Evolution of self-assembled lateral quantum dot molecules

doi:10.1016/j.jcrysgro.2006.11.280

Journal of Crystal Growth

Volumes 301–302, April 2007, Pages 812-816

https://doi.org/10.1016/j.jcrysgro.2006.11.280 Get rights and content

Abstract

Self-assembled lateral InAs quantum dot molecules (QDMs) are grown by solid-source molecular beam epitaxy (MBE) using the thin-capping-and-regrowth MBE process. Thin capping of GaAs on as-grown InAs quantum dots (QDs) at low temperatures leads to nanohole templates. The shape, size and depth of nanoholes are controlled by capping thickness. Subsequent regrowth with different amounts of InAs on the templates result in nano-propeller QDs with different blades’ dimensions. We showed that the length of the propeller blades is controlled by either the capping temperature or the thickness of capping layer. When the regrowth thickness reaches 1.2 ML, different self-assembled lateral QDMs are formed depending on the shape of nanoholes. The dot uniformity and the dot size of all QDM samples are confirmed by photoluminescence (PL) measurements at 77 K. In addition, by ramping the regrowth temperature while the In shutter is open after the deposition of the capping layer, we are able to investigate the formation of nano-propeller QDs at the beginning phases and to propose a model to explain the evolution.

Introduction

Self-assembled quantum dots (QDs) are defect-free nanostructures and are expected to be the new working horse of nanoelectronic devices. Nanoelectronics will be the future technology to extend Moore's law [1], that predicted that the number of transistors per chip will double every 18 months. QD transistors are one of the key devices to overcome the discontinuity of Moore's law that faces the difficulties based on top–down approach. Miniaturization of transistors is facing significant technological limitations, such as the short channel effects, the high power dissipation associated with quantum tunneling through the gate oxide and the depletion regions, and the difficulties in doping shallow junctions and uniform channels [2]. Consequently, a new computing architecture based on quantum dots has emerged [3], [4], [5], [6], [7].

Quantum dot molecules (QDMs) are sets of QDs in which we emphasize on QDMs having lateral close packing. The number of QDs per QDM is controlled by growth process. Specific pattern of ordered QDMs has potential application for quantum computing. One approach for quantum computing is based on Coulomb repulsion of like charges, named “quantum cellular automata (QCA)”. The unit cell of a QCA consists of four QDs positioned at the corners of a rectangle [9], [10], [11]. Another approach is based on a quantum mechanical property, a spin, of electrons in semiconductors [12]. In this paper, we study how to control dot number in QDMs by varying several growth parameters of thin-capping-and-regrowth MBE process [13], [14]. In addition, anisotropic strain during thin capping of initial QDs gives rise to elongated nanostructure along $[1 \bar{1} 0]$ crystallographic direction. Regrowth of QDMs on capped QDs, therefore, leads to self-aligned QDMs. At specific growth condition, QDMs with suitable number of QDs can be obtained and used as a basic building block for quantum computation in accordance with the principle of QCA.

In our modified MBE process, thin capping of GaAs on as-grown InAs QDs at low temperatures leads to nanohole templates. Subsequent regrowths of InAs result in nano-propeller QDs. We observed that the length of the propeller blades is controlled by the capping temperature and the capping thickness. When the regrowth of InAs is carried out at an appropriate thickness, QDMs are formed. QDMs with different dot number per molecule are obtained at different capping temperatures and thicknesses. With precise control of capping temperature at 430 °C and slight increase of regrowth thickness by ramping up the growth temperature to 500 °C while In shutter is opened under As pressure of 8×10⁻⁶ Torr, we observed the evolution of QDMs, starting from regrown QDs at nanoholes followed by the increasing number of satellite dots at the boundary of nano-propeller structures, which we believe are controlling templates of QDMs. The evolution of QDMs can be ex situ displayed by atomic force microscopic (AFM) images taken from the controlled samples, which stopped their growth, by closing the shutters immediately after the QD nucleation is seen in the reflection high energy diffraction (RHEED) pattern. Anisotropic strain at the nano-propeller blades, particularly at the blade's boundary, gives rise to the first nucleation position, and the QDM's rhombus configuration results seen from the asymmetrical strains along the $[1 \bar{1} 0]$ directions.

In this paper, we describe how the shape, size and depth of nanoholes, as well as, the length of nano-propeller blades, are related to the QDMs’ patterns in terms of dot number and dot uniformity. The analytical results are also confirmed by the photoluminescence (PL) spectra of respective samples at 77 K showing their PL peaks and respective PL full-width at half-maxima (FWHM).

