Self Organized InAs Quantum Dots on Patterned GaAs Substrates
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J19.4.1
Self organized InAs quantum dots on patterned GaAs substrates M. Schramboeck, W. Schrenk, A. M. Andrews, T. Roch, G. Fasching, W. Brezna, A. Lugstein, G. Strasser Institut fuer Festkoerperelektronik Technische Universität Wien Floragasse 7 1040 Wien, Austria
ABSTRACT We investigate the growth of InAs quantum dots on patterned GaAs substrates. The GaAs substrate has been structured using holographic lithography. Quantum dot formation along the patterns has been observed as well as an increase in homogeneity of the quantum dots. Furthermore, the use of ion beams focused to nanometer diameters for substrate patterning has been studied and showed promising results. For the investigation of vertically aligned InAs quantum dots, cross-sectional atomic force microscopy has been successfully employed.
INTRODUCTION Self-assembled semiconductor nanostructures such as quantum dots (QDs) have been under intense investigation during the last years due to their appealing electronic and optical properties. Unique physical properties have been observed in QD structures and device applications using QDs, such as lasers, detectors and memories have been explored. Using lattice mismatched material systems such as Si/Ge or In(Ga)As/GaAs, structures in the quantum range can be grown. However, QDs grown using this Stranski-Krastanow growth mode usually show an undesired fluctuation in size and density. Especially for device applications it would be desirable to have more control over the size and the density of the QDs and to have control over the lateral position of the QDs on the substrate. To achieve this, a variety of methods have already been studied, ranging from atomic force microscopy based direct positioning of QDs[1] to overgrowth on pre-patterned substrates. Especially the growth of QDs on patterned substrates has shown promising results [2, 3]. Among the lithographic techniques for pattern preparation, the most common is electron beam lithography followed by either plasma enhanced or wet chemical etching [4, 5]. In the following, we investigate two methods for pattern preparation. Holographic lithography and focused ion beam (FIB) assisted substrate preparation. Holographic lithography was studied because it provides an easy and efficient way to create nanosized patterns over large areas. Also the setup is very flexible and the pattern spacing can be varied over a large range. FIB pattern preparation has the advantage that no lithography is needed and thus contamination of the substrate coming from the resist and other chemicals used in processing can be avoided.
J19.4.2
Figure 1: AFM image and the corresponding cross-section of a nanoscale pattern with a period of 230nm and an etch depth of 24nm.
LASER HOLOGRAPHY - EXPERIMENTAL DETAILS For the holographic lithography, an experimental setup, including a HeCd Laser operating at a wavelength of 325nm was used as an ultraviolet (UV) light source. The period of the pattern is determined by the incident angle between the laser optics and the substrate. To get a peri
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