Isothermal capacitance transient spectroscopy for deep levels in Co- and Mn-doped ZnO single crystals
- PDF / 268,429 Bytes
- 7 Pages / 612 x 792 pts (letter) Page_size
- 11 Downloads / 250 Views
Junzo Tanaka, Takeshi Ohgaki,c) and Hajime Haneda Advanced Materials Laboratory,b) National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan
Mio Ozawa and Takaaki Tsurumi Department of Metallurgy and Ceramic Science, Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro, Tokyo 152-8552, Japan (Received 5 September 2001; accepted 28 March 2002)
Deep donor levels in ZnO single crystals doped with transition metal (TM; Co or Mn) were characterized by isothermal capacitance transient spectroscopy (ICTS) applied to ZnO-based Schottky junctions, Au/ZnO (0001¯) or Ag/ZnO (0001¯). The barrier height at the junction and donor concentration was not influenced by TM. A deep donor level at 0.28 eV was detected by ICTS; however, its energy dispersion and concentration was composition independent. The effect of doping with TM was found in the magnitude of leakage current; in other words, the leakage current at the Au/ZnO:Mn junction was lower than the other junctions on undoped or Co-doped crystals. I. INTRODUCTION
ZnO varistors1,2 have been used as surge cut filters in electric circuits because of their excellent nonlinear current–voltage (I–V) characteristics. It is generally accepted that the nonlinear I–V relationship is enhanced by additives, e.g., Bi1 or Pr,2 and that the formation of double depletion layer at grain boundary is the origin of varistor properties. Several models and theories for the origin of depletion layers and the current transport processes have been proposed.3–7 However, relationships between the chemical compositions of additives and properties of varistors are still vague. To understand and develop the properties of ZnO varistors, it is necessary to clarify the contribution of each additive to phenomenological parameters. However, quantitative analyses and measurements for grain boundaries in ZnO varistors are difficult because of their small grain size, random grain orientation, and inhomogeneous chemical
a)
Address all correspondence to this author. e-mail: [email protected] Moved from Tokyo Inst. Tech. to AML-NIMS on July 1st, 2000. b) National Institute for Research in Inorganic Materials was reorganized into the Advanced Materials Laboratory, National Institute for Materials Science, in April 2001. c) Moved from Tokyo Institute of Technology to AML-NIMS on April 1, 2002. J. Mater. Res., Vol. 17, No. 6, Jun 2002
http://journals.cambridge.org
Downloaded: 12 Apr 2015
composition.8–10 In fact, each grain boundary in ZnO varistor ceramics has its own property as reported in Ref. 11. Therefore, apparent I–V characteristics of polycrystalline ZnO ceramics are an average or a summation of the properties of each grain boundary. Using single crystals and epitaxially grown thin films is an appropriate way to simplify the problem. For example, bicrystals12 and stacked thin films13 were synthesized as artificially controlled model grain boundaries. In our previous studies,14,15 Co- or Mn-doped ZnO bicrystals were s
