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PHYSICAL REVIEW B 89, 165305 (2014)

Valence-band orbital character of CdO: A synchrotron-radiation photoelectron spectroscopy and density functional theory study J. J. Mudd,1,* Tien-Lin Lee,2 V. Mu˜noz-Sanjos´e,3 J. Z´un˜ iga-P´erez,4 D. J. Payne,5 R. G. Egdell,6 and C. F. McConville1,† 1 Department of Physics, University of Warwick, Coventry, CV4 7AL, United Kingdom Diamond Light Source Ltd., Harwell Science and Innovation Campus, Didcot, OX11 0DE, United Kingdom 3 Departamento de Fisica Aplicada y Electromagnetismo, Universitat de Val´encia, C/Dr. Moliner 50, 46100 Burjassot, Spain 4 Centre de Recherche sur lH´et´ero-Epitaxie et ses Applications, Centre National de la Recherche Scientifique, Parc de Sophia Antipolis, Rue Bernard Gr´egory, 06560 Valbonne, France 5 Department of Materials, Imperial College London, London, SW7 2AZ, United Kingdom 6 Inorganic Chemistry Laboratory, Department of Chemistry, University of Oxford, South Parks Road, Oxford OX1 3QR, United Kingdom (Received 4 February 2014; revised manuscript received 19 March 2014; published 11 April 2014) 2

N -type CdO is a transparent conducting oxide (TCO) which has promise in a number of areas including solar cell applications. In order to realize this potential a detailed knowledge of the electronic structure of the material is essential. In particular, standard density functional theory (DFT) methods struggle to accurately predict fundamental material properties such as the band gap. This is largely due to the underestimation of the Cd 4d binding energy, which results in a strong hybridization with the valence-band (VB) states. In order to test theoretical approaches, comparisons to experiment need to be made. Here, synchrotron-radiation photoelectron spectroscopy (SR-PES) measurements are presented, and comparison with three theoretical approaches are made. In particular the position of the Cd 4d state is measured with hard x-ray PES, and the orbital character of the VB is probed by photon energy dependent measurements. It is found that LDA + U using a theoretical U value of 2.34 eV is very successful in predicting the position of the Cd 4d state. The VB photon energy dependence reveals the O 2p photoionization cross section is underestimated at higher photon energies, and that an orbital contribution from Cd 5p is underestimated by all the DFT approaches. DOI: 10.1103/PhysRevB.89.165305

PACS number(s): 68.47.Gh, 71.15.Mb, 79.60.Bm

I. INTRODUCTION

CdO is a rocksalt metal oxide which is amenable to n-type doping resulting in high conductivity and optical transparency. It shows promise for device applications due to its ability to achieve high n-type carrier concentrations (∼1021 cm−3 ) and high electron mobilities (220 cm2 V−1 s−1 ) [1–3]. It is also a promising candidate to alloy with ZnO to form Cdx Zn1−x O, potentially achieving devices operating across the visible spectrum [4,5]. However, in order to exploit these potential applications a detailed knowledge of the electronic structure of both CdO and ZnO is required [6,7]. In particular, predicting the band gap of these materials is crucial for most applications. However, many current theoretical approaches predict CdO to be a semimetal [8]. This is a result of the underestimation of the binding energy of the Cd 4d states, which results in strong p-d hybridization in the valence band (VB) and a reduction of the band gap. There have been several theoretical attempts to correct this problem including the use of hybrid functionals [9,10] and GW quasiparticle calculations [11,12]. These approaches have had some success in predicting the location of the shallow Cd 4d states and the band gap; however, they are also computationally more complex and therefore less widely applied. An alternative approach is LDA + U [13–15], where an on-site, orbital dependent Coulomb term is added to the local density approximation (LDA) potential. This allows the position of the Cd 4d states to be corrected and should therefore improve the associated theoretical predictions.

*

[email protected][email protected] 1098-0121/2014/89(16)/165305(7)

In order to assess the merits of these theoretical approaches, comparisons with experimental data need to be made. Photoemission is an ideal technique for this as it essentially allows the VB density of states (DOS) to be observed. To the best of our knowledge there have been two previous publications comparing theoretical predictions to experimental photoemission data for CdO, King et al. [16] and Dou et al. [17]. This work significantly extends both of these comparisons, by both the use of additional theoretical approaches and synchrotron-radiation techniques. Synchrotron-radiation excited photoelectron spectroscopy (SR-PES) has been used to obtain both hard x-ray photoemission (HAXPES) data of the CdO VB and Cd 4d regions for comparison to density functional theory (DFT), and a series of CdO VB spectra over a range of photon energies to allow the VB orbital character to be investigated in more detail. With its higher incident photon energy, HAXPES is a preferred technique when comparing to theoretical calculations as it is more bulk sensitive than traditional photoemission [18], allowing for better comparison with the theoretical bulk calculations. It is also of benefit that photoionization cross sections are smoothly varying at high photon energies, allowing trends to be more easily observed. Theoretically three DFT functionals were chosen (LDA, PBE-GGA, and LDA + U), due to their widespread availability and relative computational simplicity.

II. EXPERIMENTAL DETAILS

Epitaxial CdO(100) single crystal thin films were grown on r-plane sapphire substrates using metal organic vapor-phase epitaxy (MOVPE), further details of which can be found 165305-1

©2014 American Physical Society

J. J. MUDD et al.

PHYSICAL REVIEW B 89, 165305 (2014)

elsewhere [21]. The sample was prepared in ultra-high vacuum (UHV) by annealing at 550 ◦ C for 45 min, an approach which has previously been shown to be very effective [16]. Prior to the PES measurements the surface order was confirmed by LEED, which showed a sharp (1 × 1) diffraction pattern indicating high surface quality. Additionally, no contamination was observed during the PES measurements. Photoemission spectra were obtained at the I09 beamline of Diamond Light Source, UK. The I09 beamline offers photon energies from 100 eV to 18 keV, achieved using two canted undulators, providing both soft and hard x rays focused to the same point on the sample. Hard x rays (hν  2500 eV) were monochromated using a cooled Si(111) double crystal monochromator. To maintain sufficient energy resolution at the two highest photon energies an additional Si channel-cut high resolution monochromator, Si(444) for hν = 7935 eV and Si(004) for hν = 6054 eV, was used. Soft x rays (hν = 600 eV)

were monochromated using a plane grating monochromator (300 lines/mm). The endstation is equipped with a VG Scienta EW4000 electron analyzer with ±30◦ angular acceptance. In the experimental geometry at I09 shown in Fig. 1(a), the photon beam is perpendicular to the electron emission direction. The photon beam is polarized in the plane of the orbit (horizontally) resulting in the electric vector being aligned with the electron emission direction. The sample was placed in a grazing incidence geometry (∼5◦ ), with the surface normal in the plane defined by the photon beam and electron emission direction (p-polarized), thereby significantly enhancing the count rate. The experiments were performed at room temperature. The total experimental energy resolution is