Core level analysis of the KGaP(110) interface

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Surface Science 317-379 (1997) 233-237

Core level analysis of the K/GaP( 110) interface S. D’Addato a,*, P. Bailey b, J.M.C. Thornton ‘, D.A. Evans d aIstituto Nazionale di Fisica della Materia and Dipartimento di Fisica, Universitd di Modena, Via G. Campi 213/a, 4110 Modena, Italy ’ Interdisciplinary

b CCLRC, Daresbury Laboratory, Warrington WA4 4AD, UK Research Centre for Surface Science, University of Liverpool, P. 0. Box 147, Liverpool L69 3BX, UK d AMRL, Athrofa Gogledd Ddwyrain Cymru (NEWI), Wrexham LLII 2AW, UK

Received 1 August 1996; accepted for publication 15 October 1996

Abstract

The results of a photoelectron spectroscopy study of the K/GaP( 110) interface grown at T= 120 K are presented for the hrst time. The analysis shows a big core level shift at lower binding energies (AB= 1.6 eV at a coverage 0 =O.l ML) for the P 2p core level spectra. We interpret this result in terms of a charge redistribution affecting both types of atoms of the first substrate layer, suggesting some differences from the intuitive picture of strong ionic localised bonding between the alkali and the surface Ga atom. Keywords:

Alkali metal; Metal-semiconductor

interface; Photoelectron

1. Introduction

The adsorption of alkali metals (AM) on cleaved III-V semiconductor surfaces is a subject that has been studied extensively [ 1,2], mainly because it is believed that this class of system is suitable as a model to explain the mechanism of Schottky barrier formation. The main reasons for these assumptions are the simple electronic configuration of the AM atoms (a single s valence electron) and detailed knowledge of the properties of the [llO] surfaces of the III-V compounds. Nevertheless, it was found that these systems are not simple. In fact the electronic and structural properties exhibit a surprising variety of effects, depending strongly on the conditions in which the AM-Semiconductor interfaces are grown (e.g. the * Corresponding author. Fax: + 39 59 367488; e-mail: [email protected]

spectroscopy

substrate temperature [3,4]) and also on which AM is deposited at the surface. The results of the many theoretical and experimental studies on AM/GaAs( 110) (reviewed in Ref. [2]) have shown that there is a very large amount (between 0.7 and 0.9e) of negative charge transferred to the substrate per K atom and that this charge is mainly displaced on the Ga dangling bond. The correlation effects and the Mott insulating state of the interface observed experimentally [5] and obtained with electronic structure calculations [6-S] are a consequence of this strong localisation. For many recent studies, the charge distribution around As surface atoms seems to be affected very little by the AM adsorption, although a theoretical paper by Fong et al. [9] showed that there is a polarization of the As dangling bond in the case of Na/GaAs( 110). Few experimental or theoretical studies have been extended to the adsorption of AM to the

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(1 ;O) surfaces of other III-V compounds, especially in the case of Gap. In order to have a more systematic knowledge of the role played by the different physical and chemical characteristics of the substrate atoms, we have performed the first experimental study on K/GaP( 110). The technique we used in this investigation is photoelectron spectroscopy (PES) with synchrotron radiation from the Ga 3d and P 2p core levels.

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2. Experimental All the experiments were performed at beamline 6.1 of the Synchrotron Radiation Source, Daresbury Laboratory (UK), in a chamber with a base pressure of 7 x lo-l1 mbar. GaP single crystal bars were cleaved to obtain clean surfaces and kept at low temperatures (T= 120 K) with liquid nitrogen. K layers were deposited from getter sources (SAES Getters, Italy). The cleanliness of the surface was checked by looking at the photoemission from the valence band (VB). The photoemission spectra were taken with a PerkinElmer double-pass cylindrical mirror analyser. We took PES spectra from the Ga 3d core levels, from VB (photon energy hv = 80 eV) and from the P 2p core levels (hv = 160 eV), with an escape depth fo: the P 2p and Ga 3d photoelectrons of about 5.5 A [lo]. The experimental resolution (measured from the Fermi edge of a thick Ag film deposited on the sample) was found to be approximately AE = 0.24 eV (Full Width at Half Maximum, FWHM) at hv= 80 eV and AE=O.45 eV at hv= 160 eV.

3. Results and discussion In Figs. 1 and 2 we plot the evolution of the P 2p and Ga 3d core level spectra, together with the result of a least square fitting (LSF) analysis. The rigid shifts in energy of the spectra at different coverages are caused by band bending and surface photovoltage (SPV [ 111). We observed a feature in our VB spectra in the bulk gap energy region, but since it is not very clear in our data, it is ditlicult to correct the relative Fermi position to

Kinetic Energy (eV) Fig. 1. P 2p core level photoemission spectra taken after different K evaporation times (dots) and fitting results (continuous line) with the relative components B, S and K (dashed line).

account for the SPV effect, as was done previously with similar systems [ 12,131. On the basis of the attenuation curves of the core level signal shown in Figs. 3a and 3b, and of the work function changes measured from the secondary electron cutoff (not shown here), we can deduce that 0 = 1 ML of K was deposited on the surface after 8 min of dosing, and a saturation coverage of 0 = 1.4 ML was reached after t = 16 min. In order to obtain some insight into the core level data set, we performed a least square fitting analysis of the spectra. The P 2p and Ga 3d core level spectra of the clean surface were fitted with two doublets, a bulk (B) and a surface core level shifted (S) component (with a BE shift AEs of -0.40 for P 2p and 0.31 for Ga 3d, respectively). A third-order polynomial was used to approximate

