Ternary-chalcopyrite structure ZnGeP2, CdGeAs2 (II-IV-V2) and AgGaS2, AgGaSe2 (I-III-VI2) compounds are currently of technological interest. They show the most promise for practical nonlinear optical applications in the areas of high-efficiency optical parametric oscillators and frequency up-converters for the infrared (ir) range as well as for widespectral-range optoelectronic devices. (See also the article by Schunemann, Schepler, and Budni in this issue.) However extensive realization of their potential has still not been achieved. One of the principal difficulties in the way to obtaining high-device-quality ZnGeP2, CdGeAs2, AgGaS2, and AgGaSe2 single crystals is undesired optical absorption in their transparency range near the fundamental band edge induced by lattice-related defects. This article summarizes selected aspects of dopant-incorporation techniques of these crystals including dopant choice of dopant material and monitoring of dopant incorporation as done in our laboratory.
In general for the ternary chalcopyrite compounds, doping-incorporation processes are more complicated in comparison to those of binary zinc-blende III-V compounds. The most common sources of dominant incorporation of acceptors and donors in as-grown chalcopyrites are believed to appear from (1) nonstoichiometric melts as well as by doping with different elements during the growth process and (2) incomplete removal of disorder on the cation sublattice during subsequent cooling. Furthermore the chalcopyrite structure II-IV-V2 undergoes a disorder-order phase transition upon cooling through approximately 1220 K for ZnGeP2 and 900 K for CdGeAs2. At these transition temperatures, solidification can be complicated also by supercooling phenomena, and the crystals transform from the cubic zinc-blende structure (where Zn atoms randomly fill cation sites) to the ordered chalcopyrite structure (e.g., when Zn and Ge occupy alternating cation sites in ZnGeP2).