Role of deposition parameters on the photovoltaic quality of amorphous silicon germanium alloys: correlation of microstructure with defect density and electronic transport

Commonly, the germane fraction (f = flow rate of GeH4/flow rate of SiH4 + G eH4) is changed to vary the optical gap (E-opt) of amorphous silicon german ium alloy (a-SiGe:H) films. We report that for a particular f, the change o f deposition conditions, the flow rate of diluent gas (H-2) and the radiofr equency (rf) power density can vary the optical gap (1.67-1.40 eV), the ger manium content (41.3-22.5 at%) and the microstructural defect density (0.92 -0.42), the mobility lifetime product (eta mu tau; 6.81 x 10(-6)-1.46 x 10( -8) cm(2) V-1) in a wide range. Initially, with the increase of the H-2 flo w rate up to 20 SCCM for the rf power density of 60 mW cm(-2) and up to 30 SCCM for the rf power density of 30 MW cm(-2), the microstructural. defects decreases, although, the Ge content of a-SiGe:H films increases. The micro structural defects of a-SiGe:H films becomes a direct function of the midga p defect density and a indirect function of eta mu tau. Moreover, the natur e of diluent gas is also important. We present that in a wide range of opti cal gap (1.74-1.36 eV) the defect density is lower, and eta mu tau is highe r for the optimized He diluted film compared to those of the optimized H-2 diluted films.