MORPHOLOGICAL CHANGES TO InAs ISLANDS BY OVERGROWTH

As described above the growth of highly compressed epilayers in some heteroepitaxial systems, nanometer scale 3D islands can nucleate on the surface as a strain relief mechanism at the growth front. If growth is terminated at this point and the islands are then covered with a larger bandgap material, the nanometer scale 3D geometrical confinement results in quasi-zero dimensional electronic levels inside the island. InAs islands grown in a GaAs matrix are the dominant material system for such quantum dots (QD’s). These films provided a rich testing ground for the study of zero dimensional confinement in semiconductors, and hold the potential for several interesting device applications.

Because the electronic levels of the QD’s depend on the size and shape of the island, a large body of research has focused on better understanding and controlling the InAs island morphology. The bulk of structural characterization studies have examined the surface morphology of the uncapped 3D-islands. However, research that probes the structure of the QD’s embedded inside the GaAs matrix indicates that the 3D-island morphology is altered during GaAs overgrowth. In this section we discuss the use of in-situ scanning tunneling microscopy (STM) to study how the InAs island morphology changes as a function of GaAs overgrowth.

Growth and Imaging of InAs Islands as a Function of Overgrowth

The sample preparation and InAs deposition on GaAs is described earlier.  After InAs deposition, the sample is maintained at 495ºC under an As BEP of 1x10-6 Torr for 50s to allow island ripening which yields larger and more uniform dots. The GaAs overgrowth was performed at 0.7ML/s (Ga BEP of 4x10-7 Torr, As BEP of 4x10-6 Torr) in all experiments. The sample heater was turned off immediately after GaAs deposition for each experiment, and the sample cooled under a gradually reduced As flux until its temperature fell below 350ºC.

It is useful to first consider what changes to the surface might be caused by GaAs deposition. Figure 13 illustrates how the surface profile of the 3D-island morphology might be changed by GaAs overgrowth. Figures 13-a and 13-b illustrate possible outcomes in the case that the islands are completely stable. That is, in these cases it is assumed no In-Ga intermixing occurs, so that the 3D-island morphology is preserved. Figures 13-c and 13-d illustrate the more realistic situation in which cation intermixing leads to a redistribution of the island material. In this latter scenario, as overgrowth is initiated the InAs layer becomes an alloyed InGaAs layer, effectively reducing the lattice mismatch between epilayer and substrate. Such an alloyed layer would likely possess a 2D lowest energy surface. In this case, the surface morphology observed after overgrowth, would depend on how much of the 3D-island volume had redistributed itself into the 2D region. If the sample were maintained at high temperature for sufficient time, after a few monolayers of Ga were deposited, a 2D surface would be expected. But if the sample were quenched during this change, an unstable, and intermediate 2D-3D surface would be observed.  Such a surface would exhibit a reduced island height, combined with an enlarged basal area as illustrated in Figures 13-c and 13-d.

InAs Morphology

Figure 14 shows two images of the surface morphology after 2.1ML of InAs deposition. This coverage corresponds to 0.4ML beyond the 2D to 3D critical thickness. The growth conditions we have employed produce relatively large islands which are 9.5 ± 1.5nm tall and have a base diameter of approximately 33nm. The number density of the islands is 3 x 1010 cm-2. This morphology of relatively large islands that are separated by 2D regions is well suited for studying the changes to the islands after GaAs overgrowth. Figure 14-b shows a high resolution scan of an individual island. We observe a structure for the uncapped island that is different from the disc shaped structure commonly reported by AFM, and supports the structure reported by Hasegawa et al.. This image shows four facetted planes, which form a pyramidal structure that is elongated along the (-110). This morphology is strikingly similar to that predicted by Lee et al., who derived a similar structure from quantitative analysis of the reflective high-energy electron diffraction (RHEED) pattern.  

