Influence of the Distribution of Tail States in a-Si:H on the Field Dependence of Carrier Drift Mobilities
- PDF / 207,518 Bytes
- 12 Pages / 612 x 792 pts (letter) Page_size
- 36 Downloads / 190 Views
A5.6.1
Influence of the Distribution of Tail States in a-Si:H on the Field Dependence of Carrier Drift Mobilities Monica Brinza, Evguenia V. Emelianova, André Stesmans and Guy J. Adriaenssens Laboratorium voor Halfgeleiderfysica, University of Leuven, Celestijnenlaan 200D, B-3001 Leuven, Belgium. ABSTRACT Exponential distributions of tail states have been able, within the framework of a multipletrapping transport model, to account rather well for the time-of-flight photoconductivity transients that are measured with ‘standard’ a-Si:H, i.e. material prepared by plasma-enhanced chemical vapor deposition at ∼250°C. A field-dependent carrier mobility in the dispersive transport regime is part of the observations. However, samples prepared in an expanding thermal plasma, although still exhibiting the dispersive transients, fail to show this field dependence. The presence of a Gaussian component in the density of valence-band tail states can account for such behavior for the hole transients. Nanoscale ordered inclusions in the amorphous matrix are thought to be responsible for the Gaussian density of states contribution.
INTRODUCTION It has been widely accepted that the band tails in hydrogenated amorphous silicon (a-Si:H) can be described fairly well by exponential distributions of localized states of the type g(E) = g(0)exp(-E/E0) [1]. The characteristic energy E0 is often expressed as temperature T0 = E0/k, and has been variously determined as 250 to 300 K for the conduction band tail and around 500 K for the valence band tail. The exponential band tail distributions offer a natural explanation for the anomalously dispersive non-equilibrium carrier transport that is seen in a-Si:H, and at the same time they strongly facilitate the formulation of analytical expressions for experimental quantities related to the carrier transport. The experimental results most closely linked to the assertion of exponential tails have been obtained from Time-of-Flight (TOF) transient photocurrent measurements of the type published by Tiedje et al. [2]. The full analysis by Arkhipov et al. [3] as well as the simplified formulation of the problem by Tiedje and Rose [4] are still frequently cited sources on the subject. However, an exponential density of states (DOS) only explains part of the experimental observations. It is self-evident that the exponential distribution of tail states must necessarily give way to a different distribution at or near the band edges, or deeper in the gap where the defect density plays a role, but it turns out that also with respect to material properties that are largely determined by the tail states, the exponential distribution often fails to agree with the data. For instance: in the standard multiple-trapping analysis of the TOF experiment, an exponential DOS leads to the well-known power laws i ph (t ) ∝ t −(1−α ) , t < tT ,with α = T / T0 , (1) i ph (t ) ∝ t −(1+α ) , t > t T for the transient current before and after the transit time. In practice however, the two power laws will only occasionally yield
Data Loading...
