Compensation of Native Defect Acceptors in Microcrystalline Ge and Si1-xGex Thin Films by Oxygen Incorporation: Electrical Properties and Solar Cell Performance

Undoped hydrogenated microcrystalline Ge (μc-Ge:H) films grown by plasma-enhanced chemical vapor deposition reveal high concentration of free holes (${>}10^{18}$ cm -3 ) when the films exhibit a high crystalline volume fraction. ESR and Hall-effect experiments suggest that the acceptor states arise from the native dangling bond defects at Ge crystalline grain boundaries. It is demonstrated that an intentional oxygen incorporation during the μc-Ge:H deposition reduces the hole concentration by two orders of magnitude. Furthermore, μc-Si 1-x Ge x :H ($x=0.1{\mbox{--}}0.3$) alloy p--i--n solar cells show marked improvements in photocarrier collection properties upon oxygen incorporation into the i-layer in the order of $5\times 10^{18}{\mbox{--}}10^{20}$ cm -3 . These findings are explained by the effect of the compensation of the negatively charged Ge dangling bonds by oxygen donors.