Porosity Study in ZnO Films for Carbon Monoxide Sensing

Abstract

The development of gas sensors to monitor toxic and combustible gases is imperative due to concerns for environmental pollution and safety requirements for the industry. In general, sensors provide an interface between the electronic equipment and the physical world by typically converting nonelectrical physical or chemical stimuli into electrical signals [1]. One fundamental sensing principle relies on the change of conductivity of the sensors when they are exposed to target gases at specific temperatures. One of the demands on the gas sensors is the low power consumption since the sensors work from day-to-day [2]. A low resistance material provides a lower driving power when used as a sensor. Among various metal oxide semiconductors, typical n-type Zinc oxide (ZnO) which exhibits 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 of ZnO films with controlled pore size towards carbon monoxide. Films with controlled pore size were deposited on silicon substrates by DC sputtering by using Zn targets of 2 in diameter with a purity of 99.99%. (0 0 1) P-type silicon substrates with a size of 1.5 x 1.5 cm2 were used. The substrates were cleaned using RCA conventional cleaning procedure including sulfuric acid, ammonium hydroxide and hydrochloric acid. 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 cm3/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 temperature treatment they were annealed at 800 ° C for 1 hour in three different atmospheres, the first was a 20 cm3/l dry nitrogen flow (labeled S1 ), the second was a air flow with a relative humidity of 42% (S2 ), finally the third atmosphere was a 20 cm3/min artificial dry air (nitrogen: oxygen = 70: 30) flow (S3 ). The surface morphology of the samples changed according to the oxidation atmosphere used, but in all cases, uniform porous surfaces with nanosized diameters were produced. The samples oxidized under the three atmospheres showed a uniform distribution of particles with a porosity generated after the complete sample oxidation with different pore sizes (20nm-100nm). The XRD patterns for the samples processed in dry N2 evidenced wurtzite ZnO crystallites with preferential orientation in the (002) planes. Under both, dry and wet air, the films showed the presence of the wurzite phase of ZnO according to the lines (100), (002), (101). Aluminum interdigitated contacts were defined on the surface of the films by the lift-off photolithographic method. The sensor was placed inside a non-evacuated chamber of known volume (~ 70 cm3) varying the amount of carbon monoxide. The concentrations used were 50, 100, 250 ppm, at temperature of 260°C. The current produced through the sensor by a constant voltage was recorded as a function of the time while the target gas in the atmosphere was periodically exchanged. The sensing measurements were made under carbon monoxide for the three samples in which we can see that sample S3 shows a larger response to the target gas, this can be attributed to the average pore size which turned to be 30 nm.