Microraman Study of Laser Ablated GaAs
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C.GARCIA, J.JIMINEZ, A.C. PRIETO, J.RAMOS AND L.F. SANZ Ffsica de la Materia Condensada, Cristalograffa y Mineralogfa Facultad de Ciencias and ETSII, 47011 Valladolid, Spain
ABSTRACT Morphologic and structural changes induced by UV pulsed laser beams on GaAs are studied by
means of surface inspection (optical interferometry) and MicroRaman spectroscopy. Crystal order and chemical composition (dopant distribution ) are shown to be changed by the ablation.
INTRODUCTION Laser processes are of high interest for microelectronics technology. Different technological steps involve laser / semiconductor interactions; among other, laser annealing [1], ohmic contact formation
[2], thin film growth [3], doping [4]... ; direct laser writing of micron sized structures is nowadays presenting an increasing interest for local doping or etching [5]. Both the control of the ideal
experimental conditions and the effect that laser processing can have on the properties of the materials are problems that require to be studied since they can play an important role in relation to the degradation of the electric properties of the material. Different phenomena have been reported to occur in GaAs when it is heated under infrared or visible laser beams [6]. The most usual observations are the formation of oxide layers and the out diffusion of As with the subsequent solid arsenic formation [6,7]. Most of these effects are thermally induced. The use of pulsed UV lasers is very promising for many of these purposes, since very short laser pulses and high energy, together with a very short
penetration depth are available. Different structural and morphologic changes are produced in the material by the ablation procedure; a study of these changes should allow to improve laser / semiconductor interaction processes when focused UV laser beams are used. Herein, we present a morphologic and structural study of the effects produced by pulsed UV laser beams in different GaAs bare substrates. The morphology of the ablation is studied using optical interferometry and fringe calculations. Structural changes are assessed at micron size scale using a Raman microprobe, in addition to the structural information the Raman microprobe allows to get data concerning the electric activity of dopants after laser ablation. EXPERIMENTAL AND SAMPLES Laser ablation was performed with a YAG laser with a frequency doubler at 266 nm wavelength for which the penetration depth in GaAs is -7 nm. The pulse duration is 10 ns and the laser beam was focused with a quartz lens to get a spot diameter variable from 5 to 15 pm by adjusting the focus. The
ablations were made with different laser energies and variable number of pulses. Several bare GaAs substrates were ablated, undoped semiinsulating, silicon doped (n-10 18 cm -3), 687 Mat. Res. Soc. Symp. Proc. Vol. 354 01995 Materials Research Society
tellurium doped (n-'10 17 cm -3) and indium doped (=1%). All of them were chemically polished and degreased before the ablation. Morphologic changes produced by melting were studi
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