Blue LED progress has laid the ground works of nitride technology to tackle the higher challenge of longer wavelength direct emitters of green, yellow, and orange. Use of bulk GaN substrate allows leapfrogging epitaxy development and offers crystallographic planes that allow higher crystal perfection and a control over piezoelectric polarization. Their combination allows stabilization of emission wavelength with current. Further improvement is found in substrate patterning on the micro and nano-meter length scale where we find roughly equal performance enhancement due to both, enhancement in light extraction and enhanced crystalline perfection.
Energy efficiency has been the primary driving force for solid state lighting to replace wasteful incandescent lamps by
light emitting diodes (LEDs). Recently, rising cost for rare earth metals has redoubled the push to also replace
fluorescent and compact fluorescent lighting. Phosphors in fluorescent lamps heavily rely on rare earth metals and even
first generation LEDs use such phosphors, albeit at much lower quantities. The role of phosphors to expand a narrow
wavelength source into a wider spectrum is a very lossy process in itself and can be circumvented altogether by second
generation LEDs, where the full visible solar spectrum is directly replicated by direct emitting LEDs. We here report
progress of our work towards this goal, in particular by the development of high brightness direct emitting green group-
III nitride LEDs.
In absence of piezoelectric polarization along the growth axis, a- and m-plane green GaInN light emitting diodes
manifest stable emission wavelength -- independent of the injection current density. The shift of the dominant
wavelength is less than 8 nm when varying the forward current density from 0.1 to 38 A/cm2. Furthermore, the light
emitted from the growth surface of such non-polar structures shows a very degree of linear polarization. This is
attributed to a strong valance band splitting in such anisotropically strained wurtzite GaInN quantum wells . Such light
emitting diodes show a high potential for energy efficient display applications.
The green spectral region provides a formidable challenge for energy efficient light emitting diodes. In metal organic
vapor phase epitaxy we developed GaInN/GaN quantum well material suitable for 500 - 580 nm LEDs by rigorous
defect reduction and thrive for alloy uniformity. We achieve best results in homoepitaxy on polar c-plane, and non-polar
a-plane and m-plane bulk GaN. By the choice of crystal orientation, the dipole of piezoelectric polarization in the
quantum wells can be optimized for highest diode efficiency. We report progress towards the goal of reduced efficiency
droop at longer wavelengths.
We report on the low-frequency current and light noise in 515 nm green GaInN/GaN quantum well LEDs. The current noise was the superposition of the 1/f and the generation-recombination (GR) noise. The characteristic time of the GR process was found to be proportional to the reciprocal current for the entire current range. This dependence is the characteristic for the monomolecular non-radiative recombination. The dominance of the nonradiative recombination is in agreement with the measured low external quantum efficiency (EQE) <10%. Hence, the noise measurements point out that a low EQE is caused by the low internal quantum efficiency and not by an inefficient light extraction. The noise spectra of light intensity fluctuations were close to the 1/f noise and correlated with the LED quantum efficiency and with the recombination current. Higher noise corresponded to a smaller quantum efficiency and to a higher non-radiative recombination current. The relative spectral noise densities of the light intensity fluctuations within the LED spectral line increase with the wavelength decrease. Fluctuations at different wavelengths are found to be correlated.
Increasing emission power and efficiency in green light emitting diodes is one of the big challenges towards all-solid-state lighting. The prime challenge lies in the combination of extension of wavelength from 470 nm blue to 525 nm green while maintaining the emission power level. Commonly a steep decrease in power is observed. In a broad development effort we have been able to ameliorate that decrease significantly and obtain bare die performance at 525 nm of 1.6 mW at 20 mA for 350x350 μm2 dies. Here we discuss critical die performance and wafer yield aspects of our optimization approach to the active layer of the GaInN/GaN quantum wells.
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