Students and faculty of the Optical Systems Technology Program at Monroe Community College have developed and deployed a narrative-based, mobile color science laboratory for public STEM outreach in the greater Rochester, New York region. The laboratory consists of a series of experimental demonstrations designed for hands-on self-exploration and guided instruction by trained optics students and outreach staff. Visitors to the lab learn about the hidden color world of white light and how different light sources generate color differently. Participants leave the classroom with a greater understanding of how illumination sources and imaging displays create the color present in their daily lives. Each experiment is designed to be robust, easily transportable, quick to assemble on any standard table, and safe for interaction by people of all ages. To date, the color science laboratory has been deployed at both on-campus and multiple, high-profile, off-site events, reaching several thousand members of the general public and inspiring the next generation of STEM students.
The continuing shortage of trained optics technicians has become a bottleneck for the precision optical manufacturing and assembly industry. The Optical Systems Technology (OST) program at Monroe Community College (MCC) has engaged with its industry partners to clarify the scope and timescale of training that will best address industry needs for qualified personnel. New modalities of instruction have been implemented to reduce the amount of time that employees and potential new hires are required to be at campus facilities to acquire core technical skills. In this paper, we present learnings from condensed laboratory training and plans for microcredential offerings to accelerate the certification process for optics technicians. These modalities have been applied to optical physics and manufacturing coursework.
We present III-Sb resonant cavity-enhanced (RCE) photodetectors suitable for gas detection in the mid-wave infrared. AlAsSb/GaSb DBRs and absorbers of bulk InAsSb or a type-II InAsSb-InAs SLS were grown on GaSb, allowing for operation at 3.72 μm or 4.52 μm, with linewidth Δλ < 50 nm and Δλ < 70 nm, respectively. A barrier diode structure was used, and the absorber thickness was limited to 96 nm for InAsSb – or 192 nm for the SLS – in order to limit the dark currents. High quantum efficiency was obtained through the resonant optical field, while the remainder of the cavity was grown using wide-gap AlAsSb spacer layers not contributing to the dark current. By carefully compensation doping the AlAsSb layers, the 3.72 μm device was bandgap-engineered for a flat Fermi level in the thin absorber, and hence dark currents which scale with the absorber thickness. This can equate to a >20x reduction in noise compared with a conventional nBn detector with full thickness absorber. At 3.72 μm, performance above the BLIP limit imposed on broadband photodetectors was found by calculating for the specific detectivity.
Inserting an infrared detector architecture into an optical cavity between two high-reflectivity mirrors allows incident light to reflect and pass through the detector multiple times, thereby enhancing absorption within the active region. This allows for a 40-100x thinner optical absorbing region compared to conventional infrared detector structures which reduces the detector dark current and noise and enhances SNR. We report the design, growth, fabrication and characterization of resonant cavity enhanced MWIR photodiodes on GaSb substrates. Devices on GaSb use AlAsSb/GaSb mirrors, AlAsSb spacer layers, and a narrow 96 nm InAsSb absorber. Dark current and detectivity behavior better than equivalent broadband nBn detectors in the literature have been observed. 34nm linewidth detector response is observed. Resonant cavity-enhanced photodiodes with resonant wavelengths of 3.6μm and 3.72μm are demonstrated with dark currents equal to or lower than Rule 07 over the operating temperature range of the device. D* in excess of 1×1010 cm Hz1/2W-1 at 300K and 8×1010 cm Hz1/2W-1 at 250K have been achieved. Amethyst Research has produced packaged resonant-cavity detectors. The 3.6 μm resonant-cavity enhanced photodiode was packaged within an Amethyst Research designed pre-amplifier package with an integrated TEC for detector cooling.
The conventional processing of the III-V nBn photodetectors defines mesa devices by etching the contact n-layer and stopping immediately above the barrier, i.e., a shallow etch. This processing enables great suppression of surface leakage currents without having to explore surface passivation techniques. However, devices that are made with this processing scheme are subject to lateral diffusion currents. To address the lateral diffusion current, we compare the effects of different processing approaches and epitaxial structures of nBn detectors. The conventional solution for eliminating lateral diffusion current, a deep etch through the barrier and the absorber, creates increased dark currents and an increased device failure rate. To avoid deep etch processing, a new device structure is proposed, the inverted-nBn structure. By comparing with the conventional nBn structure, the results show that the lateral diffusion current is effectively eliminated in the inverted-nBn structure without elevating the dark currents.
This work presents a fundamental investigation of the surface conduction pathways occurring along etched sidewalls in devices fabricated from InAs and GaSb. Surface leakage currents are identified by their dependence on device size and thermal activation energy, and are characterized in terms of sheet conductance. InAs is found to have a temperature-independent sheet conductance of approximately 8×10-8 mho×square. The sheet conductance of GaSb is comparable to that of InAs at room temperature, and when cooled it decreases with a thermal activation energy of 75 meV, which is approximately equal to the known separation between the valence band and surface Fermi level. The temperature dependence of the surface conductance of the two materials indicates that the surface of InAs is degenerate and the surface of GaSb is non-degenerate.
The e-SWIR wavelength band is a performance gap for infrared detectors. At both shorter and longer wavelengths, high
performance detector technologies exist: SWIR InGaAs detectors (1.7 micron cutoff), and MWIR (3-5 micron) detectors
such as InAs-based and GaSb-based Unipolar Barriers, MCT, and InSb. This work discusses development of high
performance e-SWIR detectors with cutoff wavelengths in the 2.7 - 2.8 micron range.
