Metastable Defects in Amorphous-Silicon Thin-Film Transistors

Abstract

When a positive gate voltage is applied to an amorphous-silicon thin-film transistor, electrons become trapped in states close to the silicon-dielectric interface. This is studied by a new technique involving the transient discharge current produced under illumination. It is suggested that the behavior may involve metastable dangling bonds generated within the amorphous silicon as a consequence of the field-effect-induced increase in electron concentration. This constitutes an important new instability mechanism for amorphous-silicon thin-film transistors.PACS numbers: 71.55.Fr, 73.40.Qv, 85.30.Tv Amorphous-silicon thin-film transistors (TFT's) are currently of great commercial interest. The best such devices presently available are fabricated with a hydrogenated-amorphous-silicon (a-Si:H) active layer and an amorphous-silicon-nitride gate dielectric, both grown by the glow-discharge technique. 1 The trapping of charge in the region of the semiconductor-dielectric interface is of considerable importance in determining device stability. A TFT subjected to a large positive gate voltage for a long period exhibits a large positive shift in threshold voltage. This was interpreted in terms of electron injection into the nitride. 2 Additionally, from studies of the silicon-nitride interface, Street and Thompson 3 have suggested that appreciable charge storage occurs in the silicon, involving states of depth 0.25 eV, plus deeper centers not resolved by their particular experimental technique. TFT's have also been shown to alter their characteristics as a result of prolonged illumination, 4 in line with the known creation of metastable dangling-bond states as observed in the Staebler-Wronski effect. 5 In this paper, we describe a new photoconductive technique for the study of excess trapped carriers. The devices used were prepared by the glow-discharge decomposition of silane in a capacitatively coupled reaction chamber, as described elsewhere. 6 The experimental process involves the photoinduced discharge of devices in which charge has been allowed to accumulate during a period of application of a gate voltage. To avoid complications associated with transient changes in source-drain current under illumination, these electrodes were linked [ Fig. 1(a)]. As indicated in Fig. 1 (b), a gate field was applied for a period t g and then removed. Under these conditions, the source and drain contacts are injecting, and excess electron majority carriers are drawn across the a-Si:H layer towards the silicon-nitride interface, in a time much shorter than the t g values used here. After a time delay of t d , the specimen was briefly illuminated, yielding a discharge current pulse which was detected across resistor R. Illumination for 150 msec at a flux of 10 16 cm"" 2 sec~l was sufficient to produce a complete discharge. The total charge Q, assessed by integration of the transient current through 7?, established the residual number of carriers trapped. Figure 2(a) displays the influence upon Q of delay time and temperature. To obtain...

Light-induced defects in thermal annealed hydrogenated amorphous silicon

“…It seems clear that the creation of metastable defects is produced by the recombination of electron-hole couples [2], or utmost by charge trapping [3]. It is also accepted that this effect, although influenced by the presence of high impurity concentrations [4], is intrinsic to hydrogenated amorphous silicon.…”

Bias dependence of instability mechanisms in amorphous silicon thin-film transistors

“…3 However, more recently, an alternative model involving the slow creation of states in the a-Si:H has been proposed. 5 The states are metastable, since the effect can be annealed out. The states were suggested to be Si dangling bonds, due to certain similarities to the photoinduced degradation.…”

“…The states were suggested to be Si dangling bonds, due to certain similarities to the photoinduced degradation. 5 The essential difference in the two models is that in Ref. 3 charge is slowly trapped into electron states which always exist but are in poor communication with the a-Si conduction band.…”

Relationship between defect density and charge carrier transport in amorphous and microcrystalline silicon

“…The defect density is strongly affected by the preparation conditions, e.g., when aiming at high deposition rates, and by several external influences such as light illumination or bias stress, which result in a metastable increase of N D . 2,3 Besides being therefore an important measure of the material quality and optimization parameter for the deposition process of a-Si: H, the defect density and its relationship to charge carrier dynamics and recombination processes have been of fundamental interest. For c-Si: H, which exists in a large variety of structure compositions of crystalline and disordered phase in different amounts and dimensions, structural defects such as dangling bonds are also expected to influence the charge carrier transport, recombination, and device performance.…”