Passivation and Nucleation of Ge(100) via H2O and HOOH Dosing

Germanium is a promising candidate for potential channel materials due to its higher hole and electron mobility. To minimize the oxide- semiconductor interfacial defect density, a proper passivation layer must be used before the oxide layer is deposited1. The passivation layer must be very thin, ideally one monolayer, to allow for increased scaling of the equivalent oxide thickness (EOT). H2O provides a well-ordered chemisorption monolayer (ML) at room temperature without disrupting surface Ge atoms2. In this study, a monolayer of H2O chemisorbates is shown to activate TMA chemisorption due to the Ge-OH bonds catalyzing the formation of an ultrathin passivation layer which can serve as an ideal ALD nucleation template on a Ge surface3. However, since H2O chemisorption results in equal density of Ge-H and Ge-OH sites on the Ge(100), H2O can only provide a maximum of 0.5 monolayer of Ge-OH sites, limiting the TMA nucleation density. By using HOOH dosing, the density of Ge-OH sites can be doubled thereby increasing the potential TMA nucleation density. This study compares the passivation of the Ge(100) surface via H2O and HOOH, for the application of nucleating ALD growth on the surface, using scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). This study will also look into similar H2O/HOOH passivation and TMA nucleation techniques on SiGe. Using a differentially pumped dosing system, a clean Ge(100) sample was dosed at room temperature with a saturation dose of either H2O or HOOH. The saturation H2O dose resulted in 0.85 ML coverage of -OH and -H species chemisorbed on the surface (Fig. 1a, blue box). The remaining unreacted portion of the surface is seen as fuzzy bright features randomly distributed on the surface. These bright features are attributed to single (Fig. 1a, green box) or double (Fig. 1a, red box) dangling bond sites. STS measurements of this surface are shown in Figure 1b. Both of the dangling bond sites have energy states in the band gap consistent with the presence of dangling bonds. Conversely, STS measurements of the H2O sites show the conduction band edge dangling bond states are eliminated due to the passivating Ge-OH and Ge-H bonds. But, the Ge-OH and Ge-H sites on the surface have limited thermal stability. As shown in Figure 2, when the H2O/Ge(100) sample is annealed to 100°C, the H2O coverage is significantly reduced due to the recombinative desorption of H2 or H2O. A saturation dose of HOOH on Ge(100) results in a coverage of 0.95 ML of Ge-OH species chemisorbed on the surface (Fig. 3), which is more than double that of the H2O surface. Only a few sites on the surface are unreacted dangling bond sites (Fig. 3, blue circles) while the red circles represent contaminants on the surface. STS measurements of this surface are shown in Figure 5. As seen with the H2O dosed surface, Ge-OH sites on the surface passivate the conduction band edge dangling bond states, while the few dangling bond sites show energy states in the band gap. In contrast to the H2O passivated surface, this HOOH passivated surface has increased thermal stability. Figure 4 shows the HOOH/Ge(100) surface that has been annealed to 100C. The surface coverage remains constant to within 10% demonstrating the improved thermal stability of Ge-OH on the surface in comparison to the Ge-H and Ge-OH sites on the H2O surface since the absence of Ge-H sites in HOOH dosing removed the H2O recombination desorption mechanism.