Cation-site intrinsic defects in Zn-doped CdTe

The properties of the cation vacancy and the Te antisite, two dominant defects in CdTe and ${\text{Cd}}_{1-x}{\text{Zn}}_x\text{Te}$ alloys grown in Te-rich conditions, are examined using first-principles calculations. First, the structure, electronic levels, and migration paths of ${\text{V}}_{\text{Cd}}$ and ${\text{Te}}_{\text{Cd}}$ in CdTe are studied in detail. Additionally, we analyze the evolution of the stability and electronic properties in ${\text{Cd}}_{1-x}{\text{Zn}}_x\text{Te}$ alloys, taking into account both the role of alloying in the position of the ionization levels and its effects on the equilibrium concentration of those two defects. It is shown that the formation of cation vacancies becomes progressively more favorable as $x$ increases, whereas Te antisites become less stable, backing the trend towards $p$-type conductivity in dilute ${\text{Cd}}_{1-x}{\text{Zn}}_x\text{Te}$.

Figure 1

LCAO model for the splitting of the ${\text{Te}}_{\text{Cd}}$-related levels (not to scale).

Figure 2

(Color online) Isosurface of the charge density associated with the $a_1{}^c$ state of $\text{Te}_{\text{Cd}}{}^0$.

Figure 3

(Color online) Migration mechanisms investigated for ${\text{Te}}_{\text{Cd}}$. Initial and final states (respectively, before and after a single migration step) are depicted for migration paths I and II. Te and Cd atoms are represented by large and small spheres, respectively. In the representations of the final state, the previous position of the antisite defect and its displacement direction are marked by a dashed circle and an arrow.

Figure 4

(Color online) Calculated equilibrium alloy bond lengths $R_{\text{Cd-Te}}$ and $R_{\text{Zn-Te}}$, and average bond length $R_{\text{average}}\left(x\right)=\sqrt{a_0\left(x\right)}/4$. Each point corresponds to a supercell. Lines are added to aid visualization.

Figure 5

(Color online) Mixing enthalpy for CdZnTe. Pseudorandom vs ordered structures. The results from the random model and ordered alloy calculations of Wei et al. (Ref. 46) are plotted along for comparison.

Figure 6

(Color online) Formation energies (left) and ionization levels (right) of intrinsic defects in strained CdTe, as a function of the linear compressive strain, defined as $\epsilon =\left(a_0{}^{\text{CdTe}}-a_0\right)/a_0{}^{\text{CdTe}}\times 100$. The reorientation energy of the Te antisite is also shown $\left[E_{\text{reor}}\left(\text{Te}_{\text{Cd}}{}^0\right)\right]$.

Figure 7

(Color online) Minimum formation energies of the neutral cation vacancy $\left({\text{V}}_X\right)$ and tellurium antisite $\left({\text{Te}}_X\right)$ defects in ${\text{Cd}}_{1-x}{\text{Zn}}_x\text{Te}$ alloys with different Zn concentrations, obtained for Te-rich conditions and for ${\mu }_X+{\mu }_{\text{Te}}=E\left(X\text{Te}\right)$. Curves were added to help visualization.

Figure 8

(Color online) Energy levels of the cation vacancy $\left({\text{V}}_X\right)$ and tellurium antisite $\left({\text{Te}}_X\right)$ defects in ${\text{Cd}}_{1-x}{\text{Zn}}_x\text{Te}$ for different Zn concentrations. Lines were added to help visualization.