| AVS 63rd International Symposium & Exhibition | |
| Surface Science | Wednesday Sessions |
| Session SS+AS+EM-WeA |
| Session: | Semiconductor Surfaces and Interfaces |
| Presenter: | Stacy Heslop, University of Arizona |
| Authors: | S.L. Heslop, University of Arizona A.J. Muscat, University of Arizona |
| Correspondent: | Click to Email |
Ge and SiGe are promising materials for future p-type metal-oxide semiconductor field effect transistors (MOSFETs) due to their higher hole mobilities and narrower bandgap compared to Si. In contrast to silicon, Ge and SiGe readily oxidize in ambient air forming nonstoichiometric Ge oxides that are detrimental to the electrical performance of the device. One approach is to remove these oxides and passivate the surface. SiGe with molar ratios of 25 and 75% Ge were treated with aqueous ammonium sulfide, (NH4)2S, to deposit sulfur. The composition of the surface was measured using x-ray photoelectron spectroscopy (XPS) as a function of concentration and pH. The (NH4)2S concentration was varied from 3 mM to 3 M, and the pH was varied from 10 to 8 using HCl and HF. Film thicknesses were measured with spectroscopic ellipsometry.
A fresh SiGe starting surface was produced by immersing in SC-1 (1:1:500 v/v) to form oxides and stripping the oxides using HF:HCl:H2O (1:3:300 v/v). In the case of SiGe 25%, sulfides were not detected based on the S 2p XPS state for surfaces treated with (NH4)2S (Figure 1a). The oxygen coverage increased with increasing (NH4)2S concentration, forming primarily SiO2 and a small coverage of GeO. The surface was enriched in Si and oxidized, and there was not enough Ge atoms exposed for S to bond to. HCl and HF were added to remove the Si and Ge oxides that formed. The addition of HCl and HF resulted in the deposition of sulfides on SiGe 25% (Figure 1b). The Si/Ge peak area ratio after oxide removal was 1.7. After immersion in 30 mM (NH4)2S at a pH of 10 the surface composition was unchanged (Si/Ge=1.7). For the same (NH4)2S concentration at a pH of 8 the surface was only slightly enriched with Si (Si/Ge=2.4). Overall, (NH4)2S is not an effective passivation reagent for Si-rich SiGe surfaces due to the lack of S deposited and the undesirable oxides which form during processing. By dropping the pH to 8, less than a monolayer of S is deposited but oxides still remain. In contrast, SiGe 75% did not oxidize as a function of the (NH4)2S concentration. Sulfur was detected based on the S 2p XPS state and the S coverage was independent of (NH4)2S concentration. The sulfur thickness increased from about 2.3 Å for (NH4)2S (30 mM or 1:100 v/v) at a pH of 10 to 3.4 Å for the same (NH4)2S dilution at a pH of 8 (Figure 1c and d). These film thicknesses were approximated from XPS peak areas based on a single layer model for S on Ge. The deposition of the S layer did not affect the surface stoichiometry between oxide removal steps (Si/Ge=0.19) and 30 mM passivation (Si/Ge=0.20).