In order to observe the InAs surface state after various wet chemical treatments, InAs (100) wafers were dipped in various solutions such as APM (NH₄OH + H₂O₂ + H₂O), HPM (HCl + H₂O₂ + H₂O), and FPM (HF + H₂O₂ + H₂O), and solutions with various pH levels. The pH levels were adjusted from 1 to 13 using HCl and NaOH. The etching rate of InAs surface was measured from the weight change by a microbalance, and the change of surface roughness was analyzed by atomic force microscopy (AFM). The change in the elemental composition and chemical state was analyzed by X-ray photoelectron spectroscopy (XPS), and oxide thickness of the surface was measured by ellipsometry.

The representative changes in the XPS oxidation states (In3d_5/2 and As3d) of the InAs surface treated in the solutions with different pH are shown in Fig. 1. It is obvious that bulk InAs peaks of In3d and As3d appear without their oxidation states such as In₂O₃ and As₂O₃ when InAs surface was treated in the solution which has a pH below 3. However, both In₂O₃ and As₂O₃ were observed on InAs surface from the pH level of solution over 4. To analyze the oxidation degree depending on pH level of the wet chemical solution, the oxide ratio was calculated by In₂O₃ peak area to the total (In₂O₃ + InAs) peak area. Arsenic oxide ratio was made in a same way. As shown in Fig. 1(b), for both In₂O₃ and As₂O₃ oxide ratios show similar trend: almost zero at pH 1-3 and slight decrease from neutral to pH 13. Those behaviors can be explained thermodynamically by E vs. pH diagram [4]. In the solution below pH 3, indium and arsenic are stable in the form of In³⁺, InOH²⁺_, AsO^2- and As⁰. Therefore, the formation of oxide-free surface of InAs is expected when the surface is dipped in the solutions with pH less than 3. As shown in Fig. 1(b), the oxide ratio of both In₂O₃ and As₂O₃ decreased as the pH increased from 8 to 13. Indium is stable in the form of InO₂^- over pH 11 and arsenic is stable in the form of HAsO₄^2- and AsO₄^3- over pH 9. It is suggested that the oxide ratio decreases because In and As are thermodynamically preferred as ionic forms of in high pH levels, while oxide state is thermodynamically stable at around pH 7.

The change of oxide thickness after chemical treatment was also investigated and shown in Fig. 2(a). Oxide thickness decreased as compared to pre-existing oxide prepared in DIO₃ when the pH was below 3 and over 10. Fig. 2 (b) shows that the surface roughness increased with the H⁺ and OH^- ion concentrations at higher and lower pH levels. In summary, the ellipsometry and AFM results match well with the XPS results that the oxidation state on InAs can be categorized into three groups depending on the pH of the chemical solution in the current study: oxide-free surface below pH 3, oxide-covered surface in pH 4-8, pseudo oxide-covered surface in pH over 9.

References

[1]   J. A. del Alomo, Nature, 479, 317 (2011).

[2]   D.H. van Dorp, S. Arnauts, D. Cuypers, J. Rip, F. Holsteyns, S. De Gendt, Solid State Phenom., 219, 56 (2015).

[3]   M.P. Mikhailova, Handbook Series on Semiconductor Parameters, World Scientific, London, (2011).

[4] M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solutions, National Association of Corrosion Engineers: Houston, Texas, (1974.)