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Aging under high current stress of AlGaN quantum well based light emitting diodes with high and low Al content in the wells emitting at 270 nm and 335 nm, respectively, has been studied by scanning near field optical spectroscopy and far field electroluminescence, photoluminescence and time-resolved photoluminescence. In the high Al content devices emission band related to optical transitions in the cladding involving nitrogen vacancies has been found. Evolution of this band during aging suggests that the role of N vacancies is crucial in the aging process by aiding defect generation and formation of high conductivity channels. [ABSTRACT FROM AUTHOR]

Low frequency noise of current and light intensity of ultraviolet light emitting diodes (LED) with wavelength from 265 to 340 nm are the superposition of the 1/f and generation-recombination noise. The dependence of generation-recombination noise on the LED current has a maximum caused by a relatively shallow trap level in the quantum well. The upper bound of this trap level concentration is estimated to be Nt=7×1015 cm-3. The relative spectral noise density of the light intensity fluctuations decreased with an increase of the LED forward current. At high currents, the difference in the noise level for LEDs with different wavelength is small and is of the same order of magnitude or even smaller than for visible LEDs. [ABSTRACT FROM AUTHOR]

Low-frequency fluctuations of current and light intensity were measured for different types of ultraviolet (UV) light-emitting diodes (LEDs) with wavelengths from 280 to 375 nm. These UV LEDs are suitable for studying steady-state and time-varying UV fluorescences of biological materials. The correlation coefficient between the current and light intensity fluctuations varies with the LED current and load resistance. This dependence is explained in terms of contributions to the 1/f noise from the active region and from the LED series resistance. The noise level could be reduced by operating the UV LEDs at a certain optimum current level and with large external series resistance. [ABSTRACT FROM AUTHOR]

Stimulated emission (SE) is studied in AlGaN/AlGaN multiple quantum wells (MQWs) with different Al content grown on sapphire substrate. The spectra of spontaneous and stimulated emission and their transformations with increasing temperature as well as stimulated emission thresholds were measured in the temperature range from 8 to 300 K. Phonon-assisted band broadening in low-Al-content MQWs and double-scaled potential profile in high-Al-content MQWs were observed in the samples and linked with carrier localization conditions. The temperature dependence of the stimulated emission threshold was similar in the samples where the stimulated transitions occur between extended states and in the samples where the transitions occur in localized states. The stimulated emission threshold depends predominantly on the density of nonradiative recombination centers. [ABSTRACT FROM AUTHOR]

This paper analyzes the performance and reliability of deep-ultraviolet light-emitting diodes (LEDs) on AlGaN emitting at 280 and 295 nm. By means of detailed electroluminescence characterization, we show that the optical properties of the LEDs are strongly influenced by the presence of deep-level-related radiative transitions, and we separately evaluate the contribution of each of these recombination mechanisms on the overall light emission. The reliability analysis presented in this paper shows that stress determines the gradual decrease of the output power of the LEDs, which is more prominent at low measuring current levels. Degradation is attributed to the increase of the nonradiative recombination rate. By means of C-V analysis, we give evidence of modifications of the charge distribution in the active layer, taking place as a consequence of stress: this mechanism is considered to be related to the generation of new defect states near/within the active region. [ABSTRACT FROM PUBLISHER]

A set of Al0.35Ga0.65N/Al0.49Ga0.51N multiple quantum wells (MQWs) with fixed barrier width and well widths varying from 1.65 to 5.0 nm has been grown by metal-organic chemical vapor deposition. Carrier dynamics in the MQWs were studied using time-resolved photoluminescence (PL) spectroscopy and light-induced transient grating (four wave mixing) technique. The authors observed that the lifetime of nonequilibrium carriers (excitons) increases with decreasing well width and interpreted the effect by stronger localization preventing their migration to nonradiative recombination centers. Meanwhile the radiative decay time is also influenced by screening of the built-in electric field, which spatially separates the electrons and holes. It is shown that this effect affects the initial part of PL intensity decay after pulsed excitation. It becomes more pronounced with increase in the initial carrier density but saturates when the carrier density is high enough to completely screen the built-in electric field. The screening effect on PL decay is stronger in wider quantum wells. [ABSTRACT FROM AUTHOR]

We report on 245–247 nm AlGaN-based deep ultraviolet (DUV) light-emitting diodes with continuous wave output power up to 2 mW. DUV diodes with peak emission wavelength of 245 and 247 nm exhibit turn-on voltage less than 10 V. At room temperature and cw operation the maximum external quantum efficiency was close to 0.18%, which is the highest value published to date for devices with peak emission wavelength shorter than 250 nm. A large external efficiency droop observed at current densities above 100 A/cm2 is attributed to self-heating, carrier spillover from the QWs into the barrier layers or the p-type cladding layer, and/or Auger recombination. A semiempirical equation was proposed to describe the efficiency droop in DUV diodes at a high current injection. [ABSTRACT FROM AUTHOR]

