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f Noise QWIPs and nBn detectors
Infrared Physics & Technology xxx (2014) xxx–xxx
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Infrared Physics & Technology journal homepage: www.elsevier.com/locate/infrared
1/f Noise QWIPs and nBn detectors S.D. Gunapala ⇑, S.B. Rafol, D.Z. Ting, A. Soibel, L. Höglund, C.J. Hill, A. Khoshakhlagh, J.K. Liu, J.M. Mumolo, S.A. Keo Center for Infrared Photodetectors, Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
h i g h l i g h t s 1/f Noise measurement of infrared focal plane arrays. 1/f Noise of quantum well infrared photodetectors. 1/f Noise of nBn detectors with InAsSb bulk absorber.
a r t i c l e
i n f o
Article history: Received 31 July 2014 Available online xxxx Keywords: Infrared detector QWIP nBn Focal plane array 1/f Noise
a b s t r a c t The low-frequency noise is a ubiquitous phenomenon and the spectral power density of this fluctuation process is inversely proportional to the frequency of the signal. We have measured the 1/f noise of a 640 512 pixel quantum well infrared photodetector (QWIP) focal plane array (FPA) with 6.2 lm peak wavelength. Our experimental observations show that this QWIP FPA’s 1/f noise corner frequency is about 0.1 mHz. With this kind of low frequency stability, QWIPs could unveil a new class of infrared applications that have never been imagined before. Furthermore, we present the results from a similar 1/f noise measurement of bulk InAsSb absorber (lattice matched to GaSb substrate) nBn detector array with 4.0 lm cutoff wavelength. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction The low-frequency noise (also known as pink noise or flicker noise) could be an omnipresent phenomenon. The spectral power density (PSD) of this fluctuation process is inversely proportional to the frequency of the signal. The occurrence of 1/f noise can be found almost everywhere in nature. It has been observed in science, medicine, astronomy, economics, psychology, music, and even in movies, since the first observation by Johnson [1]. Johnson discovered this low frequency 1/f noise in vacuum tubes. In terms of noise PSD, the 1/f noise falls in between the white noise (no time correlation) and the brown noise due to Brownian motion (proportional to 1/f2) with no correlation between consecutive increments. Unlike the 1/f and white noises, the brown noise is not observed in electronics devices. The importance of this in electronic devices motivated numerous studies of this noise to find its physical mechanism and thereby how to control it. However, the origin of 1/f noise remains a mystery after nearly a century from its discovery ⇑ Corresponding author at: Jet Propulsion Laboratory, M/S 302-306, 4800 Oak Grove Drive, Pasadena, CA 91109-8099, USA. Tel.: +1 818 354 1880; fax: +1 818 393 4540. E-mail address: [email protected] (S.D. Gunapala).
by Johnson [1]. Its important to know whether the origin of 1/f noise is from the bulk of the electronic device or from its surface. The origin of 1/f noise in semiconductor devices has been viewed and treated in many different ways, including generation– recombination sources in the device, fluctuations in the mobility of carriers [2], and surface effect. 1/f noise knee of the high operating temperature (HOT) HgCdTe devices are reported in the 500 Hz – 1 kHz range [3], and the very-long-wavelength HgCdTe devices are reported to have 1/f noise knee around 1 kHz [4]. 1/f Noise of QWIPs were first measured by Ressler et al. [5] when evaluating QWIP FPAs for astronomical observations which require long integration times. Ressler et al. measured the 1/f noise of a 256 256 pixel long-wavelength infrared (8–9 lm) QWIP FPA with 30 lm pixel pitch and reported the observation of 1/f noise corner frequency around 10 mHz. This low 1/f noise is a significant advantage of QWIPs over Si:As impurity band conduction detectors for ground based astronomy [5]. Additionally, this is a very useful property for infrared imagers as it avoids unnecessary frequent calibrations needed for correcting the drifting pixels in FPAs. Though this property is very useful for infrared observational instruments, there were no reports on the low 1/f noise measurements of QWIPs since the first reported observation by Ressler et al. [5].
http://dx.doi.org/10.1016/j.infrared.2014.09.031 1350-4495/Ó 2014 Elsevier B.V. All rights reserved.
