  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.
