HIGH-PERFORMANCE GLOW-DISCHARGE A-SI1-XGEX-H OF LARGE X

Radio frequency glow discharge chemical vapor deposition has been used to deposit thin films of a-Si1-xGex:H which possess optoelectronic pr operties that are greatly improved over any yet reported in the range of x greater than or equal to 0.6. These films were deposited on the c athode (cathodic deposition) of an rf discharge. Their properties are assessed using a large variety of measurements and by comparison to th e properties of alloys conventionally prepared on the anode (anodic de position). Steady state photoconductivity measurements yield a quantum -efficiency-mobility-lifetime product, eta mu tau; of (1-3) x 10(-7) c m(2) V-1 for 1.00 greater than or equal to x greater than or equal to 0.75 and (6-10) x 10(-8) cm(2) V-1 for 0.75 greater than or equal to x greater than or equal to 0.50, and photocarrier grating measurements yield ambipolar diffusion lengths several times greater than previousl y obtained for alloys of large x. It is confirmed that the improvement s in phototransport are not due to a shift in the Fermi level. In fact , results of recent measurements on lightly doped samples strongly sug gest that fur these cathodic alloys neither photocarrier is dominant [ (mu tau)(e) approximate to (mu tau)(h)]. The improvements are attribut ed in large part to the reduction of long range structural heterogenei ty observed in x-ray scattering and electron microscopy, and partly to the reduction in midgap state density. In spite of the superior prope rties, an assessment of the data of the cathodic alloys suggests that alloying introduces mechanisms detrimental to transport which are not present in a-Si:H or a-Ge:H. The Urbach tail width is 42 +/- 2 meV for cathodic a-Ge:H and 45 +/- 2 meV for cathodic a-Si1-xGex:H and is con stant with x. From differences in the band edges and tails we infer th at the atomic bond ordering is different between the cathodic and anod ic alloys. For a given composition the cathodic alloys have roughly an order of magnitude lower midgap state density than do the anodic allo ys, and both midgap densities increase exponentially with x, consisten t with defect creation models from which the lower midgap density can be attributed to a larger band gap and decreased valence band tail wid th. A photoluminescence peak is observed with an intensity roughly an order of magnitude greater than for the anodic alloys, and a significa ntly different peak energy. Section VII E provides an overview of the results and conclusions. The improved properties of these alloys have significant implications or current and future device applications. (C ) 1997 American Institute of Physics.