Improved A-Si1-Xgex:H Of Large X Deposited By Pecvd

By plasma enhanced chemical vapor deposition a-Si1-xGex:H thin films of large x have been prepared which possess optical, electrical and structural properties that are greatly improved over any yet reported. This work extends our previous work on improving the properties of a-Ge:H [W.A. Turner et al., J. Appl. Phys. 67 (1990) 7430]. Steady-state photoconductivity measurements yield an eta mu tau of (1 to 3) X 10(-7) cm(2) V-1 for 1.00 greater than or equal to x greater than or equal to 0.75 and (6 to 10) X 10(-8) cm(2) V-1 for 0.75 greater than or equal to x greater than or equal to 0.50. Photocarrier grating measurements yield an ambipolar diffusion length much greater than previously obtained for alloys of large x. The electronic state defect density, as determined by drive level capacitance measurements, decreases from 5.3 x 10(16) cm(-3) for x = 1.00 to 6.5 X 10(15) cm(-3) for x = 0.57. The Urbach parameter, E(0), was found to be 41 +/- 2 meV for a-Ge:H and 45 +/- 2 meV for the alloys. Small angle X-ray scattering measurements reveal a structure that is nearly as homogeneous as device quality a-Si:H. Much of the improvement in electronic and optical properties is associated with the reduction of heterogeneities in the structure. The elimination of columnar structure is attributed to increased ion bombardment during growth and conditions which yield a high electron temperature in the discharge plasma, resulting in favorable discharge chemistry.