Vacuum 82 (2008) 1321–1324 Contents lists available at ScienceDirect Vacuum journal homepage: www.elsevier.com/locate/vacuum Structural and physical properties of Mg-doped CuAlO2 thin ﬁlms Guobo Dong a, Ming Zhang a, *, Wei Lan a, b, Peiming Dong a, Hui Yan a a b The College of Materials Science and Engineering, Beijing University of Technology, Beijing 100022, PR China Department of Physics, School of Physical Science and Technology, Lanzhou University, Lanzhou 730000, PR China a r t i c l e i n f o a b s t r a c t Article history: Received 18 September 2007 Received in revised form 31 March 2008 Accepted 4 April 2008 The CuAl1xMgxO2 (x ¼ 0, 0.01, 0.02 and 0.05) thin ﬁlms were successfully deposited on quartz substrate by using the RF magnetron sputtering technique. XRD patterns indicate that the delafossite structure could be guaranteed for all CuAl1xMgxO2 ﬁlms. The conductivity measured at room temperature for CuAl0.98Mg0.02O2 ﬁlm is three orders of magnitude higher than that of undoped CuAlO2 ﬁlm and the band gaps of CuAl1xMgxO2 (x ¼ 0, 0.01, 0.02 and 0.05) thin ﬁlms decrease with the increase of the doping concentration, which is related to the formation of impurity energy levels with increasing the doping concentration. Ó 2008 Elsevier Ltd. All rights reserved. Keywords: CuAlO2 ﬁlms Doping concentrations Band gap Conductivity 1. Introduction Transparent conducting oxide (TCO) ﬁlms are being widely applied in liquid crystal displays, organic light-emitting diodes, and solar cells, etc [1–3]. It is well known that TCO ﬁlms usually are n-type (electron) semiconductors, no p-type (hole) TCO ﬁlms were found until the report of the delafossite-type CuAlO2 prepared by pulse laser deposition [4]. The discovery of p-type TCO ﬁlm makes it possible to fabricate transparent oxide optoelectronic devices such as transparent p–n junction diodes and transistors using an appropriate combination of p- and n-type TCO ﬁlms complementary of Si-based electronics. However, the conductivity of CuAlO2 ﬁlm was 3–4 orders of magnitude lower than that of more commonly used n-type TCO like indium-tin oxide. Therefore, the improvement of electrical property of CuAlO2 ﬁlm seems a very exigent and necessary for practical applications. Up to now it is already known that excess oxygen [4,5] and introduction of some divalent species [6–8] into the delafossite structure could effectively increase the electrical conductivity of the material. Although some doped CuMO2 (M ¼ 3d transition trivalent cation) ﬁlms, i.e., CuSc1xMgxO2þy [6], CuCr1xMgxO2 [7], CuY1xCaxO2 [8] ﬁlms were already reported and there are theoretical studies that suggest an improved p-type TCO properties upon Mg doping of CuAlO2 [9,10], few experimental reports on the improvement of conductivity for CuAlO2 thin ﬁlm by doping with divalent cation were found. * Corresponding author. Tel.: þ86 10 67392733; fax: þ86 10 67392412. E-mail address: [email protected] (M. Zhang). 0042-207X/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2008.04.010 In this article, the CuAl1xMgxO2 (x ¼ 0, 0.01, 0.02 and 0.05) thin ﬁlms were successfully deposited on quartz substrate by using the radio frequency (RF) magnetron sputtering technique. The crystal structures, optical and electrical properties of the deposited ﬁlms with different doping concentrations were investigated. 2. Experimental Polycrystalline targets of CuAl1xMgxO2 (x ¼ 0, 0.01, 0.02 and 0.05) used for sputtering were synthesized heating a stoichiometric mixture of Cu2O (99%), Al(OH)3 (99.9%) and MgO (99.9%) at 1373 K for 10 h [11]. Then the CuAl1xMgxO2 (x ¼ 0, 0.01, 0.02 and 0.05) powders with the pure delafossite structure were pressed within the Aluminums holder as the sputtering target. The RF magnetron sputtering system used in this work is shown in Fig. 1. A mixture of Ar (4N) and O2 (4N) with a ratio of 3:2 was adopted as the sputtering gas. The substrates were cleaned ultrasonically using methylbenzene, acetone and ethanol in turn and then fastened to a heated substrate holder, the distance between substrate and target was 70 mm. Prior to deposition, the chamber was pumped down to 4 103 Pa and the target was pre-sputtered for 30 min to remove contamination from the surface by rotary pump and molecular pump. After the deposition, all samples were in situ annealed in the vacuum chamber at 773 K for 1 h and then annealed under the N2 atmosphere at 1173 K for 5 h. The working pressure, the sputtering power and the substrate temperature were around 1.0 Pa, 100 W, and 773 K, respectively. The ﬁlm thickness was around 180, 160, 210, and 240 nm for the corresponding CuAl1xMgxO2 ﬁlms with x ¼ 0, 0.01, 0.02 and 0.05, respectively. 