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 (mu c-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 mu c-Ge:H deposition reduces the hole concentration by two orders of magnitude. Furthermore, mu c-Si1-xGex:H (x = 0.1-0.3) 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 x 10(18)-10(20) cm(-3). These findings are explained by the effect of the compensation of the negatively charged Ge dangling bonds by oxygen donors. (C) 2012 The Japan Society of Applied Physics