We study the effects of low-temperature capping (200-450°C) on the optical properties of GaAs/AlGaAs quantum wells. Photoluminescence measurements clearly show the formation of abundant nonradiative recombination centers in an AlGaAs capping layer grown at 200°C, while there is a slight degradation of the optical quality in AlGaAs capping layers grown at temperatures above 350°C compared to that of a high-temperature capping layer. In addition, the optical quality can be restored by post-growth annealing without any structural change, except for the 200°C-capped sample.
reflection high-energy electron diffraction
Self-assembled semiconductor nanostructures have attracted tremendous interest due to their excellent electronic and optical properties. Since the properties of nanostructures strongly depend on their size, shape, and composition, it is important to reduce the morphological change of nanostructures during the capping process. In this context, much research has recently focused on low-temperature capping with less atomic intermixing, although it is commonly believed that the crystalline quality of the capping layer deteriorates quickly with decreasing temperature.
Droplet epitaxy is a self-assembled growth technique based on the formation of metallic droplets followed by crystallization into semiconductor quantum dots (QDs) [1–13]. Droplet epitaxy allows the self-assembly of QDs in lattice-matched systems such as GaAs/AlGaAs, which is unattainable in a conventional Stranski-Krastanow growth mode. In the growth of GaAs/AlGaAs QDs, various quantum structures such as monomodal dots , single/multiple rings [4, 8, 9], and nanoholes [10–13] have been derived by controlling the As pressure and temperature during the crystallization of Ga droplets.
However, in droplet epitaxy, low-temperature processes at around 200°C are required for the formation of droplets and their crystallization, which often causes degradation of the crystalline and optical qualities of the QDs and subsequent AlGaAs capping layer. Uncapped annealing of QDs is, therefore, used as an effective way to improve the quality of the QDs . This annealing step, however, can also cause significant morphological changes in the QD. For example, GaAs QDs grown on GaAs(001) substrates elongate in the [-110] direction when annealed at temperatures higher than 400°C , and so a capping temperature below 400°C is necessary for embedding QDs with their original morphology maintained. However, such a low temperature is challenging for the growth of high-grade AlGaAs, and indeed, the effects of a low-temperature AlGaAs capping layer on the optical properties of adjacent GaAs quantum structures have not yet been clarified.
We studied the optical qualities of GaAs nanostructures capped with a low-temperature AlGaAs layer. To clarify the effects of the capping layer, we used high-quality GaAs/AlGaAs single quantum wells (QWs) capped at various temperatures. Luminescence study showed a clear difference between the sample capped at 200°C and the samples capped above 350°C, which is explained by the incorporation of excess arsenic in the AlGaAs grown at low temperatures (< 300°C).
Results and discussion
In contrast, the sample capped at 200°C exhibits faint emission around 718 nm, which is blue shifted by 60 meV compared to the QW emission from the sample capped above 350°C. The emission linewidth also increases to 30 meV. We attribute this change to the incorporation of excess As atoms into the AlGaAs capping layer during the low-temperature growth. It is well known that GaAs grown at temperatures below 300°C becomes nonstoichiometric with an excess of arsenic incorporated as a point defect in the GaAs matrix [17, 18]. The excess arsenic forms precipitates when annealed at temperatures above 500°C, but the epilayer is still highly nonradiative due to the presence of residual point defects  or resultant metallic As clusters . In our case, the AlGaAs capping layer containing As clusters was developed during the subsequent growth of the second capping layer at 580°C. Not only does the annealed low-temperature AlGaAs layer act as a nonradiative pathway, but the As clusters may modulate the QW potential, resulting in the imperceptible emission with a peak shift.
Here we would like to compare our results with previous reports on the properties of GaAs grown at low temperatures. Since the first report by Stall et al.  that the electrical properties of GaAs were degraded when grown below 480°C, many efforts have been made to obtain good quality of GaAs at low temperatures. Metze et al.  were able to grow good-quality GaAs at 450°C by reducing the growth rate to 0.2 μm h-1. Missous and Singer  pointed out the superiority of As2 in reducing the concentration of deep levels compared to As4. By contrast, our growth condition was "normal", i.e., the growth rate was 1 μm h-1 and an As4 source was used. The difference is that the epilayer was very thin and undoped in our case. In fact, our purpose is to embed nanostructures with little atomic diffusion, and the thickness (volume) of the capping layer is very small compared to that of the whole structure. Our results show that a thin capping layer does not significantly lower the quantum efficiency of the embedded nanostructure, even though the capping layer was grown at a low temperature with a normal condition. Of course the quality of the capping layer would be improved by optimizing the growth conditions such as growth rate, V/III ratio, and As species.
We have studied the effects of a low-temperature AlGaAs capping layer on the optical properties of a GaAs QW, using different capping temperatures of 200, 350, 450, and 580°C. Although a good morphology was obtained for all samples, there was a clear difference in the optical qualities between the 200°C-capped sample and the others. In the sample capped at 200°C, incorporation of excess arsenic followed by the formation of As clusters introduces many nonradiative recombination centers in the AlGaAs capping layer, which greatly reduces the PL from the QW. By contrast, the sample capped above 350°C showed clear emission from the QW, though a slight degradation in intensity was observed with decreasing capping temperature. Except for the 200°C-capped sample, the quality could be restored to that of the 580°C-capped sample without any structural change caused by post-growth annealing at 800°C. These results clearly demonstrate that the capping temperature of 350°C is high enough to obtain a quantum structure with high quantum efficiency, thus paving the way for low-temperature capping of QDs to suppress morphological changes and interdiffusion.
This study was supported in part by a Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science.
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.