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Cite this: Chem. Commun., 2014, 50, 1854 Received 20th November 2013, Accepted 18th December 2013

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Transparent p-type epitaxial thin films of nickel oxide† Pengfei Zhai,a Qinghua Yi,a Jie Jian,b Haiyan Wang,b Pingyuan Song,a Chao Dong,c Xin Lu,a Yinghui Sun,a Jie Zhao,a Xiao Dai,a Yanhui Lou,a Hao Yanga and Guifu Zou*a

DOI: 10.1039/c3cc48877b www.rsc.org/chemcomm

Transparent p-type nickel oxide (NiO) thin films have been epitaxially grown on (0001) Al2O3 substrates by a chemical solution method of polymer-assisted deposition for the first time. The films have a high optical transparency of above 95% in the wavelength range of 350–900 nm.

Nowadays transparent conducting oxides (TCOs) such as indium tin oxide (ITO), Al-doped zinc oxide and F- or Sb-doped tin oxide are widely used for transparent electrodes and window coatings.1,2 Most of the TCOs are n-type semiconductors resulting from intrinsic defects and extrinsic donors.3 However, the development of p-type TCOs is also critical to fully explore the potential applications of TCOs ranging from optoelectronic devices, smart windows to transparent electronic devices.4 To date the reports on the preparation of such p-type TCOs have been rare. With the band gap (Eg) from 3.6 to 4.0 eV,5,6 nickel oxide (NiO) thin films are a promising candidate for p-type TCOs in the visible regime because of their attractive electrical, optical, magnetic and thermoelectric properties as well as high chemical resistance.7 These unique properties of NiO films allow them to be used in many applications such as formation of p–n junctions with n-type ZnO,8 smart windows,9 solar cells,10,11 light emitting diodes12,13 and gas sensors.14 Generally, NiO thin films are prepared as randomly oriented polycrystallites with numerous defects such as grain boundaries and dislocations which act as trap sites for carriers, thus significantly reducing charge carrier mobility.15 Therefore, it is desirable to grow high-quality epitaxial NiO thin films with excellent properties. Some physical deposition methods such as pulsed laser deposition16,17 and magnetron sputtering15,18 have been used to obtain epitaxial NiO films. It is well known that chemical solution deposition is a

School of Energy and Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou 215000, P.R. China. E-mail: [email protected] b Department of Electrical and Computer Engineering, Texas A & M University, College Station, TX 77843, USA c Department of Chemistry and Chemical Biology, The University of New Mexico, Albuquerque, NM 87131, USA † Electronic supplementary information (ESI) available: Experimental details, characterization, and XPS examination. See DOI: 10.1039/c3cc48877b

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generally considered as a cost-effective thin film growth method since it does not require vacuum equipment. This method also has the ability to coat large areas and 3D objects. Up to now, several typical chemical approaches, including the sol–gel technique,19,20 liquid phase deposition,2 electrochemical deposition21 and chemical bath deposition,22 have been employed to grow NiO films. Despite the significant efforts, growing high-quality epitaxial NiO films by chemical solution deposition remains a challenge. Polymer-assisted deposition (PAD) is a new chemical solution method, especially developed to grow high-quality epitaxial thin films.23,24 Three key elements enable the epitaxial growth of the films using the PAD method. Firstly, the polymer, binding metal ions and controlling the solution’s viscosity result in the stabilization and homogeneity of the precursor solution. Secondly, filtration to remove unbound metal ions and unwanted components ensures well-controlled nucleation and a pure phase. Finally, the annealing process provides the highquality grown film with desired structural and physical properties. In this communication, we present the epitaxial growth of NiO thin films on (0001) Al2O3 substrates by PAD. To the best of our knowledge, this is the first report on the epitaxial growth of the transparent p-type NiO thin films by a chemical solution method. The demonstration of epitaxial growth of NiO using a chemical method provides not only a material platform to explore the film intrinsic properties but also a great alternative thin film growth method in the field of the transparent p–n junction and its derived devices. Fig. 1 shows the X-ray diffraction (XRD) results from the y–2y scan and the j scan for the NiO thin films. As shown in Fig. 1a, only the (111) and (222) peaks of NiO and the peaks of the Al2O3 substrates are observed. The appearance of only (lll) diffraction peaks indicates that the NiO thin film is of single phase with a preferential orientation. Fig. 1b shows the j scans of reflections of NiO (200) and Al2O3 (104). As shown in the j scans, the NiO thin film is aligned in-plane with the Al2O3 substrate as well, except for a 301 rotation with respect to Al2O3. Six peaks with an average full width at half-maximum (FWHM) value of 1.11, as compared to a value of 0.91 for the single crystal Al2O3 substrate, indicate that the film has good epitaxial quality. The in-plane alignment between the film and the substrate can be understood by considering the crystal structures of both NiO and Al2O3.

