Compositionally modulated sputtered InSb/GaSb superlattices: Crystal growth and interlayer diffusion

Abstract

Compositionally modulated single‐crystal InSb/GaSb superlattice structures have been grown by multitarget sputtering in order to investigate the effects of film growth parameters on the defect structure, abruptness, and coherence of sputtered heterojunctions. The layer thicknesses studied ranged from 12 to 70 Å. The polycrystalline–to–single‐crystal ’’transition temperature’’ T_c was found to decrease with decreasing average film‐substrate lattice mismatch and modulation period, Λ. The Λ‐dependence resulted from a decrease in the coherence between layers and the associated increased amount of plastically accommodated strain which occurred with increasing layer thickness. Increasing the film growth temperature T_S and decreasing the period of the composition modulation also resulted in a decrease in the density of microtwins and low‐angle dislocation boundaries which were observed in all films. Single and multiple {111} twins were identified. For a given set of growth conditions on cleaved BaF₂ and NaCl substrates, a significant decrease in T_c and structural defect densities could be achieved by coating the substrates with thin (in situ immediately prior to superlattice film growth. Variations in T_S between 150 and 320 °C had no effect on either the amplitude of composition modulation in deposited films or on the minimum layer thickness Λ_min obtainable from a given set of sputtering conditions. Λ_min was, however, found to be dependent on the sputtering pressure and was limited by ion‐bombardment‐enhanced diffusion during film growth. A model is discussed for determining bombardment‐enhanced diffusion coefficients D* as a function of growth conditions using measured x‐ray superlattice diffraction intensities. Values of D* obtained in this way were several orders of magnitude larger than thermal diffusion coefficients found from postannealing experiments.