Please cite this article in press as: S.D. Gunapala et al., 1/f Noise QWIPs and nBn detectors, Infrared Phys. Technol. (2014), http://dx.doi.org/10.1016/ j.infrared.2014.09.031
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2. QWIP focal plane array Several VGA format (i.e., 640 512 pixel) QWIP FPAs were fabricated as described elsewhere [6]. Twelve 640 512 pixel QWIP FPAs were processed on a 3-in. GaAs wafer. The detector pixel pitch of the FPA is 25 lm and the actual pixel area is 23 23 lm2. Indium bumps were evaporated on top of the detectors for hybridization with a silicon readout integrated circuit (ROIC). These QWIP FPAs were hybridized (via indium bump-bonding process) to a 640 512 pixel complementary metal-oxide semiconductor (CMOS) ROIC and biased at VB = 0.7 V. At temperatures below 72 K, the signal-to-noise ratio of the system is limited by array non-uniformity, ROIC noise, and photocurrent (photon flux) noise. At temperatures above 72 K, the temporal noise, due to the dark current, becomes the limitation. Charge injection efficiency into the ROIC was calculated as described in Bethea et al. [7]. An average charge injection efficiency of 90% has been achieved at a frame rate of 30 Hz. The noise equivalent temperature difference (NEDT) of the FPA estimated [6] from test structure data as a function of temperature. The background temperature TB = 300 K, the area of the pixel A = (23 lm)2, the f-number of the optical system is 2, and the frame rate is 30 Hz. The measured NEDT of the FPA is 20 mK at an operating temperature of T = 65 K, 16 ms integration time for 300 K background with f/2 optics. It closely agrees with the single element test detector results of 16 mK. A 640 512 QWIP FPA hybrid was integrated with a 330 mW integral Stirling closed-cycle cooler assembly and installed into a FLIR Phoenix™ camera-body, to make a stable camera. The camera head consists of a 640 512 format LWIR QWIP array hybridized with FLIR ISC 9803 ROIC, a cold-stop, a Stirling cooler, pre-amplifiers, and analog-to-digital converters. The optical element of the camera is a 100 mm focal length germanium lens assembly, with a 9.2° field-of-view. It is designed to be transparent in the 5–8 lm wavelength range, to be compatible with the QWIP’s 6.2 lm operation. The digital acquisition resolution of the camera is 14-bits, which determines the instantaneous dynamic range of the camera (i.e., 16,384). However, the dynamic range of QWIP is 85 dB. The measured mean NEDT of the QWIP camera system is 25 mK at an operating temperature of T = 65 K for a 300 K background with germanium f/2 optics. The uncorrected photocurrent non-uniformity of the VGA format QWIP FPA is about 5% (=sigma/mean). The non-uniformity after a two-point (17 °C and 27 °C) correction improves to an impressive 0.02%.
3. 1/f Noise measurement of QWIP detector array Three regions of interest (ROI) of the FPA were arbitrarily selected for this measurement. We let the camera run for a few hours until its cryo-cooler and the electronics get stabilized. FPA was uncorrected, because, 1/f noise is in temporal frequency domain and the spatial noise is in the spatial frequency domain. Thus, 1/f noise, which is temporal, is independent of the spatial noise of the FPA. A raw data set was collected from each ROI in every 40 s time interval (Dt) and a time series was built (i.e., a data set which evolves with the time). The sizes of left, middle, and right ROIs are 25 50, 40 50, and 60 50 pixels. Voltage of the detector common (i.e., Det_Com) of the FPA was kept at 4.0 V. The actual bias voltage on detector pixels is negligibly small when VDet_Com = 4.0 V. Thus, detector-pixels at this bias on ROIC do not produce any dark current and photo current (i.e., detector pixels turned off). This measurement provides the noise of the imaging system (i.e., including cooler, electronics, ROIC, etc.) without the detector pixels. Data was collected for approximately 2.5 days and the raw data set of all three regions are shown in
Fig. 1. The two large variations in each data set (at 1.25 and 2 day) are due to laboratory temperature variation caused by major heat-wave during the period of data acquisition. However, this fluctuation occurred at very low frequency and it should be able to identify and remove in the frequency domain. The analog-todigital-converter (ADC) counts in Fig. 1 can be easily converted to voltage using V/ADC count conversion and the output voltage can be converted to electrons using the ROIC gain. The total signal in each integration period was converted to current signal using the integration time of 16 msec. Deviation of each data point in each ROI from its mean signal is the noise. Fig. 2 shows the noise current of each ROI as a function of time. A fast Fourier transform (FFT) was performed on each data set to convert the data into frequency domain and these noise spectrums for each ROI is shown in Fig. 3. PSD was calculated using the following equation,
PSDðf Þ ¼ 2
jFFTðAi Þj2 NðDtÞ1
¼2
jFFTðAi Þj2 NF s
ð1Þ
for each noise spectrum. Where Ai is the time series data and i = 1, 2, . . .. . ., N. N is the total number of samples, Dt is the sampling time and Fs is the sampling frequency. The bumps in each noise spectrum and the PSD curves at 1.5 105 Hz are due to cry-cooler reaction to the external temperature fluctuations as discussed earlier. Fig. 4(a) shows the PSDs of all three ROIs and Fig. 4(b) shows the PSDs of all three ROIs after 8-point running average (smother curves). Both set of curves in Fig. 4 show the onset of low frequency noise (i.e., 1/f noise) of the system without the QWIP detector array, since the detector pixels were not turned on during this measurement. A similar set of data series was taken with the detector pixels turned on by increasing the Det_Com to 4.7 V which applied 0.7 V bias voltage across the detector pixels. Fig. 5(a) shows the PSDs of the QWIP camera with and without detector pixels turned on. The total noise of the camera can be written as, 2
