Infrared sensors are an essential technology in the rapid detection and identification of potential threats. Continuous improvements in performance and yield as well as reductions in cost, size, weight, and power consumption are necessary to keep pace with new and emerging threats that are increasing in number and have greater range, speed, and maneuverability. Yield and performance limiting challenges of cooled infrared detector materials and infrared focal plane array processing will be discussed. Paths to addressing some of these challenges including surface passivation, defect identification and analysis, and pixel definition will be presented.
During last decade, several Government-funded programs resulted in a groundbreaking new infrared focal plane array (FPA) material that serves as a low cost and more reliable FPA alternative for DoD military systems. These new IR FPAs use the Sb-based III-V semiconductor Type-II Superlattice (T2SL) infrared detector technology with bandgap-engineered device architectures. Follow on programs have accelerated the manufacturability of T2SL FPAs sensitive in single and multiple infrared bands with inherent operability, uniformity, and stability advantages. Today, T2SL materials grown on very-large diameter (up to 6”) gallium antimonide (GaSb) substrates with higher processing yields provide a much lower cost option for very large format staring imagers in mid- and long-wavelength infrared bands. This talk reviews the current state of T2SL FPA technology and discusses remaining challenges for further advancement of this infrared detector technology.
Actoprobe team had developed custom Tip Enhancement Raman Spectroscopy System (TERS) with specially developed Ultra High Aspect Ratio probes for AFM and TERS measurements for small pixel infrared FPA sidewall characterization. Using this system, we report on stimulated Raman scattering observed in a standard tip-enhanced Raman spectroscopy (TERS) experiment on GaSb materials excited by 637-nm pump laser light. We explain our results by TERS-inherent mechanisms of enormous local field enhancement and by the special design and geometry of the ultrahigh-aspect-ratio tips that enabled conditions for stimulated Raman scattering in the sample with greatly enhanced resonance Raman gain when aided by a microcavity to provide feedback mechanism for the Raman emission. The approach has great potential for further, orders-of-magnitude, progress in TERS enhancement by significantly increasing its nonlinear component. We report development of novel class of probes for atomic force microscopy (AFM active optical probe - AAOP) by integrating a laser source and a photodetector monolithically into the AFM probe. The AAOPs are designed to be used in a conventional AFM and would enhance its functionality to include that of the instruments (NSOM, TERS, hybrid AFM).
Mid-wavelength infrared photodetectors incorporated into a unipolar barrier architecture with a bulk InAsxSb1-x absorber and an AlSb barrier layer are demonstrated. An extended cutoff was achieved by increasing the lattice constant from 6.09 Å of the GaSb substrate to 6.13 Å using a 1.5 μm thick AlSb buffer layer. This enabled the growth of bulk absorber material with a higher antimony content, InAs0.81Sb0.19, and a greater than 5 μm cutoff. Transitioning the lattice to 6.13 Å also enabled the implementation of a simple binary AlSb layer as a unipolar barrier to block majority carrier electrons and reduce dark current noise. Individual test devices with 4 μm thick absorbers displayed 150 K dark current density, cutoff wavelength, quantum efficiency, and specific detectivity of 3 x 10-5 A/cm2, 5.31 μm, 44 % at 3.4 μm, and 4.3 x 1011 cmHz1/2/W at 5 μm, respectively. The instantaneous dark current activation energy at a given bias and temperature was determined via Arrhenius analysis from the dark current vs. temperature and bias data, and a discussion of valence band alignment between the InAsxSb1-x absorber and AlSb barrier layers is presented. The carrier concentration, mobility, and lifetime of the bulk absorber material and the device performance will be presented and a discussed.
We propose to utilize confocal Raman spectroscopy combined with high resolution atomic force microscopy (AFM) for nondestructive characterisation of the sidewalls of etched and passivated small pixel (24 μm×24 μm) focal plane arrays (FPA) fabricated using LW/LWIR InAs/GaSb type-II strained layer superlattice (T2SL) detector material. Special high aspect ratio Si and GaAs AFM probes, with tip length of 13 μm and tip aperture less than 7°, allow characterisation of the sidewall morphology. Confocal microscopy enables imaging of the sidewall profile through optical sectioning. Raman spectra measured on etched T2SL FPA single pixels enable us to quantify the non-uniformity of the mesa delineation process.
ZnSe and other zinc chalcogenide semiconductor materials can be doped with divalent transition metal ions to create a
mid-IR laser gain medium with active function in the wavelength range 2 - 5 microns and potentially beyond using
frequency conversion. As a step towards fiberized laser devices, we have manufactured ZnSe semiconductor fiber
waveguides with low (less than 1dB/cm at 1550nm) optical losses, as well as more complex ternary alloys with
ZnSxSe(1-x) stoichiometry to potentially allow for annular heterostructures with effective and low order mode corecladding
waveguiding.
Diminished performance due to poor chemical and electrical surface stability of InAs/GaSb SL photodetectors continues
to be a major hurdle to the realization of the theoretically predicted high performance of this material system. Improved
epitaxial growth conditions have yielded improvements in material quality over the past several years. However, surface
instability resulting in electrical shunt pathways across the junction, and diminished device performance over time is still
a major limiting factor for application of InAs/GaSb SL in long-wavelength infrared detectors. This study focuses on a
two-step approach towards the successful surface passivation of long-wavelength InAs/GaSb superlattice structures.
Two distinct sulfide chemical surface treatments were applied to inhibit the formation of native surface oxides and
satisfy dangling bonds. This was followed by the application of a robust SU8-2 dielectric treatment on the mesa
sidewalls to inhibit sulfide layer degradation and oxidation of the surface over time. A variable area diode analysis
(VADA) technique employing diodes of variable diameter (40-400um) enabled the investigation of surface resistivity as
a result of different passivation treatments. Temperature dependent studies of the dark current were used to understand
the dominating current mechanisms.
Integration of semiconductor and metal structures into optical fibers to enable fusion of semiconductor optoelectronic
function with glass optical fibers is discussed. A chemical vapor deposition (CVD)-like process, adapted for high pressure
flow within microstructured optical fibers allows for flexible fabrication of such structures. Integration of semiconductor
optoelectronic devices such as lasers, detectors, and modulators into fibers may now become possible.
We have recently fabricated continuous semiconducting micro and nanowires within the empty spaces of highly ordered microstructured (e.g., photonic crystal or holey) optical fibers (MOF's). These systems contain the highest aspect ratio semiconductor micro- and nanowires yet produced by any method: centimeters long and ~100 nm in diameter. These structures combine the flexible light guiding capabilities of an optical fiber with the electronic and optical functionalities of semiconductors and have many potential applications for in-fiber sensing, including in-fiber detection, modulation, and generation of light.
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