Katherine H. A. Bogart

Junction and carrier temperature measurements in deep-ultraviolet light-emitting diodes using three different methods

The junction temperature of AlGaN ultraviolet light-emitting diodes emitting at 295nm is measured by using the temperature coefficients of the diode forward voltage and emission peak energy. The high-energy slope of the spectrum is explored to measure the carrier temperature. A linear relation between junction temperature and current is found. Analysis of the experimental methods reveals that the diode-forward voltage is the most accurate (±3°C). A theoretical model for the dependence of the diode forward voltage (Vf) on junction temperature (Tj) is developed that takes into account the temperature dependence of the energy gap. A thermal resistance of 87.6K∕W is obtained with the device mounted with thermal paste on a heat sink.

Room-temperature direct current operation of 290 nm light-emitting diodes with milliwatt power levels

Ultraviolet light-emitting diodes (LEDs) have been grown by metalorganic vapor phase epitaxy using AlN nucleation layers and thick n-type Al0.48Ga0.52N current spreading layers. The active region is composed of three Al0.36Ga0.64N quantum wells with Al0.48Ga0.52N barriers for emission at 290 nm. Devices were designed as bottom emitters and flip-chip bonded to thermally conductive submounts using an interdigitated contact geometry. The ratio of quantum well emission to 330 nm sub-band gap emission is as high as 125:1 for these LEDs. Output power as high as 1.34 mW at 300 mA under direct current operation has been demonstrated with a forward voltage of 9.4 V. A peak external quantum efficiency of 0.18% has been measured at an operating current of 55 mA.

Junction Temperature in Ultraviolet Light-Emitting Diodes

The junction temperature and thermal resistance of AlGaN and GaInN ultraviolet (UV) light-emitting diodes (LEDs) emitting at 295 and 375 nm, respectively, are measured using the temperature coefficient of diode-forward voltage. An analysis of the experimental method reveals that the diode-forward voltage has a high accuracy of ±3°C. A comprehensive theoretical model for the dependence of diode-forward voltage (Vf) on junction temperature (Tj) is developed taking into account the temperature dependence of the energy gap and the temperature coefficient of diode resistance. The difference between the junction voltage temperature coefficient (dVj/dT) and the forward voltage temperature coefficient (dVf/dT) is shown to be caused by diode series resistance. The data indicate that the n-type neutral regions are the dominant resistive element in deep-UV devices. A linear relationship between junction temperature and current is found. Junction temperature is also measured by the emission-peak-shift method. The high-energy slope of the spectrum is explored in the measurement of carrier temperature.

Three dimensional silicon photonic crystals fabricated by two photon phase mask lithography

We describe the fabrication of silicon three dimensional photonic crystals using polymer templates defined by a single step, two-photon exposure through a layer of photopolymer with relief molded on its surface. The resulting crystals exhibit high structural quality over large areas, displaying geometries consistent with calculation. Spectroscopic measurements of transmission and reflection through the silicon and polymer structures reveal excellent optical properties, approaching properties predicted by simulations that assume ideal layouts.

Three-dimensional nanostructures formed by single step, two-photon exposures through elastomeric penrose quasicrystal phase masks.

We describe the fabrication of unusual classes of three-dimensional (3D) nanostructures using single step, two-photon exposures of photopolymers through elastomeric phase masks with 5-fold, Penrose quasicrystalline layouts. Confocal imaging, computational studies, and 3D reconstructions reveal the essential aspects of the flow of light through these quasicrystal masks. The resulting nanostructures show interesting features, including quasicrystalline layouts in planes parallel to the sample surfaces, with completely aperiodic variations through their depths, consistent with the optics. Spectroscopic measurements of transmission and reflection provide additional insights.

