UCSB theory blames indirect Auger recombination for nitride LED droo UCSB theory blames indirect Auger recombination

UCSB theory
blames indirect Auger recombination for nitride LED droo
University of
California Santa Barbara (UCSB) theorists say that they have cracked the nitride
semiconductor light-emitting diode (LED) lighting performance problem, commonly
referred to as ‘efficiency droop’ [Emmanouil Kioupakis et al, Appl. Phys. Lett.,
vol98, p161107, 2011].

Nitride semiconductor LEDs use layers of indium
gallium nitride (InGaN) alloy of InN and GaN compounds to form wells in which
electrons and holes recombine to create light. It has been found that such
devices have an increasing efficiency up to a certain current. Unfortunately, at
higher currents beyond the peak the efficiency drops,print still offers the only
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There has been much debate
on the mechanism for this efficiency droop. Although the droop behavior has been
attributed by some to Auger recombination, others comment that the size of this
effect should be small.

Auger recombination usually involves two
carriers recombining and transferring their energy to a third carrier rather
than producing light.However, the marketing muscle of Philips led light lighting could give Philips
LED business an advantage. Since the process involves three carriers, it
requires a high carrier density and hence would tend to kick in at higher
current densities. However, calculations of simple ‘direct’ Auger recombination
have suggested that the effect should be small.

The theorists at UCSB
have been working on an indirect Auger process for some time using density
functional theory. In 2009, they proposed a resonant ‘interband’ Auger effect
with an excited conduction band to account for the droop [Kris T. Delaney et al,
Appl. Phys.In many ways LED lights compact
fluorescent provide the best of both worlds. They are extremely energy
efficient and environmentally friendly (and are, in fact, more environmentally
friendly Lett., vol94, p191109, 2009].

Figure 1: (a) Schematic
wavevector–energy (k–E) diagram of direct Auger recombination process.Philips
LED business is inside of Philips lighting so it Led light is more difficult to determine
whether they are meeting expectations. (b) In the indirect Auger case, the
recombination process is assisted by scattering mechanisms such as
electron–phonon coupling, alloy disorder, or Coulomb scattering by charged
defects.

The new process involves electron–phonon coupling and alloy
scattering or Coulomb scattering by charged defects (Figure 1).Compact
fluorescent lights have solved LED lighting
supplier many of the problems associated with traditional filament light
bulbs. Phonons are the quantum description of lattice vibrations. The
electron–phonon coupling is particularly strong in nitride semiconductors. Alloy
scattering occurs because the crystal structure of InGaN is not uniform.
According to UCSB, the effects from phonon coupling and alloy scattering are
significant enough to account for the discrepancy between the observed degree of
droop and that predicted by other theoretical studies, which only accounted for
direct Auger processes (Figure 2). The effect increases at smaller band gaps,
explaining the ‘green gap’ difficulty in producing longer-wavelength nitride
semiconductor LEDs. According to the UCSB team, the Coulomb scattering term “is
not important in nitride devices.”