Band tail recombination

While most of the research in metastable defect formation has focussed on light-induced defects, there has recently been growing interest in thermally generated defects. Smith and Wagner (1985 Smith et al., 1986) extended the proposed Staebler-Wronski mechanism of electron-hole recombination via band tail states, resulting in the formation of dangling... 

The recombination process comprises two sequential steps, as illustrated in Fig. 8.1. An excited electron or hole first loses energy by many transitions within the band, in which the energy decrements are small but frequent. This process is referred to as thermalization. The thermalization rate decreases as an electron moves into the localized band tail states and the density of available states is lower. Eventually the electron completes the recombination by making a transition to a hole with the release of a large energy. Recombination lifetimes are generally much longer than the thermalization times, so that the two processes usually occur on distinctly different time scales. 

The recombination mechanisms which operate in a-Si H are the same as in crystalline semiconductors. The presence of disorder apparently does not lead to any new processes, but does influence which mechanisms apply and their relative contribution in different measurements. The large density of band tail localized states is the most influential factor in the recombination. Recombination at low... 

The rapid thermalization of carriers in extended states ensures that virtually all of the recombination occurs after the carriers are trapped into the band tail states. The two dominant recombination mechanisms in a-Si H are radiative transitions between band tail states and non-radiative transitions from the band edge to defect states. These two processes are described in this section and the following one. The radiative band tail mechanism tends to dominate at low temperature and the non-radiative processes dominate above about 100 K. The change with temperature results from the different characteristics of the transitions. The radiative transition rate is low, but there is a large density of band tail states at which recombination can occur. In contrast, the defect density is low but there is a high non-radiative transition rate for a band tail carrier near the defect. Band tail carriers are immobile at low temperatures, so that the recombination is... 

Examples of the low temperature luminescence spectra are shown in Fig. 8.12. The luminescence intensity is highest in samples with the lowest defect density and so we concentrate on this material. The role of the defects is discussed in Section 8.4. The luminescence spectrum is featureless and broad, with a peak at 1.3-1.4 eV and a half width of 0.25-0.3 eV. It is generally accepted that the transition is between conduction and valence band tail states, with three main reasons for the assignment. First, the energy is in the correct range for the band tails, as the spectrum lies at the foot of the Urbach tail (Fig. 8.12(6)). Second, the luminescence intensity is highest when the defect density is lowest, so that the luminescence cannot be a transition to a defect. Third, the long recombination decay time indicates that the carriers are in localized rather than extended states (see Section 8.3.3). 

Light induced ESR measures the density of band tail electrons and holes, and provides a different method of measuring the recombination... 

The quantum efficiency of the band tail luminescence is largest at low temperature, low excitation intensity, and in samples of low defect density. Other conditions cause competing non-radiative processes which quench the luminescence intensity. Direct recombination to defect states in samples of high defect density is discussed in Section 8.4.1. The other main non-radiative mechanism is thermal... 

Ej is a demarcation energy, similar to that defined in the analysis of dispersive transport (see Section 3.2.1). It is assumed that all carriers which are thermally excited recombine non-radiatively, but the same result is obtained if some fraction are subsequently retrapped and recombine radiatively. The luminescence efficiency is given by the fraction of carriers deeper than E, . An exponential band tail density of states proportional to exp (E/kf,) results in a quantum efficiency of... 

Fig. 8.20 shows the dependence of the band tail luminescence intensity on the defect density as measured by the g = 2.0055 ESR resonance in undoped a-Si H. The luminescence intensity drops rapidly when the defect density is above 10 cm" , becoming unobservable at defect densities above 10 cm" (Street et al. 1978). These data establish that the defect provides an alternative recombination path competing with the radiative band tail transition. At the low temperatures of the measurements, the electrons and holes are trapped in the band tails and are immobile. Both the radiative and the non-radiative transitions must therefore occur by tunneling. Section... 

Fig. 8.21. Illustration of the competition between band tail radiative recombination and defect recombination either by tunneling (solid line) or by thermal excitation to the band edge (dashed line).

The defect recombination is predominantly, but not completely, non-radiative. The quenching of the band tail luminescence at high temperature or at high defect density is accompanied by the onset of a weak luminescence transition at lower energy. Some typical luminescence spectra are shown in Fig. 8.25 for doped and high defect density undoped material. The peak energy at low temperature is... 

The charge-induced defect creation mechanism is too slow to be significant at low temperature and the electronic recombination effects reestablish themselves. Low temperature measurements (0-100 K) have been performed using an IR probe beam to modulate the excess carrier density that is in the band tail states (Hundhausen, Ley and Carius... 

Figure 7.5. The radiative recombination of band tail electrons e" and holes h, which yields the main 1.3 eV emission band, the radiative decay of H into neutral dangling bonds db", which yields the defect emission band at 0.9 eV, and the non-radiative decay of e" into db in a-Si H.

The first observation of a metastable component to the optically induced ESR in a-Si H was reported by Street ct a/., (1981) in heavily P- and B-doped samples and in doped and compensated samples. [For a recent review of metastable, optically induced effects in a-Si H see Pankove (1982).] As near as can be determined, this metastable signal is a fraction ( 20%) of the transient optically induced response, and the dependences of the line shapes on doping appear to be similar for the two eflFects. These ESR centers can be generated with light intensities of 150 mW cm , and they anneal in a few minutes at 80 ° K. It is particularly surprising that a subset of the photoexcited electrons are trapped in band-tail states or in a Tj configuration in p-type material in which there exists a large density of holes to facilitate rapid recombination. 

In the following, we present a model for interpreting our ODMR results The luminescence arises from radiative recombination of trapped electron -hole pairs, in which trapped electrons lie in the band-tail and gap regions and trapped holes are localized at the A centers. 

The nature of the conduction-band-tail states that are responsible for radiative recombination is not clear. Potential fluctuations occur at the conduction-band edge due to the disorder associated with the random network of Si atoms and the random distribution of foreign atoms such as H and F (Dunstan and Boulitrop, 1981). As a result, local minimum energy states are created, in which electrons are captured after band-to-band excitation. These states may be possible candidates for conduction-band-tail states. 

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