Influence of iron doping on the structural, chemical, and optoelectronic properties of sputtered zinc oxide thin films
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Heller Institute of Ion Beam Physics and Materials Research, Helmholtz-Zentrum Dresden-Rossendorf, 01328 Dresden, Germany (Received 24 April 2016; accepted 30 August 2016)
Iron (Fe)-doped zinc oxide (ZnO) thin films were deposited using two techniques: (i) radio-frequency (RF) sputtering of Fe-doped ZnO targets, and (ii) co-sputtering, where ZnO was RF-sputtered and iron was direct-current (DC)-sputtered. The as-deposited films were polycrystalline, with predominant growth along the (002) direction of hexagonal ZnO, and possessed a considerable concentration of oxygen vacancies. From an optoelectronic point of view, the films were highly transparent, with a band gap of 3.25 eV, and had electrical resistivity values in the range of 100–103 X cm. To improve the electrical conductivity of the films, they were annealed in a vacuum and in a hydrogen atmosphere. The annealing process did not affect the optical properties of the films. However, there were substantial structural and chemical changes in the films. Moreover, the electrical conductivity of the films was enhanced drastically upon annealing in hydrogen, where the electrical resistivity was reduced to 3.2 10 3 X cm.
I. INTRODUCTION
Zinc oxide (ZnO) is a material characterized by a wide direct band gap (3.37 eV), a large exciton binding energy (60 meV), a large piezoelectric response, high thermal and mechanical stability, radiation hardness, and biocompatibility.1–3 ZnO can be synthesized in the form of thin films or nanostructures4,5 and its constituent elements are abundant, nontoxic, and inexpensive.6,7 ZnO thin films have been extensively investigated for use in optoelectronic applications, such as light-emitting devices,8 photovoltaics,9 and photocatalysis.10 Nominally undoped ZnO thin films are n-type semiconductors with a background charge carrier concentration of the order of 1016 cm 3.3 Their electrical conduction characteristics are primarily determined by the electrons generated by intrinsic native defects, namely zinc interstitials due to excess zinc, oxygen vacancies, or hydrogen impurities.2,11–13 These deviations from stoichiometry generate a shallow donor level immediately below the conduction band14 and provide the free charge carriers. However, the provision of charge carriers by defects compromises the crystalline and stoichiometric quality of ZnO thin films. Moreover, these defects cannot be controlled, making the electrical properties of undoped ZnO thin films unstable
Contributing Editor: Cewen Nan a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2016.343
because of the chemisorption of oxygen at the top surface of the films and at grain boundaries.15 Thus, impurity doping is essential to overcome these drawbacks. In particular, doping with the trivalent elements of aluminum and gallium has been successfully implemented to synthesize highly conductive and stable ZnO thin films.16 This behavior stems from two factors. First, the electronegativity difference between zinc (1.65) and aluminum (1.61) or gallium
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