Self-assembling of isoelectronic impurities in GaAs doped with Al and N, AlN doped with Ga and As, Ge doped with C and Sn and ZnS doped with Mg and O was predicted and described in [1]. The equality conditions of the free energies of the random alloys and alloys with impurity tetrahedral clusters were represented. Such self-assembled cluster ordered state is a new phase state of the alloys. However, a phase transition between the cluster ordered and homogeneous alloys is not described yet. The cluster order - disorder phase transition in ZnTe doped with Mg and O is described.

The self-assembled cluster ordered state is characterized by the cluster order parameter that is a part of oxygen impurity atoms placed in 1O4Mg clusters. The ultra dilute limit for the oxygen impurity from y = 1´ 10^-8 to 1´ 10^-3 was chosen. The enthalpy is a continuous function of temperature and heat capacity undergoes the finite discontinuity at an occurrence of 1O4Mg clusters. Such temperature dependences of the enthalpy and heat capacity correspond to the second-order phase transition. The critical temperature of Mg(x)Zn(1-x)O(y)Te(1-y) with x = 1.52´ 10^-4 and y = 1´ 10^-5 is equal to 786 K. The free energies of this alloy at the completely cluster ordered and random states are equal to each other at temperature of 773 K. At this temperature the thermodynamically stable state corresponds to the alloy in which one-half of oxygen impurities is placed in 1O4Mg clusters. At the completely cluster ordered state this alloy becomes stable at temperature of 747 K. The occurrence of the complete cluster order at this temperature is also a result of the second-order phase transition.

References

1. V. A. Elyukhin et al, Appl. Phys. Lett. 85, 1704 (2004); V. A. Elyukhin et al, Physica E 28, 552 (2005); V. A. Elyukhin et al , J. Appl. Phys. 99, 033504 (2006).

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