THE EFFECTS OF SULFUR INCORPORATION ON THE OPTOELECTRONIC PROPERTIES OF HYDROGENATED AMORPHOUS-GERMANIUM

Hydrogenated amorphous germanium-sulphur (a-G(1-x)S(x):H) alloys have been prepared by the glow-discharge technique from mixtures of hydroge n, germane and sulphur hexafluoride gases. The sulphur content of the films prepared at the powered and the unpowered electrodes varied betw een 2 and 30 at.%. Addition of sulphur decreases the activation energy of the conductivity from a typical value of 0.5 eV for hydrogenated a morphous germanium (a-Ge:H) to minima of 0.14 and 0.28 eV for the allo ys deposited on the unpowered electrode and powered electrode respecti vely. Concurrent with this, for the cathode-deposited films, the eta m u tau product of electrons is increased by one order of magnitude to a maximum of 2 X 10(-6) cm(2) V-1, and the ambipolar diffusion length d ecreases from 470 to 350 Angstrom. From these trends, and with a consi deration of various additional experimental findings, it is inferred t hat n-type doping occurs when sulphur is added to a-Ge:H. Sulphur inco rporation improves the quality of films prepared at the unpowered elec trode as is evident from the optoelectronic properties; the eta mu tau product increases by more than three orders of magnitude to 10(-6) cm (2) V-1, whereas the ambipolar diffusion length is nearly constant at around 400 Angstrom.