Testing ZnO based photoanodes for PEC applications

AbstractWe report on multi layered ZnCdO photoanode structures synthesized on c-A12O3 substrates using metal organic vapor phase epitaxy and covered with a thin TiO2 protective film using atomic layer deposition and pulsed laser deposition techniques. Structural, optical and photoelectrochemical properties of the multilayers were investigated systematically in connection with their potential application in the photolysis of water. X-ray diffraction and Rutherford backscattering techniques confirmed staggered arrangement and graded Cd content of the multilayers. Temperature-dependant photoluminescence revealed excitonic nature of a broad emission band representing combined band-edge emissions from the individual layers. The photocurrent was found to increase with decreasing thickness of the TiO2 protective layer

Surface/strain energy balance controlling preferred orientation in CdZnO films

Single-phase rock-salt CdZnO films were synthesized on r-plane sapphire substrates by metal–organic chemical vapor deposition. Evolutions in growth orientations were investigated in these films as a function of Zn content and film thickness. The preferred orientation is found to black (thick) and red (thin) lines depend on the balance between the surface and strain energy accumulated in the films in accordance with the so-called overall energy model. Specifically, (100) orientation dominates below a critical Zn content and/or a critical film thickness, otherwise (111)-oriented grains nucleate on top of the (100) planes. This research was originally published in the Journal of Applied Physics. © AIP Publishin

Cd diffusion and thermal stability of CdZnO/ZnO heterostructures

Structural properties and thermal stability of CdZnO/ZnO heterostructures grown on sapphire substrate by metal-organic vapor phase epitaxy (MOVPE) have been studied. Zn/Cd interdiffusion and Cd evaporation appear as key factors limiting the thermal stability of CdZnO/ZnO heterostructures. Cd diffusion in MOVPE ZnO is found to start already at 600 °C with an activation energy of ∼2.2 eV and yields a saturation in the Cd concentration at 0.1-1 at. %. The actual Cd content remaining in the samples upon annealing is determined by a competition between Cd evaporation via the surface and diffusion/segregation in the bulk. This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing

Understanding phase separation in ZnCdO by a combination of structural and optical analysis

A phenomenon of wurtzite (w), zincblende (zb), and rock-salt (rs) phase separation was investigated in ZnCdO films having Cd contents in the range of 0%–60% settling a discussion on the phase stability issues in ZnCdO. First, low-Cd-content (⩽17%) ZnCdO was realized preferentially in a w matrix determining optimal Zn-lean conditions by tuning the precursor decomposition rates during synthesis. However, more detailed analysis of x-ray diffraction and photoluminescence (PL) data revealed that the w single-phase stability range is likely to be as narrow as 0%–2% Cd, while samples containing 7%–17% of Cd exhibit a mixture of w and zb phases. Second, high-Cd-content (32%–60%) ZnCdO samples were realized, supplying more of the Cd precursor utilizing Zn-lean growth conditions, however, resulting in a mixture of w, zb, and rs phases. Characteristic PL signatures at 2.54 and 2.31 eV were attributed to zb-ZnCdO and rs-CdO, respectively, while the band gap variation in w-Zn1−xCdxO is given by (3.36–0.063x) as determined at 10 K. The phase separation is interpreted in terms of corresponding changes in the charge distribution and reduced stacking fault energy. © 2011 American Physical Societ