posted on 2024-01-02, 21:04authored byFatimah Alreshidi, Lih-Ren Chen, Mohammed Najmi, Bin Xin, Hadeel Alamoudi, Georgian Melinte, Nimer Wehbe, Daisuke Iida, Kazuhiro Ohkawa, Tien-Chang Lu, Iman S. Roqan
We
study the impact of strain engineering by exploring the influence
of the number of superlattice (SL) layers underneath InGaN/GaN multiple
quantum wells (MQWs) on the optical properties of InxGa1–xN/GaN MQWs grown on
patterned sapphire by metal–organic chemical vapor deposition
while retaining the same composition and MQW periods. X-ray diffraction
and reciprocal space mapping show that the strain initially increases
with the number of SLs in the structure followed by a slight relaxation.
Scanning electron microscopy analysis indicates that the desired strain
is obtained by increasing the number of SL pairs up to 12 due to which
the V-pit density and size (>270 nm in diameter) increase. Scanning
transmission electron microscopy reveals that such large-sized V-pits
[with large sidewalls comprising ultrathin MQWs and SLs (<1 nm)]
emerge in the n-GaN layer below the SLs, leading
to high n-GaN quality as confirmed by temperature-dependent
photoluminescence (PL) and PL excitation measurements as defect-related
emission in n-GaN decreases as the V-pit density
increases. Low-temperature PL spectra show a higher-energy emission
centered at 402 nm besides the MQW emission at ∼454–458
nm, while room-temperature cathodoluminescence mapping reveals that
this higher-energy emission is due to the ultrathin MQW + SL structures
surrounding V-pits, forming ultrathin subquantum well (sub-QW). We
show, for the first time, that the carrier repopulation process between
MQWs and sub-QW caused by a high density of V-pits through the strain
engineering process can be a significant factor in enhancing the optical
quality and efficiency. These findings provide valuable insight into
the impact of strain engineering that can govern high-efficiency light-emitting
diode (LED) performance.