72, 222–226] and ODMR [227–235] to study the microscopic nature of transport and re- combination processes in a-Si:H. In view of the results obtained from PC, PL and QFRS spectroscopy, this section summarizes the additional information gained from ODMR and EDMR leading to the present picture of transport and recombination in a-Si:H.
3.4.1 Spin-dependent recombination via dangling bonds
EDMR has been applied not only the a-Si:H films, but also to fully processed devices such as a-Si:H homo- [67, 68, 236–238] and a-Si/c-Si heterojunction solar cells [69, 77, 239, 240], a-Si-based thin-film transistors (TFTs) [72] or Schottky-barrier diodes [71]. All types of samples and devices exhibited a characterisitic current-quenching EDMR signal at temperatures T = 100–300 K, where most of the early EDMR experiments were carried out. This signals was immediately assigned to non-radiative recombination via DB defects, which had already been proposed before as the predominant recombination channel [192] due to the anticorrelation between PL efficiency and DB defects (see sec- tion 3.3.1). Deconvolution of the observed EDMR signal by altering the modulation phase (see fig. 3.5a) revealed of two features that contain the characteristic signatures of all para- magnetic states known in undoped a-Si:H:
All samples showed a line at g = 2.0050 (ΔBpp≈ 1 mT), consistent with the super- positon of the resonances of a singly occupied CBT state (g = 2.0044) and a neutral DB defect (g = 2.0055).
In samples of low defect density, an additional line appeared at g = 2.01 (ΔBpp≈ 2 mT), identical to the resonance of a hole trapped in a VBT state. The relative intensity of the hole line increased with decreasing defect density [60].
Dersch et al. [60] were the first to associate these two signal components with a two-step recombination process, schematically depicted in fig. 3.5b: They interpreted the appear- ance of the g = 2.0050 in the EDMR spectra of all a-Si:H samples as a spin-dependent
2 mT (a) 0° 84.5° (e–D0) 90° (h–h) Magnetic field EDMR sig nal EC EV D0,D+ D− h–h e–D0 h–D− (b)
FIGURE 3.5 (a)Field-modulated CWEDMR spectra of undoped a-Si:H atT=160 K. Recorded with a modulation fre- quency of5 kHz and different modulation phases as indicated, to decompose the signal. Reproduced from ref. [27]. (b)Recombination scheme for spin-dependent recombination via DBs as proposed by Dersch et al. [60]. Dashed arrows mark tunneling transitions of electrons and holes. Arrows on electrons and holes indicate spin-dependent transitions (see text for further details).
tunneling transition of an electron trapped in a CBT state into a neutral DB defect (e–D0). The recombination cycle is completed by a the capture of a hole into the intermediately negatively charged DB (h–D−). A direct tunneling transition of a hole into the doubly occupied D−state is not spin-dependent. Thus, Dersch et al. explained the appearance of a current-quenching VBT resonance for low-defect samples with diffusion of holes by hopping through VBT states towards the DB site, thereby increasing the recombination rate. The hopping transitions can be spin-dependent if two neighbouring VBT states are singly occupied (h–h transition in fig. 3.5b). For samples of high defect density, a single direct transition to a defect state becomes more probable than diffusion through the tail states, which explains the decreasing relative intensity of the hole resonance.
A similar resonance has also been found in ODMR spectra of undoped a-Si:H [231– 233]. As expected for a non-radiative recombination process, the e–D0 resonance ap- peared as a quenching signal in ODMR. For samples of low defect density, also the hole resonance has been observed as a quenching ODMR signal [231].
Although the picture proposed by Dersch et al. explains the experimentally observed EDMR and ODMR signals and has been widely accepted in the literature, it is to be noted that the microscopic recombination mechanism appears to change when changing the excitation energy. Brandt and Stutzmann [241] found that for sub-bandgap illumination, the g-value of the e–D0resonance shifts from g = 2.0050 to g = 2.0062. Later, Stutzmann et al. [17] explained this behavior with a change of the dominant recombination mecha- nism from tunneling of an electron into the DB followed by the capture of a hole, to tun- neling of hole into DB followed by the capture of an electron. In fact, ODMR experiments using the defect luminescence band (0.9 eV) show an enhancing signal at g ≈ 2.0065, which has been assigned to a radiative recombination between VBT holes and neutral DBs [221, 229, 233]. This shows the microscopic process behind the dominant recombi- nation mechanism in a-Si:H is still subject to debate.
3.4.2 Spin-dependent hopping conduction
At low temperatures (T ≲ 100 K), reported EDMR spectra of a-Si:H exhibit resonances at g = 2.0040–2.0045 and g ≈ 2.01 [17], resembling the EPR signatures of CBT- and VBT- trapped electrons and holes. This assignment is supported by the fact the g = 2.004 reso- nance appears in n-type phosphorus-doped a-Si:H [27, 69, 77, 225], whereas the g = 2.01 signal is found in p-type boron-doped samples [27, 60, 225]. Both resonances have been identified as current-enhancing signals, which has recently been confirmed by means of PEDMR [26]. Since hopping transport via tail states is the dominant transport process at low temperatures (see section 3.3.1), both signals have been assigned to spin-dependent hopping conduction of electrons and holes in their respective tail states (see also sec- tion 2.5.5).
