Photonic Materials Technology Section (PMTS)

Nano-Electronic Materials and Devices Laboratory

Transparent Conducting Electrodes (TCEs) are thin layers of materials that combine optical transparency with electrical conductivity. They are integral components in a variety of optoelectronic devices, such as touch screens, solar cells, light-emitting diodes (LEDs), and liquid crystal displays (LCDs). Common materials for TCEs include indium tin oxide (ITO), graphene, silver nanowires, and conductive polymers like PEDOT:PSS. We have developed ZnO based low-cost and environment friendly TCOs by doping ZnO with n-types dopants such as Al, Si and Ga, which showed performance comparable to commercially available ITO electrodes.

We have grown Si doped ZnO thin films with Si concentration in the range from 0.5 to 5.8% using sequential pulsed laser deposition. The lowest resistivity obtained was ~ 4.6×10^-4 Ω-cm at a Si concentration of ~ 1.7%. Although the electron concentration in the films was higher than the Mott critical density, the temperature dependent resistivity measurements in the range from 300 to 4.2 K revealed negative temperature coefficient of resistivity (TCR) for the 0.5, 3.8 and 5.8% Si doped ZnO films in the entire measurement temperature range. The 1.0, 1.7 and 2.0% Si doped films showed a transition from negative to positive TCR with increasing temperature. The negative TCR was found to be primarily due to disorder induced weak localization of the electrons.

Figure 1: Temperature dependent resistivity (filled circles) of the Si doped ZnO films with Si doping concentrations of (a) 0.5, (b) 1.0, (c) 1.7, (d) 2.0, (e) 3.8, and (f) 5.8 %. The continuous curves show the corresponding fittings of the data according to disorder induced electron interference effects.

A clear signature of disorder induced quantum-interference phenomena leading to phase-coherent electron transport was observed in (Zn, Al)O_x thin films grown by atomic layer deposition. The degree of static-disorder was tuned by varying the Al concentration through periodic incorporation of Al₂O₃ sub-monolayer in ZnO during atomic layer deposition. All the films showed small negative magnetoresistance (MR) in the entire range of the measurement temperature (5-200K) and magnetic field (0-8T) due to magnetic field suppressed weak-localization effect as shown in a particular case of (Zn, Al)O_x with Al ~ 3.8 at.% in Figure 2 (a). The temperature dependence of phase-coherence length (lφ αT^-3/4 ) extracted from the magnetoresistance measurements as shown in Figure 2 (b), indicated electron-electron scattering as the dominant dephasing mechanism. The persistence of quantum interference at relatively higher temperatures up to 200 K is promising for the realization of phase-coherent electron transport devices.

Figure 2 (a) Magnetic field dependent (0-8 T) MR of all the (Zn, Al)Ox thin films with 3.8 at.% Al at different temperatures in the range of 5-200K. The solid lines show the fitting of the experimental data (b) Variation of phase coherence length with temperature for all the (Zn, Al)Ox thin films with Al ~ 1.4, 1.9, 3.2, and 4.7 at.%