Study of growth properties of InAs islands on patterned InP substrates defined by focused ion beam
Introduction
Quantum dots (QDs) are atom-like carrier-confining nanostructures that can be used in high efficiency LEDs, lasers and photodetectors, among other applications. The role of QDs in the efficiency gain of such devices comes from the three-dimensional confinement of the charge carriers resulting in a discrete distribution of their energy levels. Tuning shape, size, density, size distribution and composition of the QDs allow the design of devices for very specific applications [1], [2]. However, to achieve this possibility a fine control of deposition/growth variables is mandatory. Despite of the capabilities of the usual growth techniques, which provide a good control of the composition, the adsorption of precursors on multifaceted or rough surfaces may occur as a sum of distinct surface-mediated mechanisms, rendering the process uncontrollable. Growing in such scenario may lead to uncontrolled properties of the QDs. Therefore it is of paramount importance to have a fine control of the nucleation sites and their morphologies, allowing the manipulation of morphological, structural and chemical properties of QDs which have a direct impact on their electronic states [1], [2]. To control growth processes under such conditions mentioned above a combination of techniques to induce QDs localized growth is required. Among the known patterning techniques one can list lithography using ultraviolet light [3], [4], X-rays [4] or electron beam [5], as well as indentation and local chemical changes induced via scanning probe microscopy [6]. Alternatively, the Focused Ion Beam technique (FIB) is a very promising tool to induce site-controlled nucleation of QDs with an accuracy of a few tens of nm [7], [8.], [9], [10], [11], [12], [13], [14]. With FIB it is possible to implant Ga (or other) ions on a variety of substrates in specific locations to create favorable nucleation sites for the QDs growth [2], [7], [8.], [9] or generate distinct surface profiles to grow QDs [7], [8.], [9], [10]. Since the feasibility of using FIB to produce site-controlled InAs QDs on GaAs substrates was demonstrated, efforts have been driven towards the production of devices based on this technology [2], [7], [8.], [9], [10], [13]. Despite the advances on this method, the dependencies of Ga+ dose on shape, size distribution, defects, chemical composition and interdiffusion of the atomic speciments in nanostructures remains poorly understood. In the present investigation InAs islands were grown by Metal-Organic Vapor Phase Epitaxy (MOVPE) on InP (001) surfaces previously patterned by Ga+ focused ion beams, producing a regular arrangement of shallow cavities on the substrate surface. Subsequently InAs islands were found to grow on the cavity sites, and their properties were analyzed using scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), grazing incidence X-ray (GID) and Raman Spectroscopy techniques. A simplified model based on the island density dependence on the Gaussian depth profile of the cavity has been developed, allowing the understanding of the mechanism behind the observed modified island growth.
Section snippets
Experimental methods
A dual-beam Quanta 3D FEI was used to create an array of shallow cavities on the InP surface prior to the deposition of InAs islands by MOVPE. Initially the InP substrates were cleaned in ultrasonic bath with trichlorethylene (TCE) (3 min), acetone (3 min) and isopropyl alcohol (3 min). The substrates were then dried under ultrapure nitrogen flow and introduced into the FIB chamber. In order to generate the templates, the ion beam was held stationary at a single point of the InP surface for a
Results
All samples were initially studied by AFM. Fig. 1(a) illustrates the process of the array of holes fabrication by FIB. Fig. 1(c), (e) and (g) show AFM topographic images of the InP surface containing the patterns manufactured by FIB with the doses 3.7x1015 Ga+/cm2, 5.6x1015 Ga+/cm2, e 1.3x1016 Ga+/cm2, respectively. From an analysis of these images the average roughness of the flat areas was found to be 2.3 nm. Linear profiles passing through the middle of a row with holes are shown in Fig. 1
Discussion
The growth mechanisms of InAs islands on flat surfaces have been extensively studied along the past decades. For topographically modified surfaces or host crystals with defects there is a relative lack of information to build a common scenario. This is the case for our InP patterned surfaces created by ion beam bombardment in this work. Besides the surface characteristics of the substrate, the temperature and the material flow also affect the growth mechanisms. The non-uniform filling of
Conclusion
In this work the growth of InAs islands on InP(100) surfaces patterned by FIB was investigated. Regular arrays of cavities were produced under three different ion doses and InAs islands deposited with two deposition times (3 s and 4 s) of exposure to the In precursor. Scanning electron microscopy, transmission electron microscopy and grazing incidence X-ray diffraction were employed to investigate the structure of the grown islands. The formation of InAs islands and small clusters inside most of
Acknowledgements
This work was supported by the INCT-DISSE, the Brazilian agencies CNPq, CAPES, FAPEMIG and FAPERJ. We would like to thank the Microscopy Center of the Universidade Federal de Minas Gerais as well the Brazilian Synchrotron Light Laboratory (LNLS) under proposal XRD2-11795 for their technical and financial support of this work. We would like to thank also the Nano Spectroscopy Laboratory (http://www.labns.com.br) as well as the Prof. Dr. Luiz Gustavo Cançado for helping with Raman experiments. We
References (48)
- et al.
Prog. Quant. Electron.
(2007) - et al.
J. Cryst. Growth
(2011) - et al.
Physica E
(2008) - et al.
Physica E
(2010) - et al.
Ultramicroscopy
(1998) - et al.
Surf. Sci.
(1999) - et al.
Quantum Dot Heterostructures
(1999) Nanoscience and Nanotechonolgy - Lateral Alignment of Epitaxial Quantum Dots
(2007)- et al.
Quantum dots by ultraviolet and x-ray lithography
Nanotechnology
(2007) - et al.
Appl. Phys. Lett.
(2007)