J. Phys. Chem. C 2009, 113, 18331–18340
18331
Preparation and Characterization of Octadecanethiol Self-Assembled Monolayers on Indium Arsenide (100) Wout Knoben,* Sywert H. Brongersma, and Mercedes Crego-Calama Holst Centre/IMEC, High Tech Campus 31, 5656 AE EindhoVen, The Netherlands ReceiVed: July 22, 2009; ReVised Manuscript ReceiVed: August 17, 2009
Self-assembled monolayers (SAMs) of octadecanethiol (ODT) on InAs(100) were prepared and characterized in detail. The native oxide of InAs was removed by a wet chemical etching method which allows formation of ODT SAMs on bare InAs(100) without any exposure to the ambient atmosphere. Contact angle measurement, ellipsometry, Fourier transform infrared spectroscopy, and atomic force microscopy show that the resulting SAMs are smooth, continuous, and well ordered. X-ray photoelectron spectroscopy confirms bonding of ODT to the substrate as thiolate. When InAs-ODT is exposed to the ambient atmosphere, some reoxidation of the substrate takes place. Thermal stability studies demonstrate that ODT SAMs are completely stable up to 140 °C. Decomposition of ODT occurs above 250 °C. Introduction As the dimensions of electronic devices continue to shrink, they start to approach the molecular length scale. In view of this development, it is not surprising that a lot of attention is devoted to combining the inorganic materials that traditionally make up these devices with organic molecules. A good example of this is the assembly of monolayers of organic molecules on inorganic substrates.1,2 Well known examples are monolayers of alkenes on Si,3,4 alkoxy- and chlorosilanes on SiO2,3,5 and thiols on Au and other metals.6 Such self-assembled monolayers (SAMs) can be used to tune the physical, chemical, and electronic properties of the surface on which they are assembled, such as friction, wetting, and surface potential. Control of surface properties is especially important for small structures, as they have a higher ratio of surface area to volume than their bigger counterparts. Thiols on metals are perhaps the most widely studied type of SAM. They have been immensely popular ever since they were first reported in 1983,7 as they can be prepared under mild conditions (from dilute solution at room temperature) and form well organized monolayers by formation of stable thiolate-metal bonds with the substrate.1,2,6 Moreover, a wide variety of thiols is commercially available which has allowed a vast number of molecules to be attached, either directly or by coupling to a terminal reactive group of a thiol. A second development associated with the continuing miniaturization of electronic devices is the increased application of semiconductors other than Si. III-V semiconductors, which have much higher electron mobilities, are prime candidates to replace Si as the main functional material in future devices.8 III-V materials are already used in certain specific applications such as optoelectronic devices and integrated circuits operating at microwave frequencies. More recently, the interest in III-V semiconductors has extended to application in chemical sensors.9 Assembly of SAMs on III-V substrates can be achieved by the same types of molecules that are used for traditional Si and metal substrates. For attachment of molecules to oxide covered III-V substrates, SAMs of silanes10,11 and phosphonic12,13 and carboxylic14,15 acids are reported. Alkenes,16,17 aryl diazonium * To whom correspondence should be addressed. E-mail: wout.knoben@ imec-nl.nl.
salts,18 and thiols19-21 have been used to prepare SAMs on bare (oxide-free) III-V substrates. Despite the massive amount of literature on thiol SAMs on metal substrates, the number of papers about these molecules on III-V substrates is relatively small. The first reports of alkanethiol SAMs on GaAs(100) appeared in the early 1990s.22,23 After that, the vast majority of papers have also dealt with GaAs (for a comprehensive overview, see refs 19-21 and papers cited therein). Although the literature on other III-V substrates is scarce, there are some examples of thiol SAMs on InP,24-29 GaP,30 and InAs.31-34 The high electron mobility of InAs has led to a lot of recent interest in applications in nanowire based field effect transistors.32,35-37 Furthermore, InAs has a small direct band gap of 0.354 eV, which corresponds to a wavelength of approximately 3.5 µm or a wavenumber of 2850 cm-1. This makes InAs an interesting material for application in optoelectronic devices in the infrared range.38 The accumulation of conductive electrons at InAs surfaces, creating a natural two-dimensonal electron gas, also holds promise for application in chemical sensors.15,39-41 Formation of SAMs on the bare InAs surface could be a useful method to tune device properties and improve performance. Functionalization of bare InAs with thiols has some advantages over functionalization of the oxide-covered substrate. For example, the thiolate-substrate bond should be stronger than that of carboxylic acids adsorbed by hydrogen or coordination bonding that are being applied in chemical sensors.15,40 Furthermore, there is no insulating oxide layer between the functional molecules and the substrate, which could enhance device performance. Assembly of ODT SAMs on InAs has also been reported to reduce the number of surface states and band bending, which greatly enhances the performance of InAs nanowire based transistors.31,32 Despite this promising outlook, a thorough characterization of thiol SAMs on InAs has not been published. In the few previous reports where thiol SAMs on InAs are used,31-34 no full characterization was performed. In this paper, we present the detailed and extensive investigation of SAMs of octadecanethiol (ODT) on InAs(100). A wide range of techniques was used for characterizing InAs-ODT: contact angle measurement, spectroscopic ellipsometry, atomic force microscopy (tapping mode and Kelvin probe force microscopy), angle resolved X-ray photoelectron spectroscopy
10.1021/jp9069543 CCC: $40.75 2009 American Chemical Society Published on Web 09/18/2009
18332
J. Phys. Chem. C, Vol. 113, No. 42, 2009
(XPS), and Fourier transform infrared spectroscopy (FTIR). More importantly, the stability of the monolayers upon exposure to ambient atmosphere and heating, which is critical for successful application in devices, is also described here for the first time. XPS and thermal desorption gas chromatography/ mass spectrometry (GC/MS) were employed for this purpose. Experimental Section SAM Preparation. InAs wafers with a (100) crystal orientation were purchased from MTI Corporation (Richmond, CA). A dry solution of HCl in isopropanol (5-6 N) was purchased from Acros. Octadecanethiol (ODT) was ordered from SigmaAldrich. Solvents were deaerated by purging with nitrogen and dried using molsieves (type 3A). Removal of the native oxide and subsequent SAM formation was done in a nitrogen flushed glovebox (H2O
