Materials Science and Engineering B 109 (2004) 260–263 Field-effect transistor based on nanometric thin CdS films B. Mereu 1 , G. Sarau, E. Pentia, V. Draghici, M. Lisca, T. Botila, L. Pintilie ∗ National Institute of Materials Physics, PO Box MG-7, Bucharest-Magurele 76900, Romania Abstract Cadmium sulphide (CdS) thin films were deposited by chemical bath deposition (CBD) method on SiO 2 /Si (n-type) substrates. Approxi- mately, 70nm thick nano-crystalline CdS layers were obtained. Thin film field effect transistors were realised by deposition of two coplanar electrodes of Au (drain and source) on the CdS surface. The gate contact is aluminium deposited on the backside of the Si substrate. The drain current–drain voltage characteristics (I d - V d ) were performed in dark. Normal field effect transistor characteristics are obtained in case of positive gate and drain voltages, the device acting as an n-channel transistor in the accumulation mode. For negative drain voltages the characteristic is dominated by space charge limited currents (SCLC). An on/off current ratio of about 10 2 is reported, this being limited in our case by geometry. © 2003 Elsevier B.V. All rights reserved. Keywords: Thin film transistors; Cadmium sulphide; Chemical bath deposition 1. Introduction Cadmium sulphide (CdS) is a well-known A 2 B 6 semi- conductor compound with good photoconductive prop- erties in the visible domain of electromagnetic spectrum (400–600 nm) [1]. Various structures and semiconductor devices based on CdS were realised and characterised, in- cluding thin film field-effect transistors (TFT) [2]. This type of device is used for addressing flat panel displays and for storage technology. Among different methods, which are being used for the preparation of CdS films, chemical bath deposition (CBD) is the most attractive due to its low cost, easy to handle and large possibilities regarding doping and deposition on various substrates. Heterostructures, with in- teresting physical properties can be imagined, realised and tested in this way. In particular it can be used to easily ob- tain field effect devices by depositing a CdS thin film over a SiO 2 /Si substrate. Such structures were realised and have shown good transistor characteristics [2]. In the present article we report on the behaviour of CBD–CdS TFTs for positive and negative drain voltages when positive gate voltages are applied. All devices exhib- ited n-channel transistor action in the accumulation mode for positive drain (V d ) and gate (V g ) voltages. For negative ∗ Corresponding author. Tel.: +40-214930267. E-mail address: pintilie@alpha1.infim.ro (L. Pintilie). 1 Present address: MPI-Halle, Weinberg 2, Halle/Salle, Germany. V d and positive V g the transistor characteristics are not symmetrical with respect to the current axis. In this case space charge limited conduction (SCLC) occurs. The drain current–drain voltage characteristics (I d - V d ) were raised in dark. 2. Sample preparation and experimental methods The SiO 2 layer of about 150 nm was grown on single-crystalline n-type Si wafers, by oxidation at 850 ◦ C in an atmosphere containing oxygen and water vapours. CdS thin films were deposited by CBD on SiO 2 /Si (n-type) sub- strates. The raw materials for CdS deposition were: cadmium acetate, thioureea and ammonia hydroxide. Details about the deposition procedure and chemistry are given elsewhere [3,4]. The deposition was performed at 60 ◦ C for about 1 h. The as-deposited films were then annealed at 300 ◦ C for about 1 h in air. Approximately, 70 nm thick nano-crystalline CdS layers were obtained, as estimated from step-profile measurements. The CdS film was processed by pho- tolithograpy, leaving an area of 2.5 mm × 3.5 mm. Gold electrodes were vacuum evaporated on the CdS surface, the final active area being of 1.5 mm × 1.5 mm. These two elec- trodes will be further called source and drain. On the back- side (Si substrate) a continuous Al electrode was deposited, playing the role of gate contact. The schematic of the final structure is presented in Fig. 1, together with the set-up used 0921-5107/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2003.10.077