Iridium (Ir) is a refractory metal commonly seen in industrial applications, but has great potential for optical applications including metasurfaces. Metasurfaces are used to control the optical properties of an interface via sub-wavelength surface structures. These patterns require sharply defined features to create precise optical phase interactions. For high-temperature environments, most materials are insufficient candidates for metasurfaces because the sharpness of the surface structures are lost due to edge-rounding or oxidation. Ir is better suited for metasurface applications in high-temperature environments but the patterning of Ir using nanofabrication techniques has not been thoroughly investigated. In this work, Ir metasurfaces were fabricated and characterized for optical applications in the infrared.
Variable angle spectroscopic ellipsometry was used to determine the optical properties of n- and p-type GaAs over a doping range of 4.6×1016 to 9.3×1018 cm-3 and a spectral range of 190 nm to 30 μm. Increased doping concentration was observed to have several distinct effects on the samples’ optical properties: the band edge broadens and shifts to a higher energy; the E1 and (E1 + Δ1) absorption peaks blur together; the E2 absorption peak decreases; sub-bandgap, infrared absorption increases. Additionally, the doping effects are generally stronger for n-type than for p-type GaAs. These findings will help inform future design of optoelectronics.
KEYWORDS: Sensors, Antennas, Long wavelength infrared, Absorption, Dielectrics, Resonators, Infrared sensors, Chemical elements, Signal to noise ratio, Etching
Antenna coupled detectors break the intrinsic tradeoff between signal and noise by “collecting over a large area” and “detecting over a small area”. Most antenna coupled detectors in the infrared rely on a metal resonator structure. However, there are losses associated with metallic structures. We have demonstrated a novel long-wave infrared (LWIR) detector that combines a dielectric resonator antenna with an antimonide-based absorber. The detector consists of a 3D, subwavelength InAsSb absorber embedded in a resonant, cylindrical dielectric resonator antenna made of amorphous silicon. This architecture enables the antimonide detection element to shrink to deep subwavelength dimensions, thereby reducing its thermal noise. It is important to note that this concept only applies when (a) the detector noise is limited by bulk noise mechanisms with negligible surface leakage currents and (b) the dominant source of current in the device is due to dark current (such as diffusion) that scales with the volume of the detector. The dielectric resonator enhances the collection of photons with its resonant structure that couples incident radiation to the detector. We will present results on the absorption in structures with and without the dielectric resonator antenna. The signal to noise enhancement in the LWIR photodiodes integrated with the dielectric resonator antenna using radiometric characterization will be discussed.
III-V semiconductors have broad uses in optoelectronics due to their direct band gaps and high carrier motilities. GaAs(1- x)Bix and TlxGa(1-x)As ternary alloys are of interest for light emitting, light absorbing and other applications (e.g. communication lasers, photovoltaics, and high speed transistors) in the infrared spectrum due to their decreased bandgap relative to GaAs. While GaAs has been extensively studied, the optical properties of GaAsBi and TlGaAs are less documented and show significant variation with Bi and Tl content respectively. This study characterized the optical properties of GaAsBi and TlGaAs films of varying Bi and Tl composition using variable angle spectroscopic ellipsometry (VASE) in a range of temperatures from 25 °C – 300 °C. GaAsBi films were grown between 3.3% and 6.5% bismuth. TlGaAs films were grown between 1.7% and 2.7% thallium. Modeling using a superposition of Gaussian oscillators fit to the dielectric functions of sample layers was used to separate film optical properties from the pseudooptical properties of the sample. The analysis in this study directly compares the inclusion of the two largest III-V constituent atoms, Bi and Tl. Comparison of the refractive index and absorption coefficient of samples was done over a spectral range of 0.5 eV to 5 eV (250 nm to 2500 nm). This region displays the absorption edge corresponding to the bandgap of the material, which is then correlated to the incorporation of Bi and Tl in the samples. This characterization allows for better modeling of these alloys for both a fundamental understanding of their properties and for their inclusion in future devices.
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