Low-cost fabrication of tunable band gap composite indium and gallium nitrides

III-nitride materials have been linked with a vast number of exciting applications from power electronics to solar cells. Herein, polycrystalline InN, GaN and systematically controlled InxGa1−xN composite thin flms are fabricated on FTO glass by a facile, low-cost and scalable aerosol assisted chemical vapor deposition technique. Variation of the indium content in the composite flms leads to a dramatic shift in the optical absorbance properties, which correlates with the band edges shifting between those of GaN to InN. Moreover, the photoelectrochemical properties are shown to vary with indium content, with the 50% indium composite having an external quantum efciency of around 8%. Whilst the overall photocurrent is found to be low, the photocurrent stability is shown to be excellent, with little degradation seen over 1hour. These fndings demonstrate a new and low-cost method for fabricating polycrystalline III-nitrides, which have a range of interesting properties that are highly sought after for many applications

Epitaxial growth of iii-nitride nanostructures and their optoelectronic applications

Light-emitting diodes (LEDs) using III-nitride nanowire heterostructures have been intensively studied as promising candidates for future phosphor-free solid-state lighting and full-color displays. Compared to conventional GaN-based planar LEDs, III-nitride nanowire LEDs exhibit numerous advantages including greatly reduced dislocation densities, polarization fields, and quantum-confined Stark effect due to the effective lateral stress relaxation, promising high efficiency full-color LEDs. Beside these advantages, however, several factors have been identified as the limiting factors for further enhancing the nanowire LED quantum efficiency and light output power. Some of the most probable causes have been identified as due to the lack of carrier confinement in the active region, non-uniform carrier distribution, and electron overflow. Moreover, the presence of large surface states and defects contribute significantly to the carrier loss in nanowire LEDs. In this dissertation, a unique core-shell nanowire heterostructure is reported, that could overcome some of the aforementioned-problems of nanowire LEDs. The device performance of such core-shell nanowire LEDs is significantly enhanced by employing several effective approaches. For instance, electron overflow and surface states/defects issues can be significantly improved by the usage of electron blocking layer and by passivating the nanowire surface with either dielectric material / large bandgap energy semiconductors, respectively. Such core-shell nanowire structures exhibit significantly increased carrier lifetime and massively enhanced photoluminescence intensity compared to conventional InGaN/GaN nanowire LEDs. Furthermore, AlGaN based ultraviolet LEDs are studied and demonstrated in this dissertation. The simulation studies using Finite-Difference Time-Domain method (FDTD) substantiate the design modifications such as flip-chip nanowire LED introduced in this work. High performance nanowire LEDs on metal substrates (copper) were fabricated via substrate-transfer process. These LEDs display higher output power in comparison to typical nanowire LEDs grown on Si substrates. By engineering the device active region, high brightness phosphor-free LEDs on Cu with highly stable white light emission and high color rendering index of \u3e 95 are realized. High performance nickel?zinc oxide (Ni-ZnO) and zinc oxide-graphene (ZnO-G) particles have been fabricated through a modified polyol route at 250?C. Such materials exhibit great potential for dye-sensitized solar cell (DSSC) applications on account of the enhanced short-circuit current density values and improved efficiency that stems from the enhanced absorption and large surface area of the composite. The enhanced absorption of Ni-ZnO composites can be explained by the reduction in grain boundaries of the composite structure as well as to scattering at the grain boundaries. The impregnation of graphene into ZnO structures results in a significant increase in photocurrent consequently due to graphene\u27s unique attributes including high surface area and ultra-high electron mobility. Future research directions will involve the development of such wide-bandgap devices such as solar cells, full color LEDs, phosphor free white-LEDs, UV LEDs and laser diodes for several applications including general lighting, wearable flexible electronics, water purification, as well as high speed LEDs for visible light communications

The 2020 UV emitter roadmap

Solid state UV emitters have many advantages over conventional UV sources. The (Al,In,Ga)N material system is best suited to produce LEDs and laser diodes from 400 nm down to 210 nm—due to its large and tuneable direct band gap, n- and p-doping capability up to the largest bandgap material AlN and a growth and fabrication technology compatible with the current visible InGaN-based LED production. However AlGaN based UV-emitters still suffer from numerous challenges compared to their visible counterparts that become most obvious by consideration of their light output power, operation voltage and long term stability. Most of these challenges are related to the large bandgap of the materials. However, the development since the first realization of UV electroluminescence in the 1970s shows that an improvement in understanding and technology allows the performance of UV emitters to be pushed far beyond the current state. One example is the very recent realization of edge emitting laser diodes emitting in the UVC at 271.8 nm and in the UVB spectral range at 298 nm. This roadmap summarizes the current state of the art for the most important aspects of UV emitters, their challenges and provides an outlook for future developments

