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Shallow acceptor state in Mg-doped CuAlO and its effect on electrical and optical properties: An experimental and first-principles study Ruijian Liu, Yongfeng Li, Bin Yao, Zhanhui Ding, Yuhong Jiang, Lei Meng, Rui Deng, Ligong Zhang, Zhenzhong Zhang, Haifeng Zhao, and Lei Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b01354 • Publication Date (Web): 24 Mar 2017 Downloaded from http://pubs.acs.org on March 27, 2017
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ACS Applied Materials & Interfaces
Shallow acceptor state in Mg-doped CuAlO2 and its effect on electrical and optical properties: An experimental and first-principles study Ruijian Liu1, Yongfeng Li1, 2, *, Bin Yao1, 2, *, Zhanhui Ding1, Yuhong Jiang1, Lei Meng1, Rui Deng3, Ligong Zhang4, Zhenzhong Zhang4, Haifeng Zhao4, Lei Liu4 1
State Key Lab of Superhard Material and College of Physics, Jilin University, Changchun 130012, P.R. China
2
Key Laboratory of Physics and Technology for Advanced Batteries (Ministry of Education), College of Physics, Jilin University, Changchun 130012, China
3
School of Materials Science and Engineering, Changchun University of Science and Technology, Changchun, 130022, China 4
State Key Lab of Excited State Processes, Changchun Institute of Optics, Fine
Mechanics and Physics, Chinese Academy of Sciences, Changchun, 130033, P.R. China
*E-mail:
[email protected] and
[email protected] 1
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Abstract:
Shallow acceptor states in Mg-doped CuAlO2 and their effect on structural, electrical and optical properties are investigated by combining first-principles calculations and experiments. First-principles calculations demonstrate that Mg substituting Al site in CuAlO2 plays a role of shallow acceptor and has a low formation energy, suggesting that Mg doping can increase hole concentration and improve conductivity of CuAlO2. Hall effect measurements indicates that the hole concentration of Mg-doped CuAlO2 thin film is two orders of magnitude higher than that of undoped CuAlO2. The best room temperature conductivity of 8.0×10-2 S/cm is obtained. A bandgap-widening is observed in the optical absorption spectra of Mg-doped CuAlO2, which is well supported by the results from first-principles electronic structure calculations.
Keywords: CuAlO2, Mg doping, photoluminescence, optical property, electrical property, first-principles calculation
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1.
Introduction
Transparent conducting oxides (TCOs) are compounds exhibiting high electrical conductivity and transparency to visible light.1 Their applications consist of flat panel displays, solar cells, liquid crystal displays, touch screen, photodetectors, light-emitting diodes, etc.2-6 Up to now, TCOs applied in industry are primarily n-type semiconductors with electrons as charge carriers, which have been developed and are commercially available, such as indium tin oxide (ITO),7-8 zinc oxide (ZnO)9-11 and tin oxide (SnO2).12-14 The n-type TCOs usually have high electrical conductivities of ~104 S/cm and high visible-light transmittance of greater than 80%.15 By contrast, p-type TCOs are scarce due to the difficulty of finding shallow acceptors with low formation energy in wide-bandgap oxides. A fundamental reason to this difficulty is the nature of deep and localized O 2p states that typically form the valence-band edge in most oxides. The holes introduced at the edge cannot migrate within the crystal even under an applied electric field.16-18 To overcome this issue, a new theory involved modulating the valence-band edge through hybridization of the O 2p with metal orbitals, such as the hybridization between O 2p and Cu 3d in CuAlO2, has been proposed by Kawazoe et al19 and the p-type characteristic of delafossite CuAlO2 film was experimentally proved meanwhile. However, the electrical conductivity at room temperature is no more than 0.95 S/cm, which is the highest conductivity for undoped CuAlO2 film, but it is still much lower than that of n-type TCOs. The low electrical conductivity of p-type CuAlO2 severely constrains its application especially in the 3
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entire new field of transparent electronics that requires both p- and n-type TCO materials.17, 20-22 To enhance the conductivity of CuAlO2 film, a plenty variety of experiments have been studied in delafossite CuAlO2, such as partial substitution of (Ag, Ni, Zn)23 or Pt24 for Cu-sites, (Be, Mg, Ca, Zn, Ni, etc.)25-28 substitution for Al-sites and nonstoichiometric composition (excess oxygen29-30 and/or excess metal cations31). Among those doping elements, the Mg is an appropriate element due to its atom radius and electronegativity close to Al. However, the effect of Mg doping on electronic structure, electrical and optical properties of CuAlO2 were few reported. Furthermore, there exist conflicting results on the influence of Mg doping on electronic structures of CuAlO2. For instance, Zou et al32 and Dong et al25 reported that the optical bandgap increases with the increase of Mg doping concentration, while Jiang et al33 reported that bandgap decreases. In this paper, to reveal the origin of p-type conduction in Mg-doped CuAlO2 and enhance the p-type conductivity of CuAlO2, we theoretically studied the Mg substitution in CuAlO2 by the first-principles calculations and then experimentally investigated its effects on the structural, optical and electrical properties of CuAlO2 thin films.
