Keywords: electric transport, electronic structure, catalytic oxides
Summary:The operation of chemiresistors is based upon reversible changes of the electrical resistance of metallic oxides that are caused by the chemical environment. These materials are exhibit a granular form and the fundamental mechanism lying below these resistance changes is believed to be the lowering (or the enlargement) of the potential barrier that develops near the grain boundaries where grains come in contact and the corresponding decrease (or increase) of the thickness of a layer depleted of carriers. This is a macroscopic model that cannot explain the complicated phenomena involved in transport and, therefore, to allow for the choice of optimal operation conditions (e.g., temperature, bias, frequency of measurement) of chemisenors based on thin films that are broadly used today in manufacturing. In this work we investigate the electrical surface transport in molybdenum and tin oxide films at temperatures varying between 25 and 400 oC in connection with their electronic structure. For this purpose a specially designed accessory was adapted to a Woolam ellipsometer allowing for measurements within the above temperatures and simultaneous current-voltage (I-V) measurements. Electrical measurements were made with two tungsten probes applied on Al contacts formed on the samples and a programmable KEITHLEY voltage source within the range -10 to 10 V with increment of 0.1 V and various delays. The temperature range 25 to 400 oC was chosen because chemisensors are usually working within it. Although our setup is designed for measurements in environments containing various non-corrosive gases, our so far study is limited in air only. Both films studied were deposited by vapor deposition and it was found that the electronic structure of both depends on factors such as the atomic ordering at short and long range (SRO and LRO, respectively) and the exact oxygen sub-stoichiometry. Of course, these factors are inter-related so, e.g., oxygen sub-stoichiometry affects SRO and LRO and, in turn, the latter influence film morphology. In any case, it can be said that the existence of defects causes the formation of individual electronic states within the band gap which, at high concentrations may form an intermediate band (IB). The formation of defects depends on the nature of the individual material and the temperature. The existence of states and IB influences significantly the electric transport, so in Mo oxide temperatures up to 200 oC the transport is ambipolar and depends on the delay (frequency) of measurement and at higher temperatures, the transport is governed by the metal-semiconductor contact (the I-V curve is not linear). SnO2 films exhibit ohmic behavior at temperatures up to 250 oC and above that they start to reduce and exhibit ambipolar electrical conduction, so the I-V curve again deviates from linearity. From the above it is clear that resistance measurements at a single biasing voltage, which is often the case, is not always sufficient to conclude about the composition of the gas phase. Similar measurements are foreseen made at environments containing non-corrosive gases to investigate the complex phenomena involved in the electric transport under such conditions.