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76 Technology focus: Deep UV optoelectronics
Non-polar AlN growth
for enhancing deep UV
Researchers explore the best growth conditions, with a view to
creating the ideal platform for light-emitting diodes and laser diodes.
orth Carolina State University
(NCSU) has been developing
homo-epitaxy of non-polar
aluminium nitride (AlN) with a view
to deep ultraviolet (DUV, less than
300nm wavelength) optoelectronics
[Isaac Bryan et al, J. Appl. Phys.,
vol116, p133517, 2014].
DUV light emission has been
achieved using high-aluminiumcontent aluminium gallium nitride
(AlGaN). However, the efficiencies
are generally low due to high defect
levels and unwanted electric fields
that arise from the polarization of
charge in the III-nitride chemical
Defects arise in epitaxial material
that has a large lattice-constant
mismatch with the growth substrate.
The effect of polarization can be
reduced by choosing a crystal
orientation where the electric fields
are in the plane of the surface.
Also, the polarization field can be
increased by strain from latticemismatched hetero-epitaxy.
The NCSU research involved
homo-epitaxy to eliminate lattice
mismatching and growth on non–
polar (1100) m-plane substrates.
Figure 1. Low-temperature near-band-edge PL spectrum of a 1.2µm thick
(1100) homoepitaxial AlN film grown at 1450°C.
This is in contrast to the conventional route to DUV light-emitting
Another potential advantage of m-plane material is that
diodes grown in the (0001) c-plane direction on AlN
light extraction is expected to be higher compared with
templates on sapphire substrates.
c-plane material due to non-propagation of TM-polarized
The NCSU says of its work: “The growth of these
photons along the c-direction (normal to the c-plane).
high-quality non-polar AlN homo-epitaxial films will
provide an ideal platform for future deep-UV optoelecNCSU used AlN substrates created out of boules
grown through physical vapor transport (PVT). The distronic device structures.”
semiconductor TODAY Compounds&AdvancedSilicon • Vol. 9 • Issue 10 • December 2014/January 2015
Technology focus: Deep UV optoelectronics 77
location density of the boules was less
than 103/cm3. A diamond wire saw was
used to slice m-plane substrates from
the c-plane boule. The substrate surface was smoothed using mechanical
and chemical-mechanical polishing.
NCSU also implemented a wet
etch/ammonia anneal treatment that
the researchers have developed to
reduce the total oxygen content of the
substrate surface by more than 80%.
Atomic force microscopy (AFM) of the
substrate surface showed atomic-level
steps. The root mean square (RMS)
roughness of the substrate surface was
“consistently” below 100pm for
5µmx5µm scan areas.
Epitaxial layers of 1.2µm AlN thickness
were produced in a vertical cold-wall
metal-organic chemical vapor deposition (MOCVD) reactor. The source gases
were trimethyl-aluminium and ammonia in hydrogen carrier. The
nitrogen/aluminium ratio was 1000.
The growth temperature range and
pressure were 1250–1550°C and
Figure 2. Calibrated SIMS depth profile for O, Si, and C in a two layer
20Torr, respectively. Substrates with
1.2µm thick (11 00) homo-epitaxial AlN film with 600nm grown at
misorientation of 0.45° off [1100]
1450°C followed by 600nm grown at 1350°C.
toward [0001] crystal direction were
chosen for the epitaxial growth.
peaks from donor-bound (Si0X, 6.012eV) and free
The surface of all the epitaxial layers “appeared
smooth and featureless without cracking through opti(Γ1, 6.032eV, Γ5, 6.040eV) excitons (Figure 1).
cal differential interference contrast microscopy imagThere were also peaks from an oxygen bound exciton
ing,” according to the research team.
(O0X, 6.006eV), and two unidentified structures at
X-ray analysis gave rocking curve full-width at half
6.010eV and 6.003eV. The unidentified peaks are not
maximum (FWHM) values between 18 and 25arcsec
typically seen in (0001) c-plane epitaxial films.
for the (1010) peak along the [0001] direction.
A two-step process with growth of 600nm at 1450°C
The researchers comment: “These FWHM values are
followed by 600nm at 1350°C was used to study the
comparable to that of the substrates themselves,
impurity content through secondary-ion mass specdemonstrating that the epitaxial layers are of at least
trometry (SIMS, Figure 2). The main effect of the
the same quality as the substrates. This demonstrates
higher growth temperature was to reduce the oxygen
one advantage of using a high-quality native subcontent by more than an order of magnitude. It was
also found that the epitaxial layers had reduced silicon
The symmetry of the x-ray peaks suggested the
and carbon impurities compared with the substrate.
absence of strain between the epitaxial layer and
The researchers compared the oxygen incorporation
substrate. Interference fringes in the curves from
with 200nm (1100) m-plane and (0001) c-plane films
difference in carrier density (Pendellösung) suggested
grown at 1250°C. The m-plane material had oxygen
an abrupt change in free electron density between the
concentration of 4x1020/cm3, compared with just
substrate and epitaxial layer.
3x1017/cm2 for c-plane films.
An epitaxial layer grown below 1350°C showed rough
The researchers comment: “It is clear from this SIMS
surfaces with RMS values of 8–13nm over 5µmx5µm
analysis that (1100) AlN homo-epitaxial growth at
areas. The surfaces also showed preferential faceting
higher temperatures as compared to (0001) growth is
in the ±[0001] direction. Above 1350°C the surface
necessary for high-purity epitaxial films, as the impurity
became atomically smooth, with RMS roughness of
incorporation depends on surface morphology.” ■
less than 0.4nm.
Photoluminescence on 1450°C material showed
Author: Mike Cooke
semiconductor TODAY Compounds&AdvancedSilicon • Vol. 9 • Issue 10 • December 2014/January 2015