Investigation of the properties of nanostructured Li-doped NiO films using the modified spray pyrolysis method
© Chia-Ching and Cheng-Fu; licensee Springer. 2013
Received: 14 November 2012
Accepted: 22 December 2012
Published: 18 January 2013
The lithium-doped nickel oxide (L-NiO) films were synthetized using the modified spray pyrolysis method with a two-step grown process. By observing the spectra of X-ray photoemission spectroscopy of L-NiO films, the intensity of Ni 2p3/2 peak of Ni3+ bonding state increases with increasing Li concentration that causes the decrease of transparency and resistivity. The L-NiO films with optimum characteristics were obtained at Li = 8 at%, where a p-type resistivity of 4.1 × 10−1 Ω cm and optical transparency above 76% in the visible region are achieved.
N-type transparent conductive oxide (TCO) films, such as indium tin oxide, aluminum zinc oxide, indium gallium zinc oxide, etc., are widely used as transparent electrodes, solar cells, and touch panels. However, not many TCO films have the p-type properties, and they are also required in other applications. Nickel oxide (NiO) films are a promising candidate for p-type semi-TCO in the visible light with the band gap (Eg) values from 3.6 to 4.0 eV. NiO films have a wide range of applications, such as (1) transparent conductive films , (2) electrochromic display devices , (3) anode material in organic light emitting diodes , and (4) functional layer material for chemical sensors .
In the past, NiO films were prepared by various methods, including electron beam evaporation, chemical deposition, atomic layer deposition, sol–gel, and spray pyrolysis method (SPM) . Sputtering is one of the most popular methods to deposit NiO films with low resistivity of 1.4 × 10−1 Ω cm . The SPM is a very important non-vacuum deposition method to fabricate TCO films because it is a relatively simple and inexpensive non-vacuum deposition method for large-area coating. However, the resistivity of SPM deposited doped NiO films is about 1 Ω cm , which is almost 1 order of magnitude higher than that of sputter-deposited NiO thin films.
Undoped NiO has a wide Eg value and exhibits low p-type conductivity. The conduction mechanism of NiO films is primarily determined by holes generated from nickel vacancies, oxygen interstitial atoms, and used dopant. The resistivity of NiO-based films can be decreased by doping with lithium (Li) . In 2003, Ohta et al. fabricated an ultraviolet detector based on lithium-doped NiO (L-NiO) and ZnO films . However, only few efforts have been made to systematically investigate the effects of deposition parameters and Li concentration on the electrical and physical properties of SPM deposited NiO films. In this research, a modified SPM method was used to develop the L-NiO films with higher electrical conductivity. We would investigate the effects of Li concentration on the physical, optical, and electrical properties of NiO thin films.
where α is the absorption coefficient, hv is the photon energy, A is a constant, Eg is the energy band gap (eV), and n is the type of energy band gap. The NiO films are an indirect transition material, and n is set to 2 .
Results and discussion
Non-vacuum SPM method was used to deposit high quality p-type L-NiO films. The (200) preferred orientation of L-NiO films increases over (111) as the Li concentration increases, which would cause the better conductive properties and resist electrical aging in the L-NiO films. In this study, the characteristics of modified SPM deposited L-NiO films were comparable to the sputter-deposited ones, and the optimum Li doping amount is set at 8 at %.
C-CW was born in Taiwan, in 1979. He received the Ph.D. degree in electrical engineering from the National Sun Yat-sen University, Kaohsiung, Taiwan, in 2009. In 2009, he joined department of electronic engineering, Kao Yuan University, where he investigated on organic/inorganic nanocomposites materials, integrated passive devices (IPDs), transparent conductive oxide (TCO) films, electron ceramics and carbon nanotubes and graphene.
C-FY was born in Taiwan, in 1964. He received the BS, MS, and Ph.D degree in electrical engineering from the National Cheng Kung University, Tainan, Taiwan, in 1986, 1988, and 1993. In 2014, he joined department of Chemical and Materials Engineering, National University of Kaohsiung, where he investigated on ferroelectric ceramics and thin films, application ferroelectric materials in memory devices, organic/nanotubes nanocomposites, organic/inorganic nanocomposites, YZO thin films, transparent conduction oxide thin films and their applications in solar cells, microwave antennas, and microwave filters.
The authors acknowledge the financial support of the National Science Council of the Republic of China (NSC 101-2221-E-244-006 and 101-3113-S-244-001).
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