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\begin{document}

\title{BAlGaN light emitting diode emitting at 350 nm}



\author[1]{Peter  Milner}%
\author[1]{Vitaly Zubialevich}%
\author[1]{Thomas O'Connor}%
\author[1]{Sandeep Singh}%
\author[1]{Davinder Singh}%
\author[1]{Brian Corbett}%
\author[1]{Peter Parbrook}%
\affil[1]{Tyndall National Institute}%


\vspace{-1em}



  
  \date{May 23, 2024}


\begingroup
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\selectlanguage{english}
\begin{abstract}
In this work, we report the first demonstration of an ultraviolet
light-emitting diode (LED) with boron-containing multiple quantum wells.
Electroluminescence emission from the BAlGaN LED was observed at 350 nm,
with higher intensity compared to the AlGaN reference LED. A higher
operating voltage compared to the reference LED was also observed which
may be attributable to a nanomasking behaviour of boron in (Al)GaN
alloys.%
\end{abstract}%



\sloppy


BAlGaN light emitting diode emitting at 350 nm

P. Milner\textsuperscript{1,2}, V.Z. Zubialevich\textsuperscript{1}, T.
O'Connor\textsuperscript{1,2}, S.M. Singh\textsuperscript{1,2}, D.
Singh\textsuperscript{1}, B. Corbett\textsuperscript{1} and P.J.
Parbrook\textsuperscript{1, 2}

\textsuperscript{1} \emph{Tyndall National Institute, University College
Cork, Lee Maltings, Dyke Parade, Cork, T12 R5CP, Ireland}

\textsuperscript{2} \emph{Department of Electrical and Electronic
Engineering, University College Cork, Cork, T12 K8AF, Ireland}

Email: peter.milner@tyndall.ie, p.parbrook@ucc.ie

In this work, we report the first demonstration of an ultraviolet
light-emitting diode (LED) with boron-containing multiple quantum wells.
Electroluminescence emission from the BAlGaN LED was observed at 350 nm,
with higher intensity compared to the AlGaN reference LED. A higher
operating voltage compared to the reference LED was also observed which
may be attributable to a nanomasking behaviour of boron in (Al)GaN
alloys.

\emph{Introduction:} III-Nitride ultraviolet (UV) light sources are in
increasing demand to replace mercury-based lamps owing to potentially
higher efficiencies, smaller footprints, and tunable wavelengths
{[}1{]}. While significant developments have been made over the past two
decades, the performance of AlGaN-based light-emitting diodes (LEDs)
still lags behind visible (In)GaN-based devices {[}2, 3{]}. Some of the
major challenges are the crystalline quality of initial
substrates/templates, light absorption within the LED heterostructure
(due to \emph{p} -GaN contact layers), lack of reflective contacts, and
large intrinsic polarisation fields across the active region {[}4{]}.
These fields originate from a combination of the ionicity of the bonds
in the wurtzite lattice and the lattice mismatch in the crystal, which
result in a separation of the electron and hole wavefunctions in the
quantum wells (QWs), along with a redshift in the emission spectrum.
This effect is more commonly known as the quantum confined Stark effect
(QCSE) and is detrimental to the internal quantum efficiency (IQE) of
the device.

Wurtzite boron nitride is an ultra-wide indirect bandgap material with a
bandgap of \textasciitilde{}6.7 eV, while the bandgap estimate for a
direct transition is \textasciitilde{}13 eV {[}5, 6{]}. The relatively
small BN lattice constant of \emph{a} [?] 2.5 \selectlanguage{ngerman}Å allows for the possibility
of lattice-matching between well and barrier/buffer material {[}7{]}.
Therefore, the addition of small amounts of BN to the (Al)GaN QWs has
the potential to negate the polarisation field across the well and
reduce the QCSE, thereby increasing the IQE. However, boron-containing
QWs have to overcome many practical challenges in order to demonstrate
experimentally their potential. In this Letter, we report, to our
knowledge, the first UV LED with B-containing QWs.

