CVD of Nanosized ZnS and CdS Thin Films from Single-Source Precursors

Abstract

A chemical vapor deposition ͑CVD͒ route to nanosized ZnS and CdS thin films was developed. The layers were deposited on SiO 2 substrates in a N 2 atmosphere at temperatures between 473 and 723 K using M(O-i PrXan) 2 ͓M ϭ Zn,Cd; O-i PrXan ϭ S 2 COCH(CH 3 ) 2 ] as single-source precursors. Thermal decomposition and fragmentation of M(O-i PrXan) 2 compounds were investigated by thermal analyses and mass spectrometry. The sulfide films were thoroughly characterized in their composition, nanostructure, and morphology by means of several analytical techniques. Surface and in-depth chemical composition was studied by X-ray photoelectron spectroscopy, X-ray excited Auger electron spectroscopy, and secondary ion mass spectrometry. Film nanostructure and surface topography were investigated as a function of the synthesis conditions by glancing incidence X-ray diffraction and atomic force microscopy, respectively. Optical absorption properties were also studied. Nanophasic and contaminant-free ZnS and CdS thin films with an average crystallite size lower than 25 nm were obtained. The layers mainly contained the hexagonal ͑␣͒ sulfide phase and displayed a smooth and regular surface morphology. In the present work, the influence of synthesis conditions on film characteristics is analyzed and discussed.The group 12 metal sulfides ZnS and CdS are both direct bandgap semiconductors existing in two different polymorphs, the hexagonal phase ͑␣, wurtzite͒, and the cubic one ͑␤, sphalerite͒ ͓for ZnS: E G (␣) ϭ 3.67 eV; E G (␤) ϭ 3.54 eV; 1-3 for CdS: E G (␣) ϭ 2.55 eV; E G (␤) ϭ 2.44 eV 4-6 ͔. For both sulfides, the stability of the ␣ and ␤ phases is a function of temperature. As regards ZnS, the low-temperature polymorph is the cubic ␤ form, that can be converted into the hexagonal ␣ at temperatures between 773 and 1273 K, depending on the system structure and stoichiometry ͑sulfur deficiency͒. 3 In the case of CdS, the ␣ phase is the most thermodynamically stable up to 1173 K, and the ␤ phase has been obtained in the form of a thin film on suitable substrates such as GaAs͑100͒ and Si͑111͒. 6,7 Zinc͑II͒ and cadmium͑II͒ sulfide thin films have been used for a wide variety of technological applications, including solid-state cell windows, electro-optic modulators, sensors, electroluminescent and photoluminescent devices, and antireflection coatings. [8][9][10][11] In particular, such systems have been widely employed in display technology as host lattices for doped phosphor materials, whose emission wavelength can be tailored as a function of the dopant metal. 9,12-14As a general trend, phase composition, as well as thermal stability and morphology, which, in turn, strongly influence the functional behavior of ZnS and CdS-based devices, 15 are directly dependent on the deposition mechanism and processing conditions. 16 As a consequence, several techniques have been adopted for the preparation of ZnS and CdS layers, including sputtering, 17,18 pulsed laser ablation, 19,20 spray pyrolysis, 21,22 chemical bath deposition, 4,23 electros...

Spin-coated nickel doped cadmium sulfide thin films for third harmonic generation applications

“…The CdS thin films have been extensively explored by numerous techniques, for instance, sol-gel dip-coating [14], spray pyrolysis [15], chemical vapor deposition (CVD) [16], sputtering [17], electrodeposition [18], pulse laser deposition [19] and chemical bath deposition [20]. With all the techniques to coat CdS thin films, the sol-gel spin-coating method stands out because it offers several advantages like simplicity, low cost, and its ability to generally obtain uniform films with good adherence and reproducibility [21].…”

Role of Thiourea in the Kinetic of Growth of the Chemical Bath Deposited ZnS Films

“…Published October 1, 2014. ZnS is a promising semiconductor material, whose properties depend on the technique of synthesis, and it can be used for a wide variety of applications. 1 Various techniques, such as sputtering, 2 electrodeposition, 3 spray pyrolysis, 4 chemical vapor deposition (CVD), 5,6 successive ionic adsorption and reaction, 7 thermal evaporation, 8,9 and chemical bath deposition 10 have been used for synthesizing this semiconductor. However, chemical bath deposition (CBD) has been recognized as the simplest and low cost aqueous technique for preparing films with large area.…”

Synthesis and characterization of zinc bis(O-isopropylxanthate) as a single-source chemical vapor deposition precursor for ZnS

“…• C. 11,20,22 These phenomena point to the stability of Zn-S moieties in the gas phase, thus indicating the suitability of Zn(Oi PrXan) 2 as a single-source CVD precursor for zinc(II) sulfide. …”

“…25 Two sulfur atoms around each zinc center belong to a chelating xanthate group, and the remaining positions are bound to two different bidentate (CH 3 ) 2 CHOCS 2 ligands. 11,25 These tetramers are packed three-dimensionally via van der Waals interactions between sulfur-methyl and methyl-methyl groups. 24 In order to attain a deeper insight on the coordination geometry of zinc bis(O-isopropylxanthate), a preliminary solid-state characterization was performed by extended Xray absorption fine structure (EXAFS), Raman and FT-IR spectroscopies.…”