硫化镉的结构和光电性质.pdf

INVESTIGACI ´ON REVISTA MEXICANA DE F´ISICA 54 (2) 112–117 ABRIL 2008Structural, optical and electrical properties of CdSthin films obtained by spray pyrolysisC. Santiago Tepantl´anUniversidad Polit´ecnica de Tulancingo,Calle Ingenier´ıas 100 Huapalcalco, 43629, Tulancingo, Hidalgo, M´exico,e-mail: cesar upt@.mxA.M. P´erez Gonz´alez and I. Valeriano ArreolaUniversidad Popular Aut´onoma del Estado de Puebla (UPAEP),21 Sur 1103 Colonia Santiago, 72160, Puebla, Puebla, M´exico,e-mail: arllenemariana.perez@upaep.mx, iracema.valeriano@upaep.mxRecibido el 14 de agosto de 2007; aceptado el 21 de febrero de 2008Cadmium sulphide (CdS) thin films were prepared by means of the chemical spray pyrolysis technique. The substrate temperature wasvaried in the range from 200 to 400–C. The structural properties of the semiconductor were characterized by X-ray diffraction; XRD patternsindicated the presence of single-phase hexagonal CdS. Direct band gap values of 2.37-2.41 eV were obtained. The refractive index is reportedon depending on the substrate temperature, and was obtained from transmission spectra and from spectroellipsometry measurements. Asa consequence, the optical parameters of the films were determined using the Swanepoel, Cauchy, Sellmeier and Wemple models. Theresistivity of the films was found to vary in the range 103¡105›.cm, depending on the substrate temperature.Keywords: CdS thin films; spray pyrolysis; structural properties; optical properties; electrical properties.Pel´ıculas delgadas de sulfuro de cadmio han sido preparadas por el m´etodo qu´ımico de roc´ıo pirol´ıtico, variando la temperatura del substratoen el intervalo de 200 a 400–C. Las propiedades estructurales del material obtenido fueron estudiadas utilizando la t´ecnica de difracci´onde rayos X, y los patrones de difracci´on muestran la presencia de la fase hexagonal del CdS. Se obtuvo un ancho de banda entre 2.37 y2.41 eV, en funci´on de la temperatura de dep´osito. Las curvas de ´ındice de refracci´on fueron obtenidas a partir del espectro de transmisi´on ypor mediciones espectroelipsom´etricas, de manera que los par´ametros ´opticos fueron obtenidos usando los modelos de Swanepoel, Cauchy,Sellmeier y Wemple. La resistividad el´ectrica en condiciones de oscuridad var´ıo en el rango de 103-105›.cm, dependiendo de la temperaturadel substrato. Un an´alisis de los efectos del m´etodo de dep´osito en las propiedades de las pel´ıculas es presentado.Descriptores: Sulfuro de cadmio; roc´ıo pirol´ıtico; propiedades estructurales; propiedades ´opticas; propiedades el´ectricas.PACS: 61.10.Nz; 71.55.Gs; 78.20.Ci; 78.66.-w; 78.66.Hf1. IntroductionCdS thin films are regarded as one of the most promisingmaterials for heterojunction thin film solar cells. Wide bandCdS (Eg= 2.42 eV) has been used as the window material to-gether with several semiconductors such as CdTe, Cu2S andInP with 14-16% efficiency [1-3]. However, due to the highcost of this material, studies were developed towards poly-crystalline compound semiconductors and particularly thinpolycrystalline films.The deposition of CdS films has been explored by dif-ferent techniques: sputtering, thermal evaporation, chemicalbath deposition, and molecular beam epitaxy [4-8]; in eachof these methods polycrystalline, uniform and hard films areobtained, and their electrical properties are very sensitive tothe method of preparation. Spray pyrolysis [9,10] althoughit is expensive, requires the use of sophisticated materials forlarge areas of deposition, now gives good quality semicon-ductors which permit the fabrication of solar cells with satis-factory efficiency.The aim of this work is to produce CdS thin films bymeans of the spray pyrolysis technique and to investigatetheir structural, optical and electrical properties, and this de-pendence on substrate temperature. Optical properties wereobtained from transmission spectra (TS) and Spectroellip-sometry (SE) measurements.2. ExperimentalThe spray pyrolysis technique is a simple technology inwhich an ionic solution containing the constituent elementsof a compound in the form of soluble salts is sprayed ontoover-heated substrates using a stream of clean, dry air. Theapparatus used for our sprayed process is diagrammed in Fig.1, and has been described in Ref. 11. The CdS thin films wereprepared by spraying an aqueous solution of cadmium chlo-ride (CdCl2) and thiourea [CS(NH2)2] on a glass substratekept at 200, 300 and 400–C. The atomization of the chemicalsolution into a spray of fine droplets is effected by the spraynozzle, with the help of compressed air as the carrier gas. Thespray rate of about 10 cm3/min through the nozzle ensuresa uniform film thickness. The substrates are Corning 1737glass 2 £ 2 cm, and are placed in a fitted socket at the surfaceof a substrate heater when sprayed. The heater is a cylindricalstainless steel block furnace, electrically controlled to an ac-curacy of §1–C. The substrate temperature was varied, whilethe other spra。