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

Fluorine doping: A feasible solution to enhancing the conductivity of high-resistance wide bandgap Mg0.51Zn0.49O active components

N-type doping of high-resistance wide bandgap semiconductors, wurtzite high-Mg-content MgxZn1-xO for instance, has always been a fundamental application-motivated research issue. Herein, we report a solution to enhancing the conductivity of high-resistance Mg0.51Zn0.49O active components, which has been reliably achieved by fluorine doping via radio-frequency plasma assisted molecular beam epitaxial growth. Fluorine dopants were demonstrated to be effective donors in Mg0.51Zn0.49O single crystal film having a solar-blind 4.43 eV bandgap, with an average concentration of 1.0E19 F/cm3.The dramatically increased carrier concentration (2.85E17 cm-3 vs ~1014 cm-3) and decreased resistivity (129 ohm.cm vs ~10E6 ohm cm) indicate that the electrical properties of semi-insulating Mg0.51Zn0.49O film can be delicately regulated by F doping. Interestingly, two donor levels (17 meV and 74 meV) associated with F were revealed by temperature-dependent Hall measurements. A Schottky type metal-semiconductor-metal ultraviolet photodetector manifests a remarkably enhanced photocurrent, two orders of magnitude higher than that of the undoped counterpart. The responsivity is greatly enhanced from 0.34 mA/W to 52 mA/W under 10 V bias. The detectivity increases from 1.89E9 cm Hz1/2/W to 3.58eE10 cm Hz1/2/W under 10 V bias at room temperature.These results exhibit F doping serves as a promising pathway for improving the performance of high-Mg-content MgxZn1-xO-based devices.Comment: 8 page

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

Thermal Conductivity of Double Polymorph Ga2O3 Structures

Recently discovered double gamma/beta ({\gamma}/\b{eta}) polymorph Ga2O3 structures constitute a class of novel materials providing an option to modulate functional properties across interfaces without changing chemical compositions of materials, in contrast to that in conventional heterostructures. In this work, for the first time, we investigate thermal transport in such homo-interface structures as an example of their physical properties. Specifically, the cross-plane thermal conductivity (k) was measured by femtosecond laser-based time-domain thermoreflectance with MHz modulation rates, effectively obtaining depth profiles of the thermal conductivity across the {\gamma}/\b{eta}-Ga2O3 structures. In this way, the thermal conductivity of {\gamma}-Ga2O3 k=1.84{\div}2.11 W m-1K-1 was found to be independent of the initial \b{eta}-substrates orientations, in accordance with the cubic spinel structure of the {\gamma}-phase and consistently with the molecular dynamics simulation data. In its turn, the thermal conductivity of monoclinic \b{eta}-Ga2O3 showed a distinct anisotropy, with values ranging from 10 W m-1K-1 for [201] to 20 Wm-1K-1 for [010] orientations. Thus, for double {\gamma}/\b{eta} Ga2O3 polymorph structures formed on [010] \b{eta}-substrates, there is an order of magnitude difference in thermal conductivity across the {\gamma}/\b{eta} interface, which potentially can be exploited in thermal energy conversion applications

Self-assembling of multilayered polymorphs with ion beams

Polymorphism contributes to the diversity of nature, so that even materials having identical chemical compositions exhibit variations in properties because of different lattice symmetries. Thus, if stacked together into multilayers, polymorphs may work as an alternative approach to the sequential deposition of layers with different chemical compositions. However, selective polymorph crystallization during conventional thin film synthesis is not trivial; e.g. opting for step-like changes of temperature and/or pressure correlated with switching from one polymorph to another during synthesis is tricky, since it may cause degradation of the structural quality. In the present work, applying the disorder-induced ordering approach we fabricated such multilayered polymorph structures using ion beams. We show that during ion irradiation of gallium oxide, the dynamic annealing of disorder may be tuned towards self-assembling of several polymorph interfaces, consistently with theoretical modelling. Specifically, we demonstrated multilayers with two polymorph interface repetitions obtained in one ion beam assisted fabrication step. Importantly, single crystal structure of the polymorphs was maintained in between interfaces exhibiting repeatable crystallographic relationships, correlating with optical cross-sectional maps. This data paves the way for enhancing functionalities in materials with not previously thought capabilities of ion beam technology.Comment: 9 pages, 4 figure, under review, private communication for supplementary note

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