Publication No: 4254Search all ORC publications    

Ultra-high pressure MOCVD - a supercritical route to compound semiconductor materials

James William Wilson1, Jixin Yang2, Jason R.Hyde1, David C.Smith1, Steven M.Howdle2, Kanad Mallik4, Pier J.A.Sazio4, Paul O'Brien3, Mohamed Malik3, Mohammad Afzaal3 and Chinh Q.Nguyen3

1. School of Physics and Astronomy, University of Southampton, UK
2. School of Chemistry, University of Nottingham, Nottingham, UK
3. Department of Chemistry and the Manchester Materials Science Centre, University of Manchester, UK
4. Optoelectronics Research Center, University of Southampton, UK


The deposition of thin films of materials on to and in to preformed, high aspect ratio, template materials, is of significant interest to the semiconductor community. Damascene processes are vital to the electronic industries, and new synthetic methods are being developed in order to achieve modification of rationally designed templates; modifications that are limited by current MOCVD or CBD deposition technologies. Supercritical chemical fluid deposition, SCFD, offers a route to these devices by exploitation of their zero surface tension, tuneable physical properties, and ability to dissolve relatively high concentrations of reagents. In addition to Cu metalisations [1], Si-Ge core-shell wires and nanotubes[2] 3nm in diameter, can be produced by batch SCFD. However, this approach has yet to be extended to include compound-semiconductor deposition, i.e. II-VI materials.

We report the design, deposition and characterisation of high quality CdS thin-films from a SCF in a continuous flow reactor as the first synthetic stage towards deposition in complex topologies. Our approach employs a tailored single source precursor with an enhanced solubility in SCFs, eliminating the need to control individual reagent concentrations in the kinetically limited regime within the flow reactor.

Chemical composition of thin films deposited on SiO2/Si substrates have been examined by AES, and show that films deposited by this technique are close to stoichiometric. Furthermore, SEM demonstrates that these films are formed from highly compacted hexagonal pyramidal crystals, which are in the order of 200 nm. XRD confirms pure a-CdS. Judicial engineering and design has eliminated extrinsic dopants and other contamination leached from the stainless steel pressure vessels; this has been confirmed by SIMS measurements. CdS films are highly reflective and exhibit interference fringes due to small thickness variations. Photoelectrochemical spectroscopy were performed and the band-edge absorption was found to be 504 nm, in agreement with published values for CdS. Moreover, unlike most CBD deposited material, films deposited from our SCF reactors exhibit room temperature band-edge luminescence with a FWHM of 15.8 nm decreasing to 5.5 nm at 2.9 K, smaller than that of PLD material which exhibits lasing [3].

This presentation demonstrates a general approach to deposition from SCFs, by the development and design of both reagent chemistry and reactor engineering, opening the way for a wider class of semiconductors to be deposited into complex 3D-topologies conforming to a rational design.

[1] Cabanas et al., Chem. Mater., 16, 2028 (2004)
[2] Audoit et al., J. Mater. Chem., 15, 4809 (2005)
[3] Ullrich et al., J. Lumin., 8789, 1162 (2000)

MRS '08 Materials Research Society Fall Meeting Boston 1-5 Dec (2008)

Southampton ePrint id: 65515




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