Technical Insight
Ionization Density; The Answer To Absorption Loss in Green Lasers?
UCSB computational scientists who investigated the light absorption by carriers in nitride lasers found that the low ionization density of the magnesium dopant atoms was a major contributor to absorption losses.
Researchers at the University of California, Santa Barbara (UCSB) have identified the high concentration of non-ionized Mg acceptor atoms in p-type GaN as the culprit for the large absorption loss in nitride lasers.
It has previously been found that in addition to Auger recombination, free-carrier absorption could play a role in the absorption of photons by electrons or holes, causing losses in nitrides. The research team, led by Professor Chris Van de Walle commented, “This is not a big concern for LEDs, but it turns out to provide an explanation for the losses that have been observed in nitride lasers. The origin of these losses has been puzzling, and they are a particular concern at green wavelengths.” The scientists have now pinpointed the source and attributed it to absorption in the Mg-doped layers.
The research team used cutting-edge first-principles calculations to study a number of different absorption processes. These processes are usually analyzed using semiclassical expressions (such as the Drude model), which rely on empirical parameters and lack the required accuracy for quantitative predictions.
Instead, the researchers implemented a completely quantum-mechanical treatment of the absorption process and a fully microscopic modeling of the scattering mechanisms [see Phys. Rev. B 81, 241201 (2010)], a feat that they say has not been achieved before, for any semiconductor.
“At first, we were looking at direct absorption processes, trying to identify which of them could be causing the absorption loss in green lasers. All of them turned out to be quite weak in the green”, commented Emmanouil Kioupakis, the lead researcher in the study. “We then turned our attention to indirect transitions, such as those mediated by phonons. Our initial estimates indicated that these processes might be important, so we went ahead with the full calculation.”
It was already known that internal absorption in nitride lasers occurs due to the overlap of the optical mode with the p-type layers, but the exact origin of the loss remained unclear. Earlier work [see Phys. Status Solidi A 192, 329 (2002)], identified Urbach tail absorption as an origin of the loss in blue-violet lasers. However, since the Urbach tail decays exponentially with wavelength it cannot be responsible for absorption in the green.
Instead, the UCSB team point to phonon-assisted optical transitions of holes from the Mg-bound states to the valence band. The large activation energy of Mg acceptor atoms means that high Mg concentrations are required to achieve adequate free-hole concentrations. Although the remaining acceptor-bound holes do not contribute to the electrical conductivity, they do participate in optical absorption, through indirect transitions, mediated primarily by the longitudinal optical (LO) phonon modes.
“Electrons and holes couple particularly strongly to the LO phonons in the group-III nitrides”, said Patrick Rinke, who also participated in the study. “We found that the calculated electron-phonon coupling strength is in excellent agreement with the Fröhlich model, an expression that is used frequently to model electron-phonon coupling. The accuracy of this model had never been explicitly checked before.”
The researchers provide parameters to model the internal loss in devices and suggest ways to minimize its impact on the device performance. This includes engineering the optical mode profile to minimize the overlap with the p-type layers, as well as employing a non-uniform Mg-doping profile. “Now that the source of the optical absorption is known, targeted strategies to suppress or circumvent it can be devised”, concluded Chris Van de Walle, Professor of Materials, who supervised this study.
Schematic illustration of a green nitride laser diode, depicting the active region containing the InGaN quantum wells and the adjacent n- and p-type GaN layers. The image of the nitride crystal demonstrates the primary internal absorption loss process: photons are absorbed by holes bound to non-ionized Mg acceptor atoms in the p-type layers. The optical transition is indirect, mediated by the emission or absorption of a phonon which assists in the overall energy and momentum conservation. Because of the high concentration of acceptor-bound holes in p-GaN, this absorption process dominates over other free-carrier processes and determines to a large extent the overall internal loss in the laser device.
Further details of this research is reported in Appl. Phys. Express 3, 082101 (2010).
