Microstructural Evidence of Hall Mobility Anisotropy in c-Axis Textured Al-Doped ZnO Yoshiaki Kinemuchi, w,z Hiromi Nakano, y Hisashi Kaga, z Satoshi Tanaka, z Keizo Uematsu, z and Koji Watari z z National Institute of Advanced Industrial Science and Technology, Advanced Manufacturing Research Institute (AIST), Nagoya 463-8560, Japan y Cooperative Research Facility Center, Toyohashi University of Technology, Toyohashi 441-8580, Japan z Department of Materials Science and Technology, Nagaoka University of Technology, Nagaoka 940-2188, Japan The high electrical conductivity, 1150 S/cm at room tempera- ture, in the ab-plane of c-axis textured Al-doped ZnO is attrib- uted to its high Hall mobility that is almost double the mobility in the c-axis direction. Temperature-independent mobility in the ab-plane below 200 K suggests that ionized impurity dominates the scattering of electron transport, which reasonably agrees with a modified Brooks–Herring–Dingle model taking into account nonparabolic E–k dispersion. However, the pronounced anisotropy between ab-plane and c-axis cannot be expected based on the model. Detailed observations of the grain bound- ary (GB) by means of high-resolution transmission electron microscopy, high-angle annular dark-field scanning transmis- sion electron microscopy, and energy-dispersive X-ray spectros- copy revealed the existence of an Al-enriched, Zn-deficient layer near the GB traversing the c-axis direction. In contrast, the highly conductive direction encompasses a tilt grain boundary, in which coincident sites were observed and Al segregation was barely evident. We conclude that such a preferential segregation in the GB and/or GB structure itself are responsible for the an- isotropy of mobility in the textured Al-doped ZnO. I. Introduction T HE well-known zinc oxide is an n-type semiconductor whose conductivity is effectively enhanced by Al 1,2 or Ga 3 impurity doping, which is promising for practical applications such as thermoelectric materials or transparent conductive oxides. We previously reported that for highly c-axis oriented Al- doped ZnO ceramics, textured by magnetic alignment, the elec- trical conductivity at room temperature along the ab-plane, i.e., the basal plane of ZnO, increased to 41000 S/cm, which was almost twice that along the c-axis. 4–6 This high conductivity along the ab-plane originated from a high mobility of B90 cm 2 / V  s. In addition, the conductivity was found to be a function of the degree of orientation: a higher degree of orientation resulted in a higher level of conductivity. However, an analysis based on a tensor calculation indicated that the orientation of the crys- tallite was not responsible for the enhancement in conductivity. 4 A study on the Hall mobility of ZnO single crystal without doping reported nearly isotropic properties above room tem- perature, supporting our analysis. 7 A probable mechanism for the anisotropy of the textured Al– ZnO is the difference in scattering at the grain boundary, which generally originates in the structure and/or the chemical homo- geneity at the grain boundary (GB). In order to solve the uncertainty in the textured Al-doped ZnO system, we performed detailed observations on GBs using high-resolution transmission electron microscopy (HRTEM), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), and energy-dispersive X-ray spectroscopy (EDS). Hall measurement was also carried out to clarify the anisotropy in mobility. II. Experimental Procedure C-axis textured Al-doped ceramics were prepared via the mag- netic field alignment method. Nominal composition of the ce- ramics was Al 0.02 Zn 0.98 O, which was prepared from commercial ZnO powder adding g-Al 2 O 3 of 1 mol%. Details of sample preparation can be found elsewhere, 6 but briefly, the procedure was as follows. Because of the anisotropic magnetic susceptibil- ity of ZnO, particles dispersed in liquid start to align within a few seconds in correlation with the magnetic field. These aligned particles were fixed by the gelation of monomer mixed in the liquid. Upon completion of gelation, the samples were dried and sintered via standard ceramic processing. The texture strength of the ceramic was almost 100 multiple of random distribution confirmed by a pole figure of 002 diffraction. Next, the samples were sliced perpendicular to the c-axis (ab-plane) and parallel to the c-axis (see Fig. 1). Carrier con- centration and mobility were analyzed at a temperature range of 80–400 K using a DC Hall measurement system (Resitest8300, Toyo Corporation, Tokyo, Japan). Here, the van der Pauw method was adopted. The current–voltage characteristics of these contacts showed a linear relationship. The applied mag- netic field was 0.75 T during the measurement and reverse po- larity measurement was carried out to cancel the voltage offset. The microstructure of the sintered specimens was observed by transmission electron microscopy (TEM; 3000F, JEOL, Tokyo, Japan). TEM foils were prepared using the standard technique for thin ceramic foils: cutting, grinding, dimpling, and Ar-ion thinning. Energy-dispersive X-ray spectroscopy (EDS; Voyager III, NORAN Instruments Inc., Middleton, WI) was used for elemental analyses of GBs. The spot size was controlled to be o1 nm for this EDS analysis. Moreover, HAADF-STEM (JEM2100F equipped with Cs corrector, JEOL) was used to clarify the image contrast of atomic columns. EDS mapping was also carried out using JEM2100F under the conditions of the dwell time of 0.8 ms and the spot size of 0.6 nm. As to the view direction of the GB observations, refer to Fig. 1. III. Results and Discussion (1) Anisotropy of Hall Mobility The results of Hall measurement for the ab-plane and c-axis are shown in Fig. 2. The carrier concentration in each direction H.-J. Kleebe—contributing editor w Author to whom correspondence should be addressed. e-mail: y.kinemuchi@aist.go.jp Manuscript No. 28126. Received September 9, 2010; approved December 8, 2010. J ournal J. Am. Ceram. Soc., 94 [8] 2339–2343 (2011) DOI: 10.1111/j.1551-2916.2010.04373.x r 2011 The American Ceramic Society 2339