Characterization of GaN-based light-emitting diodes

Maximizing the performance of light-emitting diodes (LEDs) is essential for the widespread uptake of solid-state lighting. To contribute towards this goal, this thesis focuses on the electrical and optical characterization of InGaN/GaN-based multi quantum well LEDs. In this work a wide range of characterization methodologies are introduced. A new development is the study of the emission spectra under resonant optical pumping and varying electrical bias. This has proven to be useful to obtain insights into the carrier dynamics and is used here to investigate LED samples containing different numbers of quantum wells (QWs) with different thicknesses for the barriers. Despite having only small structural differences, these samples have shown strong differences in their performance, which are attributed to a stronger piezoelectric field in the QWs in the sample with thinner barriers. Fluorescence microscopy with selective excitation of the QWs also allows the study of the spatially dependent luminescent properties of LEDs. In this case ohmic contacts create an equipotential surface and influence the collective emission. Strong carrier escape is observed in photovoltaic mode under both open and short circuit conditions. To help identify the underlying recombination mechanisms, different voltage ideality factors are extracted and compared with each other. This thesis shows that the use of photovoltaic measurements together with biasdependent spectrally resolved luminescence is a powerful tool to investigate GaN LEDs. The methodologies presented here provide experimental tools to better understand carrier recombination processes in different LED samples. These methods can extended to samples grown on different crystallographic orientations or to study the effects of additional layers in novel LED structures

High bandwidth freestanding semipolar (11–22) InGaN/GaN light-emitting diodes

Freestanding semipolar (11–22) indium gallium nitride (InGaN) multiplequantum-well light-emitting diodes (LEDs) emitting at 445 nm have been realized by the use of laser lift-off (LLO) of the LEDs from a 50- m-thick GaN layer grown on a patterned (10–12) r -plane sapphire substrate (PSS). The GaN grooves originating from the growth on PSS were removed by chemical mechanical polishing. The 300 m × 300 m LEDs showed a turn-on voltage of 3.6 V and an output power through the smooth substrate of 0.87 mW at 20 mA. The electroluminescence spectrum of LEDs before and after LLO showed a stronger emission intensity along the [11–23]InGaN/GaN direction. The polarization anisotropy is independent of the GaN grooves, with a measured value of 0.14. The bandwidth of the LEDs is in excess of 150 MHz at 20 mA, and back-to-back transmission of 300 Mbps is demonstrated, making these devices suitable for visible light communication (VLC) applications

Combined electrical and resonant optical excitation characterization of multi-quantum well InGaN-based light-emitting diodes

We present a comprehensive study of the emission spectra and electrical characteristics of InGaN/GaN multi-quantum well light-emitting diode (LED) structures under resonant optical pumping and varying electrical bias. A 5 quantum well LED with a thin well (1.5 nm) and a relatively thick barrier (6.6 nm) shows strong bias-dependent properties in the emission spectra, poor photovoltaic carrier escape under forward bias and an increase in effective resistance when compared with a 10 quantum well LED with a thin (4 nm) barrier. These properties are due to a strong piezoelectric field in the well and associated reduced field in the thicker barrier. We compare the voltage ideality factors for the LEDs under electrical injection, light emission with current, photovoltaic mode (PV) and photoluminescence (PL) emission. The PV and PL methods provide similar values for the ideality which are lower than for the resistance-limited electrical method. Under optical pumping the presence of an n-type InGaN underlayer in a commercial LED sample is shown to act as a second photovoltaic source reducing the photovoltage and the extracted ideality factor to less than 1. The use of photovoltaic measurements together with bias-dependent spectrally resolved luminescence is a powerful method to provide valuable insights into the dynamics of GaN LEDs

REFRACTIVE INDEX AND THICKNESS EVALUATION OF MONOMODE AND MULTIMODE STEP-INDEX PLA- NAR OPTICAL WAVEGUIDES USING LONGITUDINAL SECTION MAGNETIC (LSM) AND LONGITUDINAL

Abstract—In this work, we demonstrate that the LSM and LSE modes formulation is an excellent theoretical tool for determining the refractive index and thickness of the guiding layer in planar optical waveguides with step refractive index profile. Refractive index of transparent materials capable of being deposited as a solid thin layer on a substrate for confining light can be evaluated very accurately. The method can be applied to analyze and design monomode and multimode optical waveguides, unlike the methods proposed so far, including cutoff wavelength region. This wave model only requires the experimental evaluation of the effective indices of the guided modes. In order to verify the developed formulation, the commercial software Olympios was used for theoretical comparison. Polymeric planar optical waveguides were fabricated and characterized. A prism coupling method and the Metricon system were used for effective indices measurements and to compare the accuracy. The experimental evaluation of the thickness was carried out by profilometry. In all cases a complete agreement was obtained for refractive index and thickness between theory and experiments