Electrical Characteristics of RF Sputtered ZnO/HfO 2 Interfaces in Transparent Thin Film Transistors
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Electrical Characteristics of RF Sputtered ZnO/HfO2 Interfaces in Transparent Thin Film Transistors Prem Thapaliya, Wenchao Lu and Rashmi Jha Department of Electrical Engineering and Computer Science University of Toledo, OH 43606, U.S.A. ABSTRACT In this work, we have reported the interface characterization of rf sputtered ZnO/HfO 2 in thin film transistor structure by dc current-voltage and admittance spectroscopy. The interface state density (Dit) of 1013 eV-1cm-2 was extracted from the Gp/ω vs ω plot was comparable to value obtained from the subthreshold behavior. The grain boundary trap density (NGB) of 9.12×1012 cm-2 was estimated using Levinson’s model. The interface state density distribution below the conduction band edge shows a decreasing trend with energy below the conduction band edge. We also studied the impact of introducing MgO interfacial layer between ZnO and HfO2 interface as an approach towards decreasing the interface state density. INTRODUCTION ZnO based thin film transistors (TFTs) have received much attention due to wide direct band gap of 3.37 eV, high electron mobility, high transparency in the visible range (400-700 nm), low temperature synthesis and thermal stability [1-2]. In the past few years, much of the research efforts have been inclined towards the integration of high-k gate dielectric material in ZnO TFTs in an attempt to improve the carrier mobility, decrease the gate leakage current, and improve the overall device performance. Among several high-k dielectrics, HfO2 is promising due to its high dielectric constant (20-25), wide band gap (5.6 eV), and reasonable band-offset with ZnO [2]. However, the performance of the TFTs is largely limited by the quality of the gate dielectric and high interface trap density at the interfaces between HfO2 and ZnO layer [3]. Therefore, the study of the interface properties between ZnO and high-k dielectrics such as HfO2 is essential to improve the TFT performance. To reduce trap density, a thin MgO interfacial layer between HfO2 and ZnO is studied. MgO is a high-k dielectric (k = 9.4∼9.8) with wide band gap (7.7 eV) and good chemical stability [4]. It is reported to reduce the grain boundaries in the ZnO film due to excess oxygen ions [4]. In addition, MgO has a high tolerance for plasma ion bombardment which can effectively minimize the defect density in the ZnO layer during depositions using sputtering [4]. Though there have been previous studies of ZnO/HfO2 TFT fabricated using a combination of Atomic Layer Deposition (ALD), Pulsed layer Deposition (PLD), and other techniques [5-6], our knowledge on the characteristic of interfaces formed between ZnO and HfO2 when both materials are sputtered is limited. It should be noted that sputtering technique offers advantages in terms of ability to deposit materials at low-temperature on desired substrate such as glass and plastics, and large-area scalability. However, lowtemperature deposition can also introduce unique defects in high-k and ZnO and their interface which needs to be better understood.
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