Carrier dynamics near a crack in GaN microwires with AlGaN multiple quantum wells

Abstract

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.

Toward Crack-Free Core–Shell GaN/AlGaN Quantum Wells

“…Figure 5b shows a SEM image of a typical cracked wire (also A-type structure but with 5 nm Al 0.6 Ga 0.4 N barriers and thin GaN QW thickness corresponding to a growth time of 20 s) and a CL intensity map at ∼5 K: intensity integration over the 270−328 nm range corresponds to the GaN/AlGaN MQW emission. The whole emission is quenched at the crack locations, probably due to a local decrease of the efficiency as observed by Finot et al 21 on core−shell AlGaN/AlGaN MQWs. In our study, the CL signal only comes from the regions between the cracks, showing the detrimental influence of these extended defects on the QW emission.…”

“…Indeed, one of the major challenges stems from the strain relaxation imposed by the wire core on the Al-rich shell required for UV-B,C emission that leads to crack generation. 21 As the origin of crack formation is directly related to the strain state of the AlGaN/GaN heterostructure, the framework of classical elastic theory is hereafter considered in the case of core−shell structures. Raychaudhuri et al estimate the relative core/shell thicknesses to get a coherent epitaxy from the balance between the elastic strain energy (resulting from lattice mismatch) and the edge dislocation formation energy.…”

“…The tuning of the emission wavelength down to UV-B,C emission with a GaN wire as a template remains an issue. Indeed, one of the major challenges stems from the strain relaxation imposed by the wire core on the Al-rich shell required for UV-B,C emission that leads to crack generation . As the origin of crack formation is directly related to the strain state of the AlGaN/GaN heterostructure, the framework of classical elastic theory is hereafter considered in the case of core–shell structures.…”

Surface Recombinations in III-Nitride Micro-LEDs Probed by Photon-Correlation Cathodoluminescence

“…In addition, the acceleration voltage is kept to 10 kV to efficiently probe the MQW region while having the best possible spatial resolution. The probe current has been set to 4 pA. More details on the SRTC-CL setup can be found in ref 19.…”

Photon-Correlation Cathodoluminescence Spectroscopy in a SEM: A Tool to Analyze the Performance of Optoelectronics Devices

“…Thus, to measure the g 2 (τ) of the CL signal, we built a Hanbury Brown and Twiss (HBT) interferometer to analyze the CL photon statistics [6]. We apply the method to investigate the influence of surface recombinations on the optical properties of InGaN/GaN quantum wells near a mesa edge with a spatial resolution of 100 nm and a time resolution better than 50 ps thanks to spatially-resolved time-correlated cathodoluminescence spectroscopy (SRTC-CL) [7].…”