ZnO and ZnCdO metal organic vapor phase epitaxy: epitaxy, defects and band gap engineering

Zinc oxide (ZnO) and its ternary alloys have high potential to compete with III-V nitrides for optoelectronic applications. Furthermore, oxide semiconductors receive considerable attention due to their low cost of fabrication, chemical robustness and high thermal conductance. The goal of this work was two fold: (i) to explore manufacturing route of ZnO and ZnCdO films using metal organic vapor phase epitaxy (MOVPE) in vector flow epitaxy mode and (ii) to master structural/optical properties of these films for preparing such as components in electronics, optoelectronics and solar energy conversion. The starting point was to study the influence of basic synthesis parameters on the structural and luminescence properties of pure ZnO films on c-axis oriented sapphire substrates. The samples were synthesized using previously unexplored for ZnO vector flow epitaxy mode of MOVPE employing systematic variations of fundamental synthesis parameters such as temperature, pressure, II/VI molar ratio, total carrier gas flow ratio, susceptor rotation rate, etc. It was concluded that the growth temperature affects the precursor pyrolysis and in these terms pre-determines the actual II/VI molar ratio available at the reaction zone. Concurrently, direct II/VI molar ratio variations by supplying different amount of precursors influences the properties too, for example, changing intrinsic defect balance in the films. Variations of other parameters like chamber pressure, total gas flow rate and susceptor rotation rate resulted in minor deviations in the growth, uniformity and properties. Further, exploring lower-cost substrates, ZnO films have also been successfully fabricated on Si(111) substrates by using AlN buffer layers. The process resulted in ZnO/AlN/Si heterostructures, where ZnO films were grown epitaxially on AlN buffers of different thicknesses and on Si(111) by so called domain-matching epitaxy. An optimal thickness of the AlN buffer was determined, resulting in nearly in-plane strain free ZnO films. Such films exhibited excellent crystalline quality and extremely bright excitonic emissions. The control of point defects in the crystal is essential for realization of any device, and we have specifically investigated the changes in the defect balance as a function of synthesis parameters in our films. By manipulating the growth temperature, we could achieve either Zn-lean or O-lean conditions. Positron annihilation spectroscopy and photoluminescence were employed to study point defects in such films. A range of vacancy complexes was identified from signal variations going consistently with variations in the synthesis conditions. Specifically, a synthesis temperature window has been determined allowing to control the concentration balance of zinc vacancies (VZn). Finally, manufacturing routes of wurtzite ZnCdO alloys were explored utilizing the knowledge obtained in the process of mastering VZn-enriched material. The alloys exhibited mixed wurtzite, zincblende and rocksalt phases for Cd contents > 7 % also demonstrating general decrease in excitonic luminescence. The phase separation is interpreted in terms of corresponding changes in charge distribution and reduced stacking fault energy. A narrow Cd content region (< 2%) was attributed to the wurtzite single phase equilibrium. The band gap of ZnCdO thin was found to decrease with increasing Cd concentration consistently with literature. In present work, the band gap of ZnCdO was tuned from 3.4 eV to 2.3 eV by changing Cd content up to 60 %, providing an excellent opportunity for band gap engineering in novel optoelectronic applications

Changing vacancy balance in ZnO by tuning synthesis between zinc/oxygen lean conditions

The nature of intrinsic defects in ZnO films grown by metal organic vapor phase epitaxy was studied by positron annihilation and photoluminescence spectroscopy techniques. The supply of Zn and O during the film synthesis was varied by applying different growth temperatures (325–485 °C), affecting decomposition of the metal organic precursors. The microscopic identification of vacancy complexes was derived from a systematic variation in the defect balance in accordance with Zn/O supply trends.Peer reviewe

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

Comparison of the structural properties of Zn-face and O-face single crystal homoepitaxial ZnO epilayers grown by RF-magnetron sputtering

Homoepitaxial ZnO growth is demonstrated from conventional RF-sputtering at 400 °C on both Zn and O polar faces of hydrothermally grown ZnO substrates. A minimum yield for the Rutherford backscattering and channeling spectrum, χmin, equal to ∼3% and ∼12% and a full width at half maximum of the 00.2 diffraction peak rocking curve of (70 ± 10) arc sec and (1400 ± 100) arc sec have been found for samples grown on the Zn and O face, respectively. The structural characteristics of the film deposited on the Zn face are comparable with those of epilayers grown by more complex techniques like molecular beam epitaxy. In contrast, the film simultaneously deposited on the O-face exhibits an inferior crystalline structure ∼0.7% strained in the c-direction and a higher atomic number contrast compared with the substrate, as revealed by high angle annular dark field imaging measurements. These differences between the Zn- and O-face films are discussed in detail and associated with the different growth mechanisms prevailing on the two surfacesThis work has been performed within “The Norwegian Research Centre for Solar Cell Technology” Project No. 193829, a Centre for Environment-friendly Energy Research co-sponsored by the Norwegian Research Council and research and industry partners in Norway and the Frienergi program. R.S. acknowledges the partial support from the EU 7th Framework Programme Project No. REGPOT-CT-2013- 316014 (EAgLE)

