The effect of two-temperature capping on germanium/silicon quantum dots and analysis of superlattices so composed

Abstract

It is well documented that quantum dots (QDs), grown and subsequently buried under silicon at temperatures greater than 400°C, flatten into a pancakelike shape. Although QD arrays in Si superlattices have been studied for more than a decade, the process of flattening is not well understood. Here, we examine the process by which flattening occurs, using a two-temperature capping technique. Briefly, a 300°C cold-cap layer is deposited, conformally coating the QD, followed by deposition of hot Si at 750°C. Through this process, full or partial shape retention of the buried QDs can be selectively maintained. Interestingly, we found that QDs grown with this technique do not flatten in the traditional way. In fact, the QD truncates without any associated base spreading. The material from this truncation fills in the valleys, thereby flattening the surface of the sample. We analyzed this truncation by growing a series of samples under conditions of varied cold cap thicknesses and base growth temperatures. In addition, we found annealing at or above 650°C sufficient to induce this truncation, while adatom flux was not required for truncation. We examined the samples using cross-sectional transmission electron microscopy (TEM) to determine the degree of shape retention and location of the truncated material. This technique also provided the optimal growth conditions for two additional studies: photoluminescence (PL) analysis, which is used to determine how shape retention affects recombination efficiency. In sharp contrast to earlier studies, PL data indicated that while shape retention could be maximized by low temperature overgrowth, this was accompanied by an unacceptable loss in luminescent intensity. This decrease suggested quenching by QD surface and/or overlayer traps. To retain both QD shape and PL intensity, a low/high temperature procedure was identified. These samples were then analyzed by TEM for the appearance of dislocations, degree of shape retention, and alignment/self-segregation of upper layers compared to lower layers. The findings from all of the above techniques help to elucidate the properties of QDs in complicated structures and imply possible techniques to refine current technological practices.