|M.Sc Student||Kobrinsky Victorya|
|Subject||Nanostructured ZnO Films for Gas Sensing|
|Department||Department of Materials Science and Engineering||Supervisors||Professor Emeritus Yeshayahu Lifshitz|
|Professor Emeritus Yigal Komem|
|Full Thesis text|
The aim of this research was to study the correlation between the processing parameters of ZnO thin films and their gas sensing properties.
ZnO gas sensing devices were produced. Thin ZnO films (60-90nm) were deposited by RF magnetron sputtering. These films were sputtered with varying temperatures (Room Temperature -R.T. and 2000C) and sputtering gases compositions (O2:Ar- 0:11, 4:11 and 7:11).
HRSEM and TEM images showed a polycrystalline morphology with an average grain size of 30 nm. AFM images demonstrated an RMS roughness of ~2 nm. FIB confirmed a conformal coverage on the surface. A Cross-sectional HRSEM image revealed a columnar structure of the ZnO films, which grew in a wurtzite hexagonal structure confirmed by XRD. Chemical analysis done by XPS showed a Zn-O bonding. SIMS analysis showed the films are pure. All characterization methods did not exhibit structural differences between the samples. Hall effect measurements confirmed two types of samples- n- and p-type.
Electrical characterization of the ZnO sensors was measured as a function of temperature for three different ambients: Ar (a reference gas), dry synthetic air for O2 sensing and 5% H2 in Ar. The electrical behavior varied, depending on the sensed gas and ZnO thin film sputtering conditions, despite the similar microstructure. The samples deposited at 2000C and at R.T, O2:Ar- 0:11, exhibited increased resistance upon exposure to O2. This was attributed to O vacancies or Zn interstitials leading to n-type films. In samples deposited at R.T. with additional O2, the resistance decreased upon exposure to O2 . This can be attributed to an excess of O or Zn vacancies and a corresponding p-type nature.
The resistance upon exposure to 50ppm H2 increased in p-type films and decreased in n-type films. The resistance after Exposure to 5%H2 decreased in both types of films due to the strong increase of the electron concentration.
n-type samples demonstrated a higher sensitivity (defined as the ratio between resistance values in the sensed and reference gases) to O2 while p-type samples were more sensitive to H2.
This research has clearly shown that the sensing properties of ZnO gas sensors depend on sputtering conditions. The difference in the electrical response was found to originate from the carrier type within the thin films, as determined by the preparation conditions. These findings could assist in the development of ZnO gas sensor arrays with improved sensitivity.