Abstract
Wide-bandgap oxide semiconductors are essential for the development of high-speed and energy-efficient transparent electronics. However, while many high-mobility n-type oxide semiconductors are known, wide-bandgap p-type oxides have carrier mobilities that are one to two orders of magnitude lower due to strong carrier localization near their valence band edge. Here, we report the growth of bilayer beta tellurium dioxide (β-TeO2), which has recently been proposed theoretically as a high-mobility p-type semiconductor, through the surface oxidation of a eutectic mixture of tellurium and selenium. The isolated β-TeO2 nanosheets are transparent and have a direct bandgap of 3.7 eV. Field-effect transistors based on the nanosheets exhibit p-type switching with an on/off ratio exceeding 106 and a field-effect hole mobility of up to 232 cm2 V−1 s−1 at room temperature. A low effective mass of 0.51 was observed for holes, and the carrier mobility reached 6,000 cm2 V−1 s−1 on cooling to −50 °C.
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Data availability
The data used to determine the data points shown within the plots presented in this paper, and other findings from this study, are available from the corresponding authors upon reasonable request.
Change history
07 June 2021
A Correction to this paper has been published: https://doi.org/10.1038/s41928-021-00606-9
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Acknowledgements
T.D. acknowledges funds received from the Australian Research Council (ARC) through the DECRA scheme (DE190100100). A.Z. thanks the University of Melbourne for the support received through the McKenzie postdoctoral fellowship programme. This work was supported by ARC Centre of Excellence FLEET (CE170100039) and Exciton Science (CE170100026). We thank RMIT University’s Microscopy and Microanalysis Facility (RMMF), a linked laboratory of the Australian Microscopy and Microanalysis Research Facility (AMMRF), and RMIT University’s MicroNano Research Facility (MNRF) for scientific and technical support. The Cypher ES AFM instrument was funded in part by grant no. LE170100096 from the ARC. This project was also supported by computational resources provided by the Australian government through the National Computational Infrastructure National Facility (NCI-NF) and the Pawsey Supercomputer Centre (ARC). A.E. is supported by the Jack Brockhoff Foundation (JBF grant no. 4655-2019-AE). D.L.C. is supported by the ARC under Discovery Project grant no. DP190102852.
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Contributions
The project was designed and directed by T.D., C.F.M. and A.Z. A.Z. and P.A. synthesized the chalcogen mixture and developed the synthesis procedure for 2D β-TeO2 while also conducting XPS and Raman measurements. A.Z., P.A. and B.Y.Z. performed the AFM imaging. A.E. performed atomic-resolution HR-AFM imaging. A.Z. performed TEM/SAED and HRTEM imaging. P.A. led the device fabrication with contributions from H.T., N.S., A.J., K.A.M. and J.v.E. J.G.P., A.Z. and P.A. characterized the FET devices. B.J.M. carried out UPS measurements and assisted with the XPS analysis. M.W. performed 2D nanosheet transfer experiments. D.L.C. conducted and interpreted Hall effect measurements. S.P.R. performed DFT calculations. T.D., C.F.M., K.K.-Z., A.Z. and P.A. analysed the material and device characteristics and drafted the manuscript. All authors revised the manuscript.
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Extended data
Extended Data Fig. 1 Roll-synthesis technique characterization.
a, Maximum droplet velocity achieved when droplet size was varied (Supplementary Videos 1 and 2). Lower speeds can be applied to the larger droplets as rolling too fast causes the fragmentation. b, Oxide thickness against time shows no significant change of the sheet thickness when prolonging the oxidation time. c, Rolling time-steps against substrate coverage and sheets lateral dimensions (Supplementary Video 3). Time-steps are defined as resting molten droplet before rolling a droplet-diameter length. Each error bar represents ± 1 standard deviation from four measurements. Discussion on printing parameters can be found in Supplementary Note 1.
Extended Data Fig. 2 Energy-dispersive X-ray spectroscopy (EDXS) elemental composition spectrum.
The composition of the chalcogen mixture was determined to be 95 wt% selenium and 5 wt% tellurium.
