Properties of Dopants in ZnGeP 2 , CdGeAs 2 , AgGaS 2 and AgGaSe 2
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MRS BULLETIN/JULY 1998
for ZnGeP2 and 900 K for CdGeAs2. At these transition temperatures, solidification can be complicated also by supercooling phenomena,6 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). For AgGaS2 and AgGaSe 2 , similar phase transitions exist at about 1310 K and 1120 K, respectively, though the existence of the zinc-blende modifications
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LT p-ZnGeP 2 (Bi)
o
o
3 (0
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p-ZnGeP2(TI) PL
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2.0 2.5 3.3 3.5 Reciprocal Temperature 1000/K (1/K) Figure 1. Temperature dependence of the free-hole concentration of the LT-grown p-ZnGeP2 single crystals obtained from liquid-bismuth (LT) and tellurium solutions.
requires further confirmation.2 Presence of disorder-order transitions in these ternary compounds may result in the presence of some partial disordered lattice sites at room temperature. Therefore the most probable point defects in these compounds are likely to be associated with vacancies and antisite defects. In our laboratory, single crystals of chalcopyrites were grown by the improved low temperature (LT) directional crystallization technique from nonstoichiometric melts. The LT technique was developed at the Ioffe Physico-Technical Institute for obtaining ZnGeP2 crystals in the temperature range of 870-1020 K, which is significantly below the temperature of the chalcopyrite-zinc-blende phase transition.5 This may drastically decrease the deviation from the stoichiometry. During the growth, ZnGeP2 was doped by indium, gallium and selenium—yielding only with the p-type conductivity. However in contrast to ZnGeP2, the conversion of conductivity type of the CdGeAs2 crystals was effectively adjusted by doping either with indium and tellurium donors or gallium and copper acceptors during crystal growth.
Temperature-Dependent Hall Effect and Electrical Conductivity Hall measurements provide the concentration of acceptors and donors by assuming an even distribution among inequivalent lattice sites. All the LTgrown ZnGeP2 samples exhibited p-type conductivity. Figure 1 shows the typical temperature dependence of the free-hole concentrations for LT-grown p-ZnGeP2 single crystal obtained from liquid bismuth and tellurium solution respectively. Using these data, the activation energy of 0.55 eV was obtained for a hole concentration of 1.0 X lO10 cm"3 at room temperature. The Hall scattering factor was taken to be unity for this analysis. The Hall mobility strongly depends on donor and acceptor concentrations and compensation which is the sum of concentrations over all the defect sites. We found that the Hall coefficients and electrical conductivity of the LT-grown ZnGeP2 single crystals were isotropic. This means that the tetragonal compression along the [001] direction is not enough to cause measurable anisotropy of the Hall mobility.4 Table I summarizes the native defect characteristics and typical room-temperature el
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