Investigating antireflection properties of hybrid silicon nanostructures comprising rod-like nanopores and nano-textured surface

In the present work, we have fabricated hybrid silicon (Si) nanostructures comprising vertical rod-like nanopores and nano-textured surface by metal assisted chemical etching (MACE) method at room temperature. The as-received p-type Upgraded Metallurgical grade (UMG) Si wafers were chemical polished, prior to investigating the etching effects at the metal nanoparticle semiconductor interface. The influence of metal silver nanoparticle (AgNPs) concentration on the formation of hybrid nanostructures were studied systematically. Depending on the surface morphology, the hybrid structures exhibited constant 10% average reflectance in the UV–Visible spectral region or average 7.5% reflectance in range of 200–400 nmsubmittedVersio

Dominating migration barrier for intrinsic defects in gallium oxide: Dose-rate effect measurements

Ion bombardment provides an opportunity to study basic properties of intrinsic defects in materials since the radiation-induced disorder accumulation depends on the balance between defect generation and migration rates. In particular, variation of such parameters as irradiation temperature and ion flux, known in the literature as dose-rate effect, interconnects the macroscopically measured lattice disorder with the migration barrier of the dominating defects. In this work, we measured the dose-rate effect in monoclinic gallium oxide (β-Ga2O3) and extracted its activation energy of 0.8 ± 0.1 eV in the range of 25–250 °C. Taking into account that the measurements were performed in the Ga-sublattice and considering 0.8 ± 0.1 eV in the context of theoretical data, we interpreted it as the migration barrier for Ga vacancies in β-Ga2O3, limiting the process. Additionally, we observed and took into account an interesting form of the lattice relaxation due to radiation-induced disorder buildup, interpreted in terms of the compressive strain accumulation, potentially trigging phase transitions in Ga2O3 lattice

Influence of metal assisted chemical etching time period on mesoporous structure in as-cut upgraded metallurgical grade silicon for solar cell application

In this work, upgraded metallurgical grade silicon (UMG-Si) wafer was used to fabricate mesoporous nanostructures, as an effective antireflection layer for solar photovoltaic cells. The length of the vertical Si nanostructure (SiNS) arrays was altered by varying the etching time period during metal assisted chemical etching process, using a silver catalyst. The optical, structural, morphological changes and the antireflection properties of Si nanostructures formed on UMG-Siwafer were analysed. SEM and photoluminescence studies indicate that Si nanocrystals are formed on the surface and along the vertical nanowires. The pore size depends on the Ag nanoparticle size distribution. All the samples demonstrated a luminescence band centred around 2.2 eV. From the optical results, samples etched for 45 min show strong absorption in the visible spectrum. The minimum and maximum surface reflectance in the visible region was observed for 15 min and 60 min etched SiNS. Based on the observed results, 15 min etched Si with a uniform porous structure has minimum reflectance across the entire silicon UV–Vis absorption spectrum, making it worth further investigation as a candidate for use as an antireflection layer in silicon based solar cells

Carbon-dioxide as annealing atmosphere to retain the electrical properties of indium-tin oxide

In practical applications of indium-tin-oxide (ITO) annealing at temperatures ~400 °C without degrading its electrical and optical properties is an important challenge. In the present work, commercial Indium-tin oxide (ITO) coated on glass was subjected to post-annealing treatment in the range of 200-400 °C at different annealing atmospheres; oxygen, nitrogen and carbon-dioxide. The annealed samples were characterized by X-ray diffraction, UV-visible spectroscopy and Hall measurements to evaluate the structural, optical and electrical properties. Both oxygen and nitrogen treated samples degrades the structural, optical and conducting properties of ITO, while carbon-dioxide atmosphere inhibits the degradation of ITO at 400 °C. The obtained results suggest that carbon-dioxide can be well utilized as annealing ambient to retain opto-electronic, structural and electrical properties of ITO and thereby improve the efficiency of ITO based solar cells