Study of the Electrical and Structural Characteristics of AlÕPt Ohmic Contacts on n-Type ZnO Epitaxial Layer Han-Ki Kim, a,b,c,z I. Adesida, a K.-K. Kim, b S.-J. Park, b and T.-Y. Seong b, * a Department of Electrical and Computer Engineering and Micro and Nanotechnology Laboratory, University of Illinois, Urbana, Illinois 61801, USA b Department of Materials Science and Engineering, Kwangju Institute of Science and Technology, Kwangju 500-712, Korea We have investigated Al/Pt ~20/50 nm! ohmic contacts on a n-type zinc oxide ~ZnO:Al! epitaxial layer. The samples were annealed at temperatures of 300°C and 600°C for 1 min in nitrogen ambient. Current-voltage measurements indicated that the as-deposited sample was ohmic with a specific contact resistivity of 1.25 ( 60.05) 3 10 25 V cm 2 . However, the annealing at 300°C resulted in a significantly better ohmic behavior, with a contact resistivity of 2 ( 60.25) 3 10 26 V cm 2 . A further increase in the annealing temperature to 600°C led to a decrease in specific contact resistivity due to extensive interfacial reactions between Al and ZnO. Both Auger electron spectroscopy and glancing angle X-ray diffraction were employed to investigate the nature of the interfacial reaction between the Al/Pt and ZnO layer with increasing annealing temperature. Possible explanation is given to describe the temperature dependence of the specific contact resistivity. © 2004 The Electrochemical Society. @DOI: 10.1149/1.1647576# All rights reserved. Manuscript received June 2, 2003. Available electronically February 19, 2004. ZnO and related oxide semiconductors, such as MgZnO and CdZnO, have acquired considerable importance as the basis for a new generation of optoelectronic devices due to their wide direct bandgap ( E g 5 3.37 eV), and large exciton binding energy of 60 meV at room temperature, which is much higher than that of GaN ~25 meV! and ZnSe ~18 meV!. 1-4 ZnO is an alternate candidate for optoelectronic applications, for use in the short wavelength range ~green, blue, UV!. 5 Although great progress has been achieved in the area of ZnO-based oxide semiconductors, many problems remain, such as the difficulty of p-ZnO growth and lack of high quality ohmic or Schottky contacts for n, p-ZnO. In particular, to realize commercialized ZnO-based optoelectronic devices, high quality ohmic and Schottky contacts are required. Several groups have re- ported ohmic contact schemes, however, most work is the initial stages. Akane et al. investigating In ohmic contacts to undoped n-ZnO, demonstrated that a postannealing treatment for 1 min at 300°C resulted in ohmic contact with a specific contact resistivity of 7 3 10 21 V cm 2 . 6 Kudo et al. fabricating transparent n-ZnO/p-SrCu 2 O 2 junction diode, employed n 1 -ZnO and ITO as n-type and p-type electrodes, respectively. 2 In our previous studies, we reported on Au, Ti/Au, and Ru ohmic contact scheme with spe- cific contact resistivity of 10 23 -10 25 V cm 2 . 7-9 Recently, an Al ohmic scheme was used for a metal-semiconductor-metal device structure and p-n homojunction diodes. 4,10 Although the Al have been used as n-type ohmic contact metal, there are still lack of detailed interfacial reaction study between Al and ZnO. In addition, the detailed electrical and structural examinations of the Al/Pt ohmic schemes have not been hitherto performed. In this work, we reported on electrical and structural examination of Al/Pt multilayer on Al-doped ZnO epitaxy layer with increasing annealing temperature. Using glancing angle X-ray diffraction ~GXRD! and Auger electron spectroscopy ~AES! depth profiles analysis, we suggested possible mechanism to describe temperature dependence of Al/Pt ohmic contact on n-ZnO. Experimental An Al-doped n-ZnO epitaxial layer was grown on a ~0001! sap- phire substrate by means of a high temperature epitaxy radio fre- quency sputtering system ~HTE rf sputtering system: Korea vacuum-KVS-25060! at 800°C using a 2 in. target containing 1 wt % Al 2 O 3 powder ~Pure Tech!. 11 The carrier concentration and mo- bility of the annealed layers were measured at room temperature by means of Hall effect measurements with Van der Pauw geometry. The measurements showed that the carrier concentration and Hall mobility are 2 ( 60.5) 3 10 18 cm 23 and of the order of 60 ~65! cm 2 /Vs, respectively. Prior to lithography, the samples were ultra- sonically degreased with acetone and methanol for 1 min in each step, and then rinsed with deionized ~DI! water. The native contami- nation layer was removed by treatment with a buffered oxide etchant ~BOE! solution then blown dry by nitrogen gas. The Al ~20 nm!/Pt ~50 nm! bilayer was subsequently deposited n-ZnO by electron beam evaporation. The circular pad was patterned by standard pho- tolithography techniques for measurement of specific contact resis- tivity using a circular-transmission line model ~CTLM!. 12 The use of CTLM structure is advantageous because no etching of ZnO is re- quired for feature isolation. The inner dot radius was 105 mm, and the spacing between the inner and the outer radii were 3, 4, 6, 12, 13, 16, and 21 mm. The actual gap spacing was measured by scan- ning electron microscopy and was used in the CTLM analysis. The measured total resistance, R T , between contacts for the circular model configuration can be expressed as R T 5 R s 2 p F ln S R 1 R 1 2 d D 1 L TS 1 R 1 1 1 R 1 2 d D G @1# where R s is the sheet resistance of the materials, R 1 the outer circu- lar dot radius, L T the transfer length, and d the gap spacing between the contacts. The total resistance ( R T ) was measured for various spacings and plotted as function of ln(R 1 /R 1 2 d). 13,14 The least squares curve fitting method was used to obtain a straight line plot of R T vs. ln(R 1 /R 1 2 d). Thus, the specific contact resistivity can be calculated by r c 5 L T 2 R s Current-voltage ~I-V! data were measured using a parameter ana- lyzer ~HP 4155A!. To characterize the extent of indiffusion between Al and n-ZnO, AES was used ~PHI 670 model! with an electron beam of 10 keV and 0.0236 mA depth profiles. The interfacial reac- tion products were identified by GXRD, which was carried out with a Rigaku diffractometer ~D/MAX-RC!. Atomic force microscopy ~AFM! was employed to characterize the surface morphology of the samples. Results and Discussion Figure 1 shows the I-V characteristics of the Al/Pt metallization scheme on ZnO with increasing annealing temperatures. They were measured with a circular transmission line method of 4 mm spacing. Both as-deposited and annealed samples show linear I-V character- * Electrochemical Society Active Member. c Present address: Samsung SDI, Core Technology Laboratory, Suwon, Gyeonggi-Do 442-391, Korea. z E-mail: hanki1031@samsung.com Journal of The Electrochemical Society, 151 ~4! G223-G226 ~2004! 0013-4651/2004/151~4!/G223/4/$7.00 © The Electrochemical Society, Inc. G223