ZnO Doping and Co-doping Paradigm and Properties K. Shtereva, a,b,z I. Novotny, b, * V. Tvarozek, b P. Sutta, c A Vincze, b,d and A. Pullmannova b a Department of Electronics, University of Rousse, BG-7017 Rousse, Bulgaria b Department of Microelectronics, Slovak University of Technology, 81219 Bratislava, Slovakia c New Technologies - Research Center, West Bohemia University, 30614 Plzen, Czech Republic d International Laser Centre, 84104 Bratislava, Slovak Republic Here, we report on experimental studies of the role of doping and co-doping on the properties of ZnO thin films deposited by radio-frequency diode sputtering at varying nitrogen content  0 ÷ 100% in the sputtering Ar /N 2 gas. Co-doping improved the crystalline structure, and ZnO:Al:N films maintain a c-axis texture in the direction of the surface normal. Depending on the N 2 content, the estimated crystallite size varies from 8 to 37 nm for ZnO:N and 21–33 nm for ZnO:Al:N. Nitrogen doping results in an increased absorption around the bandedge and the bandgap narrowing  E g  3.2 eV. © 2010 The Electrochemical Society. DOI: 10.1149/1.3459900 All rights reserved. Manuscript submitted January 11, 2010; revised manuscript received May 14, 2010. Published July 23, 2010. Zinc oxide ZnO is a wide bandgap 3.37 eV, group II–VI compound semiconductor with a hexagonal wurtzite structure and diverse properties that depend on doping, including a type of con- ductivity n-type or p-type, which can range from metallic to insu- lating, high transparency, piezoelectric, ferromagnetic, and sensing qualities. Therefore, this versatile material has been intensively stud- ied for a wide range of applications from optoelectronic and trans- parent electronic devices, 1 surface and bulk acoustic wave devices and piezoelectric transducers, 2 and spintronics 3 to chemical and gas sensors 4 and solar cells. 5 Moreover, ZnO has a widely abundant and inexpensive source material and is an environment friendly material. Low cost, high resistance to hydrogen plasma, and ease of pattern- ing make ZnO an alternative to indium tin oxide in transparent con- ducting oxide applications such as transparent electrodes for liquid crystal displays, in organic light emitting diodes, and in photovoltaic solar cells. 6 ZnO has been studied as a semiconductor for light emit- ting diodes and UV semiconductor lasers due to its advantages over gallium nitride GaN, namely, a larger exciton binding energy 60 meV and availability of high quality ZnO single crystals. The fabrication of ZnO-based devices and the progress of transpar- ent electronics are predetermined from the advance in quality and processing of both n-type and p-type ZnO materials. Among numer- ous methods for ZnO thin-film deposition, sputtering is often used due to its important industrial advantages, namely, simplicity, low processing temperature, and low cost. High quality n-type ZnO with electron concentrations of 10 21 cm -3 has been achieved using alu- minum Al, gallium Ga, or indium In as donor dopants. 7,8 Much effort has been put in the development of transparent p-type ZnO materials with high hole concentrations needed for transparent elec- tronics, organic optoelectronics applications, and photovoltaics. Among the possible acceptors, nitrogen has been one of the most studied potential p-type dopant, and p-type ZnO via nitrogen doping has been reported. 9,10 Nitrogen dopant source is easy to get, eco- nomically lucrative, and nontoxic. The novelty of our approach is in the use of radio-frequency rf diode reactive sputtering toward the goal of obtaining a p-type ZnO material via nitrogen doping. In general, sputter deposition is characterized by complex processes occurring i on the target bombarded by energetic ions, ii in low temperature plasma, and iii on the substrate surface and the grow- ing film. Thin-film growth is influenced by the kinetic energy of coating species on the substrate. In addition to substrate tempera- ture, a total energy flux that depends mainly on the amount and the energy of i sputtered coating species 1–20 eV, ii energetic neu- tral working gas atoms neutralized and reflected at the target 100 eV, iii energetic secondary electrons emitted from the tar- get 100 eV, and iv negative ions coming from the working gas plasma or target  1 keV is an important parameter for film mi- crostructure evolution. The effects of high energy particle bombard- ment on ZnO films O 2 , Ar, O, and O -  during diode and magnetron sputtering have been known for a long time. 11 Rf diode sputtering in Ar /N 2 discharge provides a very reactive environment and supports the formation of charged and neutral species ZnO, ZnO + , Zn, Zn + , Ar, Ar + , O, O - ,e - ,O 2 + , N, N + , and N 2 +  that can more or less affect the growing film. We found previously 12 that the crystalline struc- ture of sputtered ZnO films and, hence, film properties are deter- mined by both the substrate temperature and the total energy flux density. When rf diode sputtering in a low pressure region  p  1.3 Pa is used for film growth, the mean free path of sputtered particles 10 -2 m is comparable with the distance between the target and the substrate holder usually 4.10 -2 m, i.e., we can as- sume a “collision-less” regime, particularly for energetic particles that passed through the rf discharge. In rf diode sputtering, three important effects with regard to p-type doping of ZnO can be pre- sented. The first is an increased molecular dissociation that can lead to the formation of nitrogen atoms  N 2 ↔ 2N and NO, whose in- corporation in the growing film forms desirably for p-type doping nitrogen acceptors  N O . Moreover, N atoms can form AlN/GaN molecules when co-dopants Al and Ga are added to the growth ambient. Thus, the concentration of N that can reconvert to N 2 is reduced and more N atoms reach the growing film and substitute for O leading to p-type ZnO. Second, in rf diode sputtering, the substrate/growing film are subjected to energetic particle bombard- ment sputtered particles, reflected neutrals, and negative ions that can affect the formation of acceptor or donor complexes in the growing film. Third, there is a significant contribution of secondary electron bombardment to the atomic scale heating of the film when it is prepared by the rf diode sputtering. Our aim was to exploit the special technological conditions of rf diode sputtering in comparison, e.g., with magnetron sputtering for developing a p-type doping paradigm and for investigating the role of doping and co-doping on the properties of rf-sputtered ZnO thin films deposited at varying nitrogen contents  0 ÷ 100% in the sput- tering Ar /N 2 gas. Experimental The ZnO:N and ZnO:Al:N thin films were deposited on Corning glass 7059 substrates by rf diode sputtering in an Ar /N 2 working gas. The percentage of nitrogen in the sputtering gas varied from 0 to 100%. Other deposition parameters, such as base pressure  2  10 -5 Pa, working gas pressure 1.3 Pa, and sputtering power 500 W ZnO:N/400 W ZnO:Al:N, were maintained constant during the deposition process. The ZnO:N films were sputtered from a ZnO target purity 99.99%. The deposition rate decreased linearly with the increasing content of nitrogen from 25 to 75% in the sputtering gas Fig. 1. A sintered ceramic target, a mixture of ZnO * Electrochemical Society Active Member. z E-mail: KShtereva@ecs.uni-ruse.bg Journal of The Electrochemical Society, 157 9 H891-H895 2010 0013-4651/2010/1579/H891/5/$28.00 © The Electrochemical Society H891 Downloaded 06 Oct 2010 to 91.139.245.14. Redistribution subject to ECS license or copyright; see http://www.ecsdl.org/terms_use.jsp