Excitation-Induced Low Temperature Epitaxial Growth of Ge on Si(100)
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Excitation-Induced Low Temperature Epitaxial Growth of Ge on Si(100) Ali Oguz Er1 and Hani Elsayed-Ali2 1 Department of Physics, Old Dominion University, Norfolk, VA 23529, U.S.A. 2 Applied Research Center and ECE Department, Old Dominion University, Norfolk, VA 23529, U.S.A. ABSTRACT The effect of laser-induced electronic excitations on the self-assembly of Ge quantum dots (QD) on Si(100)-(2x1) grown by pulsed laser deposition is studied. The experiment was conducted in ultrahigh vacuum. A Q-switched Nd:YAG laser (wavelength λ = 1064 nm, 10 Hz repetition rate) was split into two beams; one used to ablate a Ge target while the other to electronically excite the substrate. In situ reflection high-energy electron diffraction (RHEED) and ex situ atomic force microscopy (AFM) were used to study the morphology of the grown QDs. The dependence of the QD morphology on substrate temperature and ablation and excitation laser energy density was studied. Electronic excitation is shown to affect the surface morphology. For Ge coverage of 22 monolayer, it was observed that the excitation laser reduces the epitaxial growth temperature to 250 °C, a temperature at which no epitaxy is possible without excitation. Applying the excitation laser to the substrate during the growth changes the QD morphology and island density and improves the size uniformity of QDs at 390 °C. Surface diffusion measurement calculated from RHEED recovery curves show that the excitation-laser increases the surface diffusion of the Ge atoms. A purely electronic mechanism of enhanced surface diffusion of the Ge adatoms is involved. INTRODUCTION Pulsed laser induced electronic processes leading to surface structural modifications have been shown to occur even when the laser intensity is significantly below the melt threshold [1-2]. Low temperature thin film growth is strongly desirable in microelectronic fabrication. In Si/Ge, it has been long recognized that one way to suppress misfit dislocations is by lowering the growth temperature [3]. To lower the epitaxial growth temperature, extrinsic assistance by energetic particles, such as ions, electrons and photons, could be used to add energy to promote the migration of adsorbed atoms at the surface. We have recently studied the effects of nanosecond pulsed laser-induced electronic excitations on the self-assembly of Ge QD on Si(100)-(2x1) grown by pulsed laser deposition (PLD) [4-5]. Electronic excitation resulted in the formation of an epitaxial wetting layer and crystalline Ge QD at ~250 ºC, a temperature at which no crystalline QD formed without excitation under the same deposition conditions. The results are consistent with an electronically driven mechanism that increases surface diffusion. EXPERIMENT Ge QDs were grown in an ultrahigh vacuum (UHV) chamber (~1x10-9 Torr) by PLD. The Ge target was mounted on a rotation stage in order to expose a fresh surface during ablation which minimizes the particulate formation. The Si(100) substrates were chemically etched by using a modified Shiraki method before bein
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