| AVS 57th International Symposium & Exhibition | |
| Energy Frontiers Topical Conference | Thursday Sessions |
| Session EN+SS+TF-ThA |
| Session: | Transparent Conductors |
| Presenter: | P. Banerjee, University of Maryland |
| Authors: | P. Banerjee, University of Maryland W.-J. Lee, Dong-Eui University, Republic of Korea G.-Y. Bae, Dong-Eui University, Republic of Korea S.-B. Lee, University of Maryland G.W. Rubloff, University of Maryland |
| Correspondent: | Click to Email |
Among various materials for thin film transparent conductor applications, Al-doped ZnO (AZO) is a particularly attractive material because of its excellent properties, such as higher thermal stability, good resistance against damage by hydrogen plasma and potentially, low cost compared to indium tin oxide (ITO). Of the various available deposition techniques, Atomic layer deposition (ALD) provides superb control at the nanoscale for thickness, uniformity, conformality and Al doping of AZO films. This is particularly attractive for use in nanostructures, as well as in more conventional applications such as liquid crystal displays.
We report here results for structural, optical and electrical properties of ultrathin ALD AZO films as a function of at% Al. AZO films of ~ 100nm thickness were deposited on quartz substrates at 150C using a commercial BENEQ TFS 500 reactor using diethyl zinc (DEZ) and H2O as precursors for ZnO, and trimethyl aluminum (TMA) and H2O as precursors for Al2O3. Al-doping was incorporated in a film by introducing a single cycle of TMA-H2O after fixed cycles of DEZ-H2O pulses. This ‘super’ cycle was repeated until the desired thickness was achieved. Al-doping was varied from 0.0at% to 24.5at%, on various samples, as determined by EDX. In addition, XRD, AFM, UV-Vis spectroscopy and temperature-dependent (80K-340K) Hall measurements were carried out to understand the structural, optical and electrical properties in these films.
Strong texture effects were observed in the AZO films on the quartz substrates as the films preferentially crystallized along the [100] direction. This texturing effect is different from the [002] normally reported for AZO films deposited using established methods other than ALD. Crystallinity and electrical conductivity peaked at 3at% Al, consistent with previous published work. AFM results show a dramatic drop in surface roughness with Al doping. Optical transmittances of over 80% were obtained for all films in the visible region.
Calculation of lattice parameter constants from XRD data and analysis within the framework of the Burstein-Moss effect, reveal that AZO films act as substitutionally doped films for Al doping less than ~7.3at%. Beyond this value of doping, phase segregation and possible formation of a low conductivity phase cause a reduction in the concentration and mobility of free carriers and hence a degradation of the electrical properties.