XPS evidence for band bending at semiconducting oxide surfaces

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Surface Science 122 (1982) L643-L647 North-Holland Publishing Company

SURFACE

SCIENCE

LETTERS

XPS EVIDENCE FOR BAND BENDING AT SEMICONDUCTING OXIDE SURFACES R.St.C.

SMART

School of Science, Griffith Unioerslty, Nathan. Received

20 September

Queensland 41 II, Australia

1982

The magnitude of the surface potential, directly influenced by defect properties and donor or acceptor chemisorption. has been studied using XPS for p-type nickel oxide surfaces. Above 200 W. the KE shifts are independent of X-ray intensity. A simple explanation of the dependence of the surface potential (I/;,,) on band bending (V,) is proposed with preliminary experimental evidence allowing derivation of values of V,.

The chemistry of oxide surfaces has been shown to depend in large part, on defect structure of the surface [ 1,2]. Defects present as atomic imperfections, i.e. low coordination sites at steps, ledges, etc., or compositional imperfections, i.e. impurities, dopants, can be crucial in adsorption and catalytic processes. Additionally, however, the great majority of oxides are semiconductors. In this case, the changes in charge carrier concentrations at the surface caused by band bending are fundamentally important to charge transfer processes. The extent of band bending is difficult to measure by any reliable, straight forward technique. X-ray photoelectron spectroscopy is one of the most useful techniques available to follow changes in surface electronic structure. XPS studies on bulk oxide samples have been regarded, however, as rather unreliable due to the problem of surface charging, producing kinetic energy (KE) shifts of all XPS lines, and leading to allegedly inaccurate binding energy (BE) values and uncertain interpretation. Earlier workers used electron flooding to remove the surface charge but this produced new uncertainties in effects on adsorbed species and on surface reactions. A series of investigations with Roberts has made this problem tractable [3-51. We have previously reported consistent KE shifts of O(ls), C(ls) and Ni(2p,,,) XPS spectra due to surface charging of polycrystalline nickel oxide prepared in different thermal pretreatments [3]. The magnitude of the surface potential, after evacuation of the sample in the spectrometer at 500°C is directly related to the defect concentration in this p-type semiconducting oxide [3,4]. The effect of chemisorption of electron acceptor (i.e. NO, 0,) and 0039-6028/82/0000-0000/$02.75

0 1982 North-Holland

electron donor (i.e. CO) molecules on the free hole concentration of the semiconductor surface has also been observed by monitoring changes in the surface potential (3,4]. Thus, NO and 0, adsorption, as negatively charged NO- and 02- species, substantially reduces the surface potential whereas CO adsorption, as CO*+ species, increases surface potential on all NiO surfaces. Values between 0.0 eV and about 5 eV have been recorded. These values are reproducible for a particular sample using defined pretreatment and adsorption conditions. To explain the systematic variation in surface charge observed with different oxide pretreatment and adsorption processes, changes in band bending have been suggested [3-51. The question of interest is the extent to which the value of the potential Vch is determined by the photoemission processes (i.e. sample charging) as opposed to band bending (i.e. V,). The first would be expected to depend on the particular spectrometer and spectrometer conditions chosen but the second is an intrinsic property of the solid surface and can contribute valuable information. To examine this question, a sample of nickel oxide heated to 1450°C (i.e. NiO,,,,) for 4 h in air (i.e. prepared as in previous work [3-51) was used. The purity, surface morphology and defect properties of this preparation have been reported [6,7]. The powder was pressed into a cavity in a Cu mount. XPS spectra were recorded on a Perkin Elmer Physical Electronics PHI 560 Spectrometer using Al Ka radiation. Before recording spectra, the sample was heated to 45O’C for 20 min, evacuated to < 1 X lOme Torr and re-cooled to room temperature. X-ray intensity was varied by varying the power between 200 and 400 W and this variation was used to obtain a correlation with KE shift, measured using the procedure described previously [3,4]. The results are summarised in table 1. The values are in good agreement with those obtained for the same sample using a Vacuum Generators ESCA-3 spectrometer at 400 W, i.e. 3.3-4.3 eV [3]. The C(ls) spectra show two components, a smaller unchanged peak at binding energy 284.9 eV and a peak from carbon contamination on the

1 Ni%5o evacuated Table

X-ray intensity

45O’C for 20 min. pressed Surface

charge

into Cu cavity Signal strength

(eV)

(W)

400 300 200 ‘) Above background

C(ls)

O(ls)

Ni(2p,,,)

3.6 3.6 3.6

3.6 3.1 3.1

3.4 3.5 3.5

8372 6328 4217

‘) (cps)

FWHM of O( Is) (eV)

