Suppression of dislocations by Sb spray in the vicinity of InAs/GaAs quantum dots
© Dai et al.; licensee Springer. 2014
Received: 3 April 2014
Accepted: 21 May 2014
Published: 30 May 2014
The effect of Sb spray prior to the capping of a GaAs layer on the structure and properties of InAs/GaAs quantum dots (QDs) grown by molecular beam epitaxy (MBE) is studied by cross-sectional high-resolution transmission electron microscopy (HRTEM). Compared to the typical GaAs-capped InAs/GaAs QDs, Sb-sprayed QDs display a more uniform lens shape with a thickness of about 3 ~ 4 nm rather than the pyramidal shape of the non-Sb-sprayed QDs. Particularly, the dislocations were observed to be passivated in the InAs/GaAs interface region and even be suppressed to a large extent. There are almost no extended dislocations in the immediate vicinity of the QDs. This result is most likely related to the formation of graded GaAsSb immediately adjacent to the InAs QDs that provides strain relief for the dot/capping layer lattice mismatch.
81.05.Ea; 81.07.-b; 81.07.Ta
Semiconductor quantum dots (QDs) have a great potential for applications in a wide variety of novel devices [1–4]. Their optoelectronic properties can be turned by careful design through the control of their size, shape, composition, and strain [5, 6]. In recent years, the III-V QDs, especially InAs/GaAs(Sb), have been drawing great interest due to their promise in wide applications beyond photovoltaics , such as quantum dot lasers [8, 9] and photodetectors [10–12]. In particular, much effort has been dedicated to develop QD laser diodes emitting at the telecommunication bands of 1.3 and 1.55 μm. A recent promising approach is to extend the emission wavelength of self-assembled InAs/GaAs to these two regions by using a GaAs capping layer by Sb incorporation [13–16], and even a longer wavelength has already been obtained [15, 16]. The strong redshift has been attributed to a type II band alignment for high Sb contents . A few studies aiming to analyze the emission evolution with the amount of Sb [18, 19], as well as the microstructures of these QDs, have been carried out recently by means of scanning transmission electron microscopy (STEM), atomic force microscopy (AFM), and conventional transmission electron microscopy (CTEM). The results demonstrate that they have the significant difference from those of GaAs-capped QDs [17, 19–21].
However, there is almost no report about the effect of Sb sprayed on the surface of InAs immediately prior to depositing the GaAs capping layer, from the perspective of crystal structure. Since Sb incorporation will result in the formation of GaSb with a larger lattice constant, this should help provide a strain relief layer effectively bridging the lattice mismatch between InAs QDs and GaAs matrix. Then, the strain induced in the QDs during capping should be reduced, which will influence the QD size, shape, composition, defect, and dislocations. It is known that the properties of promising devices relying on quantum dot properties are compromised due to the presence of defects generated when the quantum dots are capped [22–25]. Therefore, a fundamental understanding about the defects of the QDs with and without Sb incorporation before GaAs capping is very important for device applications and will lead to better methods for minimizing the impact of these defects and dislocations. High-resolution transmission electronic microscope (HRTEM) structural imaging enables us to see atoms at their real locations and thus gives us detailed information about lattice misfit, defects, and dislocations. In this work, we used cross-sectional HRTEM to see how defects and dislocations are generated during the growth of InAs/GaAs QDs and the impact of the addition of Sb atoms.
The two samples studied were grown by molecular beam epitaxy in an AppliedEpi GenIII system (Veeco, Plainview, NY, USA) on (100) GaAs substrates with a 200-nm-thick GaAs buffer layer. One sample with InAs/GaAs QDs capped by GaAs was named sample 1, and the other sample with InAs/GaAs QDs spayed by Sb flux for 30 s before the GaAs capping layer was named sample 2. Gallium and indium fluxes were supplied by conventional thermal sources, while As and Sb fluxes were provided by valved cracker sources. The growth rates determined by monitoring the RHEED oscillations were 0.4 and 0.035 monolayers/s for GaAs and InAs, respectively, and the measured beam equivalent pressure for Sb was 9.7 × 10-8 Torr. The As overpressure for all the GaAs and InAs growth steps was 2 × 10-6 Torr. The GaAs buffer layers of the two samples were grown at 580°C, followed by a 10-s rest, and the temperature was reduced to 500°C, and then approximately 2.0 monolayers of InAs were deposited. Different growth processes were then employed for the two samples. Sample 1 had a 30-s rest under As flow, while sample 2 was exposed to the Sb flow for 30 s. At the end of each group's spray regime, a 70-nm GaAs cap layer was grown immediately.
The structural characteristics of InAs/GaAs QDs with Sb and without Sb spray were investigated by cross-sectional HRTEM using a JEOL-JEM-3000 F microscope (Akishima-shi, Japan) operated at 300 kV. Cross-sectional TEM specimens were prepared using the standard procedures (mechanical thinning and ion milling). Fast Fourier transformation (FFT) was carried out using a DigitalMicrograph software package.
Results and discussion
There have been reports of InAs and GaSb intermixing with the formation of an In x Ga1 - x As y Sb1 - y alloy in the core of the QDs ; however, it was also demonstrated that the Sb atoms are distributed solely in the As atom matrix of the QDs . While the HRTEM structural imaging can allow us to see atoms at their real locations, and give us detailed information about lattice misfit, defects, and dislocations, we are exploring the feasibility of by atom probe tomography (APT) to identify how the Sb atoms distribute and interact with other atoms in and around the QDs in order to determine the exact mechanism by which the defect passivation observed in our results are realized.
HRTEM has been used to study the structural properties of self-assembled InAs/GaAs QDs with and without an Sb spray immediately prior to GaAs capping. The Sb spray process can reduce the height of the InAs/GaAs QDs and increase the QD density and, more importantly, can passivate the defects and dislocations in the dot/cap interface region and suppress dislocations to a large extent. This result is very useful for fabricating novel QD-based optoelectronic devices, in particular photovoltaic devices where ensuring a high proportion of QDs that are active is a key requirement for novel energy conversion mechanisms and to reduce losses due to recombination via defects.
The authors are grateful for the scientific and technical support from the Australian Microscopy and Microanalysis Research Facility node at the University of Sydney. This research was supported by the Australian Research Council, the financial support from the National Natural Science Foundation of China (61204088), the China Scholarship Council, and the natural science funds of China. ZL acknowledges the Australian Research Council for the funding support (DP130104231).
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