Quantum Well Infrared Photodetectors Device Research Articles

Optimization of different structural parameters of GeSn/SiGeSn Quantum Well Infrared Photodetectors (QWIPs) for low dark current and high responsivity

Dark current and responsivity are two important parameters of quantum well infrared photodetectors (QWIPs). An optimization approach is applied herein to achieve low dark current along with high responsivity for a GeSn/SiGeSn QWIP. The dark current is reduced by varying different physical parameters of well and barrier, operating temperature and bias voltage of the QWIP. To optimize the QWIP, the detectivity is calculated. The results show that the optimally designed GeSn/SiGeSn QWIP can achieve a low dark current of 2.35 pA at 2 V with a peak responsivity of 1.24 A/W at 4.3 µm and a high detectivity of 3.47 × 1012 cm · Hz1/2 W−1 at 2 V and 77 K. Finally, the frequency response of the optimally designed GeSn/SiGeSn QWIP device is estimated. The theoretically predicted higher detectivity with respect to previously reported SiGe/Si QWIPs and GeSn/SiGeSn interband QWIPs indicates a promising future for GeSn/SiGeSn intersubband QWIPs.

The opportunity of using InGaAsN/AlGaAs quantum wells for extended short-wavelength infrared photodetection

We propose and demonstrate a novel concept to address high-performance, extended short wavelength (e-SWIR) photodetectors. Our approach is based on shifting the well-developed GaAs quantum-well infrared photodetector (QWIP) technology to e-SWIR wavelengths. In order to increase the available conduction band offsets (CBOs), we suggest incorporating nitrogen (N) atoms into the quantum well material. The incorporation of N atoms into III-Vs results in dilute-nitride highly mismatched alloys with lower bandgaps and higher CBOs. In our work, we demonstrate CBO values reaching up to ∼1 eV in InGaAsN/AlGaAs QWIPs. This large CBO makes these structures suitable for e-SWIR detection. The large CBO reduces the dark current dramatically and allows efficient detection at room temperature.In our study, we devised two similar InGaAsN/AlGaAs QWIP devices with twofold, 1% and 2% N composition. Based on the measured dark current data, we extracted activation energy barriers of 780 meV and 580 meV for the 1% and 2% N devices, respectively. The dark current and photocurrent spectral response behave in correlation to the change in the barriers’ height. The photocurrent response of the 1% N device peaks at ∼2.25 μm and spans at a spectral range between 1.3 and 2.95 μm. The photocurrent response of the 2% N device is blue shifted to ∼1.42 μm and spans at a spectral range between 1.1 and 2.2 μm. The 2% N device exhibits lower dark currents and a strong photoresponse at room temperature, whereas the 1% N device exhibits a clear response only at temperatures below 150 K.The detection mechanism in the InGaAsN/AlGaAs QWIP devices is based on optical excitation of carriers from the quantum wells into highly-localized nitrogen-related E+ defect-like states located energetically above the conduction band edge. For this reason, the photoresponse is insensitive to the radiation polarization and exhibits low absorption efficiency on the order of 0.15% per quantum well. However, at the same time, the responsivity and photocurrent gain are very high due to the long lifetime of the highly localized excited states.The peak responsivity of the 2% N device is ∼70 A/W at room temperature. Significant responsivity is also available at the response tail at longer wavelengths, i.e., ∼15 and 1 A/W for 1.65 μm—the peak wavelength of the night glow emission and 2 μm—at the e-SWIR wavelengths, respectively. Corresponding detectivity values are ∼1010 and ∼5 × 108 cm·Hz1/2/W at 1.65 μm and 2.0 μm, respectively. These values are similar to those of other developing technologies. In-reach potential detectivity is estimated based on the measured data and can arrive easily to values as high as ∼2 × 1012 and ∼1 × 1011 cm·Hz1/2/W for ∼1.65 μm and 2.0 μm, respectively.The presented characteristics and potential performance indicate that InGaAsN/AlGaAs quantum wells are most suitable for efficient e-SWIR photodetection.

