Oxidation Characteristics of InAs Surface in Wet Chemical Treatment
Authors: Na Jihoon , Seo Dongwan , Lee Jinhoon , Lim Sangwoo 
Published: February 2020

With the scaling down of silicon-based complementary metal-oxide-semiconductor (CMOS), the problems such as short channel effect and current leakage become crucial. Introduction of new channel material can be a solution to solve those problems. In particular, III-V compound semiconductors can be one of the candidate materials because of their higher electron mobilities than Si [1, 2]. Because InAs has a high electron mobility as 40000 cm2/(Vs) [3], it can be applied to the channel material for the next generation semiconductor devices. In order to introduce InAs into the channel material, its surface should be well-controlled before the gate dielectric deposition. However, the effect of wet cleaning and surface preparation and reaction on the InAs has not been fully elucidated yet. Therefore, we investigated the effect of wet cleanings on the InAs surface in the current study.
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 (In3d5/2 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+
, AsO2- 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.)

Figure 1