1.4.3 Initial Stages of Overgrowth

Figure 15 shows four 200 x 200nm STM images. Figure 15-a shows the InAs surface prior to overgrowth. Figure 15 shows images taken after 0.7ML, 1.4ML and 2.7ML of GaAs overgrowth. After only 0.7ML of overgrowth we observe small terraces forming at the base of the island. This effect is more pronounced after 1.4ML of overgrowth as the island base has become significantly enlarged, and the taller 3D region of the island has been reduced in size. By 2.4ML of GaAs coverage, the mean island height has been reduced from 100Å to 20Å. From this series of images it can be concluded that extensive cation intermixing is taking place during GaAs overgrowth. As a result the lowest energy surface morphology has become 2D.  We know this to be true because we observe a complete dissolution of the 3D morphology after 2ML of deposition if the sample is maintained at 495º C for 30s. Figure 15 shows that we have cooled the sample fast enough to allow only partial intermixing. Because these surfaces are not in equilibrium, these images can not be viewed as snap shots of the growth front.

Figure 16 shows the surface profile data of representative islands taken for each surface shown in Figure 15 for both the (110) and (-110) directions. The height reduction of the 3D islands at these low coverages, compared to the uncapped island profile, demonstrate that material is leaving the 3D islands. From the (110) surface profiles, it is clear that no broadening of the island base accompanies this height reduction. In contrast, after 2.7ML of GaAs overgrowth, the island base has increased by nearly a factor of three along the (-110). This anisotropic volume redistribution points to the specific mechanism in which the In is transferred away from the 3D islands and into the GaAs capping layer. Cation surface diffusion coefficients are reported to be 100 times larger along the (-110) than along the (110) due to the corrugated surface of the 2x4 surface reconstruction. Because the zincblende lattice has non-symmorphic symmetry, the bulk diffusion rate is expected to be the same in these two directions. To summarize our results from these initial stages of GaAs overgrowth, we have confirmed that extensive In-Ga intermixing occurs, and that the specific mechanism is In surface diffusion on the 2x4 surface.

1.4.4 More Substantial Overgrowth            

Figure 17 shows more 200 x 200nm images taken after additional stages of GaAs overgrowth; the 2.7ML coverage image is shown again for comparative purposes. At 5ML the islands continue to be reduced in height while their basal area increases. At 11 ML, the islands become taller, and even taller at 21ML. In addition, depressions are observed in the center of the islands at 10 and 20 ML. Traces of this feature are also apparent at lower coverage. The depression may be explained by preferential nucleation of the capping layer in the 2D region.

This has been proposed to be the result of the 3D island locally possessing a larger lattice constant near to that of InAs, so that nucleation of the smaller GaAs bond in this region would be energetically unfavorable. The surface diffusion of In away from this region during and/or after overgrowth could also account for this depression.

These features are revealed more clearly in the line profile data shown in Figure 18. These line profiles are first normalized in the 2D region to 0, and then offset by the amount of GaAs overgrowth. This allows for meaningful comparison to the uncapped island profiles shown with these data. A very interesting observation may be reached regarding the change in height shown by these line scans as the coverage in increased from 5 to 10ML. Because, in the center of the line scan, this height increase is not due to GaAs nucleation, we conclude that a greater portion of the 3D island has been preserved after 10ML of coverage. That is, thicker or more rapid overgrowth better preserves the 3D-island morphology. Because we have observed that a small amount of overgrowth can lead to a complete loss of the 3D islands, this idea is confirmed by the simple fact that cross sectional studies of capped islands show that the 3D islands are indeed preserved inside a GaAs matrix. That is, were this observation untrue it would be unlikely that anyone had ever observed 3D islands buried inside the GaAs capping layer. Therefore, the volume redistribution process is not so rapid as to be independent of the GaAs deposition rate. Faster overgrowth buries more In beneath the growth front. This In would then have to undergo bulk diffusion through the zincblende lattice to achieve volume redistribution. This later intermixing process would occur at a rate insignificant compared to surface diffusion.

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