Richard Fu

Resonant structures for infrared detection

We are developing resonator-quantum well infrared photodetectors (R-QWIPs) for long-wavelength applications. Detector pixels with 25 μm pitch were hybridized to fan-out circuits for radiometric measurements. With a moderate doping of 0.5×1018  cm-3, we achieved a quantum efficiency (QE) of 37% and conversion efficiency (CE) of 15% in a 1.3 μm thick active material and 35% QE and 21% CE in a 0.6 μm thick active material. Both detectors are cutoff at 10.5 μm with a 2 μm bandwidth. The temperature at which photocurrent equals dark current is about 65 K under F/2 optics. The thicker detector shows a large QE polarity asymmetry due to nonlinear potential drop in the QWIP material layers.

Advanced thin conformal Al2O3 films for high aspect ratio mercury cadmium telluride sensors

Abstract. Mercury cadmium telluride (HgCdTe) processing must be performed at a low temperature in order to reduce Hg depletion. To meet demand, low-temperature plasma enhanced atomic layer deposition (PE-ALD) is an emerging deposition technology for highly conformal thin films. We comparatively studied the effectiveness of low-temperature PE-ALD by measuring the ALD film roughness, thickness, and dielectric values. Conformal deposition was investigated through scanning electron microscopy images of the Al2O3 film deposited onto high aspect ratio features dry-etched into HgCdTe. PE-ALD demonstrated conformal coatings of trenches, pillars and holes in advanced HgCdTe infrared sensor architectures.

Long wavelength resonator-QWIPs

We are developing resonator-QWIPs for long wavelength applications. Detector pixels with 25 μm pitch were hybridized to fanout circuits for radiometric measurements. With a moderate doping of 0.5 x 1018 cm-3, we achieved a quantum efficiency of 37% and conversion efficiency of 15% in a 1.3 μm-thick active material and 35% QE and 21% CE in a 0.6 μm-thick active material. Both detectors are cutoff at 10.5 μm with a 2 μm bandwidth. The temperature at which photocurrent equals dark current is about 65 K under F/2 optics. The thicker detector shows a large QE polarity asymmetry due to nonlinear potential drop in the QWIP material layers.

Photochromic cross-link polymer for color changing and sensing surface

Abstract. Photochromic cross-link polymers were developed using patented ultraviolet (UV) photoinitiator and commercial photochromic dyes. The photochromic dyes have been characterized by measuring absorbance before and after UV activation using UV-visible (Vis) spectrometry with varying activation intensities and wavelengths. Photochromic cross-link polymers were characterized by a dynamic xenon and UV light activation and fading system. The curing processes on cloth were established and tested to obtain effective photochromic responses. Both PulseForge photonic curing and PulseForge plus heat surface curing processes had much better photochromic responses (18% to 19%, 16% to 25%, respectively) than the xenon lamp treatment (8%). The newly developed photochromic cross-link polymer showed remarkable coloration contrasts and fast and comparable coloration and fading rates. Those intelligent, controlled color changing and sensing capabilities will be used on flexible and “drapeable” surfaces, which will incorporate ultra-low power sensors, sensor indicators, and identifiers.

Design and fabrication of resonator quantum-well infrared photodetector with 10.2 μm cutoff

Abstract. 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.

Mercury cadmium telluride (HgCdTe) passivation by advanced thin conformal Al 2 O 3 films

HgCdTe passivation process must be performed at low temperature in order to reduce Hg depletion. Low temperature plasma enhanced atomic layer deposition (PE-ALD) is an emerging deposition technology for thin highly conformal films to meet the demand. Room temperature PE-ALD Al2O3 films passivation on HgCdTe has been studied. Conformal film was investigated through SEM images of the Al2O3 film deposited onto high aspect ratio features dry etched into HgCdTe. Minority carrier lifetime was measured and compared by photoconductive decay transients of HgCdTe before and after deposition. Room temperature ALD Al2O3 film increased the minority carrier lifetime of HgCdTe.

Controlling defects in fine-grained sputtered nickel catalyst for graphene growth

Sputter-prepared nickel (Ni) films can lose more than half their starting thickness due to evaporation in hydrogen (H2) annealing environments. The loss rate of the sputtered Ni films during the chemical vapor deposition growth of graphene has not been reported earlier. The evaporation rate of sputtered Ni film with the amorphous, mixed, preferred ⟨111⟩ texture was experimentally determined to be 20, 11, and 6 nm/m, respectively. An increase of argon mixture in H2 was found to reduce pitting defects in the films during annealing. The quality of grown graphene on top of the Ni improved when the growth temperature was raised from 900 to 1000 °C, as monitored by Raman spectroscopy. More importantly, loss in the starting Ni film thickness can inhibit the growth of graphene layers. By maintaining the growth of the graphene to two layers or less, a high optical transparency of 95% or better can be achieved.

A systematic study of resonator quantum well infrared photodetector (QWIP)

We recently proposed a resonator structure to increase the quantum efficiency (QE) of a quantum well infrared photodetector (QWIP). In this detector systematic parameter study, we have selected two active layer thicknesses, three detector sizes, and three doping levels to investigate the Resonator-QWIP characteristics and the EM modeling in a wide range of detector parameters. To achieve the expected performances, the detector geometry must be produced in precise specification. In particular, the height of the diffractive elements (DE) and the thickness of the active resonator must be uniformly and accurately realized to within 0.05 μm accuracy and the substrates of the detectors have to be removed totally to prevent the escape of unabsorbed light in the detectors. To attain these specifications, two optimized inductively coupled plasma (ICP) etching processes are developed to fabricate a number of test detectors. Due to submicron detector feature sizes and overlay tolerance, we use an ASML stepper instead of a contact mask aligner to pattern wafers. The highest QE we found in this study is 64% obtained from a less optimized 30 μm pitch detector with 1.0×1018 cm-3 doping. In generally, the experimental result agrees with the prediction from electromagnetic (EM) modeling, and the R-QWIPs are able to maintain a relatively constant QE as the pixel size shrinks to 6 μm. The present 6 μm pitch R-QWIP FPA can potentially achieve 20 mK NETD at F/1.2 and 12 ms integration time.

Design and fabrication of resonator-QWIP for SF6 gas sensor application

The infrared absorption of SF6 gas is of narrowband and peaks at 10.6μm. This narrow band absorption posts a stringent requirement on the corresponding sensors as they need to collect enough signal from this limited spectral range to maintain a high sensitivity. Resonator-Quantum Well Infrared Photo detectors (R-QWIPs) are the next generation of QWIP detectors that use resonances to increase the quantum efficiency (QE) 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 (DE) 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 removed totally to prevent the escape of unabsorbed light in the detectors. To achieve these specifications, two optimized inductively coupled plasma (ICP) etching processes are developed. Due to submicron detector feature sizes and overlay tolerance, we use an ASML stepper instead of a contact mask aligner to pattern wafers. Using these etching techniques and tool, we have fabricated FPAs with 30 μm pixel pitch and 320x256 format. The initial test results showed promising results.

Design and fabrication of resonator-quantum well infrared photodetector for SF6 gas sensor application

Abstract. 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.