# Reduction of Schottky Barrier Height at Graphene/Germanium Interface with Surface Passivation
## 3 Results  
Figure 1 shows the Ge3d core level after 3 h annealing in UHV for the G/Ge samples. The experimental spectra were decomposed after background correction with a least-square procedure using a Lorentzian convoluted with a Gaussian to represent each of the spin-orbit-split components of the Ge3d core level. In the fitting procedure, the spin-orbit splitting of the Ge3d core level was fixed at 0.58 eV and the branching ratio was taken to be the statistical value of 3:2. The same spin-orbit-split and branching ratio were used for the various components introduced in our fitting procedure. The main peak at 29.64 eV for G/Ge and 29.60 eV for Ge is attributed to bulk Ge. Other peaks located at 0.9 eV, 1.6 eV, 2.6 eV and 3.4 eV from the main peak correspond to the photoemission signal from germanium in an oxidation state of +1, +2, +3 and +4, respectively [29].  
After HF etching, only the +1 oxidation component is observable for the Ge sample, while for G/Ge the +1 and +2 oxidation states are visible. This indicates that HF removed most of the oxide but that the etching efficiency and/or kinetics in HF are reduced by the presence of graphene on the surface in opposition to what we observed on silicon, where oxide removal is not affected by the presence of graphene [17]. The deposition of 1 nm Au on the Ge surface induces a 0.25 eV shift of the main Ge3d component towards lower binding energies (BE). This shift indicates that the gold deposition induces a band bending change as the Au/Ge interface forms. For the graphene-covered sample, no binding energy shift was observed after the deposition of gold, indicating that no further band bending took place.  
Because of the greater bond strength of Ge-Cl over Ge-H, chloride passivation is expected to be more stable than hydrogen [30]. As Ge oxides are soluble in HCl, etching in HCl would remove the oxide and passivate the surface with chloride. We can see in Figure 2 an almost similar shape of the Ge3d spectra between graphene-covered and -uncovered surfaces. Here, in the photoemission spectra, we observe only a contribution from Ge atoms in oxidation state +1 in addition to the Ge bulk component. This indicates that either the oxide removal is not complete in HCl or the surface re-oxidizes before introduction under UHV. We confirm in our survey spectrum that Cl ions are still present at the surface after introduction of the sample in UHV. After the deposition of 1 nm Au on the Ge:Cl surface, we observe a 0.34 eV shift towards lower binding energies of the Ge3d bulk component. The final BE position of the main Ge3d peak is essentially the same as for the Au/Ge:H sample. When the surface is protected with graphene the shift of the Ge3d main component is only 0.19 eV.  
Figure 1: Ge3d core level of G/Ge:H and Ge:H samples after annealing for 3 h under vacuum, after 10 mins in HF (10%) and after 1 nm Au deposition.  
Figure 2: Ge3d core level of G/Ge:Cl and Ge:Cl samples after 30 s in HCl (7.5%) and after 1 nm Au deposition.  
Figure 3 shows the experimental spectrum of the C1s core level for single-layer graphene on Ge passivated with HCl and HF. A main component corresponds to C-C sp2 bonds and a second component located at 0.7 eV higher BE is attributed to sp3 carbons [31]. One more component at a 2.4 eV higher BE (labelled C1 on the figure) is assigned to the contribution of carbon atoms from PMMA residues [32]. It can be seen that the deposition of 1 nm of Au on graphene causes a C1s core level shift towards lower BE of 0.18 and 0.10 eV for Ge:Cl and Ge:H samples, respectively. This shift is induced by an electron transfer from graphene toward Au at the graphene/Au interface [33].