Structures and properties of various phases of zinc tin oxide by ab-initio calculations

Abstract

Zinc tin oxide (ZTO) is one of the transparent conducting oxides which can be used as a channel layer of thin film transistor (TFT), solar cell, gas sensor due to the high transmittivity, high mobility, excellent chemical selectivity. For these applications, amorphous structures or nanoclusters are mainly used. However, the fundamental properties even on crystalline ZTO phases have not been thoroughly investigated. In this dissertation, the understanding on the bulk properties of crystalline phases, point defects in crystal phase, and amorphous phase of ZTO was investigated by using ab-initio calculations, which are powerful simulation tools to figure out the fundamental properties from the interactions between atoms and electrons. Various crystalline phases of ZTO have been reported to form depending on the synthesis methods and conditions. Zn2SnO4 and ZnSnO3 are the representative compositions. For Zn2SnO4, it was reported that the inverse spinel is stable and that the cation disordering at the octahedral sites by zinc and tin changes the symmetry of the structure in the unit cell scale. For ZnSnO3, lithium niobate and ilmenite phases exist which are corundum-base structures with the different occupancy at the octahedral sites by zinc, tin and vacancy. Thermodynamic stabilities of various zinc tin oxides were investigated based on the Gibbs energy obtained from density functional theory calculations with the phonon vibrational and configurational terms. The pressure-temperature (p-T) phase diagram was determined

at zero external pressure, the coexistence of ZnO and SnO2 was the most stable at low temperature, and Zn2SnO4 became stable over approximately 1000~1300 K. Pressurization mainly affected the Zn-O bonds and changed the thermodynamic stability via the following sequences: from the coexistence of ZnO and SnO2 to the coexistence of Zn2SnO4 and SnO2, then to ZnSnO3 in the lithium niobate structure. ZnSnO3 in the ilmenite structure was found to be unstable due to the relatively high energy and the negative phonon frequency. The effects of the space group of the unit cell scale for Zn2SnO4 and the exchange-correlation functionals were also investigated. The different space groups of Zn2SnO4 in the unit cell scale affected the thermodynamic conditions of Zn2SnO4. The calculated results showed good agreement with the experimental phase stability and were able to explain the experimental observation of the mixed state of Zn2SnO4, ZnO and SnO2 in the temperature and pressure midranges. The point defects of oxygen vacancy and hydrogen interstitial in the inverse spinel Zn2SnO4 phase were investigated, where Zn2SnO4 phase is the most stable phase in ternary system. The calculations were performed using hybrid density functional proposed by Heyd-Scuseria-Ernzerhof (HSE) in order to adjust the bandgap to the experimental value. The defect formation energies showed the neutral oxygen vacancy is stable when the Fermi level is located at the n-type region. The transition level ɛ(2+/0) was about 1 eV below the conductional band minimum. The oxygen vacancy can be ionized by the photon energy of 2 eV and perform as the photocurrent source. On the other hand, the hydrogen interstitial was found to exist as a singly charged state and can be act as an electron source. The amorphous phases of Zn2SnO4 and ZnSnO3 were obtained by melt-quenching method based on the ab-initio molecular dynamics. The radial distribution function results showed the coordination number of Zn-O bonds in amorphous phase decreased close to 4 as that of binary oxide, ZnO. The bond length of Zn-O becomes similar as that of ZnO. Meanwhile, the coordination number and bond length of Sn-O bonds are all similar with each other in both binary oxide and ternary oxides.