Change of Electrical Properties of Rutile- and Anatase-TiO2 Films by Atomic Layer Deposited Al2O3
Authors: Nabatame Toshihide , Yamamoto Ippei , Sawada Tomomi , Ohi Akihiko , Dao Thang Duy , Ohishi Tomoji , Nagao Tadaaki 
Published: February 2020

Introduction

Titanium dioxide (TiO2) has been widely investigated as a channel layer for oxide thin film transistors (oxide-TFTs) and an active layer of resistive random access memory [1]. TiO2 is a very attractive because high dilectric constant (k) of anatase and rutile phases are ~30 and ~80, respectively, and easily converted from an insulator to a semiconductor and conductor due to the electrons generated by formation of oxygen vacancies (Vo). TiO2 semiconductor was studied as a channel material for oxide-TFTs because of its relatively high electron mobility of 10 cm2V-1s-1 [2]. However, the semiconductor region is very narrow and Vo formation must be controlled carefully because poor and excess Vo concentrations lead to insulator and conductor regions, respectively.
Recently, MOSFETs with high-mobility InGaAs channels have exhibited good performance when native oxides (GaOx and AsOx), which caused Fermi-level pinning, were removed from the channel. The native oxides were removed during Al2O3 deposition using atomic layer deposition (ALD) with trimethylaluminum (TMA) precursor and H2O oxidant gas. Furthermore, the formation of a two-dimensional electron gas (2-DEG) in Al-based amorphous oxide/SrTiO3 heterostructures was reported to be grown by ALD using TMA and H2O gases, based on a similar mechanism of oxygen removal from the Ti-O layer of a SrTiO3 single crystal [3]. Here, we have an iteresting about how the ALD process with TMA and H2O affect to characteristics of rutile- and anatase-TiO2 films as channel metaerial for oxide-TFT.
In this paper, we deposited an Al2O3 layer on polycrystalline rutile- and anatase-TiO2 films by ALD with TMA and H2O, and examined its electrical properties, including its resistivity, Hall mobility (m
Hall), and carrier density (n).

Experiment

First, a 10-nm-thick TiO2 film was deposited on a 100-nm SiO2/p-Si substrate by ALD at 200°C using a Ti[N(CH3)2]4 precursor and H2O oxidant gas. The as-grown TiO2 film had an amorphous structure, and rutile- and anatase-phases were formed by post-deposition annealing (PDA) of the films at 800°C in N2 and 500°C in O2 atomosphere, respectively. Next, the TMA precursor and H2O oxidant gases were alternately supplied, as in an ALD Al2O3 deposition at 300°C. The number of ALD cycles was varied over the ranges of 0–50 cycles.
The electrical properties of rutile- and anatase-TiO2 films were investigated by Hall measurement with the van der Pauw method at room temperature. For Hall measurement, Au (100 nm)/Ti (10 nm) electrodes were deposited in 4 places of the samples with a single TiO2 or TiO2/Al2O3 stack structure by thermal evaporation.

Results and discussion

Initially, the rutile- and anatase-TiO2 films were an insulator, but its resistivity dropped dramatically to 10-1–10-2 Wcm and stayed low after 5 ALD cycles. The n values of both samples were 1019–1020 cm-3 in the low-resistivity region. The m
Hall values were 1–6 cm2V-1s-1, which correspond well with the literature values of 0.2–6 and 10 cm2V-1s-1 for thin-film and single-crystal anatase-TiO2, respectively. These m
Hall values are also similar to values for 2DEG of LaAlO3/SrTiO3 heterostructures (4–5 cm2V-1s-1) [3] and InOx-based oxide semiconductors (~10 cm2V-1s-1) [4]. Therefore, we found that rutile- and anatase-TiO2 films could be easily converted from an insulator to a conductor after ALD Al2O3 deposition.
Here, we considered the mechanism that generated an electron carrier in rutile- and anatase-TiO2 films under the ALD Al2O3 deposition. In an initial step of ALD process, when a TMA molecule is absorbed on the surface of the TiO2 film, the reaction of the –CH3 ligand of TMA and oxygen of TiO2 forms Al2O3 because of the negative Gibbs free energy. As a result, a Vo is formed, and electrons are generated in the TiO2 film.

Summary

We conclude that electrical properties dramatically changed as the number of ALD cycles increased above 5 cycles. The resistivity, n, and m
Hall of the TiO2 films were 10-1–10-2 Wcm, 1019–1020 cm-3, and 1–6 cm2V-1s-1, regardless of rutile and anatase phase structures.

Acknowledgement

The authors thank all staff members of the Nanofabrication group of the National Institute for Materials Science (NIMS) for their support.

References

[1] L. Alekseeva et al., Jpn. J. Appl. Phys. 55, 08PB02 (2016).
[2] T. Yajima et al., Status Solidi A 213 2196 (2016).
[3] S. W. Lee et al., Nano Lett. 12 4775 (2012).
[4] K. Nomura et al., Nature 432, 488 (2004).