Transmission electron microscopy is used to study the morphology and the composition profile of ''pure'' Ge islands grown at high temperature on Si͑001͒ by molecular beam epitaxy. An alloying process, involving mass transport from the substrate to the islands during the island growth, was identified. It was found that, as a result of Si mass transport to the Ge islands, the island/substrate interface moves towards the substrate, and trenches form on the substrate surface around the islands. Reduction of the misfit strain at the island/substrate interface is the driving force for this process. ͓S0163-1829͑99͒00748-1͔Semiconductor quantum dots ͑QD's͒ are attracting increasing interest because of their significant optoelectronic properties. 1 Although QD's can be produced in many ways, the method of coherent island formation is of great importance for materials with large lattice mismatch ͑such as Ge͑Si͒/Si 2,3 and In x Ga 1Ϫx As/GaAs 4 because of the possible combination of the QD growth and semiconductor integration techniques. Of crucial importance in determining the optoelectronic properties of the QD's are the structural parameters of the QD's including the shape, size and composition. 5 Uniformity in and control over these parameters are a prerequisite for many applications. To achieve this goal, a complete understanding of the mechanism of the QD growth is necessary. Although many investigations have concentrated on the shape and size 6,7 and evolution 8,9,10 during the QD growth, relatively less attention has been paid to the composition. 11,12,13 In the classical Stranski-Krastanow 14 ͑S-K͒ mode of coherent island formation, one material with a different lattice parameter and low interfacial energy is initially deposited on a substrate surface, layer by layer, forming a ''wetting layer''. When the wetting layer reaches a critical thickness ͓usually three to five monolayers for pure Ge on Si͑001͒, 3,15 island growth starts to partially release the mismatch strain energy between the epitaxial layer and the substrate. However, in the case of InAs/GaAs, recent investigations suggest that, for higher temperature growths, there is significant mass transport from both the wetting layer and the substrate to the islands. 16 Furthermore, the wetting layer in InAs/GaAs systems has been reported to be an ͑In,Ga͒As alloy with temperature dependent composition. 17 The composition of the alloy wetting layer will certainly affect any subsequent island growth procedure. These points suggest that the classical S-K growth mechanism needs to be modified to explain the details of island growth. In this paper, we demonstrate evidence for mass transport from the Si substrate to the Ge islands during high temperature molecular beam epitaxy ͑MBE͒ growth of ''pure'' Ge islands on the Si͑001͒ substrate. The mass transport changes the composition of the islands, moves the Ge/Si interface below the original substrate surface, and forms a trench around each island. It is proposed that the driving force for this mechanism is a reduction...

Ge͑Si͒/Si͑001͒ coherent islands grown at 700°C by molecular beam epitaxy were investigated using transmission electron microscopy. ͓001͔ on-zone bright-field diffraction contrast imaging and image simulation techniques were used to investigate the structure of these coherent islands. Comparison of simulated and experimental images indicates nonuniform composition distribution within the coherent islands when the islands were grown at high temperatures ͑700°C͒, but uniform composition for growth at lower temperatures ͑600°C͒.

This article presents optical characteristics of ZnSe/ZnS0.12Se0.88 strained-layer superlattices (SLs) with ZnS0.06Se0.94 buffers pseudomorphically grown on GaAs(100) substrates by molecular beam epitaxy. The SL samples exhibit strong blue luminescence. The main emission peaks in photoluminescence spectra can be attributed to the free exciton transitions between lowest electron subband and ground heavy-hole subband of ZnSe wells. The temperature dependence of PL was investigated in detail. The experimental results of temperature dependence of peak positions and linewidths (FWHM) were fitted to the theoretical calculations, based on Varshni’s formula and broadening model. The activation energies of the samples were derived from the temperature dependence of PL intensities. The effects of strain and quantum confinement in ZnSe/ZnS0.12Se0.88 strained-layer SLs with different well thicknesses of 30, 60, and 120 Å were studied by experiments and theoretical calculations. Theoretical calculations and experimental observations are in reasonable agreement.