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We recently proposed a resonator structure to increase the quantum efficiency (QE) of a quantum well infrared photodetector. In this paper, we performed a systematic study on its QE as a function of detector size and doping density. We found that the experimental result generally agrees with the prediction from electromagnetic modeling, and the resonant coupling scheme is effective with the pixel pitch as small as $6~\mu \text{m}$ . In addition, we modeled the potential detector performance at even smaller pixel pitches and found that the maximum QE occurs at 3- $\mu \text{m}$ pixel pitch for 9- $\mu \text{m}$ detection, where QE is predicted to be above 80%.

Mercury telluride colloidal quantum dots (CQD) hold potential for the fabrication of IR devices that operate in the range of near IR, through short-, mid-, and long wavelength (NIR, SWIR, MWIR, and LWIR, respectively). We synthesized a variety of colloidal HgTe QDs using the wet technique in a non-coordinating environment. Both a standard synthesis technique (mixing Hg and Te precursors and letting the nanocrystals grow and develop) and a more complex technique involving the controlled intermittent addition of precursors were applied. CQDs with sizes up to 20–30 nm were grown, although materials with sizes well above 10 nm did not show a distinct excitonic absorption peak. Our synthesized CQDs were used to fabricate photoconductive devices and measure their photoresponse. We present the synthesis techniques used in this work, photoconductive device fabrication and characterization. Devices with quantum efficiency of about 12.5% at a 4 V bias were obtained.

Photodetectors in the ultraviolet spectral range are of great interest for applications such as fluorescence-free Raman spectroscopy and non-line-of-sight optical communications. These applications require single-photon sensitivity, resulting in the use of intensified CCDs or photomultiplier tubes (PMTs) that are bulky, fragile, operate at high voltages, and/or require active cooling. Silicon carbide avalanche photodiodes (SiC APD) show promise as a compact, rugged replacement, as they exhibit low noise, durability and high gain. In this paper we report on detailed studies of p+-p-i-n SiC APDs operating in both Geiger and linear mode. The single photon detection efficiency (SPDE) of these devices was measured as a function of excess bias using a pulsed 280 nm light emitting diode source focused through a 20 µm pinhole to a spot size 20% with dark count rate (DCR) of < 700 Hz in both gated and continuous operation. Comparison with linear mode operation shows that avalanche multiplication gain in these devices exceeds 5x106 under these conditions. Examination of linear mode gain vs. applied bias dependence suggests that a sluggish dependence corresponds to a poorer SPDE, which is likely associated with parasitic resistance in these devices. This resistance is consistent with the observed inverse dependence of calculated linear mode gain with increasing optical flux. A peak SPDE of 37% was measured at an excess bias of 3.9 V with a DCR of 7.3 kHz.

We recently developed a finite element three-dimensional electromagnetic model for quantum efficiency (QE) computation. It is applicable to any arbitrary detector geometry and materials. Using this model, we can accurately account for the open literature experimental results that we have investigated, which include those from GaAs solar cells, GaSb type-II superlattices, and GaAs quantum wells. We applied the model to design a photon trap to increase detector QE. By accumulating and storing incident light in the resonator-QWIP structure, we observed experimental QE as high as 71%. This improvement shows that we are now able to fully determine the optical properties of QWIPs. For example, we can design QWIPs to detect at certain wavelengths with certain bandwidths. To illustrate this capability, we designed QWIPs with its QE spectrum matching well with the transmission spectrum of a medium. We subsequently produced several focal plane arrays according to these designs with 640 × 512 and 1 K × 1 K formats. In this paper, we will compare the modeled QE and the experimental results obtained from single detectors as well as FPAs.

HgTe and PbS colloidal quantum dots (CQD) with first excitonic absorption peak of about 2 μm and shorter (down to about 1 μm) have been synthesized and characterized. The synthesized CQDs were characterized using FTIR spectroscopy and TEM technique. The nanomaterials were tested for photo-electrical properties with photoconductive (PC) devices. The devices were fabricated by drop-casting a suspension of the CQDs on the fanout, followed by solid state ligand exchange (SSLE) process, and then spectral and electrical photoresponse of the device were measured. The SSLE process was evaluated thru absorption spectra of test samples. The device fabrication parameters were the number of deposited layers, the thickness of individual layers, the type of the substituting ligand, and the ligand exchange duration. For selected devices external quantum efficiency (EQE) was also determined.

