Phosphorous Diffuser Diverged Blue Laser Diode for Indoor Lighting and Communication.

An advanced light-fidelity (Li-Fi) system based on the blue Gallium nitride (GaN) laser diode (LD) with a compact white-light phosphorous diffuser is demonstrated for fusing the indoor white-lighting and visible light communication (VLC). The phosphorous diffuser adhered blue GaN LD broadens luminescent spectrum and diverges beam spot to provide ample functionality including the completeness of Li-Fi feature and the quality of white-lighting. The phosphorous diffuser diverged white-light spot covers a radiant angle up to 120(o) with CIE coordinates of (0.34, 0.37). On the other hand, the degradation on throughput frequency response of the blue LD is mainly attributed to the self-feedback caused by the reflection from the phosphor-air interface. It represents the current state-of-the-art performance on carrying 5.2-Gbit/s orthogonal frequency-division multiplexed 16-quadrature-amplitude modulation (16-QAM OFDM) data with a bit error rate (BER) of 3.1 × 10(-3) over a 60-cm free-space link. This work aims to explore the plausibility of the phosphorous diffuser diverged blue GaN LD for future hybrid white-lighting and VLC systems

Suppression of the quantum-confined Stark effect in polar nitride heterostructures

Recently, we suggested an unconventional approach (the so-called Internal-Field-Guarded-Active-Region Design “IFGARD”) for the elimination of the quantum-confined Stark effect in polar semiconductor heterostructures. The IFGARD-based suppression of the Stark redshift on the order of electronvolt and spatial charge carrier separation is independent of the specific polar semiconductor material or the related growth procedures. In this work, we demonstrate by means of micro-photoluminescence techniques the successful tuning as well as the elimination of the quantum-confined Stark effect in strongly polar [000-1] wurtzite GaN/AlN nanodiscs as evidenced by a reduction of the exciton lifetimes by up to four orders of magnitude. Furthermore, the tapered geometry of the utilized nanowires (which embed the investigated IFGARD nanodiscs) facilitates the experimental differentiation between quantum confinement and Stark emission energy shifts. Due to the IFGARD, both effects become independently adaptable.DFG, 43659573, SFB 787: Halbleiter - Nanophotonik: Materialien, Modelle, Bauelement

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

Calibration of Polarization Fields and Electro-Optical Response of Group-III Nitride Based c-Plane Quantum-Well Heterostructures by Application of Electro-Modulation Techniques

The polarization fields and electro-optical response of PIN-diodes based on nearly lattice-matched InGaN/GaN and InAlN/GaN double heterostructure quantum wells grown on (0001) sapphire substrates by metalorganic vapor phase epitaxy were experimentally quantified. Dependent on the indium content and the applied voltage, an intense near ultra-violet emission was observed from GaN (with fundamental energy gap Eg = 3.4 eV) in the electroluminescence (EL) spectra of the InGaN/GaN and InAlN/GaN PIN-diodes. In addition, in the electroreflectance (ER) spectra of the GaN barrier structure of InAlN/GaN diodes, the three valence-split bands, Γ9, Γ7+, and Γ7−, could selectively be excited by varying the applied AC voltage, which opens new possibilities for the fine adjustment of UV emission components in deep well/shallow barrier DHS. The internal polarization field Epol = 5.4 ± 1.6 MV/cm extracted from the ER spectra of the In0.21Al0.79N/GaN DHS is in excellent agreement with the literature value of capacitance-voltage measurements (CVM) Epol = 5.1 ± 0.8 MV/cm. The strength and direction of the polarization field Epol = −2.3 ± 0.3 MV/cm of the (0001) In0.055Ga0.945N/GaN DHS determined, under flat-barrier conditions, from the Franz-Keldysh oscillations (FKOs) of the electro-optically modulated field are also in agreement with the CVM results Epol = −1.2 ± 0.4 MV/cm. The (absolute) field strength is accordingly significantly higher than the Epol strength quantified in published literature by FKOs on a semipolar (112¯2) oriented In0.12Ga0.88N quantum well

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