Phase transformation and optical characteristics of porous germanium thin film T.S. Ko a, ⁎ , J. Shieh b , M.C. Yang b , T.C. Lu a , H.C. Kuo a , S.C. Wang a a Department of Photonics & Institute of Electro-Optical Engineering, National Chiao Tung University,1001 Tahsueh Rd., Hsinchu 30050, Taiwan, ROC b National Nano Device Laboratory, 26 Prosperity Road I, Science-based Industrial Park, Hsinchu 30078, Taiwan, ROC Received 30 May 2006; received in revised form 12 April 2007; accepted 5 June 2007 Available online 14 June 2007 Abstract In this study, we proposed a method to prepare GeO 2 by treating porous Ge thin film with thermal annealing in O 2 ambient. After annealing, the morphological transformation from porous thin film to an island structure was observed. The crystallization and composition of the porous Ge thin film prepared using different annealing time in O 2 ambient were confirmed by X-ray diffraction, X-ray photoelectron spectroscopy and Raman spectra. Initial Ge composition was gradually oxidized to GeO 2 with increasing annealing time. Comparing the photoluminescence (PL) results between Ge and GeO 2 , it was found that the visible photoluminescence originated from the germanium oxide. Photoluminescence measurements obtained at different temperatures exhibited a maximum integrated PL intensity at around 200 K. A possible explanation for this behavior might be the competition between radiative recombination and nonradiative hopping process. © 2007 Elsevier B.V. All rights reserved. Keywords: Chemical vapor deposition; Germanium; Phase transitions; Photoluminescence 1. Introduction Since the observation of visible luminescence, nanostructured silicon and germanium have attracted much attention recently. For example, visible photoluminescence (PL) characteristics of SiGe nanostructure with high area to volume ratio can be achieved at room temperature [1–3]. These nanostructured materials have excellent potential for applications on optical-electronic devices [4]. The earliest room-temperature luminescence for silicon was discovered from porous silicon prepared by electrochemical anodization in a dilute hydrofluoric acid solution [5]. Because bulk silicon is a semiconductor with indirect band gap energy about 1.12 eV (1100 nm), it is generally believed that the PL emission in visible light segment originated from either quantum confined effect or capture of excitons from the interface between surface oxidization film and Si crystal [6]. In contrast to silicon, germanium has lower energy band gap energy about 0.67 eV and larger effective excitonic Bohr radius so that the quantum confined effect related to limit size could be easier achieved [7]. To date many relevant researches on luminescence property of SiGe system have been carried out. For example, Liu et al. have fabricated self-organized Ge quantum dot superlattices by solid source molecular beam epitaxy, and they observed the blue shift emission peak from the decreasing size of quantum dots [8]; Zhu et al. have fabricated Si and Ge nanocrystals in Ge/Si thin film prepared by using a variation of the pulsed laser deposition method, and studied the possibility of light emission from interface between the crystal and thin film [9]; Zacharias et al. have investigated the luminescence in SiO 2 films containing Ge and GeO 2 nanocrystals [10]. In spite of many methods that have been developed to prepare nanostructured Ge materials and the optical properties that have been intensively studied, it is still not clear how the visible luminescence mechanism originated from either defects in oxide or interface between nanocrystal and oxide, even quantum confinement effects. Recently, we have reported that high quality porous Ge thin film can be prepared by low-pressure inductively coupled plasma chemical vapor deposition (ICPCVD) [11]. In this study, we Available online at www.sciencedirect.com Thin Solid Films 516 (2008) 2934 – 2938 www.elsevier.com/locate/tsf ⁎ Corresponding author. Tel.: +886 3 5712121x52962; fax: +886 3 5716631. E-mail address: tsko.eo93g@nctu.edu.tw (T.S. Ko). 0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2007.06.023