Copyright 2006 Society of Photo-Optical Instrumentation Engineers
Bispectral Thermal Imaging with Quantum Well Infrared Photodetectors and InAs/GaSb Type-II Superlattices Robert Rehm1a, Martin Walther a, Joachim Fleißner a, Johannes Schmitz a, Johann Ziegler b, Wolfgang Cabanski b, and Rainer Breiter b a
Fraunhofer-Institut für Angewandte Festkörperphysik, Freiburg i. Br., Germany b AIM Infrarot-Module GmbH, Heilbronn, Germany ABSTRACT
We report on bispectral imaging systems based on quantum-well infrared photodetectors (QWIPs) and InAs/GaSb type-II superlattices (SLs) for the mid-wavelength infrared spectral range between 3-5 µm (MW) and the longwavelength infrared regime at 8-12 µm (LW). A dual-band MW/LW QWIP imager and a dual-color MW/MW InAs/GaSb SL camera are demonstrated. The two systems offer a spatial resolution of 288x384 pixels and a simultaneous detection of both channels on each pixel. Both technologies achieve an excellent noise equivalent temperature difference below 30 mK in each channel with F#/2.0 optics. Keywords: infrared camera, focal plane array, multi-color, dual-band, dual-color, bispectral, two-color, quantum-well infrared photodetector, QWIP, InAs/GaSb type-II superlattice, thermal imaging
1. INTRODUCTION Multispectral infrared (IR) imaging systems offer access to the comprehensive information contained in a scene’s spectral signatures. Today’s standard multispectral imagers employ bulky optical and mechanical components like, e.g., filter wheels, beamsplitters and lenses in order to spectrally separate and focus the radiation on one or more monospectral IR focal plane arrays (FPAs). In order to overcome the temporal and spatial registration problem of standard multispectral imagers and to reduce complexity, weight, size and cost of the system, IR-FPAs with integrated on-chip multispectral capability are worldwide under development.
A first step into this direction are bispectral FPAs operating either in two separate atmospheric windows (“dualband”) or within the same transmission window (“dual-color”). Bispectral imaging provides several advantages like, e.g., remote absolute temperature measurement, operation in a wide range of ambient conditions, better distinction between target and background clutter and spectral discrimination of unique object features. Especially for applications which require a short-integration time, e.g., the imaging of fast moving objects, a simultaneous detection of both channels on each pixel is highly desirable, since the subsequent data processing is significantly simplified. Several bispectral demonstrators operating both in the 8-12 µm long-wavelength infrared regime (LW) and the 3-5 µm mid-wavelength infrared spectral range (MW) have been realized using either HgCdTe (MCT) technology or GaAs-based quantum-well infrared photodetectors (QWIPs) [1, 2, 3]. The prevalent MCT approach offers flexibility in cutoff wavelength and a high performance due to a high quantum efficiency and low dark current. In contrast to liquid phase epitaxy (LPE) mostly used for monospectral detectors, the dominant method for the growth of complex bispectral MCT-FPAs is molecular beam epitaxy (MBE) [4]. Yet, with MCT-MBE excellent homogeneity is difficult to achieve
because of the particularly sensitive dependence of the HgxCd1-xTe bandgap on the alloy composition, e.g., induced by a slight variation of the temperature across the generally small and irregularly shaped CdZnTe substrates. Hence, bispectral MCT-FPAs are still very expensive due to a low production yield. In contrast, QWIP-FPAs can be manufactured in the MW and the LW on large and cheap GaAs substrates with high yield. Homogeneity, as well as pixel operability are excellent. The mature production technology, combined with the 1
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narrow band absorption characteristic (∆λ/λ ~ 10-20 % for monospectral FPAs) make QWIPs well suited for multispectral IR-FPAs [2, 3]. However, the major drawback of QWIPs remains their low quantum efficiency, limiting the integration time for good thermal resolution to approximately 5-10 ms. The III/V counterpart to MCT are InAs/GaSb type-II superlattices (SLs), which offer a comparable quantum efficiency. In particular for applications at longer wavelengths, type-II InAs/GaSb SLs are expected to outperform MCT diodes due to a lower Auger recombination rate and reduced tunneling currents caused by a higher effective mass. In the present paper, we report on the worldwide first dual-color MW/MW InAs/GaSb SL imager.