In doped a-Si:H, the observation of spin-dependent hopping within the tail states is consistent with the doping-induced shift of the Fermi level. Spin-dependent hopping has, however, also been observed in undoped a-Si:H, for instance in a-Si:H TFTs under conditions of strong electron accumulation [72], or in a-Si:H solar cells under illumina- tion or strong forward-bias conditions [26, 69, 226]. In these non-equilibrium cases, the quasi-Fermi levels of electrons and holes are shifted from mid-gap into the tail states.
While spin-dependent recombination in a-Si:H has been extensively investigated in previous research, spin-dependent transport at low temperatures has gained much less
3.4 Spin-dependent transport and recombination
attention yet. This is also a result of the much lower photoconductivity of a-Si:H at cryogenic temperatures. In fact, most CWEDMR experiments on a-Si:H have been con- ducted at temperatures above 100 K, where transport predominantly occurs within the extended band states, and spin-dependent recombination via DB defects is the major spin-dependent transition probed by EDMR. With improved experimental setups and sample designs, low-temperature EDMR have come into reach. In chapter 6, we will present a comprehensive low-temperature PEDMR study, which will also reevaluate the assignment of the observed signals the spin-dependent tail-state hopping conduction.
3.4.3 Excitonic states
Besides the EDMR and ODMR signals assigned to spin-dependent recombination and transport channels via defects and localized tail states, both methods have revealed a dis- tinctively different resonance at g ≈ 2.008, characterized by an unusually broad X-band line width of about 20 mT. This line has been observed in CWODMR spectra of undoped a-Si:H at low temperatures (T ≤ 30 K) [233, 242, 243] as well as in CWEDMR spectra at temperatures of about 150 K [27, 244].
The origin of this resonance has been controversially discussed throughout the liter- ature. Brandt and Stutzmann [244] assigned the line to strongly dipolar-coupled triplet excitons and explained the broad line width with a distribution of exciton radii and cou- pling strengths. Such a distribution would lead to a line shape that consists of a superposi- tion of Pake patterns (see section 2.3.3), adding up to form an approximate Gaussian line shape. This is consistent with the observed EDMR line shape. From the line width, they estimate an average exciton radius of 5 A. The argument is, however, solely based on the line width. An alternative explanation was also based on excitonic states, but explained the signal with strongly exchange-coupled electron-hole pairs trappied in CBT and VBT states [27, 233, 235]. This interpretation is supported by the observed g-value, which is close to the arithmetic mean of the CBT and VBT resonances.
In the case of ODMR, unambiguous proof for the presence of spin-dependent pro- cesses via excitonic states is provided by the observation of a half-field resonance (g ≈ 4), which can be assigned to the ΔmS= ±2 transitions between the |T+⟩ and |T−⟩ states of an S = 1 triplet manifold [242, 243, 245]. In addition, later pulsed-ODMR studies identi- fied the S = 1 character by means of field-resolved ED-Rabi nutation experiments [246– 248]. Such evidence is, however, as yet missing for the EDMR resonance.
To date, it is unclear whether the same excitonic species give rise to the ODMR and EDMR signals. In addition, the nature of the underlying spin-dependent process is en- tirely unknown. Spin-dependent radiative recombination of the excitons could explain the ODMR signal. This is supported by the observation of radiative recombination chan- nels via geminate excitons by means of photoluminescence, as discussed in section 3.3.2. However, such a spin-dependent recombination channel via geminate triplet excitons cannot cause an EDMR signal, since geminate pairs do not contribute to photoconduc- tivity. Possible alternative explanation include spin-dependent dissociation or a three- particle Auger-like recombination mechanism. Such processes have been identified in organic semiconductors [21, 96] and recently also in µc-Si:H [98]. Answers to these open questions cannot be found from the EDMR and ODMR spectra alone. Modern PEDMR techniques, however, allow to probe the origin of this resonance in much more detail. In chapter 6, we will address this issue and present a combined EDMR/EPR study that not
only proves the excitonic origin of the EDMR resonance, but also allows to conclude on the microscopic nature of the underlying transport process.
3.5
SUMMARY
This chapter has given a brief overview of the characteristics of a-Si:H, especially focussing on the electronic-transport properties that arise from the structural disorder. This thesis lays particular emphasis on the microscopic nature of transport and recombination pro- cesses that are of fundamental importance for the efficiency of a-Si:H-based devices such as, e. g., solar cells. The immense research history on a-Si:H—part of which presented in this chapter—lays the basis for this work. In particular, the models developed for trans- port and recombination channels via defect states will be probed with the help of modern PEDMR techniques. We will use this chapter as a starting point and reference in order to set the results obtained in the course of this thesis into context.
Finally, it is to be noted that many of the mechanisms present in a-Si:H have proven to be universal properties of amorphous semiconductors. Therefor, both earlier studies and the results of this thesis are not necessarily limited to a-Si:H or the specific situation in a-Si:H-based solar cells, but can be of more general relevance, considering a-Si:H as a prototype material for disorded solids.