High resolution electron microscopy of GaAs capped GaSb nanostructures

Abstract

We show in this work that GaAs capping of 2 ML of GaSb grown by molecular beam epitaxy results in the formation of very small ͑with heights of about 1 nm͒ GaAs x Sb 1−x nanostructures surrounded by a GaAs rich layer. This conclusion is obtained by analyzing the morphology of the GaAs x Sb 1−x nanostructures by high resolution scanning transmission electron microscopy in Z-contrast mode. This result shows that a significant fraction of the Sb atoms must segregate along the growth direction during the GaAs capping process. © 2009 American Institute of Physics. ͓DOI: 10.1063/1.3077009͔Research on GaSb/GaAs quantum dots is a topic of increasing interest because this material has very attractive properties, such as its type-II band alignment, 1 opening the possibility to fabricate new devices such as quantum information devices and memories based on quantum dots. 2-4Jiang et al.5 studied by atomic force microscopy ͑AFM͒ uncapped GaSb quantum dots grown by molecular beam epitaxy ͑MBE͒ and they found quantum dots sizes of about 10 nm. Nevertheless, during the process of capping of the GaSb with GaAs, an important fraction of the formed quantum dots seems to disappear, probably due to the exchange reaction of Sb adatoms with As atoms on the GaAs surface layer. 6,7 This idea is supported by the fact that very small ͑approximately 1-2 nm in size͒ GaAs capped GaSb/GaAs nanostructures are observed by cross-sectional scanning tunneling microscopy on samples grown by metalorganic chemical vapor deposition at 470°C. In this work, we investigate by high resolution Z-contrast the nanomorphology of GaAs capped GaSb nanostructures grown at 480°C by MBE. The analysis of the photoluminescence spectra of these nanostructures in a previous work shows evidence of a type-II band alignment, with holes strongly confined ͑501 meV͒ and electrons weakly confined ͑46 meV͒ in the electrostatic potential created by the accumulation of multiple holes inside each quantum dot under optical pumping. The quantum dot photoluminescence at 1057 meV blueshifts as the excitation power is increased.8 Here we present a structural characterization study showing that GaAs capping of GaSb quantum dots results in the formation of very small ͑with heights of about 1 nm͒ nanostructures at the GaAs/ GaSb interface.The analyzed nanostructures were grown by solid source MBE at a growth rate of 0.1 ML/s at 480°C on an n-type GaAs͑001͒ substrate after deposition of an n-type GaAs buffer layer ͑Si: 1 ϫ 10 18 cm −2 ͒. The formation of the GaSb quantum dots was detected by the change in the reflection high energy electron diffraction ͑RHEED͒ pattern after deposition of 1.8 ML of GaSb. The GaSb layer, with a nominal thickness of 2 ML, was then exposed to Sb flux for 20 s and then annealed for an additional 20 s. The GaAs capping was done at 0.4 ML/s in two steps. In the first step, a 10 nm thick GaAs layer was grown at the temperature of quantum dot nucleation. In the second one, a 40 nm thick GaAs layer was deposited at 570°C. During the growth, the As and Sb beam equiv...

Role of segregation in InAs/GaAs quantum dot structures capped with a GaAsSb strain-reduction layer

“…A region depleted of both Sb and In is present at the contact point of the island and the wetting layer. Furthermore no Sb was observed inside the island, contrary to the observation of Molina et al 65 In order to gain a better understanding of the nature of the segregation effects in these structures we have analyzed the relative composition of In/Ga and As/Sb in regions away from the QD islands for the three nominal Sb contents in the sample. The experimental data is presented in Fig.…”

“…64 In QD structures, the role of Sb/As intermixing has also received attention. Segregation of group V elements was reported in GaSb/GaAs QDs, 65 with the formation of a floating layer containing Sb observed throughout the GaAs growth. The InSb/In͑As͒Sb QD system has also been investigated.…”

High-Resolution Electron Microscopy of Semiconductor Heterostructures and Nanostructures

“…This type of analysis is very helpful to determine the lateral size and height of a nano-object. In this case, it demonstrates the formation of a discontinuous layer composed of GaAs x Sb 1 x nanostructures with a lateral size in the region of 1-10 nm and heights of around 1 nm [54]. These measured dimensions mean that there has been a volume reduction of the nanostructure by three orders of magnitude during the GaAs capping process.…”

Modified qHAADF method for atomic column-by-column compositional quantification of semiconductor heterostructures

“…In semiconductor materials, quantifying the composition with large spatial resolution is needed to correlate material band structure and epitaxial growth conditions, which eventually assists the extrapolation of optimum device design parameters. Several direct and indirect analyzing techniques have been used in this regard such as-photoluminescence (PL) [18,19], energy dispersive spectroscopy (EDS) [20] or electron energy loss spectroscopy (EELS) [21,22]. HAADF-STEM can also be used with this purpose, although the quantification is not straightforward as the HAADF-STEM signal contains both composition and specimen thickness induced information [23,24].…”