Field Effect Controlled Photoresistors Based on Chemically Deposited PbS Films
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Field Effect Controlled Photoresistors Based on Chemically Deposited PbS Films Eugenia Pentia, Lucian Pintilie, Ion Matei, Ioana Pintilie, National Institute of Materials Physics, Atomistilor 105 bis, P.O. Box MG-7, 76900 Bucharest-Magurele, Romania Fax: +401-4930267; E-mail: [email protected] ABSTRACT MOS-like structures were obtained by chemical deposition of a polycrystalline PbS thin film on top of a silicon dioxide/Si substrate. Gold ohmic electrodes in coplanar configuration were subsequently deposited by vacuum evaporation on PbS surface (drain and source electrodes). The gate aluminum electrode was deposited on the back of the Si substrate. The dependence of the photoconductive signal, generated in the PbS film, on the gate voltage was studied for wavelengths ranging between 800 nm and 3000 nm at room temperature as well as at low temperatures. It was found that the relative variation of the signal could be as high as 50 % for gate voltages ranging between -30 V and +30 V. Two possible mechanisms are proposed to explain the signal variation with the gate voltage: 1) Variation of the depleted region’s thickness in the PbS film, that leads to a variation of the conduction channel’s resistance (the reference resistance called, also, the dark resistance), 2) The possible variation of the majority carriers (holes) life-time due to the electron blocking at the PbS/oxide interface when positive gate voltages are applied on the back electrode. Integrated IR detectors with controlled sensitivity in the 800-3000 nm range can be manufactured at a relatively low cost using the PbS/oxide/Si MOS-like structure. INTRODUCTION Lead sulfide (PbS) is a A4B6 semiconductor compound with very good photoconductive properties in the near infrared (NIR) spectrum. The sensitivity domain at room temperature is 600-3000 nm, with a maxima at around 2200-2400 nm [1,2]. The standard method used to obtain PbS thin films with good IR detection properties is the Chemical Bath Deposition (CBD) [3-5]. Other technique that started to be used in the last years for producing PbS based optoelectronic devices (laser diodes) is the hot wall epitaxy [6]. PbS films are mainly used to produce high sensitivity photoconductive cells for NIR. The spectral sensitivity could be as high as 1011 cmW-1Hz1/2 at maximum wavelength [7]. Even PbS was intensively studied in the past, being considered as the tamplate of polycristalline semiconductor compounds, further studies are under way on this material in the last years. The main target of these are: 1) to clarify the connection between bath composition, film morphology and physical properties of the film [3,8-10]; 2) to integrate PbS IR detectors with standard semiconductor technology [11,12]; 3) to developed new types of heterojunctions and heterostructures with enhanced detection properties by combining PbS with other materials (semiconductors or/and dielectrics) [11-13]. The last direction of research is particulary interesting because offers the possibility to manufacture different type of devices, in
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