Substitutional vs. Vacancy-Impurity Complex in Compound Semiconductors: CdSe Anant K. Ramdas and Sergio Rodriguez, Purdue University, DMR Research goal: To discover and delineate novel impurity configurations in stoichiometrically controlled compound semiconductors through high resolution Fourier- transform spectroscopy of local vibrational modes (LVM). We have demonstrated that, by manipulating stoichiometry, novel vacancy-defect impurity complexes can be generated in compound semiconductors like CdTe; oxygen replacing Te surrounded by four nearest neighbor (NN) Cd’s tetrahedrally coordinated with it, i.e., O Te, as well as O Te associated with Cd vacancy, (O Te -V Cd ), were observed by changing stoichiometry [PRL 96, (2006)]. How general is this occurrence? In the uniaxial (wurtzite) CdSe, following similar procedures, surprising results emerged. By introducing oxygen into CdSe grown under stoichiometric control, we discovered the O Cd with four Se’s as NN’s [Fig. 1(a)]. The two LVMs, µ 1 and µ 2, display a remarkable fine structure whose intensities reflect the natural isotopic abundance of Se NN’s. Indeed it is this feature which allowed us to convincingly conclude that an anti-site substitution as O Cd has occurred. If stoichiometry favors the generation of Cd vacancies, O Se is associated with a V Cd in a NN position as shown in the inset of Fig. 1(b); the three peaks  1,  2, and  3 in the infrared spectrum of (O Se - V Cd ) are displayed in the figure. The only surving symmetry element of the defect is  v, the plane of reflection on which Cd in site 1 and V Cd lie and hence with C s symmetry. The three normal modes of O Se are along  1,  2, and  3. Had V Cd occurred on site 1, two more infrared active modes would have appeared. In their absence, it appears that (O Se -V Cd ) with C s symmetry, not C 3v, is energetically favored. Fig 1

The temperature behavior of  1,  2, and  3, displayed in Fig. 2 is remarkable. Whereas the frequencies of  2 and  3 decrease that of  1 increases;  2 merges with  1 at T 1 ~ 480 K, and  12, the frequency of the coalesced mode, continues to increase and joins  3 at T 2 ~ 560 K, beyond which a single signature  123 is observed. These remarkable coalescences of  1,  2, and  3 are understood by invoking bond switching and thermally averaged higher symmetry, C s  C 3v  T d. In CdTe, the analogous acquired symmetry C 3v  T d occurs at  300 K. This unique phenomenon resembles the coalescence of the two normal modes of (O Te -V Cd ) and the temperature averaged transformation of its C 3v to T d in CdTe. Future Plans: The research highlighted in this nugget promises to develop into a systematic line of research to discover novel defect structures in compound semiconductors in general, with impurities incorporated in specific sites selected by crystal growth strategies. The insights achieved should be of importance for discovering new examples of this remarkable class of defect complexes. Education/Broader Impact: The research reported here resulted from a collaboration involving the two PIs [Ramdas (Experimenter) and Rodriguez (Theorist)], G. Chen and J. Bhosale (Graduate Students) and I. Miotkowski (Senior scientist in charge of crystal growth). A very intense collaboration with S. Tsoi (NRL), 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. A promising line of research on electronic Raman effect in ruby has been lauched by Venugopalan (SUNY, Binghamton) and X. Lu. 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 research; this experience prepares them for an exciting future in universities, national and industrial laboratories. Substitutional vs. Vacancy-Impurity Complex in Compound Semiconductors: CdSe Anant K. Ramdas and Sergio Rodriguez, Purdue University, DMR Fig 2