conference report nitrides
ICNS-9 was held on 10-15 July at the Scottish Exhibition and Conference
Centre, Glasgow.
model carrier and photon populations in the laser diodes. He then went on to explain that this work employs the well-known ABC model for determining the evolution of charge carriers – in this widely used model, which is defined in terms of the carrier density n, the carrier recombination rate is described as the sum of three terms: the Shockley-Reed-Hall non-radiative recombination rate, An; the radiative recombination rate, Bn2 Cn3
; and the Auger non-radiative recombination rate, .
According to Scheibenzuber, one advantage of working with lasers, rather than LEDs, is that it is possible to determine the injection efficiency of the device using optical gain spectroscopy. After extracting this value – in this case it was 68 percent – it is possible to separate carrier leakage from the recombination rate.
The European collaboration’s next step was to characterise their laser under very low driving currents and obtain a value for the A coefficient of 4.2 x 107
s-1 .
They then studied the dynamics of the laser, such as relaxation oscillations and turn-on delays, and were finally able to extract values for the B and C coefficients of 3 x 10-12
cm6 s-1 and 4.5 x 10-31 cm6 s-1 .
Scheibenzuber concluded his talk by pointing out that the value obtained for the C coefficient agrees with the value calculated by Chis van de Walle and co-workers from the University of California, Santa Barbara (UCSB). These West-coast theorists believe that the forms of Auger recombination that dominate LED droop involve phonons and alloy disorder.
Further support for Auger recombination as the primary cause of LED droop came from a presentation by Ted
Thrush from the University of Cambridge. He presented electroluminescence intensity plots for a commercial LED driven at current densities from 0.0001 A cm-2
to 100 A cm-2 from 77K to 385K.
Thrush and his colleagues have characterized this LED with a transmission electron microscope: Its threading dislocation density is 1.4 x 109
cm-2
features five wells with thicknesses of 3.2 nm, sandwiched between 4.8 nm-thick barriers.
The ABC model has been used to provide a good fit to the electroluminescence intensity plots, using the same B and C coefficients at all temperatures. Thrush said that consistency of the C coefficient over this temperature range indicated that the droop mechanism was not due to traps, leakage or a direct Auger process. He argued, however, that it was consistent with an impurity- or phonon-mediated Auger process, as suggested by the theoretical work of the UCSB group.
Auger recombination was also blamed as the major culprit behind LED droop in a paper given by Dmitry Zakheim from Ioffe Physico-Technical Institute, Russia, who has been working with researchers from Epi-center and STR-Group. Through a combination of theory and experiment, this partnership from St Petersburg has shown that LED efficiency can be increased by switching from a conventional active region to one based on a short-period superlattice.
Zakheim and his co-workers have used the STR Software SiLENSe 5.0 to model electron and hole distributions in a conventional LED featuring five, 3 nm- thick quantum wells sandwiched between 10 nm-thick barriers. This model – which includes drift and diffusion effects and can account for carrier delocalisation in the active region – revealed that the holes are not uniformly distributed through the active region, but predominantly loacted in the well nearest the p-type region (see Figure 1). According to Zakheim, this high degree of carrier localisation leads to high Auger recombination losses, and ultimately LEDs that suffer from significant droop.
Figure 1. A partnership between Ioffe Physico-Technical Institute, Epi-Center and STR-Group has modelled electron and hole distributions in two types of LED: (a) a device with a conventional active region, containing five, 3 nm- thick quantum wells sandwiched between 10 nm-thick barriers (b) a device with a short-period superlattice active region comprising 2.5 nm-thick wells and barriers.
16
www.compoundsemiconductor.net August / September 2011
Modelling indicates that a far more uniform hole distribution is possible with a superlattice active region comprising 2.5 nm-thick wells and barriers. Such a structure is far better at combating droop: A standard flip-chip LED with a p-type contact formed from ITO and silver had an efficiency at 1 A that was 52 percent of its peak value; in comparison, a similar device with a superlattice active region delivered 76 percent of its peak output at 1 A.
It is also possible to suppress droop by improving the capability of the electron-blocking layer. According to
at temperatures ranging
and its active region
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