Crystalline Grains and Electrical Properties of Vacuum-Evaporated SnO 2 Thin Films
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ABSTRACT We have studied grain growth and electrical properties of polycrystalline tin oxide (SnO 2) thin films prepared by vacuum-evaporation with a two-step process: evaporation of tin metal films and then oxidation of these metal films. Surface morphology of the Sn0 2 thin films was observed by atomic force microscopy. The grain size of the Sn0 2 thin films is found to increase with the film thickness and oxidation temperature. Kinetics of the grain growth is discussed in terms of a 3dimensional diffusion limited process. The diode current-voltage (I-V) characteristic of the Sn0 2/Si heterojunctions (isotype and anisotype) was measured in the temperature range of 14K-
383K. Changes in the diode ideality factor and threshold voltage with temperature are discussed. In addition, we present ambient tunnelling I-V results measured from individual Sn0 2 grains. INTRODUCTION In recent years, many studies had been focused on improvement of the properties in Sn0 2based gas sensors. It was reported that the gas sensitivity is strongly dependent on grain size of the tin oxide films and enhanced as grain size decreased [1-3].
This phenomenon has been
explained with both the two-dimenional neck model and the mixed state of grain boundary contacts and neck contacts [3,4]. However, little is known for the modification of the electrical properties under different preparation methods and the mechanism of the grain growth. Using the vacuum-evaporation method, we have reported the dependence of hillock growth and grain size on the preparation conditions [5]. With energy-dispersive spectroscopy, it was found that on the oxidized films metallic hillocks appeared for substrate temperatures below 150'C for films originally deposited as tin metal. The grain growth was identified to follow an Arrhenius behavior. In the present study, characterization including the grain size and electrical properties will be reported and discussed. 441 Mat. Res. Soc. Symp. Proc. Vol. 403 @1996 Materials Research Society
EXPERIMENT Tin oxide thin films used here were prepared by a two-step process. Tin metal films were first prepared by means of vacuum-evaporation on silicon wafers (n- and p-type with resistivity of 0.01-0.029ecm) at substrate temperature of above 150'C. Deposition rate was 1-7 nm/sec. The tin oxide thin films were then obtained by dry thermal oxidation in a furnace at oxidation temperatures ranging from 300'C to 550'C. Aluminium was subsequently deposited to be used for the electrical contacts. An unmonochromatizied Mg Kac X-ray source of a Kratos AXIS-HS XPS was used for present study. A Digital Instrument Nanoscope III atomic force microscopy (AFM) and scanning
tunnelling microscopy (STM) were employed to observe the film topography.
Surface images
were acquired in the constant force mode using standard AFM cantilevers.
For STM, an
electrochemically etched platinum-iridium tip was used for both constant current imaging and scanning tunelling spectroscopy. Both images were taken in ambient air. I-V characteristics of heteroju
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