Spatial inhomogeneities in Al_xGa_1−xN quantum wells induced by the surface morphology of AlN/sapphire templates

Abstract

The effects of the template on the optical and structural properties of Al 0.75 Ga 0.25 N/Al 0.8 Ga 0.2 N multiple quantum well (MQWs) laser active regions have been investigated. The laser structures for optical pumping were grown on planar c-plane AlN/sapphire as well as on thick epitaxially laterally overgrown (ELO) AlN layers on patterned AlN/sapphire. Two ELO AlN/sapphire templates were investigated, one with a miscut of the sapphire surface to the m-direction with an angle of 0.25°, the other with a miscut angle of 0.25°to the sapphire a-direction. The MQWs are studied by atomic force microscopy, plan-view cathodoluminescence (CL) at room temperature and 83 K as well as transmission electron microscopy using high-angle annular dark-field imaging and energy-dispersive x-ray spectroscopy. The results are compared to optical pumping measurements. It was found that the surface morphology of the templates determines the lateral wavelength distribution in the MQWs observed by spectral CL mappings. The lateral wavelength spread is largest for the laser structures grown on ELO AlN with miscut to sapphire a-direction caused by the local variation of the MQW thicknesses and the Ga incorporation at macrosteps on the ELO-AlN. A CL peak wavelength spread of up to 7 nm has been found. The MQWs grown on planar AlN/sapphire templates show a homogeneous wavelength distribution. However, due to the high threading dislocation density and the resulting strong nonradiative recombination, laser operation could not be achieved. The laser structures grown on ELO AlN/ sapphire show optically pumped lasing with a record short wavelength of 237 nm.

The 2020 UV emitter roadmap

“…A rough growth front with a high density of steps can help in this (by bending dislocations away from the +c surface normal). However, for the growth of the AlGaN device layers, high steps are detrimental since they can result in compositional modulation by a higher Ga incorporation rate at such steps [13]. To reproducibly manage the transition to a rough growth front and back to a smooth final AlN surface is a challenge.…”

Temperature Dependence of Dark Spot Diameters in GaN and AlGaN

“…[ 31,32 ] In addition, the AlGaN composition can vary at certain growth features like growth islands, [ 33 ] hillocks, [ 34 ] or growth steps. [ 35,36 ] The latter effects are rarely present in sample C, but add to the broader emission linewidth in sample D, where the studied AlGaN layer is grown under a higher compressive strain.…”

“…[ 33,35 ] Such growth features are not prominent in sample C. However, the underlying growth dynamics are still present and may cause additional emission energy extrema in Figure 2e. [ 36 ]…”

Drive High Power UVC‐LED Wafer into Low‐Cost 4‐Inch Era: Effect of Strain Modulation

“…As shown in Figure 3b, a nice step bunching microscopic morphology with a root-meansquare roughness of 3 nm is observed. However, although the crystalline quality and surface morphology of HTA-AlN both satisfy the requirement for following AlGaN layer epitaxy, the compressive strain conceivably poses a tough challenge in the subsequent AlGaN growth: Due to the initial spiral steps provided by screw-and mixed-type dislocations and the larger diffusion mobility and incorporation efficiency of Ga adatoms at steps compared with Al adatoms, AlGaN layer presents hexagonal-hillock morphology with composition inhomogeneity; [37,[45][46][47] Such a phenomenon is further enhanced by compressive strain, leading to serious hexagonal island growth mode of the n-type AlGaN layer and thus terrible surface roughening in following UVC-LED epitaxy, [30,33,[48][49][50] as shown in Figure 3a. It is worth noting that the growth mode of n-type AlGaN layer is the dominant factor to influence the quality of subsequent epitaxy.…”