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the secondary electron background. Each doublet consists of two spin-orbit (SO) split Lorentzian peaks convoluted with Gaussian lines, in good agreement with previous experimental results [lo]. The P 2p and Ga 3d spectra at increasing coverage could be best fitted with three components: the B component, the clean S component and a K-induced (K) component. The intensity, the energy position and the Gaussian breadth of each component were allowed to vary for the different spectra. The Ga 3d spectrum taken at 0 = 1 ML was fitted with one additional component to take into account an additional feature, probably caused by the surface reaction [ 3,4,13]. The most noticeable result is the large value of the K component energy shift at low coverage (AE= 1.6 eV) for the P 2p spectra. This energy shift decreases at increasing coverage, going to AE=0.7 eV at 0 = 1 ML. For 0 = 1 ML and 0 = 1.4 ML, the spectra look very similar to each other, with a high kinetic energy shoulder, a higher secondary electron back-

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Fig. 3. (a) Total intensity of the P 2p core level spectra (full circles), intensity of the surface component (triangles) and of the K-induced component (squares) plotted versus K dosing times. (b) Same as (a) for Ga 3d core level spectra. The values are joined by line segments as a guide to the eye.

ground and the appearance of satellite structures (not shown here) which we attribute to plasmon features, indicating the incipient metallicity of the interface. The Ga 3d core level spectrum follows a similar behaviour, with the exception of the appearance of the reacted component at high coverage. Although the binding energy (BE) of the K 3p core level is very close to that of the Ga 3d (BE =

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18.3 eV for metallic K [3]), the LSF analysis remains good without taking into account the presence of the K 3p peak. This can be explained by the fact that the photoionisation cross-section (0) of the K 3p core levels at the photon energy used in this experiment is lower by a factor of 6.5 than the corresponding g of the Ga 3d [ 141. The total (It> and partial intensities of the different components normalised to 1, are plotted versus the dosing time in Fig. 3. The S component intensity diminishes gradually, and it is not detectable any more at t =8 min (1 ML), while the K-induced component increases and tends to a constant value after t =8 min, revealing the absence of a high degree of interface reactivity. The behaviour of the S components for Ga 3d and P 2p suggests that the surface relaxation changes gradually without a complete removal in the submonolayer coverages. We interpret the K-induced structures in our spectra in terms of initial state components, as it was done in previous studies on similar systems [4,12,13]. In particular, they can be ascribed to charge transfer from the AM atom to the substrate, as in the case of the Na/GaP interface. We relate the BE increase of the K-component with K coverage to a charge redistribution caused by the growing adatom-adatom interaction. In earlier PES studies of AM on GaAs( 110) an AM-induced shifted component was observed only in Ga 3d core level spectra [ 31. This result was seen as confirmation of the strong local character of the bonding between the AM and the surface Ga atom. In this framework, the AM valence s electron is donated to the Ga-derived empty DB state, and a simple estimation of the Ga 3d AM-induced core level shift was done in terms of the presence of a surface dipole potential [2]. We believe that the surface As and P atom electronic environments in the (110) surfaces of III-V compounds are affected by this charge redistribution between the adatom and the substrate. Some reasons why this effect is more evident in the AM/GaP( 110) interface can be a smaller surface unit cell, a different adsorption geometry or a higher degree of delocalisation of the empty DB state occupied by the valence electron. A comparison with the Na/GaP( 110) system

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reveals that the energy shift (with respect to the B component) of the K peak in our P 2p spectra is much higher at low coverage than that of the Na component in the corresponding surface, while the shift in the Ga 3d is slightly lower (equal to 0.75 eV). If the AM-induced component is explained only as due to charge transfer, the shift of both sets of spectra should show the same behaviour. In fact, with K being less electronegative than Na, there should be a larger amount of charge transfer to the substrate, and the core level energy shifts should be in the same direction. We thus deduce that there are some differences in adsorption and interface formation. The main possibility is a slightly different K adsorption site, or the bigger size of the K than the Na atom, the ionic radius of the former being 35% larger than the latter.

4. Conclusions We have shown the results of a PES experiment on the formation of the K/GaP( 110) interface at low temperatures (T= 120 K). We observed that at this temperature a saturation coverage is achieved, corresponding approximately to 1.4 ML of K. The interface shows a minor degree of reactivity only at higher coverage. At low coverage there is a shifted component in the P 2p and Ga 3d core level photoelectron spectra which we assign to charge transfer from the adatom to the substrate. This K-induced component is present in both sets of core level spectra, revealing that there is a valence charge redistribution in the whole surface, contrary to what was deduced from earlier results for the AM/GaAs( 110) interface. A comparison with the results obtained from the Na/GaP interface shows an overall similarity, but there are some differences, possibly due to a larger size of the K atoms and a slightly different adsorption geometry. More experimental and theoretical work is needed to test these two hypotheses.

References [l] Rodolfo Miranda, in: Physics and Chemistry of Alkali Metal Adsorption, Fxls. H.P. Bonzel, A.M. Bradshaw and

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G. Ertl(l989) p. 425; P. Soukiassian and H.I. Starnberg, ibid., p. 449; M. Prietsch et al., ibid., p. 469. F. Bechstedt and M. Scheffler, Surf. Sci. Rep. 18 (1993) 145. D.M. Prietsch, M. Domke, C. Laubschat, T. Mandel, C. Xue and G. Kaindl, Z. Phys. B 74 (1989) 21. D.A. Evans, P. Bailey, J.M.C. Thornton and S. D’Addato, Surf. Rev. Lett. 1 (1994) 597. L.J. Whitman, J.A. Stroscio, R.A. Dragoset and R.J. Celotta, Phys. Rev. Lett. 66 (1991) 1338. G. Allan and M. Lannoo, J. Vat. Sci. Technol. B 9 (1991) 2135. 0. Pankratov and M. Scheffler, Phys. Rev. Lett. 70 (1993) 351.

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