Influences of substrate type on the pH sensitivity of CuS thin films EGFET prepared by spray pyrolysis deposition

Copper sulphide (CuS) thin films were deposited by spray pyrolysis using CuCl2·2H2O and Na2S2O3·5H2O precursors as sources of Cu2+ and S2- respectively, with deionized water as a solvent. Three types of substrates were used for films deposition; glass, tungsten and Si. X-ray measurements of the structural characteristics of the deposited films indicated single phase CuS covellite phase with orientations in the z-axis (001). The presence of oxygen suggested improved pH sensing characteristics. Morphological characterization showed differences in shape and size of CuS crystal structure, which affect the surface-to-volume ratio and surface site density of the substrates. The as-deposited films were used as extended gates of field effect transistor to measure the pH sensitivity for buffer solutions in the set of 2-to-12 step 2; the optimum pH sensitivity was 23.3 mV/pH for CuS/glass membrane. The hysteresis was measured for buffer solutions prepared at pH7-pH4-pH7-pH10-pH7 in order to find its pH response delay; the minimum value was 0.48 mV for glass substrate. Also, the repeatability, stability and reliability of the (CuS) thin films were measured in terms of coefficient of variation (C.V.) with the same setup used for measuring pH sensitivity. Repeatability values of 0.04%, 0.02% and 0.38% were recorded for glass, tungsten and Si, respectively, while stability and reliability values of 0.15%, 0.15% and 0.10% were recorded at pH4, pH7 and pH10, respectively, for glass substrate.