Extended Data Fig. 3 XPS spectra taken from the transferred TeO2 nanosheets and XPS elemental map.
The results reveal the composition of the 2D sheets, shown in Fig. 1a (97.7 mol% TeO2 and 2.3 mol% Se). a, The peak in the O 1 s binding energy region located at 530.6 eV is associated with TeO242 b, The Te4+ 3d5/2 peak is located at 576.4 eV42. c, A small amount of Se was detected in the Se 3d region with a peak located for the 3d5/2 at 55.2 eV43. d, XPS elemental map of a deposited flake on the right indicates the TeO2 flake, while Se is revealed as a sparse residue on the substrate.
Extended Data Fig. 4 Optical images of TeO2.
a-i, Transferred 2D TeO2 sheets on a variety of substrates. The labels I-IX refer to the different substrates, while X represents the 2D TeO2 sheet. j, A thicker TeO2 sheet can be obtained from repeated roll transfer across the same area. The optical image and AFM step height profile reveal multiple TeO2 sheets stacked on top of one another, which caused an increase in thickness. k-m, Transfer of a TeO2 flake from a GaAs substrate onto a SiO2 substrate. Optical images of the TeO2 flake on GaAs, polypropylene-carbonate (PPC) and SiO2 substrate, respectively, demonstrate the successful transfer process (See Methods section for the transfer protocol). Black scale bars are 50 μm.
Extended Data Fig. 5 High-resolution AFM.
a, HRAFM image of 2D β-TeO2 on a Si/SiO2 wafer b, The observed spacing is shown in the model crystal structure.
Extended Data Fig. 6 Hole effective mass.
The calculated hole effective mass (\(m_h^ \ast\)) of 0.51 obtained from Equation S2 utilizing data from STS measurement (see Supplementary Note 2).
Extended Data Fig. 7 2D β-TeO2 valence band spectrum obtained from UPS.
The work function of the material was located at 3.88 eV.
Extended Data Fig. 8 Atomic Orbital Projected DOS plot of the bilayer.
The results show which types of atomic orbitals of Te and O contribute to the upper valence bands. The projections are partitioned into the different atomic orbital types (s, p, d) for O and Te. The plot clearly shows that in the region near the valence band maximum, O and Te p orbitals form the major contribution to the bands, suggesting π-bonding.
Extended Data Fig. 9 Conductive path in bilayer β-TeO2.
For simplicity, the crystal structure of the bottom layer of the two layers found in unit cell thick β-TeO2 (Fig. 1c) is shown. The charge density shown in Fig. 1c suggests that conduction most efficiently occurs close to the interlayer band edge (top side of the displayed monolayer), while the DOS associated with the VBM is sparse at the bottom of the shown β-TeO2 layer. Here the conduction pathways are shown along the b-axis (green) and the a-axis (red). The DFT calculations indicate a hole mobility of 7690 cm2 V-1 s-1 along the b-axis and 436 cm2 V-1 s-1 along the a-axis, respectively. The higher mobility along the b-axis arises due to the shorter mean free path lengths and transport through regions of high DOS, while transport along the a-axis requires a longer mean free path that diverts into regions of low DOS.
Extended Data Fig. 10 Stability testing of a TeO2 field-effect transistor showing 1500 On-Off cycles.
No degradation in performance is observed. Five On-Off cycles are shown on the right.
Supplementary information
Supplementary Information
Supplementary notes 1–5, Figs. 1–6 and Tables 1–3.
Supplementary Video 1
Roll-transfer of a droplet with a diameter less than 2 mm.
Supplementary Video 2
Roll-transfer of a droplet with a diameter larger than 2 mm.
Supplementary Video 3
Roll-transfer of a droplet moving every 3 min, and the corresponding substrate coverage.
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Zavabeti, A., Aukarasereenont, P., Tuohey, H. et al. High-mobility p-type semiconducting two-dimensional β-TeO2. Nat Electron 4, 277–283 (2021). https://doi.org/10.1038/s41928-021-00561-5
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DOI: https://doi.org/10.1038/s41928-021-00561-5
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