The effect of hot electrons on electroluminescence of m ‐plane double heterostructure light emitting diodes (with a single 6 nm active layer of In0.20Ga0.80N) is investigated. Diodes with an electron blocking layer (Al0.15Ga0.85N) demonstrate from 3 to 5 times higher electroluminescence efficiency than those without a blocking layer. The lower electroluminescence efficiency in devices without the blocking layer is ex‐ plained in terms of electron overflow caused by ballistic and quasi‐ballistic transport of injected electrons across the InGaN active region. The same mechanism explains the decrease, observed at high current densities, of the electroluminescence efficiency (efficiency droop) in the In0.20Ga0.80N diodes with the Al0.15Ga0.85N blocking layer. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
The Feature Article by Morkoç and co‐workers () centers around the not so intuitive phenomena in two types of GaN based devices, namely InGaN based LEDs and InAlN barrier GaN heterojunction FETs. In terms of the LEDs, the paper uncovers that the quantum efficiency degradation observed at high current injection levels is not necessarily of Auger recombination origin. Furthermore, nearly similar behavior of LEDs on c‐plane and mplane suggests that the main driving force for the efficiency degradation is not polarization induced field either. The data along with their interpretation should set the stage for an accurate physics‐ based model to be developed. In terms of the FETs, the authors show that there is an optimum sheet density, which depends on drain bias or the electric field in the channel, at which the LO phonon lifetime is shortest, the velocity is highest, and the device degradation is least. The average optimum density is near 7 × 1012 cm−2 which challenges the proverbial notion that the higher the sheet density the better it is. Another outcome of this discussion is that heat dissipation takes the route of hot electrons giving off heat to LO phonons which in turn give it to LA phonons when they decay. Naturally, the shortest LO phonon lifetime is best for heat removal and thus the devices are more reliable in addition to electrons traversing at the highest velocity.
We have undertaken a series of experiments in InGaN light emitting diode (LED) structures both on polar (c‐plane) and non‐polar (m‐plane) GaN substrates with and without magnesium doped AlGaN electron blocking layers (EBLs) on the p‐side of the p‐n junction to shed the much needed light on the carrier injection and transport. The LEDs grown on c‐plane bulk GaN substrates without EBL show 40% peak electroluminescence (EL) intensity, while the EL peak intensity of the LEDs grown on m‐plane bulk GaN substrates without EBL is 30% of that with EBL. However, optical measurements for internal quantum efficiency (IQE) reveal that the IQE values of LEDs with and without EBL are comparable for both cases, which are in the range of 80‐85% for m‐plane variety and 50% for c‐plane variety. Furthermore, with varying Al composition (15%, 8%, 0%) in the EBL, the EL intensity for both m‐pane and c‐plane LEDs decrease progressively as the Al composition decreases. When highly conductive and transparent Ga doped ZnO layers are used as the current spreading layers for LEDs, the degradation of the efficiency with injection is significantly reduced to 27% compared to LEDs with semitransparent Ni/Au contacts having 50% efficiency degradation up to the same current density ∼3500 A/cm2. This can be due to presumably the reduction in current filamentation or current crowding. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
The concept of nonpolar ͑1100͒ m-plane GaN on Si substrates has been demonstrated by initiating growth on the vertical ͑111͒ sidewalls of patterned Si͑112͒ substrates using metalorganic chemical vapor deposition. The Si͑112͒ substrates were wet-etched to expose ͕111͖ planes using stripe-patterned SiN x masks oriented along the ͓110͔ direction. Only the vertical Si͑111͒ sidewalls were allowed to participate in GaN growth by masking other Si͕111͖ planes using SiO 2 , which led to m-plane GaN films. Growth initiating on the Si͑111͒ planes normal to the surface was allowed to advance laterally and also vertically toward full coalescence. InGaN double heterostructure active layers grown on these m-GaN templates on Si exhibited two times higher internal quantum efficiencies as compared to their c-plane counterparts at comparable carrier densities. These results demonstrate a promising method to obtain high-quality nonpolar m-GaN films on large area, inexpensive Si substrates.
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