Solution Processing of GaAs Thin Films for Photovoltaic Applications Sanjayan Sathasivam, † Ranga Rao Arnepalli, ‡ Bhaskar Kumar, ‡ Kaushal K. Singh, ‡ Robert J. Visser,* ,‡ Christopher S. Blackman, † and Claire J. Carmalt* ,† † Materials Chemistry Centre, Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, U.K. ‡ Applied Materials, Inc., 3225 Oakmead Village Drive, Santa Clara, California 95052-8039, United States ABSTRACT: In this article we present a novel route to high quality GaAs thin films via a solution processing technique (aerosol assisted chemical vapor deposition) using a novel single source precursor [Me 2 GaAs(H) t Bu] 2 . The thin films, grown on inexpensive glass substrates, were polycrystalline in nature with a Ga to As ratio of 1:1. The morphology studied via SEM showed the films to be smooth and consisting of compact domes. High-resolution transmission electron mi- croscopy (HRTEM) revealed the films to have columnar growth and an average crystallite size of 90 nm. The films also contained low levels of contaminants as determined via energy dispersive X-ray spectroscopy (EDX) mapping, X-ray photo- electron spectroscopy (XPS) depth profiling, and secondary ion mass spectrometry (SIMS). 1. INTRODUCTION Gallium arsenide (GaAs) is a semiconductor that is widely used in photovoltaic and optoelectronic devices. 1,2 It displays better theoretical and experimental properties than the ubiquitous silicon-based devices. For example, GaAs thin film solar devices show an efficiency of 28.8% compared to 20.1% that is achieved by amorphous silicon (crystalline silicon devices show an efficiency of 25.0%). 3 This is owed to GaAs having a direct bandgap of 1.43 eV, which is close to the 1.34 eV bandgap that is optimum for solar conversion for a single junction solar cell at AM1.5. 4,5 GaAs also has high electron mobility that make for better device performance, 1,6,7 and is more resistant to heat and radiation damage compared to Si due to the higher threshold energy under high energy radiation. 7 However, the high cost and difficulty associated with fabricating GaAs devices has so far limited the use of GaAs photovoltaics to space and military applications. The current methods of fabrication involve epitaxial methods such as metal-organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE) on expensive single crystal substrates, such as GaAs and germanium, involving dual source precursors. The precursors involved in such depositions are usually trimethylgallium (GaMe 3 ), which is a pyrophoric liquid, and arsine (AsH 3 ), which is a highly toxic gas. 8 Liquid tert-butyl arsine ( t BuAsH 2 ) has also been used as an alternative to AsH 3 , but it is still highly toxic. 9 Hence these precursors when used on an industrial scale present handling and safety concerns. Single-source precursors can overcome these concerns as they are much less toxic, less pyrophoric and are easier to handle if in the solid state. 10 Furthermore, they are very useful in the growth of binary semiconductor film, such as GaAs, as they contain preformed Ga-As bonds with the required elements of the film in the correct ratio. Hence allowing the formation of films at reduced temperatures and with the correct stoichiometry. The 1:1 ratio of Ga:As in the single-source precursor is advantageous as current dual source techniques such as MOCVD require an excess of arsine (up to 10:1) to obtain stoichiometric GaAs films. In the majority of cases in the literature, GaAs is grown on single crystal substrates such as GaAs and Ge. 6 There are very few examples of deposition on amorphous substrates such as glass. Imaizumi et al. reported the use of chemical beam epitaxy to deposit polycrystalline GaAs on glass using arsine and trimethylgallium at 500 °C. 11 The deposition required a precracking step of 1000 °C to enable pyrolysis of the arsine precursor. GaAs has also been grown on glass substrates via flash evaporation at 2.7 × 10 -6 mbar by Campomanes et al., the as-deposited films required a thermal annealing step to obtain crystallinity. 12 Deposition via MBE and low pressure (LP) CVD have also been reported using arsine and metallic sources of Ga on glass substrates. In the literature there are many examples of molecular GaAs precursors. Ranging from simple Lewis acid-base adducts such as [GaR 3 {AsR′ 3 }] (R = Et, t Bu; R′= SiMe 3 , i Pr) 13 to more complex dimeric and trimeric compounds such as [R 2 GaAs t Bu 2 ] 2 (R = Me, Et, n Bu) 14,15 and [Me 2 GaAs i Pr 2 ] 3 . 16 Though the synthetic routes to GaAs precursors are well documented, the deposition of GaAs films from such precursors is not as well studied, as these precursors are not all compatible with traditional deposition techniques. For example, the dimers [ n Bu 2 Ga(μ-E t Bu 2 ) 2 Ga n Bu 2 ] 2 (E = P, As) were shown to be effective precursors for deposition of Received: April 10, 2014 Revised: July 10, 2014 Published: July 21, 2014 Article pubs.acs.org/cm © 2014 American Chemical Society 4419 dx.doi.org/10.1021/cm501280e | Chem. Mater. 2014, 26, 4419-4424