Electrical and optical properties of plasma-deposited a-SiGe:H alloys: Role of growth temperature and postgrowth anneal

Basic properties of hydrogenated amorphous silicon-germanium (a-SiGe:H) alloys deposited by plasma-enhanced chemical-vapor deposition were studied. We found that there is an optimal growth temperature in the range 250-280 degrees C. Infrared-absorption spectra measurements show that in this temperature range the alloys have optimal composition of hydrogen content and bonding, providing Growth at lower a minimum in dangling bond defect density N-s and a high photoconductivity sigma(ph). Growth at lower temperatures results in hydrogen-rich alloys with high N-s. Hydrogen in these alloys is mainly bonded as SiH2 and as clusters on internal voids. Growth at temperatures above the optimal value gives less deterioration of the properties of a-SiGe:H than the growth below this optimal temperature. We found two different dependencies of sigma(ph) on N-s:in optimized alloys sigma(ph) changes proportionally to N-s(-1) and in low-temperature alloys sigma(ph) decreases more steeply with N-s(-1) due to a decrease in electron mobility. It is shown that annealing of a-SiGe:H causes a sharp increase in N-s starting at annealing temperatures about 20 degrees C below the temperature at which the alloy was grown. We observed a new phenomenon that during annealing N-s increases much more (by two orders of magnitude) than the photoconductivity decreases (only factor of 5). The entropy-based model was applied to explain this experimental phenomenon. We found that annealing shifted the centrum energy of the D-+/0 dangling bond levels to the valence-band edge. The rate of electron recombination lowers due to this shift and the change in the effective electron capture crosssection of the defects, and therefore the decrease in sigma(ph) is smaller than the increase in N-s. (C) 1996 American Institute of Physics.