MOVPE Growth of Smooth and Homogeneous Al_0.8Ga_0.2N:Si Superlattices as UVC Laser Cladding Layers

Abstract

Smooth, uniform, and conductive Al x Ga 1Àx N layers with high aluminum mole fractions of x > 0.7 are required as cladding layers for laser diodes emitting in the deep ultraviolet spectral region. In this paper, the growth of silicon-doped AlGaN/AlGaN superlattices by metal-organic vapor phase epitaxy is investigated and compared to bulk AlGaN layers. It is found that the superlattice approach enables the growth of AlGaN layers with improved lateral uniformity of composition and strain state. In order to reduce the surface roughness, growth interruptions between the superlattice layers are investigated. With increasing growth interruption time, the average aluminum content is increasing and the superlattice period thickness is decreasing. Scanning transmission electron microscopy investigations show that the growth interruptions lead to more abrupt interfaces and to the formation of one to two monolayer thin aluminum-rich AlGaN layers at each interface. This also leads to a significantly smoother surface morphology. Low resistivities of 0.025 Ω cm are obtained for AlGaN:Si superlattices with average aluminum content of x ¼ 0.8. The influence of the surface morphology of the AlGaN cladding layers on optically pumped laser heterostructures is investigated. By using a smooth AlGaN superlattice, the lasing threshold decreases by a factor of 2 compared to lasers with bulk AlGaN cladding layers.

The 2020 UV emitter roadmap

“…UVC lasers with high structural quality can be realized on low defect density bulk AlN [21,190,207,208]. Since these high aluminium mole fraction AlGaN heterostructures can be grown pseudomorphically strained on bulk AlN [213], they exhibit excellent optical properties as described in the previous paragraph. However, efficient carrier injection and confinement as well as low resistivity p-AlGaN cladding layers with an aluminium content exceeding 70% pose very difficult challenges for the realization of AlGaN QW lasers emitting in the UVC.…”

“…For UVB lasers one of the biggest challenges is the lack of high quality AlGaN substrates with a lattice constant corresponding to the medium AlGaN composition range. For example, growth on bulk AlN substrates is not ideal since the strong compressive strain in the AlGaN quantum well heterostructures leads to relaxation and defect formation during growth [213] and consequently a degradation of the optical properties. This might be the main reason for higher lasing threshold of the demonstrated UVB laser diode in comparison for UVC laser diodes grown on bulk AlN [190,204].…”

Determination of Sapphire Off‐Cut and Its Influence on the Morphology and Local Defect Distribution in Epitaxially Laterally Overgrown AlN for Optically Pumped UVC Lasers

“…Both heterostructures exhibit lasing between 270 and 275 nm as the power-dependent PL intensity shows a threshold behavior in combination with spectral narrowing (not shown here). [20] The laser structure on ELO AlN/sapphire with a smooth surface morphology exhibits lasing at a threshold power density of 1.7 MW cm À2 . This value is increased to 2.8 MW cm À2 for a laser structure grown on an ELO AlN/sapphire template exhibiting step bunching.…”

Determination of Sapphire Off‐Cut and Its Influence on the Morphology and Local Defect Distribution in Epitaxially Laterally Overgrown AlN for Optically Pumped UVC Lasers

“…The integrated PL intensity collected from the facet as a function of the excitation power density for both structures is shown in Figure b. Both heterostructures exhibit lasing between 270 and 275 nm as the power‐dependent PL intensity shows a threshold behavior in combination with spectral narrowing (not shown here) . The laser structure on ELO AlN/sapphire with a smooth surface morphology exhibits lasing at a threshold power density of 1.7 MW cm −2 .…”