Hole Contacts on Transition Metal Dichalcogenides: Interface
Hole Contacts on Transition Metal Dichalcogenides: Interface Chemistry and Band Alignments (Supporting Information) Stephen McDonnell1*, Angelica Azcatl1, Rafik Addou1, Cheng Gong1,
Corsin Battaglia2, Steven Chuang2,
Kyeongjae Cho1, Ali Javey2, and Robert M. Wallace1*
1Department
of Materials Science and Engineering, The University of Texas at Dallas, Richardson, TX, 75080
2Electrical
Engineering and Computer Sciences, University of California, Berkeley, CA, 94720 *Correspondence to: [email protected]; [email protected]
Scanning tunneling spectroscopy of WSe2
Figure S1 STS of multiple locations on the same WSe2 sample. Shows local variations in the observed band gap. The Fermi level is located at 0 V. The scanning tunneling spectroscopy information obtained from multiple locations around the same 2 × 2 mm WSe2 sample at room temperature suggest that the observed local band gap can vary from 1.1 to 2 eV. Importantly for this work, the Fermi level position appears to consistently be located at 1.15 ± 0.15 eV below the conduction band. In light of the recently observed1 hole Schottky barriers for MoOx on WSe2, the possibility that these local variations in the valance band position play a role should not be overlooked.
Core-level spectra for MoOx/MoS2 and MoOx/WSe2
Figure S2 (a) Mo 3d and (b) S 2p, core-level spectra acquired sequentially between MoOx depositions of increasing thickness. Total MoOx thickness is: initial 0 nm, (i) 0.2 nm, (ii) 0.4 nm, (iii) 2.4 nm, (iv) 6.3 nm, (v) ~10 nm. Band bending of ~0.43 eV to lower binding energy is observed after the first deposition. No new chemical states of sulfur are observed.
Figure S3 (a) W 4f, (b) Se 3d, and (c) Mo 3d core-level spectra acquired sequentially between MoOx depositions of increasing thickness. Total MoOx thickness is: initial 0 nm, (i) 0.2 nm, (ii) 0.4 nm, (iii) 2.9 nm, (iv) 6.9 nm, (v) ~ 10 nm. Band bending of ~0.69 eV to lower
binding energy is observed after the first deposition. No new chemical states of tungsten or selenium are observed in the W 4f and Se 3d core-level spectra.
Fitting constraints for spectral analysis The explanation provided for the similar hole Schottky barriers reported by Chuang et al.1 relies on the work function at the interface of MoOx/TMD being lower than the bulk MoOx work function, but also that the work function of this interlayer be lower on WSe2 than on MoS2. The above correlation between work function and Mo5+ concentration allows us to use the Mo5+/Mo6+ ratio as a way to track trends in the expected work function. Table 2 in the main text shows the extracted ratios of Mo5+/Mo6+ for both substrates as a function of thickness. Figure 2 in the main text shows the spectral analysis that allowed for this quantitative analysis. The deconconvolution required careful consideration of the substrate features spin orbit splitting intensities and line shapes. Since relying on purely minimizing chi-squared values can lead to physically meaningless fits, realistic constraints were placed on the fitting parameters. All Mo 3d3/2 components were fixed to be 66.7% of the integrated intensity of their corresponding Mo 3d5/2 component to adhere to the theoretical electron occupancy rules. The spin orbit splitting for the Mo 3d was fixed to be 3.14 eV. For the Mo6+ and Mo5+ features, the full width at half maximum was correlated to be the same. It is worth noting that while this procedure was required to produce meaningful values of the Mo6+ and Mo5+ integrated intensities, the resulting trends match well with what can be observed by even a cursory review of the raw data. For example it is clear from the thickest MoOx films that the Mo5+ concentration is higher on the WSe2 substrate than on the MoS2. It is also clear that the Mo5+ concentration generally decreases with thickness.
Evidence of island growth Following the sequential depositions and in situ characterizations of MoOx on MoS2 and WSe2 the samples were removed from vacuum for ex situ analysis by atomic force microscopy. As can be seen from the images shown in figure S4 the resultant films were not uniform and instead show large height variations indicative of island growth.
