Tunable plasmonic resonances in highly porous nano-bamboo Si-Au superlattice-type thin films

We report on fabrication of spatially-coherent columnar plasmonic nanostructure superlattice-type thin films with high porosity and strong optical anisotropy using glancing angle deposition. Subsequent and repeated depositions of silicon and gold lead to nanometer-dimension subcolumns with controlled lengths. The superlattice-type columns resemble bamboo structures where smaller column sections of gold form junctions sandwiched between larger silicon column sections ("nano-bamboo"). We perform generalized spectroscopic ellipsometry measurements and finite element method computations to elucidate the strongly anisotropic optical properties of the highly-porous nano-bamboo structures. The occurrence of a strongly localized plasmonic mode with displacement pattern reminiscent of a dark quadrupole mode is observed in the vicinity of the gold subcolumns. We demonstrate tuning of this quadrupole-like mode frequency within the near-infrared spectral range by varying the geometry of the nano-bamboo structure. In addition, coupled-plasmon-like and inter-band transition-like modes occur in the visible and ultra-violet spectral regions, respectively. We elucidate an example for the potential use of the nano-bamboo structures as a highly porous plasmonic sensor with optical read out sensitivity to few parts-per-million solvent levels in water

Observation of strongly enhanced photoluminescence from inverted cone-shaped silicon nanostuctures

Silicon nanowires (SiNWs) attached to a wafer substrate are converted to inversely tapered silicon nanocones (SiNCs). After excitation with visible light, individual SiNCs show a 200-fold enhanced integral band-to-band luminescence as compared to a straight SiNW reference. Furthermore, the reverse taper is responsible for multifold emission peaks in addition to the relatively broad near-infrared (NIR) luminescence spectrum. A thorough numerical mode analysis reveals that unlike a SiNW the inverted SiNC sustains a multitude of leaky whispering gallery modes. The modes are unique to this geometry and they are characterized by a relatively high quality factor (Q ~ 1300) and a low mode volume (0.2 < (λ/neff)3 < 4). In addition they show a vertical out coupling of the optically excited NIR luminescence with a numerical aperture as low as 0.22. Estimated Purcell factors Fp ∝ Q/Vm of these modes can explain the enhanced luminescence in individual emission peaks as compared to the SiNW reference. Investigating the relation between the SiNC geometry and the mode formation leads to simple design rules that permit to control the number and wavelength of the hosted modes and therefore the luminescent emission peaks

Fabrication and characterisation of microcavity based III-nitride optoelectronics on a microscale

In this work, the design implementation, fabrication and characterisation on III-nitride microemitters with microcavity effects grown on c-plane sapphire are presented. Highly doped GaN layers have been effectively porosified by means of electrochemical etching (EC). Different porous morphology is established by varying the doping level of the GaN layer and the EC bias voltage, enabling a wide refractive index tunability of the porous layer. Through this thesis the potential of nearly-lattice matched nanoporous GaN as a cladding layer and as a Distributed Bragg reflector (DBR) has been fundamentally studied. There are three major parts that constitute the outcome of the work. The first part is on the development and characterisation of a novel microcavity based microemitter array, which partially mitigates one of the consequences of the Quantum Confined Stark Effect (QCSE). The so-called ‘blue shifting’ of the emission wavelength that is considered to be one of the long-standing problems of III- nitrides has been reduced from 50 nm to 3 nm. The second part highlights a novel approach towards micro-Vertical Surface Emitting Lasers, which is achieved by developing a direct epitaxial method based on selective epitaxial growth. Nearly lattice-matched porous DBRs have been achieved with reflectivities over 99%. Strong cavity effects have been demonstrated via electrical and optical pumping. Although stimulated emission is not fully confirmed, the full width at half maximum of the peak emission has been reduced by a factor of 5. Finally, the last part is an introduction on the potential enhancement effect of porous GaN as a cladding layer in III-nitride green edge emitting lasers, instead of conventional AlGaN. There is an enhancement in the optical confinement by a factor of 2.5, as verified using a Finite Difference Eigenmode solver. Porous cladding layers have been effectively formed, achieving porosities of 50%. Effective current injection through the porous was demonstrated. However, the inherent challenges from the cleaving of sapphire substrates limited the facet formation. Thus, stimulated emission was not confirmed yet

