VOLUME 83, NUMBER 17 PHYSICAL REVIEW LETTERS 25 OCTOBER 1999 Observation of a Photoinduced Lattice Relaxation in CdTe:In F. J. Espinosa and J. Mustre de Leon Cinvestav, Unidad Merida, Merida, Yucatan 97310, Mexico S. D. Conradson Los Alamos National Laboratory, Los Alamos, New Mexico 87545 J. L. Peña* and M. Zapata-Torres* Cinvestav, Unidad Merida, Merida, Yucatan 97310, Mexico (Received 26 April 1999) The local atomic structure of CdTe:In at an In concentration of 6 at. % was investigated by x-ray absorption spectroscopy before and after photoexcitation at 80 K. After photoexcitation, In K edge spectra change in both the near-edge and x-ray absorption fine structure (XAFS) regions, showing a change in local structure. Cd and Te structural parameters are consistent with the structure of CdTe and did not change after photoexcitation. For In, only the first shell contribution is present in the XAFS, indicating a disordered environment beyond the first shell of neighbors. The relation of these observations with the proposed model of a DX center for CdTe:In is discussed. PACS numbers: 61.72.Ji, 61.72.Bb, 61.72.Dd, 71.55.Gs The study of metastable defects in compound semicon- ductors has received recent attention due to its implica- tions to the problem of intrinsic limits to doping [1,2] and the fact that these defects can lead to many body phenomena such as photoinduced structural changes [3– 6]. The observation of persistent photoconductivity (PPC) and persistent electron paramagnetic resonance (PEPR) in some doped semiconductors, along with the observa- tion of a large difference between the photoionization threshold energy and the thermal level position of de- fects, suggests that large lattice relaxations are relevant in these phenomena [7,8]. Among the theoretical models of metastable defects, which involve large lattice relaxations, are the DX center with a single breaking bond (SBB) [4], which has been predicted for CdTe:In, and other doped semiconductors [4,9]. This model yields to a negative Hubbard energy U with trapping of two carriers which form a pair in the defect, and can explain PPC and PEPR. Also, if the level of the SBB center lies in the conduc- tion band near the conduction band minimum (CBM) as a resonance, then, at some level of doping, some of the carriers become trapped in this relaxed state. This leads to Fermi level pinning and can explain the existence of intrinsic limits to doping [1]. In this and other models, the local atomic structure of the defect is correlated with its charge state. Hence, a change of the charge state of the defect, e.g., by illumination, will lead to a change of atomic structure [4]. There are few experimental techniques that can pro- vide quantitative information about the microscopic struc- ture around defects helping to test the predictions of theoretical models. X-ray absorption fine-structure spec- troscopy (XAFS) has proven to be a useful technique to characterize lattice distortions in semiconductors [10 – 12]. Photoinduced structural changes have been observed in Ga 0.78 Al 0.22 As:Sn using high resolution x-ray diffraction [13]. These changes are reflected in the expansion of the lattice constant under illumination. It is difficult, how- ever, to relate this expansion to the local atomic structure around the defect. Nevertheless in other systems, such as the semiconducting parents of high-temperature supercon- ductors, local structural changes produced by illumination have been recently reported [14]. These systems also ex- hibit electronic changes under photoexcitation, e.g., PPC and photoinduced superconductivity [15]. In the specific case of CdTe:In, the maximum dop- ing concentration after which carrier passivation occurs is 10 18 [3,16]. This value is consistent with the predic- tion of an energy of 40 meV above the CBM for the formation of an SBB center [4]. However, some experi- ments in CdTe:In, in the doping range #10 18 , indicate that the formation of donor-Cd vacancy centers In Cd -V Cd  can play a major role in passivation [17,18]. Nevertheless, for higher In concentrations experiments indicate that carrier passivation is controlled by the appearance of centers with large lattice relaxation [1,12,19]. Also, SBB-like centers in CdTe:In have been inferred from conductivity experi- ments under hydrostatic pressure and with alloying with Zn [7]. Consequently, we have selected a sample of CdTe:In in a heavily overdoped regime, which presents strong car- rier passivation and PPC, to study possible photoinduced structural changes. A CdTe:In with a 6 at. % indium sample was prepared using close-spaced vapor transport combined with free evaporation in the form of a 80 mm thick film, which was shaved from the substrate to produce a powder sample for the XAFS measurements. Details of the growth procedure have been previously reported [20]. Elemental concentrations were determined by energy dispersive x-ray spectroscopy using a Jeol 35C electron 3446 0031-9007 99  83(17)  3446(4)$15.00 © 1999 The American Physical Society