2. DUAL-BAND QWIPS A. Basic Principles of QWIPs In contrast to bipolar band-to-band excitation exploited in MCT detectors, QWIPs are III/V compound unipolar devices, in which the detection of infrared photons is based on intersubband transitions in quantum wells [5]. In a typical n-type QWIP (see Fig. 1a), 20 - 50 periods of GaAs or InxGa1-xAs quantum wells separated by AlyGa1-yAs barrier layers are employed. The precisely controllable width and height of the resulting potential wells determine the total number and the energy of the subbands contained in each well. Most common are wells comprised of a ground state and one excited state at an energy approximately equal to the barrier height. The ground states in the conduction band is populated by appropriate n-type doping of the well. The resonant absorption of photons raises ground state carriers into the excited state. However, normal incident absorption is forbidden by a selection rule. Inter subband absorption requires a polarization component perpendicular to the layers. Therefore, specific diffraction grating couplers are used to overcome the polarization selection rule. Due to the applied electric field, excited carriers contribute to the photocurrent. Those carriers drift along the field direction in the continuum until they reach the contact region or until they are captured in a subsequent well. Therefore, a photoconductive gain g, describing the ratio between the mean drift length and the thickness of the entire active layer, is associated with the transport process. For each generated mobile carrier the readout capacitor is charged with g electrons. Actually, a small gain is preferable under high photon flux conditions, e.g., in the LW, when only a readout integrated circuit (ROIC) with a limited charge storage capacity is at disposal [6].
Fig. 1: Conduction band edge profil and carrier transport mechanisms in a conventional photoconductive QWIP (a) and in a low-noise QWIP (b). The dual band QWIP camera is comprised of a photoconductive MW-QWIP on top of a low-noise QWIP for the LW regime. In order to reduce the photoconductive gain, i.e., the mean drift length, a different detector structure schematically shown in Fig. 1b [7] was developed. Each period consists of four different zones, i.e., an excitation zone comprised of a doped quantum well, an AlGaAs drift zone, a capture zone realized with an undoped well and a tunneling zone
including thin AlAs barriers. These barrier layers block very efficiently the carrier transport in the continuum and result in a deterministic recapture process after a carrier drift of exactly one period. Captured carriers reach the next period by a subsequent tunneling process. Hence, the noise associated with the recapture process in a standard photoconductive QWIP is reduced and these structures are therefore called low-noise QWIPs. Compared to standard photoconductive QWIPs, low-noise QWIP-FPAs enable a higher dynamic range, longer integration time and improved thermal resolution [6]. B. Manufacturing Technology Growth of the Dual Band QWIP detector structure was performed by MBE on (100)-oriented, undoped, semiinsulating 3”-LEC-GaAs substrates. A lattice-matched AlAs/AlGaAs superlattice serves as an etch stop layer for wet chemical substrate removal after hybridization with the ROIC. Next, an undoped GaAs layer for the isolation trench etching process was grown. The lower, LW sensitive detector structure is a 20 period low-noise QWIP sandwiched between two silicon doped (2×1018 cm-3) GaAs contact layers. The upper MW detector consists of 20 periods of 2.6 nm wide In0.3Ga0.7As quantum wells, each well doped with 2.1×1012 cm-2 Si and 24 nm wide Al0.32Ga0.68As barriers. A 2×1018 cm-3 silicon doped GaAs layer acts as top contact layer. For the realization of the two-dimensional grating coupler, a thin Al0.32Ga0.68As etch stop layer and 400 nm Si-doped GaAs terminate the dual band detector structure. The contact layer in between the LW and the MW sensitive layers represents the common ground for both bands. On a 3”-wafer, 12 dual band QWIP-FPAs with 288x384 pixels and 40 µm pitch were processed in a full wafer process using standard optical lithography. Processing started with the deposition of the upper ohmic contact
Fig. 2: SEM pictures showing sections of a completely processed 288x384 dual band QWIP-FPA with 40 µm pixel pitch. Each pixel comprises three contact lands for a simultaneous and pixel-registered data acquisition of both bands. The two-dimensional grating coupler is optimized for the lower, LW-sensitive low-noise QWIP. metallization. Next, the two-dimensional grating coupler was etched on top of each pixel using a highly selective reactive ion etching (RIE) process. After several tests, a parameter set of 400nm grating depth combined with a grating period of 2.85 µm, optimized to a peak wavelength of 8.5 µm, represented the best trade-off to achieve a similar noise equivalent temperature difference (NETD) within both bands. However, due to the significant area needed for the three contact lands and the large grating period, only a small number of grooves can be placed upon a dual band pixel. This proved to lower the coupling efficiency and to broaden the spectral absorption characteristics. With chlorine-based chemically assisted ion beam etching (CAIBE), deep trenches for the electrical isolation of the pixels were realized. Subsequently, two wet chemical etching steps were employed to obtain both via holes for the common ground and the lower contact layer, respectively. After the deposition of a silicon nitride passivation layer, the passivation was selectively etched by a fluorine-based RIE process to get access to the semiconductor contact layers and a contact metallization pad was deposited. With the next layer a mirror reflector metallization was evaporated on top of the pixels’ grating coupler and a short strip conductor between the metallization pads in the contact via holes and the
corresponding contact lands on top of the pixels was established. After the deposition of a second passivation layer and its selective removal, the front side process was completed by the evaporation of a final metallization layer required for the hybridization with the ROIC. Fig. 2 shows Scanning Electron Microscopy (SEM) pictures of completely processed 288x384 dual band QWIP-FPAs with 40 µm pixel pitch.
