HfZnO/ZnO Heterostructures Fabricated Using Low-Cost Large-Area Compatible Sputtering Processes
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HfZnO/ZnO Heterostructures Fabricated Using Low-Cost Large-Area Compatible Sputtering Processes Chih-Hung Li1, Jian-Zhang Chen1,*, I-Chun Cheng2 1
Graduate Institute of Applied Mechanics, National Taiwan University, Taipei City 10617, Taiwan 2 Graduate Institute of Photonics and Optoelectronics & Department of Electrical Engineering, National Taiwan University, Taipei City 10617, Taiwan *Email: [email protected] ABSTRACT We investigated the electrical properties of the rf-sputtered HfxZn1-xO/ZnO heterostructures. The thermal annealing on ZnO prior to the HfxZn1-xO deposition greatly influences the properties of the heterostructures. A highly conductive interface formed at the interface between HfxZn1-xO and ZnO thin films as the ZnO annealing temperature exceeded 500°C, leading to the apparent decrease of the electrical resistance. The resistance decreased with an increase of either thickness or Hf content of the HfxZn1-xO capping layer. The Hf0.05Zn0.95O/ZnO heterostructure with a 200-nm-thick 600°C-annealed ZnO exhibits a carrier mobility of 14.3 cm2V-1s-1 and a sheet carrier concentration of 1.93×1013 cm-2; the corresponding values for the bare ZnO thin film are 0.47 cm2V-1s-1 and 2.27×1012 cm-2, respectively. Rfsputtered HfZnO/ZnO heterostructures can potentially be used to increase the carrier mobility of thin-film transistors in large-area electronics. INTRODUCTION The ZnO is a n-type wide bandgap semiconductor [1] that has been applied in various types of optoelectronic devices such as light emitting diodes (LEDs) [2, 3], UV sensors [4, 5], gas sensors [6, 7], and thin-film transistors (TFTs) [8-11]. The optoelectronic properties of ZnO can be further modulated by alloying various types of oxides into it. For examples, alloying the group III-A dopants (like Al and In) can increase the conductivity [12-15], whereas alloying group IV-B dopants can improve the stability of ZnO [16-18]. Recently, HfZnO thin films have been studied by several groups [1, 16, 19-26]. Our previous experimental results indicated that the crystallinity of sputtered HfZnO decreased as the Hf content increased, and the thermal annealing process could release the built-in stresses of the HfZnO thin films to reduce the bandgaps [24]. We also prepared HfZnO via sol-gel method. Different from the results of sputtered HfZnO, the Hf doping in turn improved the crystallinity of HfZnO, and HfO2 precipitated in the HfZnO thin films after thermal annealing [25]. Bae et al. investigated HfZnO thin films deposited by pulsed laser deposition at various temperatures. The built-in stress status also changed from compression to tension, and the bandgap decreased with the deposition temperature [1]. The sputtered HfZnO had been applied to the active layer of TFTs. The incorporation of Hf in ZnO increased the threshold voltage of the TFT and enhanced the bias temperature stability [27, 28]. Cheol et al. reported the electrical stability of bottom-gated TFTs with a ZnO/HfZnO multilayer channel grown by atomic layer deposition. The TFTs exhibited good stabi
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