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Characterization of Electronic Transport through Amorphous TiO_2 Produced by Atomic-Layer Deposition

Nunez, Paul and Richter, Matthias H. and Piercy, Brandon D. and Roske, Christopher W. and Cabán-Acevedo, Miguel and Losego, Mark D. and Konezny, Steven J. and Fermin, David J. and Hu, Shu and Brunschwig, Bruce S. and Lewis, Nathan S. (2019) Characterization of Electronic Transport through Amorphous TiO_2 Produced by Atomic-Layer Deposition. Journal of Physical Chemistry C, 123 (33). pp. 20116-20129. ISSN 1932-7447. https://resolver.caltech.edu/CaltechAUTHORS:20190620-093003082

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Abstract

Electrical transport in amorphous titanium dioxide (a-TiO_2) thin films, deposited by atomic layer deposition (ALD), and across heterojunctions of p+-Si|a-TiO_2|metal substrates that had various top metal contacts has been characterized by ac conductivity, temperature-dependent dc conductivity, space-charge-limited current spectroscopy, electron paramagnetic resonance (EPR) spectroscopy, X-ray photoelectron spectroscopy, and current density versus voltage (J–V) characteristics. Amorphous TiO_2 films were fabricated using either tetrakis(dimethylamido)-titanium with a substrate temperature of 150 °C or TiCl_4 with a substrate temperature of 50, 100, or 150 °C. EPR spectroscopy of the films showed that the Ti^(3+) concentration varied with the deposition conditions and increases in the concentration of Ti^(3+) in the films correlated with increases in film conductivity. Valence band spectra for the a-TiO_2 films exhibited a defect-state peak below the conduction band minimum (CBM) and increases in the intensity of this peak correlated with increases in the Ti^(3+) concentration measured by EPR as well as with increases in film conductivity. The temperature-dependent conduction data showed Arrhenius behavior at room temperature with an activation energy that decreased with decreasing temperature, suggesting that conduction did not occur primarily through either the valence or conduction bands. The data from all of the measurements are consistent with a Ti^(3+) defect-mediated transport mode involving a hopping mechanism with a defect density of 10^(19) cm^(–3), a 0.83 wide defect band centered 1.47 eV below the CBM, and a free-electron concentration of 10^(16) cm^(–3). The data are consistent with substantial room-temperature anodic conductivity resulting from the introduction of defect states during the ALD fabrication process as opposed to charge transport intrinsically associated with the conduction band of TiO_2.


Item Type:Article
Related URLs:
URLURL TypeDescription
https://doi.org/10.1021/acs.jpcc.9b04434DOIArticle
ORCID:
AuthorORCID
Richter, Matthias H.0000-0003-0091-2045
Losego, Mark D.0000-0002-9810-9834
Fermin, David J.0000-0002-0376-5506
Hu, Shu0000-0002-5041-0169
Brunschwig, Bruce S.0000-0002-6135-6727
Lewis, Nathan S.0000-0001-5245-0538
Alternate Title:Characterization of Electronic Transport through Amorphous TiO2 Produced by Atomic-Layer Deposition
Additional Information:© 2019 American Chemical Society. Received: May 9, 2019; Revised: June 14, 2019; Published: June 19, 2019. This work was supported through the Office of Science of the U.S. Department of Energy (DOE) under award no. DE-SC0004993 to the Joint Center for Artificial Photosynthesis, a DOE Energy Innovation Hub. P.D.N. and C.W.R. thank the National Science Foundation for graduate research fellowships. C.W.R. also thanks the Link Energy Foundation for a graduate research fellowship. Research was in part carried out at the Molecular Materials Resource Center of the Beckman Institute and at the Microanalysis Center of the California Institute of Technology. D.J.F. acknowledges the financial support by the UK Engineering and Physical Sciences Research Council through the PVTEAM programme (EP/L017792/1). We thank Dr. Y. Guan for SIMS measurements, Dr. Angelo Di Bilio and Dr. Paul H. Oyala for EPR measurements, and K. Papadantonakis for assistance with editing this manuscript. S.H. and S.J.K. acknowledge the start-up support from the Tomkat Foundation. The authors declare no competing financial interest.
Group:JCAP
Funders:
Funding AgencyGrant Number
Department of Energy (DOE)DE-SC0004993
NSF Graduate Research FellowshipUNSPECIFIED
Link FoundationUNSPECIFIED
Engineering and Physical Sciences Research Council (EPSRC)EP/L017792/1
Tomkat FoundationUNSPECIFIED
Issue or Number:33
Record Number:CaltechAUTHORS:20190620-093003082
Persistent URL:https://resolver.caltech.edu/CaltechAUTHORS:20190620-093003082
Official Citation:Characterization of Electronic Transport through Amorphous TiO2 Produced by Atomic Layer Deposition. Paul Nunez, Matthias H. Richter, Brandon D. Piercy, Christopher W. Roske, Miguel Cabán-Acevedo, Mark D. Losego, Steven J. Konezny, David J. Fermin, Shu Hu, Bruce S. Brunschwig, and Nathan S. Lewis. The Journal of Physical Chemistry C 2019 123 (33), 20116-20129. DOI: 10.1021/acs.jpcc.9b04434
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:96580
Collection:CaltechAUTHORS
Deposited By: George Porter
Deposited On:20 Jun 2019 17:03
Last Modified:03 Oct 2019 21:23

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