Effects of nonuniform distribution of quantum well and quantum wire base on infrared photodetectors under dark conditions

This paper presents the effects of uniformity and nonuniformity distribution of quantum well (QW) and wire (QR) base on quantum infrared photodetectors under dark conditions. These detectors are quantum well infrared photodetectors (QWIP) and quantum wire infrared photodetectors (QRIP). Analytical expressions for dark current characteristics of the considered devices are implemented. Additionally, the proposed results are validated against published results in literature and high agreement is accomplished. The potential distribution in QWIP active region depends on weak non-locality approach. Also, the effect of electrons concentration above the barriers on the performance of QRIP is considered. The nonuniformity parameter of QRIP is computed. Moreover, the uniformity and nonuniformity distributions effects of QRs and QWs on dark current ratio between QRIP and QWIP are estimated. This current ratio is changed by uniformity and nonuniformity distribution of QWs and QRs. It is noted that dark current ratio between QRIP and QWIP decreases with applied bias voltage. Hence, the QWIP device is affected by applied bias voltage greater than QRIP. However, this current ratio increases with temperature. Accordingly, the QRIP device is influenced by temperature larger than QWIP. It is noticed that dark current ratio under uniformity distribution is smaller than this ratio under nonuniformity distribution for both QRs and QWs. So, the nonuniformity problem within QRs is a great challenge that must be addressed. Also, the nonuniformity distribution introduces limits to QRIP device characteristics. The results demonstrate that zero value of dark current ratio is attained with zero value of nonuniformity parameter. Also, the dark current of QRIPs is found to be greater than of QWIP structures under large variation of nonuniformity distribution. Even though, this parameter is approximated to 1 under uniform distribution of QWs and QRs. It is observed that nonuniformity distribution parameter is vanished up to 80 K. So, the dependence of the operating temperature on the number of electrons occupied by each QR is evaluated. The obtained results confirm that the applied bias voltage decreases this challenge to some extent. Besides, parameters optimization for QWIPs and QRIP is of primary concern. Therefore, the devices performance is improved. Consequently, the operations of underlined infrared photodetectors are robust against noise sources in far infrared spectrum.

Quantum Well Infrared Photodetector Technology and Applications

Quantum well infrared photodetectors (QWIPs) are known for their stability, high pixel-to-pixel uniformity, and high-pixel operability, which are essential for large area imaging arrays. In this paper, we discuss the initial demonstration of QWIP devices, and the many years of progress that propelled this technology toward the demonstration of large format focal plane arrays. In addition, we present some potential applications of this technology in science and medicine.

A gold hybrid structure as optical coupler for quantum well infrared photodetector

A hybrid structure consisting of a square lattice of gold disk arrays and an overlaying gold film is proposed as an optical coupler for a backside-illuminated quantum well infrared photodetector (QWIP). Finite difference time-domain method is used to numerically simulate the reflection spectra and the field distributions of the hybrid structure combined with the QWIP device. The results show that the electric field component perpendicular to the quantum well is strongly enhanced when the plasmonic resonant wavelength of the hybrid structure coincides with the response one of the quantum well infrared photodetector regardless of the polarization of the incident light. The effect of the diameter and thickness of an individual gold disk on the resonant wavelength is also investigated, which indicates that the localized surface plasmon also plays a role in the light coupling with the hybrid structure. The coupling efficiency can exceed 50 if the structural parameters of the gold disk arrays are well optimized.

Quantum well infrared photodetector simultaneously working in two atmospheric windows

We have demonstrated a two-contact quantum well infrared photodetector (QWIP) exhibiting simultaneous photoresponse in both the mid- and the long-wavelength atmospheric windows of 3-5 mu m and of 8-12 mu m. The structure of the device was achieved by sequentially growing a mid-wavelength QWIP part followed by a long-wavelength QWIP part separated by an n-doped layer. Compared with the conventional dual-band QWIP device utilizing three ohmic contacts, our QWIP is promising to greatly facilitate two-color focal plane array (FPA) fabrication by reducing the number of the indium bumps per pixel from three to one just like a monochromatic FPA fabrication and to increase the FPA fill factor by reducing one contact per pixel; another advantage may be that this QWIP FPA boasts broadband detection capability in the two atmospheric windows while using only a monochromatic readout integrated circuit. We attributed this simultaneous broadband detection to the different distributions of the total bias voltage between the mid- and long-wavelength QWIP parts.

