Thin films and nanomaterials are suitable for gas sensors because the sensing properties are related to the material surface where the gases are adsorbed and surface reactions occur. Surface reactions change the concentration of charge carriers in the material, creating a depletion layer and surface dipole at the interface, which results in a change in electrical resistance [2�C4,10,18,21�C23]. The high sensitivity of ZnO thin film gas elements has been attributed to reactions at grain boundaries and the metal/ZnO interface, where the depletion of carriers modifies the material transport properties [2�C4,18,21].Although ZnO itself has an active gas response, its gas-sensing performance can be enhanced by the addition of a palladium (Pd) catalyst, which can be incorporated either via doping or embedding particles within the film [4,7,24�C28].

Furthermore, Pd thin films have been studied as a Schottky barrier contact for ZnO-based gas sensors [12,29,30]. In the presence of the gas, a Schottky barrier is formed at the inter-grain boundaries of the film and Pd/ZnO interface, which dominates the conductivity of the film. Depending upon the type of gas and temperature, the Schottky barrier height changes, resulting in an increase or decrease in the conductivity. The sensing properties are found to depend on the temperature, grain size, catalyst and porosity [12]. Although there are numerous reports on the sensing properties of Schottky Pd/ZnO, there are few reports on the effect of palladium doping and embedding of Pd microparticles for this contact scheme [12,29,30].

In the present study we report on the fabrication and characterization of Pd/ZnO interdigitated MSM LPG sensors having three different arrangements. Specifically, devices with Pd Schottky contacts were fabricated with (1) un-doped ZnO active layers; (2) Pd-doped ZnO active layers; and (3) un-doped ZnO layers on top of Pd microstructure arrays. The electrical characteristics and gas response of the devices were studied and compared to explore the potential applications of these configurations as room temperature LPG sensors.2.?ExperimentFigure 1 shows a schematic diagram of the MSM photodetector geometry for the three device structures under study: (a) un-doped ZnO active layers; (b) Pd-doped ZnO; and (c) un-doped ZnO active layers on Pd microstructure arrays.

We deposited ZnO thin films using the sol-gel technique, and we fabricated MSM device contacts and microstructure arrays by thermal evaporation of Pd using a shadow mask technique.Figure 1.Schematic diagram Brefeldin_A of the metal-semiconductor-metal (MSM) gas sensor geometries for devices based on (a) un-doped zinc oxide (ZnO) films; (b) Pd-doped ZnO films; and (c) ZnO films deposited on Pd microstructures; (d) SEM image of a metal microstructure …The substrates used for deposition were p-type Si(111) (~380 ��m thick) with a resistivity of 2�C7 ��cm.