Two approaches for e-SWIR detector absorber materials were evaluated, lengthening the wavelength response of the
SWIR InGaAs technology and shortening the wavelength response of MWIR GaSb-based technology. The InGaAs e-
SWIR approach employs mismatched InGaAs absorber layers on InP substrates, using graded AlInAs buffer layers. The
GaSb-based approach uses lattice-matched InGaAsSb absorber layers on GaSb substrates. Additionally, two device
architectures were examined, pn-based photodiodes and unipolar barrier photodiodes. For both of the absorber materials,
the unipolar barrier device architecture was found to be superior.
The unipolar barrier device architecture enables both types of device to be free of effects of surface leakage currents and
generation-recombination dark currents. InGaAsSb-based devices show excellent performance, with diffusion-limited
dark current within a factor of 2-4 of the HgCdTe standard, Rule 07. They achieve background-limited (BLIP)
performance at T=210K, which is accessible by thermo-electric coolers. As expected, defects associated with latticemismatch
increase dark currents of the InP-based approach. The dark currents of the mismatched unipolar barrier
photodiodes are 30x larger than those of the lattice-matched GaSb approach, however despite the defects, the devices
still exhibit diffusion-limited operation, and achieve BLIP operation at T=190K Further improvements in the InP-based
approach are expected with refinements in the epitaxial structures. Both types of detector approaches are excellent
alternatives to conventional e-SWIR detectors.
Under elevated defect concentrations, MWIR, III-V nBn detectors exhibit diffusion limited performance with elevated dark current densities. The resulting diffusion current is limited by the generation of carriers through defect states in the neutral n-type absorber and a dark current dependence on the defect density described by one of two limits, a short absorber or long absorber limit. This characteristic contrasts that exhibited by defect limited, conventional pn junction based photodiodes which exhibit performance limited by Shockley-Read-Hall generation in the depletion layer rather than diffusion based processes.
The effect of defects on the dark current characteristics of MWIR, III-V nBn detectors has been studied. Two different types of defects are compared, those produced by lattice mismatch and by proton irradiation. It is shown that the introduction of defects always elevates dark currents; however the effect on dark current is different for nBn detectors and conventional photodiodes. The dark currents of nBn detectors are found to be more tolerant of defects compared to pn-junction based devices. Defects more weakly increase dark currents, and cooling reduces the defect-produced dark currents more rapidly in nBn detectors than in conventional photodiodes.
The unipolar barrier is a new approach for control of dark currents in infrared photodetectors. First demonstrated in the
nBn detector and then in the unipolar barrier photodiode, unipolar barriers have been shown to block surface leakage
current. Unipolar barriers can also be implemented to filter out dark current components such as Shockley-Read-Hall
current, direct band-to-band tunneling and trap-assisted tunneling, but are not useful for blocking diffusion currents.
Current density-voltage characteristics of molecular-beam-epitaxy-grown InAs based unipolar barrier photodiodes are
presented and analyzed, showing effective limiting of noise current mechanisms for different unipolar barrier photodiode
architectures. RoA data shows near Auger-limited device performance and RoA values in excess of 1x107 Ω-cm2.
Excessive surface leakage currents, and their associated noise, deteriorate the performance of infrared photodetectors.
The conventional approach to suppress surface leakage is post-epitaxy deposition of polycrystalline or amorphous
passivation layers. Disadvantages of such passivation layers are the cost and complexity of the required additional
processing steps, and the fact that they do not always work well. An alternative approach, presented here, is to design the
photodetectors' epitaxial structures so that surface leakage currents are suppressed without the need for ex-situ
deposition of passivation layers. Two examples of such epitaxial designs are the nBn detector and the unipolar barrier
photodiode.
Emerging technologies and continuing progress in research are making way for novel, high speed forms of optical data transfer. Vertical-cavity surface emitting laser (VCSEL) diodes operating at 1550nm have only recently become commercially available, while metal-semiconductor-metal (MSM) photodetectors are pushing the limits of contact lithography with interdigitated electrode widths reaching sub micron levels. We propose a novel, free-space optical interconnect operating up to 1Gbit/s utilizing commercially available 1550nm VCSEL diodes and newly fabricated InGaAs MSM photodetectors with functionality for both chip level and board level applications. We report on development, progress, and current work. Analyses of the divergent behavior and of the normalized frequency response of VERTILAS GmbH 1550nm VCSEL diodes for coupling to MSM photodetectors with a 50μm by 50μm active area are presented. The MSM photodetectors are fabricated on a pseudomorphic In0.9Ga0.1P-InP-InGaAs heterostructure and have gold interdigitated Shottky contacts with varying electrode width and spacing on the order of 1 to 3 microns. We discuss the calculated response of these MSM photodetectors as well as the fabrication and characterization of the devices. A report on bit error rate (BER) characteristics of the VCSEL diodes coupled to commercially available high-speed photodetectors and successively coupled to the novel MSM photodetectors integrated with commercially available transimpedance amplifiers (TIA) follows. The work accounted here will lead to the formation and characterization of a fully integrated 1Gbit/s free-space optical interconnect applying VCSEL diodes and MSM photodetectors operating at 1550nm for RF/microwave digital systems.
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