In this work, we describe the most recent progress towards the device modeling, fabrication, testing and system
integration of active resonant subwavelength grating (RSG) devices. Passive RSG devices have been a subject of
interest in subwavelength-structured surfaces (SWS) in recent years due to their narrow spectral response and high
quality filtering performance. Modulating the bias voltage of interdigitated metal electrodes over an electrooptic thin
film material enables the RSG components to act as actively tunable high-speed optical filters. The filter characteristics
of the device can be engineered using the geometry of the device grating and underlying materials.
Using electron beam lithography and specialized etch techniques, we have fabricated interdigitated metal electrodes on
an insulating layer and BaTiO3 thin film on sapphire substrate. With bias voltages of up to 100V, spectral red shifts of
several nanometers are measured, as well as significant changes in the reflected and transmitted signal intensities around
the 1.55um wavelength.
Due to their small size and lack of moving parts, these devices are attractive for high speed spectral sensing applications.
We will discuss the most recent device testing results as well as comment on the system integration aspects of this
In this paper, we describe progress towards a multi-color spectrometer and radiometer based upon an active resonant subwavelength grating (RSG). This active RSG component acts as a tunable high-speed optical filter that allows device miniaturization and ruggedization not realizable using current sensors with conventional bulk optics. Furthermore, the geometrical characteristics of the device allow for inherently high speed operation. Because of the small critical dimensions of the RSG devices, the fabrication of these sensors can prove challenging. However, we utilize the state-of-the-art capabilities at Sandia National Laboratories to realize these subwavelength grating devices. This work also leverages previous work on passive RSG devices with greater than 98% efficiency and ~1nm FWHM.
Rigorous coupled wave analysis has been utilized to design RSG devices with PLZT, PMN-PT and BaTiO3 electrooptic thin films on sapphire substrates. The simulated interdigitated electrode configuration achieves field strengths around 3×107 V/m. This translates to an increase in the refractive index of 0.05 with a 40V bias potential resulting in a 90% contrast of the modulated optical signal. We have fabricated several active RSG devices on selected electro-optic materials and we discuss the latest experimental results on these devices with variable electrostatic bias and a tunable wavelength source around 1.5μm. Finally, we present the proposed data acquisition hardware and system integration plans.
We present our most recent results and review our progress over the past few years regarding InAs/GaSb Type II superlattices for photovoltaic detectors and focal plane arrays. Empirical tight binding methods have been proven to be very effective and accurate in designing superlattices for various cutoff wavelengths from 3.7 μm up to 32 μm. Excellent agreement between theoretical calculations and experimental results has been obtained. High quality material growths were performed using an Intevac modular Gen II molecular beam epitaxy system. The material quality was characterized using x-ray, atomic force microscopy, transmission electron microscope and photoluminescence, etc. Detector performance confirmed high material electrical quality. Details of the demonstration of 256×256 long wavelength infrared focal plane arrays will be presented.
Dark current has become a significant limiting factor for the development of the Type II InAs/GaSb superlattices technology. Experimental results showed that at liquid nitrogen temperature the dominating dark current under reverse bias is the generation-recombination current before the tunneling current turns on. Recent research on the source of the dark current indicated that the Auger recombinations might play a very important role in the superlattice diode dark current. With proper design of the superlattice structure, we have been able to reduce the dark current several orders of magnitude in the LWIR range. The superlattice diode performance was also improved dramatically. Infrared focal plane arrays based on these superlattices will also be discussed.
Leakage currents limit the operation of high performance type II InAs/GaSb superlattice photodiode technology. Surface leakage current becomes a dominant limiting factor, especially at the scale of a focal plane array pixel (< 25 μm) and must be addressed. A reduction of the surface state density, unpinning the Fermi level at the surface, and appropriate termination of the semiconductor crystal are all aims of effective passivation. Recent work in the passivation of type II InAs\GaSb superlattice photodetectors with aqueous sulfur-based solutions has resulted in increased R0A products and reduced dark current densities by reducing the surface trap density. Additionally, photoluminescence of similarly passivated type II InAs/GaSb superlattice and InAs GaSb bulk material will be discussed.
Nanopillar devices have been fabricated from GaInAs/InP QWIP material grown by MOCVD. Using electron beam lithography and reactive ion etching techniques, large, regular arrays of nanopillars with controllable diameters ranging from 150 nm to less than 40 nm have been reproducibly formed. Photoluminescence experiments demonstrate a strong peak wavelength blue shift for nanopillar structures compared to the as-grown quantum well material. Top and bottom metal contacts have been realized using a polyimide planarization and etchback procedure. I-V and noise measurements have been performed. Optical measurements indicate photoconductive response in selected nanopillar arrays. Device peak wavelength response occurs at about 8 μm with peak device responsivity of 420 mA/W. Peak detectivity of 3×108 cmHz1/2/W has been achieved at -1V bias and 30 K.
