Humidity Effects on the Electrical Properties of Epitaxial Rutile Thin Films
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effects of humidity on the electrical properties of rutile. The crystallographic and microstructural
quality of the films decreases with increasing Ga and Nb concentrations. Heteroepitaxy is, however, maintained with Ga or Nb concentrations up to 4.5 at% for the (001) orientation and to
0.5 at% Ga for the (100) orientation. The electrical properties of the (001) oriented rutile films have been characterized from room temperature to 225 'C in dry and humid, N2 and air atmospheres. At constant temperature in dry atmospheres, the conductance of the Nb-doped rutile films is greater than that of the undoped, which is greater than the conductance of the Ga-doped films. The activation energies for conduction in the Nb-doped and undoped rutile films in dry atmospheres are similar (-0.1 eV), whereas the activation energy in Ga-doped films is much greater (-0.8 eV). The effects of humidity in reducing the resistance of rutile is greatest in the Gadoped and very thin (-150 A) undoped films. Humidity is observed to have similar effects on both the (001) and (100) oriented 0.5 at% Ga-doped films. INTRODUCTION Rutile phase TiO2 thin films have a variety of practical applications. In addition to their use as waveguides, photocatalysts and components in electrical devices, they are also of interest as chemical sensors. Typically, chemical sensors are porous polycrystalline ceramics to maximize the surface area which can interact with the gas phase [1-3]. Additives are used to change the microstructural characteristics and increase the sensitivity of the material to the gas being detected
[4-7]. In many sensor structures, however, the exact nature of these changes is not well known.
We are using oriented rutile thin films doped with acceptor (Ga) and donor (Nb) ions to examine the effects of surface orientation, space charge potential and surface morphology on the chemical sensing characteristics of rutile. This paper reports the results of our study on the effects of humidity on the electrical properties of undoped and doped (Ga, Nb), (001) and (100) oriented rutile thin films grown by the MOCVD technique on sapphire substrates. EXPERIMENTAL The films were grown by MOCVD in a high vacuum, inverted pedestal hot-walled reactor. The growth temperature was 650*C and the growth rate was nominally 40 A/min resulting in a typical film thickness of 3500 A. Solid precursors (2,2,6,6-tetramethyl-3,5-heptanedionato) were used as sources of Ti, Ga, and Nb. Doping was accomplished by mixing dopant precursor material with the Ti precursor on a per Ti basis. For further details regarding growth, please see reference [8]. The crystallographic, microstructural and surface characteristics of the films were examined using X-ray diffraction and atomic force microscopy (AFM). The doping concentrations and distributions in the films were determined using Rutherford backscattering spectrometry (RBS) of 2 MeV 4 He+ ions and secondary ion mass spectrometry (SIMS). The electrical measurements were carried out in an environmental chamber in dry and humi
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