\emph{Growth and fabrication:} A BAlGaN MQW UVA LED was grown by
metalorganic chemical vapor deposition (MOCVD) in an AIXTRON 3×2'' close
coupled showerhead reactor. A 2-inch \emph{c} -plane sapphire wafer with
25 nm sputtered AlN (Kyma Technologies) was used as the substrate, onto
which a 200 nm AlN connecting layer was grown. This was followed by 400
nm \emph{u} -Al\textsubscript{0.35}Ga\textsubscript{0.65}N, 1.8 \selectlanguage{greek}μ\selectlanguage{english}m
Al\textsubscript{0.350.22}Ga\textsubscript{0.650.78}N (upper 0.9
\selectlanguage{greek}μ\selectlanguage{english}m\emph{n} -doped) graded composition layer, five QW/QB
B\emph{\textsubscript{x}}
(Al\textsubscript{0.06}Ga\textsubscript{0.94})\textsubscript{1-\emph{x}}
N (3.5 nm)/Al\textsubscript{0.16}Ga\textsubscript{0.84}N (10 nm), 30 nm
Al\textsubscript{0.35}Ga\textsubscript{0.65}N electron blocking layer
(top 20 nm \emph{p} -type doped), 50 nm\emph{p}
-Al\textsubscript{0.350.00}Ga\textsubscript{0.651.00}N, and 10 nm
\emph{p} -GaN contact layer. The growth temperature of the active region
was 1160\selectlanguage{ngerman}°C. Trimethylgallium, trimethylaluminum, triethylboron (TEB),
and ammonia were used as the main precursors. The TEB/III ratio for the
QW was 1\%. Disilane and bis-cyclopentadienyl magnesium were used as the
\emph{n} - and \emph{p} -type dopants, respectively. A reference LED was
also grown, with the exact same structure as above, but with no TEB
flowing during QW growth.

After growth, both samples were processed into individual LEDs. First, a
surface clean consisting of buffered oxide etchant, HCl:DI 1:1 and 45\%
KOH at 100°C was carried out to prepare the \emph{p} -GaN surface for
processing. Pd was evaporated as the \emph{p} -metal, followed by the
formation of 100 \selectlanguage{greek}μ\selectlanguage{english}m diameter circular mesas by inductively-coupled
plasma etching with a Cl\textsubscript{2}-based plasma. Finally,
Ti/Al/Ti/Au was evaporated to form the \emph{n} -metal.

\emph{Effects of boron incorporation:} Test MQW stacks consisting of
B\emph{\textsubscript{x}}
(Al\textsubscript{0.05}Ga\textsubscript{0.95})\textsubscript{1-\emph{x}}
N/Al\textsubscript{0.16}Ga\textsubscript{0.84}N with different TEB/III
ratios were grown by MOCVD, and a transmission electron microscopy (TEM)
image revealed the presence of nanopipe-shaped voids in the active
region for a low TEB/III ratio (\textless{} 1\%). This effect has been
attributed to boron surface segregation during growth {[}8{]}. It is
proposed that boron acts as a surfactant, reducing the surface energy of
the semipolar and nonpolar planes to below that of polar \emph{c}
-plane. In between the nanopipes, the QWs are undisturbed and show high
uniformity. For the sample prepared with a higher TEB/III ratio (3\%),
the QWs disappear completely due to a high concentration of voids.
Hence, for this LED experiment the TEB/III was set to 1\%, using a QW
growth temperature of 1160\selectlanguage{ngerman}°C.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image1/image1}

\end{center}
\end{figure}

\textbf{Fig 1} \emph{SEM image from FIB cut of BAlGaN LED material.
Voids can be seen in the active region of the LED (some of them are
highlighted by the red arrows), showing an effect of boron.}

Fig 2 \emph{Room temperature PL of the LEDs. The BAlGaN LED shows a
reduction in intensity compared to the AlGaN reference LED.}

Figure 1 shows a scanning electron microscopy (SEM) image of a focused
ion beam (FIB) cross-section through the BAlGaN LED. The `jagged' Pt
interface is due to initial e-beam Pt evaporation and surface roughness.
Voids can be seen in the active region of the LED, confirming the same
mechanism of boron incorporation as observed in the BAlGaN test MQW
sample preparation {[}8{]}. The AlGaN reference LED showed no such
disturbance of the active region (not shown). These voids could explain
the reduced intensity of the BAlGaN LED in the photoluminescence (PL) as
seen in Figure 2. For these measurements, a frequency-doubled continuous
wave Ar-ion 244 nm laser was used as the excitation source, and
measurements were carried out at room temperature. The AlGaN LED MQW
emission has a peak at around 344 nm, and there is a slight redshift of
2 nm in the BAlGaN LED. This is likely due to the significant bowing
parameter of BGaN and BAlN alloys {[}5, 6{]}. The peak at 314 nm is
attributed to emission from the buffer layer, while the broad peak
centered around 400 nm is due to the \emph{p} -type region. The PL shows
a lower QW peak intensity with the addition of B. It is possible that
the area of the active region capable of photoexcitation has been
reduced because of these voids; however, more work is required to
understand their impact on PL performance.

