Raman and XRD studies of Ge nanocrystals in alumina films grown by RF-magnetron sputtering P. Caldelas a , A.G. Rolo a, * , M.J.M. Gomes a , E. Alves b , A.R. Ramos b , O. Conde c , S. Yerci d , R. Turan d a Centre of Physics, University of Minho, 4710-057 Braga, Portugal b Instituto Tecnolo ´gico e Nuclear (ITN), Physics Department, E.N.10, 2686-953 Sacave´m, Portugal c Department of Physics and ICEMS, Faculty of Sciences of the University of Lisbon, Campo Grande, Ed. C8,1749-016 Lisboa, Portugal d Department of Physics, Middle East Technical University, 06531 Ankara, Turkey Keywords: Germanium Nanocrystals X-ray diffraction Raman spectroscopy abstract Germanium (Ge) nanocrystals (NCs) embedded in alumina thin films were produced by deposition on fused silica and silicon (111) substrates using radio-frequency (RF) magnetron sputtering. The films were characterised by both Raman and X-ray diffraction (XRD) spectroscopy. The deposition conditions were optimised in order to obtain crystalline Ge nanoparticles. In as-deposited films, the typical NC size was w3 nm as estimated by means of X-ray diffraction. Raman spectra taken from as-deposited films re- vealed both amorphous and crystalline semiconductor phases. Annealing was performed in order to improve the crystallinity of the semiconductor phase in the films. After a 1 h annealing at 800  C the mean NC size estimated from the XRD data and Raman spectra increased to w6.5 nm. An increase in the crystallinity of the Ge phase was also confirmed by the Raman spectroscopy data. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction Over the last two decades, a lot of research has been dedicated to studying the quantum-confined electronic states in low di- mension structures of group IV semiconductors like Ge. Nano- crystals (NCs) of indirect-gap semiconductors, such as Si and Ge are widely studied, as they would open new possibilities for the ap- plication of these materials in novel integrated optoelectronics and microelectronics devices. Several techniques are being used to fabricate Ge nanocrystals, such as RF co-sputtering [1–6], dc sput- tering [7], ion implantation [8,9], evaporation–condensation [10], electron beam evaporation [11,12], chemical vapour deposition [13], and pulsed laser deposition [14]. In almost all these works the NCs have been grown inside a SiO 2 matrix, and very few studies are reported using alumina as a doped material. SiO 2 is, without any doubt, one of the materials most studied and widely used as a gate dielectric in electronic devices. However, the constant shrinking of the thickness of gate dielectrics to below 2–3 nm has led to a search for alternative materials, whose di- electric constant is higher than that of SiO 2 , but whose other properties remain similar to SiO 2 [14]. Because of its similar band gap energy value and more than twice as high dielectric constant, Al 2 O 3 is a good candidate to replace SiO 2 as a gate dielectric material. In this work we report results of structural study of Ge NCs embedded in alumina films, produced at 500  C using a RF-mag- netron co-sputtering technique. This study was performed using X- ray diffraction (XRD) and Raman scattering spectroscopy, while Rutherford backscattering spectrometry (RBS) was employed to study the elemental composition and stoichiometry of the com- posite films. 2. Experimental Ge-doped alumina films were grown by a conventional co- sputtering method using an RF-magnetron Alcatel SCM 650 system. An alumina plate (purity of 99.99%, 50 mm diameter) was used as a target. An unpolished polycrystalline germanium sheet (99.999%) was placed over and at the centre of the Al 2 O 3 plate, to produce the doped films. Fused silica (FS) and (111) siliconwafers were used as substrates and were kept at 500  C during the deposition process. Prior to sputtering, a pressure of at least 1 10 6 mbar was reached inside the chamber and in situ argon plasma treatment of target and substrates was performed in order to clean the surfaces. Samples deposited on Si (111) substrates were annealed at 800  C, during 1 h, under air pressure of approximately 4  10 3 mbar. The experimental parameters used to produce the Ge/Al 2 O 3 films are presented in Table 1 . * Corresponding author. Fax: þ351 253 604061. E-mail address: arolo@fisica.uminho.pt (A.G. Rolo). Contents lists available at ScienceDirect Vacuum journal homepage: www.elsevier.com/locate/vacuum 0042-207X/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2008.03.067 Vacuum 82 (2008) 1466–1469