354 Sensors and Actuators A. Use of the electroreflectance technique in Pt/GaAs Schottky barrier sensor characterization 32 (1992) 354-356 L. M. Lechuga, G. Armelles, A. Calle, D. Golmayo and F. Briones Centro National de A4icroelectrdnica (CSIC), CISerrano, 144, 2XMX Madrid (Spain) zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPON Abstract Electroreflectance spectra of Pt/GaAs Schottky diodes are obtained and analysed as a function of the device exposure to a hydrogen-containing atmosphere at different temperatures. The observed changes in the spectra can be related to a reduction of the diode barrier height by about 0.2 eV as a consequence of saturation of the interface states. 1. Introduction Schottky barrier devices with catalytic metals have been .shown to be excellent Hz gas sensors [l-3], with high sensitivity and a similar be- haviour to MOS and MOSFET silicon H2 gas sensors [3,4]. In a Schottky diode, the barrier height decreases when the device is exposed to a hydrogen-containing atmosphere. The HZ-induced changes are detected as a modification of the capacitance-voltage (C- V) or the current- voltage (I- V) characteristics. In this work, we present an alternative method to study the interface modification by measure- ments of the electroreflectance (ER) spectrum near the fundamental gap. Electroreflectance is a modu- lation spectroscopy technique [5] in which the change in reflectance induced by an applied elec- tric field is measured through a semitransparent Schottky electrode as a function of photon energy. This technique reveals sharp derivative type of features at critical points of the semiconductor band structure and, therefore, weak structures that may not be resolved in absolute reflectance spectra are easily detected. The exposure of the Schottky structure to hydrogen causes a modification in the electroreflectance spectrum through the metal- semiconductor interface induced electric field. 2. Experimental The Pt/GaAs diodes were fabricated on an Si- doped epitaxial layer of GaAs grown on a 0924-4247/92/$5.00 GaAs (100) n-type substrate by molecular beam epitaxy. The doping concentration (Nd) of the epitaxial layer was 5 x 10” cmp3. The Schottk barrier is a thin transparent film of Pt (75 K thick) with a Pt dot 1000 8, thick to ensure an electric contact, electron gun evaporated on GaAs through a mask in a vacuum of 10e7 Torr at 150 “C. Previously a Au-Ge ohmic back contact was deposited by thermal evaporation. The devices were located inside a stainless steel cham- ber where the atmospheric composition was con- trolled by a gas-flow panel. The hydrogen gas was diluted with nitrogen and the concentration ranged between 30 and 1000 ppm. Before the introduction of HZ, the sample was kept in a nitrogen ambient. ER spectra were obtained at room temperature and at 150 “C. The high temperature prevents water adsorption over the platinum surface and also increases the speed of the device response [2]. ER measurements were obtained by means of a conventional ER experimental set-up. The d.c. bias was varied between 0 and 3 V and the a.c bias between 50 and 200 mV. Changes in the shape of the ER spectra due to the modifica- tion of the a.c. component amplitude were not observed. 3. Results and discussion Figure 1 shows ER spectra obtained on a device before and after the introduction of 1000 ppm of Hz in Nz, for zero voltage bias at 150 “C (a) and @ 1992 - Elsevier Sequoia. AI1 rights reserved