in
2
Total
¼ in
2
ðSystem-DetectorÞ
þ in
Detector
ð2Þ
and Eq. (2) can be re-written as,
PSDDetector ¼ PSDDetector on PSDDetector off
ð3Þ
Fig. 5(b) shows the difference between the PSD with detector array turned on and PSD without detector array turned on. Thus, Fig. 5(b) shows the PSD of the QWIP detector arrays and it clearly shows the onset of low frequency noise (i.e., 1/f noise). This experimental observation shows the 6.2 lm cutoff QWIP FPA’s 1/f noise corner frequency is less than 0.5 mHz which is the lowest reported 1/f noise knee value for QWIPs. The data collected from FPA pixels in each ROI at a given time is temporal data. Its temporal data frozen at that moment. Thus, averaging the data in each ROI should reduce the white noise which is uncorrelated in time. To understand this behavior, a center pixel, 4 4 pixels and 16 16 pixels regions around the center of the center ROI were selected for this experiment. PSDs were calculated for a single pixel, 4 4 pixels area and the 16 16 pixels area selected from the center of the center ROI. Fig. 6 shows the PSDs of a single pixel, 4 4 pixels and the 16 16 pixels regions selected form the center ROI. It clearly shows that the white noise (high frequency noise) reduces as we average over pixels, however, it approached the center ROI PSD at 16 16 pixel level. Also, its worth noting that the low frequency noise behavior is similar in all three cases and its independent of the size of the region selected for noise measurements. 1/f noise of the QWIP devices published in the literature were taken from the 8–9 lm cutoff QWIP devices. The QWIP FPA used in this work has a much shorter cutoff wavelength of 6.4 lm. This may be an indication that the origin of 1/f noise in QWIP devices is due to some mid band gap traps.
Please cite this article in press as: S.D. Gunapala et al., 1/f Noise QWIPs and nBn detectors, Infrared Phys. Technol. (2014), http://dx.doi.org/10.1016/ j.infrared.2014.09.031
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Fig. 1. Raw data of the time series collected from each ROI. (a)–(c) Are left, center, and right ROI of the QWIP focal plane.
Fig. 2. This figure shows the noise current as a function of time for each ROI from left to right. (a)–(c) Are left, center, and right ROI of the FPA.
4. 1/f Noise of nBn detector array The antimonides based nBn detector with lattice-matched bulk InAsSb absorber to GaSb substrate was first demonstrated by Maimon and Wicks [8,9]. Furthermore, there has been rapid progress in the development of nBn devices which has resulted in the demonstration of single pixel detectors and imagers based on InAs– AlAsSb, InAsSb–AlAsSb, and InAs/GaSb–AlAsSb absorber–barrier material pairs, grown on InAs, GaSb, and GaAs substrates and covering both the mid-wavelength and long-wavelength infrared (MWIR and LWIR) spectral ranges [10–16]. Additionally, these nBn detector based infrared imaging cameras are already commercially available for applications of your choice [17,18]. Thus, its important to evaluate the long-term stability of these detector arrays, since some applications require long-term stability. We have grown an nBn device structure with a lattice matched 4 lm thick InAsSb absorber with AlAsSb barrier on a 3-in. GaSb substrate. 640 512 pixel detector arrays with 25 mm pixel pitch matched to FLIR ISC-9803 ROIC were fabricated using dry etching process. Pixels were defined by etching through the AlAsSb barrier, InAsSb absorber, and half-way through the heavily doped bottom
contact layer. Pixels were fully delineated to avoid possible pixelto-pixel optical and electrical cross-talk. The cutoff wavelength of the detector is 4 lm at 120 K and the quantum efficiency of the FPA is 0.67. Indium bumps were evaporated on top of the detectors for hybridization with a silicon ROIC. These QWIP FPAs were hybridized to a 640 512 pixel FLIR ISC-9803 ROIC and biased at VB = 200 mV. This initial array gave excellent images with 99.9% pixel operability. The measured mean NEDT of the FPA is 20 mK at an operating temperature of T = 120 K with 600 ls integration time at 300 K background with f/2 optics. A 640 512 nBn FPA hybrid was integrated with a 250 mW integral Stirling closed-cycle cooler to make a stable camera. The digital acquisition resolution of the camera is 14-bits. As described earlier, three ROIs of the FPA were arbitrarily selected for 1/f noise measurement and let the camera run for a several hours until its cooler reached a stable condition. A raw data set was collected from each ROI in every 2 s time interval (DT) and a time series was built. Fig. 7(a) shows the PSDs with and without detector array turned on and Fig. 7(b) shows the difference between the PSD with detector array turned on and PSD without detector array turned on. The PSD of the nBn detector arrays (see
Please cite this article in press as: S.D. Gunapala et al., 1/f Noise QWIPs and nBn detectors, Infrared Phys. Technol. (2014), http://dx.doi.org/10.1016/ j.infrared.2014.09.031
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Fig. 3. Power spectral densities of the left (a), center (b), and right (c) ROI of the QWIP FPA.
