Light-induced metastable effects in hydrogenated amorphous silicon

Solar Energy Materials 8 (1982) 141-151 North-Holland Publishing Company

LIGHT-INDUCED METASTABLE AMORPHOUS SILICON

141

EFFECTS

IN

HYDROGENATED

J.I. P A N K O V E RCA Laboratories, Princeton, NJ 08540, USA

This paper reviews various light-induced metastable effects in a-Si:H: changes in dark conductivity (Staebler-Wronski effect), increased spin resonance signal due to dangling bonds, reduced luminescence efficiency, emergence of emission at -~0.8eV, and also field-induced observations. The defects can be produced not only by irradiation with visible light, but also by X-rays, electron or ion bombardment, and even by ambipolar injection.

1. Introduction

Light-induced metastable effects such as increased absorption below the optical gap, increased spin density, reduced luminescence efficiency and changes in conductivity have been found in chalcogenide glasses [1, 2]. These effects are thermally reversible. They have been tentatively attributed to various localized properties such as lone pair non-bonding orbitals [3] which are not expected to occur in materials made mostly of tetrahedrally bonded elements [2]. The discovery of similar effects in hydrogenated amorphous silicon, a-Si:H, has stimulated the curiosity of many researchers. The interest in these effects has been enhanced by concern over their influence on the stability of solar cells [4]. The Staebler-Wronski effect, to be described below, was the first light-induced metastable effect found in a-Si:H, and it was of a spectacular magnitude [5]. Eventually, all the other light-induced metastable effects found in chalcogenides were also found in a-Si:H, and an intriguing mystery soon became a nightmare as new light-induced metastable effects of opposite polarity were uncovered, e.g., increasing instead of decreasing dark conductivity (called 'negative Staebler-Wronski effect')[6]. Some effort was devoted to separate surface from bulk effects. Here, we shall review most of the metastable effects that have been found in a-Si:H. Some new perspective has evolved that should in turn stimulate further work.

0165-1633 / 82/0000-0000/$ 02.75 © 1982 North-Holland

J.l. Pankove/Light-induced metastable effects in a-Si:H

142

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2. The Staebler-Wronski effect Staebler and Wronski [5] found that the dark conductivity of a-Si:H could drop by four orders of magnitude after prolonged illumination (fig. 1). The material could remain for a long time in the new state B but could be returned to the initial or A state by a thermal anneal at about 160 ° C. The sample could by cycled many times between the two states. The effect reaches a saturated value in a time that depends on the intensity of the illumination. This dependence is shown in fig. 1 by the dashed line that follows the dark conductivity after successive short intervals of irradiation. A measurement of the temperature dependenc6 of the dark conductivity showed that the activation energy for transport increases with irradiation (fig. 2). This important result means that irradiation moves the Fermi level closer to midgap. The temperature dependence of the annealing process, measured on one sample, gave an activation energy of 1.5 eV [7]. The Staebler-Wronski effect has been found to various degrees in doped a-Si:H, whereupon it was discovered that in the conductivity relation: ~r = ~7o e x p ( - E a / k T ) ,

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the pre-exponential factor is also related to the activation energy E a by a = a ~ exp(AEa). This relation is known as the Meyer-Neldel rule [8]. How well the data follows this rule is shown in fig. 3. Hence, one could write: a = a ~ exp[(A -

1/kT)Ea],

where A = 20 eV 1. Another remarkable observation is that the Staebler-Wronski effect also seems to obey the Meyer-Neldel rule, i.e., the dark conductivity seems to depend on the position of the Fermi level regardless whether this position is determined by doping or by irradiation. Recently, Tanielian et al. [6] have found a 'negative Staebler-Wronski effect' in slightly n-type a-Si:H where the Fermi level is slightly above midgap. Fig. 4 shows a range of enhanced light-induced dark conductivity between regions of normal behavior and the disappearance of the Staebler-Wronski effect at high doping. Some question arose about the possibility that the Staebler-Wronski effect might be a surface phenomenon [9]. In fact, by absorbing and desorbing various gases on the surface of a-Si:H, Tanielian et al.[10] have shown that the Staebler-Wronski

144

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effect is responsive to adsorbates (a surface effect). However, from studies using a sandwich structure (a-Si:H between electrodes) evidence remains that the Staebler-Wronski effect is also a bulk effect [7].

J.L Pankove/Light-induced metastable effects in a-Si:H

145

3. Light-induced absorption An absorption tail is found below the optical gap at absorption coefficients lower than - 1 0 cm 1. Using photothermal spectroscopy, Skumanich et al.[ll] have found that the light-induced metastable states increase the value of the absorption coefficient in the tail by a factor of about 3. A similar observation was made on compensated a-Si:H for which the tail absorption is two orders of magnitude lower. Although we shall consider paramagnetic effects later, let us point out now that the absorption in the tail correlates with the spin density attributable to dangling bonds [12, 131. A short-lived induced absorption has been studied at low temperatures by O ' C o n n o r and Tauc in a-Si:H [14] and in a-SixGe I x:H [15], this induced absorption having a threshold at about 0.5 eV.

