In this paper we describe the scanning electron microscopy techniques of electron backscatter diffraction, electron channeling contrast imaging, wavelength dispersive X-ray spectroscopy, and cathodoluminescence hyperspectral imaging. We present our recent results on the use of these non-destructive techniques to obtain information on the topography, crystal misorientation, defect distributions, composition, doping, and light emission from a range of UV-emitting nitride semiconductor structures. We aim to illustrate the developing capability of each of these techniques for understanding the properties of UV-emitting nitride semiconductors, and the benefits were appropriate, in combining the techniques.

The nanorod structure is an alternative scheme to develop high-efficiency deep ultraviolet light-emitting diodes (DUV LEDs). In this paper, we first report the electrically injected 274-nm AlGaN nanorod array DUV LEDs fabricated by the nanosphere lithography and dry-etching technique. Nanorod DUV LED devices with good electrical properties are successfully realized. Compared to planar DUV LEDs, nanorod DUV LEDs present >2.5 times improvement in light output power and external quantum efficiency. The internal quantum efficiency of nanorod LEDs increases by 1.2 times due to the transformation of carriers from the exciton to the free electron–hole, possibly driven by the interface state effect of the nanorod sidewall surface. In addition, the nanorod array significantly facilitates photons escaping from the interior of LEDs along the vertical direction, contributing to improving light extraction efficiency. The three-dimensional finite-different time-domain simulation is performed to further analyze in detail the TE- and TM-polarized photons extraction mechanisms of the nanostructure. Our results demonstrate the nanorod structure is a good candidate for high-efficiency DUV emitters.

This paper reviews and introduces the techniques for boosting the light-extraction efficiency (LEE) of AlGaN-based deep-ultraviolet (DUV:λ<300nm) light-emitting diodes (LEDs) on the basis of the discussion of their molecular structures and optical characteristics, focusing on organoencapsulation materials. Comparisons of various fluororesins, silicone resin, and nonorgano materials are described. The only usable organomaterial for encapsulating DUV-LEDs is currently considered to be polymerized perfluoro(4-vinyloxy-1-butene) (p-BVE) terminated with a ─CF3 end group. By forming hemispherical lenses on DUV-LED dies using p-BVE having a ─CF3 end group with a refractive index of about 1.35, the LEE was improved by 1.5-fold, demonstrating a cost-feasible packaging technique.

In this work, a GaN p-i-n diode based on Mg ion implantation for visible-blind UV detection is demonstrated. With an optimized implantation and annealing process, a p-GaN layer and corresponding GaN p-i-n photodiode are achieved via Mg implantation. As revealed in the UV detection characterizations, these diodes exhibit a sharp wavelength cutoff at 365 nm, high UV/visible rejection ratio of 1.2×104, and high photoresponsivity of 0.35 A/W, and are proved to be comparable with commercially available GaN p-n photodiodes. Additionally, a localized states-related gain mechanism is systematically investigated, and a relevant physics model of electric-field-assisted photocarrier hopping is proposed. The demonstrated Mg ion-implantation-based approach is believed to be an applicable and CMOS-process-compatible technology for GaN-based p-i-n photodiodes.

The low modulation bandwidth of deep-ultraviolet (UV) light sources is considered as the main reason limiting the data transmission rate of deep-UV communications. Here, we present high-bandwidth III-nitride micro-light-emitting diodes (μLEDs) emitting in the UV-C region and their applications in deep-UV communication systems. The fabricated UV-C μLEDs with 566μm2 emission area produce an optical power of 196 μW at the 3400A/cm2 current density. The measured 3 dB modulation bandwidth of these μLEDs initially increases linearly with the driving current density and then saturates as 438 MHz at a current density of 71A/cm2, which is limited by the cutoff frequency of the commercial avalanche photodiode used for the measurement. A deep-UV communication system is further demonstrated. By using the UV-C μLED, up to 800 Mbps and 1.1 Gbps data transmission rates at bit error ratio of 3.8×10 3 are achieved assuming on-off keying and orthogonal frequency-division multiplexing modulation schemes, respectively.

The impact of operation current on the degradation behavior of 310 nm UV LEDs is investigated over 1000 h of stress. It ranges from 50 to 300 mA and corresponds to current densities from 34 to 201 A/cm2. To separate the impact of current from that of temperature, the junction temperature is kept constant by adjusting the heat sink temperature. Higher current was found to strongly accelerate the optical power reduction during operation. A mathematical model for lifetime prediction is introduced. It indicates that lifetime is inversely proportional to the cube of the current density, suggesting the involvement of Auger recombination.

AlGaN-channel high electron mobility transistors (HEMTs) were operated as visible- and solar-blind photodetectors by using GaN nanodots as an optically active floating gate. The effect of the floating gate was large enough to switch an HEMT from the off-state in the dark to an on-state under illumination. This opto-electronic response achieved responsivity >108??A/W at room temperature while allowing HEMTs to be electrically biased in the off-state for low dark current and low DC power dissipation. The influence of GaN nanodot distance from the HEMT channel on the dynamic range of the photodetector was investigated, along with the responsivity and temporal response of the floating gate HEMT as a function of optical intensity. The absorption threshold was shown to be controlled by the AlN mole fraction of the HEMT channel layer, thus enabling the same device design to be tuned for either visible- or solar-blind detection.

AlGaN nanocrystals have emerged as the building blocks of future optoelectronic devices operating in the ultraviolet (UV) spectral range. In this article, we describe the design and performance characteristics of AlGaN nanocrystal UV light-emitting diodes (LEDs) and surface-emitting UV laser diodes. The selective-area epitaxy and structural, optical, and electrical properties of AlGaN nanocrystals are presented. The recent experimental demonstrations of AlGaN nanocrystal LEDs and laser diodes are also discussed.

We report on AlGaN-based tunnel heterojunctions grown by metalorganic vapor phase epitaxy enabling fully transparent UVC LEDs by eliminating the absorbing p-AlGaN and p-GaN layers. Furthermore, the electrical characteristics can be improved by exploiting the higher conductivity of n-AlGaN layers as well as a lower resistance of n-contacts. UVC LEDs with AlGaN:Mg/AlGaN:Si tunnel junctions exhibiting single peak emission at 268 nm have been realized, demonstrating effective carrier injection into the AlGaN multiple quantum well active region. The incorporation of a low band gap interlayer enables effective tunneling and strong voltage reduction. Therefore, the interlayer thickness is systematically varied. Tunnel heterojunction LEDs with an 8 nm thick GaN interlayer exhibit continuous-wave emission powers >3 mW near thermal rollover. External quantum efficiencies of 1.4% at a DC current of 5 mA and operating voltages of 20 V are measured on-wafer. Laterally homogeneous emission is demonstrated by UV-sensitive electroluminescence microscopy images. The complete UVC LED heterostructure is grown in a single epitaxy process including in situ activation of the magnesium acceptors.