Single Photon Counting UV Solar-Blind Detectors Using Silicon and III-Nitride Materials

Ultraviolet (UV) studies in astronomy, cosmology, planetary studies, biological and medical applications often require precision detection of faint objects and in many cases require photon-counting detection. We present an overview of two approaches for achieving photon counting in the UV. The first approach involves UV enhancement of photon-counting silicon detectors, including electron multiplying charge-coupled devices and avalanche photodiodes. The approach used here employs molecular beam epitaxy for delta doping and superlattice doping for surface passivation and high UV quantum efficiency. Additional UV enhancements include antireflection (AR) and solar-blind UV bandpass coatings prepared by atomic layer deposition. Quantum efficiency (QE) measurements show QE > 50% in the 100–300 nm range for detectors with simple AR coatings, and QE ≅ 80% at ~206 nm has been shown when more complex AR coatings are used. The second approach is based on avalanche photodiodes in III-nitride materials with high QE and intrinsic solar blindness

Single Photon Counting UV Solar-Blind Detectors Using Silicon and III-Nitride Materials

Ultraviolet (UV) studies in astronomy, cosmology, planetary studies, biological and medical applications often require precision detection of faint objects and in many cases require photon-counting detection. We present an overview of two approaches for achieving photon counting in the UV. The first approach involves UV enhancement of photon-counting silicon detectors, including electron multiplying charge-coupled devices and avalanche photodiodes. The approach used here employs molecular beam epitaxy for delta doping and superlattice doping for surface passivation and high UV quantum efficiency. Additional UV enhancements include antireflection (AR) and solar-blind UV bandpass coatings prepared by atomic layer deposition. Quantum efficiency (QE) measurements show QE > 50% in the 100–300 nm range for detectors with simple AR coatings, and QE ≅ 80% at ~206 nm has been shown when more complex AR coatings are used. The second approach is based on avalanche photodiodes in III-nitride materials with high QE and intrinsic solar blindness

Enabling Technologies for Next Generation Ultraviolet Astrophysics, Planetary, and Heliophysics Missions

