Study of the GaAs growth on pseudomorphic Si layers for the formation of self-assembled quantum dots

We employed a Si interlayer to drive a 2D-3D transition in the GaAs/AlGaAs system. We have found that self-assembled GaAs quantum dots can be obtained by this technique. The samples were prepared in a Riber 32P MBE system employing undoped SI-GaAs[100] substrates. First, a 5000 /spl Aring/ thick layer of Al/sub x/Ga/sub 1-x/As was grown with a nominal concentration x = 0.3. With the As-cell shutter closed, Si was deposited on the AlGaAs layer at a substrate temperature of 500/spl deg/C. Then, the As-cell shutter was opened, and GaAs overgrowth was started by opening the Ga-cell shutter. A series of samples were prepared varying the Si-interlayer thickness (0-1 ML), and the amount of GaAs overgrowth to study the effects on the final structures. For all the samples we observed streaky reflection high-energy electron diffraction (RHEED) patterns during the growth of the Si-interlayer. Moreover, the spacing between principal RHEED reflections from the underlying AlGaAs layer was conserved during the whole Si growth. These results indicate a 2D pseudomorphic growth of Si on AlGaAs. Now, for interlayers with Si-thickness smaller than I ML, the streaky RHEED patterns were also conserved during the GaAs overgrowth. In addition, we found that the RHEED specular spot intensity presented oscillations during the GaAs overgrowth, showing a 2D nucleation of GaAs on Si interlayers with thickness smaller than 1 ML. In sharp contrast, for the GaAs overgrowth on 1 ML-thick Si, a strong increase of the RHEED intensity with no oscillations was observed at the first stages of growth. This was caused by the appearance of intense 3D spots in the RHEED patterns, indicating the formation of 3D islands on the Si surface. As the GaAs overgrowth is continued the 3D RHEED spot intensity eventually decreased showing a smoothing out of the surface. Thus, in order to avoid the smoothing out of the 3D islands, we prepared a sample stopping the GaAs overgrowth at the time when the 3D diffraction sp- t reached the maximum intensity.