Infrared Photodetector Focal Plane Array Research Articles

Fabrication of resonator–quantum well infrared photodetector focal plane array by inductively coupled plasma etching

Inductively coupled plasma (ICP) etching has distinct advantages over reactive ion etching in that the etching rates are considerably higher, the uniformity is much better, and the sidewalls of the etched material are highly anisotropic due to the higher plasma density and lower operating pressure. Therefore, ICP etching is a promising process for pattern transfer required during microelectronic and optoelectronic fabrication. Resonator-quantum well infrared photodetectors (R-QWIPs) are the next generation of QWIP detectors that use resonances to increase the quantum efficiency (QE). To fabricate R-QWIP focal plane arrays (FPAs), two optimized ICP etching processes are developed. Using these etching techniques, we have fabricated R-QWIP FPAs of several different formats and pixel sizes with the required dimensions and completely removed the substrates of the FPAs. Their QE spectra were tested to be 30 to 40%. The operability and spectral non- uniformity of the FPA is ∼99.5 and 3%, respectively. © The Authors. Published by SPIE under a Creative Commons Attribution 3.0 Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI. (DOI: 10

Fabrication of pyramidal corrugated quantum well infrared photodetector focal plane arrays by inductively coupled plasma etching with BCl3/Ar

We developed an optimized inductively coupled plasma etching process to produce gallium arsenide (GaAs) pyramidal corrugated quantum well infrared photodetector focal plane arrays (C-QWIP FPAs). A statistically designed experiment was performed to optimize the etching parameters. The resulting parameters are discussed in terms of the effect on the etching rate and profile. This process uses a small amount of mask corrosion and the control of the etching mask gap to give a 45 deg to 50 deg V-groove etching profile, which is independent of the crystal orientation of GaAs. In the etching development, scanning electron microscope was used to observe the surface morphology and the pattern profile. In addition, x-ray photoelectron spectroscopy was used to obtain the elemental composition and contamination of the etching surface. It is found that extremely small stoichiometric change and surface damage of the etching surface can be achieved while keeping a relatively high etching rate and ∼45 degV-groove etching profile. This etching process is applied to the fabrication of pyramidal C-QWIP FPAs successfully, which are expected to have better performance than the regular prism-shaped C-QWIPs according to electromagnetic modeling.

An Optimized ICP Etching Process for Pyramidal C-QWIP FPAs

ABSTRACTWe developed an optimized inductively coupled plasma (ICP) etching process to produce GaAs pyramidal corrugated quantum well infrared photodetector focal plane arrays. A statistically-designed experiment was performed to optimize the etching parameters. The resulting parameters are discussed in terms of the effect on the etching rate and profile. This process uses a small amount of mask corrosion and the control of the etching mask gap to give a 45-50 degree V-groove etching profile, which is independent on the crystal orientation of GaAs. In the etching development, scanning electron microscope (SEM) was used to observe the surface morphology and the pattern profile. In addition, X-ray photoelectron spectroscopy (XPS) was utilized to obtain the elemental composition and the contamination of the etching surface. It is found that extremely small stoichiometric change and surface damage of the etching surface can be achieved while keeping relatively high etching rate and ~45 degree V-groove etching profile. This etching process is applied to the fabrication of pyramidal C-QWIP FPAs, which is expected to have better performance than the regular prism-shaped C-QWIPs according to electromagnetic (EM) modeling. The expected results will be verified by optical and electrical measurements. In addition to infrared detectors, this process technology can also be applied to GaAs V-groove solar cell, quantum wire light-emitting diodes, quantum wire lasers, and other GaAs –based devices.

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.

C-QWIPs for space exploration

We have extended our investigation of corrugated quantum well infrared photodetector focal plane arrays (C-QWIP FPAs) into the far infrared regime. Specifically, we are developing the detectors for the thermal infrared sensor (TIRS) used in the NASA Landsat Data Continuity Mission. This mission requires infrared detection cutoff at 12.5 μm and FPAs operated at ∼43 K. To maintain a low dark current in these extended wavelengths, we adopted a low doping density of 0.6 × 10 18 cm −3 and a bound-to-bound state detector in one of the designs. The internal absorption quantum efficiency η is calculated to be 25.4% for a pixel pitch of 25 μm and 60 periods of QWs. With a pixel fill factor of 80% and a substrate transmission of 70.9%, the external η 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/cm 2 at 43 K. For regular infrared imaging above 8 μm, the FPA will have a noise equivalent temperature difference (NETD) of 16 mK at 2 ms integration time in the presence of 260 read noise electrons, and it increases to 22 mK at 51 K. 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, which will be useful for the Jupiter-Europa deep space exploration. In this work, we also investigated the C-QWIP optical coupling when the detector substrate is thinned.

