Exciton photoluminescence in strained and unstrained ZnSe under hydrostatic pressure

Near-band-gap photoluminescence (PL) from exciton recombination in bulk crystalline ZnSe and in strained and strain-relaxed ZnSe epilayers grown on GaAs substrates is examined for pressures up to 80 kbar at 9 K in a diamond-anvil cell. The small sublinear dependence of PL energy with pressure for bulk ZnSe is attributed to the pressure dependence of the bulk modulus. In the strained ZnSe film, the change in biaxial strain with pressure is seen by the changing separation of the heavy- and light-hole exciton peaks. The heavy-hole exciton energy goes from being 12.5 meV below that of the light hole (1 bar), to the same energy as the light hole (∼29 kbar), to relatively higher enegy (>29 kbar) as pressure is increased, which shows that the strain in the ZnSe film has been tuned from being compressive to tensile. In contrast, PL suggests that strain-relaxed films slip when pressure is applied. The hydrostatic deformation potential for near-band-gap transitions in ZnSe is a=-4.37 eV, while ‖a‖ is unexpectedly larger for transitions associated with deep levels. There is evidence that the tetragonal deformation potential b is a function of either volume deformation or strain.