CdSe thin film transistor (TFT) structures which have been ion implanted with 50 keV 52Cr, 50 keV 27Al, or 15 keV 11B have a very steeply rising conductivity above some threshold dose and exhibit modulated transistor characteristics over certain ranges of implant dose, even though there is no thermal annealing during or after ion implantation. These results are interpreted using a model based on grain boundary trapping theory. The dependence of leakage current on implant dose, and of drain current (at a fixed dose) on gate voltage are described very well by this model, when the drain voltage is very small. Using this simple model, the important parameters of the polycrystalline CdSe film, namely the trap density per unit area in the grain boundary, the donor density, grain size, and electron mobility can be deduced. The effect of thermal annealing on implanted and unimplanted CdSe TFT’s has also been studied and the model appears to give a general description of the conductivity behavior in polycrystalline semiconductor TFT’s. This is illustrated by applying the model to devices fabricated by other groups from polycrystalline CdSe, poly-Si and laser-annealed poly-Si semiconductor layers.
Zn diffusion in InP has been investigated in two configurations: diffusion from an external source into uniformly n-doped substrates and diffusion between layers in n-p-n-p-n structures grown by metalorganic chemical vapor deposition (MOCVD). Alternating layers of p-type InP (0.5 μm, 4×1017<[Zn]<2×1018 cm−3) and n-type InP (0.5 μm, 1016<[Si]<3×1019 cm−3) were grown by low-pressure MOCVD at T=625 °C. The distribution of Si and Zn was determined by secondary ion mass spectrometry, using implanted standards to calibrate the data. For undoped spacer layers (n∼1016 cm−3) the diffusion profiles depended strongly on the Zn-doping level; little out-diffusion of Zn was observed for [Zn]=4×1017 cm−3, but for [Zn]>1018 cm−3, the Zn completely diffused across the spacer layers during growth (1–2 h). For doped spacer layers, the doping level (Si) had a dramatic effect on the Zn diffusion profiles. The total Zn diffusion across the grown dopant interface was not substantially affected, but an accumulation of Zn took place in the Si-doped layers, with the formation of Zn spikes, for which the increase in Zn level, as compared to that in the Zn-doped layer (∼1018 cm−3) was similar to [Si]. No diffusion of Si across the grown dopant interface was detected. Electrochemical C-V profiling indicated that the Si and Zn were electrically active. The results have been explained in terms of a model in which mobile Zn species diffusing into the Si-doped layers are immobilized by the formation of Zn-donor pairs. The model is shown to be consistent with diffusion profiles obtained for diffusion of Zn into n-InP from an external ZnGaCdIn source. The effect of this diffusion on as-grown junctions, is to displace the location of the junction. These results can have important consequences for device fabrication.
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