Scanning near-field PL spectroscopy was applied to study spatial variations of the emission spectra of AlGaN epilayers with AlN molar fractions between 0.3 and 0.7. Experiments were performed at 300 K with 100 nm spatial resolution. In general, photoluminescence spectra were found to be highly uniform with the peak energy deviation of 2 to 6 meV for different alloy compositions. In the 30% and 42% Al layers, a slightly lower Al content and a higher point defect concentration at the boundaries of growth domains were detected. These features were attributed to the higher mobility of Ga adatoms during growth. The inhomogeneous broadening beyond the random alloy distribution was found negligible for the 30% and 42% Al samples, and about 40–50 meV for the layers with a larger Al content.
We discuss factors affecting the external quantum efficiency, droop and reliability of AlGaN deep ultraviolet (DUV) light emitting diodes (LED) grown on sapphire substrates. Improvement of LED performance is achieved by suppression of the nonradiative recombination in epitaxial structures with dislocation density reduced to below 5x108 cm-2, transparent LED structure design and optimized UV encapsulation for enhanced light extraction. Relatively low light extraction efficiency remains to be a key factor limiting LED output power and quantum efficiency.
III-Nitride based deep ultraviolet (DUV) light emitting diodes (LEDs) rapidly penetrate into sensing market owing to
several advantages over traditional UV sources (i.e. mercury, xenon and deuterium lamps). Small size, a wide choice of
peak emission wavelengths, lower power consumption and reduced cost offer flexibility to system integrators. Short
emission wavelength offer advantages for gas detection and optical sensing systems based on UV induced fluorescence.
Large modulation bandwidth for these devices makes them attractive for frequency-domain spectroscopy. We will
review present status of DUV LED technology and discuss recent advances in short wavelength emitters and high power
LED lamps.
Deep UV LEDs emitting at on 280 nm with powers as high as 1 mW at 20 mA have been reported recently. These devices have mesa size of 100 μm x 100 μm to avoid current crowding due to the high Al-composition of the AlxGa1-xN buffer layers. Small mesa size results in pump current density of 200 A/cm2 for a device current of 20 mA. Small area of p-contact also leads to higher operating voltage and higher thermal impedance for the flip-chip packaged device. These factors limit the device lifetime for 50 % power reduction to only a few hundred hours. From temperature and bias dependent power degradation measurements we found the output power to decay with two characteristic time constants indicating two degradation mechanisms. The faster time constant is bias dependent and temperature independent. The slower time constant varies exponentially with junction temperature having the activation energy of 0.27 eV at 200 A/cm2 pump current density. For the devices with high thermal impedance this degradation mechanism controls the long term power degradation. To increase the device area for better reliability we used the interconnected micro-pixel device design with 10x10 array of 22 μm in diameter pixels. This design allowed for the four-fold increase of the junction area and thereby led to improved reliability performance with the operation life-time for 50 % power reduction of about 1000 hours. In this paper we will present the details of the reliability measurements and use the experimental results to determine possible degradation mechanisms.
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