Quantum Well Infrared Photodetectors Structures Research Articles

The application of a modified Levine model to quantum-well infrared photodetectors in the low-temperature regime

A different approach to the calculation of the dark current of quantum-well infrared photodetectors (QWIPs) based on a modified version of Levine’s method is presented. A quantum-well-dependent transmission probability is proposed and evaluated. The results show that the model accurately reproduces the experimental behavior of the photodetector dark current in the low-temperature regime. As a proof of concept, the presented approach is applied to elucidate the role of filter barriers in QWIP structures. As expected, a significant decrease in the dark current is observed, principally in the high bias regime, demonstrating that the modified Levine’s model captures relevant information about the behavior of the dark current of photovoltaic devices.

InGaAs/InP quantum well infrared photodetector integrated on Si substrate by Mo/Au metal-assisted wafer bonding

Integration of an InGaAs/InP quantum well infrared photodetector (QWIP) onto a Si substrate was successfully demonstrated via a metal-assisted wafer bonding (MWB) using a Mo/Au metal scheme. The Mo/Au/Mo layer, situated between the QWIP structure and the Si, has shown a well-ordered lamination. It provides a smooth surface with a roughness of about 0.8 nm, as measured by a scanning electron microscope (SEM) and atomic force microscopy (AFM). The results on crystalline quality evaluated by Raman spectroscopy and X-ray diffraction (XRD) imply that the MWB could be achieved without any measurable material degradation and residual strain. Temperature dependence of dark current revealed that there is no noticeable change in the dark current properties of the QWIP after bonding on Si, despite that the quantum wells are only 200 nm away from the bonding interface.

Ultrafast terahertz detectors based on three-dimensional meta-atoms

Terahertz (THz) and sub-THz frequency emitter and detector technologies are receiving increasing attention, underpinned by emerging applications in ultra-fast THz physics, frequency-combs technology and pulsed laser development in this relatively unexplored region of the electromagnetic spectrum. In particular, semiconductor-based ultrafast THz receivers are required for compact, ultrafast spectroscopy and communication systems, and to date, quantum well infrared photodetectors (QWIPs) have proved to be an excellent technology to address this given their intrinsic ps-range response However, with research focused on diffraction-limited QWIP structures (lambda/2), RC constants cannot be reduced indefinitely, and detection speeds are bound to eventually meet un upper limit. The key to an ultra-fast response with no intrinsic upper limit even at tens of GHz is an aggressive reduction in device size, below the diffraction limit. Here we demonstrate sub-wavelength (lambda/10) THz QWIP detectors based on a 3D split-ring geometry, yielding ultra-fast operation at a wavelength of around 100 {\mu}m. Each sensing meta-atom pixel features a suspended loop antenna that feeds THz radiation in the ~20 m3 active volume. Arrays of detectors as well as single-pixel detectors have been implemented with this new architecture, with the latter exhibiting ultra-low dark currents below the nA level. This extremely small resonator architecture leads to measured optical response speeds - on arrays of 300 devices - of up to ~3 GHz and an expected device operation of up to tens of GHz, based on the measured S-parameters on single devices and arrays.

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.

Electromagnetic Modeling and Design of Quantum Well Infrared Photodetectors

The quantum efficiency (QE) of a quantum well infrared photodetector (QWIP) is historically difficult to predict and optimize. This difficulty is due to the lack of a quantitative model to calculate QE for a given detector structure. In this paper, we found that by expressing QE in terms of a volumetric integral of the vertical electric field, the QE can be readily evaluated using a finite element electromagnetic solver. We applied this model to all known QWIP structures in the literature and found good agreement with experiment in all cases. Furthermore, the model agrees with other theoretical solutions, such as the classical solution and the modal transmission-line solution when they are available. Therefore, we have established the validity of this model, and it can now be used to design new detector structures with the potential to greatly improve the detector QE.

A study of photomodulated reflectance on staircase-like, n-doped GaAs/Al x Ga1−xAs quantum well structures

In this study, photomodulated reflectance (PR) technique was employed on two different quantum well infrared photodetector (QWIP) structures, which consist of n-doped GaAs quantum wells (QWs) between undoped AlxGa1−xAs barriers with three different x compositions. Therefore, the barrier profile is in the form of a staircase-like barrier. The main difference between the two structures is the doping profile and the doping concentration of the QWs. PR spectra were taken at room temperature using a He-Ne laser as a modulation source and a broadband tungsten halogen lamp as a probe light. The PR spectra were analyzed using Aspnes’ third derivative functional form.Since the barriers are staircase-like, the structure has different ground state energies; therefore, several optical transitions take place in the spectrum which cannot be resolved in a conventional photoluminescence technique at room temperature. To analyze the experimental results, all energy levels in the conduction and in the valance band were calculated using transfer matrix technique, taking into account the effective mass and the parabolic band approximations. A comparison of the PR results with the calculated optical transition energies showed an excellent agreement. Several optical transition energies of the QWIP structures were resolved from PR measurements. It is concluded that PR spectroscopy is a very useful experimental tool to characterize complicated structures with a high accuracy at room temperature.

