Substitutional Oxygen vs. Vacancy-Oxygen Complex in Cadmium Telluride (CdTe)

1. Substitutional Oxygen vs. Vacancy-Oxygen Complex in Cadmium Telluride (CdTe) Anant K. Ramdas and Sergio Rodriguez, Purdue University, DMR-0405082 Research goal:To discover and delineate novel impurity configurations in compound semiconductors through high resolution Fourier-transform spectroscopy of localized vibrational modes (LVM). We have demonstrated that the sharp ‘infrared’ signatures in Fig 1 arise from oxygen impurities introduced into CdTe, a tetrahedrally co-ordinated II-VI semiconductor with zincblende structure in which each Cd is surrounded by four Te and each Te by four Cd atoms. It is well known that even the purest CdTe suffers from a departure from exact stoichiometry and as a result has Cd vacancies (see inset in Fig 1(a)). The preferential association of oxygen which has replaced Te near a Cd vacancy then displays a pair of sharp LVMs: it can be argued convincingly that the spectroscopic signatures labeled 1 and 2 are, respectively, due to vibration of oxygen either along c or perpendicular to it, c being the axis of three fold symmetry of the oxygen-vacancy (OTe-VCd) center. If oxygen is introduced simultaneously with Cd excess suppressing the generation of Cd vacancies, it substitutes for Te, now surrounded by four tetrahedrally located Cd (OTe). One then observes only a single LVM at a significantly lower frequency (Fig 1(b)). OTe OTe-VCd Fig 1

2. Substitutional Oxygen vs. Vacancy-Oxygen Complex in Cadmium Telluride (CdTe) Anant K. Ramdas and Sergio Rodriguez, Purdue University, DMR-0405082 Future Plans:The research highlighted in this nugget promises to be a precursor to extended, similar studies of the LVMs in compound semiconductors in general, with impurities incorporated in specific sites selected by crystal growth strategies. LVM of SeTe and STe in CdTe and of PSb in GaSb are potentially fascinating candidates for future studies. The insights thus achieved should be of importance for discovering new examples of this remarkable class of defect complexes. The temperature dependence of 1 and 2 is striking (Fig 2) --- 1 increases with T while 2 decreases; they coalesce ~ 300K and behave as a single frequency beyond. This extra-ordinary behavior can be attributed to increased tunneling of Cd into VCd site with increasing T, producing an overall Td symmetry for the (OTe-VCd) center. Education/Broader Impact:The research for the nugget resulted from a collaboration involving the two PIs [Ramdas (Experimenter) and Rodriguez (Theorist)], G. Chen (Graduate Student) and I. Miotkowski (Senior scientist in charge of crystal growth). A very intense and productive collaboration with S. Tsoi (PhD with AKR in August 2005, now working at the Naval Research Laboratory), M. Cardona (Max-Planck Institute), M. Grimsditch (ANL), X. Lu (Graduate Student) and H. Alawadhi (UAE) has led to novel insights into electron-phonon interaction in ZnO, an opto-electronic material of considerable current interest. Extended, ongoing collaborations with Anthony (formerly of GE), RamMohan (WPI) and Haller (UCB) are further examples of the interactive ambience in which graduate students and undergraduates carry out their research. The entire research experience presages for the younger participants exciting future prospects in universities, national and industrial laboratories. Fig 2