Porosity Study in ZnO Films for Carbon Monoxide Sensing

Abstract

The development of gas sensors to monitor the toxic and combustible gases is imperative due to the concerns for environmental pollution and the safety requirements for the industry. In general, sensors provide an interface between the electronic equipment and the physical world typically by converting nonelectrical physical or chemical quantities into electrical signals [1]. The fundamental sensing principle relies on the change of conductivity of the sensors when they are exposed to certain target gases at moderate temperatures. One of the demands on the gas sensors is the low power consumption because the sensors work from day-to-day [2]. A low resistance material provides the lower driving power when it is used as a sensor. Among various metal oxide semiconductors (SMOs), typical n-type Zinc oxide (ZnO) with merit features such as wide direct band gap of 3.37 eV, good morphology properties, high electron mobility, low production cost, excellent chemical properties and appropriate response toward gaseous species such as carbon monoxide gas (CO), is widely used in the resistance-type CO gas sensors[3-4]. This work reports the response to carbon monoxide by using ZnO films with controlled pore size. Zn films were deposited on silicon substrates by DC sputtering using Zn targets of 2 in diameter with a purity of 99.99%. The (0 0 1) P type silicon substrates have a size of 1.5 x 1.5 cm2. The substrates were cleaned using the conventional cleaning procedure based on xylene, acetone and propanol to eliminate probably residues of adhesives. A 30-Watt power and a pressure of 5.5 mTorr were used for the deposition. Under these processing conditions for a sputtering period of 20 min, Zn films of 50 nm in thickness were grown as measured by profilometry. The samples were placed in a quartz furnace with a chromatographic N2 flow of 20 (cm3x l), at a temperature of 350 ° C for 30 minutes. Subsequently, with the purpose of obtaining porosity in the precursor films and improve the crystalline quality of ZnO, after the first treatment they were annealed at 800 ° C for 1 hour in three different atmospheres, the first was N2 with a flow of 20 (cm3x l) (S1 ) , the second was air with a relative humidity of 42% (S2 ), finally the third atmosphere was dry air (nitrogen: oxygen = 70: 30) (S3 ), Aluminum interdigitated contacts were defined on the surface of the films by the lift-off photolithographic method. To evaluate the response of the device, it was placed inside a non-evacuated chamber of known volume and exposed to several different amounts of carbon monoxide. The concentrations used were 50, 100, 250, at different temperatures. The current produced through the sensor by a constant voltage was recorded as a function of the time while the atmosphere was periodically exchanged. The surface morphology of the samples changed according to the used oxidation atmosphere, featuring distinct uniform porous surfaces with nanosized diameters. The samples showed a uniform distribution of particles with a porosity generated after the complete sample oxidation with different size of porous (20nm-100nm), depending on the specific oxidizing atmosphere. The XRD patterns for the samples processed in dry N2 show preferential orientation in the plane (002) of the ZnO crystallites. Dry and wet air produced the wurtzite phase of ZnO according to the diffraction of the (100), (002), (101) planes as seen in Fig. 1. The sensing measurements were made for carbon monoxide. The results from the three samples are observed in Fig. 2, in which we can see that sample 3 shows a larger response. This can be attributed to the average pore size which turned to be 30 nm.