Phonon And Photon Emission From Optically Excited InGaN/GaN Multiple Quantum Wells

1
Phonon And Photon Emission From Optically Excited
InGaN/GaN Multiple Quantum Wells
INTRODUCTION At room temperature, high quality
InGaN/GaN quantum wells exhibit strong
photoluminescence (PL) and at low temperature,
the quantum efficiency is close to unity.
Non-radiative processes compete with PL and
reduce the quantum efficiency. Such processes,
which include the relaxation of hot carriers and
excitons and non-radiative recombination at
defects, will therefore impact on optoelectronic
device performance. Since these processes
involve the emission of phonons, studies
involving direct phonon detection can provide
complementary information to that obtained by
other methods. The effect of well width on both
PL and phonon emission in InGaN / GaN MQWs has
been investigated in this work. The angular
dependence of phonon emission has been measured
using the technique of phonon imaging.
PL QUANTUM EFFICIENCY The total amount of phonon
energy, Eph, emitted by hot carriers and excitons
is Eph?E (1-?) EPL where ?E EO - EPL, and
the second term describes the energy released as
a result of non-radiative recombination. Assuming
that ? 1 in the 1.25nm and 2.5nm wells, and ?
0.6 in the 5nm MQW (the measured PL was 60 that
in the 1.25nm and 2.5nm wells), the expected
ratio of phonon intensities is 1 1.3 6.3 for
the 1.25nm, 2.5nm and 5nm wells. Experimentally,
the phonon signal ratios is 1 1.4 5.6, which
is in very good agreement with the predicted
values.
Well Width 5 nm
FIGURE 1 The PL (red dotted curves) and
bolometer signals (blue solid lines) for the
three MQWs, with the PL peak normalized to
unity. The first peak in the bolometer signals
is the PL, while the second is due to acoustic
(TA) phonons, as determined from the arrival
time (in sapphire, vTA 6000 ms-1 ). The phonon
peak appears at slightly different times due to
different substrate thicknesses in the samples.
ANGULAR DISTRIBUTION OF EMITTED PHONONS In the
1.25nm MQW (Fig. 2A), phonon emission occurs
mainly perpendicular to the plane of the wells.
This is a result of the relative magnitudes of
the in-plane, qparallel, and out of plane,
qperpendicular, components of the phonon
wavevector. The maximum qparallel 2kF and
qperpendicular ?a-1, where a is the MQW width.
In the 1.25nm MQW, qperpendicular is greater
than qparallel, and so emission is mainly
perpendicular to the plane of the MQW. As the
well width increases, qperpendicular decreases
and the anisotropy in phonon emission should be
less pronounced. This is indeed the case, as
shown in Fig. 2B, where the image is similar to
that of blank sapphire.
THE SAMPLES The samples used were grown by MOCVD
on sapphire substrates. The MQW structure
consisted of 10 InXGa1-XN (x0.13) quantum wells
separated by 7.5nm GaN spacers. Samples with
well widths 1.25nm, 2.5nm and 5nm were studied.
Phonon detectors (40mm x 40mm) were evaporated
onto the polished back surface of the substrates.
FIGURE 2A
FIGURE 2C
FIGURE 2B


THE EXPERIMENTS The experiments were performed in
a pumped liquid helium cryostat, with the samples
held at TO ? 2K, at the superconducting
transition temperature of the aluminium
bolometer. A frequency tripled NdYAG laser
(?355nm, 10ns pulse), focussed to a 50?m spot,
was used to excite the samples. For
characterisation, the PL was detected using a
single grating monochromator and a fast UV
photodiode. Non-equilibrium phonons emitted as a
result of hot carrier relaxation and
non-radiative recombination propagate
ballistically through the substrate, and are
detected by noting the change in resistance of
the bolometer. Since the bolometer is also
sensitive to photons, the pure phonon signal is
extracted by subtracting the contribution from PL
measured with the photodiode. Phonon imaging
allows the angular dependence of phonon emission
to be studied. The laser is raster scanned
across the sample, and by plotting the signal
intensity at each position in the scan, an image
of the emitted phonons is obtained.


CONCLUSIONS Comparison of the predicted and
measured ratios of phonon intensities shows that
in the 1.25nm and 2.5nm MQWs, the quantum
efficiency is close to unity. Phonon emission is
mainly due to hot carrier and exciton relaxation.
In the wider (5nm) well, non-radiative
recombination also contributes to phonon
emission. The reason for the decrease of ? in
the wide quantum well could be due to the strong
spontaneous polarization in this material band
tilting occurs resulting in confinement of the
electron-hole wavefunctions near the well edges,
which reduces the wavefunction overlap as the
well width increases. That the angular
dependence of phonon emission depends on the well
width is in good agreement with estimations based
on the theory of electron-phonon interactions in
2D semiconductor nanostructures.
FIGURE 2 The phonon images obtained with Fig.
2A) 1.25nm MQW Fig. 2B) 2.5nm MQW and Fig. 2C)
Blank sapphire substrate. In the case of the
sapphire substrate, phonons were generated by
optical excitation of a thin metal film,
resulting in an isotropic distribution of
phonons. The excitation power was the same in
all images. The differences seen in Figs. 2A and
2B show that phonon emission in the 1.25nm and
2.5nm MQW samples is anisotropic.
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Sugahara and S. Sakai, Appl. Phys. Lett. 74 3128
(1999) 2 J.A. Davidson, P. Dawson, T. Wang, T.
Sugahara, J.W. Orton and S. Sakai, Semi. Sci.
Tech. 15 497 (2000) 3 A.J. Kent in Hot
Electrons in SemiconductorsPhysics and Devices
ed. N. Balkan(Clarendon Press, Oxford) (1998)