Demonstration of Megapixel Dual-Band QWIP Focal Plane Array

Quantum well infrared photodetectors (QWIPs) are well known for their stability, high pixel-pixel uniformity and high pixel operability which are quintessential parameters for large area imaging arrays. In this paper we report the first demonstration of the megapixel-simultaneously-readable and pixel-co-registered dual-band QWIP focal plane array (FPA). The dual-band QWIP device was developed by stacking two multi-quantum-well stacks tuned to absorb two different infrared wavelengths. The full width at half maximum (FWHM) of the midwave infrared (MWIR) band extends from 4.4-5.1 ?m and FWHM of the long-wave infrared (LWIR) band extends from 7.8-8.8 ?m. Dual-band QWIP detector arrays were hybridized with direct injection 30 ?m pixel pitch megapixel dual-band simultaneously readable CMOS read out integrated circuits using the indium bump hybridization technique. The initial dual-band megapixel QWIP FPAs were cooled to 68 K operating temperature. The preliminary data taken from the first megapixel QWIP FPA has shown system NE?T of 27 and 40 mK for MWIR and LWIR bands, respectively.

The optical coupling improvement of THz quantum well infrared photodetectors based on the plasmonic induced near-field effect

Near-field effect has been recognized to play an important role in optical coupling of multiple quantum well infrared photodetectors (QWIP). Both the optical near-field effect and the plasmonic effect in the quantum well active region of QWIP device have been studied by the finite difference time-domain (FDTD) method at the optical frequency of 4.2 THz. Our numerical results indicate that both the near-field effect and the plasmonic effect can induce an evanescent wave with the electrical component being parallel to the growth direction of quantum well. The induced electrical component in the QWIP growth direction is 2.5 times larger than that of the incident optical fields. The induced polarization can increase the optical coupling efficiency of QWIP and the detectivity of QWIP can be improved at 4.2 THz.

1024 × 1024 Format pixel co-located simultaneously readable dual-band QWIP focal plane

This paper reports the first demonstration of the megapixel-simultaneously-readable and pixel-co-registered dual-band quantum well infrared photodetector (QWIP) focal plane array (FPA). The dual-band QWIP device was developed by stacking two multi-quantum-well stacks tuned to absorb two different infrared wavelengths. The full width at half maximum (FWHM) of the mid-wave infrared (MWIR) band extends from 4.4 to 5.1 μm and the FWHM of a long-wave infrared (LWIR) band extends from 7.8 to 8.8 μm. Dual-band QWIP detector arrays were hybridized with custom fabricated direct injection read out integrated circuits (ROICs) using the indium bump hybridization technique. The initial dual-band megapixel QWIP FPAs were cooled to 70 K operating temperature. The preliminary data taken from the first megapixel QWIP FPA has shown system NEΔT of 27 and 40 mK for MWIR and LWIR bands, respectively.

The influence of interface roughness on the normal incident absorption of quantum-well infrared photodetectors

Thirty-period GaAs/AlGaAs quantum-well infrared photodetector (QWIP) samples with the same device structure prepared by molecular beam epitaxy (MBE) and metal-organic chemical vapor deposition (MOCVD) are investigated in this paper. Temperature-insensitive responsivities observed for the devices are attributed to the unchanged photocurrent gain within the temperature range 10–77 K. Higher absorption for transverse electric mode infrared incident light observed for MBE-prepared QWIP is attributed to an increased interface light scattering resulted from MBE-prepared less smooth interfaces. In this case, higher normal incident absorption for the QWIP device prepared by MBE would be observed. Similar phenomenon observed for the MOCVD-prepared QWIP grown on 2° off (100) GaAs substrate is consistent with the argument.

