Theoretical modelling of exciton binding energy, steady-state and transient optical response of GaN/InGaN/GaN and AlGaN/GaN/AlGaN core–shell nanostructures

Physica Status Solidi (b)

Banerjee

et al. 2021

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III-V compound semiconductors laid the foundation for optoelectronic devices and solid-state lighting with wavelengths ranging from infrared to ultraviolet regions. Infrared to green light-emitting diodes (LEDs) were invented in the early 1960s. [1][2][3] However, blue LEDs took another three decades to get established, considering the challenges in defect-free high-quality growth, [4][5][6] and controlled p-doping of wide-bandgap semiconductors. [7][8][9][10] The success of blue LEDs was accomplished by the development of gallium-nitride (GaN) alloyed with indium and aluminum in the late 1980s. [11,12] GaN and related materials have a wurtzite crystal structure and direct bandgap, making them ideal for bright LEDs and laser diodes (LDs). In order to move the emission wavelength to the visible range (2-3.4 eV), InN is alloyed with GaN. [13,14] Advancement in GaN-based LEDs revolutionized the white broadband lighting when laminated with yellow phosphorous coating. Nakamura et al. developed the first high-quality InGaN back in 1992, which paved the way for III-nitride blue and green LEDs/ LDs. [15] In 1995-1996, Nakamura made true blue LEDs possible by developing recombination regions with InGaN. [16] III-nitride devices acquire a wide area of applications due to their wavelength tunability by varying the alloy composition of In and Al in InGaN and AlGaN, respectively. [17,18] Moreover, an additional degree of freedom can be achieved using quantum-confined nanostructures. Emission wavelength and surface/interface material properties can be altered by reducing the dimensionality/size of the devices. [19][20][21] quantum well (QW; 2D), [22][23][24] vertical/lateral nanowires (1D), [25][26][27][28][29] and quantum dots (QDs; 0D) [30][31][32][33] are among the most important quantum-confined nanostructures used in the active regions of modern optoelectronic devices including LEDs, lasers, photodetectors, and solar cells. Improved internal quantum efficiency, [34][35][36] large surface-to-volume ratio, [37] low power

Section: Introduction To Ingan/gan Quantum‐confined Heterostructuresmentioning

confidence: 99%

Investigation of Ultrafast Carrier Dynamics in InGaN/GaN‐Based Nanostructures Using Femtosecond Pump–Probe Absorption Spectroscopy

Physica Status Solidi (b)

Banerjee

et al. 2021

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“…With time the carriers decay from the QDs, and the quantum-confined Stark effect brings the absorption peak back to longer wavelengths. 5,6,23 Figure 3d shows that the spectra change sign for some wavelengths for a larger delay between the pump and the probe beams.…”

Section: Resultsmentioning

confidence: 98%

Gradual Carrier Filling Effect in “Green” InGaN/GaN Quantum Dots: Femtosecond Carrier Kinetics with Sequential Two-Photon Absorption

Aiello

ACS Appl. Mater. Interfaces

et al. 2021

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Quantum dots (QDs) allow for a significant amount of strain relaxation, which is helpful in GaN systems where a large lattice mismatch needs to be accommodated. InGaN QDs with a large indium composition are intensively investigated for light emitters requiring longer wavelengths. These are especially important for developing high-efficiency white light sources. Understanding the carrier dynamics in this large lattice-mismatched system is essential to improving the radiative efficiency while circumventing high defect density. This work investigates femtosecond carrier and photon dynamics in self-organized In0.27Ga0.73N/GaN QDs grown by molecular beam epitaxy using transient differential absorption spectroscopy, which measures the differential absorption coefficient (Δα) with and without an optical pump. Due to 3D quantum confinement and the small effective mass of InGaN, the low density of states in the conduction band is easily filled with electrons. In contrast, the GaN barrier region is replete with a high density of electrons due to a large effective mass. This contrast in carrier density creates a unique phenomenon in the dynamics, showing a change in the differential absorption coefficient (Δα) sign from negative to positive with time. The ultrafast microscopic processes indicate that right after the optical pump and first photon absorption, the valence (conduction) band states are depleted (replete) of electrons. This ground-state bleaching process makes Δα negative, and the probe beam is not absorbed. The electrons are then gradually transferred from the GaN barrier into InGaN QDs, which absorb the second photon from the probe beam (excited-state absorption), making Δα positive. The presence of excited-state carriers with a long lifetime is indicative of the enhanced availability of carriers for radiative recombination. This effect also promotes stimulated emission and amplified spontaneous emission, which can be used to develop lasers and superluminescent LEDs, respectively. Measurements with multiple pump powers and temperatures further confirm that the efficacy of InGaN QDs is enhanced by this effective mass contrast and 3D reservoir of carriers from the GaN barrier. This effect can be used to improve the internal quantum efficiency of GaN-based light emitters.

“…Auger recombination is a significant contributor to efficiency droop due to its cubic dependence on the free carrier density. Various techniques have been explored to prevent SRH and Auger recombinations; this includes core-shell nanorods , (provide reduced polarization charges), wide quantum wells (QWs), reverse polarization, semi- and nonpolar GaN, , defect-free GaN nanowires, and quantum dots − (which have less Auger coefficient than their planar counterparts). The growth of high-quality InGaN layers on a semi- or nonpolar substrate is difficult.…”

Section: Introductionmentioning

confidence: 99%

Reduced Auger Coefficient through Efficient Carrier Capture and Improved Radiative Efficiency from the Broadband Optical Cavity: A Mechanism for Potential Droop Mitigation in InGaN/GaN LEDs

ACS Appl. Mater. Interfaces

Saha

et al. 2022

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Efficiency droop at high carrier-injection regimes is a matter of concern in InGaN/GaN quantum-confined heterostructure-based light-emitting diodes (LEDs). Processes such as Shockley–Reed–Hall and Auger recombinations, electron–hole wavefunction separation from polarization charges, carrier leakage, and current crowding are identified as the primary contributors to efficiency droop. Auger recombination is a critical contributor owing to its cubic dependence on carrier density, which can not be circumvented using an advanced physical layout. Here, we demonstrate a potential solution through the positive effects from an optical cavity in suppressing the Auger recombination rate. Besides the phenomenon being fundamentally important, the advantages are technologically essential. The observations are manifested by the ultrafast transient absorption pump-probe spectroscopy performed on an InGaN/GaN-based multi-quantum well heterostructure with external DBR mirrors of varying optical confinement. The optical confinement modulates the nonlinear carrier and photon dynamics and alters the rate of dominant recombination mechanisms in the heterostructure. The carrier capture rate is observed to be increasing, and the polarization field is reducing in the presence of optical feedback. Reduced polarization increases the effective bandgap, resulting in the suppression of the Auger coefficient. Superluminescent behavior along with enhanced spectral purity in the emission spectra in presence of optical confinement is also demonstrated. The improvement is beyond the conventional Purcell effect observed for the quantum-confined systems.