High Conversion Efficiency InP/InGaAs Strained Quantum Well Infrared Photodetector Focal Plane Array With 9.7 $\mu$m Cut-Off for High-Speed Thermal Imaging

InP/InGaAs material system is an alternative to AlGaAs/GaAs for long wavelength quantum well infrared photodetectors (QWIPs). We demonstrate a large format (640 × 512) QWIP focal plane array (FPA) constructed with the strained InP/InGaAs material system. The strain introduced to the structure through utilization of In0.48Ga0.52As (instead of In0.53Ga0.47As ) as the quantum well material shifts the cut-off wavelength from ˜8.5 to 9.7 μm. The FPA fabricated with the 40-well epilayer structure yields a peak quantum efficiency as high as 12% with a broad spectral response (Δλ/p=17%). The peak responsivity of the FPA pixels is 1.4 A/W corresponding to 20% conversion efficiency in the bias region where the detectivity is reasonably high (2.6 × 1010 cm Hz1/2/W, f/1.5, 65 K). The FPA providing a background limited performance temperature higher than 65 K (f/1.5) satisfies the requirements of most low integration time/low background applications where AlGaAs/GaAs QWIPs suffer from read-out circuit noise limited sensitivity due to lower conversion efficiencies. Noise equivalent temperature differences of the FPA are as low as 19 and 40 mK with integration times as short as 1.8 ms and 430 μs (f/1.5, 65 K).

Enhanced quantum well infrared photodetector focal plane arrays for space applications

A 30 months European Space Agency project started in March 2008, whose overall purpose is to expand and assess the performance of broadband (11–15 μm) quantum detectors for spectro-imaging applications: Dispersive Spectrometers and Fourier Transform Spectrometers. We present here the technical requirements, the development approach chosen as well as preliminary experimental results. Our approach is fully compatible with the final array format (1024 × 256, pitch 50–60 μm). We expect the requested uniformity, operability and SNR levels to be achieved at temperatures close to the goal values. The performance level will be demonstrated on 256 × 256, 50 μm pitch arrays. Also, operability and uniformity issues will be addressed on large mechanical 1024 × 256 hybrid arrays.

AlGaAs/GaAs quantum well infrared photodetector focal plane array based on MOCVD technology

128 × 128, 128 × 160 and 256 × 256 AlGaAs/GaAs quantum well infrared photodetector (QWIP) focal plane arrays (FPA) as well as a large area test device are designed and fabricated. The device with n-doped back-illuminated AlGaAs/GaAs quantum structure is achieved by metal organic chemical vapor deposition (MOCVD) epitaxial growth and GaAs integrated circuit processing technology. The test device is valued by its dark current performance and Fourier transform infrared spectroscopy (FTIR) spectra at 77 K. Cut off wavelengths of 9 and 10.9 μm are realized by using different epitaxial structures. The blackbody detectivity DB* is as high as 2.6 × 109 cm⋅Hz1/2⋅W−1. The 128 6128 FPA is flip-chip bonded on a CMOS readout integrated circuit with indium (In) bumps. The infrared thermal images of some targets under room temperature background have been successfully demonstrated at 80 K operating temperature. In addition, the methods to further improve the image quality are discussed.

Large-Format Voltage-Tunable Dual-Band Quantum-Well Infrared Photodetector Focal Plane Array for Third-Generation Thermal Imagers

We report a large-format (640 times 512) voltage- tunable quantum-well (QW) infrared photodetector focal plane array (FPA) for dual-band imaging in the mid- and long-wavelength infrared (MWIR and LWIR) bands. Voltage-tunable spectral response has been achieved through a series connection of eight-well MWIR AlGaAs-InGaAs and 16-well LWIR AlGaAs-GaAs QW stacks grown by molecular beam epitaxy on GaAs substrate. The peak responsivity wavelength of the detectors is shifted from 4.8 to 8.4 mum as the bias is increased within the limit applicable by commercial read-out integrated circuits. The FPA with MWIR and LWIR cutoff wavelengths of 5.1 and 8.9 mum provides noise equivalent temperature differences of 20 and 32 mK (f/1.5) in these bands, respectively. The results are very encouraging for the development of low-cost large-format dual-band MWIR/LWIR FPA technology.

