CHARACTERIZATION OF LATTICE STRAIN FROM DX-CENTERS AND PERSISTENT PHOTOCARRIERS IN GAALAS

We discuss the results of high resolution, low temperature X-ray diffr action measurements used to determine the lattice strain caused by ele ctron emission from Sn- and Si-related DX centers in Ga1-xAlxAs. DX ce nters are deep levels associated with donors in various III-V semicond uctors, and they are responsible for persistent photoconductivity and other unusual electrical and optical properties of these materials. Al though DX centers have been extensively studied for more than a decade , information is still needed about the atomic arrangements that compr ise DX centers for various donor and host systems. Our lattice strain measurements show that there is lattice expansion attributable to loca l bonding changes that occur when the DX centers are emptied. This res ult is important in testing proposed structural models for the DX cent er, and it can be used to rule out a proposal that symmetric breathing -mode relaxation is responsible for a DX(-) state for Sn-doped Ga1-xAl xAs. The theoretical proposal by Chadi (Phys. Rev. B, 46 (1992) 6777) requires an inward relaxation when a Sn-related DX center is emptied, but our experiments show an outward relaxation.