ELSEVIER Applied Surface Science 81 (1994) 281-287 iii~i!i{ii!i!~{iii~i~ii~i~i~i~ii~i~iii~iiii~!~i~iiiii applied surface science Characterization of adsorption in flow type atomic layer epitaxy reactor J. Aarik, H. Siimon * Institute of Experimental Physics and Technology, University of Tartu, Ulikooli 18, EE2400 Tartu, Estonia Received 21 June 1994; accepted for publication 5 August 1994 Abstract Numerical model calculations describing the propagation of an adsorption wave in a low-pressure channel-type ALE reactor are carried out. The effect of channel parameters and flow rate of the carrier gas as well as diffusion and sticking coefficients of the precursor on the propagation of the adsorption wave are studied. It is shown that the surface density of adsorption sites and the diffusion coefficient of the precursor molecules can be determined from the precursor pulse delay time measurements. Using the time dependence of the precursor concentration or that of the surface coverage, both measured at the channel outlet, the sticking coefficient of the precursor can be calculated. 1. Introduction Growth of thin solid films in the chemical vapor deposition (CVD) and, particularly, in the atomic layer epitaxy (ALE) process is strongly dependent on the reactions between the solid surface and the pre- cursor molecules. However, possibilities for investi- gation of surface reactions in the flow-type reactors are rather limited. Unlike ultra-high vacuum equip- ment for molecular beam epitaxy (MBE) and chemi- cal beam epitaxy (CBE) the apparatus for CVD and gas transport ALE cannot be equipped with the traditional surface analysis means based on electron scattering. Thus alternative techniques such as mass spectrometry of reaction products [1], some optical * Corresponding author. Tel: 372-7-433276; Fax: 372-7-435440. methods for surface [2-6] and gas phase [7] analysis, deposit mass monitoring by quartz microbalance [8- 11] and gravimetry [12] have been used for investiga- tion of adsorption-desorption processes and surface reactions in gas transport reactors. Mass spectrometry yields information about com- position and formation kinetics of gaseous reaction products while the surface coverage changes can be described by surface photo-absorption (SPA) [2-4] and reflection difference spectroscopy (RDS) [5,6]. None of these methods enables us to receive direct information about the growth rate. This information can be achieved from spectro-ellipsometry measure- ments [13]. Unfortunately, the last method is rather complicated and therefore other methods like gravimetry and deposit mass monitoring by quartz microbalance are frequently used. In situ gravimetry has enabled Koukitu et al. [12] to record the overall weight increase which occurred 0169-4332/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0169-4332(94)00286-X