Controlled oxide removal and surface morphology on InSb(100) using gas phase HF/H₂O

The native oxide removal, surface termination, composition, and morphology of InSb(100) surfaces etched in gas phase HF/H₂O mixtures at a total pressure of 100 Torr and 29 °C were investigated using quantitative X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and scanning electron microscopy (SEM) as a function of the HF to water partial pressure ratio. Gas phase HF/H₂O etching preferentially removed antimony oxide and left a residue that was indium fluoride rich. Water played a critical role in the process by directly participating in the etching reaction and promoting the desorption of fluoride etching products. The lowest HF to H₂O ratio studied was 0.3, and this mixture removed over 90% of the native oxide, producing a smooth surface that consisted of an Sb-terminated substrate, an elemental Sb layer (2 Å), and an indium fluoride-rich overlayer (<10 Å). Excess elemental Sb was only observed at low oxygen coverages, suggesting that its formation was a byproduct of HF preferentially reacting with In atoms from the substrate. Moderate HF to H₂O ratios of 0.8-1.2 removed the oxide and yielded the same surface composition as the lowest ratio; yet, oblate spheroid islands were produced on the surface with heights of 4-60 nm and composed of indium fluoride that had agglomerated due to the surface energy of the underlying elemental Sb layer. These islands covered about 10% of the surface area and were imaged using AFM and SEM. The highest HF to H ₂O ratio studied was 2.3, and this mixture removed about 70% of the native oxide. The surface was smooth and terminated with Sb covered by a ∼27 Å thick indium fluoride-rich overlayer. The accumulation of indium fluoride on the surface at high HF to H₂O ratios must have created a barrier for transport of the etchants to the surface. Control of both the composition and the morphology of the InSb(100) surface was possible with gas phase HF, providing more process latitude than observed with liquid phase HF etching, which was used as a baseline.