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 het ero-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 mu m Hetero-interfacial stress effects were investigated by photorefle ctance (PR) and reflectance-difference spectroscopy (RDS). From a compariso n between PR spectra and the second energy-derivative of the RDS spectra (S DRD) we conclude that both PR and RDS spectra have two components: (1) a bu lk-like signal as for bare GaAs and (2) a signal coming from a strained reg ion near the ZnSe-GaAs hetero-interface. From the theory of PR we estimate that the observed compressive strain giving rise to the second component ha s a value epsilon congruent to - 0.0010 +/- 0.0004, independent of the thic kness 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 cros s-section profiles, a situation that tends to preclude coherent growth betw een the ZnSe and the GaAs. We calculate that there has to be present a stra in in the upper atomic layers of the GaAs due to the incoherent growth of Z nSe inside the GaAs pits and to the difference in thermal expansion coeffic ients between the GaAs and the ZnSe, Both phenomena are expected to produce a total strain of same magnitude as that observed by PR. (C) 1999 Publishe d by Elsevier Science B.V. All rights reserved.