2.3 2.3 2.3

R.St.C. Smart / XPS evidence for band bendrng

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charged surface near apparent binding energy 288.5 eV. The 0( Is) spectra have three components, at apparent binding energies of 530.4, near 533.4 and near 535.2 eV. From our previous work, the last two peaks correspond to oxygen ions, 02- and O- , found at 529.7 and 531.4 eV respectively on uncharged and on charged surfaces. The first peak at 530.4 eV obviously represents an oxygen species, probably 02-, in an environment of much lower charge (- 0.7 eV). This may occur close to the metal support. Similar peaks were observed for NiO ,450 O(ls) spectra from our previous ESCA-3 work [3,4]. The Ni (2p,,,) spectra show no evidence for reduction to nickel metal. The important result from table 1 is that the KE shift appears to be independent of X-ray intensity above 200 W. It is worthwhile to briefly review the origin of surface charge in XPS spectra. The systematic experimental work of Ebel and Ebel [8], with thin, discontinuous metal films on insulating substrates, has established that there are three contributions to the charge flow: (a) Electron emission from the sample (i,) caused by the X-ray photon flux and hence, i, is proportional to the X-ray tube current (I) and to the X-ray tube voltage V, i.e. i, = K,I and i, = K,V. (b) Secondary electron emission from the X-ray tube window (i2) principally caused by back scattered electrons from the X-ray target with i, proportional to I but independent of V in the 3-6 kV range, i.e. i, = K,I and Z, = K,. (c) Electron conduction to the sample surface (i3). The significant differences between their studies and ours are that they used a different XPS spectrometer (McPherson ESCA 36) but, more importantly, that band bending was not a significant factor for their metal samples. In contrast, our experimental results can be summarised as follows: - At constant X-ray flux (n), conductivity to the surface (a) is changed by band bending (V,) from the flat band potential due to electron-donor or electron-acceptor adsorption. We expect that u should be proportional to exp( - eV,/kT). - At constant X-ray flux (RI), conductivity to the surface is changed by bulk defect concentration changes; i.e. by bulk conductivity changes. - At constant V, (i.e. no change in adsorption or defect levels), conductivity to the surface is changed by X-ray flux to maintain the surface potential (Vch) constant. We expect that u should be proportional to n, or oru1 and WV. Suppose now for the dependence of 0 on I, we have o=A(c~,)+B(u,)exp(-eV,/kT)Z, where aa is the bulk conductivity of the sample, A(u,) is the conductivity with no X-ray flux, B( q,) is a constant for a particular sample and i, = V&/R = V&T, then, according to Kirchoffs Law, i, = i, + i,, giving K,I=K,I+ =K,I+

v,,,u V,,[A(u,,)+B(u,)exp(-eV,/kT)I],

L646

R.St.C.

Smurr / XPS evidence for hand hendq

so that

I/,,=(K,-K,)I/[A(a,)+B(ao)exp(-eV,/kT)Il. At low values of 1, this reduces to K,, = (K, - K,) Z/A(u,). Ebel and Ebel [8] found a linear relationship of the surface potential with I in the case where V, is not significant and I is < 5 mA. At high values of I. V.ch =

(K’- K3) exp( eV/kT). B(%)

s

Fig. 1 shows a plot of In Vch against l/T for NiO,,,, obtained with the VG ESCA-3 at Bradford University. The linear portion of the curve gives a slope of 2.68 x lo3 K from which the value of Vs = 0.231 V is obtained. This value appears to be reasonable in comparison with values from the literature. At high temperature, the deviation from linearity may be due to increasing influence of a, which is itself temperature dependent. At low temperature, a mechanism like Zener breakdown giving a large i, when Vchapproaches the band gap has been suggested [9]. For adsorbate-free single crystal samples, Ley et al. [9] have found that the magnitude of the potential in II-VI semiconductors is close to the band gap (i.e. - 5 eV for NiO). The potential V,, depends on the bulk conductivity, in accord with experimental results, but it is clear that the dependence is not simple since our results [3] have shown that, for NiO,,,, Vch- 1.1 eV with estimated a, = 3.1 X IO-' 9-l m-’ whilst, for NiO,,,,, P’&= 3.8 V with estimated (Jo= 5.0 X 10 ' L'~'

I/T

lx 10-3K-’

Fig. 1. Dependence of Vfh on temperature, plotted pre-evacuated at 45O’C in the ESCA-3 spectrometer.

as In V,, versus

l/T.

for a NiO,,,O

sample

R.St.C. Smart / XPS eoidencefor band bending

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m- ‘. A functional dependence on a, is indicated, but qualitatively we see that high a, reduces &, and low a0 increases k& as observed experimentally. The close correspondence between charge values on the same sample treated by the same procedures but determined on different spectrometers (ESCA-3 and PHI 560) is important evidence that changes in surface charge can be used to follow changes in band bending caused by surface reactions, adsorption processes or semiconductor modification. The removal of surface charge by electron-accepting adsorbed species as when NO adsorbs on NiO,,,, giving a &, 3.6 to 1.4 V change, then corresponds to a change in V, of 0.23 1 to 0.207 V. The increase of surface charge by electron-donating adsorbed species, e.g. CO adsorbed on Ni idso gives I$, 3.6 to 4.3 V change, corresponds to V, change of 0.231 to 0.236 V. The systematic variation of surface charge with estimated conductivity of t’ne sample further supports this simple model. The work to test the relative contributions of charging mechanisms arose from a helpful suggestion of one of the referees for our earlier work [4]. The spectra were kindly obtained by Dr. W.C. Johnson at Perkin Elmer Physical Electronics Division Laboratories, Eden Prairie, Minnesota, USA.

References [I] [2] [3] [4] [5] [6]

G.T. Surratt and A.B. Kunz, Phys. Rev. B19 (1979) 2352. V. Bermudez, Progr. Surface Sci., 11(1981) I. M.W. Roberts and R.St.C. Smart, Chem. Phys. Letters 69 (1980) 234. M.W. Roberts and R.St.C. Smart, Surface Sci. 100 (1980) 590. M.W. Roberts and R.St.C. Smart, Surface Sci. 108 (1981) 271. C.F. Jones, R.L. Segall, R.St.C. Smart and P.S. Turner, J. Chem. Sot. Faraday I, 73 (1977) 1710. [7] C.F. Jones, R.L. Segall, R.St.C. Smart and P.S. Turner, J. Chem. Sot. Faraday I, 74 (1978) 1615. [8] M.F. Ebel and H. Ebel, J. Electron Spectrosc. 3 (1974) 169. [9] L. Ley, R.A. Pollak, F.R. McFeely, S.P. Kovalczyk and D.A. Shirley, Phys. Rev. B9 (1974) 600.