Fig. 4. (a) PSDs of all three ROIs; (b) PSDs of all three ROIs after 8-point running average (smother curves). Both set of curves in this figure show onset of low frequency noise of the system without the QWIP detector array.
Fig. 5. (a) PSDs of QWIP camera with and without detector pixels turned on; (b) the difference between the PSD with and without detector array turned on. Thus, (b) shows the PSD of the QWIP detector arrays and it clearly shows the onset of low frequency noise.
Please cite this article in press as: S.D. Gunapala et al., 1/f Noise QWIPs and nBn detectors, Infrared Phys. Technol. (2014), http://dx.doi.org/10.1016/ j.infrared.2014.09.031
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Fig. 6. (a) PSDs of a single pixel and the 4 4 pixels and 16 16 pixels regions and (b) PSDs of a single pixel and the 4 4 pixels and 16 16 pixels regions after 8-point running average. (b) Clearly shows the white noise reduces as we average over pixels, however, onset of 1/f noise is unchanged.
Fig. 7. (a) PSDs of nBn imaging system with and without detector pixels turned on; (b) the difference between the PSD with and without detector array turned on. Thus, (b) shows the PSD of the nBn detector arrays and it clearly shows the onset of low frequency noise.
Fig. 7(b)) clearly shows the onset of the 1/f noise. This experimental observation shows the 4 lm cutoff nBn detector array 1/f noise corner frequency is between 50 and 90 mHz.
Conflict of interest There is no conflict of interest in reference to this manuscript.
5. Conclusion Acknowledgments The 1/f noise could be found almost everywhere in nature and it has been observed in science, medicine, astronomy, economics, psychology, music, and even in movies. We have measured the 1/f noise of both QWIP and nBn detectors, but their 1/f noise knees are at extremely low frequencies. There are two major models that can be useful in predicting the trends of 1/f noise in semiconductor devices. The McWhorter model attributed 1/f-noise to fluctuations in the trap occupancy and charge carriers tunneling back and forth between the host semiconductor and the defects. This model described these fluctuations using a simple SRH-based model and extending it by the effect of charge carrier tunneling [19]. The Hooge model attributed 1/f noise to fluctuations carrier mobility in the semiconductor devices [20]. However, we do not have sufficient data to fit with either model within the scope of this paper. Data fitting to these models may able to predict the trends of 1/f noise of QWIP and nBn detectors. The general rule is that the 1/f noise of the system has to be less than the Nyquist frequency of the system of interest. Lower frequency stability allows longer integration time which improves the signal-to-noise-ratio of the sensor. Thus, this kind of low frequency stability, QWIP and nBn detectors could unveil a new class of infrared applications that have never been imagined before. Frequent sensor calibration becomes redundant with these kinds of stable detector arrays and they could be very useful for space infrared instruments due to elimination of frequent calibrations and added reliability comes from the elimination of moving parts associated with the calibration sub-system.
The authors thank M. Herman and E. Kolawa, T. Luchik, and J. Fanson of JPL, and M. Tidrow and S. Bandara of the U.S. Army Night Vision Electronics Sensor Directorate. The research described in this paper was carried out at the Jet Propulsion Laboratory, California Institute of Technology, through an agreement with the National Aeronautics and Space Administration. Copyright 2014 California Institute of Technology.
