Electron-Nuclear Double Resonance Study of the Zinc Vacancy in Zinc GERMANIUM PHOSPHIDE (ZnGeP 2 )
- PDF / 379,898 Bytes
- 6 Pages / 414.72 x 648 pts Page_size
- 59 Downloads / 185 Views
ABSTRACT As-grown crystals of ZnGeP 2 are highly compensated and contain significant concentrations of donors and acceptors. The dominant acceptor in ZnGeP 2 is believed to be the zinc vacancy. This center is paramagnetic in its normal singly ionized state, and gives rise to an electron paramagnetic resonance (EPR) signal characterized by a resolved primary hyperfine interaction with two equivalent phosphorus nuclei adjacent to the vacancy. The present investigation has focused on electron-nuclear double resonance (ENDOR) measurements of additional hyperfine interactions which are not resolved in the regular EPR spectra. Principal values and principal axes directions for four additional phosphorus nuclei are determined from the ENDOR angular dependence. These parameters support the zinc-vacancy assignment for the acceptor and they provide an experimental check of wave functions generated in future computational modeling efforts. INTRODUCTION The identification and characterization of point defects continues to be a primary focus of research in zinc germanium phosphide (ZnGeP 2). Much of this interest arises because of the detrimental effect that the point defects can have on the performance of nonlinear optical devices which employ this material. It has been demonstrated that an optical parametric oscillator (OPO) based on ZnGeP 2 crystals can be tuned across large regions of the mid-infrared [1,2]. The performance, however, of such an OPO is affected by an unwanted optical absorption band in the crystal which overlaps the desirable 2-gtm pump region and thus limits the maximum pump intensity that can be used. This broad defect-related absorption band extends from 0.7 to 2.5 ýtm and must be eliminated, or greatly reduced, if ZnGeP 2 -based OPOs are to reach their expected potential. A recent study has suggested that this near-edge absorption in ZnGeP 2 is associated with the presence of the zinc-vacancy acceptor [3]. A very intense electron paramagnetic resonance (EPR) spectrum is present in every ZnGeP 2 crystal. Initially, it was suggested [4] that this spectrum could be due to either a singly ionized zinc vacancy (Vjn) or a singly ionized zinc on a germanium site (Znfe). A subsequent investigation using electron-nuclear double resonance (ENDOR) provided lattice-distortion evidence which supported the zinc vacancy assignment [5]. Because of the importance of this acceptor in the extrinsic optical absorption and the need to further substantiate its model, we have extended the ENDOR investigation to include several additional weak phosphorus hyperfine interactions. EXPERIMENT The ZnGeP 2 crystals used in the present investigation were grown by the horizontal gradient freeze technique at Sanders, a Lockheed Martin Company (Nashua, NH). A typical sample had 549 Mat. Res. Soc. Symp. Proc. Vol. 484 © 1998 Materials Research Society
approximate dimensions of 3 x 3 x 3 mm 3 with the faces perpendicular to the high symmetry directions. The EPR data were taken on a Bruker ESP-300 spectrometer operating at 9.45 GHz with 1
Data Loading...
