Group-ΙΙA Impurities In SnO₂ From Fist-principles Calculations And Preparation Of P-SnO₂ Thin Films

The transparent devices based on wide band gap metal oxides（Eg ≥ 3.1 e V） have extensive applications in flat panel displays and solar cells. Metal oxides could be insulators, semiconductors or conductors by controlling the impurity of doping. At present, most of the reported TCOs and TSOs are based on n-type oxide. However, seldom desired p-type oxides are available, because of their poor properties comparing to their n-type counterparts, which is the main obstacle to open up a new field in “Transparent Electronics”. Moreover, taking optoelectronic devices with the p-layer oxide, such as solar cells, p-tye TFT and LED into account, we are also facing exactrly problems achieving ohmic contact with proper p-type oxides. Therefore, much attention has been given to p-type wide band gap metal oxides and developing their performances.Tin dioxide（SnO₂） is a classic oxide material that combines low electrical resistance with high optical transparency in the visible range of the electromagnetic spectrum. N-type SnO₂ thin films, such as SnO₂:F and SnO₂:Sb, have excellent electrical performances but in an opposite way in p-type films. Any founding of a p-type conducting oxide must be important for “transparent circuit technology”. The optoelectronic properties of p-SnO₂ are affected by native defects and impure elements. A possible route for achieving p-type doped SnO₂ would consist of IIA elements having lower valence states impurities on the Sn site. In this study, we research the first-principle calculation of structural and electronic properties of IIA elements doped SnO₂ to investigate the possibility of forming p-type SnO₂ and prepared the p-type Mg-doped SnO₂ thin films by e-beam evaporator. The main works as well as obtained results of this thesis can be summarized as follow:（1） The first-principles calculations of the formation energy and transition level of native defects of SnO₂ and the influence of defects on achieving p-type SnO₂.Tin dioxide crystallizes in the rutile structure which has a tetragonal symmetry with the phase. The valence band maximum is mainly formed from O 2p orbitals while the conduction band minimum is dominated by the Sn 5s and 5p states. The native defects in SnO₂ include oxygen vacancies（Vα）, tin interstitials（Sn_i）, tin antisites（Sn O）, tin vacancies（V_Sn）, oxygen interstitials（O_i） and oxygen antisites（O_Sn）. Under O-poor conditions, the formation energies of（Vα）2+ and（Sn_i）4+are negative so that they can form spontaneously during the growth of not-intentionally doped SnO₂. However, the transition level（2+/0） of the O vacancy is located at 1.21 e V above the VBM indicating that the O vacancy acts as a deep donor and cannot be the origin of n-type conductivity in SnO₂. In addition, the O vacancy is stable in 2+ charge state near the VBM, acting as a compensation center for p-type conductivity, which is unfavorable for p-type doping. Under O-rich conditions, the formation energy of O vacancy is much higher, indicating that the O-rich conditions are favorable for p-type conductivity.（2） The first-principles calculations of investigating the effects of Be, Mg, Ca and Sr impurities on the structural and electronic properties of SnO₂, focusing on their behavior as acceptors or donors.The possibility for achieving p-type conductivity in SnO₂ by doping group-IIA impurities（Be, Mg, Ca and Sr） on the Sn site have been discussed in this section. Based on atomic size and electronic-structure considerations, Be is expected to prefer the interstitial site in SnO₂ and acts as an donor. Magnesium has an atomic size very close to that of Sn. Thus Mg preferentially occupies the Sn site in SnO₂, causing relatively small lattice relaxations. Under O-rich conditions, Mg Sn is stable in the（Mg_Sn）2- configuration and the formation energy of（Mg_Sn）2- is low（Ef =0.69 e V） enough to obtain greater possible Mg Sn concentration. The ionization energy is determined by the position of transition level ε（0/-2） and is calculated to be 21 me V, indicating that Mg Sn acts as a shallow acceptor in SnO₂. The atomic sizes of Calcium and Strontium are much larger than that of Sn, resulting in the relatively large lattice relaxations. Under O-rich conditions, the calculated formation energies of Ca Sn and Sr Sn configuration in SnO₂ are much lower than that under O-poor conditions. However, it is found that the formation energies of Cai（or Sri） and Ca Sn-VO（or Sr Sn-VO） are very close to that of Ca Sn（or Sr Sn）, which will limit the solubility of Ca Sn（or Sr Sn）. In this study, we find that the group-IIA impurities are able to give rise to p-type conductivity: with Mg being the most desirable dopant for generating p-type conductivity in SnO₂.（3） According to the above theoretical calculation results, p-type Mg-doped SnO₂（MTO） thin films have been successfully prepared by e-beam evaporation.Firstly, MTO targets were prepared via a high energy planetary ball mill. The influence of ball-milling time, annealed temperature and Mg concentration on the structural properties of MTO targets had been investigated. With increasing the milling time, the average crystallite size and lattice strain of the Mg Sn O powder decreased and increased, respectively. X-ray diffraction results revealed that the Mg₂SnO₄ phase was formed after calcinations of the as-milled MgSnO powder at temperatures higher than 1000 °C. XRD characterization showed that the Mg-doped SnO₂ films exhibited the rutile structure of tin oxide but the crystallinity of SnO₂ decreased with increasing Mg percentage in the films. Secondly, MTO thin films were deposited on the glass substrate by electron beam evaporation technique and the influence of increasing the Mg concentration upon their structural, optical and electrical properties were investigated. As a result, the surface of the initial MTO film samples is sufficiently flat and show small roughness. The optical transmittance of the films was above 80 % in the visible region. The bandgap of the films was in a range of 3.44 3.77 e V with increasing Mg concentration. Hall-effect results indicated that undoped SnO₂ is n-type conductivity, while Mg-doped SnO₂（Mg concentration, 1 %, 3 % and 4 %） is p-type conductivity, exhibiting a high hole concentration of 2.01 × 1018 cm-3, 8.24 × 1018 cm-3 and 3.27 × 1018 cm-3, respectively. The carrier mobility of films（Mg concentration, 1 %, 3 % and 4 %） are 3.01 cm2V-1s-1, 3.29 cm2V-1s-1 and 5.80 cm2V-1s-1, respectively. The resistivity of p-type Mg-doped SnO₂ films（Mg concentration, 1 %, 3 % and 4 %） are 0.103 Ω·cm、0.023 Ω·cm and 0.022 Ω·cm. The nature of conductivity changes from p-type to n-type when the Mg doping level is above 5 %.