Figure S4 Ex situ AFM and line profiles after ~ 10 nm MoOx depositions on (a) MoS2 and (b) WSe2. The large roughness suggests that initial growth was likely non- uniform. Scale bars correspond to 250 nm.
Work function variations with MoOx composition In previous studies by Greiner et al.2, 3 the work function and composition of MoOx deposited on a range of substrates were studied. It was shown that for all of the substrates studied the work function of the initial film was lower than its bulk value of almost 7 eV. Similarly the composition was also seen to vary with the initial Mo5+ concentration decreasing with increasing thickness. By comparing substrates with different MoOx thin film compositions, such as Mo with 65% Mo5+ and Au with 40% Mo5+ one could observe that their initial work functions are also different, with the MoOx on Mo showing ~5.3 eV and the MoOx on Au showing 5.8 eV. From this it might be inferred that the concentration of Mo5+ is directly related to the work function of the MoOx, however it is difficult to discount substrate effects on measured work function as the change transfer between the substrate metals and of the thin films will necessarily be different and also the structure of the thin film may be substrate dependent. It is well known that the work function of metals varies with crystal structure.4 To strengthen our conclusion that an interfacial MoOx layer may have a lower work function due to it higher Mo5+ concentration we utilized data acquired on MoOx films grown ex situ on silicon, described in a previous study.5 The films were 5 nm and 40 nm and the Mo5+ concentration was found to decrease with thickness. Also the work function was seen to increase with thickness. To rule out a contribution from thickness we deliberately exposed these films to a reducing atomic hydrogen atmosphere followed by in situ analysis by XPS in a system described elsewhere.6 The results are shown in figure S5 for the 5 nm film. Figure S5 (a) shows that the low energy cut off (from which the work function can be extracted) decreased after the atomic hydrogen exposure. Concurrently the Mo5+
concentration was seen to increase in figure S5 (b). This process was not seen to increase the carbon concentration (Figure S5 (c)). This is important since the work function MoOx is extremely sensitive to carbon concentration. Finally the calculated work functions for both the 5 nm and 40 nm MoOx before and after atomic hydrogen exposure are plotted as a function of Mo5+/Mo6+ ratio in figure S5 (d). These work functions are lower than those observed during in situ work do to the surface carbon contamination, however the trend observed in the work function with increasing Mo5+ concentration is clear.
Figure S5 XPS analysis of MoOx on Silicon before and after atomic hydrogen treatment. (a) Low energy cut of for 5 nm MoOx, (b) Mo 3d for 5 nm MoOx, (c) C 1s for 5 nm MoOx, (d) calculated work function for both 5 nm and 40 nm MoOx. A clear decrease in work function is observed with increasing Mo5+ concentration.
Figure S6 MoS2 and WSe2 crystals mounted side by side on a sample holder to ensure identical MoOx deposition conditions. The sample holder is approximately 1 cm in diameter.
References 1. Chuang, S.; Battaglia, C.; A. Azcatl; McDonnell, S.; Kang, J. S.; X. Yin; Tosun, M.; H. Fang; Kapadia, R.; Wallace, R. M., et al. Mos2 P-Type Transistors and Diodes Enabled by High Workfunction Moox Contacts. Nano Lett 2014, 14, 1337-1342. 2. Greiner, M. T.; Lu, Z.-H. Thin-Film Metal Oxides in Organic Semiconductor Devices: Their Electronic Structures, Work Functions and Interfaces. NPG Asia Materials 2013, 5, e55.
3. Greiner, M. T.; Chai, L.; Helander, M. G.; Tang, W. M.; Lu, Z. H. Metal/Metal-Oxide Interfaces: How Metal Contacts Affect the Work Function and Band Structure of MoO3. Adv. Funct. Mater. 2013, 23, 215-226. 4. Michaelson, H. B. The Work Function of the Elements and Its Periodicity. J. Appl. Phys. 1977, 48, 4729-4733. 5. Battaglia, C.; Yin, X.; Zheng, M.; Sharp, I. D.; Chen, T. L.; Azcatl, A.; McDonnell, S.; Carraro, C.; Maboudian, R.; Wallace, R. M., et al. Hole Selective MoOx Contact for Silicon Solar Cells. Nano Lett. 2014, 14, 967-971. 6. Wallace, R. M. In-Situ Studies of Interfacial Bonding of High-K Dielectrics for CMOS Beyond 22nm. ECS Transactions 2008, 16, 255-271.
