Manipulating InAs Dots with GaAs Patterns: Effect of GaAs Buffer Layer Growth and Pattern Profiles
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ABSTRACT We report on our efforts to selectively place chemical beam epitaxy grown StranskiKrastanov InAs dots on GaAs patterns. The pattern profiles for placement depends on the growth conditions of a GaAs buffer layer grown on the lithographic patterns. Because we seek to suppress dot formation on (100)-oriented surfaces outside of the features, we investigated the effects buffer layer growth conditions have on dot nucleation using reflected high energy electron diffraction, atomic force microscopy and photo-luminescence. We conclude that buffer conditions favorable for patterns have negligible effect on the dots formed on the (100)- oriented surface, and that selective dot placement can be engineered by As pressure, InAs deposition and buffer growth conditions. INTRODUCTION Strained InAs islands, formed by the Stranski-Krastanov growth mode, on GaAs [1, 2] have a quantum-dot like electronic structure and are interesting as building blocks in nanoscale devices. However, to be of potential use as building blocks in devices requires both selective positioning and control of the electronic structure by manipulating dot size and coupling [3]. Position control has recently been demonstrated by growth on patterned substrates [4-7], and the electronic structure of dots has been shown to depend on pattern profile and orientation [8]. It has been demonstrated that the Stranski-Krastanov growth of InAs dots on (100)-oriented GaAs is affected during deposition by arsenic pressure [9]. In the first part of this paper, we report the effects that GaAs buffer layer growth temperature and arsenic pressure have on the formation of dots on an (100)-oriented GaAs surface, for a given InAs deposition. Low-temperature photo-luminescence, (PL), atomic force microscopy, (AFM) and reflective high electron energy diffraction, (RHEED), were used to evaluate dot sizes and size distribution. In the second part of the paper, we present AFM data from electron beam (EB) patterned samples that show the GaAs buffer growth effects on pattern profile and selective dot formation for different depositions at our standard dot growth conditions. EXPERIMENTAL The samples were grown by chemical beam epitaxy on semi-insulating (100)-oriented GaAs. Trimethyl-indium (TMI), triethyl-gallium (TEG), and tertiary-butylarsine (TBAs), thermally cracked to As 2 were used as sources. A TBAs pressure of 1 mbar corresponds to 0.21 ML/s. A TEG pressure of 0.5 mbar gives a GaAs growth rate of 0.24 MI/s. The TMI growth rate is 0.011 MIJs at 0.03 mbar pressure. The GaAs buffer was 90 nm and consisted of a hot buffer grown for 5 min. at 600' C, followed by a 20 min. buffer grown at 500, 550 or 600 'C respectively. For each temperature three samples were grown with the TBAs pressure 319
Mat. Res. Soc. Symp. Proc. Vol. 571 0 2000 Materials Research Society
at 1.2, 1.6 and 2.0 mbar. The TEG pressure remained constant at 0.5 mbar. The surface reconstruction during growth was monitored with the RHEED beam directed in the direction. After the buffer growth there was a 10 mi
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