simulated results qualitatively consistent with the data.
2. Experimental The sample is an undoped a-Si:H thin film, 1 lm thick, deposited on glass at a substrate temperature of 250 °C by plasma enhanced chemical vapor deposition (PECVD). Photodegradation was performed with light from a tungsten–halogen lamp at an incident power density of 100 mW cm2 . Isothermal annealing was performed at 140 °C in the dark. Optical absorption spectra were obtained by the constant photocurrent method (CPM) for different light-exposure and annealing times. The CPM-derived spectra were calibrated by matching to the optical absorption coefficient obtained from optical transmission spectroscopy for the band-gap energy of a-Si:H around 1.7 eV. The ambipolar diffusion length was measured by means of the steady state photocarrier grating technique (SSPG) using a
E. Morgado / Journal of Non-Crystalline Solids 338–340 (2004) 386–389
-6
10
µeτe (cm2V-1)
2 mW He–Ne laser (632.8 nm). In order to minimise photodegradation effects during the CPM and SSPG experiments, the pre-annealed sample was exposed to the lamp light over 10 min before the first measurements. All measurements were performed at room temperature.
387
10
-7
3. Results 3.1. Experimental results
light soaking annealing
-8
10
10-9
µhτh (cm2V-1)
Fig. 1. Relationship between mobility–lifetime products of electrons le se and holes lh sh during photodegradation and annealing. Lines are guides to the eye.
1.4 eV 1.3 eV 1.2 eV 1.1 eV 1.0 eV 0.9 eV
5
4
α (cm-1)
From the CPM spectra, estimates of deep defect density around 1016 –1017 cm3 and Urbach slope of 52 ± 5 meV were obtained. The mobility–lifetime products of electrons and holes both decrease during light exposure and increase along the subsequent annealing [6]. The mobility–lifetime product of electrons, ðlsÞe was deduced from the photon flux at the gap energy 1.7 eV in the CPM measurements, as follows. Assuming uniform illumination through the sample the secondary photocurrent, Iph is given by Iph eAUag½ðlsÞe þ ðlsÞh , where e is the electron charge, A accounts for reflection at interfaces and geometry, U is the photon flux, a is the optical absorption coefficient, g is the quantum efficiency, and ðlsÞe , ðlsÞh are the mobility–lifetime products of electrons and holes, respectively. Since all the CPM spectra were taken for the same constant photocurrent value and were matched at the a-Si:H gap energy, one has ðlsÞe / 1=Uð1:7 eVÞ, under the common assumption that ðlsÞe ðlsÞh . The mobility–lifetime productpofffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi holes, ðlsÞh was deduced from the relation Lamb ð2kT =eÞðlsÞh , where Lamb is the ambipolar diffusion length obtained from the SSPG measurements and kT is the thermal energy [7]. The position of the Fermi level relative to the conduction band edge, EC EF was estimated from the measurement of the dark conductivity at room temperature by using the conductivity prefactor r0 ¼ 200 X1 cm1 . Fermi level approaches mid gap during lightsoaking and shows the opposite trend during annealing, varying in the range 0.7–0.8 eV [6]. In a previous paper [6] we have reported a hysteresislike dependence of the mobility–lifetime product of both carriers on the CPM-derived optical absorption coefficient at 1.2 eV, considered often as a measure of the deep defect density. Hysteresis-like behaviour also appears when the electron and hole mobility–lifetime products, measured at each step of the light-soaking/ annealing process, are plotted one as a function of the other as is shown in Fig. 1. The plot in Fig. 1 has the advantage of being independent of any measure of the defect density in revealing the hysteresis effect. In Fig. 2 it is shown the evolution of the CPM-derived absorption, during photodegradation (Fig. 2(a)) and subsequent isothermal annealing (Fig. 2(b)). Dif-
(b)
(a)
3
2
1
10
100
1000 10000
t exposure(min)
100
1000
t
10000
(s)
annealing
Fig. 2. CPM-deduced optical absorption coefficient at different hm as a function of (a) light exposure time, texposure , and (b) annealing time, tannealing .
