Characterization and application of multilayer diffraction gratings as optochemical sensors N.L. Dmitruk a,* , O.I. Mayeva a , S.V. Mamykin a , O.B. Yastrubchak a , M. Klop¯eisch b a Institute for Physics of Semiconductors, National Academy of Sciences, 45 Prospect Nauki, 03028 Kyiv-28, Ukraine b Technical University, Ilmenau, Germany Received 19 June 2000; received in revised form 26 September 2000; accepted 26 September 2000 Abstract This paper is devoted to the surface characterization of phase diffraction gratings which are the key optical components of optochemical sensors. Surface plasmon resonance (SPR) in a Schottky junction with the diffraction grating at the interface is used as a basic principle underlying the device operation. With the aim of optimization of its technical parameters, the optical characterization of thin metal ®lms is necessary. The adequate characterization tools were demonstrated: variable angle spectroscopic ellipsometry (VASE); atomic force microscopy (AFM); re¯ectance/transmittance spectroscopy; spectral and angular characteristics of Schottky junction photoresponse. Along with investigation technique, the most physically correct theoretical model (Bruggeman-EMA) for describing the optical and structural features of multilayer systems was employed. Potentialities of the optochemical sensor have been demonstrated as well. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Diffraction grating; Surface plasmon resonance; Optochemical sensor; Ellipsometry; Photocurrent; Schottky diode 1. Introduction As it is known that the corrugated surface optical element (phase diffraction grating) is the key component for the advanced optoelectronic devices. Optochemical sensor, which is described in this paper, uses surface plasmon resonance (SPR) on a thin metal ®lm (with or without dielectric overlayer) evaporated on a periodically textured surface of semiconductor. This metal±semiconductor con- tact forms a Schottky junction. A resonant enhancement of the short-circuit photocurrent generated under SPR condi- tions in a Schottky junction is the basic principle underlying optochemical sensor operation [1]. By choosing the appro- priate metal ®lm and the optimal grating geometric para- meters, the signi®cant sensitivity increasing can be achieved. The adequate characterization tool for multilayer systems must be depended on the required application. Variable angle spectroscopic ellipsometry (VASE) was com- bined with others (e.g. AFM technique, re¯ectance/trans- mittance spectroscopy, spectral and angular characteristics of Schottky junction photoresponse, and model calculations) in order to enhance the diagnostic power. The aim of this paper is two-fold: (i) characterization of multilayer coatings of diffraction gratings for optimi- zation of devices parameters and (ii) demonstration of application of optochemical sensors based on such multi- layer structures. 2. Fabrication and characterization 2.1. Materials and technology Fabrication process starts with manufacturing of the phase diffraction grating on GaAs substrate by holographic tech- nique resulting pro®le of which is close to sinusoidal (Fig. 1a). Thermally evaporated metal ®lms were deposited onto substrates held at various temperature either through the mask to form Schottky junction or overall surface of sample for re¯ectance and VASE measurements. Various metal ®lms were used as plasmon-carrying systems.  Silver is the best for SPR sensing because the imaginary part of its dielectric constant is very small, resulting in a very sharp absorption peak. But this material undergoes corrosion, i.e. it is environmental unstable.  Gold was also chosen, because it is stable against oxida- tion or other chemical treatments. Sensors and Actuators A 88 (2001) 52±57 * Corresponding author. Tel.: 38-44-2656486; fax: 38-44-2658342. E-mail addresses: nicola@dep39.semicond.kiev.ua (N.L. Dmitruk), man- fred.klopfleisch@physic.tu-ilmenau.de (M. Klopfleisch). 0924-4247/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII:S0924-4247(00)00504-5