Investigation of Electronic Surface States and its Correlation to Surface Modifications in Femtosecond UV-Laser Treated
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electronic properties of the semiconductor [1-3]. Use of ultrafast lasers (with characteristic time scales shorter than about 10- 12 s) for material processing has several distinct advantages over their conventional counterparts, and is discussed in detail elsewhere [4,5]. The need for lasers as tools for material processing finds its origins in the continued demands for miniaturization, along with a significant performance improvement. Although GaAs offers advantages over silicon, such as a direct and larger bandgap, its use for device applications poses some challenges too. Unlike Si, GaAs does not have a stable and protective natural oxide, which can function as a barrier against further oxidation, and as a surface passivation layer. Consequently, several attempts towards passivating the GaAs surface have been attempted [6-10]. However, the use of lasers for generating such a protective layer is an attractive option, due to its non-contact nature, thereby avoiding chemical contamination. Also, the use of femtosecond lasers for this purpose is distinctly advantageous over conventional lasers, due to the characteristic time scale that is far shorter than that of atomic vibrations in the processed solid. In this paper, we describe the passivating effects observed at the surface of n-(100) GaAs as a consequence of its interaction with an unfocussed femtosecond laser beam (1.3 mJ/cm 2, at 248 nm and -380 fs). GaAs samples were treated in an air ambient and were characterized by xray photoelectron spectroscopy (XPS) for chemical analysis of the surface followed by in-situ depth profiling. Energy dispersive x-ray analysis (EDAX) measurements were used to measure 209 Mat. Res. Soc. Symp. Proc. Vol. 585 ©2000 Materials Research Society
the relative percentage of Ga and As atoms, before and after the laser treatment. Thermally Stimulated Exoelectron Emission (TSEE) measurements characterized the electronically active surface and interface defects. This is a relatively novel technique that is described in detail elsewhere [7,8]. Earlier TSEE measurements have demonstrated their capability of detecting electronically active surface and interface defects in GaAs [9], as well as modifications to the GaAs surface, as a consequence of various types of processing steps [11,12]. EXPERIMENT a) fs Laser Treatment Samples of n(100) GaAs, 600 ,um thick, were used in the present study. The samples (1 cm x 1 cm in dimensions) were degreased with a rinse in warm methanol, for about two minutes. They were then rinsed in flowing de-ionized water, dried in flowing nitrogen gas, and were treated with an unfocussed femtosecond laser beam. The laser treatment was performed in air in an effort to make the laser treatment as simple as possible, with the following specifications: 248 nm, -380 fs, 1.3 mJ/cm 2. The sample temperature was not monitored, but based on the available literature that correlated incident energy density to the temperature rise, it was assumed that the rise in temperature was practically constant [13]. The GaAs samples w
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