Section snippets

Sample preparation

Epi-ready (0 0 1)-GaAs substrates are used as the starting material in all our growths. 1.8 ML of InAs is deposited on 300 nm GaAs buffer layer at 500 °C, resulting in randomly distributed QDs across the substrate surface. The as-grown InAs QDs is then capped by a thin GaAs layer at low temperatures that leads to nanohole templates. Low-temperature capping is used to improve the homogeneity of QDs. Subsequent regrowth of InAs on nanoholes results in nano-propeller QDs with alignment along the $[1 \bar{1} 0]$

Results and discussion

The 7% lattice mismatch between InAs and GaAs causes a large strain during epitaxial growth. The strain is partially relieved by the formation of InAs dots. However, some strains remain and affect the properties of dots and their surroundings. After the deposition of the GaAs capping layer on the as-grown InAs QDs, the GaAs would cover the sides of the InAs QDs, yet some InAs QDs’ remains are left on the surface. During the regrowth interruption, the dot with capping layer collapses because

Conclusions

We show that the length of the propeller blades can be controlled by varying the thickness of the capping layer in addition to the capping temperature in our previous studies. The surface elastic energy of QDs will be modified by increasing the partial capping, which results in different QDMs’ shapes after deposition of amounts of InAs regrowth layer. We found that the capping thicknesses between 15 and 25 ML and the regrowth thicknesses between 1.2 and 1.5 ML at 470 °C are the optimum conditions

Acknowledgments

This research is financially supported by Asian Office of Aerospace Research and Development (AOARD), US Air Force, Thailand Research Fund (TRF) through the Royal Golden Jubilee (Ph.D. scholarship program) and Senior Researcher Award. The authors are also grateful to Center of Excellence in Electrical Power Technology for supporting “Siskonkuti” scholarship.

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Cited by (13)

Bimodal optical characteristics of lateral InGaAs quantum dot molecules
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Citation Excerpt :
The temperature is further reduced to 470 °C and a thin (25-ML) GaAs capping layer deposited, transforming each seed QD into a ∼0.4-nm deep nanohole with mounds elongated along the [1–1 0] direction. Subsequent regrowth of 1.2- (sample A) or 1.5-ML (B) InAs at 470 °C results in the formation of QDMs, which is covered by 100-nm GaAs [11]. The formation of QDs and QDMs is confirmed in situ by real-time observation of streaks and spots in reflection high-energy electron diffraction patterns.
Lateral InGaAs quantum dot molecules (QDMs) formed by thin-cap-and-regrowth molecular beam epitaxial technique comprise large, central quantum dots (cQDs) and small, satellite quantum dots (sQDs) in close proximity. Temperature-dependent photoluminescent (PL) measurements show that the bimodal size distribution gives rise to bimodal optical characteristics: the cQDs ground-state (GS) emissions vary slowly with temperature while the full-width at half maximum (FWHM) remains almost constant; the sQDs GS emissions, on the other hand, exhibit a sigmoidal temperature shift while the FWHM shows an anomalous temperature behaviour. The bimodal optical characteristics are well described in the existing framework of spatially localised excitons in QDs and inter- and intramolecular carrier redistributions in each and among the QDMs via non-resonant multi-phonon assisted mechanisms.
Temperature-dependent photoluminescent characteristics of lateral InGaAs quantum dot molecules
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Lateral InGaAs quantum dot molecules (QDMs) are grown by molecular beam epitaxy. Each QDM comprises a large, central quantum dot (cQD) and several small, satellite quantum dots (sQDs). Temperature-dependent photoluminescent measurements show low-energy (∼1–1.2 eV) emissions resulting from cQDs and mid-energy (∼1.2–1.3 eV) emissions from sQDs. The ground-state and excited-state energy peaks and spacings of the QDMs simply follow bandgap temperature dependencies as shown in effective one-dimensional simulations of coupled Schrödinger–Poisson equations. Transfers of luminescence from sQDs to cQDs observed at low- (20–77 K) or mid- (50–100 K) temperatures for samples with near-merged or well-separated sQD–cQD QDMs are attributed to strong or weak sQD–cQD electronic coupling through the wetting layer, respectively.
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Therefore, it is our objective to create an optimum shape of nanoholes upon which subsequent QDs, both center and satellite dots, can be grown to the same size with good uniformity, which is the ideal case for practical QDMs. After conventional QDMs are formed by the growth of 25 ML GaAs capping thickness and 1.2 ML InAs regrowth thickness as reported earlier [7], we continue to modify QDM by increasing the regrowth temperature while keeping the In shutter open. By doing so, we achieve an optimized annealing procedure which results in the reduction of size differential between satellite dots and the center dot.
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Evolution of self-assembled lateral quantum dot molecules

Abstract

Introduction

Section snippets

Sample preparation

Results and discussion

Conclusions

Acknowledgments

Microelectron. Eng.

Fellow

IEEE, Proc. IEEE

Nature

Phys. Rev. Lett.

J. Appl. Phys.

Phys. Rev. B