THE EFFECT OF ANNEALING AND ILLUMINATION ON THE FIELD EFFECT CONDUCTANCE OF AMORPHOUS SILICON

Abstract

We have measured the effect of white light illumination and annealing to 180°C on the field effect conductance of glow discharge a-Si:H. Annealing produces a change in the off conductance by up to a factor of 30, and moves the threshold field by 2 x lo4 ~cm-l. These changes are reproducibly reversed upon illuminating with white light. A major part of the effect of illumination is most likely due to a movement of the Fermi level in the bulk of the material. Annealing to 180°C reverses this effect. Furthermore excess electrons can be trapped at the amorphous siliconsilicon nitride interface by a field assisted trapping mechanism and these are also de-trapped upon annealing to 180°C. Reversible changes in the dark,conductance of thin films of glow discharge amorphous silicon, due to annealing at T > 150°C and illumination with white light, have been reported by a number of groups. These have been interpreted as due to metastable changes in the density or occupancy of defect states which cause a movement of the bulk Fermi level (1)-(3) or conversely as due to the creation and elimination of space charge regions at one or other of the two surfaces of the film (4),(5). We have performed similar measurements on field effect devices, where one surface of a 0.5 um film of a-Si:H is separated from a gate electrode by a 0.5 pm layer of plasma deposited silicon nitride and the other surface is covered by a 0.5 um plasma deposited Si02 layer, but has no gate electrode (6). Such devices are currently of considerable interest as thin film transistors (TFTs) for matrix addressing of displays (7). In our devices, annealing to 180°C (with zero gate volts) produces a room temperature conductance between 5 and 30 times the room tempera?-ure conductance after illuminating the sample with white light from a 50 Watt quaftz-halogen lamp for 90 minutes. Figure 1, shows an example where a conductance change of 30 is observed. Subsequent re-illumination reproduces the initial condition (indicated by the vertical arrow in figure 1) and further cycles of annealing and illumination reproduce the same results. Although the magnitude of the effect in our samples is much smaller than originally reported by Staebler and Wronski (I), it is very comparable with results observed in other laboratories (5). In figure 2 we show the field effect conductance (at 4O0C) in each of the two states. State A is after annealing the sample to 180°C and state B after illumination. The field effect conductance is also reproducible on repetitive cycles of annealin and illumination. In this figure the ordinate is the sheet conductance f G( $210)and the abscissa is the surface field in the semiconductor at the amorphous silicon-silicon nitride interface, given by F = E ~ ~ ~ V ~ / E ~ ~ ~ ~ . V is the gate voltage, tins the dielectric constant and dins the thickness of the insulator and E the dielectric constant of the semiconductor. In figure 2, we plot the same results on both a log and a linear scale. In both states, the field effect conductance shows little change with negative gate voltages, but for increasing positive gate voltages the conductance first increases approximately exponentially, then at higher gate voltages increases approximately linearly. The onset of the linear region occurs at a threshold field Ft. Clearly the cycle of annealing and illumination has two effects, to change the threshold field Ft and to change the off-conductance Go (the approximately constant Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1981481 C4-380 JOURNAL DE PHYSIQUE Fig 1. Temperature dependence of Fig 2 . Field effect conductance a t 4 0 ' ~ the dark conductance of a 0.5pm after annealing to 1 8 0 ~ ~ (state A ) conductance at negative gate voltages). These two parameters Go and Ft are of paramount importance in the characterisation of TFTs, and changes in these quantities represent instabilities which are highly'undesirable for device applications. Although the changes are brought about by conditions which are well outside the normal operating conditions of TFTs, an understanding of the origin of the effects is of considerable interest.