David P. Forrai

Electromagnetic Modeling of Quantum Well Infrared Photodetectors

Rigorous electromagnetic field modeling is applied to calculate the quantum efficiency of various quantum well infrared photodetector (QWIP) geometries. We found quantitative agreement between theory and experiment for corrugated-QWIPs, grating-coupled QWIPs, and enhanced-QWIPs, and the model explains adequately the spectral lineshapes of the quantum grid infrared photodetectors. After establishing our theoretical approach, we used the model to optimize the detector structures for 12-micron pixel pitch focal plane arrays.

Corrugated Quantum-Well Infrared Photodetector Focal Plane Arrays

Corrugated quantum-well infrared photodetectors (C-QWIPs) have been proposed for long-wavelength infrared detection. In this work, we optimize the detector structure and produce a number of large format focal plane arrays (FPAs). Specifically, we adopt one-corrugation-per-pixel geometry to increase the active detector volume and incorporate a composite cover layer to preserve the large sidewall reflectivity, which results in a large detector quantum efficiency. We also optimize the detector material structure such as the final state energy, the doping density, and the number of quantum well periods to improve the FPA operation under the existing readout electronics. As a result, high FPA sensitivity has been achieved, and their characteristics are in agreement with the detector model. Based on this model, we perform a systematic analysis on the FPA performance with a wide range of detector and system parameters. We find that C-QWIP FPAs are capable of high-speed imaging especially for those with longer cutoff wavelengths.

Electromagnetic Modeling and Design of Quantum Well Infrared Photodetectors

The quantum efficiency (QE) of a quantum well infrared photodetector (QWIP) is historically difficult to predict and optimize. This difficulty is due to the lack of a quantitative model to calculate QE for a given detector structure. In this paper, we found that by expressing QE in terms of a volumetric integral of the vertical electric field, the QE can be readily evaluated using a finite element electromagnetic solver. We applied this model to all known QWIP structures in the literature and found good agreement with experiment in all cases. Furthermore, the model agrees with other theoretical solutions, such as the classical solution and the modal transmission-line solution when they are available. Therefore, we have established the validity of this model, and it can now be used to design new detector structures with the potential to greatly improve the detector QE.

Optimization of corrugated-QWIPs for large format, high-quantum efficiency, and multicolor FPAs

Previously, we demonstrated a large format 1024 x 1024 corrugated quantum well infrared photodetector focal plane array (C-QWIP FPA). The FPA has a cutoff at 8.6 μm and is BLIP at 76 K with f/1.8 optics. The pixel had a shallow trapezoidal geometry that simplified processing but limited the quantum efficiency QE. In this paper, we will present two approaches to achieve a larger QE for the C-QWIPs. The first approach increases the size of the corrugations for more active volume and adopts a nearly triangular pixel geometry for larger light reflecting surfaces. With these improvements, QE is predicted to be about 35% for a pair of inclined sidewalls, which is more than twice the previous value. The second approach is to use Fabry-Perot resonant oscillations inside the corrugated cavities to enhance the vertical electric field strength. With this approach, a larger QE of 50% can be achieved within certain spectral regions without using either very thick active layers or anti-reflection coatings. The former approach has been adopted to produce a series single color FPAs, and the experimental results will be discussed in a companion paper. In this paper, we also describe using voltage tunable detector materials to achieve multi-color capability for these FPAs.

Characterization of a C-QWIP LWIR camera

Large format corrugated quantum well infrared photodetector (C-QWIP) focal plane arrays (FPAs) have been developed over the past two years. The results of this development have demonstrated the potential for this technology to satisfy requirements for very large format high performance long-wave infrared (LWIR) imaging systems. One particular C-QWIP design has focused on developing an FPA that operates in the 8 to 10 &mgr;m spectrum with integration times in the millisecond regime when used against warm backgrounds. This FPA is very suitable for many LWIR applications and has been integrated into a camera system. The specifications of that camera are described in this paper. The characterization of this camera system includes standard electro-optical tests and compares the results of those tests to theoretical models for the FPA. This paper concludes by describing the ongoing effort to tailor the system specifically for the C-QWIP. This includes design features of the read-out integrated circuit (ROIC), dewar-cooler design and interfacing electronics, and video processing. This thorough characterization of the camera has demonstrated the utility of the C-QWIP FPA for LWIR imaging and has established a path forward to further improve the performance of imaging systems implementing this technology.

Wiregrid micro-polarizers for mid-infrared applications

Simultaneous detection of intensity and polarization at the pixel-level has many important applications in the mid-infrared region. In this work a large-format aluminum wire grid micro polarizer array has been fabricated and tested on silicon substrates. The arrays were made on 150mm silicon wafers using a 193nm deep-UV stepper, with each array spanning over 1-million pixels. A unique multilayer design and a large-area nanoscale projection lithography combined with high-aspect ratio wire-grid structures were utilized to achieve optimum extinction coefficient and transmission. Measured extinction coefficients on test samples exceeded 30-dB, with maximum transmission around 90%. These arrays could be designed to match the focal-plane array geometry for integration with mid-IR imagers.

