Chemical Passivation of In0.53Ga0.47As(100) Using Ammonium Sulfide and Thiols

III-V compound semiconductors have superior electron and hole mobilities compared to silicon, which makes them alternative materials to fabricate transistors that are faster and use less energy. Since the oxides of III-V’s contain many defects and have undesirable electrical properties, aqueous passivation chemistries are needed to incorporate these materials into existing semiconductor processing lines. This study seeks to determine whether a thin (~2-3 nm) layer of sulfur or a sulfur-containing molecule can chemically passivate clean InGaAs surfaces to prevent oxide regrowth upon ambient exposure. The goal is to devise a liquid phase process to remove carbon contamination from the surface, etch the native oxides, and deposit a thin film to prevent the diffusion of molecular oxygen and water vapor to the clean surface of the semiconductor for at least one hour. In_0.53Ga_0.47As(100) on InP substrates (RMS=0.18 nm by atomic force microscopy) were cleaned in dilute SC1 solutions and etched for 1 min in 0.3 M HF or 1.0 M HCl followed by immersion in dilute solutions of 1-eicosanethiol (ET, 20 carbon atoms in chain) in ethanol or ammonium sulfide ((NH₄)₂S) in water. The chemical composition of the passivated surfaces was analyzed by x-ray photoelectron spectroscopy (XPS). The presence of InGaAs oxides was evaluated by comparing the In 3d, Ga 2p, As 2p and O 1s region of the XPS spectra of the substrates after different processes. The analysis of InGaAs etched for 1 min in HF or HCl showed that the surface reoxidized completely after only 4 min of ambient exposure (no surface roughening occured after etching). The main oxides present on the surface after the acid etching were the As³⁺ and Ga³⁺ states. When clean substrates were etched in HF or HCl and immersed for 20 min 4 mM ET and 0.02% (NH₄)₂S solutions, oxides were detectable on the surface after 4 min of atmospheric exposure, but the reoxidation of InGaAs was slowed down (after 60 min of ambient exposure, less oxides were present on the surface in comparison with the substrate  that was not passivated, as shown in Figure 1). Additional tests showed similar results for 0.02% (NH₄)₂S passivation for 1 min and for 60 min, or for 20 min in 20% (NH₄)₂S, suggesting that the passivation is independent of (NH₄)₂S concentration or deposition time. From the XPS analysis, the passivation layer is bonding to the InGaAs surface in the form of sulfide (As¹⁺ states observed in the passivated samples only). Only when InGaAs was passivated for 20 h in ET or for 20 min in (NH₄)₂S under a nitrogen atmosphere in a glove box, no detectable oxygen was observed in the O 1s region after exposing to air for a short time (4 min). Oxide regrowth was observed for longer ambient exposure times. To estimate the thickness of the passivation layer deposited, spectroscopic ellipsometry was used to measure the overlayer (ET, S and InGaAs oxides) on InGaAs after the passivation processes. An overlayer thickness of 0.8 nm was measured on InGaAs after passivating for 20 min in 0.02% (NH₄)₂S. The thickness of the overlayer on InGaAs after passivating for 20 h in 4 mM ET was 2.6 nm (the calculated molecular length of the ET molecule is 2.7 nm). These results show that the sulfur-based chemistries are bonding to the III-V’s but the resulting layer is not thick enough to prevent oxygen or water vapor from diffusing and reacting at the surface. We are working to understand how to densify the ET layer and control the thickness of layers deposited using (NH₄)₂S.

Figure 1. XPS spectra of In_0.53Ga_0.47As(100) etched in 0.3 M HF and passivated for 20 min in 0.02% (NH₄)₂S in a nitrogen atmosphere (glove box). A control experiment where a water rinse was done instead of the passivation, exposed for 4 min to atmosphere, is shown in a). The passivated surface was exposed to atmosphere for b) 4 min, c) 8 min, d) 20 min and e) 60 min