Journal of Crystal Growth 290 (2006) 597–601 Carbon coating of fused silica ampoules Mark J. Harrison, Adam P. Graebner, Walter J. McNeil, Douglas S. McGregor à S.M.A.R.T. Laboratory, Department of Mechanical and Nuclear Engineering, Kansas State University, Manhattan, KS 66506, USA Received 22 September 2005; received in revised form 5 January 2006; accepted 10 January 2006 Available online 20 March 2006 Communicated by T. Hibiya Abstract A reliable method for depositing a layer of carbon on the inner walls of a fused silica ampoule is described and characterized. Carbon deposition rates were found to be 0.33 mm/h at mid-length and range from 0.26 to 0.55 mm/h at the ends of a 150 mm long ampoule. Deposition rate was found to vary along the length of the ampoule, but not along the radial perimeter. r 2006 Elsevier B.V. All rights reserved. Keywords: A1. Ampoule preparation; A1. Carbon coating; A2. Growth from melt; B1. Quartz 1. Introduction Fused synthetic silica (quartz) is often used in the construction of the crucible or ampoule in crystal growth or materials purification applications for a number of reasons. Quartz is readily available in very pure form while possessing the ability to withstand high temperatures and high thermal gradients. The material also maintains good mechanical strength at elevated temperatures. However, for all of the beneficial properties of quartz, devitrification and other deleterious effects occur in the presence of some materials. Certain processes benefit from a thin coating of carbon deposited on the quartz sidewalls as the carbon can act as a scavenger of oxygen (O 2 ) and water (H 2 O) impurities and as a protective buffer between the material of concern and the quartz [1]. In the case of Bridgman growth of cadmium telluride (CdTe) or cadmium zinc telluride (CdZnTe), the ingot tends to adhere to the sidewalls of the quartz vessel in which it is grown. Presumably, cadmium oxide (CdO) reacts with the silicon in the sidewall to form cadmium metasilicate (CdSiO 3 ) [1]. However, at elevated tempera- tures, carbon reacts easily with O 2 and H 2 O to form carbon dioxide (CO 2 ) and carbon monoxide (CO), thus tying up otherwise free O 2 molecules. Without the presence of free oxygen, the reaction between CdO and quartz is much less severe since CdO does not initially form. Therefore, the inclusion of carbon within the ampoule serves two purposes. The first purpose is to getter any O 2 or H 2 O. Second, the carbon layer physically prevents CdO from contacting the inner ampoule walls. Similar benefits are reported for the growth of copper indium selenide (CuInSe 2 ) [2]. The carbon coating process and its effects on crystal growth are mentioned several times in the literature [1–9]. However, a definitive procedure for obtaining uniform, adherent coatings of carbon is not present. This work describes a reliable, reproducible carbon coating technique and presents data regarding the thickness of coatings at various locations along the ampoule. 2. Experimental procedure The process described here requires four basic steps: ampoule cleaning, ampoule annealing, carbon deposition, and film annealing. The cleaning and annealing procedures were found to greatly impact carbon film quality and are therefore included. Carbon deposition is the process of cracking hydrocarbon molecules to produce an amorphous carbon layer. The deposited carbon film must be then heated to sufficiently high temperatures to densify the layer into a glass-like form of lower porosity. Each of these four steps for obtaining a uniform, well-adhered carbon film on ARTICLE IN PRESS www.elsevier.com/locate/jcrysgro 0022-0248/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2006.01.014 à Corresponding author. Tel.: +1 785 532 5284; fax: +1 785 532 7057. E-mail address: mcgregor@ksu.edu (D.S. McGregor).