Optical absorption loss processes in semiconductor lasers.
A laser video projector operates by modulating three laser beams (red, green, and blue) to project an image without the need for a screen or optical lens. Semiconductor lasers, already very successful in optical storage applications, are an ideal solution for small and portable laser projectors. Although red and blue semiconductor lasers are widely available, green ones have only recently started to emerge. However, large internal losses limit their output power. This loss is primarily due to the overlap of the optical mode with the p-type layers of the device, but its exact origin is unclear. With this question in mind, we developed a computationally-tractable first-principles formalism to study free-carrier absorption in nitride materials and assess whether it could be the microscopic mechanism responsible for the loss. Our results show that the culprit for the optical loss is absorption by the high concentration of holes bound to non-ionized acceptor atoms (in this case Mg) in p-GaN. Although these bound holes do not contribute to the conductivity, they introduce empty states just above the valence band edge and enable optical absorption processes for every wavelength in the visible. These optical transitions are indirect, mediated by the electron-phonon-interaction which is particularly strong in the nitrides. Based on this finding, we proposed ways to reduce the absorption loss and increase the laser efficiency and power output.
E. Kioupakis, P. Rinke, and C. G. Van de Walle, Determination of internal loss in nitride lasers from first principles, Appl. Phys. Express 3 (2010) 082101
E. Kioupakis, P. Rinke, A. Schleiffe, F. Bechstedt, and C. G. Van de Walle, Free-carrier absorption in nitrides from first principles, Phys. Rev. B 81 (2010) 241201(R)
Auger recombination and efficiency droop in nitride light emitters.
Light-emitting diodes (LEDs) are efficient, non-toxic, and long-lasting white light sources that can replace incandescent and fluorescent light bulbs for general lighting applications. However, existing nitride-based white LEDs show a dramatic efficiency decrease when operated at the high powers required for general lighting, the origin of which is a matter of debate. Using a first-principles computational formalism that we developed, we found that the efficiency loss comes from Auger recombination, a three-carrier non-radiative recombination mechanism that dominates over the radiative process at high carrier densities. Contrary to other optoelectronic semiconductors, Auger recombination in the nitrides occurs via indirect processes, mediated by electron-phonon coupling and alloy scattering. Identifying the primary loss mechanism suggests device designs that reduce its impact and is the first step towards the engineering of efficient high-power LEDs.
In addition, we have investigated the interplay of polarization fields and nonradiative Auger recombination on the droop and green-gap problems. An important conclusion is that device designs that minimize the polarization fields lead to higher efficiency, not because the internal quantum efficiency is improved at a given carrier density but because they can be operated at a lower carrier density for a given current density.
In addition, investigations into the fundamentals of Auger recombination have continued, including studies of the influence of temperature, carrier density and quantum-well confinement.
"Researchers say they've solved the mystery of LED lighting "Droop"", May 2011, Department of Energy blog
"LED lighting comes out of the dark", April 2011, NERSC website
"UCSB theory blames indirect Auger recombination for nitride LED droop", April 2011, Semiconductor Today
"Theorists claim to have solved LED droop puzzle", April 2011, Compound Semiconductor
"LED efficiency puzzle reportedly solved", April 2011, Photonics.com
"Theorists crack LED lighting performace problem", April 2011, UCSB Engineering
"LED efficiency puzzle solved by UC Santa Barbara theorists", April 2011, UCSB Center for Energy Efficient Materials
"LED efficiency puzzle solved by UC Santa Barbara theorists", April 2011, UCSB Institute for Energy Efficiency
Auger recombination in III-V semiconductors
In addition to III-nitrides, we are studying Auger recombination in traditional III-V materials. These materials are used in long-wavelength light emitters in the red and infrared. Also, studying these materials provides insight into the basic mechanisms of Auger recombination in materials, such as the relative importance of the direct and indirect process, as well as the phonon modes that mediate indirect Auger. We have performed calculations for GaAs. Surprisingly, even though GaAs has a smaller gap and a significantly different band structure compared to nitrides, the phonon-assisted indirect recombination is again found to be an important or even dominant contribution to the recombination rate; the direct (purely Coulomb-mediated) recombination is not sufficient to explain experimental Auger coefficients.
D. Steiauf, E. Kioupakis, and C.G. Van de Walle, 1, 643 (2014).
Loss due to Shockley-Read-Hall recombination at defects
We are exploring the microscopic origin of Shockley-Read-Hall (SRH) recombination in nitride light emitters, which limit the maximum internal quantum efficiency of the devices. A SRH recombination process consists of two carrier capture events: electron capture and hole capture. Thus, the study of carrier capture processes is a prerequisite to the study of SRH recombination. We have developed a practical first-principles methodology to calculate carrier capture coefficients based on parameters from hybrid functional calculations. These have been benchmarked with available experimental data for selected defects in GaN and ZnO, and have demonstrated excellent agreement.
A. Alkauskus, Q. Yan, and C. G. Van de Walle, Phys. Rev. B 90, 075202 (2014). [Editor's suggestion]
Photoluminescence lineshapes of defects
Though nonradiative transitions result in loss from Shockley-Read-Hall recombination, the experimental study of radiative transitions provides a lot of useful information about defect geometries, vibrational properties, etc. However it is often difficult to determine the chemical nature of defects responsible for a given experimental observation. Therefore, the ability to determine the luminescence properties of defects from first principles is a powerful tool for defect identification. We have developed a first-principles methodology to calculate luminescence lineshapes for defects with very strong electron-phonon coupling and extended it to treat the more complicated situation of defects with moderate electron-phonon coupling, which have lineshapes with more structure, and often many contributing phonon frequencies. We have benchmarked these techniques and now are using them to explore various defects in nitride materials and beyond.
A. Alkauskas, J. Lyons, D. Steiauf, and C. Van de Walle, Phys. Rev. Lett., 109, 267401 (2012).
A. Alkauskas, B. B. Buckley, D. D. Awschalom, and C. G. Van de Walle, New J. Phys. 16, 073026 (2014).
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