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Please cite this article in press as: S.D. Gunapala et al., 1/f Noise QWIPs and nBn detectors, Infrared Phys. Technol. (2014), http://dx.doi.org/10.1016/ j.infrared.2014.09.031
Contents lists available at ScienceDirect
Infrared Physics & Technology journal homepage: www.elsevier.com/locate/infrared
1/f Noise QWIPs and nBn detectors S.D. Gunapala ⇑, S.B. Rafol, D.Z. Ting, A. Soibel, L. Höglund, C.J. Hill, A. Khoshakhlagh, J.K. Liu, J.M. Mumolo, S.A. Keo Center for Infrared Photodetectors, Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
h i g h l i g h t s 1/f Noise measurement of infrared focal plane arrays. 1/f Noise of quantum well infrared photodetectors. 1/f Noise of nBn detectors with InAsSb bulk absorber.
a r t i c l e
i n f o
Article history: Received 31 July 2014 Available online xxxx Keywords: Infrared detector QWIP nBn Focal plane array 1/f Noise
a b s t r a c t The low-frequency noise is a ubiquitous phenomenon and the spectral power density of this fluctuation process is inversely proportional to the frequency of the signal. We have measured the 1/f noise of a 640 512 pixel quantum well infrared photodetector (QWIP) focal plane array (FPA) with 6.2 lm peak wavelength. Our experimental observations show that this QWIP FPA’s 1/f noise corner frequency is about 0.1 mHz. With this kind of low frequency stability, QWIPs could unveil a new class of infrared applications that have never been imagined before. Furthermore, we present the results from a similar 1/f noise measurement of bulk InAsSb absorber (lattice matched to GaSb substrate) nBn detector array with 4.0 lm cutoff wavelength. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction The low-frequency noise (also known as pink noise or flicker noise) could be an omnipresent phenomenon. The spectral power density (PSD) of this fluctuation process is inversely proportional to the frequency of the signal. The occurrence of 1/f noise can be found almost everywhere in nature. It has been observed in science, medicine, astronomy, economics, psychology, music, and even in movies, since the first observation by Johnson [1]. Johnson discovered this low frequency 1/f noise in vacuum tubes. In terms of noise PSD, the 1/f noise falls in between the white noise (no time correlation) and the brown noise due to Brownian motion (proportional to 1/f2) with no correlation between consecutive increments. Unlike the 1/f and white noises, the brown noise is not observed in electronics devices. The importance of this in electronic devices motivated numerous studies of this noise to find its physical mechanism and thereby how to control it. However, the origin of 1/f noise remains a mystery after nearly a century from its discovery ⇑ Corresponding author at: Jet Propulsion Laboratory, M/S 302-306, 4800 Oak Grove Drive, Pasadena, CA 91109-8099, USA. Tel.: +1 818 354 1880; fax: +1 818 393 4540. E-mail address: [email protected] (S.D. Gunapala).
by Johnson [1]. Its important to know whether the origin of 1/f noise is from the bulk of the electronic device or from its surface. The origin of 1/f noise in semiconductor devices has been viewed and treated in many different ways, including generation– recombination sources in the device, fluctuations in the mobility of carriers [2], and surface effect. 1/f noise knee of the high operating temperature (HOT) HgCdTe devices are reported in the 500 Hz – 1 kHz range [3], and the very-long-wavelength HgCdTe devices are reported to have 1/f noise knee around 1 kHz [4]. 1/f Noise of QWIPs were first measured by Ressler et al. [5] when evaluating QWIP FPAs for astronomical observations which require long integration times. Ressler et al. measured the 1/f noise of a 256 256 pixel long-wavelength infrared (8–9 lm) QWIP FPA with 30 lm pixel pitch and reported the observation of 1/f noise corner frequency around 10 mHz. This low 1/f noise is a significant advantage of QWIPs over Si:As impurity band conduction detectors for ground based astronomy [5]. Additionally, this is a very useful property for infrared imagers as it avoids unnecessary frequent calibrations needed for correcting the drifting pixels in FPAs. Though this property is very useful for infrared observational instruments, there were no reports on the low 1/f noise measurements of QWIPs since the first reported observation by Ressler et al. [5].
http://dx.doi.org/10.1016/j.infrared.2014.09.031 1350-4495/Ó 2014 Elsevier B.V. All rights reserved.