ferent sub-gap absorption kinetics are found for different photon energy ranges. Combining the data of Fig. 2(a) and (b) by elimination of the explicit dependence on time, anti-clockwise hysteresis is observed in the relationship between high-energy absorption and low-energy absorption in the CPM subgap spectra. This is illustrated in Fig. 3 for the case of the absorption coefficient at 1.0 eV and at 1.4 eV, a1:0 and a1:4 , respectively. 3.2. Simulation analysis The data in Figs. 1 and 2 suggest distinct evolutions of the gap-states density along the photodegradation and annealing portions of the metastability cycle. The simplest, commonly accepted, gap-states description for
388
E. Morgado / Journal of Non-Crystalline Solids 338–340 (2004) 386–389
1
-7
10
(4)
(3)
60
4 -8
10
3
2
Ann.
E0V (meV)
4
τe(s)
α1.4 (cm-1)
5
L.S.
55
light soaking annealing
(1) 15
10
3 0.2
0.4
0.6
0.8
1.0
1.2
α1.0 (cm-1) Fig. 3. High-energy absorption a1:4 versus low-energy absorption a1:0 along the light-soaking/annealing cycle. Lines are guides to the eye.
a-Si:H is the standard dangling bond model with exponential band-tails. We have performed simulation studies with a numerically solved recombination model involving these two kinds of states: dangling bonds with density ND represented by correlated discrete levels Dþ , D0 , D [8], and conduction and valence band-tail exponential distributions with characteristic energies Eoc and Eov , respectively. Basic equations of the model are the equilibrium between photo-generation rate and total recombination rates, and charge neutrality condition [9]. Assuming that light-soaking, as well as annealing, are two step processes, changes in the dangling-bond density and in the tail states were introduced in the model in different sequences. We were able to qualitatively reproduce the hysteresis pattern in Fig. 1 described by the two carrier lifetimes, as well as the Fermi level movement, by considering the following sequence of events: light-soaking is represented by two steps, step 1 where ND increases (with Eov constant) and step 2 where ND as well as Eov both increase; reversibly, annealing is made up of step 3 where ND decreases (Eov constant), and step 4 that closes the cycle by decreasing ND and Eov . Results of the calculations are shown in Fig. 4 where the electron-, se , and hole-, sh , lifetimes were calculated for the values of ND and Eov indicated in the insert. Parameter values are: band tail slopes Eoc ¼ 0:025 eV, Eov ¼ 0:055–0:060 eV, 0 ED ¼ EC 1 eV, correlation energy EU ¼ 0:3 eV, capture cross sections of 1016 cm2 for band-tails, 1015 cm2 for neutral D0 states, 1014 cm2 for charged Dþ and D states, and photogeneration rate G ¼ 1018 cm3 s1 . The position of the equilibrium Fermi level relative to the conduction band edge, EC EF , was calculated from charge neutrality condition for each pair of values (ND ; Eov ) considered in the simulation: EC EF ¼ 0:792 ! 0:811 ! 0:815 eV during simulated light-soaking; EC EF ¼ 0:815 ! 0:796 ! 0:792 eV along the annealing steps.
10 -9
10
(2)
1016 ND(cm-3) -8
τh(s) Fig. 4. Calculated evolution of electron se and hole sh lifetimes along the metastability cycle for the sequence (ND ; Eov ) in the insert. (1-2-3) corresponds to photodegradation. (3-4-1) corresponds to annealing. See details in the text.
