Abstract
We present a study of undoped AlGaN/GaN separate confinement heterostructures designed to operate as electron beam pumped ultraviolet lasers. We discuss the effect of spontaneous and piezoelectric polarization on carrier diffusion, comparing the results of cathodoluminescence with electronic simulations of the band structure and Monte Carlo calculations of the electron trajectories. Carrier collection is significantly improved using an asymmetric graded-index separate confinement heterostructure (GRINSCH). The graded layers avoid potential barriers induced by polarization differences in the heterostructure and serve as strain transition buffers which reduce the mosaicity of the active region and the linewidth of spontaneous emission.
Highlights
There is a high demand of ultraviolet (UV) lasers for applications in the fields of medicine and biotechnology [1], as well as in 3D printing [2] and non-line-of-sight communication [3]
The design of the structures was carried out taking into account both optical and electrical considerations: (i) the bottom cladding layer must be thick enough to prevent waveguide losses through the absorbing GaN substrate, (ii) the penetration depth of the electron beam in the structure imposes a limit to the thickness of the upper layers, and (iii) the multi-quantum well (MQW) must emit at the targeted wavelength
To compare the mosaicity of the samples, the full width at half maximum (FWHM) of the ω-scan around the (0002) X-ray reflection of the MQW is presented in Table 1 (∆ω) for all the samples
Summary
There is a high demand of ultraviolet (UV) lasers for applications in the fields of medicine and biotechnology [1], as well as in 3D printing [2] and non-line-of-sight communication [3]. The p-type doping of AlGaN is challenging [6], and the high doping concentrations required to achieve p-type conductivity degrade the material quality and increase absorption losses. This is one of the main reasons why laser diodes emitting below 370 nm show a dramatic increase in threshold current density [7,8,9,10,11]
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