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

Abstract

Commonly, the germane fraction (f = flowrateofGeH4/flowrateofSiH4 + GeH4) is changed to vary the optical gap (Eopt) of amorphous silicon germanium alloy (a-SiGe:H) films. We report that for a particular f, the change of deposition conditions, the flow rate of diluent gas (H2) and the radiofrequency (rf) power density can vary the optical gap (1.67-1.40 eV), the germanium content (41.3-22.5 at%) and the microstructural defect density (0.92-0.42), the mobility lifetime product (ηµτ; 6.81×10-6-1.46×10-8 cm2 V-1) in a wide range. Initially, with the increase of the H2 flow 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 microstructural defects of a-SiGe:H films becomes a direct function of the midgap defect density and a indirect function of ηµτ. Moreover, the nature of diluent gas is also important. We present that in a wide range of optical gap (1.74-1.36 eV) the defect density is lower, and ηµτ is higher for the optimized He diluted film compared to those of the optimized H2 diluted films.

High‐performance a‐SiGe:H thin film prepared by plasma‐enhanced chemical vapor deposition with high plasma power for solar‐cell application

“…When the plasma power is low, the dissociation of SiH 4 and GeH 4 is low. The sticking coefficient of SiH 2 radial is greater than that of SiH 3 radical 3. And SiH 2 and GeH 2 can also react rapidly with the parent gases, producing higher silanes, germanes, and silylgermanes 24.…”

“…The increase of the plasma power increases the average energy of the electrons in the plasma and thus enhances the dissociation cross sections of silane and germane into neutral as well as ionic radicals. Besides, the increasing plasma power transfers high momentum and energy to the precursors in the growing surface, and promotes the surface diffusion of the reactive radicals 3, 15, 16. All these lead to the above growth‐rate enhancement.…”

“…The E g of hydrogenated amorphous silicon germanium (a‐SiGe:H) thin films can be easily varied to match the long‐wavelength range by changing the Ge content. However, the increase of Ge content usually results in the decrease of photosensitivity 3, 4. So how to obtain narrow E g a‐SiGe:H thin films with high photosensitivity has become an important issue in the fabrication of the Si‐based multijunction solar cells 4–8.…”

“…And they found that hydrogen (H 2 ) dilution could improve the photoelectrical properties 10. Hazra et al 3 investigated the influence of the H 2 flow rate on the quality of the a‐SiGe:H films systematically. Xu et al 8 prepared narrow bandgap a‐SiGe:H alloys by alternately repeating thin‐film deposition and sequent exposure to hydrogen plasma.…”

Effect of Pressure on Bonding Environment and Carrier Transport of a-Si:H Thin Films Deposited Using 27.12 MHz Assisted PECVD Process

“…These polysilane chains affect the microstructure and electronic properties of the material and favour the formation of voids where hydrogen gets incorporated [15]. The microstructure parameter (R*) is a measurement of voids present within the material (which eventually has effects on the H bonding) and was estimated from R * = I 2070−2100 I 2070−2100 + I 2000 (4) where, I 2000 is the integrated absorption intensity due to Si-H bonds and I 2070−2100 is the integrated absorption intensity due to different bonding configurations such as clustered monohydride and dihydride bonds [16]. The stronger the symmetric stretch band the lower is the microstructure parameter.…”