Strain in GaAs at the heterointerface of ZnSe/GaAs/GaAs

GaAs at interfaces of molecular beam epitaxy (MBE) grown ZnSe/GaAs/GaAs fil ms with ZnSe layers of different thicknesses is studied by photoreflectance (PR) spectroscopy. We can separate two different near-band-edge optical fe atures originating from two different regions of the heterostructure by usi ng in-phase and out-phase PR measurements as well as the results of two dif ferent wavelength pumping lasers. One of the transitions is a bulk-like sig nal as for bare GaAs and another signal is attributed to a strained region adjacent to the ZnSe/GaAs interface. The bulk-like signal originates in a r egion that encompasses the buffer layer/substrate GaAs interface, as is als o revealed by the observation of Franz-Keldysh oscillations in the transiti ons from this region, which manifest the existence of an electric field wit hin it. An electric field which is larger in magnitude is also visible in t he PR signal from the heterointerface. Results for the second derivative of reflectance difference spectra (SDRD) further supports the existence of tw o spatially separated regions in the GaAs that produces two independent ove rlapping optical modulated signals in these heterostructures. From the theo ry of PR we estimate that the observed compressive strain giving rise to th e second component has a value epsilon congruent to -0.0010 +/- 0.0004, whi ch is independent of the thickness of the ZnSe epilayer. Atomic force micro scopy (AFM) measurements were carried out on the GaAs epilayer prior to ZnS e growth revealing an almost uniform density of pits for all samples observ ed. These have irregular cross section profiles, a situation that tends to preclude coherent growth between the ZnSe and the GaAs. We calculate that a strain in the upper atomic layers of the GaAs has to be present due to the incoherent growth of ZnSe inside the GaAs pits and to the difference in th ermal expansion coefficients between the GaAs and the ZnSe. Both phenomena are expected to produce a total strain of the same magnitude as that observ ed by PR.