Test techniques for high performance thermal imaging system characterization

Recent requirements for modern low noise thermal imaging systems demand higher performance and more detailed characterization of the system. The statistical uncertainty inherent to the test system can often provide misleading information about system performance. An example would be a test that eliminates pixels based on certain performance parameters such as noise or responsivity. If the test uncertainty exceeds the true variance of the parameter, the test will yield results indicative of the test system rather than the parameter. This results in good pixels being eliminated that potentially impacts operability goals. A sign that test uncertainty dominates the test results is when operability remains nearly uniform between multiple tests while the pixels marked bad by the test changes between tests. In order to minimize the uncertainty in a test, one must consider all aspects of the test system that can affect test results. Those aspects include the physical construction of the test station as well as the underlying statistics associated with the measurement. This paper will show ongoing efforts at L-3 Cincinnati Electronics to lower test uncertainty, increase test repeatability, and qualify test systems for both focal plane array and system level electro-optics testing of thermal imagers.

Corrugated quantum well infrared photodetectors for far infrared detection

We have extended our investigation of corrugated quantum well infrared photodetector focal plane arrays (FPAs) into the far infrared regime. Specifically, we are developing the detectors for the thermal infrared sensor (TIRS) used in the Landsat Data Continuity Mission. To maintain a low dark current, we adopted a low doping density of 0.6×1018 cm−3 and a bound-to-bound state detector. The internal absorption quantum efficiency (QE) is calculated to be 25.4%. With a pixel fill factor of 80% and a substrate transmission of 70.9%, the external QE is 14.4%. To yield the theoretical conversion efficiency (CE), the photoconductive gain was measured and is 0.25 at 5 V, from which CE is predicted to be 3.6%. This value is in agreement with the 3.5% from the FPA measurement. Meanwhile, the dark current is measured to be 2.1×10−6 A/cm2 at 43 K. For regular infrared imaging above 8 μm, the FPA will have an noise equivalent temperature difference (NETD) of 16 mK at 2 ms integration time in the presence of 260 read noise electrons. The highest operability of the tested FPAs is 99.967%. With the CE agreement, we project the FPA performance in the far infrared regime up to 30 μm cutoff.

Transitioning large-diameter Type II Superlattice detector wafers to manufacturing

The tri-service Vital Infrared Sensor Technology Acceleration (VISTA) program rapidly matured III-V semiconductor epitaxy to produce tactically viable detectors using Type II Superlattice (T2SL) structures. The T2SL material system allows tunable band gaps for creating lattice-matched heterojunction devices. Heterojunction devices are integral to suppressing sources of dark currents, such as internal Shockley Reed Hall (SRH) and device surface currents. Once the VISTA program demonstrated that T2SL detectors offered competitive performance to traditional indium antimonide (InSb) detectors at an operating temperature 40K to 50 K higher, many opportunities emerged. This elevation in operating temperature provides two benefits to infrared (IR) sensors. The first is to miniaturize the integrated Dewar-electronicscooler assembly (IDECA) such that it can support small aerial vehicle and soldier mounted sensors. The second is to increase the mean time to failure (MTTF) of an existing InSb IDECA. To benefit from T2SL higher operating temperature (HOT) detectors, the overall cost of the IDECA must be competitive with InSb. This drives a manufacturing capability that is equivalent to InSb. At the L3 Space and Sensors Technology Center (L3 SSTC), the III-V detector foundry processes 125 mm diameter InSb wafers. The development of 125 mm diameter T2SL detector wafers started with the gallium antimonide substrates. The greater size and weight of these substrates required extra care to avoid breakage. Leveraging the learning reported from the silicon industry, we developed a specification for the substrate thickness and edge bevel to provide a robust platform for wafer processing. Next, we worked with commercial III-V epitaxy suppliers to develop multi-wafer growth capability for 125 mm diameter substrates. The results of this effort, funded by the Office of the Secretary of Defense (OSD) Defense-wide Manufacturing Science and Technology (DMST) program through the Army Night Vision and Electronic Sensor Directorate (NVESD), we were able to improve focal plane array (FPA) yield from virtually zero to InSb manufacturing levels.

High-quantum-efficiency C-QWIP FPA-based IR cameras

Current generation QWIP detectors, although very cost effective, have relatively narrow spectral range and low quantum efficiencies. Tactical operation is generally limited to a single spectral band. These limitations arise from the design approach and restrict applications to those that can tolerate these performance limitations. Using recent device design improvements, a novel material, and special processing approaches, High Quantum Efficiency Dual Band C-QWIP detectors are currently being developed. These are expected to overcome traditional limitations in the QWIP design approach and deliver extremely high performance. In the first phase of the program, single color LWIR and VLWIR C-QWIP FPAs in large (1024x1024) format will be demonstrated with targeted peak quantum efficiency of 35%, and correspondingly high BLIP operating temperatures. In the next phase of the program, the team will continue to improve QE towards 50% with conversion efficiency of 75%, and demonstrate dual band MW/LW FPAs. The detector gain will be optimized for operation in either low background or high background applications. These goals will be accomplished using highly producible/low cost materials and processes. System considerations include ROIC well capacity, noise performance, as optics configuration and other concerns will be addressed. A robust design for high performance in a variety of applications will be shown. This work is being performed by the Army Research Laboratory (ARL) and L-3 Cincinnati Electronics (CE), with funding provided by the Missile Defense Agency.