High crystalline quality coaxial
GaInN/GaN multiple quantum shells
(MQSs) grown on dislocation-free nanowires are highly in demand for
efficient white-/micro-light-emitting diodes (LEDs). Here, we propose
an effective approach to improve the MQS quality during the selective
growth by metal–organic chemical vapor deposition. By increasing
the growth temperature of GaN barriers, the cathodoluminescent intensity
yielded enhancements of 0.7 and 3.9 times in the samples with GaN
and AlGaN spacers, respectively. Using an AlGaN spacer before increasing
the barrier temperature, the decomposition of GaInN quantum wells
was suppressed on all planes, resulting in a high internal quantum
efficiency up to 69%. As revealed by scanning transmission electron
microscopy (STEM) characterization, the high barrier growth temperature
allowed to achieve a clear interface between GaInN quantum wells and
GaN quantum barriers on the c-, r-, and m-planes of the nanowires. Moreover, the
correlation between the In incorporation and structure features in
MQS was quantitatively assessed based on the STEM energy-dispersive
X-ray spectroscopy mapping and line-scan profiles of In and Al fractions.
Ultimately, it was demonstrated that the unintentional In incorporation
in GaN barriers was induced by the evaporation of predeposited In-rich
particles during low-temperature growth of GaInN wells. Such residual
In contamination was sufficiently inhibited by inserting low Al fraction
(∼6%) AlGaN spacers after each GaInN well. During the growth
of AlGaN spacers, AlN polycrystalline particles were deposited on
the surrounding dummy substrate, which suppressed the evaporation
of the predeposited In-rich particles. Thus, the presence of AlGaN
spacers certainly improved the uniformity of In fraction through five
GaInN quantum wells and reduced the diffusion of point defects from n-core to MQS active structures. The superior coaxial GaInN/GaN
MQS structures with the AlGaN spacer are supposed to improve the emission
efficiency in white-/micro-LEDs.
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Broadened emission was demonstrated in coaxial GaInN/GaN multiple quantum shell (MQS) nanowires that were monolithically grown by metalorganic chemical vapor deposition. The non-polar GaInN/GaN structures were coaxially grown on n-core nanowires with combinations of three different diameters and pitches. To broaden the emission band in these three nanowire patterns, we varied the triethylgallium (TEG) flow rate and the growth temperature of the quantum barriers and wells, and investigated their effects on the In incorporation rate during MQS growth. At higher TEG flow rates, the growth rate of MQS and the In incorporation rate were promoted, resulting in slightly higher cathodoluminescence (CL) intensity. An enhancement up to 2–3 times of CL intensity was observed by escalating the growth temperature of the quantum barriers to 800 °C. Furthermore, decreasing the growth temperature of the quantum wells redshifted the peak wavelength without reducing the MQS quality. Under the modified growth sequence, monolithically grown nanowires with a broaden emission was achieved. Moreover, it verified that reducing the filling factor (pitch) can further promote the In incorporation probability on the nanowires. Compared with the conventional film-based quantum well LEDs, the demonstrated monolithic coaxial GaInN/GaN nanowires are promising candidates for phosphor-free white and micro light-emitting diodes (LEDs).
The morphology and crystalline quality
of p-GaN shells on coaxial
GaInN/GaN multiple quantum shell (MQS) nanowires (NWs) were investigated
using metal–organic chemical vapor deposition. By varying the
trimethylgallium (TMG) flow rate, Mg doping, and growth temperature,
it was verified that the TMG supply and growth temperature were the
dominant parameters in the control of the p-GaN shape on NWs. Specifically,
a sufficiently high TMG supply enabled the formation of a pyramid-shaped
NW structure with a uniform p-GaN shell. The ratio of the growth rate
between the c- and m-planes on the
NWs was calculated to be approximately 0.4545. High-angle annular
dark-field scanning transmission electron microscopy characterization
confirmed that no clear extended defects were present in the n-GaN
core and MQS/p-GaN shells on the sidewall. Regarding the p-GaN shell
above the c-plane MQS region, only a few screw dislocations
and Frank-type partial dislocations appeared at the interface between
the serpentine c-plane MQS and the p-GaN shell near
the tips. This suggested that the crystalline quality of the MQS structure
can trigger the formation of screw dislocations and Frank-type partial
dislocations during the p-GaN growth. The growth mechanism of the
p-GaN shell on NWs was also discussed. To inspect the electronic properties,
a prototype of a micro light-emitting diode (LED) with a chip size
of 50 × 50 μm2 was demonstrated in the NWs with
optimal growth. By correlating the light output curve with the electroluminescence
spectra, three different emission peaks (450, 470, and 510 nm) were
assignable to the emission from the m-, r-, and c-planes, respectively.
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