DOI: 10.1007/s00339-003-2224-3 Appl. Phys. A 77, 775–778 (2003) Materials Science & Processing Applied Physics A t. okato 1 p.a. atanasov 2 r.i. tomov 2 m. obara 1, ✉ Fabrication of Nd:KGW film on a Si substrate with a CeO 2 buffer layer 1 Keio University, Department of Electronics and Electrical Engineering, 3-14-1, Hiyoshi, Kohoku-ku, Yokohama-shi, 223-8522 Japan 2 Institute of Electronics, Bulgarian Academy of Sciences, 72 Tsarigradsko Shose Blvd, Sofia 1784, Bulgaria Received: 8 May 2003/Accepted: 20 May 2003 Published online: 27 June 2003 • © Springer-Verlag 2003 ABSTRACT Highly textured single-oriented (110) Nd-doped potassium gadolinium tungstate [Nd:KGd(WO 4 ) 2 or Nd:KGW] thin films were successfully fabricated on (100) Si substrate by introducing a (100) CeO 2 buffer layer using KrF-excimer-laser pulsed laser deposition (PLD) at precisely controlled experi- mental conditions. The CeO 2 buffer layer was also prepared by PLD. Home-made potassium-enriched ceramic targets were prepared and used in order to prevent K deficiency in the film during the deposition. Depositions were performed in both Ar and O 2 environments. The optimal growing conditions achieved from the viewpoint of best crystallinity, optical properties and surface morphology were: d T–S = 4 cm, P(O 2 ) = 0.08 mbar and T sub = 700 ◦ C, respectively. Improvement of the properties of the as-grown films was examined by post-deposition anneal- ing between 700 ◦ C and 900 ◦ C in air. Optical waveguide loss and the photoluminescence spectrum in the 800–1400-nm range were measured. PACS 81.15.Fg; 42.79.Gn; 42.79.Hj 1 Introduction Pulsed laser deposition (PLD) is a suitable way for preparing high-quality thin films. Simple and flexible ex- perimental apparatus, feasibility of growth in various con- ditions and high deposition rate are the advantages of this method [1]. However, some disadvantages of this method such as non-exact stoichiometric transfer of the light elem- ents from the target to the films and particulates on the sur- faces are observed. A solid-state laser crystal film (i.e. wave- guide laser) has the potential to be an important device in the field of opto-electronic integrated circuits (OEICs). Ezaki et al., for the first time, reported the fabrication of Nd:YAG films on various substrates by means of the PLD method [2]. Subsequently, many reports have focused on the fabrica- tion of waveguide laser films using the PLD method, e.g. Nd:GGG [3], Nd,Cr:GGG [4], Yb:YAG [5], Ti:sapphire [6], and so forth. Recently, the fabrication of Nd:KGW films was reported [7–10]. These reports present great progress in some of these fields. However, most of them are grown on dielec- tric substrates, except for [2] and [9]. If Si substrates are used, ✉ Fax: +81-45/566-1529, E-mail: obara@obara.elec.keio.ac.jp more useful applications will appear, especially for prepar- ation of OEIC, since Si is the most important material for the fabrication of electronic devices and is a very technological material, allowing easy and technically matured processing. Rare-earth-doped potassium gadolinium tungstate [RE:KGd(WO 4 ) 2 or RE:KGW] is an attractive waveguide laser material, since it offers not only high-quality prop- erties as a laser medium but also has ultra-low threshold stimulated Raman scattering [11–14]. It should be pointed out that the preparation of Nd:KGW films presents difficul- ties due to the preferential scattering of potassium in the plasma, which causes K deficiency, especially when a single- crystal Nd:KGW is used as an ablation target [7–9]. However, Nd:KGW films have very recently been successfully grown in better quality by using K-enriched ceramic targets [10]. Si, particularly, has a high refractive index, which does not allow for a waveguide structure on it by direct growth. To overcome this problem, we have introduced a CeO 2 buffer layer between the KGW and Si for the first time. In this pa- per we will focus on the fabrication of Nd:KGW films on (100) CeO 2 /(100) Si substrates and investigation of the film properties. 2 Experimental The ceramic targets were prepared by mixing WO 3 , K 2 CO 3 , Gd 2 O 3 and Nd 2 O 3 powders in various ratios. The detailed description of target preparation is described in [10]. The subsequent K-content was analyzed using an EDX technique equipped with environmental scanning elec- tron microscopy (XL-30 ESEM-FEG, Philips). A target with N K / N W = 1.16 was used in this study, where N K and N W represent the number of K and W atoms, respectively. The (100) CeO 2 /(100) Si substrates were also prepared by the PLD method, which will be presented elsewhere [15]. The thickness of these buffer layers was several-hundred nm. The physical parameters of KGW, CeO 2 and Si are shown in Table 1 [11, 16–18]. A KrF excimer laser (LPX150, Lambda Physik), which operated at λ = 248 nm, f = 20 Hz, τ = 27 ns was used for ablation. Its beam was introduced inside the vacuum chamber with an incident angle of 45 ◦ . The laser fluence on the tar- get was adjusted to between 1.2 and 1.5 mJ/cm 2 . The films were deposited on the CeO 2 /Si substrates, which were situ-