Atomic-Scale Insights into Light Emitting Diode

In solid-state lightning, GaN-based vertical LED technology has attracted tremendous attention because its luminous efficacy has surpassed the traditional lightning technologies, even the 2014 Nobel Prize in Physics was awarded for the invention of efficient blue LEDs, which enabled eco-friendly and energy-saving white lighting sources. Despite today’s GaN-based blue VLEDs can produce IQE of 90% and EQE of 70-80%, still there exist a major challenge of efficiency droop. Nonetheless, state-of-the-art material characterization and failure analysis tools are inevitable to address that issue. In this context, although LEDs have been characterized by different microscopy techniques, they are still limited to either its semiconductor or active layer, which mainly contributes towards the IQE. This is also one of the reason that today’s LEDs IQE exceeded above 80% but EQE of 70-80% remains. Therefore, to scrutinize the efficiency droop issue, this work focused on developing a novel strategy to investigate key layers of the LED structure, which play the critical role in enhancing the EQE = IQE x LEE factors. Based on that strategy, wafer bonding, reflection, GaN-Ag interface, MQWs and top-textured layers have been systematically investigated under the powerful advanced microscopy techniques of SEM-based TKD/EDX/EBSD, AC-STEM, AFM, Raman spectroscopy, XRD, and PL. Further, based on these correlative microscopy results, optimization suggestions are given for performance enhancement in the LEDs. The objective of this doctoral research is to perform atomic-scale characterization on the VLED layers/interfaces to scrutinize their surface topography, grain morphology, chemical composition, interfacial diffusion, atomic structure and carrier localization mechanism in quest of efficiency droop and reliability issues. The outcome of this research advances in understanding LED device physics, which will facilitate standardization in its design for better smart optoelectronics products

Theoretical study of segregation kinetics of indium in indium gallium nitride and magnesium in magnesium-gallium nitride grown by molecular beam epitaxy

A rate equation model including all physically relevant surface processes is developed for the study of In segregation in InGaN and Mg segregation in Mg-GaN MBE growth. In InGaN growth, the simulations were carried for a variety of growth conditions spanning the growth parameter space: substrate temperature in the range of 500--700°C: Ga flux in the range of 1.17--8.98 nm/min: In flux in the range of 0.39--28.74 nm/min and N flux in the range of 4.7--12 nm/min. Results of In incorporation obtained from simulations are within 1% agreement with the experiments reported in the literature. In segregation is found to be negligible below 580°C . Above 640°C, the segregation dominates the kinetics. This temperature dependence is found to be independent of the fluxes. In Mg-GaN growth, simulations were carried for various growth temperatures in the range of 600--750°C with constant flux rates of Mg, Ga and N. For the given flux rates, it is found that Mg segregates to the surface with the increase in temperature. Above 750°C a dopant depleted zone is formed below the surface layer. Results obtained from simulations are in good qualitative agreement with the experimental data

Degradation mechanisms of devices for optoelectronics and power electronics based on Gallium Nitride heterostructures