4. Discussion The hysteresis patterns in Figs. 1 and 3 suggest that different kinds of states with different kinetics are involved in the light-soaking/annealing process. CPM measures only transitions contributing to the photocurrent, i.e., leading to the excitation of carriers to the extended states, and is dominated by the carrier with the higher mobility–lifetime product. In undoped aSi:H, CPM-derived optical absorption at photon energy hm, ahm , essentially accounts for transitions between electron occupied gap-states with energy E P EC hm, where EC is the conduction band mobility edge, and conduction band extended states. We may assign the absorption coefficient in the range 0.9–1.1 eV, to occupied dangling bond states near the middle of the gap. For the discussion, we take the absorption coefficient at 1.0 eV, a1:0 , as a measure of the deep dangling-bond density. In the higher subgap energy range, the 1.3–1.4 eV absorption curves in Fig. 2 show a steeper increase when compared to the lower energy curves, which may be attributed to other kind of metastable states. The absorption at 1.4 eV includes contributions from all electron occupied states distant from EC by less or equal than 1.4 eV, and therefore include the contribution of dangling bond states. However, since a1:4 a1:0 , we may consider that changes in a1:4 mainly reflect changes in the states located around this energy, close to the exponential region of the absorption spectrum. Under these assignments, the hysteresis-like dependence of the absorption coefficients in Fig. 3 may be described as follows. In the beginning of light soaking the increase in dangling-bond density has some preference and is followed by the increase in both deep dangling-bonds and states located around EC 1:4 eV; reversibly, metasta-
E. Morgado / Journal of Non-Crystalline Solids 338–340 (2004) 386–389
ble dangling-bonds are more sensible to annealing before both species of states are reduced. The described gap-states evolution corresponds, in a schematic way, to the sequence of events used in the simulation study and represented in Fig. 4, where changes in the states around EC 1:4 eV are simulated by changes in the characteristic energy of the valence band tail, Eov ¼ 0:055–0:060 eV. It should be noted that the assumed change in the magnitude of Eov stays within the experimental error in the estimation of the Urbach slope from the CPM spectra. The results of the calculations depicted in Fig. 4 are qualitatively consistent with the data in Fig. 1 as far as a similar clock-wise hysteresis cycle in the (sh ; se ) space is obtained. The associated movement of the Fermi level also agrees with the data trend. Hysteresis effect has been commonly found in the dependence of electron dominated photoconductivity on defect density along the light-soaking/annealing process [10–12]. It has been related to a kinetics, based on a distribution of defect annealing energies, where the defects close to midgap are annealed out preferentially [10], in similarity with our above analysis. It has also been proposed that native defects and light-induced defects have different capture cross sections or different correlation energies [11]. Recently, it is becoming apparent that light-soaking and annealing are two step processes determined by the competition between bondbreaking and network disorder [13,14]. The essential feature of our tentative approach was the consideration of two species of gap states, deep defects and disorder related valence-band-tail-like centers, with different sensitivities to light-exposure and annealing. The interesting qualitative parallelism between the simulation results and data cannot discard other alternative interpretations. Including minority carrier lifetime data in the analysis of the SW effect shall always be helpful.
affect preferentially those states controlling the electron lifetime before creation of hole recombination centers becomes dominant. Subsequent isothermal annealing begins to reduce preferentially the electron recombination states before recovery of both carrier lifetimes occurs. This can be considered as a clear signature, either of distinct evolutions of different kinds of gap-states, or of changes in the electronic parameters of the defects, along the metastability cycle. Simultaneously, hysteresis was also found in the relationship between high-energy and low-energy sub-gap absorption in the CPM spectra. A simulation study predicts a similar hysteresis behaviour of the carrier lifetimes if one attributes to deep dangling-bond and shallow valence band tail states different sensitivities to light-exposure and annealing.
Acknowledgements We thank Professor Reinhard Schwarz for the a-Si:H sample produced at Technical University of Munich, Physics Department. The present work is supported by Fundacß~ao para a Ci^encia e Tecnologia.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
5. Conclusions Hysteresis-like behaviour was observed in the mutual dependence of the mobility–lifetime products of both carriers along the light-soaking/isothermal annealing process in undoped a-Si:H. Photodegradation begins to
389
[11] [12] [13] [14]
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