LOCAL MODES OF THE H-2 DIMER IN GERMANIUM

Local vibrational modes of the H-2 defect in crystalline germanium ar e identified by a combination of infrared-absorption spectroscopy, uni axial stress measurements, and ab initio theory. Germanium crystals ar e implanted with protons and/or deuterons at 30 K, and subsequently an nealed at room temperature. A number of local vibrational modes of hyd rogen are revealed by infrared-absorption spectroscopy. In particular, modes at 765, 1499, 1774, and 1989 cm(-1) originate from the same def ect which has trigonal symmetry according to the uniaxial stress measu rements. The 765-cm(-1) mode is two dimensional, while the 1774- and 1 989-cm(-1) modes are one dimensional. Measurements on samples coimplan ted with protons and deuterons show that the defect contains a pair of weakly coupled and inequivalent hydrogen atoms. The 765-, 1499-, 1774 -, and 1989-cm(-1) modes are ascribed to the H-2 defect. The 765-cm(- 1) mode is a Ge-H bend mode with an overtone at 1499 cm(-1) and the mo des at 1774 and 1989 cm(-1) are Ge-H stretch modes. An excellent fit t o the stretch frequencies is obtained with a simple model based on two coupled Morse-potential oscillators. In addition, the model gives int ensity ratios in fair agreement with those observed. The structure, th e local-mode frequencies, and the isotope shifts of H-2 are calculate d with ab initio local-density-functional cluster theory. The theoreti cal frequencies are consistently 5-10 % too high, as expected from the theory which often leads to overbinding. The isotope shifts, however, are in fair agreement with observations. These results provide additi onal support for our assignments, and show that the 765- and 1774-cm(- 1) modes primarily involve the hydrogen at the antibonding site, while the 1989-cm(-1) mode is related mainly to vibration of the hydrogen n ear the bond-center site.