Corsin Battaglia2, Steven Chuang2,
Kyeongjae Cho1, Ali Javey2, and Robert M. Wallace1*
1Department
of Materials Science and Engineering, The University of Texas at Dallas, Richardson, TX, 75080
2Electrical
Engineering and Computer Sciences, University of California, Berkeley, CA, 94720 *Correspondence to: [email protected]; [email protected]
Scanning tunneling spectroscopy of WSe2
Figure S1 STS of multiple locations on the same WSe2 sample. Shows local variations in the observed band gap. The Fermi level is located at 0 V. The scanning tunneling spectroscopy information obtained from multiple locations around the same 2 × 2 mm WSe2 sample at room temperature suggest that the observed local band gap can vary from 1.1 to 2 eV. Importantly for this work, the Fermi level position appears to consistently be located at 1.15 ± 0.15 eV below the conduction band. In light of the recently observed1 hole Schottky barriers for MoOx on WSe2, the possibility that these local variations in the valance band position play a role should not be overlooked.
Core-level spectra for MoOx/MoS2 and MoOx/WSe2
Figure S2 (a) Mo 3d and (b) S 2p, core-level spectra acquired sequentially between MoOx depositions of increasing thickness. Total MoOx thickness is: initial 0 nm, (i) 0.2 nm, (ii) 0.4 nm, (iii) 2.4 nm, (iv) 6.3 nm, (v) ~10 nm. Band bending of ~0.43 eV to lower binding energy is observed after the first deposition. No new chemical states of sulfur are observed.
Figure S3 (a) W 4f, (b) Se 3d, and (c) Mo 3d core-level spectra acquired sequentially between MoOx depositions of increasing thickness. Total MoOx thickness is: initial 0 nm, (i) 0.2 nm, (ii) 0.4 nm, (iii) 2.9 nm, (iv) 6.9 nm, (v) ~ 10 nm. Band bending of ~0.69 eV to lower
binding energy is observed after the first deposition. No new chemical states of tungsten or selenium are observed in the W 4f and Se 3d core-level spectra.
Fitting constraints for spectral analysis The explanation provided for the similar hole Schottky barriers reported by Chuang et al.1 relies on the work function at the interface of MoOx/TMD being lower than the bulk MoOx work function, but also that the work function of this interlayer be lower on WSe2 than on MoS2. The above correlation between work function and Mo5+ concentration allows us to use the Mo5+/Mo6+ ratio as a way to track trends in the expected work function. Table 2 in the main text shows the extracted ratios of Mo5+/Mo6+ for both substrates as a function of thickness. Figure 2 in the main text shows the spectral analysis that allowed for this quantitative analysis. The deconconvolution required careful consideration of the substrate features spin orbit splitting intensities and line shapes. Since relying on purely minimizing chi-squared values can lead to physically meaningless fits, realistic constraints were placed on the fitting parameters. All Mo 3d3/2 components were fixed to be 66.7% of the integrated intensity of their corresponding Mo 3d5/2 component to adhere to the theoretical electron occupancy rules. The spin orbit splitting for the Mo 3d was fixed to be 3.14 eV. For the Mo6+ and Mo5+ features, the full width at half maximum was correlated to be the same. It is worth noting that while this procedure was required to produce meaningful values of the Mo6+ and Mo5+ integrated intensities, the resulting trends match well with what can be observed by even a cursory review of the raw data. For example it is clear from the thickest MoOx films that the Mo5+ concentration is higher on the WSe2 substrate than on the MoS2. It is also clear that the Mo5+ concentration generally decreases with thickness.
Evidence of island growth Following the sequential depositions and in situ characterizations of MoOx on MoS2 and WSe2 the samples were removed from vacuum for ex situ analysis by atomic force microscopy. As can be seen from the images shown in figure S4 the resultant films were not uniform and instead show large height variations indicative of island growth.
Figure S4 Ex situ AFM and line profiles after ~ 10 nm MoOx depositions on (a) MoS2 and (b) WSe2. The large roughness suggests that initial growth was likely non- uniform. Scale bars correspond to 250 nm.
