Near Room Temperature PE-ALD of Nanostructured Gold for Enhanced Raman Scattering

Abstract

Currently only two Au ALD processes exist, using two different precursors. The first Au ALD process, reported by Griffiths et al. [1], is a three step process using Me3AuPMe3 as the precursor in combination with an oxygen plasma and water vapour as the reactants. The deposition of metallic gold was reported at a deposition temperature of 120°C, with only some carbon and oxygen impurities present in the film (6.65% C and 1.83% O). A growth per cycle of 0.05 nm per cycle was achieved. The other Au ALD process, reported by Mäkelä et al. [2], uses Me2Au(S2CNEt2) as the precursor and ozone as the reactant. Deposition between 120-180°C was reported with self-limiting growth at 180°C and a growth rate of 0.09 nm per cycle. The deposited films had low resistivity values (4-16 µΩ cm) and were chemically pure with few impurities, O (2.9%), H (0.9%), C (0.2%), and N (0.2%). A new plasma enhanced ALD process has been developed using Me3AuPMe3 and H2 plasma as the precursor and reactant, respectively. Saturating behaviour for both precursor and reactant is achieved. The growth per cycle for this process is 0.03 nm per cycle. The process exhibits a broad saturating temperature window, from 50°C to 120°C. This temperature window is limited by decomposition of the precursor above 120°C. X-ray diffraction measurements show that the as-deposited gold films are poly crystalline (face centered cubic). The deposited gold films are very pure, with no phosphorous present in the film and very few carbon impurities (0.3%). Low resistivity values (5.85 µΩ cm), close to the bulk value of gold (2.44 µΩ cm), were achieved. The surface chemistry and growth mechanism are investigated using in-situ RAIRS measurements, optical emission spectroscopy, and mass-spectrometry. Scanning electron microscopy measurements show that the initial growth starts with the nucleation of gold particles on the surface, as is the case for most noble metal ALD processes. The formed gold nanoparticles grow and coalesce during the ALD process. The spacing of the gold particles makes this process interesting for surface enhanced Raman spectroscopy (SERS). Free space Raman measurements were performed on some of the samples and these showed excellent surface enhancement of the Raman signal of a monolayer of 4-nitrothiophenol bound to the ALD gold films. As far as we know is this the first report of an ALD gold film that shows SERS properties. In contrast to other SERS substrate fabrication methods, often involving lithography, this ALD process provides a direct way to fabricate SERS substrates on both planar and complex substrates, without the need for a lot of process steps. [1] M. B. E. Griffiths, P. J. Pallister, D. J. Mandia, and S. T. Barry, Chem. Mater. 28(1) (2016) 44-46 [2] M. Mäkelä, T. Hatanpää, K. Mizohata, J. Räisänen, M. Ritala, and M. Leskelä, Chem. Mater. 29(14) (2017) 6130-6136