UHV-TEM

Ultra high vacuum transmission electron microscopy

Formation of nanoscale islands during strained layer epitaxy During lattice mismatched epitaxy a range of interesting structures can form spontaneously to minimise the total strain and surface energy. A common growth mode is the formation of individual, coherently strained three-dimensional islands. The material at the top of the islands is partially elastically relaxed, reducing the total energy even though the surface area of the system increases. For a lattice mismatch of several percent the islands formed have dimensions in the nanoscale regime (up to several tens of nanometers) and this restricted size can lead to interesting optical and electronic properties. This makes such self-assembled islands, or quantum dots, useful for a range of applications ranging from solid-state lasers to quantum cellular automata. In all applications it is important to control the size, and, to some extent, the shape of the islands, in order to optimise the properties of the confined electrons, while in some applications the position of the islands must be specified as well. Precise position and size control of self-assembled islands formed during lattice mismatched epitaxy is particularly difficult to achieve because of the spontaneous nature of island growth, and this problem has led to an intense interest in understanding the factors which control the sizes and shapes of self-assembled islands and in devising ways to control the positions at which they nucleate. Control of island growth in the Ge/Si system is of particular interest because of the ease of integration of potential devices with existing technology. Two island shapes exist The SiGe system is surprisingly complex. Scanning probe microscopy [Medeiros-Ribeiro et al., Science 279, 353 (1998)] showed that during deposition of Ge on Si(001) the islands grow through a series of different shapes. Islands of diameter below 50nm are pyramidal in shape and are made up of four [105] facets, while islands above this diameter exhibit higher index facets. Furthermore, the size distribution and the island shapes are related, with a bimodal distribution often seen in which the smaller volume peak is composed of pyramids and the higher peak of domes. Figure 1: TEM images of islands showing the distribution of sizes. These are strain contrast images recorded after deposition of about 4ML Ge from digermane gas at 1x10-8 Torr and 600°C. Below are shown the pyramid and dome shapes of islands. These images were recorded in the low energy electron microscope after deposition of a Si0.7Ge0.3 alloy at 690°C. Pure Ge islands show the same shapes but have smaller diameters. A typical bimodal size distribution is also shown. Observing Ge island growth in situ We clean chemically thinned Si(001) specimens by flashing to 1250°C under UHV, cool them to the growth temperature of 550-650°C, and then observe them at video rate as Ge (or a SiGe alloy) is grown using chemical vapour deposition from digermane (or digermane/disilane) gas at pressures up to 10-6 Torr. The main results are summarised here but for more details of the behaviour of this interesting system please see the references. Coarsening is important during growth (Video 1: ge_deposition.mp4) The first video shows the process of chemical vapour deposition in real time. It was recorded during deposition at 650°C and 5x10-8 Torr (deposition rate about 1ML per minute) using a weak beam (g, 3g) image with g=220. The field of view is 400nm. The images show the strain field around each island. They are not sensitive to the island shapes, but several phenomena are visible: Figure 2: Still from Video 1 (ge_deposition.mp4) Nucleation occurs over several seconds (although not at steeper parts of the surface). Approximately 3ML of Ge have already been deposited to form the wetting later; we do not see this layer in this imaging condition. Coarsening is significant. The smaller islands disappear, losing atoms to the larger islands by surface diffusion. This process is similar to classical Ostwald ripening. After about 30 seconds the bimodal distribution develops. Finally all the small island disappear, leaving a narrow size distribution. Later appearance of nuclei is actually nucleation on the back surface of the specimen due to the small flux of Ge which reaches the back of the specimen. The size distribution changes during growth Analysis of images covering much larger areas and greater numbers of islands confirm that the size distribution changes during growth. A bimodal distribution develops, then the smaller peak of the distribution disappears leaving a single peak. Figure 3: Images and histograms recorded during deposition of Ge at 640°C and 2x10-8 Torr digermane for the times shown; the weak beam images with g=(220) show the strain field around each island. Note that the smallest islands have very weak contrast and do not show in the distributions. A model for growth It is clear that the coarsening which occurs during growth is modified by the change in island shape and results in a bimodal size distribution. We can understand the growth kinetics with an "anomalous Ostwald ripening" model. We first calculate the energy of islands with different aspect ratios. The energy curves cross, showing that islands change shape at a critical volume VC. We then derive the chemical potential from these curves. Finally we use this chemical potential in a mean-field Ostwald ripening calculation to give the evolution of islands. Figure 4: The energy per atom, shown for two different island shapes with facet angles a1 and a2, and the chemical potential, both shown as a function of island volume. Figure 5: Stills from the video, from which the diameter of each island is extracted and shown below. This data is compared with the results of the simulation. The main features of the experimental data (both here and in figure 2) are captured in the simulation. How does the shape change occur? Although the model allows only two island shapes, we can extend it by allowing multifaceted islands to occur. Calculations lead us to expect that steeper facets will be introduced from the edges of the island. A video shows this process in real time. It was recorded in the low energy electron microscope during growth of SiGe alloyed islands, which undergo the shape transition at a larger volume than pure Ge islands making them easier to see. This video illustrates that islands undergo the shape transition through a series of asymmetric intermediates as steep facets appear one at a time. The shape transition takes several minutes to be completed. Why have transition shapes not been observed previously? If we cool a specimen in which transition shapes are present, they transform to the final dome shape, presumably due to a temperature-dependent facet energy, and the transition process is therefore only visible during in situ observations. Video 2 (gesi_deposition.mp4) Several interesting phenomena are visible on this video, which was recorded in the low energy electron microscope during deposition of a Si0.7Ge0.3 alloy from a mixture of disilane and digermane gases (growth rate about 5 ML/min) at a specimen temperature of 690°C, with a bright field imaging condition at electron energy 5-10eV. The video is speeded up 20x and the field of view is 4 microns. Figure 6: Still from Video 2 (gesi_deposition.mp4) Step flow during Si deposition As we turn on the digermane the surface becomes rough The roughness develops into ripples which then form into closely packed pyramid shaped islands The islands coarsen with smaller ones disappearing The largest islands start changing shape; they become more circular and later, as we change the imaging condition, the steeper facets become visible as bright pairs of dots The islands transform to the symmetrical pyramid shape by addition of the steep facets one at a time. Stages in development from pyramid to dome Figure 7: Stills extracted from the video showing the shape transformation from pyramid to dome, and diagrams of the intermediate shapes. Figure 8: A partly transformed specimen, recorded at the growth temperature where intermediate shapes are visible, and again at a lower temperature where the intermediate shapes have transformed into symmetrical domes. Summary Real time observations of Ge and SiGe island growth have shown directly the important processes which occur during growth. Coarsening is significant, and as they grow the islands change shape by addition of steeper facets from the perimeter. A simple model can account for the processes we see, and this model also allows us to understand the factors controlling island size distribution. For example, the narrowest distribution is achieved by stopping growth just after the pyramids have disappeared. However, it is also clear that the initial size distribution determines the subsequent island evolution: controlling the positions at which islands nucleate would help is to create a narrowly distributed array of islands.