$\mathrm{Zn}\mathrm{Se}∕\mathrm{Ga}\mathrm{As}\left(001\right)$ heterostructures with defected interfaces: Structural, thermodynamic, and electronic properties

We have performed accurate ab initio pseudopotential calculations for the structural and electronic properties of $\mathrm{Zn}\mathrm{Se}∕\mathrm{Ga}\mathrm{As}\left(001\right)$ heterostructures with interface configurations accounting for charge neutrality prescriptions. In addition to the simplest configurations with atomic interdiffusion we consider also some configurations characterized by As depletion and cation vacancies, motivated by the recent successful growth of $\mathrm{Zn}\mathrm{Se}∕\mathrm{Ga}\mathrm{As}$ pseudomorphic structures with minimum stacking fault density characterized by the presence of a defected (Zn,Ga)Se alloy in the interface region. We find that—under particular thermodynamic conditions—some defected configurations are favored with respect to undefected ones with simple anion or cation mixing, and that the calculated band offsets for some defected structures are compatible with those measured. Although it is not possible to extract indications about the precise interface composition and vacancy concentration, our results support the experimental indication of (Zn,Ga)Se defected compounds in high-quality $\mathrm{Zn}\mathrm{Se}∕\mathrm{Ga}\mathrm{As}\left(001\right)$ heterojunctions with low native stacking fault density. The range of measured band offset suggests that different atoms at interfaces rearrange, with possible presence of vacancies, in such a way that not only local charges but also ionic dipoles are vanishing.

Figure 1

Composition and bare ionic charge profiles (in unit of electronic charge per zinc blende unit cell) in the interface region of the defected heterostructures 1HV, 1HV-swap, 1V, 1V-swap, 2V, 2V-swap along the (001) growth direction. The ionic charge is averaged over pairs of adjacent (001) atomic layers and normalized to the bulk zincblende unit cell. Interface cationic planes are emphasized in boldface.

Figure 2

Interplanar distances (in Å) for the different structures considered including cation vacancies. The average value of the interplanar distances are indicated by closed symbols, and the error bar indicate the range of local variation due to the possible buckling of the atomic planes.

Figure 3

Top: Formation energy [in $\mathrm{eV}∕1\times 1$ surface] of the mixed and defected interfaces as a function of the difference of the Zn and Ga chemical potentials, for lower (upper) limit of ${\tilde{\mu }}^{\mathrm{Se}}$. Dotted lines refer to the undefected 1C, 1A, 2CA-ZnAs, 2CA-GaSe interfaces, which appear degenerate on this energy scale. Hatched areas indicate the range of interface formation energy for interfaces with the same stoichiometry: right (left) hatched areas represent the range of variation of formation energy for 1HV, 1HV-swap, 2V, 2V-swap (1V, 1V-swap). Those with the lowest formation energy are 1HV-swap and 1V for the two families respectively. Bottom: as before, as a function of the difference of the Se chemical potential for the lower (upper) limit of ${\tilde{\mu }}^{\mathrm{Zn}}-{\tilde{\mu }}^{\mathrm{Ga}}$. The chemical potentials have been rescaled with respect to their correspondent bulk value (see text).

Figure 4

Total DOS (upper panels) and atomic projected density of states over layers along the growth direction for the 1HV [panels (a)] and 1HV-swap [panels (b)] interface. The “excess” DOS is shown as black area (see text). Arrows indicate the energy position of the cation and anion $s$ states in each layer. Energies are referred to the Fermi energy for 1HV and to the Valence Band Maximum for 1HV-swap. Solid/dashed line in the topmost panels are for relaxed/unrelaxed structure.

Figure 5

Scheme of the band alignments for the different interfaces investigated. The relative energy scale is referred to the band edges of GaAs, which are indicated on the left, and the relative position of ZnSe is then indicated for the different morphologies. The horizontal dotted lines refer to GaAs band edges. The label “Exp(1)” refers to experimental values in Ref. 7 and “Exp(2)” to all the other measurements reported in the literature (Ref. 15).