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Fig. 3: Schematic cross-sectional view of a backside-illuminated dual band QWIP pixel. After dicing the wafers into single chips, the detector arrays are hybridized with a custom-designed silicon ROIC by flip-chip re-flow indium soldering technology. The ROIC is optimized for high charge handling capacity using only two capacitors with equal storage capacity per unit cell and supporting simultaneous snapshot integration and individual bias control for both bands [8]. After hybridization, the GaAs substrate is completely removed by a combination of mechanical lapping and wet chemical etching. Since only single, free-standing pixels remain, thermally induced stress between the detector chip and the ROIC is eliminated and the optical cross talk between neighboring pixels is minimized. Fig. 3 schematically depicts a cross section of a dual band QWIP-FPA pixel. The hybrid is finally mounted into an integrated detector cooler assembly (IDCA). C. Electro-optical Characterization In addition to the detector arrays, each wafer contains various test devices to characterize the electro-optical performance. For the initial optimization of the coupling efficiency, mesa structures of the same size as regular array pixels were included with a set of different grating parameters [9]. In addition, mesas without grating coupler and different sizes are processed. The latter devices are exploited for the electro-optical standard characterization with 45° facet coupling. Fig. 4a shows the normalized photocurrent spectra of both bands with 45° facet coupling. At 77 K, the MW and LW bands show a 50%-cutoff wavelength of 5.1 µm (+1.0 V bias) and 8.7 µm (-1.0 V bias), respectively. The dark current densities at a typical operating bias of +1.0 V for the MW and –1.0 V for the LW are 3.4×10-7 A/cm2 and 1.7×10-4 A/cm2, respectively.
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D. Camera Performance The NETD of the 288x384 dual band QWIP camera has been determined experimentally for a 300 K scene with F#/2.0 optics and 58 K detector temperature. Fig. 5 summarizes the results achieved with an integration time of 6.8 ms, which allows for a 100 Hz frame rate or a 2 × 2 microscan at 25 Hz for sub-pixel resolution (768 × 576 effective pixels). The gaussian fit function to the histogram data reveals excellent NETD values of 26.7 mK (MW) and 20.6 mK (LW), respectively. The pixel yield in both bands was lager than 99.5 %.
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Fig. 6: Images delivered by the 288x384 dual band QWIP camera. ( FGAN-FOM, Ettlingen, Germany).
3. DUAL-COLOR INAS/GASB SUPERLATTICES A. Basic Principles of Type-II InAs/GaSb Superlattices InAs/GaSb superlattices (SLs) are characterized by a broken-gap type-II band alignment. The GaSb valence band and the InAs conduction band exhibit an overlap of about 140 meV [10]. As indicated in Fig. 7a, for InAs/GaSb SLs with confinement energies exceeding this band overlap, a spatially indirect bandgap opens, which is smaller than the bandgap of its constituents. The effective band gap EG of these structures can be tailored from 0.3 eV to values below 0.1 eV by varying the thickness of the InAs layer or by introducing indium in the (GaxIn1-x)Sb layers. InAs/GaSb SLs for mid-IR detection typically consist of InAs and GaSb layers with a thickness between five and fifteen monolayers (ML). The holes are largely confined in the GaSb layers, whereas electron wave functions overlap considerably from one InAs layer to adjacent InAs layers. As illustrated in Fig. 7a, the overlap of the electron wave functions results in the formation of an electron miniband in the conduction band. Spatially indirect transitions between the localized holes in the GaSb layers and the electron miniband are employed for the detection of infrared photons in the InAs/GaSb SL.