Our study sought to create a new paradigm in UV instrument design, detector technology, and optics that will form the technological foundation for a new generation of ultraviolet missions. This study brought together scientists and technologists representing the broad community of astrophysicists, planetary and heliophysics physicists, and technologists working in the UV. Next generation UV missions require major advances in UV instrument design, optics and detector technology. UV offers one of the few remaining areas of the electromagnetic spectrum where this is possible, by combining improvements in detector quantum efficiency (5-10x), optical coatings and higher-performance wide-field spectrometers (5-10x), and increasing multiplex advantage (100-1000x). At the same time, budgets for future missions are tightly constrained. Attention has begun to turn to small and moderate class missions to provide new observational capabilities on timescales that maintain scientific vitality. Developments in UV technology offer a comparatively unique opportunity to conceive of small (Explorer) and moderate (Probe, Discovery, New Millennium) class missions that offer breakthrough science. Our study began with the science, reviewing the breakthrough science questions that compel the development of new observational capabilities in the next 10-20 years. We invented a framework for highlighting the objectives of UV measurement capabilities: following the history of baryons from the intergalactic medium to stars and planets. In astrophysics, next generation space UV missions will detect and map faint emission and tomographically map absorption from intergalactic medium baryons that delineate the structure of the Universe, map the circum-galactic medium that is the reservoir of galaxy-building gas, map the warm-hot ISM of our Galaxy, explore star-formation within the Local group and beyond, trace gas in proto-planetary disks and extended atmospheres of exoplanets, and record the transient UV universe. Solar system planetary atmospheric physics and chemistry, aurorae, surface composition and magnetospheric environments and interactions will be revealed using UV spectroscopy. UV spectroscopy may even detect life on an exoplanet. Our study concluded that with UV technology developments within reach over the next 5- 10 years, we can conceive moderate-class missions that will answer many of the compelling science questions driving the field. We reviewed the science measurement requirements for these pioneering new areas and corresponding technology requirements. We reviewed and evaluated the emerging technologies, and developed a figure of merit based on potential science impact, state of readiness, required investment, and potential for highly leveraged progress in a 5-10 year horizon. From this we were able to develop a strategy for technology development. Some of this technology development will be subject to funding calls from federal agencies. A subset form a portfolio of highly promising technologies that are ideal for funding from a KISS Development Program. One of our study’s principal conclusions was that UV detector performance drives every aspect of the scientific capability of future missions, and that two highly flexible detector technologies were at the tipping point for major breakthroughs. These are Gen-2 borosilicate Atomic Layer Deposition (ALD) coated microchannel plate detectors with GaN photocathodes, and ALDantireflection (AR) coated, delta-doped photon-counting CCD detectors. Both offer the potential for QE>50% combined with large formats and pixel counts, low background, and sky-limited photon-counting performance over the 100-300 nm band. Ramped AR coatings for spectroscopic detectors could achieve QE’s as high as 80%! A second conclusion was that UV coatings are on the threshold of a major breakthrough. UV coatings permeate every aspect of telescope and instrument design. Efficient, robust, ultra-thin and highly uniform reflective coatings applied with Atomic Layer Deposition (ALD) offer the possibility of high-performance, wide-field, highly-multiplexed UV spectrometers and a broadband reach covering the scientifically critical 100-120 nm range (home of 50% of all atomic and molecular resonance lines). Our study concluded that UV coating advances made possible by ALD is the principle technology advance that will enable a joint UV-optical general astrophysics and exoEarth imaging flagship mission. A third conclusion was that the revolution in micro- and nano-fabrication technology offers a cornucopia of new possibilities for revolutionary UV technology developments in the near future. An immediate example is the application of new microlithography techniques to patterning UV diffraction gratings that are highly efficient and designed to enable wide-field, high-resolution spectroscopy. These techniques could support the development of new detectors that could discriminate optical and UV photons and potentially energy-resolving detection. Relatively modest investments in technology development over the next 5-10 years could provide advances in detectors, coatings, diffractive elements, and filters that would result in an effective increase in science capability of 100-1000! The study brought together a diverse community, led to many new ideas and collaborations, and brought cohesion and common purpose to UV practitioners. This will have a lasting and positive impact on the future of our field

Low Voltage Low Light Imager and Photodetector

Highly efficient, low energy, low light level imagers and photodetectors are provided. In particular, a novel class of Della-Doped Electron Bombarded Array (DDEBA) photodetectors that will reduce the size, mass, power, complexity, and cost of conventional imaging systems while improving performance by using a thinned imager that is capable of detecting low-energy electrons, has high gain, and is of low noise

Materials and process development for the fabrication of far ultraviolet device-integrated filters for visible-blind Si sensors

In this work, we show that the direct integration of ultraviolet metal-dielectric filters with Si sensors can improve throughput over external filter approaches, and yield devices with UV quantum efficiencies greater than 50%, with rejection ratios of visible light greater than 10^3. In order to achieve these efficiencies, two-dimensional doping methods are used to increase the UV sensitivity of back-illuminated Si sensors. Integrated filters are then deposited by a combination of Al evaporation and atomic layer deposition of dielectric spacer layers. At far UV wavelengths these filters require the use of non-absorbing dielectrics, and we have pursued the development of new atomic layer deposition processes for metal fluorides materials of MgF_2, AlF_3 and LiF. The performance of the complete multilayer filters on Si photodiodes and CCD imaging sensors, and the design and fabrication challenges associated with this development are demonstrated. This includes the continued development of deep diffused silicon avalanche photodiodes designed to detect the fast 220 nm emission component of barium fluoride scintillation crystals, while optically rejecting a slower component at 300 nm

Enabling Technologies for Next Generation Ultraviolet Astrophysics, Planetary, and Heliophysics Missions

