Similarities and peculiarities in the relaxation of Urbach-tail and above-bandgap excitations in a-Se A. Mishchenko1,2,a) , G. P. Lindberg3, B. A. Weinstein3 , A. Reznik1,2 1
Department of Physics, Lakehead University, 955 Oliver Road, Thunder Bay, Ontario P7B5E1, Canada 2 Thunder Bay Regional Research Institute, 290 Munro Street, Thunder Bay, Ontario P7A7T1, Canada 3 SUNY at Buffalo, Department of Physics, Buffalo, NY 14260-1500 USA ABSTRACT The relaxation of photodarkening (PD) brought about by above-bandgap and Urbach-tail excitations is studied in a wide temperature range in a-Se films designed for avalanche photodetectors. The experimental results suggest that in contrast to Urbach-tail excitation, abovebandgap excitation does not cause the formation of self-trapped excitons, viz. photoinduced transformation of the a-Se ground state configuration into a metastable higher-energy configuration. For above-bandgap excitation only transient PD is observed, and its subsequent relaxation can be explained by thermalization and energy relaxation of uncorrelated carriers through the band-tails in order to restore the ground state configuration. In contrast, Urbach-tail excitation causes both transient and reversible PD, with the latter controlled by the formation of self-trapped excitons whose relaxation requires overcoming an energy barrier of 0.78±0.02eV either by thermal activation (at elevated temperatures) or by configurational tunneling (for temperatures below room temperature). Amorphous selenium (a-Se) is one of the best-known chalcogenide glasses with a long history of successful practical implementation ranging from xerography (in the 1950’s), to modern flat-panel mammographic detectors with unsurpassed imaging performance [1], and ultrasensitive High-gain Avalanche Rushing Photoconductor (HARP) video tubes with such high sensitivity that they can capture night-time images in dense darkness [2]. Despite the intense commercial use of a-Se, and extensive fundamental investigations of this material over the years, our understanding of its properties is not complete. In particular, this is true for the photoinduced structural metastability observed in a-Se, which manifests itself in a variety of phenomena including the topic of the present work, photodarkening (PD) – the red shift in the a-Se absorption edge as a result of prolonged exposure to light. It is believed that the photoinduced structural metastability in a-Se is driven by strong electron-phonon coupling [3], in which optical excitation brings about sufficient lattice relaxation [4] to produce new metastable local structural configurations. It has been proposed by Tanaka that in a-Se the local energy landscape has a double-well ground-state potential, with a)
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local minima X and Y separated by a potential barrier [5,6]. Upon excitation, carriers can either recombine restoring the initial X-state configuration, or can be trapped in an upper potential well Y. This configuration-coordinate picture (or proposed variations that, e.g., include a 3rd intermediate configuration [7]) is quite general. It can encompass the formation of bonding defects [4,8], as well as models that involve self-trapped excitons, as proposed by Street [9], in which correlated electron-hole pairs are trapped into the Y configuration. Accumulation of sites in the Y configuration enhances the density of deep states in the bandtails, and causes the amorphous structure to become more disordered, thereby giving rise to a wide range of photoinduced phenomena, including the PD effect [5, 6, 7, 10, 11, 12, 13, 14]. The majority of experimental results on PD have been done with Urbach-tail, i.e. subbandgap, illumination on elemental and binary chalcogenide glasses. The detailed effects of above-bandgap illumination on PD have been studied much less, particularly for a-Se, where it has been found that the amplitude of PD levels off or decreases for short wavelength excitation [15]. Here we demonstrate that for 405 nm wavelength excitation (i.e. well above the 2.0 eV bandgap of a-Se [1]), the experimental results on the temperature dependence of the PD kinetics differ dramatically from those for Urbach-tail excitation. This allows us to clearly connect the different relaxation rates observed for the PD to the participation of metastable exciton configurations in the recombination that gives rise to these rates. In the present work we utilize a two-laser-beam technique for the PD experiments, where a powerful pumping beam (either 150 mW/cm2 at 655nm for sub-bandgap illumination, or 200 mW/cm2 at 405nm for above-bandgap illumination) is used to create the photodarkening, and a weak probing beam (0.29 mW/cm2 at 655 nm) constantly monitors the changes in transmission brought about by the pumping radiation. The