Arnaud Verger

Electron-electron interactions based metal-insulator transition in Ga doped ZnO thin films

We report on the charge carrier transport mechanisms of undoped and Ga doped (2 wt. % and 4 wt. %) ZnO thin films grown by pulsed dc magnetron sputtering technique. Temperature dependent resistivity measurements showed typical semiconducting behaviour for undoped ZnO thin films where as Ga doped ZnO thin films showed metallic nature at higher temperatures and insulating nature with a metal to insulator transition at lower temperatures. The observed transition temperatures are 91 K and 140 K for 2 wt. % and 4 wt. % Ga doped ZnO films respectively. The observed metal insulator transition is attributed to the electron-electron interactions at low temperatures. The variations in the transition temperatures are explained based on the disorderness induced in the system due to the doping effect. ZnO doped with 4 wt. % Ga showed the lowest resistivity of 5.7 × 10−4 Ω cm with a carrier concentration of 1.2 × 1021/cm3. Undoped and doped ZnO thin films are about 90% transparent in the visible region. Blue shift is observed in the absorption edge with the effect of doping and it is explained based on B-M shift. The Fermi level measured from valance band spectroscopy showed a shift of +0.6 eV for 2 wt. % Ga doped ZnO thin film and +0.7 eV for 4 wt. % Ga doped ZnO thin film compared to the Fermi edge of undoped ZnO thin films. This ascertains the movement of Fermi level in to the conduction band with the effect of doping.

Stability Studies on Nitrogen Doped p-ZnO (NZO) Thin Films Grown by Reactive Magnetron Sputtering

Nitrogen doped ZnO (NZO) thin films, at different N2 flow rates have been deposited on glass substrates by pulsed DC reactive magnetron sputtering technique. The effect of N2 flow rate (1.0 sccm - 3.0 sccm) on the structural, optical, electrical and chemical state of N has been studied. With the effect of N2 flow rate: the crystallinity of the films decreased, tensile stress is developed, optical transmittance decreased (80% to 60%), conductivity decreased till 1.5 sccm and films were n-type conducting. At 2.0 sccm and 2.5 sccm of N2 flow rates, NZO thin films showed p-type conductivity. The changes in the magnitude and type of conductivity have a direct relation with the changes observed in N-chemical state in ZnO lattice. p- NZO thin films are electrically unstable; this instability has been explained based on the changes occurred in the N chemical states, resulting from the stress release in NZO lattice.

Metal insulator transition in Ga doped ZnO thin films

Gallium (Ga) doped Zinc Oxide (ZnO) thin films are grown at room temperature (300 K) by reactive pulsed DC magnetron sputtering technique using 2 wt% and 4 wt% Ga doped ZnO targets. The temperature dependent resistivity studies have shown a semiconducting behavior with negative temperature coefficient of resistance for 2wt % Ga doped samples. A metal insulator transition (MIT) is observed for 4 wt % Ga doped targets, the possible reason for MIT may be attributed to the dopant disorder based localization for charge carriers at low temperatures.