Emission from a 285 nm AlGaN quantum well light emitting diode has been studied by scanning near-field optical spectroscopy. The scans revealed micrometer-size domainlike areas emitting with a higher intensity and at a longer wavelength; presumably, because of a lower AlN molar fraction in these regions. Experiments performed on different days have shown that with time, intensity from these spots increases and emission wavelength shifts to the red, indicating a further change in the quantum well alloy composition. This has allowed distinguishing an aging mechanism that involves locally increased current, heating, and atom migration. [ABSTRACT FROM AUTHOR]

We report on the development of deep-ultraviolet light-emitting diodes with AlGaN multiple-quantum-well active region for real-time fluorescence lifetime sensing in natural biofluorophores. The peak wavelengths of the devices are 340 and 280 nm, linewidth at half maximum approximately 10 nm, wall-plug efficiency up to 0.9%, output power in the milliwatt range, peak-to-background ratio up to four orders of magnitude, and cutoff frequencies for electrical modulation in the range of 100 MHz. Devices with high-frequency modulated output were demonstrated for frequency domain fluorescence lifetime measurements in basic biological autofluorophores (nicotinamide adenine dinucliotide, riboflavin, tyrosine, and tryptophan) with subnanosecond resolution. [ABSTRACT FROM AUTHOR]

Optimization of the migration-enhanced metalorganic chemical vapor deposition and further optimization of the contact and active layer design for 280 nm light-emitting diodes resulted in large improvement of cw and pulsed output power and in a superior spectrum purity. The ratio of the main peak to the background luminescence determined by the detection system is higher than 2000:1 at 20 mA dc. The on-wafer cw power was measured to be 255 μW at 20 mA dc. The power popped up exceeding 1 mW for a packaged device under 25 mA dc and 9 mW under pulse 200 mA. The maximum wall-plug-efficiency of 0.67% was obtained for the packaged device at 25 mA dc. [ABSTRACT FROM AUTHOR]

We report on 269 nm emission deep ultraviolet light-emitting diodes (LEDs) over sapphire. The material quality, device design, and contact processing sequence yielded devices with external quantum efficiencies as high as 0.4% for a pumped pulse current of 200 mA and 0.32% for a dc pump current of 10 mA. For a module of two LEDs connected in series, a record continuous-wave power of 0.85 mW (at 40 mA) and a wall plug efficiency of 0.16% (at 10 mA dc) were measured. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We present a study on the time evolution of the electreluminescence (EL) spectra of AlGaN-based deep ultraviolet light-emitting diodes (LEDs) under pulsed current pumping. The EL spectra peaks at 285 nm and 330 nm are found to result from recombination involving band-to-band and free carriers to deep accepter level transitions. The 330 nm long-wavelength transitions to deep accepter levels in the p-AlGaN layer as well as the nenradiative processes significantly influence the LED internal quantum efficiency. [ABSTRACT FROM AUTHOR]

We report on AlGaN single-quantum-well light-emitting diodes (LEDs) on sapphire with peak emission at 285 nm. A study is presented to identify the key material parameters controlling the device quantum efficiency. At room temperature, for a 200 μm×200 μm square geometry mesa type device, we obtain a power as high as 0.25 mW for 650 mA pulsed pumping. The LEDs show significantly higher output powers at temperatures below 100 K. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A stochastic method of optimization of a white-light source that relies on additive color mixing of the emissions from colored light-emitting diodes (LEDs) was developed. The method allows for finding the optimal wavelengths of LEDs in order to obtain the best possible trade off between luminous efficacy and the general color rendering index (CRI) of the white source for an arbitrary number of primary LEDs. Optimal solid-state lamps composed of two, three, four, and five different LEDs were analyzed. We show that a dichromatic LED lamp can only provide high efficacy with a general CRI close to zero, whereas trichromatic and quadrichromatic lamps are able to cover the entire range of reasonable general CRI values. The optimization of quintichromatic LED lamps and lamps with a higher number of primary color LEDs yields a negligible benefit in improving CRI but provides for quasicontinuous spectra that might be required for special lighting needs. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

An ultraviolet light-emitting diode with peak emission wavelength at 340 nm is reported. The active layers of the device were comprised of quaternary AlInGaN/AlInGaN multiple quantum wells, which were deposited over sapphire substrates using a pulsed atomic-layer epitaxy process that allows precise control of the composition and thickness. A comparative study of devices over sapphire and SiC substrates was done to determine the influence of the epilayer design on the performance parameters and the role of substrate absorption. © 2001 American Institute of Physics. [DOI: 10.1063/1.1425453]. [ABSTRACT FROM AUTHOR]

