* Corresponding author. E-mail: valerieb@crhea.cnrs.fr. Journal of Crystal Growth 192 (1998) 102—108 Defect density in ZnSe pseudomorphic layers grown by molecular beam epitaxy on to various GaAs buffer layers V. Bousquet*, E. Tournie´, J.-P. Faurie Centre de Recherche sur l+He & te & ro-Epitaxie et ses Applications, Centre National de la Recherche Scientifique, (CRHEA/CNRS), Rue Bernard Gre & gory, Parc Sophia Antipolis, F-06560 Valbonne, France Received 17 March 1998 Abstract We have investigated the influence of both the GaAs surface stoichiometry and the nucleation procedure on the defect density in ZnSe pseudomorphic layer. We have shown that the use of migration enhanced epitaxy (MEE) at low temperature in the first steps of the growth decreases the defect density by two orders of magnitude. For a ZnSe layer grown by conventional MBE on a (24)-GaAs surface the etch pit density (EPD) is equal to 510 cm but it reaches 510 cm when MEE is used at the beginning of the growth. The same behavior is reported for the growth on a c(44)-GaAs surface but the use of MEE decreases only the density down to 410 cm. Atomic force microscopy shows that the surface roughness and morphology are also improved. We have tried to incorporate a thin BeTe layer between the GaAs buffer and the ZnSe layer. The ZnSe growth start is two-dimensional but the defect density is only of 810 cm.  1998 Elsevier Science B.V. All rights reserved. PACS: 81.15.H; 68.55.Jk; 61.72.F Keywords: MBE; Defect density; ZnSe 1. Introduction ZnSe-based semiconductors and heterostruc- tures have been intensively studied for laser diodes in the blue spectral region. In a very early step, ZnSe growth runs were realized on bare GaAs substrate. The difference of covalency, the chemical reactivity between the two compounds and the lack of control in terms of substrate surfaces stoichio- metry gave rise to a three-dimensional growth mode at the initiation of the growth [1] along with a high defect density in the layers [2]. It was then demonstrated that the GaAs surface reconstruction had a drastic effect on the defect density [3] and also that defects were responsible for nonradiative recombination centers [4] and a short device life- time [5]. The growth of a GaAs buffer epilayer before the ZnSe growth helped to initiate a two- dimensional nucleation and enabled one to obtain the first ZnSe-based laser diode [6]. More recently, the reduction in defect density down to the mid 0022-0248/98/$19.00  1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 9 8 ) 0 0 4 4 7 - 3