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The room-temperature fit result, comprising the three resonances of DBs and CBT/VBT states, is consistent with earlier EDMR studies on a-Si:H. Dersch et al. [60] were the first to interpret the signal in terms of a spin-dependent recombination of charge carriers via DBs (see section 3.4.1). A recombination process is in accordance with the negative sign of the EDMR signal, indicating a current-decreasing process. Dersch et al. assumed a two-step recombination process: In a first step, an electron trapped in a CBT state tun- nels into a nearby neutral (singly occupied) DB. This transition (e–D0_{) is spin-dependent} and explains the observation of the CBT and DB resonances. The second step, which completes the recombination cycle, is the capture of a hole into the intermediate D−_state (doubly occupied, negatively charged DB). Since this transition is not spin-dependent, Dersch et al. explained the presence of the VBT resonance with spin-dependent hopping of holes through the tail states towards the DB site, thereby promoting the recombination rate (see section 3.4.1 and fig. 3.5b).

This picture for spin-dependent recombination in a-Si:H satisfactorily explains the presence of all three resonances in the EDMR signal and has therefore been adopted throughout the literature. Nevertheless, it is not necessarily the only explanation for the observed signal. In fact, the spectral weights resulting from our fit to some extent contra- dict this recombination picture. Assuming that the spin-dependent recombination step is the initial e–D0_{transition, one would expect equal intensities of the CBT and DB res-} onances, since flipping either one of the two spins would induce the resonant transition. On the other hand, the intensity of the VBT line would be independent from the other two lines if the resonance originated from h–h transitions within the VBT states. From our fit, we obtain a set of intensity weights of about 0.30/0.14/0.57 for the CBT/VBT/DB lines, both for the X-band and for the 263 GHz spectrum. Certainly, the exact values are afflicted with a non-negligible degree of uncertainty, as intensity weights are very sensitive to slight changes in the line shape (e. g., Gaussian or Lorentzian broadening functions). However, the fact that we obtain almost equal, independently fitted sets of weights for the X-band and 263 GHz spectra enhances the reliability of the results. Our results suggest that DB resonance is almost twice as large in intensity than the CBT line, which conflicts with the recombination picture of Dersch et al.

A possible alternative explanation could be that the spin-dependent recombination via DBs occurs in two different ways: Either, as proposed by Dersch et al., an initial spin- dependent e–D0 _{transition is followed by the capture of hole, or, vice versa, the initial} step is a spin-dependent transition of a VBT hole into the neutral DB (h–D0_{), completed} by capturing an electron into the intermediate D−_{state. In this picture, the intensities of} the CBT and VBT lines would still be independent from each other, but the DB intensity would be expected to match the combined CBT and VBT line intensities, much closer to our experimental results. Our fit results thus speak for two independent spin-dependent

5.4 Spectral fitting

recombination processes of both CBT electrons and VBT holes via DB defects. Never- theless, based on the EDMR spectrum alone, it is difficult to definitively choose between the two recombination models.

While the interpretation of the room-temperature signal in terms of spin-dependent recombination channels via DBs provides a solid explanation of the experimental data, the situation for the low-temperature case is less clear. The global fitting strategy revealed that the CBT and VBT resonances are present at both temperatures, whereas the DB line is only observed at room temperature. The positive signal sign at low temperature in- dicates a current-enhancing process. Since, at temperatures T ≲ 100 K, charge-carrier transport in a-Si:H is dominated by hopping conduction within the band-tail states (see section 3.3.1), the observation of current-enhancing CBT and VBT resonances has com- monly been assigned to spin-dependent hopping transitions e–e and h–h via the respec- tive tail states [27, 69, 72, 77, 226].

The origin of the broad TE line, which we fitted with a simple Gaussian broadening function with a FWHM of 18 mT, remains elusive. Based on the unusually large field- independent line width, we will tentatively follow the assignment of previous studies to strongly dipolar coupled S = 1 states. A distribution of spin-spin distances and dipolar coupling strength can explain the observed line shapes in terms of a superposition of Pake patterns. It is tempting to assign the resonance to TEs formed by strongly coupled electron-hole pairs trapped in CBT and VBT states in close proximity, since the observed g-value is close to the arithmetic mean (g_CBT+ g_VBT) /2 = (2.0044 + 2.0100) /2 = 2.0072. The nature of the spin-dependent transport channel, however, remains unclear. Evidence for recombination channels of geminate triplet excitons has been obtains from photo- luminescence spectrocopy [206, 210] (see section 3.3.2) and pulsed ODMR [246, 248]. Geminate-pair recombination, however, does not affect photoconductivity. Moreover, the TE line is observed as a current-enhancing signal, such that an excitonic recombina- tion channel alone cannot explain the EDMR signal. Possible explanations include spin- dependent dissociation of strongly coupled geminate pairs or three-particle processes, e. g., an Auger-recombination channel. Meier et al. [98] recently identified an Auger-like spin-dependent recombination process as the origin of similar EDMR signal in micro- crystalline silicon (µc-Si:H). An excitonic EDMR signal has also been identified from ED-Rabi nutations on amorphous silicon nitride (a-Si_1−xN_x:H) [266]. Based on the data presented in this chapter, however, we can neither prove the excitonic nature of the TE line in a-Si:H, nor draw conclusions regarding a potentially similar origin as those S = 1 signals found in µc-Si:H or a-Si_1−xN_x:H. We will address this issue in chapter 6, where we present conclusive evidence for strongly coupled electron-hole pairs, based on a compre- hensive PEDMR study carried out at low temperature.

A number of open question remain concerning the interpretation of the two EDMR signals observed at high and low temperature. However, the results discussed in this sec- tion clearly show that two different spin-dependent processes lead to the EDMR signals at different temperatures. In the section, we will investigate the transition between the two temperature regimes by analyzing EDMR signals recorded over the full temperature range from 10 K to room temperature.