Dark currents of GaAs/AlGaAs quantum-well infrared photodetectors

We study the dark current of the GaAs/AlGaAs quantum-well infrared photodetector (QWIP) by assuming a drift-diffusion carrier transport in the barriers where the electric fields are obtained by the current continuity condition and the self-consistent energy band structure. It has been shown that due to the current continuity condition, the dark currents across the QWIP devices are determined by the thermionic emission from the emitter to the multiple quantum well (MQW) region. The self-consistent calculation of the Schrodinger and Poisson equations shows a weak electric field in the barrier region connecting to the emitter (much smaller than the average field across the QWIP at low bias) due to the accumulation of carriers in the triangle quantum well formed at the emitter-MQW interface, which results in a very small dark current at low bias. The numerical results explain well our experimental observation.

Near room temperature operation of a highly strained short wavelength (2.1 µm) AlAs/In0.84Ga0.16As/AlAs/InAlAs QWIP

A strain-compensated AlAs/InxGa1−xAs/AlAs/InyAl1−yAs (x ≈ 0.8, y ≈ 0.5) quantum well infrared photodetector (QWIP) structure was grown by molecular beam epitaxy (MBE). Conditions of exact stoichiometric growth were applied at a temperature of ∼420 °C to produce structures capable of detecting IR radiation in the 2–5 µm mid-infrared spectrum. Double crystal x-ray diffraction (DCXRD) and room temperature photoluminescence (PL) experiments confirmed the excellent structural characteristics of the grown material system. A strong room temperature intersubband absorption peak was observed at a wavelength of 2.16 µm. Current–voltage (I–V) measurements as a function of temperature were carried out to electrically characterize the fabricated QWIP devices yielding devices working under background limited infrared photodetection (BLIP) conditions at 270 K. From the I–V curves, an activation energy of 270 meV at zero bias was extracted. This is in good agreement with a current transport mechanism which is dominated by thermionic emission. Photocurrent measurements were carried out and we demonstrate devices that are capable of working at a temperature as high as 270 K at a wavelength of 2.1 µm. The experimental results are in excellent agreement with the modelled values.

Resonant Terahertz Photomixing in Integrated High-Electron-Mobility Transistor and Quantum-Well Infrared Photodetector Device

We propose a novel heterostructure device for generating of terahertz (THz) radiation based on the integration of a high-electron-mobility transistor (HEMT) and a quantum-well infrared photodetector (QWIP) and utilizing the photomixing of middle- or far-infrared optical signals. The operation of the photomixer under consideration is associated with the resonant excitation of plasma oscillations in the device HEMT section brought about by the transient photocurrent produced in the QWIP by incident infrared radiation. Using the developed device model for HEMT–QWIP photomixers which combines both the analytical description of electron processes and their ensemble Monte Carlo particle modeling, we evaluate the device performance.

A microscopic model of quantum well infrared photodetectors (QWIP)

A microscopic model of quantum well infrared photodetectors (QWIPs) is presented, based on quantum scattering rate equation approach. The macroscopic parameters and figures of merit such as current density, responsivity, capture probability and drift velocity, can be estimated directly from a first principles calculation. For conventional GaAs/AlGaAs QWIP device, a very good agreement was found between the calculated and the measured data taken from the literature. The model is general and can be applied to any other material system or QWIP structure.

Detection wavelength of very long wavelength quantum-well infrared photodetector

Based on a detailed discussion of the energy band structure and optical transitions in verylong wavelength () GaAs/AlGaAs quantum well infrared photodetectors (QWIPs), we have built a practicalQWIP model. We have studied various key factors that determine the photogeneratedcarriers and detection wavelength of very long wavelength QWIPs. Consequently, we havefound and confirmed a distinctive difference between the photocurrent of QWIPs with onlyone confined state in the quantum well (QW) and those binding two confined states, whichresulted in a different dependence of the detection wavelength on the QW width. Also, wehave discussed the dependence of the response wavelength on several other parameters forvery long wavelength QWIPs, such as barrier width and Al mole fraction. Afterwards, aseries of very long wavelength QWIP devices have been fabricated and measured, whosephotocurrents are well reproduced by our calculation. Based on the theoreticalmodel and experimental data, a simple and convenient polynomial equation hasbeen deduced, using structure parameters of very long wavelength QWIPs tocalculate the detection wavelength and offering convenience in device designing.