Mid-ultraviolet light-emitting diode detects dipicolinic acid

Dipicolinic acid (DPA, 2,6-pyridinedicarboxylic acid) is a substance uniquely present in bacterial spores such as that from anthrax (B. anthracis). It is known that DPA can be detected by the long-lived fluorescence of its terbium chelate; the best limit of detection (LOD) reported thus far using a large benchtop gated fluorescence instrument using a pulsed Xe lamp is 2 nM. We use a novel AlGaN light-emitting diode (LED) fabricated on a sapphire substrate that has peak emission at 291 nm. Although the overlap of the emission band of this LED with the absorption band of Tb-DPA (λmax doublet: 273, 279 nm) is not ideal, we demonstrate that a compact detector based on this LED and an off-the-shelf gated photodetection module can provide an LOD of 0.4 nM, thus providing a basis for convenient early warning detectors.

Ohmic contacts to plasma etched n-Al0.58Ga0.42N

Plasma etching is required to expose n-AlxGa1−xN layers for bottom-emitting ultraviolet light emitting diodes grown on sapphire. However, etching can increase the difficulty of forming Ohmic contacts. X-ray photoelectron spectroscopy and cathodoluminescence reveal how the semiconductor changes with etching and help explain why it becomes more difficult to form an Ohmic contact. A V∕Al∕V∕Au metallization has been investigated for Ohmic contacts to n-Al0.58Ga0.42N etched with a BCl3∕Cl2∕Ar chemistry. Increased V thickness and higher annealing temperatures were required to obtain a specific contact resistance of 4.7×10−4Ωcm2 for etched n-Al0.58Ga0.42N compared to optimized contacts on unetched films.

Optimization and performance of AlGaN-based multi-quantum well deep UV LEDs

In this paper, we overview the critical materials challenges in the development of AlGaN-based deep ultraviolet light emitting diodes (LEDs) and present our recent advances in the performance of LEDs in the 275-290 nm range. Our primary device design involves a flip-chip, bottom emitting, transparent AlGaN (Al = 47-60%) buffer layer structure with interdigitated contacts. To date, under direct current operation, we have demonstrated greater than 1 mW of output power at 290 nm with 1 mm x 1 mm LEDs, and greater than 0.5 mW output power from LEDs emitting at wavelengths as short at 276 nm. Electroluminescence spectra demonstrate both a main peak from quantum well emission as well as sub-bandgap emission originating from radiative recombination involving deep level states. The heterostructure designs that we have employed have greatly suppressed this deep level emission, resulting in deep level peak intensities that are 40-125X lower than the primary quantum well emission for different LED designs and applied current densities.

Junction Temperature Measurements in Deep-UV Light-Emitting Diodes

The junction temperature of AlGaN/GaN ultraviolet (UV) Light-Emitting Diodes (LEDs) emitting at 295 nm is measured by using the temperature coefficients of the diode forward voltage and emission peak energy. The high-energy slope of the spectrum is explored to measure the carrier temperature. A linear relation between junction temperature and current is found. Analysis of the experimental methods reveals that the diode-forward voltage is the most accurate method (± 3 °C). A theoretical model for the dependence of the diode junction voltage ( V j ) on junction temperature ( T ) is developed that takes into account the temperature dependence of the energy gap. A thermal resistance of 87.6 K/W is obtained with the AlGaN/GaN LED sample mounted with thermal paste on a heat sink.

Device performance of AlGaN-based 240-300-nm deep UV LEDs

Deep ultraviolet light emitting diodes (LEDs) with emission wavelengths shorter than 300 nm have been grown by metalorganic vapor phase epitaxy. A bottom emitting LED design is used which requires a high-Al content AlxGa1-xN (x = 0.5 - 0.8 ) buffer layer which has sufficient conductivity and is transparent to the quantum well emission wavelength. LEDs were flip chip mounted to a silicon submount which provides for good thermal performance as well as improved light extraction. For large area 1 mm x 1 mm LEDs emitting at 297 nm, an output power as high as 2.25 mW under direct current operation has been demonstrated at 500 mA with a forward voltage of 12.5 volts. For shorter wavelength LEDs emitting at 276 nm, an output power as high as 1.3 mW has been demonstrated under direct current operation at 300 mA with a forward voltage of 9.2 volts. Recent improvements in heterostructure design have resulted in quantum well emission at 276 nm with a peak intensity that is 330 times stronger than the largest sub-bandgap peak. LEDs with emission wavelengths as short as 237 nm have also been demonstrated.