Chemical Vapor Deposition of Hexagonal Gallium Selenide and Telluride Films from Cubane Precursors: Understanding the Envelope of Molecular Control

Abstract

Gallium selenide (GaSe) thin films have been grown at 325-370°C by atmospheric pressure metal-organic chemical vapor deposition (AP-MOCVD) using the single-source precursors [(R)Ga(μ3-Se)]4 (R = CMe3, CEtMe2, and CEt2Me). In contrast, the growth of gallium telluride from [(R)Ga(μ3-Te)]4 is accomplished at 285-310°C by low-pressure metal-organic chemical vapor deposition (LP-MOCVD). Characterization of the films by Auger electron spectroscopy (AES), Rutherford backscattering (RBS), and wavelength-dispersive spectroscopy (WDS) microprobe analysis shows all the films to have Ga:E (E = Se, Te) compositions of 1:1 with a low degree of impurities (C < 2%; O < 0.1%). From X-ray diffraction (XRD) and transmission electron microscopy (TEM) the GaSe and GaTe films were found to be polycrystalline hexagonal layered GaE structures, with a preferred c-axis orientation independent of substrate. For GaSe this represents the thermodynamically stable phase, while hexagonal GaTe is a metastable phase that converts to the thermodynamic monoclinic form on annealing at 500°C. Diffraction results yield appropriate lattice parameters of a = 4.1 Å (TEM) and c = 16.38 Å (XRD) for hexagonal GaTe. Crystallite and particle size measurements reveal that the as-deposited films consist of oriented ca. 10 nm crystalline particles. The formation of these hexagonal layered structures is proposed to occur as a consequence of the fragmentation of the cubane's Ga4E4 core during deposition, and the formation of trimeric "Ga3E3" building blocks.