The Uniformity of Surface Passivation After (NH 4 ) 2 S Treatment Studied by Near-Field Scanning Optical Microscopy
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thin AlGaAs cap layer also protects the GaAs surface from oxidation, this structure serves as an alternative passivation. It is of interest to study the passivation uniformity of this surface and compare with the as grown and (NH 4 ) 2 S passivated samples. EXPERIMENT SETUP The experimental set up is shown is Fig. 1. The sample is mounted on a piezo electric scanner. The probe is a pulled fiber tip coated with aluminum. The argon ion laser light is coupled into the fiber from the cleaved end. The PL or reflected light is collimated by the collector and re-focused onto the entrance slit of a spectrometer. The collector is a special Q PMT parabolic reflector with the tip at its focal focu point. This design maximizes the collection unit efficiency. The control of the scanning is fl
[scanning done by a commercial scanning tube
He-Ne laser,ND filter
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unit and software [10]. We use the standard shear force distance regulation [11]. The HeNe laser light used for the shear force
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Figure 1: NSOM experimental setup filter reducing its intensity in order to avoid its interference with the PL experiments. After the tip engages into the feedback, a spectrum is taken and the grating is then set to the PL peak position or the excitation wavelength. Simultaneous topography and optical images are then collected. SAMPLE PREPARATION The samples were grown by metal-organic vapor phase epitaxy. The GaAs epitaxial layer was 1 lim thick with p type Zn doping density of 9 10'9 cm 3 . An additional sample consisted of similarly doped p-GaAs with a 3 nm thick Al 0.65Ga 0.35As cap layer. Immediately after the growth, the samples were stored in a desiccator. Before passivation, the native oxide layer was removed in a hot 1:1 H 2 SO 4 and H20 solution. After a brief DI water rinse, a shallow ( 300 A ) etching of GaAs layer was done by immersing the sample in a dilute solution of H 2 SO 4 , H 2 0 2 and H20 (1:8:500). The etching was quenched by DI water rinse for 5 min. The sample was then passivated in 22% solution of (NH 4 )2 S in H20 for 5 min. After rinsing, the sample was blown dry with N 2. Five hours between the sample preparation and NSOM measurements allowed the sample to reach stabilized condition. EXPERIMENTAL RESULTS Some of the experimental results will be discussed in this section. Figs. 2 and 3 are the simultaneously collected topography and PL images of an as grown sample and a (NH 4 )2S passivated sample respectively. The sub-micron scale NSOM topography features are very similar to those on the AFM images. The RMS roughness analysis in Table I shows good agreement between these two techniques. This implies good NSOM distance regulation and high sensitivity of the NSOM topography measurement to sub-nanometer height variation. The features appearing on the images are intrinsic to the passivation process and not caused by improper handling. On the PL images, variations on the sub-micron spatial scale are clearly
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