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By appropriately doping the SL, p-i-n photodiodes are realized. For the fabrication of two-dimensional FPAs, SL wafers are structured into a matrix of mesa detectors. A cross sectional view of a typical monospectral mesa diode is shown in Fig. 7b. Since the holes are localized within the GaSb layers, the responsivity in p-i-n InAs/GaSb SL photodiodes is generated by optically excited minority electrons in the intrinsic and p-doped layers, respectively [11]. In contrast to QWIPs, the absorption of normal incident radiation is not forbidden in type-II InAs/GaSb SLs. This advantage allows to omit the fabrication of grating couplers, which generally become less effective with a shrinking pixel size. The most important advantage compared to QWIPs, however, consists in a substantially higher quantum efficiency, which is comparable to the values achieved with typical MCT detectors [12, 13, 14]. Hence, with type-II SLs infrared imagers with a significantly shorter integration time, i.e., a higher frame rate, can be obtained. Type-II SL-FPAs profit from a high homogeneity accomplished with MBE growth and a mature III/V process technology. The most demanding technological challenge is the suppression of surface leakage currents. For the first time, these difficulties have been overcome with the demonstration of a monospectral 256x256 MW SL camera in 2004 [15]. Meanwhile, also in the LW a stable surface passivation based on the overgrowth of mesa-structured devices with lattice-matched, high bandgap AlGaAsSb has been demonstrated [16]. Compared to MCT, type-II SL’s advantages include the commercial availability of low defect density GaSb substrates, a superior electro-optical homogeneity without large cluster defects, a reduced sensitivity to thermal treatments and a comparable performance at reduced production costs. B. Manufacturing Technology To facilitate a simultaneous and pixel-registered detection of both colors, a vertical design based on two “back-toback” InAs/GaSb SL photodiodes separated by a common ground contact layer has been chosen. The detectors are grown by MBE on undoped (100)-GaSb substrates with 2” diameter. The MBE machine is equipped with standard group-III effusion cells for gallium, aluminum and indium and valved cracker cells for arsenic and antimony. For n- and p-type doping of the superlattice regions and the contact layers Si, GaTe and Be are used. The growth sequence starts with a 200 nm isolating and lattice matched AlGaAsSb buffer layer followed by 700 nm of n-type doped GaSb. Next, the “blue channel” for the absorption of the higher energy photons, consisting of 330 periods of a 7.5 ML InAs / 10 ML GaSb SL, is realized. After this, a common ground contact layer comprised of 500 nm of p-type GaSb follows. For the detection of the lower energy radiation the “red channel” containing 150 periods of a 9.5 ML InAs / 10 ML GaSb superlattice is grown. 20 nm n-type InAs terminate the structure. The thickness of the entire vertical detector structure is only 4.5 µm, which significantly reduces the technological challenge compared to bispectral MCT-FPAs with a typical total layer thickness around 15 µm [1]. Although not perfectly matched to the signal levels of both colors, the same ROIC already in use for the dual band QWIP (see section 2) was chosen for the initial proof of concept. The processing of dual-color FPAs with 288x384 detector elements at 40 µm pixel pitch is performed in a full wafer process with standard optical lithography. Three electrical contacts to each pixel ensure temporally and spatially coincident detection. Each wafer contains four dualcolor FPAs with a chip size of 16.1 mm × 12.1 mm and a set of additional test diodes for materials characterization. Fig. 8 illustrates the processing. At first, via holes to the common p-type contact layer and to the n-type contact layer of the lower diode are etched by a chlorine-based CAIBE process. Next, another CAIBE step is used to fabricate deep trenches for a complete electrical isolation of each pixel. Fig. 8a shows a SEM image after this step. The high fill factor achieved with the dry chemical etching processes is evident. After the deposition of the diode passivation, a RIE process is employed to selectively open the passivation in order to get access to the semiconductor contact layers (see Fig. 8b). Next, the contact metallization is evaporated (see Fig. 8c). This metallization layer serves as a mirror reflector on top of the backside illuminated FPA. After the deposition of several further metallization layers and an additional passivation layer the front side process is completed. A part of a fully processed dual-color SL-FPA is shown in Fig. 8d. After completion of the front side process, the wafers are diced and the FPA chips are flip-chip hybridized to the silicon ROIC with the indium solder bump technology described in section 2B. To reduce the free carrier absorption in the substrate and minimize the thermal stress between the dual-color detector chip and the silicon ROIC under cooleddown condition the substrates are thinned by a combination of mechanical lapping, polishing and wet chemical etching to a remaining thickness of around 20 µm. The hybrids are mounted into an IDCA equipped with a 1.5 W linear Stirling cooler.
Fig. 8: SEM images illustrating the processing of 288x384 dual color InAs/GaSb SL-FPAs for the MW. At a pixel pitch of 40 µm, three contact lands per pixel permit simultaneous and spatially coincident detection of both colors.
