Real-time observation of delayed excited-state dynamics in InGaN/GaN quantum-wells by femtosecond transient absorption spectroscopy.

This work employs femtosecond transient absorption spectroscopy to investigate the ultrafast carrier dynamics of bound states in In0.14Ga0.86N/GaN quantum wells. The ground state (GS) dynamics usually dominate these characteristics, appearing as a prominent peak in the absorption spectra. It is observed that the excited state also contributes to the overall dynamics, with its signature showing up later. The contributions of both the ground and excited states in the absorption spectra and time-resolved dynamics are decoupled in this work. The carrier density in the GS first increases and then decays with time. The carriers populate the excited state only at a delayed time. The dynamics are studied considering the Quantum-Confined Stark Effect-induced wavelength shift in the absorption. The relevant microscopic optoelectronic processes are understood phenomenologically, and their time constants are extracted. An accurate study of these dynamics provides fundamentally essential insights into the time-resolved dynamics in quantum-confined heterostructures and can facilitate the development of efficient light sources using GaN heterostructures.

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.

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.

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

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.

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

Here, we present an efficient 1D model to describe carrier confinement in GaN/InGaN/GaN and AlGaN/GaN/AlGaN core-shell nanostructures (CSNs) within the effective mass framework. A self-consistent procedure combined with hydrogenic model is implemented to estimate exciton binding energy in these CSNs, as a function of CSN dimensions, polarization charge and alloy composition. A 3-fold higher exciton binding energy in these CSNs than that in planar counterparts is attributed to an increased electron-hole overlap. The trend exhibited by the exciton binding energy with polarization charge and alloy composition in the two types of CSNs is significantly different, owing to a drastic difference in the piezoelectric polarizations. A detailed investigation of the steady-state and transient optical response from these CSNs suggests that GaN/InGaN/GaN CSNs emit a wide spectrum. However, that is not the case with AlGaN/GaN/AlGaN CSNs owing to a relatively weaker quantum confined Stark effect. This study is aimed at providing accurate design strategies for UV-blue III-N CSN light-emitting diodes.

Enhanced luminescence from InGaN/GaN nano-disk in a wire array caused by surface potential modulation during wet treatment.

Here we have demonstrated the profound impact of surface potential on the luminescence of an array of InGaN/GaN nano-disk in a wire heterostructure. The change in surface potential is brought about by a combination of dry and successive wet-processing treatments. The photoluminescence (PL) properties are determined as a function of size and height of this array of nano-disks. The observed characteristics are coherently explained by considering a change in quantum confinement induced by the change in surface potential, quantum-confined Stark effect, exciton binding energy and strain relaxation for varying surface potential. The change in hole bound state energy due to parabolic potential well near the side-wall is found to be the dominating factor. The PL peak position, full width at half-maximum, strain relaxation and integrated PL intensity are studied as a function of incident power and temperature. The devices demonstrate higher integrated PL intensity and slope efficiency.

Determination of strain relaxation in InGaN/GaN nanowalls from quantum confinement and exciton binding energy dependent photoluminescence peak.

GaN based nanostructures are being increasingly used to improve the performance of various devices including light emitting diodes and lasers. It is important to determine the strain relaxation in these structures for device design and better prediction of device characteristics and performance. We have determined the strain relaxation in InGaN/GaN nanowalls from quantum confinement and exciton binding energy dependent photoluminescence peak. We have further determined the strain relaxation as a function of nanowall dimension. With a decrease in nanowall dimension, the lateral quantum confinement and exciton binding energy increase and the InGaN layer becomes partially strain relaxed which decreases the piezoelectric polarization field. The reduced polarization field decreases quantum confined Stark effect along the c-axis and increases electron-hole wave-function overlap which further increases the exciton binding energy. The strong dependency of the exciton binding energy on strain is used to determine the strain relaxation in these nanostructures. An analytical model based on fractional dimension for GaN/InGaN/GaN heterostructures along with self-consistent simulation of Schrodinger and Poisson equations are used to theoretically correlate them. The larger effective mass of GaN along with smaller perturbation allows the fractional dimensional model to accurately describe our system without requiring first principle calculations.