Voltage-Tunable Two-Color Corrugated-QWIP Focal Plane Arrays

We demonstrated a 256 times 256 voltage-tunable two- color corrugated quantum-well infrared-photodetector focal plane array. The detector operation is based on photocurrent asymmetry in a double-superlattice quantum well structure. By using a broadband corrugated light coupling scheme, we obtained a quantum efficiency of 24% in the mid-wave (MW) band and 26% in the long-wave (LW) band without AR coating. Operating at 50 K, the measured noise equivalent temperature difference is 27 mK at 33-ms integration time for the MW and 90 mK at 2 ms for the LW with f/2.44 optics. The NETD operability values are 98.4% and 95.9%, respectively.

Dual spectral InGaAs/InP quantum-well infrared photodetector focal plane array

The realisation of an InGaAs/InP quantum-well infrared photodetector focal plane array imaging device is reported. The long-wave infrared response is based on the intersubband transition (intraband) in the quantum wells. In addition, a near infrared (interband) response is demonstrated. This work shows the potential of the InGaAs/InP materials system for multi-spectral imaging applications.

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

Previously, we demonstrated a large format 1024×1024 corrugated quantum well infrared photodetector focal plane array (C-QWIP FPA). The FPA has a cutoff at 8.6μm and is BLIP at 76K 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 two FPAs, and the preliminary experimental results will be discussed. In this paper, we also describe using voltage tunable detector materials to achieve multi-color capability for these FPAs.

High-temperature operation normal incident 256/spl times/256 InAs-GaAs quantum-dot infrared photodetector focal plane array

In this letter, a 256/spl times/256 midwavelength infrared focal plane array (FPA) based on 30-period InAs-GaAs quantum-dot infrared photodetectors (QDIPs) is fabricated. The demonstrated original real-time nonuniformity corrected thermal images of hot soldering iron head with 30-Hz frame rate for the FPA are observed. Without additional light-coupling scheme, the QDIP FPA module is first operated at temperatures higher than 135 K under normal-incident condition with a 30/spl deg/ field of view and f/2 optics. For single device performances, a similar QDIP device with a 30-period InAs-GaAs QD structure is fabricated under the same processing procedure. High specific detectivity D/sup */ 1.5/spl times/10/sup 10/ cm/spl middot/Hz/sup 1/2//W and low noise current density 5.3/spl times/10/sup -13/ A/Hz/sup 1/2/ at applied voltage 0.3 V are observed.

Designs and applications of corrugated QWIPs

In this paper, we will describe the performance of two long wavelength 1024 × 1024 corrugated quantum well infrared photodetector focal plane arrays (C-QWIP FPAs) with cutoff wavelengths at 8.6 and 9.0 μm, respectively. The FPAs are background limited (BLIP) at around 76 K in an f/1.8 optical system. In addition to the high performance of these C-QWIPs, the corresponding FPAs are also easily producible, making them ideal for large production. We will discuss the optimization of the detectors for different applications. Since corrugated coupling is wavelength insensitive, it is capable of broadband and multi-color detection. We will present a GaAs/AlGaAs broadband detector based on a binary superlattice design. Incorporating the broadband characteristic in a high gain InGaAs/InP material, C-QWIPs with large background photocurrent can be obtained for high speed applications. For multi-color detection, we have investigated two different approaches. One is based on a voltage-tunable, two-color QWIP material, which can be switched between two detection wavelengths simply by changing the detector bias. Stacking two of these similar QWIPs together and separating them with a middle contact layer, a voltage tunable, four-color detector array can be fabricated. A second approach is to combine a broadband QWIP material with a wavelength-selective light coupling method. Using a light coupling geometry to control the detection wavelength of individual pixels, a large number of wavelengths can be detected based on a single broadband detector material.

Assessment of large format InP/InGaAs quantum well infrared photodetector focal plane array

We report the fabrication and characteristics of large format (640 × 512) InP/In 0.53Ga 0.47As long wavelength infrared (LWIR) quantum well infrared photodetector (QWIP) focal plane array (FPA). The FPA, which is hybridized to a read-out integrated circuit having a charge capacity of 1.1 × 10 7 electrons, yielded a mean noise equivalent temperature difference (NETD) of ∼40 mK at a cold finger temperature as high as 77 K. The performance of the FPA, being comparable to that of AlGaAs/GaAs QWIP FPAs, clearly demonstrates the feasibility of the InP/InGaAs material system as an Al-free alternative to AlGaAs/GaAs system for large format LWIR QWIP FPAs.

