Capping process of InAs∕GaAs quantum dots studied by cross-sectional scanning tunneling microscopy (original) (raw)

In situ scanning tunneling microscopy of InAs quantum dots on GaAs() during molecular beam epitaxial growth

Surface Science, 2003

Arrays of InAs quantum dots (QDs) have been studied using in situ scanning tunneling microscopy (STM) during their growth by molecular beam epitaxy on GaAs(0 0 1). At a substrate temperature of 400°C under As 4 flux, both the QDs and the underlying step-terrace structure of the wetting layer (WL) are found to be static, with neither step-flow nor QD ripening observed. Higher resolution images of the mature WL show slightly different [1 1 0] periodicities to those observed in quenched STM studies.

Formation of InAs quantum dots and wetting layers in GaAs and AlAs analyzed by cross-sectional scanning tunneling microscopy

2005

We have used cross-sectional scanning-tunneling microscopy (X-STM) to compare the formation of self-assembled InAs quantum dots (QDs) and wetting layers on AlAs (1 0 0) and GaAs (1 0 0) surfaces. On AlAs we find a larger QD density and smaller QD size than for QDs grown on GaAs under the same growth conditions (500 1C substrate temperature and 1.9 ML indium deposition). The QDs grown on GaAs show both a normal and a lateral gradient in the indium distribution whereas the QDs grown on AlAs show only a normal gradient. The wetting layers on GaAs and AlAs do not show significant differences in their composition profiles. We suggest that the segregation of the wetting layer is mainly strain-driven, whereas the formation of the QDs is also determined by growth kinetics. We have determined the indium composition of the QDs by fitting it to the measured outward relaxation and lattice constant profile of the cleaved surface using a three-dimensional finite element calculation based on elasticity theory.

Modification of InAs quantum dot structure by the growth of the capping layer

Applied Physics Letters, 1998

InAs quantum dots inserted at the middle of a GaAs quantum well structure have been investigated by transmission electron microscopy and scanning transmission electron microscopy. We find that the growth condition of the overlayer on the InAs dots can lead to drastic changes in the structure of the dots. We attribute the changes to a combination of factors such as preferential growth of the overlayer above the wetting layers because of the strained surfaces and to the thermal instability of the InAs dots at elevated temperature. The result suggests that controlled sublimation, through suitable manipulation of the overlayer growth conditions, can be an effective tool to improve the structure of the self-organized quantum dots and can help tailor their physical properties to any specific requirements of the device applications.

Influence of an ultrathin GaAs interlayer on the structural properties of InAs∕InGaAsP∕InP (001) quantum dots investigated by cross-sectional scanning tunneling microscopy

Applied Physics Letters, 2008

Cross-sectional scanning tunneling microscopy is used to study at the atomic scale how the structural properties of InAs/ InGaAsP / InP quantum dots ͑QDs͒ are modified when an ultrathin ͑0-1.5 ML͒ GaAs interlayer is inserted underneath the QDs. Deposition of the GaAs interlayer suppresses the influence of the As/ P exchange reaction on QD formation and leads to a planarized QD growth surface. A shape transition from quantum dashes, which are strongly dissolved during capping, to well defined QDs takes place when increasing the GaAs interlayer thickness between 0 and 1.0 ML. Moreover, the GaAs interlayer allows the control of the As/ P exchange reaction, reducing the QD height for increased GaAs thicknesses above 1.0 ML, and decreases the QD composition intermixing, producing almost pure InAs QDs.

GaAs cap layer growth and In-segregation effects on self-assembled InAs-quantum dots monitored by optical techniques

Journal of Crystal Growth, 1998

The overgrowth of InAs islands by a GaAs cap layer has been investigated by optical in situ measurements at various growth conditions. A better smoothing of the surface has been found for low growth rates. This effect is attributed to a larger diffusion length of the gallium during growth leading to a smoother final surface. The status of the surface during and after cap layer growth can be determined from RAS and SE measurements during growth. This correlation between the optical data and the surface morphology was confirmed by ex situ AFM measurements. Post growth annealing of samples with different cap layer thickness showed indium interdiffusion from the islands to the cap layer over several nanometers. This effect is also observed during cap layer growth at higher temperatures and can be attributed to indium segregation during growth.

