Postdeposition annealing of NiOx thin films: A transition from n-type to p-type conductivity for short wave length optoe
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Monika Tomar Physics Department, Miranda House, University of Delhi, Delhi 110007, India
Vinay Guptaa) Department of Physics and Astrophysics, University of Delhi, Delhi 110007, India (Received 18 August 2012; accepted 27 December 2012)
The present work highlights the effect of postdeposition annealing (373–773 K) on the tunability of electrical conductivity of radio frequency sputtered NiOx thin films with both n-type and p-type behavior. The NiOx thin films were polycrystalline with preferred (200) orientation with high optical transmission. The as-grown NiOx thin film exhibits an n-type behavior with room temperature resistivity of 4.80 103 X-cm and majority carrier (electrons) concentration of about 3.90 1020 cm3. Film annealed at 473 K was p-type having resistivity of 1.54 101 X-cm and majority carrier (hole) concentration of about 4.45 1018 cm3. Hall effect and thermoelectric power measurements confirm a transition in electrical conduction from n-type to p-type with postdeposition annealing at 473 K. The observed tunability of electrical conductivity of NiOx thin film will pave way toward realization of p-n homojunction for short wave length optoelectronic device applications.
I. INTRODUCTION
Nickel oxide (NiOx), a wide band gap (.3.0 eV) semiconductor, has a stable cubic structure (NaCl type) with octahedral Ni(II) and O2 sites having lattice constant, a 5 4.177 Å.1–3 NiOx is a potential candidate for transparent electronics, next-generation blue- and UV-light emitting and lasing devices, bio-sensors, antiferromagnetic layers, electrochromic devices, supercapacitor, etc.1–5 Nowadays, NiO attracts great attention because of its powerful application in organic photovoltaics by serving as the hole-transporting/electron-blocking interfacial layer to minimize interfacial power losses and hence suppression of the dark current owing to its high work function.6–8 NiO in bulk form is classified as a Mott–Hubbard insulator with very low conductivity (;1011 X1 m1) at room temperature.6–8 The conductivity of NiO can be increased by controlling the processing conditions or by doping of monovalent impurities such as lithium.9 NiO is synthesized in general with p-type conductivity by hole transport originated from nickel vacancies and oxygen interstitials.10 To fabricate p-n junctions, which is critical for the realization of next-generation oxide electronics, p-type NiO and doped n-type zinc oxide (ZnO) semiconductors have been exploited.11 However, these heterojunctions showed unsatisfactory rectifying characteristics, i.e., large reverse-bias a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2012.443 J. Mater. Res., Vol. 28, No. 5, Mar 14, 2013
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leakage current due to the presence of interface states or defects between two different oxide layers. Improved rectification in these heterojunctions has been demonstrated using sandwiched intrinsic layer of ZnO but at the cost of increased complexities and higher cos
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