The deep-center-assisted tunneling of carriers in
p–n
structures of light-emitting diodes (LEDs) with InGaN/GaN quantum wells (QWs) makes smaller the effective height of the injection barrier, but ...leads to a dependence of the radiation efficiency on the density and energy spectrum of defects in GaN. In the case of hopping conduction across the space charge region, the forward voltage mainly drops near the QW boundary, where the density of deep states at the quasi Fermi-level is the lowest. As a result, band bending at the boundary decreases, and, with increasing current, the direction of the electric field also changes, which leads to a weaker confinement of holes, to their non-radiative recombination in the
n
barrier, and to an efficiency droop. The low efficiency of green GaN LEDs is associated with the dominance of deep centers and insufficient density of shallow centers in the energy spectrum of defects in barrier layers near the boundaries with the QW. The proposed model is confirmed by the stepwise experimental dependences of the current, capacitance and efficiency of green and blue LEDs in the case of forward bias, which reflect the contribution of color centers responsible for the defect photoluminescence bands in GaN.
Gaussian impurity bands in GaN responsible for intracenter optical absorption and photoluminescence, give rise to defect-assisted carrier tunneling (hopping) through the barriers in the pn ...nanostructures with InGaN/GaN quantum wells (QWs). The tunneling injection of majority cariers into the QW results in the current humps and in rapid increase in the radiative recombination efficiency at low enough forward bias. As the bias increases, the carrier confinement in the QW weakens, leading to tunneling injection of minority cariers into the barriers, which results in the emission efficiency saturation and droop.
The current dependences of the spectral noise density and quantum efficiency in green and blue light-emitting diodes with InGaN/GaN quantum wells (QWs) are measured. It is shown that the noise level ...greatly increases at high currents at which there is a quantum efficiency droop. The mechanism by which the current noise is formed is associated with hopping transport via the deep states of color centers in GaN across the
n
barrier of an InGaN/GaN QW. The source of the noise is the hopping resistance of the space-charge region, which limits the current of thermally activated electrons into the QW. The efficiency droop and the increase in noise level are attributed to a change in the electric-field direction near the QW at high injection levels and to an increase in the tunneling leakage of holes from the QW. It is shown that the experimental frequency-related noise spectra having the shape of a Lorentzian spectrum at the working currents are related to the frequency of hopping between deep centers near the InGaN/GaN QW and to Maxwell relaxation in the space-charge region.
A free-standing bulk gallium nitride layer with a thickness of 365 μm and a diameter of 50 mm was obtained by hydride vapor phase epitaxy on a sapphire substrate with a carbon buffer layer. The ...carbon buffer layer was deposited by thermal decomposition of methane in situ in the same process with the growth of a bulk GaN layer. The bulk GaN layer grown on the carbon buffer layer self-separated from the sapphire substrate during the cooling after the growth. The dislocation density was 8 · 106 cm−2. The (0002) X-Ray rocking curve full width at half maximum was 164 arcsec.
The phenomenon of the emission efficiency droop of InGaN/GaN quantum wells (QWs) in light-emitting diode
p
-
n
structures is studied. The influence exerted by two basic processes on the emission ...efficiency is considered: tunnel injection into a QW and incomplete lateral carrier localization in compositional fluctuations of the band-gap width in InGaN. The sharp efficiency peak at low currents and the rapid efficiency droop with increasing current are due to tunneling leakage currents along extended defects, which appear as a result of a local increase in the electron hopping conductivity via the depletion
n
region and a corresponding local decrease in the height of the injection
p
barrier. A less sharp efficiency peak and a weak, nearly linear, decrease in efficiency with increasing current are caused by incomplete lateral carrier localization in the QW due to slowing-down of the carrier energy-relaxation rate and to the nonradiative recombination of mobile carriers.
The intense absorption of CO
2
laser radiation in sapphire is used to separate GaN films from GaN templates on sapphire. Scanning of the sapphire substrate by the laser leads to the thermal ...dissociation of GaN at the GaN/sapphire interface and to the detachment of GaN films from the sapphire. The threshold density of the laser energy at which
n
-GaN started to dissociate is 1.6 ± 0.5 J/cm
2
. The mechanical-stress distribution and the surface morphology of GaN films and sapphire substrates before and after laser lift-off are studied by Raman spectroscopy, atomic-force microscopy, and scanning electron microscopy. A vertical Schottky diode with a forward current density of 100 A/cm
2
at a voltage of 2 V and a maximum reverse voltage of 150 V is fabricated on the basis of a 9-μm-thick detached
n
-GaN film.
It is shown that the emission efficiency and the 1/
f
noise level in light-emitting diodes with InGaN/GaN quantum wells correlate with how the differential resistance of a diode varies with ...increasing current. Analysis of the results shows that hopping transport via defect states across the
n
-type part of the space-charge region results in limitation of the current by the tunneling resistance at intermediate currents and shunting of the
n
-type barrier at high currents. The increase in the average number of tunneling electrons suppresses the 1/
f
current noise at intermediate currents. The strong growth in the density of current noise at high currents,
S
J
∝
J
3
, is attributed to a decrease in the average number of tunneling electrons as the
n
-type barrier decreases in height and width with increasing forward bias. The tunneling-recombination leakage current along extended defects grows faster than the tunneling injection current, which leads to emission efficiency droop.
PDF
