Transparent Conductive Oxide Nanocrystals Coated with Insulators by ALD

Here is an interesting paper on transparent conductive oxide (TCO) nanocrystals coated by ALD Department of Energy, St. Louis and University of Texas at Austin, USA. As stated in teh paper, it has recently been demonstrated that filling in initially insulating films comprised of TCO nanocrystals with another insulator by ALD dramatically increases the conductivity by many orders of magnitude. 

The researchers report on the mechanism how ALD coating increases conductivity for Al2O3 and HfO2 ALD coating of ZnO TCO nanocrystals. Please check the graphical abstract and linked publication below for full details.

Transparent Conductive Oxide Nanocrystals Coated with Insulators by Atomic Layer Deposition

John Ephraim, Deanna Lanigan, Corey Staller, Delia J. Milliron, and Elijah Thimsen

Chem. Mater., 2016, 28 (15), pp 5549–5553
DOI: 10.1021/acs.chemmater.6b02414
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Abstract

Thin films comprised of transparent conductive oxide (TCO) nanocrystals are attractive for a number of optoelectronic applications. However, it is often observed that the conductivity of such films is very low when they are in contact with air. It has recently been demonstrated, somewhat surprisingly, that filling in initially insulating films comprised of TCO nanocrystals with another insulator by atomic layer deposition (ALD) dramatically increases the conductivity by many orders of magnitude. This work aims to elucidate the mechanism by which the ALD coating increases conductivity. We examined the effect of removing two adsorbed oxygen species (physisorbed molecular water and chemisorbed hydroxide) on sheet resistance and compared this result to the results with thin films comprised of ZnO nanocrystals coated with Al2O3 and also HfO2 by ALD. Although both insulating infills decrease the sheet resistance and increase the stability of the films, there is a stark discrepancy between the two. From the in situ measurements, it was found that coating with Al2O3 removes both physisorbed water and chemisorbed hydroxide, resulting in a net reduction of the ZnO nanocrystals. Coating with HfO2 removes only physisorbed water, which was confirmed by Fourier transform infrared spectroscopy. A similar phenomenon was observed when thin films comprised of Sn-doped In2O3 nanocrystals were coated, suggesting Al2O3 can be used to reduce and stabilize metal oxide nanocrystals in general.

[Reprinted with permission from American Chemical Society. Copyright 2016 American Chemical Society,Chem. Mater., 2016, 28 (15), pp 5549–5553,  Account #: 3000915597]

ALD of Transparent Semiconducting Oxide CuCrO2 for Invisible electronics

So you learn something new every day and as before and so many times also this time from Aalto University, Helsinki Finland! ALD of Transparent Semiconducting Oxide CuCrO2 for Invisible electronics! Some basic facts from this excellent ALD paper :

• Copper chromium oxide CuCrO2 is a member of the delafossite family CuAO2 (A = Al, Cr, Fe, Co, Ga, Y, In, La, Nd, and Eu.
• With predominantly antiferromagnetic interactions, members of this family possess a geometrically frustrated (!) triangular lattice
• The discovery of simultaneous p-type electrical conductivity and optical transparency in CuAlO2 thin films opened the new venue of research among delafossites for ‘invisible electronics’
• CuCrO2  is reported to have the highest electrical conductivity (220 S/cm) among the delafossite materials.
• The need for p-type TCOs stems from the realization of transparent electronics; that by far depends on the p-type semiconductor materials for use in transparent p–n and p–i–n diodes.
• Materials with a wide bandgap (>3 eV), high electrical conductivity, high mobility, low fabrication cost along with controllable transparency would be a boom for transparent electronics. 

ALD  CuCrO2 : check, check, check, check, check

So there is an industrial need for high performing TCO material (we knew that, see Mr Lytle below) with low production cost (always the case) with additional material properties and a upcoming disruptive technology - Invisible Electronics. Doesn´t look like CVD and PVD can make all this  - As always ALD must be up for the challenge!

William O. Lytle the inventor of Electric glass so called Nesa coated glass (Science 1946, Sep. 16) showing of his product which in a way is one of teh first invisible electronic devices. Note that he is using rather thick rubber gloves  to not disturb the device... [http://publica.fraunhofer.de/documents/N-313429.html]

Atomic Layer Deposition of Transparent Semiconducting Oxide CuCrO2 Thin Films

Tripurari S. Tripathi, Janne-Petteri Niemelä and Maarit Karppinen
J. Mater. Chem. C, 2015, Accepted Manuscript
DOI: 10.1039/C5TC01384D

Atomic layer deposition (ALD) is a vital gas-phase technique for atomic-level thickness-controlled deposition of high-quality thin films on various substrate morphologies owing to its self-limiting gas-surface reaction mechanism. Here we report the ALD fabrication of thin films of the semiconducting CuCrO2 oxide that is a highly prospective candidate for transparent electronics applications. In our process, copper 2,2,6,6-tetramethyl-3,5-heptanedionate (Cu(thd)2) and chromium acetyl acetonate (Cr(acac)3) are used as the metal precursors and ozone as the oxygen source. Smooth and homogeneous thin films with an accurately controlled metal composition can be deposited in the temperature range of 240–270 oC; a post-deposition anneal at 700-950oC in an Ar atmosphere then results in well crystalline films with the delafossite structure. The direct bandgap is determined from UV-vis spectrophotometric measurements to be 3.09 eV. The observed transmittance is greater than 75% in the visible range.