Anodic sulfidation and model characterisation of GaAs (100) in (NH 4 ) 2 S x solution
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Mat. Res. Soc. Symp. Proc. Vol. 573 ©1999 Materials Research Society
Figure 1: Experimental arrangement for anodic passivation.
The XPS scans were performed using a VG-Microtech x-ray source (Al1(Ka). The depth profiling was performed using Ar+ bombardment at a milling rate of 2 nnm/min. SIMS depth profiling was also performed on a SIMS analyzer (cameca IMS-3f) with primary ion beam 50 nA (14.5 keV) and a heavy bombardment of cesium ions. Raman backscattering was recorded in the z(x, x + y)• geometry using the 512 nm line of an 30 mW Ar+ laser with a 2 •tm 2 probe area. AFM was recorded in contact mode, yielding the average deviation of the average height, Ra. 3: Experimental results and discussion In this work molarity and sulfur-ion concentration of (NI{a)2 x solution including the anodic scan direction have profound effects upon the anodic passivation of n-type GaAs (100). Careful control was necessary to achieve both a chemically and electronically stable anodic passivation. The following cases have been considered for anodic characterization: (i) At high anodic potential as shown in the reverse anodic scan (Figure 2, Region II), XPS characterization has shown no evidence of sulfur deposition after DI water rinse and blow-dry in nitrogen. In this case the anodic potential and sulfur-ion concentration were high, with the result that the anodic treatment was not better than the conventional dipping of GaAs surfaces in (NH 4 )2 S solutions, leaving behind only an atomic layer of sulfur which is removed by DI water rinsing.
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Figure 2: Cyclic voltammogram of n-type lxlO' 8 cm"3 GaAs (100) in 3M (NH4)2 Sx solution (x=5g S/100 ml) at anodic scan rate 20 mV/s. (ii) However, at the sulfur characteristic peak (Figure 2, Region I), anodic passivation takes place in characteristic steps at which the thickness of the sulfide overlayer can be controlled by time and/or current density. This characteristic peak is mainly dependent upon the sulfur-ion
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concentration and positioned according to the electrochemical cell parameters for a given substrate doping concentration. It has been found that the reverse anodic scanning i.e. from high potential to low potential, is the best way to achieve good surface quality with low surface roughness compared to the forward scan (from low to high anodic scan potential) as shown in AFM later. The structure of the deposited overlayer has been analyzed using XPS and SIMS depth profiling and AES. The depth profiling revealed the atomic concentration of Ga, As, 0, S and C. Rinsing the sample in DI water after anodic passivation has dissolved the arsenic compounds at the top of the deposited layer. As shown in Figure 3, Ga3d and As3d core level spectra show evidence of strong Ga-S and As-S bonding at approximate depth of 120 nm. The chemical shifts at 1.36 eV and 2.8 eV were attributed to Ga-S bond and 1.61 eV and 3.0 eV to As-S bond. The gallium and arsenic sulfide chemical shifts were consistent with previous work as su
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