Large format voltage tunable dual-band QWIP FPAs

Third generation thermal imagers with dual/multi-band operation capability are the prominent focus of the current research in the field of infrared detection. Dual band quantum-well infrared photodetector (QWIP) focal plane arrays (FPAs) based on various detection and fabrication approaches have been reported. One of these approaches is the three-contact design allowing simultaneous integration of the signals in both bands. However, this approach requires three In bumps on each pixel leading to a complicated fabrication process and lower fill factor. If the spectral response of a two-stack QWIP structure can effectively be shifted between two spectral bands with the applied bias, dual band sensors can be implemented with the conventional FPA fabrication process requiring only one In bump on each pixel making it possible to fabricate large format dual band FPAs at the cost and yield of single band detectors. While some disadvantages of this technique have been discussed in the literature, the detailed assessment of this approach has not been performed at the FPA level yet. We report the characteristics of a large format (640 × 512) voltage tunable dual-band QWIP FPA constructed through series connection of MWIR AlGaAs–InGaAs and LWIR AlGaAs–GaAs multi-quantum well stacks, and provide a detailed assessment of the potential of this approach at both pixel and FPA levels. The dual band FPA having MWIR and LWIR cut-off wavelengths of 5.1 and 8.9 μm provided noise equivalent temperature differences as low as 14 and 31 mK ( f/1.5) with switching voltages within the limits applicable by commercial read-out integrated circuits. The results demonstrate the promise of the approach for achieving large format low cost dual band FPAs.

Modelling and simulation of electronic and optical responses of quantum well infrared photodetectors (QWIPs)

A first principles theory of electronic transport in quantum well infrared photodetectors (QWIPs) is presented. The model is based on the cascade scattering rate equations approach and uses the translation symmetry to reduce the computational cost of the problem. The model delivers a non-equilibrium electron distribution in both dark conditions and under illumination. The model was tested by simulating a non-conventional broadband InGaAs/AlGaAs QWIP and good agreement with experimental data available in the literature was achieved. The model can be applied to any QWIP structure and can be used for extending the range of materials employed.

Characterization of QWIP structures prepared on GaAs-patterned substrates

In this work we investigate the preparation of quantum well infrared photodetectors (QWIP) on planar and patterned GaAs substrates. Mesa ridges with various angles between sidewall and substrate (100) plane were prepared by wet chemical etching. The QWIP structures were grown at a temperature of 700°C by use of low-pressure MOVPE. Electrical properties and spectral sensitivity of QWIP structures prepared on tilted sidewalls were measured. Our results showed that mesa ridges confined at the sides by facets tilted at 30° to (100) were most suitable for the QWIP preparation. Asymmetry in room temperature I–V characteristics and a small photovoltaic effect observed at 77K was ascribed to asymmetric position of delta doping plane in the quantum well.

Detection wavelengths and photocurrents of very long wavelength quantum-well infrared photodetectors

Based on detailed studies of the energy band structure and the optical transitions in very long wavelength (>14 μm) GaAs/AlGaAs quantum-well (QW) infrared photodetectors (QWIPs), we have built a practical QWIP model. We study the factors that determine photogenerated carriers and response wavelengths of photocurrents of very long wavelength QWIPs. The material structures of QWIPs are first characterized by the photoluminescence measurements (PL) at room temperature and 77 K respectively. We have found and confirmed a distinctive difference between photocurrent of QWIPs with only one confined state in the quantum well (QW) and those binding two confined states, which resulted in different dependence of detection wavelength on the quantum well width. Also, we have investigated the dependence of response wavelength on several other parameters for very long wavelength QWIPs, such as barrier width and Al mole fraction. By calculating the density of photogenerated carriers in the continuum above the energy barriers using the PL calibrated QWIP structures, we have demonstrated that due to the high sample quality, the photocarriers can be either in miniband states (Bloch states in the multiple quantum wells), or they transport from one quantum well to the next in the form of propagating waves. We have further calculated the densities of photocarriers in the QWIPs reported in the literature. It is shown that the Bloch wave boundary conditions are appropriate for QWIPs with narrow QWs, whereas propagating wave boundary conditions are appropriate for wide QWs.

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.

