Shi et al. Nanoscale Res Lett 2011, 6:50 http://www.nanoscalereslett.com/content/6/1/50 Page 4 of 5 Table 1 XPS CL spectra fitting results and VBM positions obtained by linear extrapolation of the leading edge to the extended base line of the VB spectra Sample Diamond InN InN/diamond State C1 s VBM In 3d5/2 VBM In 3d5/2 C1 s Binding energy (eV) 284.90 286.00 1.32 443.42 444.21 445.27 0.66 442.59 443.50 283.80 284.50 All the binding energies are referenced to the Fermi level (0 eV) subjected to a surface clean procedure by Ar+ bombard- ment with a voltage of 1 kV at a low sputtering rate of 0.5 nm/min, which alleviates damages to the sample. The reduced thickness (less than 1 nm) is calculated by the sputtering rate, and the O-related bondings were absent in cleaned InN/diamond heterojunction because of the sputtering process. In Figure 1e, the lower-bind- ing energy component (442.59 eV) and the higher-bind- ing energy component (443.50 eV) are attributed to be screened In–N bonding and unscreened In-N bonding respectively. Finally, in Figure 1f, we suggest assign- ments of screened C–C bonding and unscreened C–C bonding for the lower- (283.80 eV) and higher-binding (284.50 eV) energy components, respectively. The VBM of the two thick samples are determined to be 0.66 and 1.32 eV, respectively. All of them are summarized in Table 1. The lower-binding energy components related to “screened” final-state are chosen for VBO calculation because the peak and line width of higher-binding energy ("unscreened” final-state) depend on the excita- tion of bulk, surface plasmon, and surface treatment [22,23], as is mentioned above. The VBO values can be calculated by substituting those measured values in Table 1 into Eq. 1. The average InN/diamond VBO (ΔEv) is -0.39 ± 0.08 eV. The CBO (ΔEC) is given by the formula ΔEC = ( E dgiamond − E IgnN ) − ΔEV . Here E diamond (~5.45 eV) and E InN (~0.64 eV) are respec- g g tively the bandgap of diamond and InN at room tem- perature. So the band lineup can be determined, with a conduction band offset (CBO) of 4.42 ± 0.08 eV, as shown in Figure 2. As XPS measurements are spatially averaged due to the finite mean free path of elastic electrons (1.5–2 nm), band bending could induce a systematic error in our measurements. Due to the lattice mismatch between InN and diamond, especially the small linear pressure Bonding C–C C–O – In–N (screened) Adsorbed In–O In–N (unscreened) – In–N (screened) In–N (unscreened) C–C (screened) C–C (unscreened) FWHM (eV) 1.21 1.89 1.09 1.09 2.45 1.26 2.19 1.28 2.43 coefficient of InN (~0.06 meV/GPa) [30], the band gap change induced by the interface strain could be neglected. So the systematic error related to band bend- ing is expected to be much smaller than the average standard deviation of 0.08 eV given above. Another fac- tor that may affect the precision of the VBO value is the strain-induced piezoelectric field in the overlayer of the heterojunction, as described in the III-nitrides system [31]. By using the constants and equation in Martin’s work [31], the field magnitude is estimated to be in the order of 107 V/m. Assuming the heterojunction InN overlayer thickness of ~4 nm after Ar+ bombardment, the error of VBO induced by lattice mismatch is less than 60 meV. Besides, practically all nitride epitaxial layers are characterized by dense networks of threading defects extending from the substrates to the surfaces [31], the strains in pseudomorphic epi-films relieve mostly, which means the “residual” effect of piezoelec- tric field is greatly reduced. As a result, the strain- induced piezoelectric effect can be neglected here. Figure 2 The VBM and CBM line-up of InN/diamond heterojunction at room temperature. A type-I band heterojunction is formed in straddling configuration.