Recombination ina-Si: H: Transitions through defect states

Abstract

To obtain detailed information about recombination processes near room temperature in a-Si:H we have measured the steady-state and transient response of luminescence, light-induced ESR, and photoconductivity in lightly doped and undoped samples. T¹ low-energy luminescence is at a different energy in nand p-type a-Si:H, and has an intensity-dependent decay. The results lead us to propose a new modelthat the radiative transition is the capture of a majority carrier into a neutral dangling bond, having a low radiative efficiency and only a small Stokes shift. Measurements of the quantum efficiency for generating light-induced ESR (or LESR) confirm that a transition through a danghng bond is the dominant recombination mechanism in all samples, and is predominately nonradiative. %'e discuss the trapping mechanism and conclude that a single multiphonon process does not seem possible. Instead wc suggest that thc ITlcchanlsm may bc a cascade through as yct unidentified excited states. Transient luminescence, LESR, and photoconductivity each show that the response time is much longer in doped samples than in undoped samples. I. INTRGDUCTIONA great deal of information has been obtained about the recombination of excess carriers in a-Si:H. The principal experiments used in these studies are luminescence, ' hght-1Ilduced ESR (LESR), ' photoconductlv1ty, and induced absorption.There are several recombination processes, both radiative and nonradiative, with ihe relative importance of each depending on the experimental conditions. Previous papers in this series have mostly explored the low-temperature mechanisms which are often dominated by a luminescence transition. This paper further investigates the high-temperature regime in which the recombination is predominately nonradiative. In particular, we compare undoped and doped samples and discuss the details of the recombination.At low temperature the dominant recombination processes are a radiative tunneling transition between bandtail states and nonradiative tunneling to dangling-bond defects. ' Surface and Auger recombination are present, but tend to be less important. ' Some recent measurements also suggest another process of, as yet, unknown origin. The low-temperature recombination is characterized by tunneling rather than extensive diffusion of carriers because the presence of 'a large density of band-tail states causes rapid trapping in states much deeper than kT. ' Near room temperature, carriers are more mobile so that carrier transport is an important factor in the recombination.The band-edge luminescence is strongly quenched, this effect being attributed to trapping at dangling bonds. ' There is a second weak luminescence transition at lower energy that tends to dominate near 300 K and which has also been associated with recombination at the defects. ' This band is seen particularly in doped samples in which the defect density is known to be high.The association of dangling bonds with recombination has been confirmed recently by time-of-fhght photoconductivity. " The...

Nd3+ photoluminescence and its implication on the excitation mechanisms of Nd3+ in Nd doped hydrogenated amorphous silicon alloyed with carbon

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“…Fig. 2 shows the PL spectra of instrinsic, Nd-doped, and Er-doped a-Si:H:C, taken at 20 K. We observe a strong, broad luminescence peak from the intrinsic a-Si:H:C film in the 0.6-1.1 lm range that is typical of bandtail to bandtail transitions in a-Si:H and related films [24]. The apparent peaks near $0.7 and $0.9 lm are optical artifacts due to multiple reflections.…”

Visible photoluminescence from silicon-incorporated diamond like carbon films synthesized via direct current PECVD technique

“…Hence the enhancement of PL properties was attributed to the alteration of the electronic structure by the incorporation of substitutional defect states and the donor activity of silicon. Earlier, Street et al [30] reported that PL of hydrogenated amorphous silicon near room temperature occurred by recombination of dangling bonds and also because dangling bonds acted as luminescence sites. It is to be noted that recombination process in a-C:H is believed to be similar to that of a-Si:H [24]; so based on the above discussions it can be inferred that, as due to doping, the percentage of sp 3 hybridized carbon increases in our case, the number of carbon dangling bonds also increases.…”

Photoluminescence of very thin oxide/a-Si:H structures passivated in HCN solutions

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