Two-photon-absorption-induced nonlinear photoresponse in GaAs∕AlGaAs quantum-well infrared photodetectors

Using a free-electron laser(FEL) source, we have studied the two-photon-absorption (TPA) effect in GaAs∕AlGaAs quantum-well infrared photodetector (QWIP). The TPA-induced photoresponse in QWIPs has been measured under different FEL excitation power by the photoconductivity method. The effective-mass approximation theory is used for the QWIP structure to explain the photoresponse behavior. It is demonstrated that the TPA-induced photocarrier density is proportional to the square of the excitation power. Based on the experimental results, the TPA coefficients of QWIPs were obtained to be 0.0045, 0.0030, 0.0103, and 0.0061cm∕MW for the excitation lines of 10.6, 10.7, 11.9 and 13.2μm, respectively. The dependence the TPA coefficients on the excitation wavelength is explained by our theoretical model.

Highly strained [formula omitted] layers for short wavelength QWIP and QCL structures grown by MBE

Highly strained quantum cascade laser (QCL) and quantum well infrared photodetector (QWIPs) structures based on In x Ga (1−x) As−In y Al (1−y) As (x>0.8,y<0.3) layers have been grown by molecular beam epitaxy. Conditions of exact stoichiometric growth were used at a temperature of ∼420°C to produce structures that are suitable for both emission and detection in the 2– 5 μm mid-infrared regime. High structural integrity, as assessed by double crystal X-ray diffraction, room temperature photoluminescence and electrical characteristics were observed. Strong room temperature intersubband absorption in highly tensile strained and strain-compensated In 0.84Ga 0.16As/AlAs/In 0.52Al 0.48As double barrier quantum wells grown on InP substrates is demonstrated. Γ– Γ intersubband transitions have been observed across a wide range of the mid-infrared spectrum (2– 7 μm ) in three structures of differing In 0.84Ga 0.16As well width (30, 45, and 80 A ̊ ). We demonstrate short-wavelength IR, intersubband operation in both detection and emission for application in QC and QWIP structures. By pushing the InGaAs–InAlAs system to its ultimate limit, we have obtained the highest band offsets that are theoretically possible in this system both for the Γ– Γ bands and the Γ–X bands, thereby opening up the way for both high power and high efficiency coupled with short-wavelength operation at room temperature. The versatility of this material system and technique in covering a wide range of the infrared spectrum is thus demonstrated.

Demonstration of 256 x 256 focal plane array based on Al-free GaInAs-InP QWIP

We report the first demonstration of an infrared focal plane array based on aluminum-free GaInAs-InP quantum-well infrared photodetectors (QWIPs). The long wavelength QWIP structure was grown via a low-pressure metal-organic chemical vapor deposition. Corrugated light coupling structure of QWIP was fabricated with a dry etching process. A 256/spl times/256 detector array was also fabricated with dry etching and hybridized to a Litton readout integrated circuit via indium bumps. A unique positive lithography method was developed to perform indium-bump liftoff. The noise equivalent differential temperature (NE/spl Delta/T) of 29 mK was achieved at 70 K with f/2 optics.

Time-resolved electron transport studies on InGaAs/GaAs-QWIPs

Due to the short internal response time, quantum-well infrared photodetectors (QWIPs) are interesting for high-speed applications such as heterodyne spectroscopy or laser pulse monitoring. We studied the photocurrent transients of InGaAs/GaAs-QWIPs after irradiation with infrared laser pulses of 250 fs duration. The excitation wavelength of about 9 μm matches the peak wavelength of the QWIP structure. The photocurrent transient consists of two different dynamical components, representing the fast photoionization in the quantum-wells and the slow injection current that compensates the remaining space charge. The investigations of the different components as a function of temperature and bias voltage were performed on a nanosecond time-scale. The experimental separation of the two photocurrent contributions allows us to determine the photoconductive gain. The Fourier transform of the photocurrent transient was compared with other experimental methods including heterodyne detection and microwave rectification. The quantitative agreement between these different measurement techniques is excellent.

Photocurrents of 14 μm quantum-well infrared photodetectors

We study the factors that determine photogenerated carriers and response wavelengths of photocurrents of long wavelength (∼14 μm) quantum well (QW) infrared photodetectors (QWIPs). The material structures of QWIPs are first characterized by the photoluminescence measurements (PL). By calculating the density of photogenerated carriers in the continuum above the energy barriers using the PL calibrated QWIP structures, we have demonstrated that due to the sample quality, the photocarriers can be either in miniband states (Bloch states in the multiple quantum wells), or they transport from one quantum well to the next in the form of running waves. By including possible scattering processes at the QWIP working temperature to link the theoretically calculated photocarrier density with the experimentally measured photocurrent, it is shown that the width of the photocurrent peaks of 14 μm GaAs/AlGaAs QWIPs under investigation is determined by the optical phonon emissions of photocarriers. We have further calculated the densities of photocarriers in the QWIPs reported in the literature. It is shown that the Bloch wave boundary conditions are appropriate for QWIPs with narrow QWs, whereas running wave boundary conditions are appropriate for wide QWs.

