Journal of Alloys and Compounds 506 (2010) 661–665 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom Comparative study of electrical properties of Cd and Te-enriched CdTe thin ﬁlms at cryogenic temperature Nazar Abbas Shah ∗ Thin Films Research Laboratory, Department of Physics, COMSATS Institute of Information Technology, Park Road, Islamabad 45320, Pakistan article info Article history: Received 12 February 2010 Received in revised form 5 July 2010 Accepted 7 July 2010 Available online 15 July 2010 Keywords: Thin ﬁlms Coatings Resistivity Cadmium telluride abstract Cd and Te-enriched cadmium telluride (CdTe) polycrystalline thin ﬁlms were grown on corning glass sub- strates by Close Spaced Sublimation (CSS) technique. The structural investigations performed by means of X-ray diffraction (XRD) technique, scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDX) showed that the deposited ﬁlms exhibit a polycrystalline structure with 111 as pre- ferred orientation. The optical transmittance for Te-enriched CdTe sample was above 0.8 in the range of 1500–2500 nm, which was signiﬁcantly below 0.8 for Cd-enriched CdTe sample. The electrical properties of these samples were analyzed as a function of the Cd and Te concentration at cryogenic temperature. The electrical resistivity dropped several orders of magnitude. These properties are signiﬁcantly changed at cryogenic temperature. The comparative study revealed that using this deposition technique, n-type, and p-type Cd and Te-enriched CdTe polycrystalline ﬁlms can be produced. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Search for efﬁcient photo-receiving materials is a long stand- ing demand of scientiﬁc community. CdTe is II–VI semiconductor material quite appropriate for applications to solar energy conver- sion devices [1–3]. Though this semiconductor has been focused as solar cell material but could not get much attention commercially due to its cost factors. An important reduction in the cost of solar cells can be achieved by preparing doped polycrystalline thin ﬁlms of appropriate quality. CdTe thin ﬁlms have been prepared by sev- eral growth techniques [4–19]. However, the best CdTe thin ﬁlms based solar cells have been fabricated using CSS method which is one of the various techniques with large area manufacturing poten- tial due to its high throughput and efﬁcient material utilization [20]. Thin ﬁlms prepared by the CSS method have distinct novel features such as large grain sizes, well-deﬁned preferential orientation and relatively high absorption coefﬁcients needed in the fabrication of solar cells material [20]. In this paper we present some of the electrical properties of Cd and Te-enriched CdTe thin ﬁlms prepared by the CSS method. The experiments were carried out by taking Hall measurements at cryo- genic temperature. Furthermore, comparative analysis of measured physical properties is presented. To our knowledge the compara- tive study of Hall measurements on Cd and Te-enriched CdTe thin ﬁlms at liquid nitrogen temperature are not reported earlier. ∗ Tel.: +92 519235036; fax: +92 514442805. E-mail address: nabbasqureshi@yahoo.com. 2. Experimental The deposition of CdTe thin ﬁlms by the CSS technique is based on the following reversible dissociation of CdTe at high temperature: 2CdTe (s) ⇔ 2Cd (g) + Te 2(g) (1) The schematic diagram of Close Space Sublimation (CSS) experimental set up is shown in Fig. 1, where a small distance of about 5 mm separates the CdTe source from a corning glass substrate. The source temperature was kept higher than that of the substrate, because the source CdTe dissociates into its constituent elements, i.e. Cd and Te and then recombines on the substrate surface, which was kept at a lower temperature. The source and substrate could be maintained at the desired temper- ature as they were heated and controlled separately. Two K-type thermocouples were used to monitor the temperatures of the source and the substrate during the evaporation process. The temperatures of the source and substrate were maintained at 500 and 400 ◦ C, respectively. The source material (CdTe 99.999% purity) was placed in a graphite sublimation cast as shown in Fig. 1. The substrate was ﬁrst cleaned with IPA bath in an ultrasonic cleaner for about 60 min and further cleaned by lint free tissues with the help of tweezers and ﬁnger cots to avoid ﬁngerprints on the glass substrate. The substrate was then supported by the cast which was made of heat insulating mica sheet and kept in a close proximity to the source material. Initially the vacuum chamber was allowed to evacuate by a rotary vane pump. The chamber was kept for evacuation for about an hour each time to reach approximately 10 −3 mbar. The optimized time for deposition was 5 min for each thin ﬁlm. The thin ﬁlm was then kept at substrate temperature until the temperature of source became lower than the substrate temperature after that the substrate heater was switched off to allow cooling to ∼40 ◦ C before opening the vacuum chamber. By using the automatic temperature controller for heating the source and substrate, the quality of the thin ﬁlm was optimized. In the next step, these as-deposited CdTe thin ﬁlms were used as substrate for varying excess Cd mass (99.99% purity) deposition by the same technique to get Cd-enriched samples. The source (Cd) and substrate (CdTe) temperatures were kept at 350 and 250 ◦ C, respectively. 0925-8388/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2010.07.038