Gallium Nitride is rapidly emerging as a promising material for electronic devices in various fields. Since it is a direct bandgap semiconductor it can be used for highly efficient light emitting devices (Light Emitting Diodes and Laser Diodes) and the possibility of growing alloys containing Aluminum and Indium allow for the selection of the peak wavelength along the whole UV-green part of the radiation spectrum. Moreover, the high electron mobility, the ability of withstand high electric fields and the good thermal dissipation make GaN-based diodes and transistors devices with a good potential for high frequency and power applications. Before final products containing Gallium Nitride devices can permeate the international market, it is required to guarantee that they are reliable enough to have long lifetimes to appeal potential customers, and that their performance/cost relationship is superior compared to other competitors, at least in some specific fields of application. Aim of this thesis is to investigate the strong points of Gallium Nitrides by means of characterization and reliability tests on various different structures (LEDs, laser diodes, blocking diodes, HEMTs, GITs, MISs), in order to analyze the behavior of the material from different points of view. Within this work is reported a detailed study of the gradual degradation of InGaN-based laser diodes and Light-Emitting Diodes submitted to electro-thermal stress. The purpose is to compare the behavior of the two devices by means of electro-optical measurements, electroluminescence characterization, near field emission measurements and Deep-Level Transient Spectroscopy (DLTS) investigation in order to give a deeper understanding of the mechanisms involved in LD degradation. Particular attention is given to the role of injection efficiency decrease and non-radiative recombination. The comparison of the degradation kinetics and an analysis of the degradation modes of the two device structures allowed a complete study of the physical mechanisms responsible for the degradation. It was found that the degradation of the devices can be ascribed to an increase of the defect density, which has a strong impact on non radiative recombination kinetics. The activation energy of the detected deep level is 0.35 - 0.45 eV. As an effect of combined electrical and thermal stress tests on commercially-available InGaN-based blue laser diodes, it has been found that sometimes there is an initial decrease of the threshold current, which is ascribed to the increase of the activation of p-type dopant, promoted by the temperature and the flow of minority carriers. In order to investigate the effects of the creation of defects, two different commercial blue InGaN-based LEDs were submitted to 3 MeV proton irradiation at various fluencies (10^11, 10^12 and 10^13 p/cm2). The degradation process was characterized by combined current-voltage (I - V), optical power-current (L - I) and capacitance-voltage (C - V) measurements, in order to investigate the changes induced by the irradiation and the recovery after annealing time at high temperature (150 °C). The experimental data suggest the creation of non-radiative recombination centers near or into the active region of the LEDs, due to atomic displacement. This hypothesis is confirmed by the results of the recovery tests: the increase of the optical power and its correlation with the recovery of the forward current is consistent with the annealing of those defects. Part of the activity on high electron mobility transistors was devoted to the realization of measurement setups in order to carry out novel characterization techniques. Were analyzed the advantages and limitations of the current-transient method used for the study of the deep levels in GaN-based high electron mobility transistors (HEMTs), by evaluating how the procedures adopted for measurement and data analysis can influence the results of the investigation. The choice of the measurement parameters (such as the voltage levels used to induce the trapping phenomena and monitor the current transients and the duration of the filling pulses) and of the analysis procedure (the method used for the extrapolation of the time constants of the processes) can influence the results of the drain current transient investigation and can provide information on the location of the trap levels responsible for current collapse. Moreover, was collected a database of defects described in more than 60 papers on GaN and its compounds, which can be used to extract information on the nature and origin of the traps in AlGaN/GaN HEMTs. Using this newly developed technique and other more common tests, several reliability and lifetime test were carried out on various structures, in order to gain a better understanding of their problematic aspects and possible improvements. One potential variation is the composition of the gate stack. Degradation tests were performed at Vgs = -5 V and increasing Vds levels on GaN HEMTs with different gate materials: Ni/Au/Ni, ITO and Ni/ITO. At each step of the stress experiment, the electrical and optical characteristics of the transistors were measured in order to analyze the degradation process. It was found that stress induces a permanent degradation of the gate diode, consisting in an increase in the leakage current. This change is due to the generation of parasitic conductive paths, as suggested by electroluminescence (EL) mapping, and devices based on ITO showed higher reliability. These data strongly support the hypothesis that the robustness is influenced by processing parameters and/or by the gate material, since all analyzed devices come from the same epitaxial wafer. Other than varying the gate material, it is possible to add a p-type layer under the gate in order to achieve normally-off operation. This change produces a benefit in terms of performances, but can give birth to unusual trapping phenomena. It was carried out an extensive analysis of the time and field-dependent trapping processes that occur in GaN-based gate injection transistors exposed to high drain voltage levels. Results indicate that, even if the devices do not suffer from current collapse, continuous exposure to high drain voltages can induce a remarkable increase in the on-resistance (Ron). The increase in Ron can be recovered by leaving the device in rest conditions. Temperature-dependent analysis indicates that the activation energy of the detrapping process is equal to 0.47 eV. By time-resolved electroluminescence characterization, it is shown that this effect is related to the capture of electrons in the gate - drain access region. This is further confirmed by the fact that charge emission can be significantly accelerated through the injection of holes from the gate. A first-order model was developed to explain the time dependence of the trapping process. Using other deep levels characterization techniques, such as drain current transients, gate frequency sweeps and backgating, several other trap states were identified in these devices. Their activation energies are 0.13, 0.14, 0.25, 0.47 and 0.51 eV. During the accelerated lifetime tests of these devices, it was found a variation of the relative amplitude of the transconductance peaks, well correlated with the increase of the electroluminescence. This effect can be explained by the activation of the p-type dopant, a phenomenon which was detected also in laser diodes. It is possible to develop diodes able to withstand very high reverse voltages using a similar structure, deprived of the gate region and with an additional Schottky diode (Natural superjunction). In this case, the activation energies of the detected deep levels were 0.35, 0.36, 0.44 and 0.47 eV. These values are very similar to the ones found in GITs, and this fact, along with the presence of the p-dopant activation in very different devices, confirms that it is useful to study different structures based on the same material in order to gain more knowledge on its performances, possibilities and reliability aspects