1322 G. Dong et al. / Vacuum 82 (2008) 1321–1324 Fig. 2. XRD patterns of Mg-doped CuAlO2 ﬁlms with different doping concentrations (0, 1, 2, 3 and 5 at.%). The hump around 22 and the peak around 26.7 are due to the quartz substrate. Fig. 1. The schematic of the sputtering device. The structural analysis of Cu–Al–O thin ﬁlms was carried out by a BRUKER-AXS D8 X-ray diffraction (Cu Ka, l ¼ 0.154056 nm). The thickness was determined by the surface roughness automatic tester (Surfcom 480A). The optical transmittance (T) and the reﬂectance (R) of the ﬁlms were measured using UV-3101PC UV–vis double-beam spectrophotometer in the wavelength (l) range 200– 800 nm. Electrical conductivity was performed by the four-probe method using the automatic measurement system assembled by Agilent E5273 and Lakeshore 340. 3. Results and discussion 3.1. The crystal structure of Mg-doped CuAlO2 thin ﬁlms Fig. 2 shows the X-ray diffraction (XRD) patterns of the Mgdoped CuAlO2 thin ﬁlms. All peaks are identiﬁed to the reﬂections of CuAlO2 with the rhombohedral crystal structure (PDF 35-1401) and no diffraction corresponding to the second phase is obviously presented, which clearly indicates that the doping could not induce any impurity phases except for CuAlO2 ﬁlms with the delafossite structure. Although the weak signal of the diffraction, if the results of the Rutherford Backscattering Spectroscopy measurement (not shown here) are taken into account, the delafossite structure of CuAlO2 ﬁlms might be conﬁrmed. Furthermore, the crystalline quality of the ﬁlms declines when the Mg concentration increases up to 5 at.%, Selim and Youssef reported there was a marked decrease in the crystallinity of 2% Na doped CuAlO2 after annealing at 1000 C [12]. Because Mg2þ is chemically close to Naþ, it is the possibility that substituting Mg2þ for Al3þ results in the decomposition of CuAlO2, which decreases the crystal quality of ﬁlms. The hump about 22 and the peak located at 26.7 could be ascribed to crystalline quartz possibly formed during the annealing process. As seen from the inset of Fig. 2, the (101) peaks for all ﬁlms shift toward lower angles with an increase of the Mg content, which reﬂects the fact that the CuAlO2 lattice expands for all doped samples. According to the data of the ion radii of Mg2þ, Cuþ and Al3þ (Mg2þ: 0.066 nm, Cuþ: 0.095 nm and Al3þ: 0.055 nm) and the fact of the cell expansion, it can be concluded that the substituted Mg is present on the Al site of CuAlO2 lattice to form substitutional solid solution. 3.2. The optical properties of the Mg-doped CuAlO2 thin ﬁlms Fig. 3 presents the optical transmittance and the reﬂectance spectra of all Mg-doped CuAlO2 ﬁlms. The undoped CuAlO2 ﬁlm exhibits the relatively high transparency (>80% at around 700 nm) in the visible light range and the transparency decreases sharply with increasing the doping concentration, the decline of the crystalline quality may be one of reasons for the strong reduction in transmittance while increasing the Mg content. Absorption edge for all samples can be clearly observed and it shifts to the long wavelength with the increase of Mg concentration. The optical band gap (Eg) of the Mg-doped CuAlO2 ﬁlms could be estimated from the optical transmittance (T) and reﬂectance (R) spectra. In detail, optical absorption coefﬁcients (a) of the ﬁlms were calculated by the following equation [13]: 1 1R a ¼ ln (1) d T Fig. 3. Optical transmittance and reﬂectance spectra of Mg-doped CuAlO2 ﬁlms. G. Dong et al. / Vacuum 82 (2008) 1321–1324 1323 where the T was corrected for reﬂection losses at the air–ﬁlm and ﬁlm–substrate interface, and d is the ﬁlm thickness. Then Eg could be deduced by the formula [14]: ðahnÞ1=n ¼ A hn Eg (2) where hn is the