Characterization of amorphous zinc tin oxide semiconductors
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Shail Sanghavi, Ponnusamy Nachimuthu, Vaithiyalingam Shutthanandan, and Tamas Varga Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, Washington 99352
Brendan Flynn School of Chemical, Biological, and Environmental Engineering, Oregon State University, Corvallis, Oregon 97331
Suntharampillai Thevuthasan Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, Washington 99352
Gregory S. Hermana) School of Chemical, Biological, and Environmental Engineering, Oregon State University, Corvallis, Oregon 97331 (Received 3 February 2012; accepted 4 May 2012)
Amorphous zinc tin oxide (ZTO) was investigated to determine the effect of deposition and postannealing conditions on film structure, composition, surface contamination, and thin-film transistor (TFT) performance. X-ray diffraction results indicated that the ZTO films remain amorphous even after annealing to 600 °C. Rutherford backscattering spectrometry indicated that the bulk Zn:Sn ratio of the sputter-deposited films were slightly tin rich compared to the composition of the ceramic sputter target. X-ray photoelectron spectroscopy indicated that residual surface contamination depended strongly on the sample postannealing conditions where water, carbonate, and hydroxyl species were adsorbed to the surface. Electrical characterization of ZTO TFTs indicated that the best devices had mobilities of 17 cm2/Vs, threshold voltages of 1.5 V, subthreshold slopes of 0.9 V/dec, turn-on voltages of 12 V, and on-to-off ratio of .107. Annealing ZTO in vacuum assisted in the removal of adsorbed species, which may reduce defects in the films and improve device performance. I. INTRODUCTION
Thin-film transistors (TFTs) with transparent amorphous oxide semiconductors (TAOS) represent a major advance in the field of thin-film electronics.1 TAOS materials containing heavy metal cations with (n 1)d10ns0 (n $ 4) electronic configuration have demonstrated relatively high mobilities (e.g., .10 cm2/Vs) as a TFT channel material.1–3 It has been proposed that high electron transport of TAOS materials is due to the overlap of empty metal s orbitals with spherical symmetry. These metal s orbitals are unaffected by disorder due to the nondirectionality of metal–metal s orbital overlap and the nearly constant metal–metal distance in amorphous and crystalline materials.2 Several investigations have demonstrated that TAOS materials have relatively high electron mobilities in spite of being amorphous.3–8 Examples of TAOS materials include zinc tin oxide (ZTO),3 indium gallium zinc oxide (IGZO),9 indium zinc oxide,10 a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2012.170 J. Mater. Res., Vol. 27, No. 17, Sep 14, 2012
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and indium zinc tin oxide.11 Of these materials, ZTO does not contain indium or gallium and is relatively inexpensive by comparison. ZTO has been used primarily as transparent conductors,12 but with pot
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