Operation of InGaN multiple-quantum-well (MQW) light-emitting diodes (LEDs) with quaternary AlInGaN barriers at room and elevated temperatures is reported. The devices outperform conventional GaN/InGaN MQW LEDs, especially at high pump currents. From the measurements of quantum efficiency and total emitted power under dc and pulsed pumping, we show the emission mechanism for quaternary barrier MQWs to be predominantly linked to band-to-band transitions. This is in contrast to localized state emission observed for conventional InGaN/InGaN and GaN/InGaN LEDs. The band-to-band recombination with an increased quantum-well depth improves the high-current performance of the quaternary barrier MQW LEDs, making them attractive for high-power solid-state lighting applications. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The UV-LED controlled phase shifter of an RF signal based on a GaN-sapphire surface-acoustic-wave (SAW) filter has been implemented. At the optical wavelength 298 nm and SAW frequency 307 MHz, the UV-induced relative change in SAW velocity per unit optical power density is 2×10-6 (µW/mm2)-1 corresponding to 3.7° phase shift. The phase modulation of an RF signal by rectangular UV pulses has been demonstrated. The efficiency of the phase shifter can be considerably improved by proper selection of sheet-resistivity of the GaN layer. [ABSTRACT FROM AUTHOR]

Photoluminescence studies of carrier dynamics in AlGaN epilayers with different degrees of carrier localization and densities of nonradiative recombination centers show that the prevailing droop mechanism in AlGaN epilayers with strong carrier localization and comparatively high density of nonradiative recombination centers is enhanced nonradiative recombination due to the carrier delocalization at elevated carrier density. The photoluminescence was investigated under quasi-steady-state excitation in the temperature range from 8 to 300 K. The results proved that the onset of this droop effect is below the threshold for the droop due to high-density effects in the epilayers, such as carrier heating, phase space filling, nonradiative Auger recombination, and stimulated emission. [ABSTRACT FROM AUTHOR]

We report on quaternary AlInGaN/InGaN multiple quantum well (MQW) light emitting diode structures grown on sapphire substrates. The structures demonstrate high quality of the p-n junctions with quaternary MQW. At low forward bias (below 2 V), the temperature dependent of current-voltage characteristics are exponential with the ideality factor of 2.28, which is in a good agreement with the model of the injected carrier recombination in the space charge region. This ideality factor value is approximately three times lower than for conventional GaN/InGaN light emitting diodes (LEDs). The obtained data indicate the recombination in p-n junction space charge region to be responsible for a current transport in LED structures with quaternary quantum wells. This is in contrast to InGaN based LEDs, where carrier tunneling dominates either because of high doping of the active layer or due to the high density of localized states. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We report on fabrication and characterization of blue GaN-InGaN multi-quantum well (MQW) light-emitting diodes (LEDs) over (111) silicon substrates. Device epilayers were fabricated using unique combination of molecular beam epitaxy and low-pressure metalorganic chemical vapor deposition growth procedure in selective areas defined by openings in a SiO[sub 2] mask over the substrates. This selective area deposition procedure in principle can produce multicolor devices using a very simple fabrication procedure. The LEDs had a peak emission wavelength of 465 nm with a full width at half maximum of 40 nm. We also present the spectral emission data with the diodes operating up to 250 °C. The peak emission wavelengths are measured as a function of both dc and pulse bias current and plate temperature to estimate the thermal impedance. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Abstract: With this paper we describe an extensive analysis of the electro-optical degradation of deep ultraviolet Light-Emitting Diodes emitting at 310nm, submitted to accelerated stress test. The results obtained within this study indicate that: (i) high current stress can induce a significant decrease in the optical power emitted by the LEDs; (ii) optical power decrease is more prominent at low measuring current levels, indicating that degradation is related to an increase in the concentration of defects, with subsequent decrease in the radiative efficiency of the active layer; (iii) stress can induce a significant increase in the operating voltage of the LEDs, due to the increase in the resistivity of the ohmic contacts or p-type semiconductor; and (iv) after stress, LEDs show a significantly increased green parasitic emission. This result suggest that stress induced an increase in the defectiveness of the active layer. Information on the location of the degraded region is achieved by the analysis of the Apparent Charge Distribution curves obtained by capacitance–voltage measurements. [Copyright &y& Elsevier]

Abstract: A concept design of an ultraviolet (UV) light-emitting diode (LED) based “detect-to-warn” bioparticle sensor operating in the two modes of real-time recognition of spectral and decay-time signatures of autofluorescence of selectively excited aromatic amino acids within a specific environment of bioparticles is presented. The sensing scheme is optimized for the discrimination of Bacillus globigii spores against common airborne interferants. It contains a deep-UV LED emitting at 280nm that is harmonically modulated at a frequency of 70MHz and a dual-channel fluorescence detector with the spectral windows at 320nm and 400nm that match the characteristic peak and dip of the spore fluorescence spectrum, respectively, wherein the phase of fluorescence signal waveform simultaneously resolved in the 320-nm channel. The output parameters of the sensor are the ratio of fluorescence intensities in the two spectral windows and the phase shift of the 320-nm fluorescence waveform in respect to that of excitation. The proposed concept sensor is expected to feature an improved ability of the recognition of bioparticles in comparison with only fluorescence spectral discrimination based sensors. [Copyright &y& Elsevier]