Please cite this article in press as: S.D. Gunapala et al., 1/f Noise QWIPs and nBn detectors, Infrared Phys. Technol. (2014), http://dx.doi.org/10.1016/ j.infrared.2014.09.031
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2. QWIP focal plane array Several VGA format (i.e., 640 512 pixel) QWIP FPAs were fabricated as described elsewhere [6]. Twelve 640 512 pixel QWIP FPAs were processed on a 3-in. GaAs wafer. The detector pixel pitch of the FPA is 25 lm and the actual pixel area is 23 23 lm2. Indium bumps were evaporated on top of the detectors for hybridization with a silicon readout integrated circuit (ROIC). These QWIP FPAs were hybridized (via indium bump-bonding process) to a 640 512 pixel complementary metal-oxide semiconductor (CMOS) ROIC and biased at VB = 0.7 V. At temperatures below 72 K, the signal-to-noise ratio of the system is limited by array non-uniformity, ROIC noise, and photocurrent (photon flux) noise. At temperatures above 72 K, the temporal noise, due to the dark current, becomes the limitation. Charge injection efficiency into the ROIC was calculated as described in Bethea et al. [7]. An average charge injection efficiency of 90% has been achieved at a frame rate of 30 Hz. The noise equivalent temperature difference (NEDT) of the FPA estimated [6] from test structure data as a function of temperature. The background temperature TB = 300 K, the area of the pixel A = (23 lm)2, the f-number of the optical system is 2, and the frame rate is 30 Hz. The measured NEDT of the FPA is 20 mK at an operating temperature of T = 65 K, 16 ms integration time for 300 K background with f/2 optics. It closely agrees with the single element test detector results of 16 mK. A 640 512 QWIP FPA hybrid was integrated with a 330 mW integral Stirling closed-cycle cooler assembly and installed into a FLIR Phoenix™ camera-body, to make a stable camera. The camera head consists of a 640 512 format LWIR QWIP array hybridized with FLIR ISC 9803 ROIC, a cold-stop, a Stirling cooler, pre-amplifiers, and analog-to-digital converters. The optical element of the camera is a 100 mm focal length germanium lens assembly, with a 9.2° field-of-view. It is designed to be transparent in the 5–8 lm wavelength range, to be compatible with the QWIP’s 6.2 lm operation. The digital acquisition resolution of the camera is 14-bits, which determines the instantaneous dynamic range of the camera (i.e., 16,384). However, the dynamic range of QWIP is 85 dB. The measured mean NEDT of the QWIP camera system is 25 mK at an operating temperature of T = 65 K for a 300 K background with germanium f/2 optics. The uncorrected photocurrent non-uniformity of the VGA format QWIP FPA is about 5% (=sigma/mean). The non-uniformity after a two-point (17 °C and 27 °C) correction improves to an impressive 0.02%.
3. 1/f Noise measurement of QWIP detector array Three regions of interest (ROI) of the FPA were arbitrarily selected for this measurement. We let the camera run for a few hours until its cryo-cooler and the electronics get stabilized. FPA was uncorrected, because, 1/f noise is in temporal frequency domain and the spatial noise is in the spatial frequency domain. Thus, 1/f noise, which is temporal, is independent of the spatial noise of the FPA. A raw data set was collected from each ROI in every 40 s time interval (Dt) and a time series was built (i.e., a data set which evolves with the time). The sizes of left, middle, and right ROIs are 25 50, 40 50, and 60 50 pixels. Voltage of the detector common (i.e., Det_Com) of the FPA was kept at 4.0 V. The actual bias voltage on detector pixels is negligibly small when VDet_Com = 4.0 V. Thus, detector-pixels at this bias on ROIC do not produce any dark current and photo current (i.e., detector pixels turned off). This measurement provides the noise of the imaging system (i.e., including cooler, electronics, ROIC, etc.) without the detector pixels. Data was collected for approximately 2.5 days and the raw data set of all three regions are shown in
Fig. 1. The two large variations in each data set (at 1.25 and 2 day) are due to laboratory temperature variation caused by major heat-wave during the period of data acquisition. However, this fluctuation occurred at very low frequency and it should be able to identify and remove in the frequency domain. The analog-todigital-converter (ADC) counts in Fig. 1 can be easily converted to voltage using V/ADC count conversion and the output voltage can be converted to electrons using the ROIC gain. The total signal in each integration period was converted to current signal using the integration time of 16 msec. Deviation of each data point in each ROI from its mean signal is the noise. Fig. 2 shows the noise current of each ROI as a function of time. A fast Fourier transform (FFT) was performed on each data set to convert the data into frequency domain and these noise spectrums for each ROI is shown in Fig. 3. PSD was calculated using the following equation,
PSDðf Þ ¼ 2
jFFTðAi Þj2 NðDtÞ1
¼2
jFFTðAi Þj2 NF s
ð1Þ
for each noise spectrum. Where Ai is the time series data and i = 1, 2, . . .. . ., N. N is the total number of samples, Dt is the sampling time and Fs is the sampling frequency. The bumps in each noise spectrum and the PSD curves at 1.5 105 Hz are due to cry-cooler reaction to the external temperature fluctuations as discussed earlier. Fig. 4(a) shows the PSDs of all three ROIs and Fig. 4(b) shows the PSDs of all three ROIs after 8-point running average (smother curves). Both set of curves in Fig. 4 show the onset of low frequency noise (i.e., 1/f noise) of the system without the QWIP detector array, since the detector pixels were not turned on during this measurement. A similar set of data series was taken with the detector pixels turned on by increasing the Det_Com to 4.7 V which applied 0.7 V bias voltage across the detector pixels. Fig. 5(a) shows the PSDs of the QWIP camera with and without detector pixels turned on. The total noise of the camera can be written as, 2