Compressively strained p-type InGaAs/AlGaAs quantum-well infrared photodetectors

The compressively strained p-type In0.15Ga0.85As/Al0.33Ga0.67As quantum-well infrared photodetectors (QWIPs) with different Be concentrations in their wells, which detect normal infrared incidence, were investigated. The QWIPs with a Be doping density of 1018 cm−3 in the wells show a cutoff wavelength of 7.9 μm and basically symmetric detectivities of about 8×108 cm Hz1/2/W at 600 Hz. By increasing the Be doping density in the wells to 2×1019 cm−3, the cutoff wavelength is blueshifted to about 7.25 meV and the photoresponsivity and detectivity become asymmetric. The detectivity is increased to about 1.4×109 cm Hz1/2/W at positive biases but significantly reduced at negative biases. The blueshift in the cutoff wavelength for the QWIP devices with heavy doping concentration in the wells is mainly due to the band-gap shrinkage and the increased well width while the asymmetric behavior in the photoresponsivity and detectivity is likely due to the inhomogenity resulting from dopant diffusion at high doping.

IMAGING PERFORMANCE OF ADVANCED QWIP FOCAL PLANE ARRAYS

Since the first demonstration of the quantum well infrared photodetector (QWIP) in the 1980s, there has been much progress in the application of QWIPs to the production infrared (IR) imaging systems. At this time, focal plane arrays (FPAs) made from QWIPs are readily available for insertion in IR cameras with formats as large as 640 × 480 pixels. Several organizations now have commercially available IR camera systems using QWIPs. In spite of the low single-pixel quantum efficiency relative to MCT, excellent IR imagery has been demonstrated with large format (640 × 480 pixels) single-band and moderate format (256 × 256 pixels) dual-band FPAs. With a large-format staring FPA, one can integrate the signal current for a relatively long time to produce images of similar quality to that from a scanned line array run at the same frame rate. In fact, it can be shown that due to the nature of the noise in a QWIP device, the noise performance of a QWIP FPA can be better than that of MCT FPA as long as the conversion efficiency (the product of the quantum efficiency and the photoconductive gain) is high enough for the read-out integrated circuit (ROIC) integration capacitor to be filled in a frame time. In this chapter the results of laboratory and field tests on large-format single-color QWIP FPAs operating in the LWIR band and dual-band FPAs operating in both the MWIR and LWIR bands simultaneously will be shown. Single-color and dual-band arrays will be shown to give excellent imaging performance and that dual-band FPAs offer unique capabilities to investigate the phenomenology of targets and backgrounds. The performance of the FPAs will be presented from a system performance perspective over a wide range of operating conditions (temperature, bias, integration time, etc.). Results of measurements of noise-equivalent temperature difference (NEΔT), minimum resolvable temperature difference (MRTD measured as a function of target spatial frequency), responsivity, and dark current will be reported. Imagery collected in the field will show the utility of large-format LWIR FPAs for increasing the range at which targets can be identified over previous-generation scanning imagers. Dual-band imagery collected using a QWIP FPA will show how such an array as part of a future imaging system may be able to exploit differences in the IR signatures of targets and backgrounds in the MWIR and LWIR bands to enhance the visibility of targets in cluttered environments. We also show how such an array can be used to make accurate remote temperature measurements. Finally, we will compare the performance of state-of-the-art FPAs made from QWIPs and MCT.

