Article pubs.acs.org/JPCC
Experimental Observation and Computer Simulation of Al/Sn Substitution in p‑Type Aluminum Nitride-Doped Tin Oxide Thin Film Po-Ming Lee,† Yen-Shuo Liu,† Luis Villamagua,‡,§ Arvids Stashans,‡ Manuela Carini,§ and Cheng-Yi Liu*,† †
Department of Chemical and Materials Engineering, National Central University, Jhong-Li, Taiwan Grupo de Fisicoquímica de Materiales, Universidad Técnica Particular de Loja, Apartado 11-01-608, Loja, Ecuador § Dipartimento di Ingegneria per l’Ambiente e il Territorio e Ingegneria Chimica, Università della Calabria, 87036 Rende, Cosenza, Italy ‡
ABSTRACT: In this study, the Al3+−Sn4+ substitution reaction in the AlN-doped SnO2 thin films is confirmed by photoluminescence and X-ray photoelectron spectrum analysis. Also, both Al3+−Sn4+ and N3−−O2− substitution reactions are verified by computational simulation, Vienna ab initio simulation package (VASP). The computational simulation shows that both Al and N impurity dopants generate an unoccupied band at the upper valence band maximum, which produces holes within the upper valence band region. Both Al3+−Sn4+ and N3−−O2− substitution reactions contribute to the p-type conversion of AlN-doped SnO2 thin films. Annealing AlN-doped SnO2 (Al content is 14.65%) thin films at high-temperature (larger than 350 °C), N outgassing would occur and cause the p-type conduction of the annealed AlN-doped SnO2 thin films back to n-type conduction. Yet, in this work, we found that the Al3+−Sn4+ substitution reaction in the high Aldoping concentration of Al-doped and AlN-doped SnO2 (the Al content is between 29% and 33.2%) thin films would be activated considerably, as they are annealed at a temperature over 500 °C. With a higher Al-doping concentration (Al concentration is 33.2%) in the Al-doped SnO2 thin films, we found that the critical annealing temperature for the n-to-p conduction transition decreases to 500 °C. The Al dopants in the AlN-doped SnO2 thin films annealed at high annealing temperature not only stabilize the N3−−O2− substitution reactions but also produce hole carriers by the Al3+−Sn4+ substitution reactions. The Al3+−Sn4+ substitution makes the AlN-doped SnO2 retain the p-type conduction in the high-temperature annealing.
■
INTRODUCTION Transparent conducting oxides (TCOs) have been extensively used in many applications in optoelectrics devices, such as light emitting diodes, flat panel displays, solar cells, lithium ion batteries, and gas sensors.1−5 Moreover, recently, the transparent thin film transistors made by TCOs garner serious attention for UV-solar cells and display technologies, often referred to as invisible electronics.6,7 Among the available TCOs, wide band gap SnO2 oxide (∼3.8 eV) has many merits, for instance, (1) a high transmittance in the range from the ultraviolet to the visible wavelength regime, (2) low resistivity (1 × 10−3 Ω cm), and (3) excellent thermal and chemical stability.3,4 Thus, the SnO2 oxide seems to be a potential oxide for advanced invisible electronics. The SnO2-based oxides usually exhibit n-type conduction. The free electrons are contributed from the intrinsic defects (oxygen vacancies or cation interstitials). To produce the transparent electronic devices, a high-quality p-type transparent conducting oxide is required.8−10 Therefore, if the n-type SnO2-based oxides can be converted to the p-type conduction, SnO2-based transparent electronics could be realized. Usually the mobility of the hole carrier is considerably lower than the mobility of the electron carrier.1−8 It can be ascribed to the different transport paths. The transport paths of electrons are the conduction band minima, which be composed © XXXX American Chemical Society
mainly by spatially spread s orbitals of metal cations. On the other hand, the transport path of hole carriers is composed mainly by localized O 2p orbitals with the hopping conduction mechanism. It is difficult to increase the conductivity of SnO2 oxide by promotion of its mobility. Increasing the hole carrier concentration of p-type SnO2-based oxides may be the key approach to produce a high-quality p-type transparent conducting SnO2-based oxide. Many researchers have attempted to produce p-type SnO2 phase by doping IIIA elements M (Al, Ga, In) in the SnO2 phase to substitute Sn4+ atomic sites.11−18 This is the most common way to achieve the p-type SnO2 phase. Every M3+− Sn4+ substitution would yield an electrical hole. Besides doping cations into the SnO2 phase, S. S. Pan et al. found that p-type SnO2 can also be achieved by doping N in SnO2, i.e., anionic substitution.19 However, the bonding of Sn−N is not stable compared to Sn−O bonding. Thus, the outgassing of N in SnO2 causes a poor electrical property of the N-doped SnO2 film. In this work, to further improve the p-type conductivity of SnO2 we made an attempt to codope IIIA elements (Al in this study) and N in the SnO2 thin film. We found that the Al Received: November 4, 2015 Revised: February 5, 2016
A
DOI: 10.1021/acs.jpcc.5b10791 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C dopants in the AlN-doped SnO2 thin films can stabilize the N3−−O2− substitution reaction with high annealing temperature, and Al dopants also produce hole carriers by the Al3+− Sn4+ substitution reaction. We analyzed the annealed Al-doped SnO2 and AlN-doped SnO2 thin films by PL and XPS. In addition, to have a better understanding of the electronic conduction behavior of SnO2 and AlN-doped SnO2, computation simulations were carried out through the use of the generalized-gradient approximation within density-functional theory (DFT-GGA) as implemented in the Vienna ab initio simulation package (VASP).20 The GGA exchange-correlation term was used according to Perdew− Burke−Ernzerhof,21 while the interaction between the core electrons and the valence electrons was schemed through the projector augmented wave (PAW) pseudopotential method22 proposed by Bloch23 and adapted by Kresse and Joubert.24
proposed by Bloch23 and adapted by Kresse and Joubert.24 To minimize the errors related to the description of d electrons by the DFT-GGA, we included an intra-atomic interaction term for the strongly correlated electrons by an unrestricted Hartree−Fock (UHF) approximation, resulting in the so-called DFT+U approximation. The U value for Sn 3d electrons and the lattice parameters, a and c, for SnO2 used throughout this work are 4.0 eV, 4.73 Å, and 3.16 Å, respectively. These values were computed previously and are detailed elsewhere.25 A cutoff kinetic energy of 480 eV is used by converging the total energy to less than 1 meV/atom. The Γ-centered Monkhorst−Pack (MP) grid scheme with a 0.035 Å −1 separation was applied, which corresponds to a k-point mesh of 6 × 6 × 8 for the 6-atom primitive unit cell. These parameters, also detailed in ref 25, were obtained through atomic relaxation until all forces were
