Materials Science and Engineering B 147 (2008) 284–288 Investigation of nanocrystalline Pbs/n-Si heterostructures for optoelectronic applications Ma. Buda a,∗ , V. Stancu a , G. Iordache a , L. Pintilie a , I. Pintilie a , Mi. Buda b , T. Botila a a National Institute of Materials Physics, Str. Atomistilor 105 bis Magurele, Ilfov, P.O. Box MG-7, cod 077125 Bucharest, Romania b Department of Applied Physical Chemistry and Electrochemistry, Faculty of Applied Chemistry and Materials Science, “POLITEHNICA” University of Bucharest, Calea Grivitei 132, 010737 Bucharest, Romania Received 24 May 2007; received in revised form 30 July 2007; accepted 13 September 2007 Abstract The nano-crystalline PbS/n-Si heterostructure was studied using photocurrent spectra, I–V and C–V characteristics and admittance spectroscopy. The frequency dependent junction capacitance and conductance measurements show the presence of two contributions: first, a defect related mechanism which we attribute to a deep trap level with a large cross-section in nano-crystals with smaller sizes around 5 nm at the interface with Si and a second mechanism with an activation energy of about 250 meV, attributed to carrier transport in relatively large PbS grains of about 15 nm in size. © 2007 Elsevier B.V. All rights reserved. Keywords: Chalcogenides; Heterostructures; p–n junctions; Semiconductors 1. Introduction The PbS polycrystalline semiconductor material deposited by chemical methods has important applications at room tem- perature (RT) as infrared (IR) detector in the 1000–3000 nm range [1,2]. This chalcogenide material is a good p-type pho- toconductor with a bandgap of 0.42 eV (2950 nm) at RT. Its heterojunction with Si is of great interest in case of integration with traditional Si-based devices. There are very few reports regarding the behavior of such heterojunctions [3,4]. The chemical bath deposition (CBD) method was success- fully used for depositing macrocrystalline PbS with very good photoconductive properties. CBD can be extended to obtain nano-crystalline PbS by varying the deposition parameters such as reaction time, temperature, pH and impurities content in the deposition bath [5]. Most of the reported nano-crystalline PbS are related to PbS nanoparticles imbedded in a glass-ceramic, glass or organic matrix [6–8]. The PbS material studied here was obtained by direct deposition of the nano-crystalline PbS ∗ Corresponding author. E-mail address: mbuda@infim.ro (Ma. Buda). on n-type Si, forming a p–n heterojunction. Quantum effects in nano-sized particles offer an additional degree of freedom given by the variation of the material bandgap as a function of nanocrystals size. 2. Experimental details The deposition bath consists of a soluble lead salt (PbNO 3 ) 2 , thiourea SC(NH 2 ) 2 , NaOH, and a reducing agent (hydroxy- lamine hydrochloride, NH 2 OH·HCl) [5]. The reduction of the reaction time to 17 ′′ and the removal of the doping element (Bi) from the deposition bath lead to the formation of crystallites with smaller sizes than for the standard macroscopic material. The PbS layer was deposited at 24 ◦ C directly on n-type N/N + Si substrate, with a N + substrate resistivity of 5 m cm and a 7 m thick N epitaxial layer of 6  cm (10 15 cm -3 ) resistivity. Refer- ence samples of nano-crystalline PbS deposited on glass were also made. Au ohmic contacts with an active area of 1.5 mm 2 were deposited on the nano-crystalline PbS surface by thermal evaporation. The photocurrent was measured using a standard lock-in set- up, including a mechanical chopper and a bulb light source. A Ge filter was used before the entrance slit of the monochroma- 0921-5107/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2007.09.070