4. Metastable luminescence effects Intense irradiation of a-Si:H with - 5 W / c m 2 of visible light induces luminescence fatigue within several minutes. The fatigue effect usually reduces the luminescence efficiency by 10 to 20070 [16], although a decrease by 40°7o at 4.2 K has been reported by Morigaki et al. [17]. Simultaneously with the decrease of the luminescence peak, an enhancement of up to 100°70 is obtained at = 0 . 8 e V , emerging as a new luminescence band (see fig. 5) [16]. These light-induced effects disappear after annealing at ---200 °C, whereupon the luminescence spectrum is fully recovered. These effects are reversible and reproducible. Note that the fully reproducible recovery of the luminescence spectrum after thermal annealing signifies that there is no loss of hydrogen. The magnitude of the light-induced luminescence effects, for both fatigue and enhancement, increase identically with the temperature during irradiation (fig. 6). This suggests that a constant fraction of the new states that reduce the luminescence efficiency become radiators at 0.8 eV. Above 100°C, the competing annealing process dominates. Note that these metastable effects, fatigue of the main emission peak and enhancement at 0.8-0.9 eV, have been observed after irradiation with an electron beam [18, 19] or with a beam of He + ions [19].

5. Light-induced Electron Spin Resonance (ESR) and Optically Detected Magnetic Resonance (ODMR) A persistent ESR signal has been observed after irradiation. This signal could be reduced to the initial value by annealing above ---150° C [19, 20]. The ESR spectrum is

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J.l. Pankove/Light-induced metastable effects in a-Si.'H

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6. Electrically induced effects In deep level transient spectroscopy, DLTS, an electric field is used to periodically fill traps; a temperature ramp is used to empty these traps and provide a signature o f their depth. Lang et a1.[25] report that the light-induced effect increases the density of states below midgap and causes the Fermi level to move away from the conduction band edge. On the other hand Crandall [26] reports that light produces

148

J.l. Pankove/Light-induced metastable effects in a-Si:H

1014 to 1016 new states/cm 3 that appear in a narrow band, and suggests that this band of photo-induced states is approximately at the same potential as the conduction band edge. The reason for this placement of traps is that the thermal activation energy, ET, to fill or to empty the traps is very similar in a given material. E T has a value ranging from 0.5 to 1.5 eV depending on the sample. We shall return to this model later. Another important observation is the possibility to induce the S t a e b l e r - W r o n s k i effect by forward biasing a pin diode or a Schottky barrier to inject electron-hole pairs into the i-layer where they recombine and produce states that mimic those produced by light [4, 27]. Presumably, some of the energy from recombining electron-hole pairs, whether photogenerated or injected, can be used up in creating traps. This could involve an energy exchange similar to that invoked for the degradation of GaAs and other I I I - V diodes [28]. Conversely, if a reverse bias is applied while the diode is illuminated, the electrons and holes are swept out before they can recombine, and no Staebler-Wronski effect results [4, 27].

7. Models The S t a e b l e r - W r o n s k i effect can be interpreted in terms of the formation of new states (possibly near midgap) that move the Fermi level away from the extended states. This model is supported by the increased activation energy for dark conductivity and also by DLTS results. The ESR data indicate that the new states are silicon dangling bonds. The increased dangling bond concentration would be sufficient to explain a decrease in luminescence efficiency, in solar cell performance and in conductivity. A more specific mechanism for the formation of dangling bonds is the excitation of an electron from a weakly bonded Si-Si pair (one that is strained by non-ideal atomic spacing or by non-ideal angular orientation - ideality occurring in a perfect single crystal) [16, 20]. Excitation by X-rays [23], by energetic electrons [18, 19] or ions [19] has enough energy to break even stronger bonds, producing a larger number of dangling bonds. In fact, the luminescence efficiency can be so strongly affected that even the 0.8 eV emission is reduced [18]. Crandall's observation of equal activation energy to fill or to empty the traps led him to propose an electron trap having the same energy as the conduction band edge (or mobility edge) but a trap surrounded by a repulsive barrier [26]. Crandall considered a model that he attributed to Lucovsky wherein a Si a t o m is surrounded by four oxygen atoms. An electron in that SiO4 site would see a large potential barrier that would prevent its escape. Note that all four bonds of the central Si must connect to an oxygen atom or else the electron would find a lower energy escape path. An electron trapped in the S i O 4 center would be endowed with a spin. Its ESR

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8. Conclusion

Light-induced metastable effects result from the formation of dangling bonds and deep traps and the population of shallow traps surrounded by a high barrier. The weakest bonds between adjacent Si atoms can be broken by any process that generates electron-hole pairs: visible photons, X-rays, energetic electrons or ions,

150

J.l. laankove/Light.induced metastable effects in a-Si:H

or by electrical injection. The creation of dangling bonds is sufficient to explain most of the observations: decreased dark conductivity (Staebler-Wronski effect), increased ESR characteristic of dangling bonds, decreased luminescence efficiency, lowered solar cell efficiency. The Influence of high barrier shallow traps on these effects is less obvious. To reduce the susceptibility of a-Si:H to the creation of metastable states, one needs to make a-Si:H with stronger bonds, a condition difficult to achieve when the addition of impurities is required.

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[30] T. Nakashita, M. Hirose and Y. Osaka, Presented 2nd Photovoltaic Science and Engineering Conf., Japan (1981). [31] V.L. Dalal, C.M. Fortmann and E. Eser, AIP Conf. Proc. No. 73, eds. R.A. Street, D.K. Biegelsen and J.C. Knights (AIP, New York, 1981) p. 15. [32] A. Madan, S.R. Ovshinsky and E. Benn, Phil. Mag. B40 (1979) 259.