Coherent {001} interfaces between rocksalt and zinc-blende crystal structures

We study coherent, two-dimensional (2D) rocksalt PbTe epilayers in a zinc-blende CdTe matrix. The embedded material transforms during an annealing step into highly symmetric quantum dots with coherent interfaces. The atomic structure of the polar {001} interfaces between the two tellurides is investigated by high-resolution transmission electron microscopy in combination with multislice simulations. Both the original 2D PbTe epilayers and the resulting PbTe quantum dots are terminated by distinctively different cation-terminated [(001)A] and anion-terminated [(001)B] interfaces at opposite faces, whereas for the structurally similar ErAs/InGaAs heterosystem as yet only the (001)A interface was reported in the literature. Ab initio simulations were performed to assess these seeming differences, which were found to be mainly caused by the different polar bonding character of the involved II-VI and III-V compounds.

Figure 1

(a) TEM image of coherently precipitated PbTe QDs with the three characteristic interfaces. (b) HRTEM images of opposite interfaces of a PbTe SQW embedded in CdTe. Clearly visible are the distinctively different symmetry-allowed lattice fringes (002) and (220) in $rs$—PbTe, and (111) and $\left(\overline{1}\overline{1}1\right)$ in $zb$—CdTe.

Figure 2

(Color online) (a) Stick-and-ball representation of the bonding configurations of $rs$—PbTe and $zb$—CdTe. 3D and projected representations of the atoms in a CdTe/PbTe/CdTe heterostructure with both interfaces (b) Cd terminated, or (c) Te terminated. The geometry represents the experimental situation with growth along (001) and TEM images recorded along a $⟨110⟩$ zone axis.

Figure 3

(Color online) Comparison of HRTEM images (left) with HR simulations (right) for the four possible combinations of interface termination and zone axes. The stick-and-ball models identify the atomic positions derived from the respective simulations.

Figure 4

(Color online) Interface-formation energy ${\gamma }^{\text{form}}$ vs the relative abundance of Te atoms, which is represented by the chemical potential $\Delta {\mu }_{\text{Te}}$. The line pairs stand for different terminations at opposite interfaces (horizontal lines and blue online), complete Cd termination (inclining and red online), and complete Te termination (declining and green online). Solid and dashed lines are results for two different PbTe layer thicknesses, as indicated.

Figure 5

Phase diagram for the stability of different interfaces in $\text{ErAs}/{\text{In}}_{1-x}{\text{Ga}}_x\text{As}$ as a function of the chemical potentials $\Delta {\mu }_{\text{Ga}}$ and $\Delta {\mu }_{\text{In}}$. Only interfaces with the same termination on opposite faces were found to be stable.