Progress in Violet Light-Emitting Diodes Based on ZnO/GaN Heterojunction

Progress in light-emitting diodes (LEDs) based on ZnO/GaN heterojunctions has run into several obstacles during the last twenty years. While both the energy bandgap and lattice parameter of the two semiconductors are favorable to the development of such devices, other features related to the electrical and structural properties of the GaN layer prevent an efficient radiative recombination. This work illustrates some advances made on ZnO/GaN-based LEDs, by using high-thickness GaN layers for the p-region of the device and an ad hoc device topology. Heterojunction LEDs consist of a quasicoalesced non-intentionally doped ZnO nanorod layer deposited by chemical bath deposition onto a metal–organic vapor-phase epitaxy -grown epitaxial layer of p-doped GaN. Circular 200 μm-sized violet-emitting LEDs with a p-n contact distance as low as 3 μm exhibit a turn-on voltage of 3 V, and an emitting optical power at 395 nm of a few microwatts. Electroluminescence spectrum investigation shows that the emissive process can be ascribed to four different recombination transitions, dominated by the electron-hole recombinations on the ZnO side

p-n Junction Diode Fabricated From ZnO Nanorod Grown By Aqueous Solution Method

Ph.DDOCTOR OF PHILOSOPH

Luminescence study of III-nitride semiconductor nanostructures and LEDs

In this work, cathodoluminescence (CL) hyperspectral imaging, photoluminescence (PL) and electroluminescence are used to study the optical properties of III-nitride semiconductor materials. III-nitride semiconductors have successfully opened up the solid-state lighting market. Light-emitting diodes (LEDs) fabricated using III-nitrides, however, still suffer from numerous deficiencies such as high defect densities, efficiency droop and the 'green gap'. In order to investigate the type and properties of the defects, CL and electron channelling contrast imaging (ECCI) were performed on the same micron-scale area of a GaN thin film. A one-to-one correlation between isolated dark spots in CL and threading dislocations (TDs) in ECCI showed that TDs of pure edge character and TDs with a screw component act as non-radiative recombination centres. Secondary electron imaging of planar InGaN/GaN multiple quantum well (MQW) structures identified trench defects of varying width. CL imaging revealed a strong redshift (90 meV) and intensity increase for trench defects with wide trenches compared with the defect-free surrounding area. Narrower trench defects showed a small redshift (10 meV) and a slight reduction in intensity. The optical properties of nanorods fabricated from planar InGaN/GaN MQW structures were investigated using PL and CL. PL spectroscopy identified reduced strain within the MQW stack in the nanorods compared with the planar structure. CL imaging of single nanorods revealed a redshift of 18 meV of the MQW emission along the nanorod axis and provided an estimate of 55 nm for the carrier diffusion length. Colour conversion using novel organic compounds as energy down-converters was studied. The first molecules absorbed in the ultra-violet and emitted in the yellow spectral region. Further modification of the organic compound shifted the absorption into the blue and white light generation was investigated by coating blue-emitting nanorods and blue LEDs. Determination of the colour rendering index and colour temperature showed "warm white" light emission with values of 70 and 3220 K, respectively.In this work, cathodoluminescence (CL) hyperspectral imaging, photoluminescence (PL) and electroluminescence are used to study the optical properties of III-nitride semiconductor materials. III-nitride semiconductors have successfully opened up the solid-state lighting market. Light-emitting diodes (LEDs) fabricated using III-nitrides, however, still suffer from numerous deficiencies such as high defect densities, efficiency droop and the 'green gap'. In order to investigate the type and properties of the defects, CL and electron channelling contrast imaging (ECCI) were performed on the same micron-scale area of a GaN thin film. A one-to-one correlation between isolated dark spots in CL and threading dislocations (TDs) in ECCI showed that TDs of pure edge character and TDs with a screw component act as non-radiative recombination centres. Secondary electron imaging of planar InGaN/GaN multiple quantum well (MQW) structures identified trench defects of varying width. CL imaging revealed a strong redshift (90 meV) and intensity increase for trench defects with wide trenches compared with the defect-free surrounding area. Narrower trench defects showed a small redshift (10 meV) and a slight reduction in intensity. The optical properties of nanorods fabricated from planar InGaN/GaN MQW structures were investigated using PL and CL. PL spectroscopy identified reduced strain within the MQW stack in the nanorods compared with the planar structure. CL imaging of single nanorods revealed a redshift of 18 meV of the MQW emission along the nanorod axis and provided an estimate of 55 nm for the carrier diffusion length. Colour conversion using novel organic compounds as energy down-converters was studied. The first molecules absorbed in the ultra-violet and emitted in the yellow spectral region. Further modification of the organic compound shifted the absorption into the blue and white light generation was investigated by coating blue-emitting nanorods and blue LEDs. Determination of the colour rendering index and colour temperature showed "warm white" light emission with values of 70 and 3220 K, respectively

