Industries such as the automotive, aerospace or military, as well as environmental and biological research have promoted the development of ultraviolet (UV) photodetectors capable of operating at high temperatures and in hostile environments. UV-enhanced Si photodiodes are hence giving way to a new generation of UV detectors fabricated from wide-bandgap semiconductors, such as SiC, diamond, III-nitrides, ZnS, ZnO, or ZnSe. This paper provides a general review of latest progresses in wide-bandgap semiconductor photodetectors.

https://iopscience.iop.org/article/10.1088/0268-1242/18/4/201/pdf

Wide-bandgap semiconductor ultraviolet photodetectors

System, devices and methods are presented that integrate stretchable or flexible circuitry, including arrays of active devices for enhanced sensing, diagnostic, and therapeutic capabilities. The invention enables conformal sensing contact with tissues of interest, such as the inner wall of a lumen, a the brain, or the surface of the heart. Such direct, conformal contact increases accuracy of measurement and delivery of therapy. Further, the invention enables the incorporation of both sensing and therapeutic devices on the same substrate allowing for faster treatment of diseased tissue and fewer devices to perform the same procedure.

Systems,methods, and devices having stretchable integrated circuitry for sensing and delivering therapy

The need for efficient, compact and robust solid-state UV optical sources and sensors had stimulated the development of optical devices based on III–nitride material system. Rapid progress in material growth, device fabrication and packaging enabled demonstration of high efficiency visible-blind and solar-blind photodetectors, deep-UV light-emitting diodes with emission from 400 to 250 nm, and UV laser diodes with operation wavelengths ranging from 340 to 350 nm. Applications of these UV optical devices include flame sensing; fluorescence-based biochemical sensing; covert communications; air, water and food purification and disinfection; and biomedical instrumentation. This paper provides a review of recent advances in the development of UV optical devices. Performance of state-of-the-art devices as well as future prospects and challenges are discussed.

https://iopscience.iop.org/article/10.1143/JJAP.44.7191/pdf

III-Nitride UV Devices

Overmolded lenses and certain fabrication techniques are described for LED structures. In one embodiment, thin YAG phosphor plates are formed and affixed over blue LEDs mounted on a submount wafer. A clear lens is then molded over each LED structure during a single molding process. The LEDs are then separated from the wafer. The molded lens may include red phosphor to generate a warmer white light. In another embodiment, the phosphor plates are first temporarily mounted on a backplate, and a lens containing a red phosphor is molded over the phosphor plates. The plates with overmolded lenses are removed from the backplate and affixed to the top of an energizing LED. A clear lens is then molded over each LED structure. The shape of the molded phosphor-loaded lenses may be designed to improve the color vs. angle uniformity. Multiple dies may be encapsulated by a single lens. In another embodiment, a prefabricated collimating lens is glued to the flat top of an overmolded lens.

LED with phosphor tile and overmolded phosphor in lens

Recent advances in the research work on III-nitride semiconductors and Al/sub x/Ga/sub 1-x/N materials in particular has renewed the interest and led to significant progress in the development of ultraviolet (UV) photodetectors able to detect light in the mid- and near-UV spectral region (/spl lambda//spl sim/200-400 nm). There have been a growing number of applications which require the use of such sensors and, in many of these, it is important to be able to sense UV light without detecting infrared or visible light, especially from the Sun, in order to minimize the chances of false detection or high background. The research work on short-wavelength UV detectors has, therefore, been recently focused on realizing short-wavelength "solar-blind" detectors which, by definition, are insensitive to photons with wavelengths longer than /spl sim/285 nm. In this paper the development of Al/sub x/Ga/sub 1-x/N-based solar-blind UV detectors will be reviewed. The technological issues pertaining to material synthesis and device fabrication will be discussed. The current state-of-the-art and future prospects for these detectors will be reviewed and discussed.

Short-wavelength solar-blind detectors-status, prospects, and markets

We report the electrical and optical characteristics of avalanche photodiodes fabricated in GaN grown by metalorganic chemical vapor deposition. The current–voltage characteristics indicate a multiplication of >25. Experiment indicates and simulation verifies that the magnitude of the electric field at the onset of avalanche gain is ⩾3 MV/cm. Small-area devices exhibit stable gain with no evidence of microplasmas.

GaN avalanche photodiodes

A novel impact-ionization-engineered multiplication region for avalanche photodiodes is reported. By pseudograding the multiplication region with materials of different ionization threshold energies, the impact ionization of the injected carrier type is localized, while that of the feedback carrier type is suppressed. Low noise (k/sub eff/<0.1), low dark current, and high gain were achieved.

Low-noise avalanche photodiodes with graded impact-ionization-engineered multiplication region

We report low noise multiplication region structures designed for avalanche photodiodes grown on InP substrates. By either implementing a single heterostructure or using a pseudograded structure in the multiplication region, better control of spatial distribution of impact-ionization for both injected and feedback carriers can be achieved; localization of the carrier impact ionization process has resulted in very low excess noise.

Low-noise impact-ionization-engineered avalanche photodiodes grown on InP substrates

We report the fabrication and characterization of GaN avalanche photodiodes grown on sapphire by metalorganic chemical vapor deposition. Current-voltage characteristics indicate a gain higher than 23. The photoresponse is independent of the bias voltage prior to the onset of avalanche gain which occurs at an electric field of /spl sim/4 MV/cm. Near avalanche breakdown, the dark current of a 30-/spl mu/m diameter device is less than 100 pA. The breakdown shows a positive temperature coefficient of 0.03 V/K that is characteristic of avalanche breakdown.

Low dark current GaN avalanche photodiodes

A dark count rate in InP-based single photon counting avalanche photodiodes is a limiting factor to their efficacy. The temperature dependence of the dark count rate was studied to understand its origin in In0.53Ga0.47As∕In0.52Al0.48As separate-absorption-charge-multiplication avalanche photodiodes. The dark count rate was observed to be a very weak function of temperature in the range from 77Kto300K. Various mechanisms for dark count generation were considered. Simulations of band-to-band tunneling in the In0.52Al0.48As multiplication layer were found to agree well with the experimental temperature dependence of dark count rate at various excess biases. To reduce tunneling-induced dark counts, a suitable design change to the detector structure is proposed.

S. Wang

Papers

GaN avalanche photodiodes

Low-noise avalanche photodiodes with graded impact-ionization-engineered multiplication region

Low-noise impact-ionization-engineered avalanche photodiodes grown on InP substrates

Low dark current GaN avalanche photodiodes

Origin of dark counts in In0.53Ga0.47As∕In0.52Al0.48As avalanche photodiodes operated in Geiger mode