Interplay between Thermodynamics and Kinetics in the Capping of InAs/GaAs(001) Quantum Dots

Physical Review Letters, 2006

A microscopic picture for the GaAs overgrowth of self-organized InAs=GaAs001 quantum dots is developed. Scanning tunneling microscopy measurements reveal two capping regimes: the first being characterized by a dot shrinking and a backward pyramid-to-dome shape transition. This regime is governed by fast dynamics resulting in island morphologies close to thermodynamic equilibrium. The second regime is marked by a true overgrowth and is controlled by kinetically limited surface diffusion processes. A simple model is developed to describe the observed structural changes which are rationalized in terms of energetic minimization driven by lattice mismatch and alloying.

Size, shape, composition, and electronic properties of InAs/GaAs quantum dots by scanning tunneling microscopy and spectroscopy

Journal of Applied Physics, 2010

InAs/GaAs quantum dot heterostructures grown by molecular-beam epitaxy are studied using cross-sectional scanning tunneling microscopy and spectroscopy. The images reveal individual InAs quantum dots (QDs) having a lens shape with maximum base diameter of 10.5 nm and height of 2.9 nm. Analysis of strain relaxation of the QDs reveals an indium composition varying from 65% at the base of the QD, to 95% at its center, and back to 65% at its apex. Room-temperature tunneling spectra acquired 3-4 nm from the center of a dot show a peak located in the upper part of the GaAs bandgap originating from the lowest electron confined state of the QD, along with a tail in the conductance extending out from the valence band and originating from QD hole states. A computational method is developed for simulating the tunneling spectra using effectivemass bands treated in an envelope-function approximation. By comparison of the computations to low-current spectra, the energy of the lowest electron and highest hole QD states are determined. These energies are found to be in reasonably good agreement both with optical measurements and prior theoretical predictions of Wang et al. [Phys. Rev. B 59, 5678 (1999)].

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.

Deep levels in GaAs(001)/InAs/InGaAs/GaAs self-assembled quantum dot structures and their effect on quantum dot devices

Journal of Applied Physics, 2010

Currently lattice mismatch strain-driven three-dimensional coherent island based quantum dots, dubbed self-assembled quantum dots ͑SAQDs͒, constitute the most developed class of quantum dots with successful applications to lasers and considerable potential for infrared detectors in the 1-12 m regime. This is in no small part a consequence of the extensive studies on the formation and control of the islands and on their capping by appropriate overlayer materials under optimal growth conditions. By contrast, surprisingly few studies have been reported on the presence and nature of the deep levels in SAQD structures, much less direct studies of the impact of deep levels on SAQD based device characteristics. The latter is of particular significance to devices such as detectors that require large numbers of SAQD layers ͓i.e., multiple quantum dot ͑MQD͒ structures͔ and are thus increasingly prone to accumulating strain-induced defect formation with increasing numbers of quantum dot layers. In this paper, we report the results of a study of the density, energy profile, and spatial profile of deep levels in different regions of GaAs͑001͒/InAs/InGaAs/GaAs SAQD structures in which the InGaAs/GaAs capping layers have been grown at different growth conditions. Different types of deep levels are found in different regions and, as expected, their densities are found to increase in the presence of the SAQDs. The study shows that it is the density of deep levels in the GaAs capping layer, forced to be grown at the low temperature of ϳ500°C to suppress In outdiffusion, which has a significant adverse impact on quantum dot device characteristics. Their density can be reduced by growth conditions such as migration enhanced epitaxy that permit high quality overgrowths at temperatures as low as ϳ350°C. Nevertheless, the ultimate performance limitation of thick MQD based devices resides in the ability to realize low density of the deep levels relative to the density of SAQDs.