Elsevier

Current Applied Physics

Volume 15, Issue 9, September 2015, Pages 1010-1014
Current Applied Physics

A study of electrical enhancement of polycrystalline MgZnO/ZnO bi-layer thin film transistors dependence on the thickness of ZnO layer

https://doi.org/10.1016/j.cap.2015.06.006Get rights and content

Abstract

A polycrystalline MgZnO/ZnO bi-layer was deposited by using a RF co-magnetron sputtering method and the MgZnO/ZnO bi-layer TFTs were fabricated on the thermally oxidized silicon substrate. The performances with varying the thickness of ZnO layer were investigated. In this result, the MgZnO/ZnO bi-layer TFTs which the content of Mg is about 2.5 at % have shown the enhancement characteristics of high mobility (6.77–7.56 cm2 V−1 s−1) and low sub-threshold swing (0.57–0.69 V decade−1) compare of the ZnO single layer TFT (μFE = 5.38 cm2 V−1 s−1; S.S. = 0.86 V decade−1). Moreover, in the results of the positive bias stress, the ΔVon shift (4.8 V) of MgZnO/ZnO bi-layer is the 2 V lower than ZnO single layer TFT (ΔVon = 6.1 V). It reveals that the stability of the MgZnO/ZnO bi-layer TFT enhanced compared to that of the ZnO single layer TFT.

Introduction

Zinc oxide based materials are a promising material for use in the next generation of flat panel displays and transparent electronic devices because of, for example, its high transparence, low-temperature processing, and higher field-effect mobility than shown by a-Si:H-TFTs [1], [2], [3]. Among of ZnO-based materials, Mg incorporation into ZnO reported that the conduction band edge increased in energy band gap, potentially away from the intrinsic shallow donor state [4]. It can control the net electron carrier concentration by suppressing carrier generation via oxygen vacancy formation, so shifts of the threshold voltage during on-state bias stressing can be prevented. However, previous our works of MgZnO TFT, the field effect mobility were decreased from 9.12 cm2 V−1 s−1 to 3.11 cm2 V−1 s−1, as Mg incorporation was increased [5]. Recently, a Monte Carlo simulation combing the grain boundary scattering effect and the 2-D finite-element method Poisson and drift-diffusion solver of polycrystalline MgZnO/ZnO bi-layer by Huang et al. [6] led to the observation of improved the mobility of MgZnO on ZnO due to the screening effect of the grain boundary potential by the polarization field and modulated doping. Chin et al. [7] also reported that the MgZnO/ZnO structures deposited by RF sputtering showed the enhancement of Hall mobility, despite of polycrystalline structures.

When the MgZnO/ZnO hetero-structure is formed, even though the MgZnO/ZnO film is poly-crystal, the conduction band offset between MgZnO and ZnO can be formed at the junction interface. Therefore, we expect that most of currents can flow along the interface of MgZnO/ZnO that has low scattering of electron when gate voltages apply through source-drain and it estimate that the field effect mobility will be improved.

In this study, we implemented the thin film transistor that the polycrystalline MgZnO/ZnO bi-layer used as the active layer and investigated the transistor properties of MgZnO/ZnO bi-layer TFTs dependence on the thickness of ZnO layer. We assume that the thickness of ZnO in MgZnO/ZnO bi-layer would be one of the most important factors on the electrical characteristics. If the MgZnO/ZnO interface is far from the insulator, it is difficult to effectively accumulate electrons to use in the channel region. On the other hand, if the thickness of ZnO layer is too thin, the ZnO layer will be formed with poor crystallinity and surface morphology. It might be degraded of electrical characteristics by the increasing scattering by grain boundaries. In our work, we investigated the ZnO thickness effect in MgZnO/ZnO bi-layer TFT significantly on the electrical characteristics and stability of TFTs and discussed the role of the MgZnO layers and the dependency of the thickness of ZnO layers.

Section snippets

Experimental

ZnO and MgZnO films were sequentially deposited by RF co-sputtering onto silicon substrates (100) that had been thermally oxidized in an O2 ambient (resistivity 0.005 Ω cm). A Nanospec system and cross-sectional transmission electron microscopy (TEM) images confirmed the SiO2 thickness to be 70 nm. Prior to deposition, the substrates were cleaned by acetone methanol, and de-ionized water in an ultrasonicator for 5 min, before being placed in the sputtering chamber. The base pressure was ∼106

Results and discussion

The bright field cross-section TEM image of the MgZnO (48 nm)/ZnO (12 nm)/SiO2/Si structure (Fig. 1(a)) mainly shows the c-directional columnar structure of the ZnO and MgZnO. The components of the bi-layer are difficult to distinguish because it maintained a hexagonal wurtzite structure throughout and the Mg content was low. Selective area electron diffraction (SAED) (Fig. 1(b)) was taken along the <110>Si zone axes (z) and beam direction (B), and have shown ZnO or MgZnO (0002) spots near the

Conclusions

MgZnO/ZnO bi-layer TFTs were shown to have higher mobilities and lower S.S. values than a similar single-layer ZnO TFT. The optimal bi-layer structure had a localized conduction band-gap region that formed a current pathway for improved mobility and low SS. The thickness of the ZnO layer affected the TFT parameters; ZnO thickness was optimized at about 10–20 nm.

Acknowledgments

This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2012R1A1A4A01011424).

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