Defects in nanostructures with ripened InAs/GaAs quantum dots (original) (raw)
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Effects of the quantum dot ripening in high-coverage InAs/GaAs nanostructures
Journal of Applied Physics, 2007
We report a detailed study of InAs/ GaAs quantum dot ͑QD͒ structures grown by molecular beam epitaxy with InAs coverages continuously graded from 1.5 to 2.9 ML. The effect of coverage on the properties of QD structures was investigated by combining atomic force microscopy, transmission electron microscopy, x-ray diffraction, photoluminescence, capacitance-voltage, and deep level transient spectroscopy. In the 1.5-2.9 ML range small-sized coherent QDs are formed with diameters and densities that increase up to 15 nm and 2 ϫ 10 11 cm −2 , respectively. For Ͼ 2.4 ML large-sized QDs with diameters of 25 nm and densities ranging from 2 ϫ 10 8 to 1.5 ϫ 10 9 cm −2 coexist with small-sized QDs. We explain the occurrence of large-sized QDs as the inevitable consequence of ripening, as predicted for highly lattice-mismatched systems under thermodynamic equilibrium conditions, when the coverage of the epitaxial layer exceeds a critical value. The fraction of ripened islands which plastically relax increases with , leading to the formation of V-shaped defects at the interface between QDs and upper confining layers that propagate toward the surface. Island relaxation substantially affects the properties of QD structures: ͑i͒ free carrier concentration is reduced near the QD plane, ͑ii͒ the QD photoluminescence intensity is significantly quenched, and ͑iii͒ deep levels show up with typical features related to extended structural defects.
Can misfit dislocations be located above the interface of InAs/GaAs (001) epitaxial quantum dots?
Nanoscale Research Letters, 2012
InAs/GaAs(001) quantum dots grown by droplet epitaxy were investigated using electron microscopy. Misfit dislocations in relaxed InAs/GaAs(001) islands were found to be located approximately 2 nm above the crystalline sample surface, which provides an impression that the misfit dislocations did not form at the island/substrate interface. However, detailed microscopy data analysis indicates that the observation is in fact an artefact caused by the surface oxidation of the material that resulted in substrate surface moving down about 2 nm. As such, caution is needed in explaining the observed interfacial structure.
Formation of dislocation defects in the process of burying of InAs quantum dots into GaAs
Semiconductors, 2009
Evidence given by electron microscopy of dislocation relaxation of stresses near InAs quantum dots buried into GaAs is presented. It was found that dislocation defects not emerging to the film surface are formed in some buried quantum dots. This suggests that stress relaxation occurs in the buried state of the quantum dot, rather than at the stage of the formation and growth of an InAs island on the GaAs surface. Models of internal dislocation relaxation of buried quantum dots are presented.
MRS Proceedings
ABSTRACTTransmission electron microscopy (TEM), and photoluminescence (PL) have been used to evaluate defects and the efficiency of defect-reduction techniques in structures with InAs quantum dots (QDs) for the 1.55 µm range grown at low substrate temperature (LT) using molecular beam epitaxy (MBE). We show that capping of the QDs with thin GaAs layer accompanied by growth interruption at 600oC (flash) allows to eliminate large islands, containing dislocations, while the smaller islands containing local defects (e.g. dislocation dipoles) still remain. If the flash procedure is accompanied with further depositing of thin AlAs cap layer, and followed by high temperature (~700oC) annealing (HTA), an almost complete elimination of defects is observed. The structures emit in the range of 1.55 µm due to lateral agglomerates of LTQDs. Simultaneously bright luminescence due to isolated QDs and GaAs matrix are detected at high excitation densities.
2001
Transmission electron microscopy (TEM), and photoluminescence (PL) have been used to evaluate defects and the efficiency of defect-reduction techniques in structures with InAs quantum dots (QDs) for the 1.55 µm range grown at low substrate temperature (LT) using molecular beam epitaxy (MBE). We show that capping of the QDs with thin GaAs layer accompanied by growth interruption at 600 o C (flash) allows to eliminate large islands, containing dislocations, while the smaller islands containing local defects (e.g. dislocation dipoles) still remain. If the flash procedure is accompanied with further depositing of thin AlAs cap layer, and followed by high temperature (~700 o C) annealing (HTA), an almost complete elimination of defects is observed. The structures emit in the range of 1.55 µm due to lateral agglomerates of LTQDs. Simultaneously bright luminescence due to isolated QDs and GaAs matrix are detected at high excitation densities.
Suppression of dislocations by Sb spray in the vicinity of InAs/GaAs quantum dots
Nanoscale Research Letters, 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.
Semiconductors, 2014
Electron microscopy studies of GaAs structures grown by the method of molecular beam epitaxy and containing arrays of semiconductor InAs quantum dots and metallic As quantum dots are performed. An array of InAs quantum dots is formed using the Stranski-Krastanow mechanism and consists of five layers of vertically conjugated quantum dots divided by a 5 nm thick GaAs spacer layer. The array of As quantum dots is formed in an As enriched GaAs layer grown at a low temperature above an array of InAs quantum dots using postgrowth annealing at temperatures of 400-600°C for 15 min. It is found that, during the course of structure growth near the InAs quantum dots, misfit defects are formed; these defects are represented by 60°o r edge dislocations located in the heterointerface plane of the semiconductor quantum dots and penetrating to the surface through a layer of "low temperature" GaAs. The presence of such structural defects leads to the formation of As quantum dots in the vicinity of the middle of the InAs conjugated quantum dots beyond the layer of "low temperature" GaAs.
Scripta Materialia, 2013
A double-layer InAs/GaAs(0 0 1) quantum dot structure grown by droplet epitaxy was found to have V-shaped defects, with the two arms of each defect originating from a buried quantum dot and extended to the top surface. Quantum dots on the sample surface nucleated and grew preferentially on top of the arms of the V-shaped defects. The mechanism behind the observed phenomenon was discussed.
Instability of electrical characteristics of GaAs/InAs quantum dot structures
physica status solidi (c), 2005
We report on GaAs samples containing an InAs quantum dot matrix grown from 3 monolayers of InAs. In this case, most of the mechanical stress relaxes by misfit dislocations. Capacitance measurements have been performed on macroscopic devices and, also, from a scanning microscope in which the capacitance is measured between the probe and sample surface. It was found that the electrical characteristics dramatically change during the capacitance measurements. This is explained by a degradation of the quantum dot layer which is attributed to the generation of point defects and/or dislocations. These results draw attention to the fact that, at the microscopic scale measurement, a small current may result in a large local current density which, in turn, degrades the device.