Nanoscale Fast Ge Diffusion in Laser Irradiated SiGe Thin Films V.S. Teodorescu * , C. Ghica * , A.V. Maraloiu * , A.M. Lepadatu * , I. Stavarache * , M.L. Ciurea * ** N.D. Scarisoreanu ** , A. Andrei ** , M. Dinescu ** * National Institute of Material Physics, 105 bis Atomistilor Street, 077125 Bucharest-Măgurele, Romania, teoval@infim.ro , cghica@infim.ro , maraloiu@infim.ro , lepadatu@infim.ro , stavarache@infim.ro , ciurea@infim.ro ** National Institute of Plasma Lasers and Radiation, 409 Atomistilor Street, 077125 Bucharest-Măgurele, Romania, snae@nipne.ro , dinescum@nipne.ro , andreic@yahoo.com ABSTRACT We report the presence of a fast Ge diffusion effect produced during laser crystallization of Si 0.45 Ge 0.55 films deposited by magnetron sputtering on Si(100) substrates. The laser irradiation was performed using the 266 nm wavelength radiation of the Nd-YAG laser. After the irradiation with 10 laser pulses at a fluence of 30 mJ/cm 2 the top part of the film shows two zones. Zone I, with a thickness of about 20 nm is crystallized and rich in Ge. The next zone in depth, i.e. Zone II, of about 30 nm thicknesses, has a mixed quasi-amorphous structure and is deficient in Ge, as the EDX scan-line analysis reveals. This cannot be explained considering the usual value of the Ge diffusivity in SiGe and the short time of the laser heating. The redistribution of Ge by fast diffusion suggests a real diffusion coefficient of the order of magnitude of 10 -4 cm 2 /s, which is an usual value for liquids. However, in this zone, the SiGe structure was not melted by the laser pulse irradiation. Keywords: laser irradiation, SiGe thin films, Ge diffusion, transmission electron microscopy, AFM INTRODUCTION Laser processing of the thin films is a well known method for developing new technologies. SiGe is an interesting material from both technological and fundamental point of view, with applications in band-gap tailoring, solar cells and non volatile memory devices. The Ge nanocrystallite formation in SiGe, Ge:Si or Ge:SiO 2 mixed systems or other dielectric matrix is intensively studied in the last decade for quantum well devices. In some cases, pulse laser processing has been applied [1-10]. In all these systems, the diffusion of Ge is an important factor related with the formation and stability of the nanostructure. The Ge diffusion in the SiGe system was studied in details [11-13]. However, some aspects related to the changes in nanostructures made by Ge diffusion are not well known or not yet understood. One of the advantages of the pulse laser annealing is the limited time of the heating and the limited diffusion length associated with. It is well known that the species diffusion length is (Dτ) 1/2 , were D is the diffusion coefficient of the atomic species in the matrix and τ is the time duration of the laser pulse. Under normal conditions, if D is of the order of magnitude of 10 -14 cm 2 /s [12] and τ about 10 ns, the diffusion length becomes less than one picometer, so practically there is no diffusion. However, the diffusion takes place at the nanometer scale in condition of a high number of laser pulses, leading to formation of a large variety of surface nanostructures [14-16], mainly due to Ge diffusion to the top layer of the laser target. Laser irradiation of SiGe films at small laser fluence value leads to the formation of a network of nano-areas reached in Ge on the film surface [14]. The goal of our experiment is related to the study of the laser crystallization of amorphous SiGe films. We report on a new interesting Ge diffusion effect obtained in pulsed laser irradiation experiments made on Si 0.45 Ge 0.55 films deposited on Si(100) wafer substrates. EXPERIMENTAL The SiGe thin films with a thickness of 170 nm were prepared by magnetron sputtering deposition using a co- sputtering from two targets of Si and Ge, respectively. The deposition was performed on Si[100] wafers and carried out in Ar atmosphere at a pressure of 4 mTorr [6]. The chosen Si:Ge composition is 45:55. The as deposited SiGe films structure is amorphous and is presented in Fig 1. The laser irradiations were performed in air perpendicular to the film surface using the 266 nm radiation of the Nd-YAG laser. The laser beam on the sample was round with a 7 mm diameter and homogenious in intensity with a local variation less than 10%. The laser fluence used for the experiments was varied between 20 and 50 mJ/cm 2 . The structure of the SiGe films before and after laser irradiation experiments was studied by transmission electron NSTI-Nanotech 2013, www.nsti.org, ISBN 978-1-4822-0581-7 Vol. 1, 2013 109