Different types of buffer layers like InGaN underlayer (UL) and InGaN/GaN superlattices are now well-known to significantly improve the efficiency of c-plane InGaN/GaN based light emitting diodes (LEDs). The present work investigates the role of two different kinds of pregrowth layers (low In-content InGaN UL and GaN UL namely "GaN spacer") on the emission of core-shell m-plane InGaN/GaN single quantum well (QW) grown around Si-doped !̅-GaN microwires obtained by silane-assisted MOVPE. According to photo-and cathodoluminescence measurements performed at room temperature, an improved efficiency of light emission at 435 nm with internal quantum efficiency > 15 % has been achieved by adding a GaN spacer prior to the growth of QW. As revealed by scanning transmission electron microscopy, an ultra-thin residual layer containing Si located at the wire sidewall surfaces favors the formation of highdensity of extended defects nucleated at the first InGaN QW. This contaminated residual incorporation is buried by the growth of GaN spacer and avoids the structural defect formation, therefore explaining the improved optical efficiency. No further improvement is observed by adding the InGaN UL to the structure, which is confirmed by comparable values of the effective carrier lifetime estimated from time-resolved (TR) experiments. Contrary to the case of planar cplane QW where the improved efficiency is attributed to a strong decrease of point defects, the addition of an InGaN UL seem to have no influence in the case of radial m-plane QW.
The present work reports high quality non-polar GaN/Al0.6Ga0.4N multiple quantum wells (MQWs) grown in core-shell geometry by metalorganic vapor phase epitaxy on the m-plane sidewalls of ̅ -oriented hexagonal GaN wires. Optical and structural studies reveal UV emission originating from the core-shell GaN/AlGaN MQWs. Tuning the mplane GaN QW thickness from 4.3 to 0.7 nm leads to a shift of the emission from 347 to 292 nm, consistent with Schrödinger-Poisson calculations. The evolution of the luminescence with temperature displays signs of strong localization, especially for samples with thinner GaN QWs and no evidence of quantum confinement Stark effect, as expected for non-polar m-plane surfaces. The internal quantum efficiency derived from the photoluminescence intensity ratio at low and room temperature is maximum (~7.3 %) for 2.6 nm-thick quantum wells, emitting at 325 nm and shows a large drop for thicker QWs. An extensive study of the PL quenching with temperature is presented.Two non-radiative recombination paths are activated at different temperatures. The low temperature path is found to be intrinsic to the heterostructure, whereas the process that dominates at high temperature depends on the QW thickness and is strongly enhanced for QWs larger than 2.6 nm, causing a drop of the internal quantum efficiency.
III-nitride micro-LEDs are promising building blocks for the next generation of high performance micro-displays. To reach a high pixel density, it is desired to achieve micro-LEDs with lateral dimensions below 10 µm. With such pixel downscaling, sidewall effects are becoming important and an understanding of the impact of non-radiative surface recombinations is of vital importance. It is thus required to develop an adapted metric to evaluate the impact of these surface recombinations with a nanoscale spatial resolution. Here, we propose a methodology to quantitatively assess the influence of surface recombinations on the optical properties of InGaN/GaN quantum wells based on spatially-resolved time-correlated cathodoluminescence spectroscopy. By coupling this technique to a simple diffusion model, we confirm that the combination of KOH treatment and Al 2 O 3 passivation layer drastically reduces surface recombinations. These findings emphasize the need for nanoscale time-resolved experiments to quantify the local changes in internal quantum efficiency of micro-devices.
Relaxation of tensile strain in AlGaN heterostructures grown on GaN template can lead to the formation of cracks. These extended defects locally degrade the crystal quality resulting in a local increase of non-radiative recombinations. The effect of such cracks on the optical and structural properties of core-shell AlGaN/AlGaN multiple quantum wells grown on GaN microwires are comprehensively characterized by means of spectrally and time-correlated cathodoluminescence (CL). We observe that the CL blueshifts near a crack. By performing 6x6 k.p simulations in combination with transmission electron microscopy analysis, we ascribe this shift to the strain relaxation by the free surface near cracks. By simultaneously recording the variations of both the CL lifetime and the CL intensity across the crack, we directly assess the carrier dynamics around the defect at T = 5 K. We observe that the CL lifetime is reduced typically from 500 ps to less than 300 ps and the CL intensity increases by about 40% near the crack. The effect of the crack on the optical properties is therefore of two natures. First, the presence of this defect locally increases non-radiative recombinations while at the same time, it locally improves the extraction efficiency. These findings emphasize the need for time-resolved experiments to avoid experimental artifacts related to local changes of light collection.
Strain relaxation of nonpolar GaN/Al0.6Ga0.4N multiple quantum wells grown in core–shell geometry by metal–organic vapor-phase epitaxy on GaN wires is investigated. Cracking along the a-direction is observed on the sidewalls of c̅-oriented hexagonal GaN wires. To overcome this issue, an undershell including AlGaN gradient and cladding layers is grown before the active region. While a decrease of the crack density is observed with the undershell, the increase of GaN QW thickness acts as a key parameter to limit the crack formation. In agreement with previous studies performed on AlGaN planar layers, a relaxation criterion is found for a threshold strain energy density of ∼4 J/m2. Considering the quantum well structure as a single AlGaN layer with an average composition, a solution to keep the strain energy density below this relaxation limit is identified by reducing the AlGaN barrier thickness from 5 to 3 nm. Combining the undershell and reduced barrier thickness, a crack-free core–shell AlGaN-based structure is demonstrated with an emission at 280 nm corresponding to the UV-B/C limit.
The growth of non-polar AlGaN digital alloy (DA) is achieved by metal-organic vapor phase epitaxy using GaN microwire m-facets as the template. This AlGaN DA consisting of five periods of two monolayer-thick layers of GaN and AlGaN (approximately 50% Al-content) is integrated into the middle of an n-p GaN/AlGaN junction to design core-shell wire- μLED. The optical emission of the active zone investigated by 5 K cathodoluminescence is consistent with the AlGaN bulk alloy behavior. Several contributions from 295 to 310 nm are attributed to the lesser thickness and/or composition fluctuations of AlGaN DA. Single-wire μLED is fabricated using a lithography process, and I–V measurements confirm a diode rectifying behavior. Room temperature UV electroluminescence originating from m-plane AlGaN DA is accomplished at 310 nm.
Core-shell GaN/AlGaN multiple quantum wells (MQWs) embedded in a p–n junction are integrated on the upper part of GaN microwires grown by silane-assisted metal organic vapor phase epitaxy. Dispersed wires are then contacted by electron beam induced deposition for fabrication of single wire UV-LED devices. Rectifying diode-like behavior is first demonstrated for both GaN and GaN/AlGaN p-n junctions without a MQW active region. A weak leakage current in the GaN/AlGaN core-shell heterostructure is attributed to an additional conduction path along wire sidewalls. Electroluminescence at 340 nm in UV-A is demonstrated using a GaN (2.6 nm)/Al0.3Ga0.7N (3 nm) heterostructure embedded in a GaN/Al0.3Ga0.7N p–n junction. This value is even decreased to 310 nm by reducing the well thickness to 0.9 nm and increasing the Al-content of barriers (up to 60%) integrated in the GaN/Al0.3Ga0.7N p–n junction. This work demonstrates UV-B emission based on single wire core-shell UV-LEDs.
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