Stress in GaAs at the hetero-interface of ZnSe/GaAs/GaAs: A possible effect of pit filling and difference in thermal expansion coefficients

We report on the observation of stress effects on GaAs at the ZnSe-GaAs hetero-interface. Samples with the structure ZnSe/GaAs/GaAs(100) were grown by molecular beam epitaxy (MBE). ZnSe epilayers thickness ranged from 0.08 to 0.6 μm. Hetero-interfacial stress effects were investigated by photoreflectance (PR) and reflectance-difference spectroscopy (RDS). From a comparison between PR spectra and the second energy-derivative of the RDS spectra (SDRD) we conclude that both PR and RDS spectra have two components: (1) a bulk-like signal as for bare GaAs and (2) a signal coming from a strained region near the ZnSe-GaAs hetero-interface. 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, 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 there has to be present a strain in the upper atomic layers of the GaAs 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 same magnitude as that observed by PR.

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