PL and XPS depth profiling of Si/Al 2 O 3 co-sputtered films and evidence of the formation of silicon nanocrystals I. Dogan a,Ã , I. Yildiz a,b , R. Turan a a Department of Physics, Middle East Technical University, 06531 Ankara, Turkey b Central Laboratory, Middle East Technical University, 06531 Ankara, Turkey article info Available online 22 August 2008 PACS: 73.22.f 78.67.Bf 81.15.Cd Keywords: Si nanocrystals Aluminum oxide Photoluminescence X-ray diffraction X-ray photoelectron spectroscopy Depth profiling abstract We have studied the correlation between light emission and structural properties of silicon nanocrystals formed in Al 2 O 3 matrix by co-sputtering followed by an annealing in the temperature range of 900–1100 1C. X-ray diffraction (XRD) experiments revealed the nanocrystal formation which is a function of Si sputtering power and annealing temperature. The chemical structure and atomic compositions of Si, Al, O and their variations with depth from the sample surface have been investigated by X-ray photoelectron spectroscopy (XPS). Formation of Si nanocrystals is identified from the presence of Si–Si bonds whose strength increases with the annealing temperature. However, Si atoms resting in the region close to the surface of the sample is found to be oxidized. Photoluminescence (PL) spectroscopy is employed as a function of Si power, annealing temperature and sample depth. The origin of various PL peaks is discussed in terms of defect formation in Al 2 O 3 , emission from nanocrystals and interfaces. Defect related peaks are attributed to Cr 3+ , Ti 3+ and F centers which are commonly observed in Al 2 O 3 . The emission from the Si nanocrystals is clearly identified in the wavelength range of 700–900 nm. The variation of this emission with the depth is in good correlation with the depth profile of Si–Si bonds as determined by the XPS analysis. & 2008 Elsevier B.V. All rights reserved. 1. Introduction Semiconductors exhibit different electronic, optical and struc- tural properties when downsized to nanoscale due to quantum size effects at least in one dimension [1]. To date, many different semiconducting nanostructures have been of interest because of their wide range of applications in microelectronic industry [2–4]. Among them, nanocrystalline Si has attracted special attention due to its compatibility with the present microelectronic technology [4,5]. Moreover, observation of intense photolumines- cence (PL) [6] from Si nanocrystals has created new excitements for the development of new electro-optical components. As a host matrix to nanocrystals, SiO 2 has been extensively studied [7–9] due to its importance and wide use in microelectronic compo- nents. It has a relatively high dielectric constant and can be grown on Si thermally. However, with continuous shrinking of the device dimensions, the thickness of SiO 2 has reached its lower limit for reliable device operations. Search for alternative materials having similar electrical properties of SiO 2 and higher dielectric constant has been intensified in recent years. Aluminum oxide (Al 2 O 3 ) appears to be a good candidate for this purpose as it has a larger dielectric constant compared to SiO 2 [10]. Furthermore, due to optical transparency, Al 2 O 3 matrix is an ideal host material for optical devices including solar cells. The visible PL from Si nanocrystals formed in Al 2 O 3 matrix has been reported previously [11]. Since then, a number of research groups have studied this system [10,12–15]. While there are uncertainties about the origin of the PL, several different reports have attributed the light emission to size dependent emission from Si nanocrystals, radiative recombination centers at the nanocrystal–matrix inter- face and defects in the Al 2 O 3 matrix such as F centers, Cr 3+ and Ti 3+ impurities [10–16]. Si nanocrystals in Al 2 O 3 matrix can be formed by different techniques such as pulsed laser ablation [12], electrochemistry [13], ion implantation [10,14], co-sputtering [15,16] and electron beam evaporation [17]. In this paper, light emission and structural variation of Si nanocrystals with respect to depth are reported. Si nanocrystals were formed in Al 2 O 3 matrix by rf-magnetron co-sputtering method and subsequent annealing between 900 and 1100 1C. X-ray diffraction (XRD) was employed to observe the nanocrystal formation as a function of Si dc-power and annealing tempera- ture. Formation of Si nanocrystals and their variation with depth from the sample surface have been studied by the evolution of Si–Si bonds and higher oxidation states with respect to annealing temperature by using X-ray photoelectron spectroscopy (XPS). In a ARTICLE IN PRESS Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/physe Physica E 1386-9477/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2008.08.036 Ã Corresponding author. Tel.: +90 312 210 4314; fax: +90 312 210 5099. E-mail address: dogan@metu.edu.tr (I. Dogan). Physica E 41 (2009) 976–981