Magnolia Optical Technologies (United States)

The development of a scalable, low cost, low power, and room temperature operating detector technology capable of high spatial resolution imaging over IR bands of interest can advance and extend space and satellite sensing capabilities such as remote sensing and earth observation. Conventional infrared (IR) band photodetectors based on HgCdTe material operating over short-wave infrared (SWIR), mid-wave infrared (MWIR), and/or long-wave infrared (LWIR) bands often require external cooling to achieve high performance IR sensing and imaging. By integrating bilayers of doped graphene to function as a high mobility channel to aid the recombination of photogenerated carriers, a graphene-enhanced IR photodetector with HgCdTe absorbing layer can be developed providing high performance uncooled detection over SWIR, MWIR, and LWIR (~1.3-14 μm) bands. Development of the high-performance IR photodetector technology involves ptype doping of graphene bilayers deposited on Si/SiO₂ with boron using a spin-on dopant (SOD) process, and then transfer of the doped bilayer graphene onto HgCdTe substrates. The graphene-enhanced HgCdTe SWIR/MWIR/LWIR band detectors were analyzed and characterized material and optoelectronically to demonstrate high performance IR detection at room temperature for advanced NASA Earth Science, defense, and commercial applications.

To improve BOL performance with EOL degradation in mind, nip InGaP-GaAs Heterojunction PV devices were grown and tested investigating a variety of epitaxial designs. Various numbers of quantum wells were added i-region, ranging from 6-45x in combination with a distributed Bragg reflector to promote additional current production at and beyond the GaAs band-edge. Utilizing the extra current production from the wells, the GaAs base was thinned from an optically thick 2,680 nm to 1,500 nm to prepare the devices for radiation-induced minority carrier lifetime degradation. Under AM0, the 45xSBQWs with a 1,500 nm base increased Jsc by 1.3mA/cm2 compared to the optically thick GaAs device containing 0xQW, as well as maintaining a Voc within 8mV.

Dual junction upright solar cells composed of a homojunction InGaP top cell and a heterojunction InGaP/GaAs bottom cell incorporating 50 layers of In0.1Ga0.9As with GaAs0.9P0.1 and a 12-pair A10.9Ga0.1As/A10.1Ga0.9As distributed Bragg reflector were designed for operation under either AM0 or AM1.5G and grown by MOVPE. The QWs allowed an increase in current density of 1.98 mA/cm2 under AM0 (1.07 mA/cm2, AM1.5G design), with an exhibited Voc reduction of only 10 mV as compared to a control tandem without QWs or a DBR. This lead to a efficiency increase of 2.55% absolute under AMO (1.79% absolute, AM1.5G design). Details on growth considerations for this system of strain balanced QWs, and development of single and dual junction device designs, will be presented.

The performance of optical and imaging systems may be limited considerably by losses due to reflection of signals off substrates and optical components. Nanoengineered optical layers offering tunable refractive index properties provide broadband and omnidirectional suppression of light reflection/scattering with increased optical transmission for enhanced detector and system performance. These nanostructured antireflection (AR) coatings custom designed for specific wavebands from the ultraviolet (UV) to infrared (IR) have many potential optical applications, particularly in maximizing light and IR radiation transmitted onto the surfaces of detectors to increase their sensitivity, for various NASA systems. Through fabrication of these AR nanostructures at various tilt angles of deposition, optimized AR coatings having high laser damage thresholds and reliable in extreme low temperature environments and under launch conditions may be realized. We have developed and advanced such AR nanostructures on GaSb substrates as well as GaSb-based detector devices for 8-14 µm LWIR applications. In this paper we review the latest findings and measurements in the development of these high-performance nanostructure-based AR coatings primarily for advanced LWIR band to NASA Earth Science sensing and imaging applications.

Broadband antireflection (AR) optical coatings covering the ultraviolet (UV) to infrared (IR) spectral bands have many potential applications for various NASA systems. The performance of these systems is significantly limited by signal loss due to reflection off substrates and optical components. Tunable nanoengineered optical layers offer omnidirectional suppression of light reflection/scattering with increased optical transmission to enhance detector and system performance particularly over IR band wavelengths. Nanostructured AR coatings enable the realization of optimal AR coatings with high laser damage thresholds and reliability in extreme low temperature environments and under launch conditions for various NASA applications. We are developing and advancing high-performance AR coatings on GaSb and various other substrate types for spectral bands ranging from UV to LWIR. The nanostructured AR coatings enhance transmission of light through optical components and detector devices by greatly minimizing reflection losses over range of incidence angles, providing substantial improvements over more conventional thin film AR coating technologies. In this paper, we review our latest developments in high performance nanostructurebased AR coatings, focusing primarily on recent efforts in designing and fabricating AR coatings for the LWIR spectral band for performance improvements in airborne and space detector applications.

