Recently developed deep-UV light-emitting diodes (LEDs) are already used in prototype fluorescence sensors for detection of hazardous biological agents. However, increasing of the sensor ability of discrimination against common interferents requires further development of measurement technique. In particular, LED-based fluorescence lifetime measurements are to be considered as a technique supplementary to fluorescence spectral and excitation measurements. Here we report on application of UVTOP<sup>®</sup> series deep-UV LEDs developed by Sensor Electronic Technology, Inc. for real-time measurements of fluorescence lifetime in the frequency domain. LEDs with the wavelengths of 280 nm (targeted to protein excitation) and 340 nm (for excitation of coenzymes NADH and flavins) were used. The output of the LEDs was harmonically modulated at frequencies up to 100 MHz and fluorescence lifetime on the nanosecond and subnanosecond scale was estimated by measuring the phase angle of the fluorescence signal in respect of the LED output. Dual-wavelength LED-based phase-resolved measurement technique was tested for discrimination of <i>B. globigii</i> against a variety of interferents such as diesel fuel, paper, cotton, dust, etc. We conclude that fluorescence phase measurements have potential to improve the discrimination ability of the "detect-to-warn" optical bioparticle sensors.
Compact ultraviolet light sources are currently of high interest for applications in solid-state lighting, short-range communication, and bio-chemical detection. Our nitride-based light-emitting diode (LED) includes AlGaN quantum wells with an emission wavelength of approximately 340 nm. In this paper, we analyze internal device physics by three-dimensional (3D) numerical simulation. The simulation incorporated a 3D drift-diffusion model for the carrier transport, the quantum well (QW) energy band-structure including interface polarization charges, the local QW spontaneous emission spectrum, as well as 3D raytracing for photon extraction. The simulation results showed good agreement with measurements. Internal physical mechanisms such as current crowding, carrier leakage, and carrier recombination were investigated. Nanoscale effects exhibited a strong influence on the LED performance.
Ultra-violet light emitting diodes with a peak wavelength of 293 nm were grown by MOCVD on AlN on sapphire. The maximum output power was 15 μW at 100 mA DC current injection for on wafer, room temperature testing. We have shown that by forming an interdigitated multi-fingered n-contact compared to a square geometry LED, the series resistance is reduced by ~ 8 - 15 Ω at 100 mA. This results in a 2 - 4 V reduction in drive voltage at 100 mA. The quantum wells exhibit a sharp electroluminescence peak at 293 nm with a 9 nm full-width at half maximum, but deep level related emission was observed at 2.56, 2.80, 3.52, and 3.82 eV. The high energy peaks, 3.52 and 3.82 eV, saturate with increasing drive current while the low energy peaks, 2.56 and 2.80 eV, increase with drive current proportional to the quantum well emission. This indicates the recombination mechanism for the low energy and high energy peaks is fundamentally different. We have also shown that forward bias leakage current in these devices is another factor limiting the quantum efficiency.