Development of aluminum gallium nitride-based emitters in the form of graded-index separate confinement heterostructure (GRINSCH)

The development of ultraviolet semiconductor emitters (LEDs and lasers) will enable a large number of industrial and medical applications. AlGaN alloys are ideally suited for the development of such devices since their energy gap can be tuned from the near UV (365 nm) to deep UV (200 nm). However, the doping of such materials n- and p-type is difficult. Another problem is the generally poor light extraction efficiency from both UV and visible LEDs. This research addressed the first problem by developing UV emitters in the form of graded-index-separate-confinement-heterostructure (GRINSCH). In these device the active region is embedded in two compositionally graded wave guiding layers. Due to the polar nature of nitride semiconductors these compositionally graded AlGaN films are doped p- or n-type if the grading changes from high to low concentration or from low to high concentration respectively. Thus, a p-n junction is automatically formed without the incorporation of dopants. The polarization induced doping level in these structures was calculated to be 1018cm-3 for the p- and n-sides. A number of devices, whose active region is either 75 nm Al0.72Ga0.28N bulk film or multiple QWs have been grown on 6H-SiC substrates by Molecular-Beam Epitaxy (MBE) and investigated. The emission properties of these structures were investigated by cathodoluminescence (CL) and by measuring their optical gain. A maximum net modal gain in excess of 80 cm-1 was measured with an optical gain threshold of 14 µJ / cm2. Some of these structures, emitting in the near UV, were also electrically pumped. The second problem was addressed by incorporating dielectric (TiO2) photonic crystals on the phosphor plates of white LEDs in order to increase the light extraction efficiency upon illumination with blue LEDs. The two-dimensional (2D) hexagonal-lattice of TiO2 photonic crystal was formed by e-beam lithography on low-scattering (Y1-xCex)3Al5O12 (YAG:Ce) ceramic phosphor plates. Yellow light extraction enhancement by a factor of 4.4 was achieved with a 2D photonic crystal nano-cylinders having diameter 430 nm, lattice constant of 580 nm and height of 350 nm. Simulations using a three-dimensional finite difference time domain are consistent with our measured data

III-Nitride Blue Laser Diode with Photoelectrochemically Etched Current Aperture

Group III-nitride is a remarkable material system to make highly efficient and high-power optoelectronics and electronic devices because of the unique electrical, physical, chemical and structural properties it offers. In particular, InGaN-based blue Laser Diodes (LDs) have been successfully employed in a variety of applications ranging from biomedical and military devices to scientific instrumentation and consumer electronics. Recently their use in highly efficient Solid State Lighting (SSL) has been proposed because of their superior beam quality and higher efficiency at high input power density. Tremendous advances in research of GaN semi-polar and non-polar crystallographic planes have led both LEDs and LDs grown on these non-basal planes to rival with, and with the promise to outperform, their equivalent c-plane counterparts. However, still many issues need to be addressed, both related to material growth and device fabrication, including a lack of conventional wet etching techniques. GaN and its alloys with InN and AlN have proven resistant essentially to all known standard wet etching techniques, and the predominant etching methods rely on chlorine-based dry etching (RIE). These introduce sub-surface damage which can degrade the electrical properties of the epitaxial structure and reduce the reliability and lifetime of the final device. Such reasons and the limited effectiveness of passivation techniques have so far suggested to etch the LD ridges before the active region, although it is well-known that this can badly affect the device performance, especially in narrow stripe width LDs, because the gain guiding obtained in the planar configuration is weak and the low index step and high lateral current leakage result in devices with threshold current density higher than devices whose ridge is etched beyond the active region. Moreover, undercut etching of III-nitride layers has proven even more challenging, with limitations in control of the lateral etch distance. In this dissertation it is presented the first nitride blue edge emitting LD with a photoelectrochemical etched current aperture (CA-LD) into the device active region.Photoelectrochemical etching (PECE) has emerged as a powerful wet etching technique for III-nitride compounds. Beyond the advantages of wet etching technique, PECE offers bandgap selectivity, which is particularly desirable because it allows more freedom in designing new and advanced devices with higher performances. In the first part of this thesis a review of PECE is presented, and it is shown how it can be used to achieve a selective and controllable deep undercut of the active region of LEDs and LDs, in particular the selective PECE of MQW active region of (10-10) m-plane and (20-2-1) plane structures is reported.In the second part of this thesis, the fabrication flow process of the CA-LD is described. The performance of these devices is compared with that of shallow etched ridge LDs with a nominally identical epitaxial structure and active region width and it is experimentally shown that the CA-LD design has superior performance. CW operation of a (20-2-1) CA-LD with a 1.5 µm wide active region is demonstrated.Finally, in the third and last part of this thesis, the CA-LD performance is discussed in more details, in particular, an analysis of optical scattering losses caused by the rough edges of the remnant PEC etched active region is presented

