Improvement in the properties of a-SiGe:H films: Roles of deposition rate and hydrogen dilution

Abstract

Device quality a-SiGe:H thin films have been deposited by radio-frequency plasma-assisted. decomposition of silane and germane diluted with and without hydrogen. Improvement in structural atid electronic'prop@ies have been achieved employing low deposition rate and high hydrogen dilution. It has been observed that low deposition rate can reduce the preferential attachment of H to silicon throughout the alloy region while the beneficial effect of hydrogen dilution is more effective in a low band gap region (E,< 1.40 eV) . The photoconductivities of the good quality aSiGe:H alloy films under white light illumination ( 50 mW/cm') are 1.34~ lo-' S cm-' and 1.9X lo-' S cm-' at the optical gaps of 1.51 and 1.35 eV, respectively. The changes in midgap defect density and tail states have been correlated with the photoconductivities of the samples deposited under different conditions. An attempt has been made to explain the results from the growth kinetics of the films.

Development of Hydrogenated Microcrystalline Silicon-Germanium Alloys for Improving Long-Wavelength Absorption in Si-Based Thin-Film Solar Cells

“…One possible reason may relate to the sticky nature of GeH 3 species more than the SiH 3 species. The diffusion length of GeH 3 species is less than SiH 3 species during the growth of SiGe alloy [24], which makes it more difficult to reach the energetically favorable sites on the film surface. As a result, Ge is easier to form weak bonds than Si in SiGe binary network.…”

Raman scattering and electrical characterizations studies of hydrogenated amorphous silicon–germanium alloys prepared by 40MHz plasma-enhanced CVD

“…Therefore, the preparation of high quality a-SiGe:H thin film becomes an important research issue. Over the past years, several studies have been made on improvement in the properties of a-SiGe:H films, such as Middya et al which showed that improvement in structural and electronic properties have been achieved by employing low deposition rate and high hydrogen dilution in a diode PECVD [7]. Xu et al showed that the hydrogen plasma annealing method eliminates the a-SiGe:H defect states to improve films quality by conventional RF-PECVD [8].…”

Properties of a-(Si,Ge) films and devices deposited at high growth rates

“…It is believed [ 17,18] that helium dilution effects are due to the enhanced bombardment of the film surface by helium ions during growth. This also can explain the seemingly lesser Ge incorporation at lower pressure.…”

“…Several seemingly partly contradictory optimum growth conditions have been suggested which include, among others: a) reducing short lifetime radicals through reaction with H2 or avoiding depletion of germane [ 10]; b) enhancing surface diffusion of Ge related radicals by hydrogen coverage [ 11]; c) using an alternative Ge dangling bond terminator [12]; d) needing no [13], versus high energy ion bombardment (or high ion bombardment at high deposition rate [14]) to reduce heterogeneities and obtain optimum performance [9,[15][16] e) using higher electron temperatures for the PECVD process to obtain a favourable discharge chemistry [ 16]; f) needing low [17,18] versus high [19,20,21] deposition rates.…”