Improvement of Mobility in ZnO Thin Film Transistor with an Oxygen Enriched MgO Gate Dielectric

Abstract

The performance of bottom-gate ZnO thin film transistors (TFTs) using MgO gate dielectrics evaporated with and without introducing oxygen have been investigated. The oxygen introduced during MgO deposition improves the field-effect mobility significantly as compared to the device without introducing oxygen during MgO deposition. The oxygen-introduced MgO exhibits a dielectric constant of 10.9 and the field-effect mobility of the TFT device is enhanced to 78.3 cm 2 /V s. The threshold voltages can also be related to whether the oxygen is introduced into MgO or not. The interface between oxygen-introduced MgO and ZnO is examined and its connection with mobility enhancement is discussed. © 2012 The Electrochemical Society. [DOI: 10.1149/2.003205ssl] All rights reserved. Oxide semiconductor-based thin film transistors (TFTs) as a replacement for conventional Si-based TFTs in electrical and optical devices have been extensively studied due to their excellent optical and electrical properties.1-3 Among the numerous oxide semiconductors, ZnO has been widely used as channel layer in TFTs in recent years, 4-6 owing to its simple composition and expeditious fabrication. In order to accumulate a sufficient number of charges in the TFTs channel, the use of high-dielectric constant (k) materials is suggested, which leads to a reduction in the operating voltage.7-9 Magnesium oxide (MgO) is a candidate of high-k gate dielectrics (k = 9.4∼9.8) 10,11 because of its good isolation 10,12 and chemically inert properties. In addition, MgO films can sustain the plasma damage effectively because it is well-known as an excellent protecting layer of a plasma display panel (PDP) from erosion by plasma ion bombardment. 13,14 Many researches indicate that MgO can play a significant role in controlling the polarity of ZnO film.15-17 Additionally, TFT device parameter such as field effect mobility is mostly related to the crystallinity and defect density of the channel materials. In general, MgO films can possess a large number of excess oxygen ions. 18 In this work we attempted to utilize these excess oxygen ions to compensate for the defect density, such as oxygen vacancies in the channel material. Therefore, MgO thin films that served as gate dielectric in ZnObased TFTs were prepared with and without introducing oxygen by electron beam evaporation. The material and electrical characteristics of the MgO gate dielectrics for the ZnO-based TFTs are investigated to explore the function of the MgO dielectric.The bottom-gate, top contact TFT devices were fabricated on glass substrates. Prior to the MgO deposition, an Al gate electrode was first deposited by electron beam evaporation onto a glass substrate through a shadow mask. MgO films were then deposited with a thickness of 180 nm by electron beam evaporation from the MgO slugs (Kurt J. Lesker, 99.99% purity) with and without introduction of an O 2 gas flow of 50 sccm. The ZnO active layer of a 30 nm thick was deposited on the MgO dielectric by sputtering from a ZnO target (Elecmat, 99.995% ...

Suppression of Oxygen Vacancy and Enhancement in Bias Stress Stability of High-Mobility ZnO Thin-Film Transistors with N2O Plasma Treated MgO Gate Dielectrics

“…On the contrary, the area ratios of the Our previous study showed that the excess oxygen in MgO dielectric layer will compensate for the oxygen deficiencies near the ZnO/MgO interface, leading to an enhanced mobility of ∼50 cm 2 /Vs. 21 XPS analysis therefore indicates that compensated oxygen near to the interface will be driven back to MgO under positive gate bias and generate oxygen vacancies in ZnO. As a result, the ZnO channel becomes more conductive and it leads to a high I off as well as a negative V TH of the devices after PBS.…”

“…21 Excess oxygen in MgO will not only increase the alignment of the ZnO channel layer along the (002) planes, but also compensate for the oxygen deficiencies near the ZnO/MgO interface, enhancing the mobility of TFTs. However, similar to the other high-k TFTs, the device suffers from the stability problem under gate bias stress.…”

Synthesis of Mg and Zn diolates and their use in metal oxide deposition

“…[9][10][11] Magnesium oxide is a promising candidate for the generation of thin lms of inorganic high-k metal oxide dielectrics, due to its wide band gap (7.8 eV), low refractive index (1.72), low dielectric constant (9.8), high melting point (2900 C), chemical and thermal stability, as well as its outstanding diffusion barrier properties. [12][13][14][15][16] Moreover, MgO thin lms can be used for the protection of plasma display panels from erosion by ion bombardment. 17 Several deposition methodologies for MgO and ZnO layer formation exist, including the sol-gel process in combination with spin-coating, 18,19 spray pyrolysis, 20,21 pulsed laser deposition, 22,23 molecular beam epitaxy, 24 as well as metal-organic gas phase deposition techniques such as CVD (¼ chemical vapor deposition) [25][26][27] and ALD (¼ atomic layer deposition).…”

ZnO TFTs prepared by chemical bath deposition technique with high‐k La2O3 gate dielectric annealed in ambient atmosphere

“…ZnO is also widely used in high‐temperature electronics devices that are reliable for space and other high environments. Nowadays, ZnO is widely used as the active material in TFTs because of its transparency and high field effect mobility, high mechanical, thermal, and chemical stability . Many investigators have reported that silicon‐based TFTs have limited applications due to their low field effect mobility, light sensitivity, and small drain current.…”