details of the experimental setup for the measurements of PD can be found in Ref. 13. The kinetics of PD is studied by exposing the a-Se to the pumping beam for intervals of 200 seconds separated by 200-second periods of rest, intervals that were found to be optimal for our investigation. On this time scale, depending on the pumping wavelength and temperature, the PD relaxation can exhibit distinct fast and slow components, termed transient and reversible, respectively. Throughout the experiment, the relative changes in the probing light transmittance T/T0 are measured in the wide temperature range from -40oC to 50oC. This procedure can, in principle, allow both defect accumulation and defect relaxation processes to be monitored, but we concentrate here on the relaxation. The experiments are carried out on 15 μm thick stabilized a-Se (0.5 wt. % of arsenic) films deposited by thermal evaporation on indium tin oxide (ITO) covered glass substrates. The collected data in Figs. 1 and 2 demonstrate that the kinetics and magnitude of PD are temperature and wavelength dependent, and there is a distinct qualitative difference in the observed behavior for sub- and above-bandgap excitation. The results of the PD measurements with sub-bandgap illumination at three selected temperatures are shown in figure 1 for four or five pump/rest cycles. It was found that, at all temperatures below the glass transition (Tg ~ 42oC or 315 K [16]), the transparency drops during the pumping period followed by partial restoration during the rest period (the results for -20oC 2
and 20oC are shown in fig. 1 as most illustrative). The lower the temperature, the larger the overall decrease in transmittance, which is consistent with previously reported results [5]. Partial recovery indicates the presence of both the transient PD and the reversible PD components [13]. For the former component, full recovery is almost immediate on the time scale of our experiments. For the latter component, the transmittance can be restored, but thermal annealing near Tg is required to reverse the decrease in T/T0 [4]. This decomposition of the PD kinetics is supported by the noticeably different PD response measured at 45oC (i.e., close to the glass transition temperature for stabilized a-Se). For this temperature, T/T0 is completely recovered at the end of each 200s rest period, and the required time of restoration is short, showing that the reversible component of PD has vanished, and only the transient PD is present. This agrees well with the kinetics of PD reported previously for a-Se samples that were not As-stabilized [17].
1.0
T/To
0.9
0.8
0.7
0
0.6
T=-20 C
0
400
0
0
T=20 C
800
T=45 C
1200
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2000
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FIG 1. PD for selected temperatures: -20oC; 20oC; 45oC in the case of 655nm excitation For 405 nm pump excitation the temperature behavior is markedly different. Only transient PD is observed in the wide temperature range from -40oC to 30oC: once the pump light is turned off the initial transparency is completely recovered (fig.2). However, the temperature dependence of the magnitude of the transient PD remains qualitatively similar to that found for sub-bandgap illumination: the lower the temperature, the smaller the magnitude of the effect (i.e., drop of the transparency within each pump cycle).
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1.00
T/To
0.96
0.92
0.88
o
o
-40 C
0
o
0C
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30 C
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FIG 2. PD for selected temperatures: -40oC; 0oC; 30oC in the case of 405nm excitation To describe the observed relaxation kinetics of the PD we assume that the amplitude of darkening is linearly proportional to the number of excited electron-hole pairs (whether or not correlated). Consider first the situation for above-bandgap excitation. When the pumping light is switched off the time dependence of the transmission can be described by the equation: ( )
(
(1)
)
where τ1 is the characteristic time constant of relaxation, and A is the maximum PD amplitude. From the experimental data in fig. 2, we determine the characteristic time constant for each recovery cycle at different temperatures. It is found that for 405 nm wavelength excitation, T/T0 recovers with τ1 = 2.5s, identical for all temperatures under investigation here. In fact, averaging τ1 for 32 cycles at every temperature gives a standard deviation of only + 0.2s, showing that τ1 has essentially no temperature dependence over a wide temperature range. In contrast, the single exponential decay fails for the longer wavelength (655nm) sub-bandgap excitation, and a doubleexponential decay is needed to fit the experimental results shown in figure 1. ( )
(
)
(
)
(2)
Here A0 (
