High diffusion length silicon germanium alloy thin films deposited by pulsed rf PECVD method

Hydrogenated amorphous silicon germanium (a-Si1−xGex:H) alloy thin films were deposited in a rf (13.56 MHz) powered plasma CVD system from a mixture of silane, germane and hydrogen. The rf power was pulsed at a frequency of 1356 Hz with the duty cycle (Q) ranging from 50% to 100%. The Ge content in the films, found in the range 40–45%, was deduced from Raman spectra. The band gaps Eg of the films were in between 1.44 and 1.48 eV. We find a good correlation between the ambipolar diffusion length (Ld) and the microstructure factor (R∗R∗) for different duty cycles. At Q = 75% the R∗R∗ value is lowest (0.14) which correlates well with the maximum value of Ld (100 nm), the highest Ld value reported for a-Si1−xGex:H films with Eg = 1.44 eV. It was also found to be very stable under light soaking. Powder incorporation within the films was also studied. Quality of the films is determined by two opposing effects viz. size of the powders incorporated into the films from the plasma and the bombardment of energetic ions and neutrals. Best transport properties were found at an optimum duty cycle of 75%.