Transport and surface conductivity in ZnO

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Control of the electrical properties of ZnO is difficult to achieve. Doping is affected by the presence of a n-type background. Magnetotransport measurements can extract detailed information on donors and acceptors, but characterization is complicated by effects such as the surface conductivity. This conducting layer can be activated by ambient illumination or by heating in the absence of oxygen. There are considerable differences in the behavior of the various polar and nonpolar crystal faces. This paper provides an overview of the properties of ZnO surface conductivity, as well as the methods which have been implemented to account for it while interpreting carrier transport measurements.

I. INTRODUCTION

ZnO is a material of interest for many applications. Its deep exciton binding energy makes it attractive for ultraviolet (UV) lasers and light-emitting diodes (LEDs), and its surface reactivity and piezoelectric properties make it useful for gas sensors and transducers.1 There are many fine reviews concerning ZnO,2 its electrically active impurities and defects,3,4 as well as its processing and doping.5 Of particular interest is the difficulty of p-type doping.6 Challenges in binary doping are commonly encountered in wide band gap materials. While Schottky and heterojunction devices have been possible for some time, ZnO homojunction LEDs remain in their infancy.7 The ability to characterize both n-type and p-type doping with certainty is of value. Additionally, an understanding of carrier transport is useful in general for the fabrication of devices. Carrier mobility in ZnO and its magnetotransport characterization will be reviewed. Particular attention will be paid to the nature of the surface conductivity channels on the various faces of ZnO, as this can be a major obstacle to the understanding of the electronic properties of ZnO if not properly accounted for. II. SURFACE CONDUCTIVITY

It has long been known that ZnO can exhibit a surface conducting layer with a sheet conductivity on the order of 104 X1. It is created or exposed by heating above 500 K in vacuum or inert gas8 or by exposure to above band gap UV without heating. Exposure to oxygen, particularly in the presence of humidity, quenches this conducting layer, as shown in Fig. 1. The adsorbed oxygen atoms were

a)

Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2012.133 J. Mater. Res., Vol. 27, No. 17, Sep 14, 2012

believed to act as acceptors, creating a negative surface charge layer of trapped electrons, inducing a depletion region near the surface of a typical n-type crystal and decreasing the conductivity. The size of the depletion region alone is not enough to account for this drop in conductivity, which is measurable even when the thickness of the sample is much greater than that of any depletion region. Rather, some highly conducting surface layer is eliminated or masked by the oxygen.9 Heat or UV exposure was found to thermally desorb or photodesorb oxygen through photogenerated holes.10 The layer

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