InGaAs/InGaP quantum-dots have been grown by low-pressure metalorganic chemical vapor deposition technique on GaAs substrate. The important growth parameters, such as growth temperature, V/III ratio, etc, have been optimized. A 10-stack quantum-dot infrared photodetector based on these InGaAs dots showed a detectivity of 3.6x1010 cmHz1/2/W at 95K. The peak photoresponse was 4.7 μm with a cutoff at 5.2μm. A 256x256 middle-wavelength infrared focal plane array based on our quantum-dot detectors was fabricated via dry etching technique. The detector array was bonded to a silicon readout integrated circuit via flip chip bonding with indium bumps. A noise equivalent temperature difference of 509 mK was achieved for this array at 120K. With the goal of improving array uniformity, exploratory work into nanopillar structure IR detectors was also performed. Experimental methods and characterization results are presented here.
The absorption or emission wavelength in optoelectronic devices such as quantum well infrared photodetectors, quantum cascade lasers, and type II superlattice photodiodes can be controlled by the thickness and composition of the quantum wells that constitute their active layers. By further confining the charge carriers, for instance in a quantum dot, even more control can be gained over energy transitions within the semiconductor crystal. We propose a method for manipulating the semiconductor band structure by confining carriers within nanopillar structures. Using electron beam lithography and dry plasma etching, we can precisely control the pillar placement, density and dimensions, and thus the performance characteristics, of the optoelectronic device. Furthermore, by patterning different size structures, it is possible to create arrays of multi-color devices on the same substrate, a technique that lends itself to large-scale monolithic integration. We demonstrate the fabrication of nanopillar arrays in the GaSb, GaInP, GaInAs, and type II InAs/GaSb superlattice material systems and show initial photoluminescence data, which seems to indicate quantum confinement within these structures.
In the very long wavelength infrared (VLWIR) band, λ>14 microns, the detector materials are currently limited to extrinsic semiconductors. These extrinsic materials can be either heavily doped bulk semiconductor, like silicon or germanium, or a doped quantum well heterostructure. An alternative choice that provides the opportunity for higher temperature operation for VLWIR sensing is an intrinsic material based on a type-II InAs/Ga(In)Sb superlattice. There are many possible designs for these superlattices which will produce the same narrow band gap by adjusting individual layer thicknesses, indium content or substrate orientation. The infrared properties of various compositions and designs of these type-II superlattices have been studied. In the past few years, excellent results have been obtained on photoconductive and photodiode samples designed for infrared detection beyond 15 microns. An overview of the status of this material system will be presented. In addition, the latest experimental results for superlattice photodiodes with cut-off wavelengths as long as 30 microns will be covered.
The authors report the most recent progress in Type II InAs/GaSb superlattice materials and photovoltaic detectors developed for focal plane array applications with a cutoff wavelength of ~8 μm. No turn-on of tunneling current was observed even at a reverse bias of -3 V for a 3 μm thick p-i-n photodiodes. The thermally-limited zero bias detectivity under 300 K 2 π FOV was 2~3×1011 cm•Hz1/2/W at liquid nitrogen temperature, with a current responsivity of 2~3 A/W and a mean quantum efficiency of ~50%. Initial passivation using SiO2 has shown to decrease the dark current by ~30% at a reverse bias of -1 V. The same detector structure was used for focal plane arrays with silicon readout integrated circuit. Concept proof of imaging was demonstrated with a format of 256×256 at liquid nitrogen temperature.
New infrared (IR) detector materials with high sensitivity, multi-spectral capability, improved uniformity and lower manufacturing costs are required for numerous long and very long wavelength infrared imaging applications. One materials system has shown great theoretical and, more recently, experimental promise for these applications: InAs/InxGa1-xSb type-II superlattices. In the past few years, excellent results have been obtained on photoconductive and photodiode samples designed for infrared detection beyond 15 microns. The infrared properties of various compositions and designs of these type-II superlattices have been studied. The infrared photoresponse spectra are combined with quantum mechanical modeling of predicted absorption spectra to provide insight into the underlying physics behind the quantum sensing in these materials. Results for superlattice photodiodes with cut-off wavelengths as long as 25 microns will be presented.
The authors report the most recent advances in type II InAs/GaSb superlattice materials and photovoltaic detectors. Lattice mismatch between the substrate and the superlattice has been routinely achieved below 0.1%, and less than 0.0043% as the record. The FWHM of the zeroth order peak from x-ray diffraction has been decreased below 50 arcsec and a record of less than 44arcsec has been achieved. High performance detectors with 50% cutoff beyond 18 micrometers up to 26 micrometers have been successfully demonstrated. The detectors with a 50% cut-off wavelength of 18.8 micrometers showed a peak current responsivity of 4 A/W at 80K, and a peak detectivity of 4.510 cm x Hz1/2/W was achieved at 80K at a reverse bias of 110mV under 300K 2(pi) FOV background. Some detectors showed a projected 0% cutoff wavelength up to 28~30 micrometers . The peak responsivity of 3Amp/Watt and detectivity of 4.2510 cm x Hz1/2/W was achieved under -40mV reverse bias at 34K for these detectors.