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Fig. 1 XRD patterns of a NiO film on the (0001) Al2O3 substrate: (a) y–2y scan; (b) j-scans of (200) reflection of NiO and (104) reflection of Al2O3.

Cubic NiO has a space group of Fm3m. Six peaks in the j scan are expected for epitaxial (111) NiO films. Meanwhile, three peaks in the j scan are consistent with the rhombohedral structure with a space group of R3% m for the (0001) Al2O3 substrate. A 301 rotation with respect to Al2O3 is expected by considering the lattice match between the film and the substrate. The heteroepitaxial relationships between the NiO film and the Al2O3 substrate can be described as (111)NiOJ(0001)Al2O3 and [2% 11]NiOJ[12% 10]Al2O3. In addition, the ratio of Ni and O in the NiO film could be estimated to be 1.3 : 1 on the basis of XPS measurements in the ESI.† High resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) were performed on the NiO thin film grown on the c-cut Al2O3 substrate. Fig. 2a shows the HRTEM image of the interface of NiO and Al2O3 taken along the Al2O3 [112% 0] zone axis. The HRTEM image shows epitaxial growth of NiO film along the (111) orientation. The bright contrast at the interface indicates a possible lattice strain relaxation for the initial epitaxial growth. The corresponding SAED pattern of the interface area with the same zone axis was recorded to confirm the orientation relationships between the film and the substrate. There are mainly two sets of diffraction patterns: one set attributed to the Al2O3 substrate and another set attributed to NiO film, as indexed in Fig. 2b. Diffraction dots from NiO are sharp and distinguishable, indicating high-quality growth of the NiO film on the c-cut Al2O3 substrate. The epitaxial relationships between the film and the substrate determined from the SAED patterns are (111)NiOJ(0006)Al2O3 and (022% )NiOJ(2% 200)Al2O3 as shown in Fig. 2b, which are consistent with the XRD results. The surface morphology of epitaxial NiO thin films was investigated by scanning electron microscopy (SEM) and atomic force microscopy (AFM). As shown in Fig. 3a, the SEM image indicates

Fig. 2 (a) A cross-sectional HRTEM image taken from the interface between the NiO film and the Al2O3 substrate; (b) corresponding SAED patterns of NiO and Al2O3.

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Fig. 3 (a) SEM image and (b) AFM image (1 mm  1 mm) of the epitaxial NiO thin film on the (0001) Al2O3 substrate. The inset shows SEM cross-section of the epitaxial NiO thin film on the (0001) Al2O3 substrate.

that the film is uniform with no detectable micro-cracks. The average value of the grain size is estimated to be 30.0 nm (not shown here). The cross-section of the NiO film in the inset shows that the film is flat and the thickness is observed to be around 50.0 nm. The crosssection also shows the dense film from another direction. The AFM image of epitaxial NiO films is shown in Fig. 3b. Huge quantities of grains with a size of about 30.0 nm are observed on the film surface, which is consistent with the result from SEM. Meanwhile, the film has a root-mean-square (rms) surface roughness of 0.7 nm, which also proves that the epitaxial NiO thin film is flat and uniform. Surprisingly, as-grown NiO films have a distinguished transparency and show an optical transparency as good as the pristine glass. Fig. 4a shows the photograph of the NiO thin film on a labeled paper, and the symbol on the paper can be clearly observed. The film has an optical transmittance of above 95% in the 350–900 nm range (Fig. 4b), which indicates that the film is transparent. A sharp absorption edge is observed at the wavelength of around 350 nm. The fundamental absorption resulting from the electron excitation from the valence band to the conduction band has been widely used to determine the optical band gap of semiconductor materials.3 The relationship between the optical absorption coefficient (a) and the photon energy (hn) is given by the following equation: (ahn) = A(hn

