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A systematic study of the GaN epitaxial lateral overgrowth (ELO) of the focused ion beam (FIB) patterned sapphire substrate is presented. The FIB technology with its atom at a time removal principle is a flexible and high-resolution maskless processing technique, unique to fabricate trenches of the highest quality without surface damage, debris, and cracks, therefore, best suited to investigate the improvement of the maskless GaN ELO on the patterned sapphire. Arrays of the square (size varying from 1 × 1 μm2 to 5 × 5 μm2) and rectangular (size varying from 20 × 1 μm2 to 20 × 5 μm2) trenches with the inter-trench distance varying from 1 to 5 μm, aligned parallel/perpendicular to sapphire ⟨ 11 2 ¯ 0 ⟩ direction were overgrown and investigated structurally and electrically using Raman scattering, atomic force microscopy (AFM), and electron beam induced current (EBIC). To find out the optimized ELO of the array, the threading dislocation density and the remaining strain in the ELO epilayer was analyzed. The epilayer improvement was demonstrated in a correlation between the trench size, the array geometry (the trench shape and inter-trench distance), and the epilayer thickness. A homogeneous overgrowth of the entire array was observed for the arrays of the largest trenches and the reduced inter-trench distance. [ABSTRACT FROM AUTHOR]

In this work, we report on the fabrication of a GaN/AlGaN waveguiding structure dedicated to modal phase matching, where GaN waveguide has planar polarity inversion. First, we optimized the growth conditions for the AlGaN epilayer. Second, on top of the AlGaN epilayer, we fabricated the waveguiding structure starting with the growth of the Ga-polar GaN epilayer followed by atomic layer deposition (ALD) of an Al 2 O 3 layer, then, continuing with the growth of N-polar GaN epilayer. We tested several layer thicknesses, but with 20 nm we managed to inverse the GaN polarity from Ga to N. To confirm the N-polarity, we etched the GaN epilayer surface in aqueous KOH solution. We performed out-of-plane (0002) and in-plane (11–20) X-ray diffraction and rocking curve measurements to estimate the crystalline quality of the AlGaN epilayer, Ga- and N-polar GaN epilayer. Atomic force microscopy measurement lets us evaluate the epilayer surface morphology and roughness. Optical and scanning electron microscopy inspection revealed characteristic hexagonal N-polar GaN epilayer surface. We used high-resolution transmission electron microscopy to investigate the crystallinity and orientation of the Ga- and N-polar GaN epilayer, also the Al 2 O 3 ALD layer, the interface quality of the waveguide structure. [ABSTRACT FROM AUTHOR]

An extended study of charge-carrier localization and delocalization in blue and green InGaN light-emitting diode (LED) test structures has been performed. Using the frequency-domain lifetime measurement (FDLM) technique based on direct harmonic modulation of photoluminescence excitation in the frequency range from 1 Hz to 100 MHz, carrier lifetimes were estimated at scales spanning from milliseconds to nanoseconds. The time resolution was determined using fast Fourier transform analysis. A system comprising a radiative and several nonradiative recombination channels was used to describe the complex photoluminescence decay. Due to the broad timescale, even stretched exponential decays from 2 ns to 4 ns up to 1.5 μs (stretching parameter 0.5 to 0.6) were revealed. A higher degree of carrier delocalization was observed for the blue compared with the green light-emitting structure, providing qualitative insight into disorder, which is tentatively assigned to spatial fluctuations of the indium concentration in the quantum wells. A nanosecond nonradiative recombination channel for the green light-emitting structure was found to be unsaturated throughout the entire photoexcitation power density range and was interpreted as being related to the higher defect density and lower internal quantum efficiency of the sample. To expand the study of lifetimes to much higher photoexcitation power density, time-resolved photoluminescence kinetics were measured. [ABSTRACT FROM AUTHOR]

InN and In-rich InGaN layers have been grown on GaN templates using the pulsed metalorganic chemical vapor deposition (MOCVD) technique and compared with analogous layers grown by conventional MOCVD. Structural investigations were performed using x-ray diffraction, scanning electron microscopy, and transmission electron microscopy. Surface morphology was studied using atomic force microscopy. Electrical and optical properties were studied using Hall measurements and photoluminescence (PL) spectroscopy. All layers were free of metal droplets. InN grown using pulsed MOCVD showed fairly low background electron concentration ( n = 8 × 10 cm to 9 × 10 cm) and high electron mobility ( μ = 644 cm V s). Structural studies revealed increase of size (from 100 nm to 500 nm) and decrease of density of InN islands at higher growth temperatures. For In-rich InGaN layers (In content 68% and 80%) the density of islands was similar to that in InN, while the diameter varied from 50 nm to 150 nm. Inhomogeneities of In and Ga distribution in the layers resulting in broadened XRD lines and PL bands are discussed. [ABSTRACT FROM AUTHOR]

Using metalorganic chemical vapor deposition, we heteroepitaxially grew undoped gallium nitride epilayers on sapphire. Assessing the epilayers at different growth stages, we investigated changes in epilayer strain and the lifetime of minority nonequilibrium charge carriers. The in-plane compressive strain was evaluated by x-ray diffraction and bandgap photoluminescence. The epilayer thickness ranged from 200 nm (islets) to 3.5 μm (continuous structure). The carrier lifetimes, measured using a light-induced transient grating technique, revealed a correlation between strain and the density of edge-type threading dislocations. This dislocation density was 10 cm to 10 cm, corresponding to the dominant mechanism for nonradiative carrier recombination. How the carrier lifetime depended on the growth stage differed between the surface and interfacial measurements. On the surface side, the carrier lifetime increased monotonically up to ~500 ps with thickness; on the interface side, the lifetime changed little with thickness, except in the thickest sample, where the carrier lifetime increased with thickness. We attributed this behavior to defect healing aided by long-term annealing, leading to mutual lateral motion and annihilation of mixed threading dislocations. [ABSTRACT FROM AUTHOR]

We demonstrate a broadband second-harmonic generator for the green spectral range based on a periodical polarity GaN waveguide structure. We grew the structure with a periodicity of 4 μ m using an MOCVD (metal-organic chemical vapor deposition) reactor. We tested the structure using femtosecond laser pulse pumping and obtained the second harmonic ( λ SH =512 nm) conversion efficiency 0.79 · 10 - 7 % · W - 1 for the pump peak power of 440 kW inside the waveguide. Based on the theoretical modeling of the dispersion curves of the pump and the second harmonic, we found multiple crossings, including an extended crossing. We relate this fact to the experimentally measured broadened second harmonic spectrum. [ABSTRACT FROM AUTHOR]