Atomic Force Microscopy Characterization of Interface Roughness of V-Shaped AlGaAs/GaAs Quantum Wire
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Fig.l. Flattened topography AFM images of V-grooved GaAs substrates (a) without and with (b) the NH40H etching treatment.
EXPERIMENT The V-grooved substrates were formed on (001))±0.10 semi-insulating GaAs substrates by photolithography and wet chemical etching using a NI-LtOH:H 20 2 :H20=I :3:50 solution (NH4 OH solution, hereafter) along the [01-1] direction. A V-groove period of 2 or 4 jim was used for AFM observation and optical characterization, respectively. Before epitaxial growth, the substrates were first cleaned ultrasonically by organic solvents, then they were either immersed in HC1 solution for removing native oxide film or etched by the same NH 4 OH solution for 10 - 20 sec at room temperature. The epitaxial growth was performed using a 76 Torr MOVPE system at 630 'C. Trimethylaluminum (TMAI), triethylgallium (TEGa) and AsH 3 were used as group III and group V sources, respectively. Other growth conditions can be found in previous publication [4]. RESULTS AND DISCUSSION
Effects of NHEOH Etching on Reducing Surface Roughness of the Initial V-grooved Substrate We first investigated the change of surface morphology of V-grooved substrates with and without NH 4 0H etching treatment by AFM. Figures 1(a) and 1(b) show the topography AFM image of substrates treated with HCl oxide film removal and NH4 OH etching, respectively. Here, the images were flattened along the [011] direction in order to put all facets on the same average 48
height level, and this makes observation of small height variations on the (II 1)A side wall facets and at the V-groove bottom possible. The scan area is 5 jim x 5 jim. In the sample without NH 4 OH etching, we can observe long-range height modulation on the (IIl )A side wall facets (the yellow and the red contrast difference) with a typical length scale of several microns. The amplitude of the height modulation, depending on measurement positions, is in the range of several 10s to 100 nm. Within these long-range height modulation, short-range roughness was observed especially at the region where the (001) flat and the (111 )A side wall facets intersect. These roughnesses were generated during the V-groove preparation processes due to the limited controllability of the processes used. To the contrary, on the surface of the sample etched with NH 4 OH solution, the short-range roughness disappeared almost completely, though long-range height modulation exists also in this case. This result clearly indicates that the NH 4 OH etching treatment is very effective for reducing roughness generated on the V-groove surface during V-groove pattern formation processes.
AFM Observation of the Surface Morpholoev of Thick OWRs We next investigated the influences of surface roughness on the initial V-grooved substrate on the interface uniformity of QWRs by directly observing QWR surface using AFM. The grown QWR samples consist of a 0.2 gim thick GaAs buffer layer, a I jim thick AIGaAs barrier layer, and an 80 nm thick FME-grown GaAs wire layer. Figures 2(a), 2(b) and 2(c), 2(d) show the AFM
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