Quantum-well infrared photodetector structure synthesis: methodology and experimental verification

A numerical method for global optimization of quantum-well infrared photodetector (QWIP) performance parameters is presented and experimentally verified. The single-band effective-mass Schroedinger equation is solved by employing the argument principle method (APM) to extract both the bound and quasibound eigen-energies of the quantum heterostructure. APM is combined with a simulated annealing algorithm to determine a set of device design parameters such as potential barrier height V/sub i/, layer thickness d/sub i/, number of material layers N, total device length, applied bias V/sub Bias/ etc., for which the QWIP performance is within a predetermined convergence criterion. The method presented incorporates the effect of energy-dependent effective mass of electrons in nonparabolic conduction bands. The present model can handle many optimization parameters and can incorporate fabrication constraints to achieve physically realizable devices. In addition, the method is not limited to the optimization of absorption structures, and can be used for other intersubband devices such as electron-wave Fabry-Perot filters and quantum-cascade lasers. The strength and versatility of the present method are demonstrated by the design of a bicolor equal-absorption-peak QWIP structure, and experimental verification of the zero-bias absorption spectrum is presented.

MULTI-COLOR, BROADBAND QUANTUM WELL INFRARED PHOTODETECTORS FOR MID-, LONG-, AND VERY LONG-WAVELENGTH INFRARED APPLICATIONS

Quantum well infrared photodetectors (QWIPs) have been widely investigated for the 3–5 μm mid-wavelength infrared (MWIR) and 8–12 μm long-wavelength infrared (LWIR) atmospheric spectral windows as well as very long wavelength infrared (VLWIR: λc &gt; 14 μm) imaging array applications in the past decade. The mature III-V compound semiconductor growth technology and the design flexibility of device structures have led to the rapid development of various QWIP structures for infrared focal plane arrays (FPAs) applications. In addition to the single-color QWIP with narrow bandwidth, multi-color or broadband QWIPs required for advanced IR sensing and imaging applications have also emerged in recent years. Using band gap engineering approach, the multi-color (2, 3, and 4-color) QWIPs with multi-stack quantum wells and voltage-tunable asymmetrical coupled quantum well structures for detection in the MWIR, LWIR, and VLWIR bands have been demonstrated recently. The triple-coupled (TC-) QWIP employs the quantum confined Stark effect to tune the peak detection wavelength by the applied bias voltage, A typical single-color QWIP exhibits a rather narrow spectral bandwidth of 1 to 2 μm. For certain applications, such as spectroscopy, sensing of a broader range of infrared radiation is highly desirable. Using the stacked quantum wells with different well width and depth, the digital-graded superlattice barrier (DGSLB) or the linear-graded barrier (LGB) structures, broadband (BB-) QWIPs covering the 8–14 μm atmospheric spectral window have been reported recently. In this chapter, the basic operation principles of a QWIP, and the design, fabrication, and characterization of multi-color and broadband QWIPs based on the GaAs/AlGaAs and InGaAs/AlGaAs material systems for the MW/LW/VLWIR applications are depicted.

Self-consistent model for quantum well infrared photodetectors with thermionic injection under dark conditions

We present a self-consistent analytical model describing transport processes in quantum well infrared photodetectors (QWIPs) under dark conditions. The model takes into account electron thermionic emission from the quantum wells, thermionic injection from the emitter contact, and features of transport and capture in the self-consistent electric field in the QWIP active region. Using an assumption that the rates of the electron escape from and capture into a QW are functions of the electric fields only in the barriers sandwiching this QW, we calculate the electric field and charge distributions as well as dark current–voltage characteristics. We clarify the origin of steep dark current characteristics. It is confirmed that the effect of the emitter contact substantially weakens with increasing number of QWs in the QWIP structure.

Towards a voltage tunable two-color quantum-well infrared photodetector

Two-color quantum-well infrared photodetectors (QWIPs), based on electron transfer between coupled QWs, suffer from the presence of the shorter wavelength peak at all bias voltages Vb. We investigate this problem by studying the bias dependence of absorption coefficient α and photoconductive gain g as a function of wavelength at 10 K. We fabricate such a detector with a peak wavelength of 8 μm for both bias polarities but a voltage tunable cutoff wavelength (9 μm for Vb&amp;gt;0 and 11 μm for Vb&amp;lt;0). We use corrugated QWIPs with different corrugation periods to extract α and g for different values of Vb. We find α≈0.1 μm−1 in the 6–12 μm range with small peaks at 8 and 9.8 μm for Vb&amp;gt;0 and 10 μm for Vb&amp;lt;0. g has a pronounced peak at 7.8 μm for both bias polarities and determines the line shape of the QWIP spectral responsivity. These results are attributed to insufficient electron transfer between the coupled QWs and to low tunneling probability of the longer wavelength photoelectrons. A modified QWIP structure has been proposed for complete switching of spectral responsivity peak when the bias voltage polarity is reversed.