Cubic phase gallium nitride photonics integrated on silicon(100) for next-generation solid state lighting

Semiconductors made of gallium nitride (GaN) and its compounds (AlInGaN) have transformed the visible light emitting diode (LED) industry thanks to their direct bandgap across the entire visible and ultraviolet spectra. Despite its success, the conventional hexagonal-phase GaN has fundamental disadvantages in performance and cost that hinder market adoption. These include: internal polarization field ( MV/cm2), high acceptor activation energy (260 meV), low hole mobility (20 cm2/V), and expensive substrates (Al2O3, SiC). Gallium nitride also crystallizes in the cubic crystal that has a higher degree of symmetry. This leads to some advantageous properties for light emitting applications: polarization-free, lower acceptor energy (200 meV), and higher hole mobility (150 cm2/V). These advantages are critical for the development of the next-generation solid state lighting. Difficulty in its synthesis stemming from the large crystal lattice mismatch, chemical incompatibility, and phase metastability has prohibited the growth of high quality semiconductor crystals that are device-worthy. This thesis explores a method of synthesizing phase-pure, high-quality cubic GaN crystals on nanopatterned Si(100) substrates via hexagonal-to-cubic phase transition, and the thesis presents a comprehensive material characterization of the crystals. Crystal growth geometry modeling of GaN on nanopatterned Si(100) substrates is used to estimate the necessary patterning parameters to facilitate complete phase transition. The cubic GaN material is then studied using structural characterization techniques including scanning electron microscopy, electron backscatter diffraction, and transmission electron microscopy. The carrier recombination properties are studied using photoluminescence, Raman spectroscopy, and cathodoluminescence. The cubic GaN synthesized using the phase transition method on carefully patterned Si(100) substrates is shown to be phase-pure, defect-free, and optically superior. Material properties such as internal quantum efficiency, Varshni coefficients, and defect levels are extracted from the experiments. Other work on hexagonal GaN light emitters on silicon substrates, chamber conditioning for metalorganic chemical vapor deposition of III-nitrides, and space-based laser instruments for NASA missions is also discussed. Class lab module development and outreach activities are included

Scanning probe techniques as an investigation tool for semiconductor nanostructures and devices

Semiconductor nanostructure based devices provide new opportunities for contributing to a sustainable energy usage. This includes harvesting of energy (solar cells) and saving of energy, e.g. in lighting (light-emitting diodes, LEDs) and transfer of energy (power devices). However, development and improvement of nanostructure devices requires thorough characterization and understanding on a single nanostructure level. At nanometer dimensions, surface effects start dominating device performance. Therefore, macroscopic bulk characterization techniques are insufficient, and surface-sensitive tools are needed. Here, I used various types of scanning probe microscopy to investigate and manipulate surface and material properties of nanostructure devices that are relevant for energy saving and harvesting.In(Ga)P nanowire diodes are promising candidates for photovoltaic applications. Kelvin probe microscopy (KPM) and scanning tunneling microscopy (STM) were used to investigate surface band alignment of a nanowire p-n junction. In addition, a novel method for optoelectronic characterization of individual nanowires without any processing steps is demonstrated. STM was utilized to both image and contact individual upright standing InGaP nanowires, obtaining current-voltage characteristics and solar cell figures of merits in dark and under illumination, for as-grown nanowires and after in-situ surface modification.InGaN nanostructures are attractive for LEDs, because the luminescence of InGaN alloys could potentially cover the entire visible range by tuning the In content. The nanowire geometry allows combination of lattice mismatched materials. Nanowire-based InGaN platelet LEDs with varying In content as well as In quantum dots were studied by atomic force microscope (AFM), correlating surface corrugation with optical properties and investigating nucleation of surface facets.GaN offers principally superior material properties for power electronic devices compared to the currently used Si and SiC, but a reduction of the defect density is required. We investigated low-defect GaN planar layers formed by reformation of GaN nanowire arrays. AFM, conductive-AFM, KPM, and scanning capacitance microscopy were utilized to investigate formation and distribution of different types of defects and their influence on GaN electrical properties. Furthermore, the revelation of defect-related conductive paths through AFM-induced anodic oxidation was explored