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

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 (c) over bar -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 similar to 4 J/m(2). 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.