\emph{LED characterization and discussion:} Figure 3\emph{a} shows the
IV characteristics of the LEDs. A noticeable increase in operating
voltage upon addition of B in the QWs is clear. Inset is the IV on a
semi-log scale, in which the BAlGaN LED demonstrates a higher turn on
voltage. One possibility is that this phenomenon is due to the voids
providing a more restricted path to carriers.

In the electroluminescence (EL) shown in Figure 3\emph{b} , contrary to
the PL, the BAlGaN LED shows an integral intensity almost 4 times higher
than that of the AlGaN reference LED. This was consistent over multiple
devices.

Fig 3 \emph{IV characteristics of the BAlGaN QW LED and AlGaN QW
reference LED (a); their EL spectra measured at injection currents of 30
mA; and LI characteristics (c) (Inset shows the relative efficiencies in
arbitrary units versus current).}

The MQW peak from the BAlGaN LED (350 nm) shows a slight redshift
compared the AlGaN LED (347 nm), similar to the PL. The exact reasons
for this improvement in performance of the BAlGaN LED under electrical
injection are not clear, but improved localization of carriers due to
the presence of boron, light scattering from voids, as well as higher
current density in the BAlGaN LED are all possible contributing factors.
This latter point is worth further discussion: under electrical
injection, the voids (assumed to be formed because of segregated BN
{[}8{]}) likely act as insulating regions, forcing the carriers into the
undisturbed areas of the active region. This has the effect of
increasing the effective current density in the active region, resulting
in an increase in operating voltage compared to the AlGaN reference but
also a more favourable ratio between radiative recombination and
Shockley-Reed-Hall non-radiative recombination {[}9{]}, which could
explain the higher EL intensity from the BAlGaN LED. This is supported
by the LI characteristic in Figure 3\emph{c} . At lower currents, the
BAlGaN LED has a higher intensity, however it then shows an increased
droop in output power at significantly lower injection currents than the
AlGaN reference LED. This droop is likely due to increased Joule heating
in the device. In addition, this droop could be related to non-radiative
Auger recombination beginning to dominate with higher current density
(\emph{C} term in the `\emph{ABC} ' recombination model proportional
to\(n^{3}\)) {[}10{]}.

A relative wall plug efficiency (WPE) of the LEDs is plotted against
current in the inset of Figure 3\emph{c} . While the boron-containing
LED has a higher operating voltage, it has a peak efficiency value
approximately 3 times higher than that of the AlGaN reference LED. This
suggests that the IQE of the LED has been improved with the addition of
small amounts of BN, although the potential effects of light scattering
on extraction efficiency must be considered.

\emph{Conclusion:} We report the first demonstration of a UV LED
containing BAlGaN QWs. EL shows QW emission at around 350 nm with
diode-like IV behaviour. SEM images revealed the presence of voids in
the active region of this LED, as seen in previous test MQW samples.
While there are other potential effects of these voids, we postulate
that they act as insulating regions, forcing the current into the
undisturbed areas of the active region. This ultimately increases the
current density, resulting in higher EL intensity compared to the AlGaN
reference LED for the same drive current. The observed increase in the
operating voltage would also tend to support this theory. LI
characteristic shows a drop in output power at lower currents in the
BAlGaN LED, however, a relative WPE plot shows that it is around 3 times
more efficient, indicating potential beneficial effect of BN in the QWs.
Further work is required to better understand the mechanism of these
voids in the active region and how to control them with growth
conditions. This work demonstrates successful operation of a
boron-containing LED, although the question of benefits in terms of
mitigating QCSE in III-Nitride based UV LEDs -- or even boron
incorporation in the undisturbed parts of BAlGaN QWs remains open.

\emph{Acknowledgments:} The authors gratefully acknowledge the funding
from Science Foundation Ireland (SFI) under grant no. 12/RC/2276\_P2,
from Frontiers for the Future Award (SFI-21/FFP-A/9014), as well as a
joint award with the Engineering and Physical Sciences Research
Council-Centre Doctoral Training (EPSRC-CDT) (grant no.
SFI-18/EPSRC-CDT/3585-PIADS).

? 2021 The Authors. \emph{Electronics Letters} published by John Wiley
\& Sons Ltd on behalf of The Institution of Engineering and Technology

This is an open access article under the terms of the Creative Commons
Attribution License, which permits use, distribution and reproduction in
any medium, provided the original work is properly cited.

\emph{Received:} xx January 2021 \emph{Accepted:} xx March 2021

doi: 10.1049/ell2.10001

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{[}dataset{]} P. Milner, V.Z. Zubialevich, T. O'Connor, S.M. Singh, D.
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https://doi.org/10.5281/zenodo.8239500

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