in
2
Total
¼ in
2
ðSystem-DetectorÞ
þ in
Detector
ð2Þ
and Eq. (2) can be re-written as,
PSDDetector ¼ PSDDetector on PSDDetector off
ð3Þ
Fig. 5(b) shows the difference between the PSD with detector array turned on and PSD without detector array turned on. Thus, Fig. 5(b) shows the PSD of the QWIP detector arrays and it clearly shows the onset of low frequency noise (i.e., 1/f noise). This experimental observation shows the 6.2 lm cutoff QWIP FPA’s 1/f noise corner frequency is less than 0.5 mHz which is the lowest reported 1/f noise knee value for QWIPs. The data collected from FPA pixels in each ROI at a given time is temporal data. Its temporal data frozen at that moment. Thus, averaging the data in each ROI should reduce the white noise which is uncorrelated in time. To understand this behavior, a center pixel, 4 4 pixels and 16 16 pixels regions around the center of the center ROI were selected for this experiment. PSDs were calculated for a single pixel, 4 4 pixels area and the 16 16 pixels area selected from the center of the center ROI. Fig. 6 shows the PSDs of a single pixel, 4 4 pixels and the 16 16 pixels regions selected form the center ROI. It clearly shows that the white noise (high frequency noise) reduces as we average over pixels, however, it approached the center ROI PSD at 16 16 pixel level. Also, its worth noting that the low frequency noise behavior is similar in all three cases and its independent of the size of the region selected for noise measurements. 1/f noise of the QWIP devices published in the literature were taken from the 8–9 lm cutoff QWIP devices. The QWIP FPA used in this work has a much shorter cutoff wavelength of 6.4 lm. This may be an indication that the origin of 1/f noise in QWIP devices is due to some mid band gap traps.
Please cite this article in press as: S.D. Gunapala et al., 1/f Noise QWIPs and nBn detectors, Infrared Phys. Technol. (2014), http://dx.doi.org/10.1016/ j.infrared.2014.09.031
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Fig. 1. Raw data of the time series collected from each ROI. (a)–(c) Are left, center, and right ROI of the QWIP focal plane.
Fig. 2. This figure shows the noise current as a function of time for each ROI from left to right. (a)–(c) Are left, center, and right ROI of the FPA.
4. 1/f Noise of nBn detector array The antimonides based nBn detector with lattice-matched bulk InAsSb absorber to GaSb substrate was first demonstrated by Maimon and Wicks [8,9]. Furthermore, there has been rapid progress in the development of nBn devices which has resulted in the demonstration of single pixel detectors and imagers based on InAs– AlAsSb, InAsSb–AlAsSb, and InAs/GaSb–AlAsSb absorber–barrier material pairs, grown on InAs, GaSb, and GaAs substrates and covering both the mid-wavelength and long-wavelength infrared (MWIR and LWIR) spectral ranges [10–16]. Additionally, these nBn detector based infrared imaging cameras are already commercially available for applications of your choice [17,18]. Thus, its important to evaluate the long-term stability of these detector arrays, since some applications require long-term stability. We have grown an nBn device structure with a lattice matched 4 lm thick InAsSb absorber with AlAsSb barrier on a 3-in. GaSb substrate. 640 512 pixel detector arrays with 25 mm pixel pitch matched to FLIR ISC-9803 ROIC were fabricated using dry etching process. Pixels were defined by etching through the AlAsSb barrier, InAsSb absorber, and half-way through the heavily doped bottom
contact layer. Pixels were fully delineated to avoid possible pixelto-pixel optical and electrical cross-talk. The cutoff wavelength of the detector is 4 lm at 120 K and the quantum efficiency of the FPA is 0.67. Indium bumps were evaporated on top of the detectors for hybridization with a silicon ROIC. These QWIP FPAs were hybridized to a 640 512 pixel FLIR ISC-9803 ROIC and biased at VB = 200 mV. This initial array gave excellent images with 99.9% pixel operability. The measured mean NEDT of the FPA is 20 mK at an operating temperature of T = 120 K with 600 ls integration time at 300 K background with f/2 optics. A 640 512 nBn FPA hybrid was integrated with a 250 mW integral Stirling closed-cycle cooler to make a stable camera. The digital acquisition resolution of the camera is 14-bits. As described earlier, three ROIs of the FPA were arbitrarily selected for 1/f noise measurement and let the camera run for a several hours until its cooler reached a stable condition. A raw data set was collected from each ROI in every 2 s time interval (DT) and a time series was built. Fig. 7(a) shows the PSDs with and without detector array turned on and Fig. 7(b) shows the difference between the PSD with detector array turned on and PSD without detector array turned on. The PSD of the nBn detector arrays (see
Please cite this article in press as: S.D. Gunapala et al., 1/f Noise QWIPs and nBn detectors, Infrared Phys. Technol. (2014), http://dx.doi.org/10.1016/ j.infrared.2014.09.031
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Fig. 3. Power spectral densities of the left (a), center (b), and right (c) ROI of the QWIP FPA.