Work function variations with MoOx composition In previous studies by Greiner et al.2, 3 the work function and composition of MoOx deposited on a range of substrates were studied. It was shown that for all of the substrates studied the work function of the initial film was lower than its bulk value of almost 7 eV. Similarly the composition was also seen to vary with the initial Mo5+ concentration decreasing with increasing thickness. By comparing substrates with different MoOx thin film compositions, such as Mo with 65% Mo5+ and Au with 40% Mo5+ one could observe that their initial work functions are also different, with the MoOx on Mo showing ~5.3 eV and the MoOx on Au showing 5.8 eV. From this it might be inferred that the concentration of Mo5+ is directly related to the work function of the MoOx, however it is difficult to discount substrate effects on measured work function as the change transfer between the substrate metals and of the thin films will necessarily be different and also the structure of the thin film may be substrate dependent. It is well known that the work function of metals varies with crystal structure.4 To strengthen our conclusion that an interfacial MoOx layer may have a lower work function due to it higher Mo5+ concentration we utilized data acquired on MoOx films grown ex situ on silicon, described in a previous study.5 The films were 5 nm and 40 nm and the Mo5+ concentration was found to decrease with thickness. Also the work function was seen to increase with thickness. To rule out a contribution from thickness we deliberately exposed these films to a reducing atomic hydrogen atmosphere followed by in situ analysis by XPS in a system described elsewhere.6 The results are shown in figure S5 for the 5 nm film. Figure S5 (a) shows that the low energy cut off (from which the work function can be extracted) decreased after the atomic hydrogen exposure. Concurrently the Mo5+
concentration was seen to increase in figure S5 (b). This process was not seen to increase the carbon concentration (Figure S5 (c)). This is important since the work function MoOx is extremely sensitive to carbon concentration. Finally the calculated work functions for both the 5 nm and 40 nm MoOx before and after atomic hydrogen exposure are plotted as a function of Mo5+/Mo6+ ratio in figure S5 (d). These work functions are lower than those observed during in situ work do to the surface carbon contamination, however the trend observed in the work function with increasing Mo5+ concentration is clear.
Figure S5 XPS analysis of MoOx on Silicon before and after atomic hydrogen treatment. (a) Low energy cut of for 5 nm MoOx, (b) Mo 3d for 5 nm MoOx, (c) C 1s for 5 nm MoOx, (d) calculated work function for both 5 nm and 40 nm MoOx. A clear decrease in work function is observed with increasing Mo5+ concentration.
Figure S6 MoS2 and WSe2 crystals mounted side by side on a sample holder to ensure identical MoOx deposition conditions. The sample holder is approximately 1 cm in diameter.
References 1. Chuang, S.; Battaglia, C.; A. Azcatl; McDonnell, S.; Kang, J. S.; X. Yin; Tosun, M.; H. Fang; Kapadia, R.; Wallace, R. M., et al. Mos2 P-Type Transistors and Diodes Enabled by High Workfunction Moox Contacts. Nano Lett 2014, 14, 1337-1342. 2. Greiner, M. T.; Lu, Z.-H. Thin-Film Metal Oxides in Organic Semiconductor Devices: Their Electronic Structures, Work Functions and Interfaces. NPG Asia Materials 2013, 5, e55.
3. Greiner, M. T.; Chai, L.; Helander, M. G.; Tang, W. M.; Lu, Z. H. Metal/Metal-Oxide Interfaces: How Metal Contacts Affect the Work Function and Band Structure of MoO3. Adv. Funct. Mater. 2013, 23, 215-226. 4. Michaelson, H. B. The Work Function of the Elements and Its Periodicity. J. Appl. Phys. 1977, 48, 4729-4733. 5. Battaglia, C.; Yin, X.; Zheng, M.; Sharp, I. D.; Chen, T. L.; Azcatl, A.; McDonnell, S.; Carraro, C.; Maboudian, R.; Wallace, R. M., et al. Hole Selective MoOx Contact for Silicon Solar Cells. Nano Lett. 2014, 14, 967-971. 6. Wallace, R. M. In-Situ Studies of Interfacial Bonding of High-K Dielectrics for CMOS Beyond 22nm. ECS Transactions 2008, 16, 255-271.
