Electrical passivation by hydrogen plasma treatment of transition metal impurities in germanium

Transition metal impurities in germanium introduce deep levels in the band gap, which may influence the lifetime of carriers and leakage currents of devices. In this work it is shown that Ti, Cr and Fe centres in germanium can be passivated using plasma hydrogenation. The metals have been implanted at 90 keV in n- and p-type wafers and in-diffused during a 5 min thermal anneal at 500 °C. Samples have been hydrogenated using a DC plasma for 4 h at 200 °C and Schottky diodes were made for measurement using DLTS. It is found that the levels of metal impurities are passivated by hydrogenation. Characteristic hole and electron traps are assigned to the irradiation damage induced by the direct plasma exposure. Metal-specific levels are tentatively assigned to transition metal–hydrogen-related centres. Two hole traps at 0.05 and 0.10 eV above the valence band are only present in the Cr-doped samples and are tentatively assigned to chromium–hydrogen complexes. A comparison is made with copper–hydrogen in germanium.