Characteristics of Oxide TFT Using Carbon-Doped Ιn2O3 Thin Film Fabricated by Low-Temperature ALD Using Ethylcyclopentadienyl Indium (Ιn-EtCp) and H2O & O3
Authors: Kobayashi Riku , Nabatame Toshihide , Kurishima Kazunori , Onaya Takashi , Ohi Akihiko , Ikeda Naoki , Nagata Takahiro , Tsukagoshi Kazuhito , Ogura Atsushi 
Published: February 2020

Introduction

In2O3-based metal oxide semiconductors have been widely investigated as active channel materials for oxide thin film transistors (TFTs) in flexible devices. The bond dissociation energy (BDE) between metal and oxygen element in the In2O3-based metal oxide has been recently focused because a low BDE easily occurs excess oxygen vacancy (Vo) and results in metal like conductor. In-Ti-O, In-Si-O, and In-Si-C-O have been studied in attempts to suppress Vo by a high BDE such as Ti-O (667 kJmol-1), Si-O (779 kJmol-1), and C-O (1076 kJmol-1) [1, 2].

In2O3 is an especially promising material because of a simple binary metal oxide and a high carrier mobility (~ 65 cm2V-1s-1). PET film was generally employed as substrate for flexible devices and limited to a fabrication process temperature below 200 °C. Considering to this process temperature, atomic layer deposition (ALD) technique is essentially suitable to produce the In2O3 films, and many In compounds such as InCl3, TMIn, In(tmhd)3, InCp, and InEtCp have been employed as precursors [3]. As-grown In2O3 film generally exhibited metal-like conductor because of excess Vo into the In2O3 film [4]. To compensate Vo sites in as-grown In2O3 film, the post-metal deposition annealing (PMA) process in air, O2, and O3 was employed during final process of TFT fabrication. However, the PMA was generally carried out at a high temperature above 300 °C [4].

In this paper, we studied growth of In2O3 films as a function of InEtCp and H2O/O3 doses using ALD at 150 °C with InEtCp precursor and H2O/O3 gas. We also demonstrate transistor performance of carbon-doped In2O3 (In-O-C) TFT with SiO2/Al2O3 dielectric after PMA at 150 °C in O3 gas.

Experiment

The In2O3 films were typically deposited on Si/SiO2 substrates by ALD at 150 °C using InEtCp and H2O/O3. The thickness of the In2O3 films as a function of InEtCp and H2O/O3 doses were measured using a spectroscopic ellipsometer. The chemical bonding state, carbon content and O/In ratio of the In2O3 films were also analyzed by XPS.

Back-gate-type In-O-C TFTs were fabricated as follows: Firstly, a 5-nm-thick Al2O3 films deposited on p++Si/SiO2 (250 nm) by ALD using trimethylaluminium precursor and O3 gas. Next, a 5-nm-thick In-O-C channel layer was deposited on the Al2O3 films by the same ALD condition at 150 °C, and subsequently patterned by lithographic process. Ti/Au source and drain electrodes were deposited. Finally, PMA was performed at 150 °C in O3 for 0-150 min.

Results and Discussion

At first the In2O3 film thickness as a function of H2O/O3 dose per pulse was measured while the InEtCp dose was 2.8×10-5 mol by ALD at 150 °C. The thickness of the In2O3 film was saturated when H2O/O3 was 0.9×10-4/2.9×10-3 mol (an inset graph of Fig. 1). According to this ALD condition, the film thickness as a function of number of ALD cycle was examined and the estimated growth per cycle (GPC) was 0.11 nm/cycle as shown in Fig. 1. The In2O3 films had a carbon content and O/In ratio were about 1.7 at.% and 1.2/1 (InO1.16C0.04), respectively, by XPS (not shown).

The electrical properties of InO1.16C0.04 TFT changed dramatically from metal like conductor to semiconductor for PMA time between 0 and 90 min. Threshold voltage (V
th) shifted from 4.1 to 4.3 V as PMA time increases from 90 to 150 min. Figure 2 shows the I
d-V
d characteristics of InO1.16C0.04 TFTs (L/W = 50/5 μm) after PMA for 150 min. The I
d was linearly increased at low V
d, indicating that ohmic contacts were consistently formed between the InO1.16C0.04 films and the source-drain electrodes. I
d also shows saturation at a high V
d, which indicates that the electron transport in the channel is well controlled by the V
g. The TFT exhibited superior transistor performance such as relatively small S.S. of 0.37 V/decade, a small V
th shift of +0.3 V, and a large μ
FE of 20.4 cm2/Vs. This is due to the suppression of excess Vo in InO1.16C0.04 channel layer.

Conclusions

We studied characteristics of as-grown In2O3 films and its InO1.16C0.04 TFT. The excellent characteristics of InO1.16C0.04 TFT, which was fabricated even in a low temperature fabrication process at 150 °C, could be obtained due to the carbon doping into In2O3 film and relatively strong oxidation strength of O3 during PMA.

References

[1] K. Kurishima et al., J. Vac Sci. Technol. B 36, 061206 (2018).

[2] S. Aikawa et al., Appl. Phys. Lett.
103, 172105 (2013)

[3] F. Mizutani et al., To be published in AIP advance.

[4] J. Sheng et al., ACS Appl. Mater. Interfaces,
8, 31136 (2016).

Figure 1