The heavy-hole and light-hole excitons of a pseudomorphic ZnSe film grown on a GaAs epilayer by molecular-beam epitaxy, are studied as a function of applied hydrostatic pressure using photomodulated reflectance spectroscopy. At ambient pressure, the signature in the spectrum due to the heavy-hole exciton occurs at an energy lower than that of the light-hole exciton, a consequence of the compressive biaxial strain in ZnSe due to its lattice mismatch with GaAs. As the pressure is increased, the two signatures approach each other in energy and coalesce at 36.2 kbar. The difference in the compressibility of ZnSe from that of GaAs generates a tensile strain that progressively compensates the lattice-mismatch-induced compressive strain and finally, at 36.2 kbar, the heterostructure is strain free. Beyond this pressure, the strain in ZnSe transforms from biaxial compression to biaxial tension, the light-hole signature now occurring at the lower energy. The transformation of strains via pressure tuning is continuous and reversible. The separation between the heavy-hole and light-hole signatures is superlinear in pressure, suggestive of a pressure-dependent shear-deformation-potential constant.
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