incident photon energy, A is a constant and the exponent n depends on the type of transition, n ¼ 1/2 and 2 for direct and indirect transitions, respectively. Thus, the indirect (Egi) and direct (Egd) optical band gaps of Mg-doped CuAlO2 ﬁlms could be deduced from the extrapolation of the linear portion of the plots of (ahn)1/2 vs. hn and (ahn)2 vs. hn to a ¼ 0, respectively. The results for undoped CuAlO2 ﬁlm are illustrated in Fig. 4, Egd and Egi are around 3.6 and 2.0 eV, respectively, which are basically consistent with ones of polycrystalline CuAlO2 thin ﬁlm (Egd and Egi are around 3.5 and 1.8 eV, respectively) deposited by pulsed laser deposition if taken into account the error induced by the measurement [4,15]. Fig. 5 shows the Egd and Egi deduced from Fig. 3 and for Mg-doped CuAlO2 ﬁlms with different concentrations. It is clear that the Egd and Egi of Mg-doped CuAlO2 ﬁlms decrease with increasing doping concentration, which is probably due to the increase of the amount of impurities and/or defects with increasing the heavy acceptor doping concentration and ﬁnally formation of the impurity energy levels that effectively reduces the optical band gaps. 3.3. The electrical properties of Mg-doped CuAlO2 ﬁlms Fig. 6 presents ln s vs. 1000/T plot of the Mg-doped CuAlO2 ﬁlms measured from 150 to 300 K. The conductivity of all Mg-doped CuAlO2 thin ﬁlms is higher than that of the undoped ﬁlm and especially the conductivity of CuAl0.98Mg0.02O2 is about three orders of magnitude higher than that of the undoped ﬁlm. Nevertheless, the conductivity decreases when the doping concentration is further increased up to 5%, which might be ascribed to the excess Mg atoms acting as defects in the CuAlO2 lattice. Although all CuAl1xMgxO2 thin ﬁlms annealing in a N2 environment and it is already proved that N dopant can improve the electrical conductivity of CuAlO2 thin ﬁlm (from 3.8 102 to 5.4 102 Scm1 with N-doping effect) [16], in this paper, the deposition and the annealing condition for the CuAl1xMgxO2 thin ﬁlms are uniform, it can be ensured the main difference in electrical properties between the undoped and doped samples is mainly due to the Mg-doping effect. Above 235 K, the linear dependence relationship is shown in the Arrhenius plot of ln s vs. 1000/T, indicating that CuAl1xMgxO2 Fig. 4. Plots of the direct and indirect optical band gaps for an undoped CuAlO2 ﬁlm (inset: determination of indirect band gap). Fig. 5. The direct and indirect band gaps of Mg-doped CuAlO2 ﬁlms with different doping concentrations (0, 1, 2, 3 and 5 at.%). thin ﬁlms are of semiconducting thermal-activation type, the activation energy of CuAl1xMgxO2 is evaluated as 0.11, 0.053, 0.005 and 0.095 eV with x ¼ 0, 0.01, 0.02 and 0.05, respectively. On the other hand, ln s decreases linearly as a function of 1/T1/4 (not shown here) below 235 K. These observations indicate that temperature dependence above 235 K is of a thermal-activation type, but below 235 K a variable-range hopping mechanism [17] becomes dominant. Hall measurements conﬁrm p-type conductivity and the carrier densities estimated from the Hall measurement are on the order of 1015–1016 cm3 for all ﬁlms. 4. Conclusions The Mg-doped CuAlO2 thin ﬁlms were ﬁrstly successfully deposited on quartz substrate by RF magnetron sputtering, all the ﬁlms exhibit the delafossite structure. Additional, the decline of the crystalline quality possibly caused by Mg-doped effect induces the strong reduction in transmittance while increasing the Mg content. The band gaps of Mg-doped CuAlO2 thin ﬁlms decrease with increasing the doping concentration. The temperature dependence of conductivity is of semiconducting type at around room temperature and the conductivity has a notable improvement after Mg-doped and reaches 8.3 102 Scm1 for x ¼ 0.02 measured at Fig. 6. Temperature dependence of the conductivity of Mg-doped CuAlO2 ﬁlms with different doping concentrations (0, 1, 2, 3 and 5 at.%). 1324 G. Dong et al. / Vacuum 82 (2008) 1321–1324 room temperature, which is around three orders of magnitude than that of the undoped CuAlO2 ﬁlm, Hall measurements conﬁrm the p-type conduction of all ﬁlms. 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