Low Temperature Atomic Layer Deposition of Tin Oxide

Abstract

Atomic layer deposition (ALD) of tin oxide (SnOx) films was achieved using a newly synthesized tin precursor and hydrogen peroxide. We obtained highly pure, conductive SnOx films at temperatures as low as 50 °C, which was possible because of high chemical reactivity between the new Sn precursor and hydrogen peroxide. The growth per cycle is around 0.18 nm/cycle in the ALD window up to 150 °C, and decreased at higher temperatures. Self-limited growth was demonstrated for both the Sn and O precursors. Thickness is linear in the number of cycles, with an induction period of not more than a few cycles. Rutherford backscattering spectroscopy (RBS) and X-ray photoelectron spectroscopy (XPS) measurements showed that the composition ratio of O/Sn is ∼2 and that the films do not contain any detectable carbon or nitrogen impurities. X-ray and electron diffraction analyses identified crystallites of SnO2 with the rutile structure and average grain size 5−10 nm. The density of the films is 83% of the bulk rutile phase. The surfaces are very smooth, with roughness about 3% of the film thickness. The lowest resistivity is about 10−2 ohm·cm. The mobility is over 7 cm2/V·s, and the free electron concentration reaches nearly 1020 cm−3. The dependence of mobility on temperature suggests that grain boundary scattering is the dominant electron scattering mechanism. The optical transmission of a 100 nm film is 87.8% and its absorption is 3.3% when averaged over the wavelengths from 400 to 800 nm. Over 80% uniformity of thickness was achieved inside holes with aspect ratios up to 50:1. This successful low temperature growth of conductive nanocrystalline SnOx films by ALD allows it to be exploited in transparent electrodes for displays, organic light emitting diodes, solar cells, conductive and protective coatings on plastic, microchannel electron multiplier plates, or as a semiconductor layer in transparent transistors.