The role of sequential tunnelling in the dark current of quantum well infrared photodetectors (QWIPs)

A quantum mechanical approach is taken to investigate the contribution of sequential tunnelling as a component of the dark current in quantum well infrared photodetectors (QWIPs). Calculations are performed on three different experimentally reported QWIP devices made for different detection wavelengths. The results show that the sequential tunnelling component remains rather constant with different devices, however it is swamped by the thermionic emission components of the dark current at longer wavelengths. The lack of a local maximum in the dark current due to resonant LO phonon emission, which should be observed at short wavelengths, suggests that interface roughness and alloy disorder could be destroying the coherence of the electron wavefunctions between quantum wells.

Quantum well infrared photodetectors for low background applications

Quantum well infrared photodetectors (QWIPs) afford greater flexibility than the usual extrinsically doped semiconductor IR detectors. The wavelength of the peak response and cutoff can be continuously tailored over any wavelength range between 6–20 μm. The spectral band width of these detectors can be tuned from narrow ( Δλ/ λ∼10%) to wide ( Δλ/ λ∼40%) allowing various applications. Also, QWIP device parameters can be optimized to achieve extremely high performances at high back ground at lower operating temperatures (∼30 K). However, for low-background irradiance levels at low operating temperatures, high resistivity of the active region could leads to non-linear responsivity behavior. A new structure with a photosensitive superlattice and a thick blocking barrier has been tested, and is expected to avoid anomalous behavior under low backgrounds at low temperatures.

Theory, fabrication and characterization of quantum well infrared photodetectors

Abstract The microscopic physics, device physics, and system performance of quantum well infrared photodetectors (QWIPs) are reviewed. QWIPs which respond to normally incident radiation without the need for an optical grating are of particular interest because they can be fabricated with fewer process steps. Recent demonstrations of n-type QWIPs (n-QWIPs) which show a significant detectivity of 4×1010 cm Hz /W without the use of an optical grating are discussed here. This detectivity is significant because it is large enough for focal plane array (FPA) performance to be limited by the uniformity of processing rather than the size of the single pixel detectivity. Studies of the microscopic physics of quantum wells are summarized to elucidate the physical origin of the intersubband absorption of normally incident radiation. The selection rules for intersubband absorption by holes in a p-doped QWIP (p-QWIP) and electrons in an n-QWIP are reviewed. In particular, it is shown that the hole intersubband absorption is typically weaker than both the conduction intersubband absorption and the valence band-to-conduction band absorption. It is also shown that uniaxial strain does not have a large effect on the strength or the selection rules of intersubband absorption because the Hamiltonian describing uniaxial strain has the same (tetragonal) symmetry as that describing the confinement of carriers in the quantum wells along the growth direction. Also reviewed are device models which yield analytical expressions for the number of, and the distance over which, carriers are depleted from quantum wells under conditions of insufficient carrier injection. This carrier depletion becomes important when the incident photon flux is large or when the QWIP operating temperature is low. Uniformity of QWIP device parameters is important in determining the ultimate array signal-to-noise ratio (SNR). Examples of high-resolution X-ray diffraction methods used to find the layer width variations of QWIPs grown by molecular beam epitaxy are reviewed. The spread of the measured full-width at half-maxima (FWHM) of superlattice diffraction peaks with the diffraction order was used with Bragg’s Law to obtain the measured layer width variation in the growth direction. A theoretical study of different noise mechanisms which contribute to QWIP performance was carried out. A key result is that, when the SNR is limited by either fixed pattern noise or thermal leakage arrival noise, the largest expected QWIP SNR occurs when the number of quantum wells in the QWIP is at the optimal value of about η1−1, where η1 is the quantum efficiency of a QWIP having only one quantum well. Common QWIP designs used in industry are evaluated. In particular, different physical models for the leakage (sequential tunneling, thermionic and thermionic field assisted leakage) are reviewed. A new result is a physical model, derived from the Kronig–Penney model, for the tunneling leakage in existing QWIP designs in which the confinement barrier is a semiconductor superlattice. The tunneling leakage in such QWIPs is shown to vary exponentially with the average (rather than the full) height of the superlattice barrier.