The morphology and transition thickness (tc) for InAs quantum dots (QDs) grown on GaAs (311) B and (100) substrates were investigated. The morphology varies with the composition of buffer layer and substrate orientation. Andtcdecreased when the thin InGaAs was used as a buffer layer instead of the GaAs layer on (311) B substrates. For InAs/(In)GaAs QDs grown on high miller index surfaces, both the morphology andtccan be influenced by the interfacial bonds configuration. This indicates that buffer layer design with appropriate interfacial bonds provides an approach to adjust the morphologies of QDs grown on high miller surfaces.
KeywordsInAs Transition thickness High miller index Strain Interfacial bonds
Self-assembled quantum dots (QDs) have been intensively studied over the past decades in both fundamental and application fields. To date, several systems have exhibited great optical properties and find their applications, such as laser diodes  and optical detectors . The InAs/GaAs should undoubtedly be the most widely studied one among these systems. In recent years, room temperature emission of InAs QD laser around 1.3 μm for the fiber optical communication waveband  and optical absorption at 8–12 μm for the long-wavelength infrared detecting  had been achieved by means of employing a so-called dots-in-a-well (DWELL) structure. In this structure, the QDs are first grown on a thin InGaAs buffer layer and then finally with an InGaAs capping layer. So, the nucleation and growth dynamic of InAs QDs grown on the alloy layer are of central importance. And much attention has been paid to these important research fields [5–7].
However, most of the studies focused on the structures grown on GaAs (100) substrates. Recently, many high index polarized surfaces, such as GaAs (311) A  and (311) B [9–13], GaAs (411) A , and (411) B , have drawn greater attention because QDs grown on theses surfaces have some unique properties, such as the narrow size distribution, high QDs density, and so on. These structure properties can further show their efforts in improving the device performances. However, the growth mechanism of QDs is still a controversial subject, especially with regard to the high index surfaces. Apparently, for the superiority of QDs grown on these high index surfaces, a deeper research into these high index surfaces grown QDs is clearly needed.
In this research, we have conducted a comparative study on the effect of buffer layer and the substrates' orientation on the equilibrium structure and the critical transition thickness (tc) of InAs QDs grown on both GaAs (311) B and (100) substrates by molecular beam epitaxy (MBE).
The samples were grown in a conventional MBE system equipped with 12-keV Reflection High Energy Electron Diffraction (RHEED). GaAs (311) B and (100) substrates were held side by side with indium on same molybdenum holder. For the InAs/InGaAs samples, after deoxidizing the surface oxide at 630 °C, a 500-nm GaAs buffer layer was grown, then 2.3-ML InAs QDs layer was grown on top of a 2-nm In0.15Ga0.85As layer, at the rate of 0.022 ML/s. Both the QDs layer and the buffer layer were grown at 530 °C. For the InAs/GaAs samples, only the 2-nm In0.15Ga0.85As layers were changed to a GaAs buffer layer, and the coverage of InAs was 2.1 ML. As2was used during the whole growth process, and the As2/In beam effective pressure–flux ratio was fixed at 40; the growth rates were determined by the RHEED oscillation technique on the (100) plane. The RHEED pattern has been imaged by a charge-coupled device camera, then digitized, and analyzed by software. When the streak pattern turned into the spots of the three-dimensional (3D) QDs which demonstrated the transition of 2D–3D growth mode, the intensity of one spotty pattern was recorded. The atomic force microscopy (AFM) test was conducted in a contact mode in air.
Results and Discussion
For the self-assembled QDs, tc is an important parameter. For it determines when the islands were formed during the growth, which therefore has a great impact on the morphology of QDs at a given coverage. It had been confirmed that the growth parameters have very little influence on tc. But tc is rather sensitive to the substrate orientation, as shown by many studies that have been conducted to check the effect of substrate orientation on tc [18, 19]. Besides, it had been found that the effect of interfacial (IF) bonds can influence tc of the noncommon anion heteroepitaxy system (III1V1/III2V2, such as InAs/GaSb and InP/GaAs) greatly. Take the InAs/GaSb superlattice for example: tc of this system was much thinner when the IF bonds consisted of In–Sb bonds rather than the Ga–As bonds [20, 21]. This is due to additional IF strain offered by the higher atom sizeof In and Sb than that of Ga and As. However, one cannot observe this effect for the common anion system (III1V/III2V, such as InAs/GaAs and InAs/InGaAs) because the GaAs (100) surfaces are As terminated under common growth, and the IF bond configurations are no different from those of the film . So one cannot find the effect of IF bonds in the InAs/GaAs or InAs/InGaAs system grown on (100) surfaces, which is the case of our InAs/GaAs QDs grown on GaAs (100). Since the In0.15Ga0.85As layers we had grown were so thin (2 nm) that they should be fully strained, tc should have no difference between the InAs/GaAs and InAs/In0.15Ga0.85As samples grown on the GaAs (100) substrates [20, 21].
Thus, when the InGaAs buffer layer was used instead of the GaAs buffer layer, tc decreased on the GaAs (311) B substrates but remained constant on the GaAs (100) substrates. One thing that should be noted in conclusion is that the morphologies of InAs/GaAs and InAs/InGaAs QDs grown on GaAs (100) substrates are clearly very different despite the difference in InAs coverage being negligible (2.1 ML–2.3 ML). This may partly be due to the change of growth environment. After all, these two samples were not grown at the same time. Besides, this difference suggests that there may be other factors that contribute to the equilibrium shape of QDs grown on GaAs and InGaAs buffer layers: for example, the morphology differences in different buffer layers may modify the migrate length of adatoms. However, we argue that the difference in tc still at least partly induced different equilibrium morphologies of QDs as measured by AFM. This result shows that tc of InAs/GaAs QDs grown on high miller surfaces, i.e., GaAs (311) B, can be adjusted through modifying the type and amount of IF bonds and further to modify the equilibrium structures. These structural characteristics would surely induce different properties. So this effect offers one parameter for the design and fabrication of self-assembled QDs, and should be regarded as an advantage for the InAs QDs grown on high miller index surfaces compared to the conventional GaAs (100) surfaces. And also, due to the often-observed morphology instability when the highly mismatched epitaxy was conducted, this study provides the information that the effect of IF bonds should be taken into consideration in this field .
In conclusion, the morphology andtcof the self-assembled InAs QDs grown on GaAs (311) B and GaAs (100) substrates with (In)GaAs buffer layer were investigated. It was found that the configuration of IF bonds plays an important role in the morphology andtcof InAs QDs. For common anion systems, such as InAs/(In)GaAs, this effect can only be observed at high miller index surfaces, which can be used to adjust the morphology in the QDs grown on high miller index surfaces.
The study was financially supported in part by the NSFC (Under Grant Numbers: 50502014), and the program for New Century Excellent Talents in University (NCET).