Formation of Submicron Single Crystal Particles and Dots by Laser Ablation
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HONG WU, R. D. VISPUTE AND J. NARAYAN Department of Materials Science and Engineering, North Carolina State University, Raleigh, NC 27695-7916
ABSTRACT We have investigated the formation of micron-sized single crystal dots of germanium by pulsed laser ablation. The laser ablation of a Ge target (KrF excimer laser X=248 nm, pulse rate 10Hz, pulse duration 25x10- 9 seconds, and energy 10J /cm 2 ) results in the formation of micron and submicron liquid droplets which are ejected from the target. These droplets can be crystallized into single crystal dots on lattice-matched substrates by rapid liquid-phase recrystallization. We report the details of microstructure as a function of dot size. It is found that under these conditions, below a critical size (about 2prm), the dots are crystalline;above which dots become polycrystalline. We discuss the implications of the results in producing doped and undoped single-crystal quantum dots for device applications.
1.Introduction Pulsed Laser Ablation (PLA) has been successfully used to deposit a wide range of materials including dielectrics, semiconductors, metals, and superconductors on various substrates. This technique offers several unique advantages, such as film stoichiometry close to that of the target, low temperature growth, low contamination levels, and nonequilibrium processing, etc. 1 . One of the unique features observed in laser deposited films is the presence of droplets. These droplets are submicron or micron sized spherical particles which are believed to originate from the characteristics of the laser-target interactions. The dimension and number density of these particles are found to vary with different deposition parameters including laser pulse energy density, laser spot size, and physical properties of the target 2 . In this paper, we report the investigation of the microstructure of laser produced droplets by Transmission Electron Microscopy (TEM). 3 The TEM is particularly suitable for the investigation of crystal structures. It is found that the particles are crystalline below a critical size of 21gm, and become polycrystalline above 2pm. This phenomenon involves supercooling of the liquid and nucleation and growth of the crystalline phase out of the supercooled liquid. The understanding of the mechanism of the growth of droplets may eventually lead to applications in producing doped and undoped single-crystal quantum dots4 for device purposes.
145 Mat. Res. Soc. Symp. Proc. Vol. 358 e1995 Materials Research Society
2.EMxodrnntal details The pulsed laser ablation chamber is shown schematically in Fig. 1. During the deposition, a KrF excimer laser (X=248nm) was used with a pulse repetition frequency of 10 Hz and a pulse duration of 25 ns. The energy density of the laser beam was 10 J/cm 2 . The Ge single crystal target was placed 5cm away from the (001) Si substrate, and the substrate temperature was kept at 300°C. These parameters resulted in the growth of an approximately 100A thick Ge epilayer on the Si substrate and the formation of submicron a
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