Resonator-Quantum Well Infrared Photo detectors (R-QWIPs) are the next generation of QWIP detectors that use resonances to increase the quantum efficiency (QE). Recently, we are exploring R-QWIPs for broadband long wavelength applications. To achieve the expected performance, two optimized inductively coupled plasma (ICP) etching processes (selective and non-selective) are developed. Our selective ICP etching process has a nearly infinite selectivity of etching GaAs over Ga1-xAlxAs. By using the etching processes, two format (1Kx1K and 40x40) detectors with 25 μm pixel pitch were fabricated successfully. In despite of a moderate doping of 0.5 × 1018 cm-3 and a thin active layer thickness of 0.6 or 1.3 μm, we achieved a quantum efficiency 35% and 37% for 8 quantum wells and 19 quantum wells respectively. The temperature at which photocurrent equals dark current is about 66 K under F/2 optics for a cutoff wavelength up to 11 μm. The NEΔT of the FPAs is estimated to be 22 mK at 2 ms integration time and 60 K operating temperature. This good result thus exemplifies the advantages of R-QWIP.

Silicon carbide Schottky diodes with thick i-regions are reported. Compared with previously reported p-i-n photodiodes, a shift of the absorption peak from 270 nm to 350 nm was observed. The responsivity curves of the Schottky diode are modeled and compared with the experimental data.Silicon carbide Schottky diodes with thick i-regions are reported. Compared with previously reported p-i-n photodiodes, a shift of the absorption peak from 270 nm to 350 nm was observed. The responsivity curves of the Schottky diode are modeled and compared with the experimental data.

The infrared absorption of SF6 gas is narrowband and peaks at 10.6 μm. This narrowband absorption posts a stringent requirement on the corresponding sensors as they need to collect enough signal from this limited spectral bandwidth to maintain a high sensitivity. Resonator-quantum well infrared photodetectors (R-QWIPs) are the next generation of QWIP detectors that use resonances to increase the quantum efficiency for more efficient signal collection. Since the resonant approach is applicable to narrowband as well as broadband, it is particularly suitable for this application. We designed and fabricated R-QWIPs for SF6 gas detection. To achieve the expected performance, the detector geometry must be produced according to precise specifications. In particular, the height of the diffractive elements and the thickness of the active resonator must be uniform, and accurately realized to within 0.05 μm. Additionally, the substrates of the detectors must be completely removed to prevent the escape of unabsorbed light in the detectors. To achieve these specifications, two optimized inductively coupled plasma etching processes were developed. Due to submicron detector feature sizes and overlay tolerance, we used an advanced semiconductor material lithography stepper instead of a contact mask aligner to pattern wafers. Using these etching techniques and tool, we have fabricated focal plane arrays with 30-μm pixel pitch and 320×256 format. The initial test revealed promising results.

Through a three-way collaboration between the Institute for Collaborative Biotechnologies (ICB) at the University of California, Santa Barbara; The Aerospace Corporation; and the U.S. Army Research Laboratory, we investigated the use of a pyroelectric perovskite based material for a novel two-dimensional uncooled infrared focal plane array. Barium strontium titanate (BSTO) was chosen by the ICB as the perovskite material to be used due to its stable nanocrystals, which would allow a high quality, homogenous, crack-free film to be dispersed onto sensor readouts. This ferroelectric ceramic material, once transformed into the pyroelectric crystalline form, would be used as the active material in thermal imaging devices. An in-house process for the film deposition was developed for this purpose.

This report describes the highlights and summary of the year 2, quarter four (Y2Q4) progress for the 6.2 project Improved Sensitivity Low-Cost Uncooled IR Detector Focal-Plane Arrays. This work occurred during the January 2010 May 2010 time period. The program goals for Y2Q4 corresponded to the continued expansion of our barium titanium oxide (BaTiO3) nanomaterial synthesis capabilities and deposition methods; laser-induced pyroelectric phase conversion studies and nanoscale characterization of the pyroelectric activation process; and fabrication of infrared (IR) test heterostructures for pyroelectrical analysis, along with the development of a technique for generating the requisite IR absorbing layer.