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Experimental and first-principles calculations details
Polycrystalline targets of CuAl1-xMgxO2 (x=0, 0.02, 0.04 and 0.06) used for sputtering were synthesized by heating a stoichiometric mixture of CuO (99.99%), Al2O3 (99.999%) and MgO (99.99%) at 1473 K under air atmosphere for 10 h. 4
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Single-crystal sapphire substrates with c-axis orientation were cleaned ultrasonically using acetone, ethanol and de-ionized water in turn and then fastened to a substrate holder. The substrate was kept in room temperature and no intentional heating was carried out during the sputtering process. High pure Ar (99.999%) was used as sputtering gas. The distance between the substrate and target is about 6 cm. Prior to deposition, the chamber was pumped down to 8.0×10-4 Pa and the target was pre-sputtered for at least 10 minutes to remove contamination from the surface. The working pressure and power are 0.40 Pa and 80 W, respectively. The sputtering time is 60 min. After deposition, all samples were annealed under the Ar atmosphere at 1173 K for 4 h and then cooled down to the room temperature rapidly. The crystal structures of all films were characterized by x-ray diffraction (XRD) using a DX-2700 x-ray diffractometer equipped with Cu Kα radiation (λ=1.5406 Å). The room temperature carrier concentration, mobility, resistivity and conduction type of these films were obtained from the Hall effect measurements in the Van der Pauw configuration using a Hall measurement system (Lakeshore 7707). The surface morphology and composition was characterized by HITACHI S-4800 field-emission scanning electron microscopy (FE-SEM) equipped with an energy-dispersive X-ray spectroscopy (EDS) system (EDAX Genesis 2000). The optical properties were characterized by Unico 2802s UV/Vis spectrophotometer. The room temperature photoluminescence (PL) measurements were performed using HORIBA Scientific’s LabRAM HR Evolution Raman spectrometer with a He-Cd laser (325-nm line) as the excitation source. The X-ray photoelectron spectroscopy (XPS) was performed using 5
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an X-ray electron spectrometer (ESCALAB 250) with Al Kα (hν = 1486.6 eV) radiation. The thin films were subjected to Ar ion etching to remove away the surface contaminants. All XPS spectra were calibrated by the C 1s peak (284.6 eV). For XPS peak fitting, a Voigt line profile and a Shirley background were used. First-principles calculations were carried out using the VASP code with the projector augmented wave (PAW) potentials and the general gradient approximation (GGA) was taken to obtain a relatively reasonable result. The cutoff energy for the plane-wave basis set is 450 eV. To simulate Mg doping states in CuAlO2, a 108-atom 3×3×1 CuAlO2 supercell was constructed with the delafossite structure as a reference and 4 types of Mg-related defects were considered: (i) interstitial Mg (Mgi), created by inserting a Mg atom at the interstitial site inside the supercell, (ii) Mg substituting Cu (MgCu) and (iii) Mg substituting Al (MgAl), created by replacing a Cu and Al atom with a Mg atom, respectively, (iv) MgCu-MgAl complex, built by replacing a Cu and an adjacent Al atom with two Mg atoms. The crystal structures with various Mg-related defects are shown in figure 1. In all calculations, all the atoms are allowed to relax until the Hellmann−Feynman forces acting on them are less than 0.01 eV/Å and self-consistency is achieved with a tolerance in total energy of 10–4 eV. The optimized a- and c-axis lattice constants of perfect CuAlO2 are 2.849 Å and 17.037 Å with a small error of 0.3% and 0.5% with respect to experimental values (a=2.858 Å, c=16.958 Å) reported in literature for CuAlO234, indicating our calculation is reasonable and reliable.
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3.
Results and discussion
3.1
First-principles calculations of electronic structure and optical properties as well as the formation energy of Mg-related defects
The formation energy of a defect or impurity α in charge state q is defined35 as ∆ , = , − ℎ − ∑ + + + ∆
(1)
where Etot(α, q) is the total energy derived from a supercell calculation with one impurity or defect α in the cell, and Etot(host) is the total energy for the equivalent supercell containing only bulk CuAlO2. The ni indicates the number of atoms of type i (~host atoms or impurity atoms) that have been added to (ni>0) or removed from (ni