Advanced monolithic quantum well infrared photodetector focal plane array integrated with silicon readout integrated circuit

Today, most infrared focal plane arrays (FPAs) utilize a hybrid scheme. To achieve higher device reliability and lower cost, monolithic FPAs with Si based readout integrated circuits (ROICs) are the trend of the future development. In this paper, two approaches for monolithic FPAs are proposed: double sided integration and selective epitaxy integration. For comparison, the fabrication process for hybrid quantum well infrared photodetectors (QWIP) FPAs are also described. Many problems, such as the growth of QWIPs on Si substrate and processing incompatibility between Si and III–V semiconductors, need to be solved before monolithic FPAs can be realized. Experimental work on GaInAs/InP QWIP-on-Si is given in this paper. A record high detectivity of 2.3 × 10 9 cm Hz 1/2/W was obtained for one QWIP-on-Si detector at 77 K.

640 × 512 pixel narrow-band, four-band, and broad-band quantum well infrared photodetector focal plane arrays

A 9 μm cutoff 640 × 512 pixel hand-held quantum well infrared photodetector (QWIP) camera has been demonstrated with excellent imagery. A noise equivalent differential temperature (NEDT) of 10.6 mK is expected at a 65 K operating temperature with f/2 optics at a 300 K background. This focal plane array has shown background limited performance at a 72 K operating temperature with the same optics and background conditions. In this paper, we discuss the development of this very sensitive long-wavelength infrared camera based on a GaAs/AlGaAs QWIP focal plane array and its performance in quantum efficiency, NEDT, uniformity, and operability. In the second section of this paper, we discuss the first demonstration of a monolithic spatially separated four-band 640 × 512 pixel QWIP focal plane array and its performance. The four spectral bands cover 4–5.5, 8.5–10, 10–12, and 13.5–15 μm spectral regions with 640 × 128 pixels in each band. In the last section, we discuss the array performance of a 640 × 512 pixel broad-band (10–16 μm full-width at half-maximum) QWIP focal plane.

Commercialization of quantum well infrared photodetector focal plane arrays

Many commercial and government applications need high performance, large format, long-wavelength infrared (LWIR) detector arrays in the range of 6–20 μm. NASA and the ballistic missile defense organization (BMDO) have devoted a significant effort to develop highly sensitive infrared (IR) detectors and large format focal plane arrays (FPAs) based on novel ‘artificial’ low effective bandgap semiconductor material systems such as GaAs/AlGaAs. Caltech’s Jet propulsion laboratory (JPL) under contract from NASA and BMDO has extensively pursued GaAs/AlGaAs based multi-quantum wells (MQWs) for IR radiation detection. Optimization of the detector design, light coupling schemes, large format FPA fabrication and packaging techniques have culminated in the realization of portable LWIR cameras with a mid format (256×256 pixel) FPA of QWIP detectors and the demonstration of TV format (i.e., 640×486) QWIP camera. QWIP technologies LLC, under an exclusive agreement with Caltech is currently manufacturing the QWIP-Chip TM, a 320×256 element FPA. In this, presentation, we will discuss the advantages of MQW technology and our experience in the commercialization of QWIP FPAs.

Two-color quantum well infrared photodetector focal plane arrays

We summarize the current state of two-color quantum well infrared photodetector (QWIP) focal plane array (FPA) technology. Our two-color FPA architecture features 3 bumps/pixel to permit two vertically-stacked QWIPs to be separately biased and the two photocurrents to be simultaneously integrated. Pixel-registered imagery is simultaneously obtained in two spectral bands. We have successfully applied this architecture to realize two-color FPAs in three separate formats: LW/LW, MW/LW, and MW/MW. Fabrication and performance details are presented.

Recent developments and applications of quantum well infrared photodetector focal plane arrays

One of the simplest device realizations of the classic particle-in-the-box problem of basic quantum mechanics is the quantum well infrared photodetector (QWIP). In this paper we discuss the optimization of the detector design, material growth and processing that has culminated in realization of large format long-wavelength QWIP cameras, holding forth great promise for many applications in 6–25 μm wavelength range in science, medicine, defense and industry. In addition, we present the recent developments in long-wavelength/very long-wavelength dualband QWIP imaging camera for various applications.