Our study sought to create a new paradigm in UV instrument design, detector technology, and optics that will form the technological foundation for a new generation of ultraviolet missions. This study brought together scientists and technologists representing the broad community of astrophysicists, planetary and heliophysics physicists, and technologists working in the UV. Next generation UV missions require major advances in UV instrument design, optics and detector technology. UV offers one of the few remaining areas of the electromagnetic spectrum where this is possible, by combining improvements in detector quantum efficiency (5-10x), optical coatings and higher-performance wide-field spectrometers (5-10x), and increasing multiplex advantage (100-1000x). At the same time, budgets for future missions are tightly constrained. Attention has begun to turn to small and moderate class missions to provide new observational capabilities on timescales that maintain scientific vitality. Developments in UV technology offer a comparatively unique opportunity to conceive of small (Explorer) and moderate (Probe, Discovery, New Millennium) class missions that offer breakthrough science. Our study began with the science, reviewing the breakthrough science questions that compel the development of new observational capabilities in the next 10-20 years. We invented a framework for highlighting the objectives of UV measurement capabilities: following the history of baryons from the intergalactic medium to stars and planets. In astrophysics, next generation space UV missions will detect and map faint emission and tomographically map absorption from intergalactic medium baryons that delineate the structure of the Universe, map the circum-galactic medium that is the reservoir of galaxy-building gas, map the warm-hot ISM of our Galaxy, explore star-formation within the Local group and beyond, trace gas in proto-planetary disks and extended atmospheres of exoplanets, and record the transient UV universe. Solar system planetary atmospheric physics and chemistry, aurorae, surface composition and magnetospheric environments and interactions will be revealed using UV spectroscopy. UV spectroscopy may even detect life on an exoplanet. Our study concluded that with UV technology developments within reach over the next 5- 10 years, we can conceive moderate-class missions that will answer many of the compelling science questions driving the field. We reviewed the science measurement requirements for these pioneering new areas and corresponding technology requirements. We reviewed and evaluated the emerging technologies, and developed a figure of merit based on potential science impact, state of readiness, required investment, and potential for highly leveraged progress in a 5-10 year horizon. From this we were able to develop a strategy for technology development. Some of this technology development will be subject to funding calls from federal agencies. A subset form a portfolio of highly promising technologies that are ideal for funding from a KISS Development Program. One of our study’s principal conclusions was that UV detector performance drives every aspect of the scientific capability of future missions, and that two highly flexible detector technologies were at the tipping point for major breakthroughs. These are Gen-2 borosilicate Atomic Layer Deposition (ALD) coated microchannel plate detectors with GaN photocathodes, and ALDantireflection (AR) coated, delta-doped photon-counting CCD detectors. Both offer the potential for QE>50% combined with large formats and pixel counts, low background, and sky-limited photon-counting performance over the 100-300 nm band. Ramped AR coatings for spectroscopic detectors could achieve QE’s as high as 80%! A second conclusion was that UV coatings are on the threshold of a major breakthrough. UV coatings permeate every aspect of telescope and instrument design. Efficient, robust, ultra-thin and highly uniform reflective coatings applied with Atomic Layer Deposition (ALD) offer the possibility of high-performance, wide-field, highly-multiplexed UV spectrometers and a broadband reach covering the scientifically critical 100-120 nm range (home of 50% of all atomic and molecular resonance lines). Our study concluded that UV coating advances made possible by ALD is the principle technology advance that will enable a joint UV-optical general astrophysics and exoEarth imaging flagship mission. A third conclusion was that the revolution in micro- and nano-fabrication technology offers a cornucopia of new possibilities for revolutionary UV technology developments in the near future. An immediate example is the application of new microlithography techniques to patterning UV diffraction gratings that are highly efficient and designed to enable wide-field, high-resolution spectroscopy. These techniques could support the development of new detectors that could discriminate optical and UV photons and potentially energy-resolving detection. Relatively modest investments in technology development over the next 5-10 years could provide advances in detectors, coatings, diffractive elements, and filters that would result in an effective increase in science capability of 100-1000! The study brought together a diverse community, led to many new ideas and collaborations, and brought cohesion and common purpose to UV practitioners. This will have a lasting and positive impact on the future of our field

