Shi et al. Nanoscale Res Lett 2011, 6:50 http://www.nanoscalereslett.com/content/6/1/50 our experiment are undoped, while the InN films are unintentionally n-type doped, with carrier concentration and Hall mobility being 6.2 × 1019 cm-3 and 370 cm2/Vs respectively, as determined by Hall effect measurement in the InN/sapphire film. The XPS measurements were performed on a PHI Quantera SXM instrument with Al Ka (energy 1486.6 eV) as the X-ray radiation source, which had been carefully calibrated utilizing work function and Fermi energy level (EF). A large number of electrons are excited and emitted from the sample during the test, so the sample is always positively charged and the conse- quent electric field can affect the measured kinetic energy of photoelectron. A low-energy electron flood gun was utilized to achieve charge compensation. The total energy resolution of this XPS system is about 0.5 eV, and the accuracy of the observed binding energy is within 0.03 eV after careful calibration. The measure- ments were as follows: first, low-resolution survey scan mode was used to determine which elements were pre- sent on the sample surfaces. Then, very-high-resolution spectra were acquired to determine the binding energy (i.e., chemical state) in the survey spectra. Since only the relative energy position in each sample is needed to determine the VBO, the absolute energy calibration for a sample has no effect on the universal energy reference. Results and Discussion The VBO (ΔEv) can be calculated from the formula ΔEV = ΔECL − ( EIInn3Nd5/2 − E VInBNM ) + ( ECdi1asmond − E VdiBaMmond ) (1) where ΔECL = ( EIInn3Nd5/2 − ECdi1asmond ) is the energy difference between In3d5/2 and C1s core levels (CLs) in InN and diamond, which are measured in the InN/diamond heterojunction. ( EIInn3Nd5/2 − E VInBNM ) and ( ECdi1asmond − E VdiBaMmond ) are the InN and diamond bulk constants respectively, measured from the two cor- responding thick films. VBM stands for valance band maximum. The In3d5/2 spectra for the InN and InN/dia- mond samples, the C1s spectra for the diamond and InN/diamond samples, and the valence band photoemis- sion for both InN and diamond samples are shown in Figure 1. All peaks have been fitted using a Shirley back- ground and Voigt (mixed Lorentzian-Gaussian) line shapes. The position of the VBM with respect to the surface Fermi level was determined by the intersection of linear fitting to the leading edge of the valence band photoemission and the background [20]. All the Page 2 of 5 parameters deduced from Figure 1 are summarized in Table 1 for clarity. As illustrated in Figure 1a, e, the In3d5/2 core-level lineshapes are slightly asymmetric, with a high binding energy shoulder on the main peaks. This phenomenon has been reported by several groups [20-22]. Due to the high carrier density in unintentionally n-type doped InN and the surface electron accumulation effect, the photo- emitted electrons will lose energy by coupling with the free electron plasmas at the surface of the samples. As we know, plasmons lead to the quantization of a collec- tive excitation of the electron gas in a solid. In metals, however, Plasmon satellites are commonly observed in photoemission spectra of core-level peaks on the high binding energy side. The unrelexed Koopaman’s state produced by removal of a core electron is not an eigen- state and is projected onto “screened” and “unscreened” final eigenstates [21], the latter corresponding to a plas- mon satellite at higher binding energies than the screened state. The unscreened final state usually gives a peak with a broader Lorentzian peak profile whose width reflects the plasmon lifetime, which in turn depends on the conduction electron relaxation time [21]. According to this, we attribute the component with lower binding energy and smaller half-width to “screened” final-state, while that with higher binding energy and broader half-width to “unscreened” final- state, as is shown in Table 1. Indeed, the “unscreened” higher-binding energy components are much broader than the “screened” lower-binding energy components in our XPS spectra. Similar plasmon loss features have also been observed in the materials SnO2 when heavily doped with Sb [23,24], indium-tin-oxide [21] and PbO2 [25]. Wertheim [26,27] calculated the influence of sur- face plasmon to binding energy in narrow band metal, from which we can estimate that influence to our sys- tem. In Wertheim’s model, the surface plasmon energy, designated as  sp is considered to be ⎛ ne 2 ⎞ 1/2  sp = ⎜ ⎟ ⎝⎜ (  ( ∞ ) + 1 ) 0m* ⎟⎠ (2) Here n is the carrier concentration, ε(∞) is the high frequency dielectric constant, and m* is the effective mass of the conduction electrons. Compared with the metals in his model, the carrier concentration in our sample surface is much lower, which means that only small energy separations exist between the screened and unscreened core-level components. This results in an asymmetric core-level XPS lineshape with just a weak high binding energy tail due to plasmon losses, which is consistent with our experimental results.