Development of Long-Cavity III-Nitride Vertical-Cavity Surface-Emitting Lasers

GaN vertical-cavity surface-emitting lasers (VCSELs) show promise for numerous lighting, display, communications, and sensor applications due to their visible wavelength emission, low threshold current, high beam quality, and arraying capabilities. Primarily, research has been focused on short to medium cavity (L&lt;5λ) VCSEL designs, prioritizing single longitudinal mode operation. However, GaN VCSELs struggle with thermal management due to self-heating from higher input power requirements, high optical losses from p-type GaN and current spreaders, and poor heatsinking from the typically low thermal conductivities of the bottomside distributed Bragg reflectors (DBRs). These issues result in a high thermal impedance, generally &gt;1000 K/W, quick thermal rollover, and low device lifetimes. Recently, long cavity (L&gt;&gt;5λ) GaN VCSEL designs have shown significant promise towards addressing the issues of thermal stability and cavity length control but require substrate polishing and complex fabrication, limiting scalability for mass production. To address these issues, a topside lens fabrication method is developed. Then, a 65λ GaN VCSEL with a topside lens, a buried tunnel junction current aperture, and bottomside epitaxial nanoporous GaN DBR was fabricated using standard microfabrication techniques. First, a topside GaN lens was demonstrated, with CW lasing achieved at lower current densities than comparable planar cavity VCSELs. However, the output power was limited by the high temperature regrowth required to fabricate the GaN lens as well as the high turn-on voltage. Next, a topside dielectric lens was developed which enabled CW lasing performance above 2mW for a GaN VCSEL with a partially etched porous DBR, and single transverse mode operation for other VCSELs with fully etched porous DBRs. The devices show high thermal stability due to the long cavity, with an estimated thermal impedance of 600K/W measured on-chip