This report describes the highlights and summary of the year two, quarters two and three progress for the 6.2 project Improved Sensitivity Low-Cost Uncooled IR Detector Focal-Plane Arrays. This work occurred during the August 2009 January 2010 time period. The program goals for year 2-quarters 2&3 corresponded to the continued expansion of our BaTiO3 nanomaterial synthesis capabilities and deposition methods, laser-induced pyroelectric phase conversion studies and nanoscale characterization of the pyroelectric activation process, and fabrication of IR test heterostructures for pyroelectrical analysis, along with the development of a technique for generating the requisite IR absorbing layer.

We report a 50% increase in power efficiency for our novel InAs/GaAs quantum dot with built-in charge (Q-BIC) solar cells. We found that n-doping the inter-dot space of a quantum dot solar cell (QDoSC) increases the short circuit current density from 15.07 mA/cm2 in undoped QDoSC to 24.30 mA/cm2 in the device doped to provide approximately six electrons per dot. To identify the physical mechanisms that provide this significant improvement, we investigate the photovoltaic response and its spectral characteristics in GaAs reference cell, undoped, n-doped, and p-doped QDoSCs. We found that the photovoltaic efficiency of the undoped QDoSC is almost the same as that of the reference cell. The efficiency monotonically improves with increasing n-doping, while the p-doping deteriorates the photovoltaic conversion. Studies of the photoluminescence of p- and n-doping show that the photoelectron capture into QDs is substantially faster than the hole capture, which leads to an accumulation of electrons in QDs. The built-in-dot electron charge enhances electron inter-subband QD transitions, suppresses the fast electron capture processes, and together with charged donors, forms the potential profile which precludes degradation of the open circuit voltage. All of these factors lead to the enhanced harvesting of IR energy and to a radical improvement of the QDoSC efficiency. Even higher efficiencies are anticipated for higher n-doping levels.

The program goals for year 2-quarter 1 (Y2Q1) Institute for Collaborative Biotechnologies project 'Lower Cost Uncooled IR Detector Focal Plane Arrays' included (1) the continued expansion of our barium titanium oxide (BaTiO3) nanomaterial synthesis capabilities and deposition methods, (2) laser-induced pyroelectric phase conversion studies and spectroscopic end-point control scheme, and (3) comprehensive material characterization studies, along with electrical property characterization of BaTiO3 thin films. This report covers progress that took place in the three month period from May 1, 2009-July 30, 2009.

We report the results of a study of the temperature-dependent photoluminescence (PL) and leakage current characteristics of a set of type II idium arsenide (InAs)-gallium antimonide (GaSb) superlattice (SL) photodiode structures. We find that the PL efficiency of high-quality structures is determined by Shockley-Read and trap-assisted tunneling nonradiative recombination processes. Our results suggest a possible correlation between trap-assisted tunneling in some SL structures and an anomalous decrease in the PL efficiency with increasing temperature over the range 40-78 K, and provide insight into the effect of defects and SL homogeneity on the PL and transport characteristics of the photodiodes.

Recently, we have developed a detector structure known as the resonator quantum-well infrared photodetector or R-QWIP. With this structure, we demonstrated quantum efficiency as high as 70% in single detectors and 30% to 40% in focal plane arrays (FPAs) with a 9-μm cutoff. We designed a broadband, 10-μm cutoff R-QWIP FPA using a more accurate refractive index. To achieve the theoretical prediction, the substrates of the detectors have to be removed completely to prevent the escape of unabsorbed light out of the detectors. The height of the diffractive elements (DE) and the thickness of the active resonator must also be uniformly produced within a 0.05-μm accuracy. To achieve these specifications, two optimized inductively coupled plasma (ICP) etching processes are developed. Using these etching techniques, a number of single detectors were fabricated to verify the analysis before FPA production. In general, test data support the theoretical predictions.

An optimized detector fabrication process is developed for resonator-quantum well infrared photodetectors (R-QWIPs). The R-QWIPs are the next generation of QWIP detectors that use resonances to increase the quantum efficiency (QE). To achieve the expected performance, the detector geometry must be produced in precise specification. In particular, the height of the diffractive elements and the thickness of the active resonator must be uniformly and accurately realized to within 0.05-μm accuracy. To achieve this specification, an optimized inductively coupled plasma etching process with a nearly infinite etching selectivity for the GaAs over the Al x Ga 1−x As etch-stop layer was developed. Using this etching technique, we have fabricated a number of R-QWIP test detectors with the required dimensions. Their QE spectra were tested to be in close agreement with the QE predictions.