Eg)m

(1)

where A is a constant, Eg is the optical band gap width of the materials and m depends on the nature of the electronic transition. The value of m is 1/2 for a direct band transition and m = 2 for an indirect band transition. As shown in the inset of Fig. 4b, a linear

Fig. 4 Optical transmittance measurements of an epitaxial NiO film on the (0001) Al2O3 substrate: (a) transparent film on a labeled paper; (b) UV-Vis spectroscopy. The inset shows the (ahn)2 versus hn plot.

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Fig. 5

The temperature dependence of resistivity of the NiO film.

relationship between (ahn)2 and hn indicates a direct energy band gap of the epitaxial NiO film. The optical band gap width is estimated to be 3.71 eV by extrapolating the straight-line portion of the plot at a = 0. This value is in the proximity of 3.6–4.0 eV reported for NiO films.2,5,6,15,18,20 As shown in Fig. 5, the NiO thin film shows semiconductive resistivity versus temperature behavior, that is, the resistivity decreases with increasing temperature. The NiO thin film has a room temperature resistivity of 117.5 O cm. In addition, the Hall effect measurement of the NiO thin films has also been carried out using a physical property measurement system. The Hall coefficient is +20.2 cm3 C 1, which suggests the semiconduction to be p-type. Usually, highly resistive NiO films prepared by pulsed laser deposition or magnetron sputtering were also reported.15,16 There are two main possible mechanisms to explain the high resistivity of epitaxial NiO thin films. The first scenario is based on the electrical conduction from the grain boundaries.25,26 That is, the electrical resistivity should increase as the number of the grain boundaries decreases. According to the above structural analyses of HRTEM and SAED, the grown NiO film shows greatly epitaxial character, and there is no visible grain boundary in the films. Therefore, the fewer grain boundaries might lead to the highly resistive properties of the NiO films. The other possible scenario is based on tailing the band edge via the degradation of the entire film crystallinity.16 Imperfections or defects in thin films not only work as trap sites but cause band tailing, and these bands merge with the nearest parent bands. The high crystallinity of epitaxial NiO films can reduce the extended states. As a result, it causes higher resistivity.15,27 In our case, characterization results (including XRD and HRTEM) show that as-grown NiO thin films have high crystallinity. Therefore, the high resistivity of the NiO thin films might be closely related to the high crystallinity. To improve the conductivity of the NiO thin films, a lot of work has been done by other groups such as adjusting the thickness of the film,2 increasing the substrate temperature19 and doping the NiO films with Li, K, and Al.28–30 The experiments of doping the epitaxial NiO thin films by PAD are underway. It is expected to effectively improve the electrical conductivity of the films so that epitaxial NiO thin films would be practically applied in TCO devices. In summary, we reported the first demonstration of the growth of epitaxial NiO thin films on (0001) Al2O3 substrates by a chemical solution approach. XRD and HRTEM results have confirmed the desired crystal and microstructures of the films. The epitaxial relationships between the film and the substrate are (111)NiOJ(0001)Al2O3 and [2% 11]NiOJ[12% 10]Al2O3. The surface morphology of the films is smooth

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and uniform. At the wavelength range of 350–900 nm, the optical transparency of the film is above 95%. The energy band gap of the epitaxial NiO film is estimated to be 3.71 eV. We gratefully acknowledge the support of the National Natural Science Foundation of China (21101110, 1004145, 11274237, 51228201), Key Program of Science and Technology of Ministry of Education of China (212063), Jiangsu Specially-Appointed Professor Program (SR1080042), and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). The TEM work at Texas A&M University is funded by the U.S. National Science Foundation (NSF-0846504).

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Transparent p-type epitaxial thin films of nickel oxide.

Transparent p-type nickel oxide (NiO) thin films have been epitaxially grown on (0001) Al2O3 substrates by a chemical solution method of polymer-assis...
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