STM Study of Self-assembly of Quantum Dot Formed by Selective Laser Heating
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STM Study of Self-assembly of Quantum Dot Formed by Selective Laser Heating Haeyeon Yang and Casey M. Clegg Nanoscience and Nanoengineering Department, South Dakota School of Mines and Technology, Rapid City, SD 57701-3995, USA
ABSTRACT Scanning Tunneling Microscope (STM) was used to examine the morphologies of selfassembled InGaAs quantum dots (QDs). In order to induce the self-assembly, unlike the conventional Stranski-Krastanov (S-K) growth method, spatial thermal modulations in nanoscale were created in-situ on strained-but-flat InGaAs surfaces in a Molecular Beam Epitaxy (MBE) growth reactor by applying interferential irradiations of laser pulses (IILP). As-irradiated surfaces were examined using an attached ultra-high vacuum (UHV) STM. STM images indicate that the irradiation of 7 nano second laser pulse induces self-assembly of QDs. The average size of laser-induced QDs is smaller while their density is larger than that of QDs formed by annealing strained but flat epilayers conventionally. Furthermore, the dot density is modulated sinusoidally with a periodicity commensurate with that of the interference, which suggests that the placement of QDs can be controlled on the scale of the optical wavelength used. QD volume analysis suggests that dots grow faster laterally than vertically so that dots become flattened as they get larger. INTRODUCTION STM has been useful to provide us the necessary insights on the epitaxial growth of strained films[1] and evolution of wetting layers[2] for novel nanostructures as well as to visualize the critical correlations at the metal-insulator transition of GaMnAs.[3] Also STM has been helpful to identify and optimize the growth processes for novel nanostructures such as quantum dashes[4] and dots via annealing strained-but-flat epilayers.[5] We report InGaAs QDs[6] that are smaller than those produced by the typical S-K growth mode, with high lateral density. A high “lateral” density is desirable for SAQDs in order to increase the optical absorption, in addition, low lateral density in a single layer is compounded by vertical organization.[7] Selective nanoscale heating of InGaAs surfaces in-situ has resulted in the small QDs with high lateral density. Interferential irradiation of laser pulses (IILP) is employed to produce the nanoscale thermal modulation on the epitaxial growth front. IILP is a simple process to control the number of “heated” atoms at the interference maxima while the control of QD position is also possible via the control of the interferential parameters. It can be applied to any substrate and coupled with various growth techniques. For example, IILP has been used to create and align self-assembled metallic nano dots on glass substrates.[8, 9] The dots are aligned along interference lines due to mass transport arising from selective heating on the nanoscale. IILP has also been used to selectively ablate surfaces. GaN nano lines[10] has been reported due to ablation at interferential maxima when two beam interference is applied to the surface. SAQDs create
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