Doping induced structural and compositional changes in ZnO spray pyrolysed films and the effects on optical and electrical properties
Introduction
High quality doped ZnO is grown using reactive and non-reactive sputtering [1], metal organic chemical vapour deposition [2] and pulsed laser ablation deposition [3]. Thereby resistivities can be achieved down to 2 × 10−4 Ω cm. The highest reported Hall mobilities are ~ 60 cm2V−1 s−1 [4]. With non-vacuum methods, however, such as spray pyrolysis [5] and sol–gel deposition [6] resistivities are normally much higher (5 × 10−3 to 10 Ω cm) since it is more difficult to control grain growth, stoichiometry and purity with such deposition techniques.
In this work we use a simple non-vacuum spray pyrolysis method to deposit ZnO. Extremely high quality films are achieved as the method is not droplet deposition spray pyrolysis, where the droplets wet the substrate, but is an aerosol assisted chemical vapour deposition (CVD) [7].
Low resistivity ZnO film can be achieved by the creation of intrinsic donors by lattice defects, for example oxygen vacancies or zinc atoms on interstitial lattice sites, or by introduction of extrinsic dopants such as Al3+ on substitutional metal lattice sites or halogens, such as Cl− on oxygen lattice sites [8]. ‘Intrinsic’ doping can be achieved during the deposition by control of the oxygen source [8] and temperature. In reality, in doped oxide films both intrinsic and extrinsic doping mechanisms occur simultaneously. The films presented here have been ‘extrinsically’ doped by adding aluminium acetylacetonate and/or zinc chloride to the zinc acetate precursor solution. In spray pyrolysis the dopants affect not only the free charge carrier density but the composition, structure and growth mechanism of the film [9].
Previously [10], it was shown that low resistivities and significant deposition rates can be achieved only above a temperature of 400 °C. This paper determines which precursor solutions at a set temperature of 400 °C produce both high charge carrier density and mobility, and correlates measured electrical and optical properties to the structure and chemical composition of the film.
Section snippets
Experimental details
The set up of the laboratory spray pyrolysis was based on apparatus described in detail elsewhere [11]. The spray chamber is purged thoroughly with nitrogen to ensure an oxygen and water-deprived atmosphere. A precursor solution is then nebulised using a MHz ultrasonic source to form an aerosol which passes at an angle over a heated substrate surface, arranged such that the precursor droplets do not impact the surface but evaporate to gaseous reactants which subsequently react and deposit on
Results and discussion
Firstly the microstructure and chemical composition of the films are discussed and then, in the following sections, correlated to the results of the electrical and optical measurements.
Summary
ZnO is grown by a spray pyrolysis process. In our setup the aerosol of the precursor solution evaporates before landing on the substrate and hence is in principle an aerosol assisted CVD method. For a non-vacuum technique this produces particularly high quality films.
Without intentionally extrinsically doping the films and using only Zn(ac)2 in the precursor solution, films with a relatively high carrier concentration (~ 8 × 1018 cm−3) are obtained. The films exhibit poor carrier mobility due to
Acknowledgments
We would like to thank Kathrin Schatke for the use of the Hall probe at the Technical University of Berlin and Dr. Michael Gledhill for proof reading.
References (30)
- et al.
Thin Solid Films
(2009) - et al.
Thin Solid Films
(2006) - et al.
Colloids Surf. A
(2002) Sol. Energy Mater.
(1982)- et al.
Thin Solid Films
(1999) - et al.
Thin Solid Films
(2006) - et al.
J. Electron. Spectrosc. Relat. Phenom.
(1976) - et al.
Thin Solid Films
(1998) - et al.
Thin Solid Films
(2004) - et al.
Thin Solid Films
(1998)
Thin Solid Films
Sol. Energy Mater. Sol. Cells
Jpn. J. Appl. Phys.
J. Phys. D Appl. Phys.
Aerosol Processing of Materials
Cited by (19)
Material characterizations of Al:ZnO thin films grown by aerosol assisted chemical vapour deposition
2016, Journal of Alloys and CompoundsProperties of In-, Ga-, and Al-doped ZnO films grown by aerosol-assisted MOCVD: Influence of deposition temperature, doping level and annealing
2015, Surface and Coatings TechnologyDoped zinc oxide films grown by hot-wire chemical vapour deposition
2015, Thin Solid FilmsCitation Excerpt :The cost of fabrication method is also a significant factor for the choice of the most appropriate TCO material. Many techniques have been used to prepare In-, Ga- and Al-doped ZnO films: magnetron sputtering (In — [4], Ga — [5–8], Al — [9–12]), pulsed laser deposition (In — [13], Ga — [14,15], Al — [16,17]), molecular beam epitaxy (Ga — [18], Al — [19]), sol–gel (In — [20,21], Ga — [22,23], Al — [24]), atomic layer deposition (In — [25], Ga — [26], Al — [27–29]), solution spray-pyrolysis (pneumatic nozzle or ultrasonic aerosol generator) using inorganic precursors (In — [30–33], Ga — [33], Al — [33–36]), atmospheric pressure chemical vapour deposition (CVD) from inorganic precursors (Ga — [37]), atmospheric pressure metal-organic chemical vapour deposition (MOCVD) that uses metal-organic (MO) precursors (Ga — [38]), aerosol-assisted atmospheric pressure MOCVD (In — [39], Al — [40]) and low pressure MOCVD (In — [41], Ga — [42–47], Al — [48,49]). The quality of the deposited TCO films highly depends on the deposition method, which mainly influences the crystallinity and microstructure of films, and as a consequence, their electrical and optical properties.
Effect of nickel doping on structural, optical, electrical and ethanol sensing properties of spray deposited nanostructured ZnO thin films
2014, Ceramics InternationalCitation Excerpt :In addition to its inherent properties like wide band gap (3.37 eV), large exciton energy (60 meV), better thermal and chemical stability, transparent conducting nature, etc., its adaptability towards metal ion doping has widened its applications in the fields of thin film transistor, solar cell, gas sensors, ultraviolet light emitting diodes, lasers, field emitters, piezoelectric devices, photo detectors, flat screen displays, sun screens, spintronics, etc. [6]. Doping ZnO with different kinds of anions or cations is one of the best ways to modulate or change the morphology, carrier concentrations, density of states, transmittance, band gap, electrical conductivity, magnetic properties, etc. [8]. Especially doping with transition metal ions has a profound effect on the materials characteristics.
The effects of carrier gas and substrate temperature on ZnO films prepared by ultrasonic spray pyrolysis
2013, Materials Science in Semiconductor ProcessingCitation Excerpt :The values of band gaps are reported in a range of 3.26–3.28 eV [14,16]. Besides air, nitrogen gas (N2) is also used as carrier gas in the USP technique [12,18]. Under the N2 atmosphere, ZnO thin films deposited using zinc acetate dihydrate show the (002) preferred orientation for deposition temperature of 300–500 °C with a bandgap of 3.26 eV [18].