Journal of Crystal Growth 214/215 (2000) 684}689 Be-induced CdSe island formation in CdSe/ZnSe sub-monolayer superlattices M. Keim!, M. Korn!, F. Bensing!, A. Waag!,*, G. Landwehr!, S.V. Ivanov!,", S.V. Sorokin!,", A.A. Sitnikova", T.V. Shubina", A.A. Toropov" !Physikalisches Institut der Universita( t Wu( rzburg, Am Hubland, D-97074 Wu( rzburg, Germany "Iowe Physico-Technical Institute, St. Petersburg 194021, Russia Abstract We report on the formation of CdSe quantum dots (QDs) in short-period superlattices (SLs) of CdSe sub-monolayers in ZnSe, grown by migration-enhanced epitaxy (MEE) on (0 0 1)GaAs substrates. Coverage of the initial ZnSe starting surfaces with a fractional monolayer (FM) of beryllium leads to enhanced islanding below a CdSe thickness of 0.6 ML corresponding to the onset of a CdSe-rich island formation in the Be-free layers. X-ray di!raction and re#ectometry measurements on CdSe/ZnSe superlattices with a similar CdSe layer thickness reveal a roughening of the CdSe layers in the case of the beryllium coverage, caused by the formation of CdSe-enriched islands. Cross-sectional transmission electron microscopy on the SLs with BeSe FM exhibits Cd-induced stress modulation with a lateral scale of &4 nm, that can be interpreted as CdSe quantum dots. ( 2000 Elsevier Science B.V. All rights reserved. PACS: 81.15.Hi; 78.66.Hf; 78.55.Et; 61.10.Kw Keywords: MBE; Island; Quantum dot; CdSe/ZnSe; XRD; Re#ectometry; TEM 1. Introduction The self-organised formation of nanoislands has been observed in MBE-grown CdSe/ZnSe struc- tures with the CdSe thickness w both above [1] and below [2] the critical one (&3 monolayer (ML)). The 2.5}2.8 ML thick CdSe layers displayed the highest photoluminescence e$ciency, which allowed the fabrication of room temperature green * Corresponding author. Tel.: #49-931-888-5777; fax #49- 931-888-5142. E-mail address: waag@physik.uni-wuerzburg.de (A. Waag). laser diodes with the active region based on a single CdSe sheet [3]. However, the average lateral size of the CdSe-rich islands (20}30 nm) is large compared to the exciton Bohr radius (&5 nm), which pre- vents one from considering these objects as elec- tronic quantum dots (QDs). In addition, the array of the islands generally exhibits extremely large dispersion of the lateral size. CdSe/ZnSe nanostruc- tures with the CdSe thicknesses in a sub-monolayer range (w(1 ML) generally behave as a two-dimen- sional excitonic system involving both the broadened ZnCdSe alloy-like quantum well (QW) and CdSe-enriched large #at islands with small- scale ((10 nm) Cd composition #uctuations inside [4}6]. 0022-0248/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 0 ) 0 0 1 7 8 - 0