Design, fabrication and characterization of III-nitride PN junction devices

Design, fabrication and characterization of III-Nitride pn junction devices Jae Boum Limb 94 pages Directed by Dr. Russell D. Dupuis This dissertation describes an investigation of three types of III-nitride (AlInGaN) based p-n junction devices that were grown by metalorganic chemical vapor deposition (MOCVD). The three types of devices are Ultra-Violet (UV) avalanche photodiodes (APDs), green light emitting diodes (LEDs), and p-i-n rectifiers. For avalanche photodiodes, a material growth on low-dislocation density GaN substrates, processed with low-damage etching receipes and high quality dielectric passivations, were proposed. Using this technology, GaN APDs with optical gains greater than 3000, and AlGaN APDs showing true avalanche gains have been demonstrated. For green LEDs, the use of InGaN:Mg as the p-layer, rather than employing the conventional GaN:Mg has been proposed. Green LEDs with p-InGaN have shown higher emission intensities and lower diode series resistances compared to LEDs with p-GaN. Using p-InGaN layers, LEDs emitting at green and longer wavelengths have been realized. For p-i-n rectifiers, design, fabrication and characterization of device structures using the conventional mesa-etch configuration, as well as the full-vertical method have been proposed. High breakdown devices with low on-resistances have been achieved. Specific details on device structures, fabrication methods, and characterization results are discussed.Ph.D.Committee Chair: Russell Dupuis; Committee Member: David Citrin; Committee Member: Joy Laskar; Committee Member: Srinivas Garimella; Committee Member: William Doolittl

Advanced imaging capabilities by incorporating plasmonics and metamaterials in detectors

Ultraviolet detection is often required to be made in the presence of a strong background of solar radiation which needs to be suppressed, but materials limitations at these wavelengths can impact both filter and sensor performance. In this work, we explore the use of 1D photonic bandgap structures integrated directly onto a Si sensor that can operate with solar blindness. These filters take advantage of the improved admittance with silicon to significantly improve throughput over conventional stand-alone bandpass filter elements. At far ultraviolet wavelengths these filters require the use of non-absorbing dielectrics such as the metal fluoride materials of MgF_2, AlF_3 and LiF. The latest performance of these 1D multilayer filters on Si photodiodes and CCD imaging sensors is demonstrated. We have also extended these 1D structures to more complex multilayers guided by the design concepts of metamaterials and metatronics, and to 2D patterned plasmonic hole array filters fabricated in aluminum. The performance of sensors and test filter structures is presented with an emphasis on UV throughput

AlxGa1-xN based solar blind Schottky photodiodes

Cataloged from PDF version of article.Photodetectors are essential components of optoelectronic integrated circuits and fiber optic communication systems. AlxGa1−xN is a promising material for optoelectronics and electronics. Applications include blue and green LEDs, blue laser diodes, high power-high frequency electronics, and UV photodetectors. Photodetectors that operate only in the λ < 280 nm spectrum are called solarblind detectors due to their blindness to solar radiation within the atmosphere. In this thesis, we present our efforts for the design, fabrication and characterization of Al0.38Ga62N/GaN based solar blind Schottky photodiodes. We obtained very low dark current, high quantum efficiency, high detectivity performance. Under 25 V reverse bias, we measured a maximum quantum efficiency of 71 percent at 254 nm and a maximum responsivity of 0.15 A/W at 253 nm for a 150 micron diameter device. To our knowledge, these are the best values reported in the literature. For a 30 micron device, 50 ps FWHM pulse response is observed. When the scope response is deconvoluted, a maximum 3-dB bandwidth of 4.0 GHz is obtained for 30 micron diameter Schottky photodiodes.Tut, TurgutM.S