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* Physics of XPS 

X-ray Photoelectron Spectroscopy is a quantitative analysis tool that were discovered by K. Siegbahn in the mid of 1960. As illustrated in Figure 1, Either incoming eletron or X-ray with sufficiently high energy is impinge or interact with eletrons in different shells and a material is charaterized by the Binding Energy(BE) and the formula is BE = hv - KE - F.

Figure 1 - Principle of XPS
                                                                                                                                                                                                                                                                                           
 
 
 
 
 
 
 
 
 
 
 
XPS is used to measure:

elemental composition of the surface (top 1-10 nm usually)
empirical formula of pure materials
elements that contaminate a surface
chemical or electronic state of each element in the surface
uniformity of elemental composition across the top surface
uniformity of elemental composition as a function of ion beam etching

Example of XPS
X-ray photoelectron spectroscopy study on the CrN surface grown on sapphire substrate to control the polarity of ZnO by plasma-assisted molecular beam epitaxy
- Decripion of sample preparation

Before growing CrN PSL, (0 0 0 1) Al2O3substrates were degreased in acetone and methanol for 10 min at room temperature. Chemical etching was performed by using H2SO4:H3PO4 = 3:1 solution at 160 _ C for 10 min. After finishing the surface etching, the substrate was rinsed in deionized water for 1 min then dried by a spin drier. We have used two-chamber molecular beam epitaxy (MBE) system, which has III-Nitride and II-oxide chambers connected with vacuum transfer chamber. Thermal treatment was performed in an III-Nitride MBE chamber for 10 min at 800 _C. CrN films (_7 nm) were grown at 700 _ C by using a solid source Cr effusion cell and nitrogen plasma cell. After that the samples were transferred into II-oxide MBE chamber through the vacuum transfer chamber, then surface treatment was performed in IIOxide chamber using Zn-knudesen cell and O-plasma source. Two specimens were prepared for this experiment [6,10]: sample-A is O-treated CrN/Al2O3, and sample-B is Zn-treated CrN/Al2O3. Otreatment was performed at 650 _ C for 10 min to form Cr2O3by oxygen plasma source with the oxygen flow rate of 1 sccm and RF power of 300 W. Zn-treatment was performed at 400 _ C for 20 min to prevent surface oxidation of CrN. Zn beam flux was monitored by a quartz crystal thickness monitor and controlled to be 1.6 A° /s. Each treatment condition was optimized and described in theprevious report [10]. Although, the samples were immediately transferred into X-ray photoelectron spectroscopy (XPS) chamber after taken out from load-lock chamber in MBE system, we have used nitrogen filled carrier box to minimize contamination by exposure to air. The XPS results were obtained by Al K radiation (1486.6 eV) (SSX-100, Surface Science Inc.). The spot size of the Xray was about 300 mm _500mm. The energy spectrum of the photoelectrons was analyzed by a hemispherical analyzer with anenergy resolution of 0.1 eV. In the measurement system, the binding energy of Au 4 f 7/2 was 84.0 eV and the full-width at halfmaximum (FWHM) Au 4 f 7/2 peak was 1.1 eV. The spectra were measured in the order of C, O, N, Cr, and Zn. We did not perform surface cleaning process, because it could modify the chemical state of the surface.


- Steps of XPS Study 


- Data Analysis
Figs. 1 and 2 show XPS spectra for Cr 2p and Cr 3s, respectively. In case of sample-A (O-treated CrN), Cr 2p 3/2 peak and Cr 2p ½ peak are observed at 576.6 eV and 586.3 eV, respectively (Fig. 1(a)). Those peaks are exactly matched with the reported peak positions of Cr2O3[7,11]. Also multiplet splitting of the Cr 3s line is observed (Fig. 2(a)). The Cr 3s peak is observed at 74.5 eV and Cr2O3peak arise from 79 eV, which also corresponds well with Ref. [12]. Those results can be regarded as a strong evidence for Cr2O3formation by oxygen-plasma treatment. While in case of sample-B (Zn-treated CrN), Cr 2p 3/2 peak and Cr 2p 1/2 peak positions (Fig. 1(b)) are mismatched with those for both Cr2O3and CrN. Also the splitting of Cr 3s peaks (in Fig. 2(b)) are incomplete. There have been a lot of studies on the chemical status of Cr-nitride and oxide compounds, since Cr-compounds are well known material for surface protection, The Cr 2p 3/2 peak was known to have various binding energies such as 574.1 eV (Cr), 574.5 eV (CrN), 575.3 eV (air exposed CrN), and 576.3 eV (Cr2O3) [11--13]. Note that the observed Cr 2p 3/2 peak position of sample-B (575.5 eV) is nearly matched with that of air exposed CrN. Those results are indicating contamination by air although we had paid special care to protect it. Interestingly, the Cr 3s spectra (Fig. 2) from both samples indicate an existence of metallic phase Cr with considerable composition and the reason will be discussed further in Section 3.2.Fig. 3 shows Zn 2p transition spectra, which clearly shows the difference of surface treatment process. Sample-A (Fig. 3(a)) displays negligible emission
References[1] D.M. Bagnall, Y.F. Chen, Z. Zhu, T. Yao, S. Koyama, M.Y. Shen, T. Goto, Appl. Phys.

Lett. 70 (1997) 2230.[2] A. Chowdhury, H.M. Ng, M. Bhardwaj, N.G. Weimann, Appl. Phys. Lett. 83 (2003)

1077.