We present a comprehensive study of the performance of GaN single-nanowire photodetectors containing an axial p-n junction. The electrical contact to the p region of the diode is made by including a p + /n + tunnel junction as cap structure, which allows the use of the same metal scheme to contact both ends of the nanowire. Single-nanowire devices present the rectifying current-voltage characteristic of a p-n diode, but their photovoltaic response to ultraviolet radiation scales sublinearly with the incident optical power. This behavior is attributed to the dominant role of surface states. Nevertheless, when the junction is reverse biased, the role of the surface becomes negligible in comparison to the drift of photogenerated carriers in the depletion region. Therefore, the responsivity increases by about three orders of magnitude and the photocurrent scales linearly with the excitation. These reverse-biased nanowires display decay times in the range of » 10 µs, limited by the resistance ´ capacitance time constant of the setup. Their ultraviolet/visible contrast of several orders of magnitude is suitable for applications requiring high spectral selectivity. When the junction is forward biased, the device behaves as a GaN photoconductor, with an increase of the responsivity at the price of a degradation of the time response. The presence of leakage current in some of the wires can be modeled as a shunt resistance which reacts to the radiation as a photoconductor and can dominate the response of the wire even under reverse bias.

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.

Electron beam pumping is a promising technique to fabricate compact and efficient light emitters (lamps or lasers) in those spectral ranges where electrical injection is problematic due doping, transport and contacting issues. Interest in this technology has increased in recent years, particularly driven by the demand for ultraviolet sources and the difficulties in developing efficient AlGaN devices to cover the spectral range of 220-350 nm. The use of a highly energetic electron beam enables the semiconductor structure to be pumped without the need for doping or contacting. The active volume is defined by the accelerating voltage, which allows the homogeneous excitation of a large active volume. The efficiency of cathodoluminescent lamps can compete and even outperform LEDs in the deep ultraviolet window, and lasers can deliver high optical power (up to around 100 W). Here, we analyze the advantages and challenges of this technology platform, and discuss its potential applications.