Fig. 4. (a) PSDs of all three ROIs; (b) PSDs of all three ROIs after 8-point running average (smother curves). Both set of curves in this figure show onset of low frequency noise of the system without the QWIP detector array.
Fig. 5. (a) PSDs of QWIP camera with and without detector pixels turned on; (b) the difference between the PSD with and without detector array turned on. Thus, (b) shows the PSD of the QWIP detector arrays and it clearly shows the onset of low frequency noise.
Please cite this article in press as: S.D. Gunapala et al., 1/f Noise QWIPs and nBn detectors, Infrared Phys. Technol. (2014), http://dx.doi.org/10.1016/ j.infrared.2014.09.031
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Fig. 6. (a) PSDs of a single pixel and the 4 4 pixels and 16 16 pixels regions and (b) PSDs of a single pixel and the 4 4 pixels and 16 16 pixels regions after 8-point running average. (b) Clearly shows the white noise reduces as we average over pixels, however, onset of 1/f noise is unchanged.
Fig. 7. (a) PSDs of nBn imaging system with and without detector pixels turned on; (b) the difference between the PSD with and without detector array turned on. Thus, (b) shows the PSD of the nBn detector arrays and it clearly shows the onset of low frequency noise.
Fig. 7(b)) clearly shows the onset of the 1/f noise. This experimental observation shows the 4 lm cutoff nBn detector array 1/f noise corner frequency is between 50 and 90 mHz.
Conflict of interest There is no conflict of interest in reference to this manuscript.
5. Conclusion Acknowledgments The 1/f noise could be found almost everywhere in nature and it has been observed in science, medicine, astronomy, economics, psychology, music, and even in movies. We have measured the 1/f noise of both QWIP and nBn detectors, but their 1/f noise knees are at extremely low frequencies. There are two major models that can be useful in predicting the trends of 1/f noise in semiconductor devices. The McWhorter model attributed 1/f-noise to fluctuations in the trap occupancy and charge carriers tunneling back and forth between the host semiconductor and the defects. This model described these fluctuations using a simple SRH-based model and extending it by the effect of charge carrier tunneling [19]. The Hooge model attributed 1/f noise to fluctuations carrier mobility in the semiconductor devices [20]. However, we do not have sufficient data to fit with either model within the scope of this paper. Data fitting to these models may able to predict the trends of 1/f noise of QWIP and nBn detectors. The general rule is that the 1/f noise of the system has to be less than the Nyquist frequency of the system of interest. Lower frequency stability allows longer integration time which improves the signal-to-noise-ratio of the sensor. Thus, this kind of low frequency stability, QWIP and nBn detectors could unveil a new class of infrared applications that have never been imagined before. Frequent sensor calibration becomes redundant with these kinds of stable detector arrays and they could be very useful for space infrared instruments due to elimination of frequent calibrations and added reliability comes from the elimination of moving parts associated with the calibration sub-system.
The authors thank M. Herman and E. Kolawa, T. Luchik, and J. Fanson of JPL, and M. Tidrow and S. Bandara of the U.S. Army Night Vision Electronics Sensor Directorate. The research described in this paper was carried out at the Jet Propulsion Laboratory, California Institute of Technology, through an agreement with the National Aeronautics and Space Administration. Copyright 2014 California Institute of Technology.
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Please cite this article in press as: S.D. Gunapala et al., 1/f Noise QWIPs and nBn detectors, Infrared Phys. Technol. (2014), http://dx.doi.org/10.1016/ j.infrared.2014.09.031