Doping Mechanism in Transparent, Conducting Tantalum Doped ZnO Films Deposited Using Atomic Layer Deposition

Abstract

The effect of tantalum (Ta) addition to ZnO films has been investigated. The films are synthesized using atomic layer deposition. The precursor for Ta is pentakis‐dimethylamino Ta (PDMAT) whereas, diethyl Zn (DEZ) is used for ZnO deposition with H2O as an oxidant. The surface reactions are studied using in situ downstream quadrupole mass spectrometry. X‐ray photoelectron spectroscopy and photoluminescence show that at low Ta doping, oxygen vacancies are eliminated from the film, whereas at higher Ta doping, Zn vacancies are formed. These observations are supported by density functional theory calculations. Hall measurements show the lowest resistivity for a 30 nm Ta doped ZnO film to be 4 × 10−3 Ω cm with a peak carrier concentration of 1.2 × 1020 cm−3. The ideal cycle ratio used to achieve these properties is 40:1 of (DEZ+H2O):(PDMAT+H2O) pulses. Optical measurements show an average transmittance of 85% in the visible regime. The efficiency of Ta doping the ZnO, which represents the fraction of Ta5+ contributing to conductivity, is found to be at least 6.3%.

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“…Thus, an “ideal” story for substitution of zinc for tantalum, illustrated in Figure 1C, when Ta 5+ occupies the crystallographic positions of Zn 2+ , is not expected to result in significant variations of the lattice parameters. The literature data often suggests a contradictory behaviour of the a and c parameters on Ta doping in ZnO-based nanostructures and thin films [20,21,22,23,24,25]. The unit cell size can be significantly affected by the presence of additional defects in ZnO-based matrix, which, in turn, can be promoted both by the different dopant incorporation mechanisms and processing conditions (e.g., [6,29]).…”

“…Up to now, the doping with tantalum was shown to be promising to enhance relevant optical properties and photocatalytic activity of ZnO-based thin films and nanoparticles [20,21,22,23,24,25]. Although the used processing and synthesis procedures apparently allow to achieve quite significant doping level for ZnO (even several at.% of tantalum), the studies show that only a part of the introduced tantalum acts as an efficient electronic donor [24]. Consolidation of highly-doped nanoparticles into ceramics, suitable as thermoelectric elements, requires elevated temperatures, which may significantly alter the doping level achieved in nanostructures.…”