Applied Surface Science 256 (2009) 407–413 Contents lists available at ScienceDirect Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc Self-assembled and electrochemically deposited mono/multilayers for molecular electronics applications S.K Gupta *, S.P Koiry, A.K Chauhan, N Padma, D.K Aswal, J.V Yakhmi Technical Physics and Prototype Engineering Division, Bhabha Atomic Research Centre, Mumbai 400085, India A R T I C L E I N F O A B S T R A C T Article history: Available online 11 June 2009 For the development of molecular electronics, it is desirable to investigate characteristics of organic molecules with electronic device functionalities In near future, such molecular devices could be integrated with silicon to prepare hybrid nanoelectronic devices In this paper, we review work done in our laboratory on study of characteristics of some functional molecules For these studies molecular mono and multilayers have been deposited on silicon surface by self-assembly and electrochemical deposition techniques Both commercially available and specially designed and synthesized molecules have been utilized for these investigations We demonstrate dielectric layers, memory, switching, rectifier and negative differential resistance devices based on molecular mono and multilayers ß 2009 Elsevier B.V All rights reserved Keywords: Molecular electronics Electrochemical grafting Self-assembly Functional molecules Introduction In recent years, there has been significant progress in data processing and information technology This has been possible by scaling down typical size of electronic devices, mainly transistor that enables to increase the number of devices per chip and consequently increases computational capacity and speed However, there are several factors that limit the continuing miniaturization of silicon-based devices to about 100 nm range These include technological limitations of device patterning, limitations to size of metal interconnects and insulator thickness and p–n junction depletion region width, etc Further, reduction in size also leads to increased statistical errors in device characteristics as well as reduced switching energy where quantum and thermal effects may increase errors Need to develop nanoscale electronic devices has resulted in the concept of molecular electronics [1] where single or a small group of organic molecules may serve as functional building blocks of electronic circuits providing electronic functions such as rectification, switching, memory and transistor action, etc Important problems in nanoscale molecular electronics that need to be overcome before these devices may be put to applications include development of methods to place the molecules at desired positions and connecting them to other molecules [2] It is clear that the development of molecular electronics requires, design and synthesis of molecules with electronic functionality, study of the electrical properties of such molecules and techniques for self-assembly of circuits * Corresponding author Tel.: +91 22 25593863; fax: +91 22 25505151 E-mail address: drgupta@barc.gov.in (S.K Gupta) 0169-4332/$ – see front matter ß 2009 Elsevier B.V All rights reserved doi:10.1016/j.apsusc.2009.06.014 In view of significant challenges to be overcome before fully functional electronic circuits can be fabricated, an interim possibility is to integrate molecular devices with silicon electronics to yield hybrid devices where part of the circuit is based on molecules [3] In this regard, the study of self-assembly techniques for deposition of organic molecules on silicon and measurement of characteristics of mono or multilayers of these molecules are important in that the studies will demonstrate molecules with electronic functionality and techniques of self-assembly that are useful for hybrid electronics as well as future placing of molecules at desired locations and interconnects In this paper, results of some studies carried out in our laboratory towards (a) synthesis of molecules, (b) self-assembly and electrochemical deposition techniques for preparation of mono and multilayers of these molecules on silicon and (c) study of their electrical properties have been presented Depending upon the nature of interaction between organic molecules and substrate (Si in present studies), there are two types of methods for deposition of mono and multilayers Thermal evaporation or Langmuir–Blodgett (LB) deposition is based on physisorption of molecules on substrate [4] Physisorbed films prepared in this manner have low mechanical and chemical stability On the other hand, self-assembly and electrochemical deposition techniques result in chemisorption due to covalent bond between substrate and molecules [5] These bonds are strong and are better suited for further processing of the prepared devices Therefore, self-assembly and electrochemical deposition techniques have been employed for the present studies Various chemical processes and type of molecules can be used for the preparation of self-assembled monolayers (SAMs) on silicon substrates with native silicon oxide or bare silicon surface [6] In 408 S.K Gupta et al / Applied Surface Science 256 (2009) 407–413 the method used by us, high density of silanol (Si–OH) groups are prepared by Piranha cleaning process with some variations on native silicon oxide or thin thermally grown oxide layer Organic molecules that end in silane group (–SiX3, X5 5Cl, OCH3 or OC2H5) can be self-assembled on such surfaces by dipping substrates in $10À3 M concentration of molecules in solvents such as dicyclohexyl or n-hexane Direct deposition of SAMs on silicon can be carried out by preparation of hydrogen terminated substrates followed by hydrosilylation reaction with organic molecules having unsaturated carbon bond (C5 5C) as end group These organic molecules can also be deposited on hydrogenated silicon by electrochemical techniques [7] We have used some of these techniques for preparation of mono/multilayers Various molecules that have been reported to have electronic functionality include alkanes as insulating material [8], redox active molecules for memory devices [9,10,11], s–p and donor bridge acceptor (D-s-A) structures for molecular diode [6,12], and s–p–s structure for negative differential resistance (NDR) [6,13] Some of these devices have been fabricated and characterized by us will be described Devices developed Preparation techniques and characteristics of mono and multilayers of several molecules have been studied Various devices such as (a) dielectric layers, (b) memory and switching devices, (c) rectifier and (d) negative differential resistance devices will be described 2.1 Dielectric material Dielectic monolayers of different molecules were prepared by self-assembly and electrochemical deposition techniques and results obtained by both methods are discussed 2.1.1 Self-assembly Three different alkyltrichlorosilane molecules have been used to prepare and study their dielectric properties [14] SAMs of octyltrichlorosilane (C8), dodecyltrichlorosilane (C12) and octadecyl trichlorosilane (C18) have been deposited on the native oxide of heavily doped n type (n++) silicon (resistivity: $1 mV cm) Si substrates were cleaned in piranha solution (2:1 solution of H2SO4 and H2O2) to obtain OH terminated surface and were immersed in a mM concentration of desired alkyltrichlorsosilane solution in toluene, for a duration of 3–48 h for different molecules For electrical measurements, a mercury drop was used as top electrode The thicknesses of different films were measured by ellipsometry and were found to be 1.5, 1.9, 2.3 and 3.0 nm for C8, C12, C18 and native Si oxide, respectively These are in good agreement with expected values for alkyl chains [15,16] Water contact angle for freshly cleaned Si was found to be