Here, we have demonstrated strong size dependency of quasi-equilibrium and nonequilibrium carrier and photon dynamics in InGaN/GaN single nanowalls using photoluminescence and transient absorption spectroscopy. We demonstrate that two-dimensional carrier confinement, strain relaxation, and modified density of states lead to a reduced Stokes shift, smaller full width at half-maxima, increased exciton binding energy, and reduced nonradiative recombination. The ultrafast transient spectroscopy shows that carrier capture is a two-step process dominated by optical phonons and carrier-carrier scattering in succession. The carrier capture is a strongly size-dependent process and becomes slower due to modulation of the density of available states for progressively decreasing nanowall sizes. The slowest process is the electron-hole recombination, which is also extremely size-dependent and the rate increases by almost an order of magnitude in comparison to that of quantum-well structures. Electron-hole wave function overlap and modified density of states are among the key aspects in determining all the properties of these structures.

Small signal modulation bandwidth characterized by relaxation oscillations (ROs) of GaN-based vertical cavity surface emitting laser diodes are of great importance in high-speed applications. In this work, we investigate the impact of distributed Bragg reflectors (DBRs) on ROs by developing a simulation framework, which can accommodate the reflected photons self-consistently in the rate equation model. Carrier and photon dynamics in the active region and at the transmitting end have been evaluated for a typical GaN/InGaN multiple quantum well heterostructure having a dielectric DBR. Temporal broadening in photon dynamics is observed due to the phase difference. Interestingly, an enhanced radiative recombination rate is also noticed due to the increased confinement, which leads to a reduced turn-on delay. Transient absorption spectroscopy has been performed to obtain the decay rates of captured carriers in the well and it turns out be in harmony with the simulation results. Photoluminescence spectroscopy also validates the increased radiative recombination. © 2019 The Japan Society of Applied Physics ( ) ( )where N b and N w are the carrier densities in the barrier and well regions, respectively, and N p is the photon density inside

We have demonstrated an electrically injected ultra-low threshold (8.9 nA) room temperature InGaN/GaN based lateral nanowire laser. The nanowires are triangular in shape and survived naturally after etching using boiling phosphoric acid. A polymethyl methacrylate (PMMA) and air dielectric distributed mirror provide an optical feedback, which together with one-dimensional density of states cause ultra-low threshold lasing. Finite difference eigen-mode (FDE) simulation shows that triangular nanowire cavity supports single dominant mode similar to TE01 that of a corresponding rectangular cavity with a confinement factor of 0.18.

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.

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

Understanding the ultrafast processes corresponding to carrier capture, thermalization and relaxation is essential to design high speed optoelectronic devices. Here, we have investigated a size dependent carrier capture process in InGaN/GaN 20, 50 nm nanowires and quantum well systems. Femto-second transient absorption spectroscopy reveals that the carrier capture is a two-step process. The carriers are captured in the barrier by polar optical phonon (POP) scattering. They further scatter into the active region by electron–electron and POP scatterings. The capture is found to slow down for quantum confined structures. A significant number of carriers are found to disappear from the barrier during the diffusion process. All the experimental observations are explained in a simulation framework depicting various scattering mechanisms.

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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.