While CdGeAs2 single crystals were promising for infra-red second harmonic generation, it was difficult to reproducibly grow low loss optical material, particularly in the 5μm region. In addition, a correlation between processing conditions and absorption had thus far eluded those studying the problem. The present work described a series of etching experiments which were intended to test whether there might be a relationship between dislocation density and optical absorption. Single crystals were grown from stoichiometric melts along the common [001] direction by using the horizontal gradient freeze method. Etch pit densities of more than 106/cm2 were observed at the edges of the boules, while almost no etch pits were found at the center. Infra-red transmission images indicated that the higher etch pit densities regions had lower optical absorption. It was also found that the absorption coefficients (α) varied from 2.1 to 18.2/cm along the boule axis. Moreover, the etch pits, which formed near the edge of the crystal adjacent to the crucible walls, were created by slip along the {112} planes, which make up the growth interface. These results suggested that in order to obtain a more uniform absorption within each boule, dislocation formation should be minimized. The first approach involved changing the orientation of the growing crystal to the [112] direction. The results of these experiments were very encouraging, showing that a considerably improved radial and axial absorption uniformity was achieved, as well as the lowest optical losses measured (minimum α = 0.18/cm) compared with similar samples grown along the [001]. It could be concluded that the growth orientation played an important role in the absorption behavior of CdGeAs2.

Correlation between Dislocation Etch Pits and Optical Absorption in CdGeAs2. K.Nagashio, A.Watcharapasorn, K.T.Zawilski, R.C.DeMattei, R.S.Feigelson, L.Bai, N.C.Giles, L.E.Halliburton, P.G.Schunemann: Journal of Crystal Growth, 2004, 269[2-4], 195-206