Atomic layer deposition of ZnS via in situ production of H 2 S J.R. Bakke a , J.S. King a , H.J. Jung b , R. Sinclair b , S.F. Bent a, ⁎ a Department of Chemical Engineering, 381 North South Axis, Stanford University, Stanford, California 94305, USA b Department of Materials Science and Engineering, Stanford University, Stanford, California 94305, USA abstract article info Article history: Received 30 September 2009 Received in revised form 1 March 2010 Accepted 16 March 2010 Available online 31 March 2010 Keywords: Zinc sulfide Atomic layer deposition Hydrogen sulfide Band gap Stacking fault Zincblende Wurtzite Transmission electron microscopy Atomic layer deposition (ALD) of ZnS films utilizing diethylzinc and in situ generated H 2 S was performed over a temperature range of 60 °C–400 °C. This method for generating H 2 S in situ was developed to eliminate the need to store high pressure H 2 S gas. The H 2 S precursor was generated by heating thioacetamide to 150 °C in an inert atmosphere, producing acetonitrile and H 2 S as confirmed with mass spectroscopy. ALD behavior was confirmed by investigation of growth behavior and saturation curves. The properties of the films were studied with X-ray diffraction, transmission electron microscopy, ellipsometry, atomic force microscopy, scanning electron microscopy, ultraviolet–visible spectroscopy, and X-ray photoelectron spectroscopy. The results show a growth rate that monotonically decreases with temperature, and films that are stoichiometric in Zn and S. The root mean square roughness of the films increases with temperature above 100 °C. A change in crystal phase begins at ∼ 300 °C. The band gap is dependent on the crystal phase and is estimated to be 3.6–4 eV. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Atomic layer deposition (ALD) is a thin-film growth technique whose applications have dramatically increased over the past decade. Since the precursors are supplied sequentially into a reactor and the half-reactions are surface-reaction rate limited, ALD has the capability to produce uniform thin films over large areas. The technique is termed atomic layer deposition because films are typically deposited up to a maximum of one monolayer per cycle, due to the self-limiting nature of precursor adsorption during each step of the ALD process. Commonly, submonolayer growth (i.e. a rate lower than the lattice constant) is observed in ALD, an effect which can stem from steric and/or electronic effects. The ALD process is characterized by a growth rate that is linear as a function of the number of cycles. Moreover, the self-limiting nature of ALD can be seen by saturation curves, which show that the growth rate per cycle reaches a maximum even when a large excess of precursor is dosed into the reactor. ALD features some advantages over other common methods for deposition of films, including chemical bath deposition (CBD) and chemical vapor deposition (CVD). In ALD, films are deposited via the vapor phase whereas CBD is solution based; thus, ALD is easier to integrate into a process that is already performed in vacuum. Moreover, ALD is often performed at lower temperatures than CVD, allowing for finer control of growth rate. Doping of complex materials and uniform deposition on high aspect ratio substrates are two additional advantages of ALD. The atomic layer deposition of sulfide films with H 2 S as a reactant has been demonstrated for many material systems including CaS [1], Cu x S [2], ZnS [3], SrS [1], CdS [4], BaS [1], and PbS [5]. The H 2 S gas used in these processes is typically purchased from a supplier as a compressed gas, and is flammable within the limits of 4–46% by volume in air. Moreover, H 2 S gas is highly toxic with an LC50 of ∼ 700 ppm; thus, its use is largely restricted by regulatory agencies and universities. This paper details a process for generating H 2 S in situ using thioacetamide (TAA) as the sulfur source, eliminating the need for gas storage [6]. There are several advantages to using TAA as the sulfur source in place of compressed H 2 S for thin-film deposition. TAA is less hazardous than H 2 S (its hazardous materials identification system rating for health, flammability, and reactivity rating is 1/1/1 compared to 4/4/0 for pure H 2 S), and its use decreases the risk of exposure to toxic and flammable compressed gas. The H 2 S gas does not require handling since it is produced in situ under vacuum in a vial containing TAA attached to a reactor. Setup of the system is straightforward, and the quantity of gas produced can precisely be controlled, making the system ideal for small scale research laboratories. We will show that the purity of the H 2 S gas produced by this method is very high, and that this source can be used for ALD of sulfide films. Thioacetamide has previously been used as a sulfur source for several different applications. In the presence of acids, particularly Thin Solid Films 518 (2010) 5400–5408 ⁎ Corresponding author. Tel.: +1 650 723 0385(office); fax: +1 650 723 9780. E-mail address: sbent@stanford.edu (S.F. Bent). 0040-6090/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2010.03.074 Contents lists available at ScienceDirect Thin Solid Films journal homepage: www.elsevier.com/locate/tsf