Front-illuminated <i>p-i-n</i> GaN-based ultraviolet (UV) avalanche photodiodes (APDs) were grown by metalorganic chemical vapor deposition (MOCVD) on 25 mm dia. bulk Ammono® <i>n</i>-GaN substrate having a low etch pit density (EPD) less than 5 × 10⁴ [cm^-2] and processed into 6×6 APD arrays. The devices employed N-ion implantation to achieve sidewall passivation. Evaluation of these 6×6 arrays will help to confirm the uniformity of the epitaxial materials and device processing. The maximum avalanche gain reached ~ 3×10⁵ at the breakdown (current limited). The dark current density was 10^-9 A/cm² at reverse bias up to -20 V and the APDs exhibited a reverse breakdown voltage of 81 &plusmn; 1 V for all 36 devices without any leaky devices, confirming a high uniformity of the growth and fabrication processes.

High performance detector technology is being developed for sensing over the mid-wave infrared (MWIR) band for NASA Earth Science, defense, and commercial applications. The graphene-based HgCdTe detector technology involves the integration of graphene with HgCdTe photodetectors that combines the best of both materials, and allows for higher MWIR (2-5 μm) detection performance compared with photodetectors using only HgCdTe material. The interfacial barriers between the HgCdTe-based absorber and the graphene act as a tunable rectifier that reduces the recombination of photogenerated carriers in the detector. The graphene layer also acts as high mobility channel that whisks away carriers before they recombine, further enhancing detection performance. This makes them much more practical and useful for MWIR sensing applications such as remote sensing and earth observation, e.g., in smaller satellite platforms (CubeSat) for measurement of thermal dynamics with better spatial resolution. The objective of this work is to demonstrate graphene-based HgCdTe room temperature MWIR detectors and arrays through modeling, material development, and device optimization. The primary driver for this technology development is the enablement of a scalable, low cost, low power, and small footprint infrared technology component that offers high performance, while opening doors for new earth observation measurement capabilities.

Polycrystalline PbSe is an established low cost material for mid wave-IR photodetection. In this work, we describe a novel design of PbSe MWIR detector that incorporates a graphene layer resulting in three possible distinct modes of operation, depending on the interface design between the functionalized PbSe and Graphene. Using Density Functional Theory (DFT) based simulations we investigate the chemistry of this interface, in terms of back-bonding as well as van-der Waals force, that give rise to rich behavior that is obtainable from this material stack, promising radical new designs for scaled high performance CMOS integrable MWIR photodetectors.

Photodetectors based on germanium which do not require cooling and can provide good near-infrared (NIR) detection performance offer a low-cost alternative to conventional infrared sensors based on material systems such as InGaAs, InSb, and HgCdTe. As a result of the significant difference in thermal expansion coefficients between germanium and silicon, tensile strain incorporated into Ge epitaxial layers deposited on Si utilizing specialized growth processes can extend the operational range of detection to 1600 nm and longer wavelengths. We have fabricated Ge based PIN photodetectors on 300 mm diameter Si wafers to take advantage of high throughput, large-area complementary metal-oxide semiconductor (CMOS) technology. This device fabrication process involves low temperature epitaxial deposition of Ge to form a thin <i>p+</i> (boron) Ge seed/buffer layer, and subsequent higher temperature deposition of a thicker Ge intrinsic layer. This is followed by selective ion implantation of phosphorus of various concentrations to form <i>n+</i> Ge regions, deposition of a passivating oxide cap, and then top copper contacts to complete the PIN detector devices. Various techniques including transmission electron microscopy (TEM) and secondary ion mass spectrometry (SIMS) have been employed to characterize the material and structural properties of the epitaxially grown layers and fabricated detector devices, and these results are presented. The I-V response of the photodetector devices with and without illumination was also measured, for which the Ge based photodetectors consistently exhibited low dark currents of around ~1 nA at -1 V bias.