Strain in GaAs at the heterointerface of ZnSe/GaAs/GaAs
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GaAs at interfaces of molecular beam epitaxy (MBE) grown ZnSe/GaAs/GaAs films with ZnSe layers of different thicknesses is studied by photoreflectance (PR) spectroscopy. We can separate two different near-band-edge optical features originating from two different regions of the heterostructure by using in-phase and out-phase PR measurements as well as the results of two different wavelength pumping lasers. One of the transitions is a bulk-like signal 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 region that encompasses the buffer layer/substrate GaAs interface, as is also revealed by the observation of Franz-Keldysh oscillations in the transitions from this region, which manifest the existence of an electric field within it. An electric field which is larger in magnitude is also visible in the PR signal from the heterointerface. Results for the second derivative of reflectance difference spectra (SDRD) further supports the existence of two spatially separated regions in the GaAs that produces two independent overlapping optical modulated signals in these heterostructures. From the theory of PR we estimate that the observed compressive strain giving rise to the second component has a value ε≈-0.0010±0.0004, which is independent of the thickness of the ZnSe epilayer. Atomic force microscopy (AFM) measurements were carried out on the GaAs epilayer prior to ZnSe growth revealing an almost uniform density of pits for all samples observed. 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 thermal expansion coefficients between the GaAs and the ZnSe. Both phenomena are expected to produce a total strain of the same magnitude as that observed by PR.