C. Electro-optical Characterization The optical characterization of each dual-color SL wafer is performed with a Fourier transform infrared spectrometer on test diodes with an optical window processed into the top contact metallization. The solid lines in Fig. 9a show normalized photocurrent spectra of both channels at 77 K and 0 V bias. The 50%-cutoff wavelengths are 4.0 µm and about 5.0 µm for the blue and red channel, respectively. In order to calibrate and occasionally monitor the cutoff wavelength, photoluminescence (PL) measurements are carried out during the growth campaign. The dashed lines in Fig. 9a compare the PL signal of as-grown, unprocessed SL photodiodes at 10 K with the 77 K photocurrent spectra of diodes grown with the same set of parameters. The PL-peak of the unprocessed reference material corresponds roughly to the 50%-cutoff wavelength of the photodiodes. This practical rule of thumb has been verified empirically on a large number of binary InAs/GaSb SL photodiodes in the MW and the LW. For the electrical characterization a set of test diodes with different size and geometry is available on each wafer. Fig. 9b and Fig. 9c present the dark current density at 77 K and the corresponding RA-product of the differential resistance R and the diode area A for both colors. At -50 mV, which is a typical operating bias for IR-cameras, the red channel exhibits a dark current density of 2.2×10-7 A/cm2 resulting in a RA-product of 2.5×105 Ωcm2. Thus, around the typical operating bias for camera applications, both, the red channel of the dual-color SL detector and the MW channel (5.1 µm cutoff) of the dual-band QWIP camera (see section 2C), exhibit comparable dark current densities. Due to the higher bandgap of the blue channel SL, the dark current is between one and two orders of magnitude below the value of the red channel. At a bias of -50mV, the dark current density of the blue channel is 1.9×10-9 A/cm2 corresponding to RA = 2.3×107 Ωcm2. Due to the low absolute dark current, the larger bandgap of the blue channel causes more noisy data.
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D. Camera Performance Fig. 10 shows the excellent NETD histogram data measured at 73 K using F#/2.0 optics and 2.8 ms integration time for both channels. The blue channel reaches a NETD of 29.5 mK and pixel outages are below 1 %. The red channel achieves a NETD of 16.5 mK with less than 2% of defective pixels.
Fig. 11: Sample images delivered by the 288x384 dual-color InAs/GaSb SL camera. The excellent imagery delivered by the 288x384 InAs/GaSb dual-color SL camera is presented in Fig. 11. The scene consists of a man, a hot soldering iron and the burning flame of a cigarette lighter producing carbon dioxide. Most parts of the scene yield a similar thermal signature in both channels. However, the carbon dioxide surrounding the burning gas flame only emits in the red channel. By performing simple image processing, i.e., subtracting the blue from the red channel (see Fig. 11c), by far the highest signal is caused by carbon dioxide. Using a threshold value in the subtraction image, hot and broadband emitting objects can be suppressed efficiently, whereas parts of the scene containing carbon dioxide are clearly identified. The presented dual-color InAs/GaSb SL technology will be used for a Multi-Color Infrared Alerting Sensor for military aircrafts. [17].
4. CONCLUSION We have established a mature production process for MW/LW dual-band QWIP imagers and MW/MW dual-color InAs/GaSb SL cameras. Both offer simultaneous, pixel-registered data acquisition of both channels at a spatial resolution of 288x384 pixels with a high fill factor. With F#/2.0 optics, both systems show excellent NETD values below 30 mK in each channel and a highly uniform performance with a low pixel outage rate. At an integration time of 6.8 ms and a detector temperature of 58 K, the dual-band QWIP camera reaches a NETD of 26.7 mK in the MW and 20.6 mK in the LW, respectively. With 2.8 ms integration time and 73 K detector temperature the dual color InAs/GaSb superlattice camera achieves 29.5 mK for the blue channel (3.4 µm ≤ λ ≤ 4.1 µm) and 16.5 mK for the red channel (4.1 µm ≤ λ ≤ 5.1 µm). As one of the first representatives of the 3rd generation of infrared imagers, the dual-color SL technology will be commercialised in a missile approach warning system.
ACKOWLEDGMENTS The authors are grateful to M. Korobka, H. Güllich, K. Schwarz, L. Kirste and K. Windscheid for assistance in detector processing and characterization. We also express our thanks to F. Fuchs, H. Schneider, P. Koidl and G. Weimann for fruitful discussions. Project funding by the Federal Ministry of Defence is gratefully acknowledged.
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