Integrating Optical Emitters into Silicon Photonic Waveguides

This thesis reports work targeting the integration of Si light emitters with optical waveguides. Such integrated devices would find utility in a number of applications including telecommunications, optical interconnects, and biological and chemical sensors. Much research has been directed by others on how to improve the emission efficiency and achieve lasing in VLSI (very large scale integration) compatible sources. Here, the focus is on how such devices can be integrated with planar waveguides. Two enhancement techniques were selected for potential integration; defect engineering (DE), and Si nanocrystals (Si-nc) embedded in Si02• Defect engineered light emitting diodes (LEDs) made on silicon-on-insulator (SOI) and emitting at 1.1 μm were successfully demonstrated. In addition, surface photoluminescence from SOI was analyzed to account for interference from the SOI cavity. However, it was determined that the emission efficiency of defect engineered LEDs studied during the course of this work is below that which was reported previously, and that the fabrication procedure thus suffers from irreproducibility. Barring an enormous advancement in the DE technique, it is concluded that the emission efficiency is too small to make use of its integration potential. A more successful approach was obtained from the Si-nc system fabricated using electron-cyclotron resonance plasma enhanced chemical vapor deposition (ECR-PECVD). Optically pumped edge emitting devices were designed, fabricated and characterized. The devices are comprised of Si-ncs emitting at 800 nm, integrated with slab silicon nitride waveguides. This work is the first report of edge emission from Si-ncs integrated with silicon nitride waveguides. Edge emission and waveguide properties were characterized in the ~850 nm emission band of the Si-ncs. The edge emission was well described as a propagating mode, attenuated primarily by the Si-nc film. Propagation losses of a typical air/Si-nc/SiNx/Si02 waveguide were measured to be 11 ± 2 dB/cm and 20 ± 2 dB/cm at 850 nm in the TE and TM polarizations respectively. A wavelength dependent loss of -0.14 ± 0.03 dB/(cm*nm) was found to exist in the material loss of Si-nc films. In addition, the Si-nc films were found to undergo a partially recoverable photo-induced degradation of PL efficiency during exposure to pump light. Processing techniques compatible with both high efficiency Si-nc and low loss silicon nitride were developed and described. A two-sectioned photonic device was also designed, fabricated and characterized. The device contained an optically pumped Si-nc emitting waveguide section integrated with a low loss silicon nitride slab waveguide. The potential for optically pumped Si-nc emitters integrated with silicon nitride photonic circuits thus appears promising.ThesisDoctor of Philosophy (PhD

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 development of micropillars and two-dimensional nanocavities that incorporate an organic semiconductor thin film

Photonic crystals (PC) are periodic optical structures containing low and high refractive index layers that influence the propagation of electromagnetic waves. Photonic cavities can be created by inserting defects into a photonic crystal. Such structures have received significant attention due to their potential of confining light inside volumes (V) smaller than a cubic wavelength of light (λ/n)3 which can be used to enhance light-matter interaction. Cavity quality factor (Q) is useful for many applications that depend on the control of spontaneous emission from an emitter such quantum optical communication and low-threshold lasing. High Q/V values can also result in an enhancement of the radiative rates of an emitter placed on the surface of the cavity by means of the Purcell effect. This thesis concerns the fabrication and study of two types of optical cavity containing an organic-semiconductor material. The cavities explored are; (1) one-dimensional micropillar microcavities based on multilayer films of dielectric and organic materials, and (2) two-dimensional nanocavities defined into a photonic crystal slab. Firstly, light emission from a series of optical micropillar microcavities containing a thin fluorescent, red-emitting conjugated polymer film is investigated. The photoluminescence emission from the cavities is characterized using a Fourier imaging technique and it is shown that emission is quantised into a mode-structure resulting from both vertical and lateral optical confinement within the pillar. We show that optical-confinement effects result in a blue-shift of the fundamental mode as the pillar-diameter is reduced, with a model applied to describe the energy and distribution of the confined optical modes. Secondly, simulation, design, and analysis of two dimensional photonic crystal L3 nanocavities photonic crystal are presented. Nanocavities were then prepared from silicon nitride (SiN) as the cavity medium with the luminescence emitted from an organic material at red wavelengths that was coated on the cavity surface. To improve the quality factor of such structures, hole size, lattice constant and hole shift are systematically varied with their effect as cavity properties determined. Finite Difference Time Domain (FDTD) modelling is used to support the experimental work and predict the optimum design for such photonic crystal nanocavity devices. It is found that by fine-tuning the nearest neighbour air-holes close to the cavity edges, the cavity Q factor can be increased. As a result, we have obtained a single cavity mode having a Q-factor 938 at a wavelength of 652 nm. Here, the cavity Q factor then increases to 1100 at a wavelength of 687 nm as a result of coating a red-emitting conjugated polymer film onto the top surface of the nanocavity. We propose that this layer planarizes the dielectric surface and helps reduce optical losses as a result of scattering