Theoretical study of cation-related point defects in ZnGeP 2 Xiaoshu Jiang, M. S. Miao, and Walter R. L. Lambrecht Department of Physics, Case Western Reserve University, Cleveland, Ohio 44106-7079 USA Received 4 June 2004; revised manuscript received 17 December 2004; published 27 May 2005 First-principles calculations are presented for the V Zn and V Ge cation vacancies and the Zn Ge and Ge Zn antisites in ZnGeP 2 , using full-potential linearized muffin-tin orbital method supercell calculations in the local-density approximation to density-functional theory. Under Zn-poor conditions, the lowest Gibbs energy defects are found to be the Ge Zn and V Zn defects, leading to a compensated p-type material in agreement with experimental evidence. The occupation energy levels of the defects are determined and compared with avail- able experimental information. As expected, the Ge Zn is found to be a donor while the other three are acceptors. Good agreement is obtained with optical quenching and activation of electron paramagnetic resonance signal studies if a direct transfer of electrons from V Zn 2- to Ge Zn 2+ is assumed rather than a process via the conduction band. This suggests a close association of the dominant acceptors and donors. This is further confirmed by showing that the formation of complexes consisting of two V Zn - with a single Ge Zn 2+ antisite are favorable in energy. The V Ge on the other hand is found to have high energy of formation under any chemical potential conditions and is found to be unstable toward formation of a V Zn and Zn Ge pair. Structural relaxation of all defects is performed but no symmetry breaking distortions are found. As a result, the defect wave functions of the unpaired electron in the V Zn - is found to be spread equally over the four neighboring P atoms, in disagree- ment with electron nuclear double resonance data which indicate primary localization on a pair of P atoms. Several possible origins for this discrepancy are discussed. DOI: 10.1103/PhysRevB.71.205212 PACS numbers: 71.55.Ht I. INTRODUCTION ZnGeP 2 is an important material for nonlinear optical fre- quency conversion with target wavelengths in the midinfrared. 1 It has not only a large  2 but also sufficient birefringence to allow for phase matching by angular tuning. However, the efficiencies of frequency doubling and optical parametric oscillators OPO based on ZnGeP 2 are hampered by undesirable defect related optical absorption forming an absorption tail below the band gap. Much experimental work has already been done in identifying the origin of this ab- sorption and correlating it with electron paramagnetic reso- nance EPR and electron nuclear double resonance ENDOR studies. 2–10 The prevalent model emerging from this work is that zinc vacancies in a negative charge state V Zn -  are responsible for the dominant EPR spectrum AL1 in as-grown crystals and that this defect is also strongly corre- lated with a broad optical absorption band. The g shifts as- sociated with the AL1 EPR center are positive, thus indicat- ing that the defect is an acceptor and the strong distortion obtained from the ENDOR analysis suggests a vacancy rather than a Zn Ge antisite. This contrasts earlier ideas that in chalcopyrite semiconductors antisite disorder should be dominant and has already led to improved material by focus- ing the growth efforts and post-growth annealing treatments on producing less Zn-deficient material. Nevertheless, the Ge Zn antisite was also found to be an important defect in ZnGeP 2 . It was found to have an associ- ated EPR signal which appears under optical excitation. 8 Furthermore the interaction of these defects was studied by optically induced EPR Refs. 6 and 7 and used to determine defect energy levels in the gap. An EPR signal associated tentatively with the V Ge was discovered only recently in ir- radiated samples. 9 To the best of our knowledge no previous systematic com- putational studies have been done of the electronic structure of the point defects in ZnGeP 2 using first-principles calcula- tions. Atomistic modeling studies were presented by Zapol et al. 11 but do not address the electronic structure. Here we present the results of such a study in which we focus on the cation related point defects: the V Zn and V Ge vacancies and the Zn Ge and Ge Zn antisites. After presenting some details on our computational approach and establishing its accuracy in Sec. II, we first establish the range of chemical potentials that needs to be considered Sec. III A. The results for neutral defect formation energies as a function of chemical poten- tials presented in Sec. III B allow us to discuss the expected abundancy of the defects. Next, we determine the energies of formations of charged defect states and deduce from them the occupation energy levels in the band gap Sec. III C and provide a discussion of the associated experimental informa- tion in Sec. III D. Next, we discuss the possibility of com- plex formation between the dominant donor Ge Zn  and ac- ceptor V Zn in Sec. III E. We discuss the stability of the high energy of formation defect V Ge which is found to be unstable toward formation of V Zn +Zn Ge in Sec. III F. We then turn to a more detailed discussion of our results for the V Zn in the context of the EPR-ENDOR data which provide information on the localization of the defect wave functions and the structural distortions in Sec. III G. An important discrepancy is found here: The ENDOR data show that the defect wave function is primarily localized on a pair of P atoms wheras the calculations show it to be spread equally over the four nearest-neighbor P atoms. This suggests a Jahn-Teller distor- tion occurs. Possible explanations for the failure of the cal- culations to find an energy lowering distortion are discussed. PHYSICAL REVIEW B 71, 205212 2005 1098-0121/2005/7120/20521212/$23.00 ©2005 The American Physical Society 205212-1