Elsevier

Thin Solid Films

Volume 520, Issue 10, 1 March 2012, Pages 3764-3768
Thin Solid Films

Amorphous structure and electrical performance of low-temperature annealed amorphous indium zinc oxide transparent thin film transistors

https://doi.org/10.1016/j.tsf.2011.06.082Get rights and content

Abstract

The effect of low-temperature (200 °C) annealing on the threshold voltage, carrier density, and interface defect density of amorphous indium zinc oxide (a-IZO) thin film transistors (TFTs) is reported. Transmission electron microscopy and x-ray diffraction analysis show that the amorphous structure is retained after 1 h at 200 °C. The TFTs fabricated from as-deposited IZO operate in the depletion mode with on–off ratio of > 106, sub-threshold slope (S) of ~ 1.5 V/decade, field effect mobility (μFE) of 18 ± 1.6 cm2/Vs, and threshold voltage (VTh) of − 3 ± 0.7 V. Low-temperature annealing at 200 °C in air improves the on-current, decreases the sub-threshold slope (1.56 vs. 1.18 V/decade), and increases the field effect mobility (μFE) from 18.2 to 23.3 cm2/Vs but also results in a VTh shift of − 15 ± 1.1 V. The carrier density in the channel of the as-deposited (4.3 × 1016 /cm3) and annealed at 200 °C (8.1 × 1017 /cm3) devices were estimated from test-TFT structures using the transmission line measurement methods to find channel resistivity at zero gate voltage and the TFT structures to estimate carrier mobility.

Introduction

Electronics based on amorphous metal oxide semiconductor (AOS) thin film transistors (TFTs) offer an alternative to amorphous Si-based devices. These materials are optically transparent and have important application to organic light emitting-based diodes which, as current-driven devices, require a higher on/off current ratio than field-driven liquid crystal display devices. Amorphous oxides with the (n − 1)d10ns0 (n  4) electronic configuration of heavy-metal cations such as zinc tin oxide [1], zinc-rich indium oxide [2], indium gallium oxide [3], indium-rich zinc oxide (a-IZO) [4], [5], [6], [7], [8] and indium gallium zinc oxide (a-IGZO) [9], [10], [11] have been demonstrated by a number of groups. Amorphous oxide channel TFTs have higher field effect mobility than amorphous Si-based devices and, therefore, should yield faster TFT switching speeds and deliver higher on-current [8], [12].

Of the AOS materials, In2O3-10wt.%ZnO (IZO) provides the highest reported Hall electron mobility, and allows controlled growth of both conducting (degenerate doping of 5×1020/cm3) and semiconducting (1015/cm3) materials from the same IZO sputter target by control of the oxygen content in the sputter gas during deposition [4], [5], [13]. In the fabrication of a-IZO channel devices, it is difficult to achieve low enough carrier concentrations in the channel [4], [5], [14] and this can lead to high TFT off-state currents. Consequently, most reports of IZO-based TFT devices use very thin [4], [5] (10–30 nm is typical) channel layers while others add Ga as a third cation species (IGZO) [14], [15] to suppress native defect-based carrier formation. The addition of a third cation species increases carrier scattering and significantly decreases channel carrier mobility.

We have reported compositionally homogeneous semiconducting IZO channel/IZO metallization structured TFTs. In this configuration, both the semiconducting channel and source/drain metallization layers are deposited from the same IZO target by adjusting the process oxygen during deposition to tune the IZO carrier density between ~ 1016/cm3 for the channel to ~ 5 × 1020/cm3 for the metallization layer. As homojunctions, these compositionally homogeneous IZO/IZO interfaces have a well-aligned band structure and, consequently, low contact resistance is expected [13].

Amorphous oxide-based IZO TFTs have been fabricated using only room temperature processing [4], [7], [8], [9]; however, post-annealing has been shown to improve field effect mobility [11], [16], electrical stability [17] and contact resistance between channel and source/drain metallization [13], [18], [19]. Nomura et al.[17], for example, reported post-deposition annealing at 400 °C to improve IGZO TFT stability but they also found that thermal annealing results in up to ~−7 V threshold voltage shift. According to Kikuchi et al. [20], both pulsed-laser deposition (PLD) and sputter processed IGZO TFTs show negative threshold voltage shift up to ~−48 V after dry and wet annealing at 150–400 °C which they attribute to the decrease of deep trap density. Barquinha et al.[7] studying IZO TFTs reported a large negative threshold voltage shift of ~−13 V after annealing at 125 °C but a smaller shift of ~−6 V after 250 °C annealing. The cause of threshold voltage shift and instability are not yet established in these oxide materials.

Section snippets

Experiment

The structure of IZO films and the performance of IZO based TFTs were evaluated before and after annealing at 100 and 200 °C. The amorphous/crystalline structure of the film was determined using glancing incident angle x-ray diffraction (GIAXRD, Siemens D5000) and transmission electron microscopy (TEM, JEOL JEM-2010) while test-TFTs were fabricated and characterized to determine field effect mobility, threshold voltage, and sub-threshold slope. Transmission line measurements (TLM) were made on

Results

Sputter deposition of IZO thin films onto unheated substrates results in the formation of an amorphous phase that is much more stable [4], [5] with respect to crystallization than either pure amorphous Indium oxide or ITO. In the amorphous state, the octahedral coordination unit of InO6 that forms the structural basis of the 80-atom crystalline bixbyite unit cell is preserved but medium and long range order are absent. Fig. 2(a) shows GIAXRD of as-deposited and annealed 100-nm-thick IZO films

Discussion

The change in IZO resistivity with annealing at 200 °C from 8.2 to 0.33 Ωcm amounts to a 20-fold increase in carrier density. This increase in carrier density is unexpected since annealing in air should consume oxygen vacancies (which contribute 2 free electrons) in the IZO and thereby should decrease the total carrier density. However, at the low anneal temperature used in this work (200 °C) the kinetics of diffusion of oxygen into the film (or vacancies to the surface of the film) is likely to

Conclusions

We have investigated the effect of low-temperature annealing in air on the electrical performance of room temperature processed amorphous IZO TFTs. No change in the amorphous structure of the annealed film was detectable by GIAXRD or HRTEM/SAD analysis. A negative 15 V shift in the TFT threshold voltage was observed with annealing at 200 °C which can be attributed to an increase in carrier density in the channel and secondarily to changes in the density of interface states. The increase in

Acknowledgements

The authors gratefully acknowledge the financial support of the National Science Foundation (NSF) Award No. DMR-0804915 and Fuji Film Corporation.

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