Structural and transport properties of CdS films deposited on flexible substrates M.S. Shur a,b , S. Rumyantsev a,b, * , R. Gaska b , B.Q. Wei b , R. Vajtai b , P.M. Ajayan b , J. Sinius b a Biological and Information Technologies, Inc., 10 Starboard Way, Latham, NY 12110, USA b Rensselaer Polytechnic Institute, 110 8th Street, Troy, NY 12180-3590, USA Received 21 November 2001; received in revised form 17 December 2001; accepted 15 January 2002 Abstract We report on structural and electrical properties of CdS films chemically deposited at temperatures 60–70 °C from solutions containing cadmium citrate complex-ions and thiourea. We demonstrated the material deposition on a variety of flexible substrates, including viewfoils, cloth, and threads and fabricated devices on the deposited films including solar cells, stress sensors, and photoconductive sensors. Scanning electron microscopy and high resolution transmission electron microscopy data show that the films consist of nanocrystalline grains. The X-ray diffraction data identify crystalline CdS with hexagonal structure with a very high degree of crystallinity. The relative intensity of the peaks in the X-ray spectrum shows that the crystallites are oriented. The nanostructure of the CdS films results in a giant re- producible sensitivity to stress (tension) under UV illumination. Ó 2002 Elsevier Science Ltd. All rights reserved. 1. Introduction Recently, we reported on depositing semiconductor materials, such as CdS, CdSe, and PbS and semicon- ducting and metallic Cu x S films on large flexible sub- strates and threads at temperatures close to room temperature [1]. This technology should enable numer- ous applications in electrotextiles [2], robotics, medicine, biology, industrial controls, and defense by covering complex and flexible surfaces with intelligent semicon- ductor films with embedded multiple sensor and data processing arrays [3,4]. We also reported on the photo- voltaic effect in polycrystalline semiconductor CdS cells deposited on viewfoils [5] and threads [6]. Such solar cells could provide a renewable energy source (energy harvesting) for electrotextiles and ‘‘sensitive skins’’ [3,4]. In this paper, we report on the structural and elec- trical properties of CdS films deposited on viewfoil and trylene threads. This study revealed that these films might serve as highly sensitive strain sensors. The CdS films were deposited at temperatures 60–70 °C from solutions containing cadmium citrate complex- ions and thiourea. Because of low deposition temper- atures, we were also able to use a variety of flexible materials, including viewfoils, cloths, and threads. We coveredtheareasuptoseveraltensofsquareincheswith 0.2–0.5 lm thick CdS films. The layer thickness was measured using ‘‘Talystep’’ profilometer on the selec- tively etched CdS layer. Fig. 1 shows the result of such measurement for the 0.2 lm thick CdS layer. More details of our film deposition process are described in Ref. [7]. A scanning electron microscope (SEM) study re- vealed that the CdS films deposited on the viewfoil are very homogeneous. As seen in Fig. 2a, no distinguish- able structure of the layers has been found with the characteristic dimension of the fraction of microns. The structure of the layers deposited on the thread was more complicated. Fig. 2b shows the SEM picture of the CdS film de- posited on thread. It is seen that the film is not uniform and contains some insertions and cracks. Solid-State Electronics 46 (2002) 1417–1420 www.elsevier.com/locate/sse * Corresponding author. Address: Rensselaer Polytechnic Institute, Room 9015, CII, 110 8th Street, Troy, NY 12180- 3590, USA. Tel.: +1-518-276-2908; fax: +1-518-276-2990. E-mail address: roumis2@rpi.edu (S. Rumyantsev). 0038-1101/02/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. PII:S0038-1101(02)00073-4