Strain-Induced Spectral Red-Shifting from Nanoscale Frustum Arrays Fabricated over InGaN/GaN Quantum Wells for Light-Emitting Applications

Abstract

While spectral blue-shifting caused by nanostructuring of InGaN/GaN quantum wells has been widely reported for altering the emission color of light-emitting diodes, the same cannot be said for spectral red-shifting. It is well-known that nanostructuring of the quantum wells gives rise to relaxation of the strain incurred in the quantum wells, reducing the quantum confined stark effect with a consequence of spectral shifting to shorter wavelengths. In this report, we demonstrate a nanostructure configuration that produces the opposite effect, spectral red-shifting, by increasing the strain in the quantum wells through the formation of an inverted nanoconical-frustum array over the quantum wells, without allowing the nanostructures to penetrate through the quantum wells. Under such conditions, spectral red-shifting of the photoluminescence spectrum can be observed, consistent with the prediction of strain induction in the quantum wells by molecular dynamics simulations. Experimentally, spectral red-shift of the photoluminescence spectrum by up to 7.6 nm has been observed, when the proposed nanostructures are fabricated on InGaN/GaN quantum wells grown on a c-plane sapphire substrate with a nominal emission wavelength of ∼560 nm.

Comparison of lasing characteristics of GaN microdisks with different structures

“…The charge carrier lifetimes τ tot are determined from the time-resolved PL decay spectra as shown in figure 5, measured at room temperature using a time-correlated single photon counting system. Details of the measurements are reported in [55]. As the InGaN/GaN MQW is a multilevel system, a bi-exponential decay fitting is used to determine the longer τ 1 and shorter τ 2 lifetimes of the as-grown wafers, of which the longer decay lifetime τ 1 which involves mainly radiative recombinations [56] is used for the calculations as listed in table 1.…”

Monolithic InGaN Multicolor Light‐Emitting Devices

“…Broadening of the emission was also observed when nanostructures were on top of the MQWs, as the state of the quantum wells gradually changed from strain relaxation on the top layer to strain induction on the bottom layer. [ 52 ] Such a spectral red‐shifting technique can potentially be used in conjunction with strain‐relaxing nanostructures to realize higher IQE monolithic RGB displays via top‐down nanostructuring. [ 50 ]…”

“…The starting LED wafer also dictates the IQE of the top‐down approach, which can improve to a certain degree after nanostructuring due to strain relaxation reducing the QCSE. In addition, strain‐inducing nanostructures that enable spectral red‐shift [ 50,52 ] may potentially circumvent the bottlenecks of IQE and long‐wavelength emission. The nanowire approach has the best color rendition, as it natively supports much higher indium incorporation than planar films, though the issues of current injection may ultimately limit its efficiency.…”

“…A typical quantum well is capable of absorbing photons at an energy higher than its bandgap and re-emitting photons at a Reproduced with permission. [52] Copyright 2021, American Chemical Society. lower energy via photoluminescence.…”

“…The measured spectral red-shift of strained nanostructures with respect to the etch depth is shown on the right. Cross-sectional TEM image above the graph shows the epitaxial structure at the corresponding etch depths.Reproduced with permission [52]. Copyright 2021, American Chemical Society.…”