Electrodeposition of zinc oxide nanostructured films

ZnO nanostructures have great promise in a wide range of applications such as sensors, optoelectronics, piezoelectronics, healthcare. Preparation of oxide films by electrodeposition from aqueous solution presents several advantages over other techniques such as controlling the rate and morphology through several well-defined parameters (electrode potential, current, temperature, pH, etc.), the fact that electrolytic processing is a well-established technology and readily scalable for production, and the non-equilibrium nature of the electrochemical interface often gives rise to morphologies and compositions not attainable through other, usually high-temperature, routes. Despite a large amount of research in this area the detailed mechanism of nucleation and growth is still controversial. Only a good understanding of it will allow the expected industrial applications to be achieved. One of the main difficulties to overcome is that tiny amounts of material are involved and the required in-situ measurements are thus very delicate. The ability of synchrotron radiation to probe material structure during deposition makes it the ideal tool for the study of nucleation and growth of these materials as a function of the processing parameters. Here we will present two synchrotron-based approaches involving both X-ray absorption and scattering. The first method, together with ex-situ characterisation, provides detailed information about how the kinetics of the growth and/or dissolution is influenced by the electrochemical parameters. The effect of time, potential, zinc ions concentration, oxygen precursor, temperature and electrolyte composition have been studied. Following this understanding of the influence of the parameters, films of desired structure can be synthesised and new structures have been made. Beside the electrochemical parameters, the growth of the film is influenced by the interaction with substrate in the early stage of nucleation. The second synchrotron technique allows the direct observation of the development of the crystal orientation of the films during the deposition. It gives promising results to study how the substrate influences the growth and thus the properties of the films

SYNTHESIS OF ZINC OXIDE FILMS AND NANOWIRES FOR NANOELECTRONICS APPLICATIONS

Ph.DDOCTOR OF PHILOSOPH

Influence of Silicon Nanostructures on the Growth of GaN on Silicon

Ph.DDOCTOR OF PHILOSOPH

Electrochemical Deposition, Characterisation and PhotoVoltaic Application of Undoped and Aluminium Doped Zinc Oxide Nanostructures

Zinc oxide (ZnO) is an n-type II-VI semiconductor with a reported band gap of 3.2-3.6 eV [1, 2, 3] and electrical resistivity of ~ 50 Ωcm [4]. Ideal for use in devices such as Photovoltaics (PVs), Light Emitting Diodes (LEDs) and detectors, ZnO has the advantage that it can be electrochemically deposited. This enables the quick and cheap controlled growth of ZnO nanostructures, which can potentially enhance performance in electronic applications over thin films. ZnO doping with a group III element e.g. Aluminium, can increase ZnO conduction by several orders of magnitude whilst having only a subtle effect on its optical properties, therefore further enhancing device performance. For the first time, this thesis presents a unique in-depth study into the potentiostatic electrochemical deposition of well defined zinc oxide nanostructures (nanorods and platelets), their controlled aluminium doping and application in PV devices. This work addresses the mechanism of doping and examines the relationship between the opto-electronic properties, composition, structure, morphology and growth. The results show that arrays of crystalline wurtzite ZnO nanorods with strong (002) preferential orientation can be deposited on ITO and Au using a 1 mM Zn(NO3)2 system. Doping has been successfully carried out using Al(NO3)3 with a doping mechanism confirmed for the first time. This study shows that doped nanorods contain < 5 at. % Al3+, where Al3+ is incorporated in the ZnO lattice as interstitial and/or substitutional ions. This results in a subtle increase in the band gap, and is believed to increase the ZnO conduction by several orders of magnitude. The application of these nanorod arrays in PV devices has improved device efficiency by ~ 1080 %. Furthermore, platelets have been successfully deposited using a 5 mM Zn(NO3)2 system. A critical dopant content ~ 5 at. % Al3+ has been found, above which there is a transition in the doping mechanism towards spontaneous Al2O3 formation in addition to interstitial and substitutional Al3+ ion locations. This results in a gradual decrease in the optical band gap towards that of undoped ZnO. This mechanism occurs in platelets, where at. % Al3+ > 5 %. Platelet formation is associated with small quantities of impurities such as Al2O3, ZnCl2, Zn(ClO4)2 Zn5(OH)8Cl2.H2O and Au3Zn, arising from deposition conditions. Both impurities and dopants result in increased ZnO polycrystallinity and decreased ZnO (002) preferential orientation. The performance of PV devices with nanorod arrays has been shown to be better than previously reported equivalent thin film devices. This work illustrates the significance of electrochemical deposition as a technique for cheap and quick, controlled mass production of high quality tailor-made ZnO semiconductor nanostructures

Growth and Characterisation of ZnO Nanostructures: Excitonic Properties and Morphology

The growth mechanism of aligned ZnO nanostructures, grown by the vapour phase transport (VPT) growth method, specifically with nanorod and nanowall morphologies has been studied. The thesis begins with an introductory chapter on ZnO nanostructures and related topics, and the second chapter introduces the various experimental techniques used. The main thesis work involves five distinct studies. Firstly, the conditions to grow nanorod and nanorod/nanowall structures on sapphire using a ZnO/graphite powder mixture as a growth source are studied and the optimum conditions for each morphology identified. Secondly, the effects on ZnO nanostructure growth on sapphire of using activated carbon and carbon black powders, rather than graphite powder are studied. Nanostructures can be grown at significantly lower temperatures with carbon black and activated carbon, though with different morphologies, compared to graphite. Thirdly, low temperature cathodoluminescence spectroscopy measurements of ZnO nanostructures grown on Si substrate are presented. These data show significant inhomogeneity in the spatial distribution of emission throughout the sample for the Al-related donor bound exciton emission at 3.3605 eV and the Al-related emissions are compared to the other spectral features seen for these samples. The possible origin of this inhomogeneity is discussed. Fourthly, the microscopic origin of a unique photoluminescence peak at ~3.367 eV, which is known as surface exciton peak, has been studied in detail and its behaviour is studied after samples have been subjected to various post-growth treatments such as plasma treatment, UV exposure in vacuum and exposure to high voltages. Finally, post-growth passivation of nanostructures has been done using PVP and HF on ZnO nanostructure samples. The effects of these chemicals on the optical emission from these samples are studied and the potential for these to act as effective passivation agents is discussed. The thesis concludes with a summary of the work done, some general conclusions and comments on possible future directions