QWIPs DESIGNED FOR HIGH ABSORPTION AND HIGH OPERATING TEMPERATURE

Rapid progress in quantum well IR photodetectors (QWIPs) technology has made it possible to develop large format, and high uniformity QWIP focal plane arrays (FPAs) for IR imaging sensor applications. So far, all QWIP FPAs are made of n-type GaAs/AlGaAs material systems grown on GaAs substrates due to the maturity of this material system and excellent detector performance. However, due to quantum selection rules, normal incidence absorption is forbidden in n-type QWIPs. As a result, n-type QWIP FPAs are required to use dielectric or metal gratings to efficiently couple the IR radiation into the quantum wells under normal incidence illumination. On the other hand, normal incidence absorption is allowed in p-type QWIPs without using the grating couplers. This property makes p-type QWIPs potentially more attractive for large format FPA applications. This paper will review the recent development in both n-type and p-type QWIPs for mid-wavelength, long-wavelength and very long wavelength IR detections. The basic device physics, structures, and performance parameters such as dark current, spectral responsivity, and detectivity for both n-type and p-type QWIPs will be depicted. Finally, a comparison of the advantages and drawbacks of n-type and p-type QWIPs will also be given.

Reduction of dark current in an n-type In 0.3Ga 0.7As/GaAs quantum well infrared photodetector by using a camel diode structure

We have studied the dependence of the well doping density in n-type GaInAs/InP quantum well IR photodetectors (QWIPs) grown by low-pressure metalorganic chemical vapor deposition. Three identical GaInAs/InP QWIP structures were grown with well sheet carrier densities of 1 by 10<SUP>11</SUP> cm<SUP>-2</SUP>, 3 by 10<SUP>11</SUP> cm<SUP>-2</SUP>, and 10 by 10<SUP>11</SUP> cm<SUP>-2</SUP>; all three samples had very sharp spectral response at (lambda) equals 9.0 micrometers . We find that there is a large sensitivity of responsivity, dark current, noise current, and detectivity with the well doping density. Measurements revealed that the lowest-doped samples had an extremely low responsivity relative to the doping concentration while the highest-doped sample had an excessively high dark current relative to doping. The middle-doped sample yielded the optimal results. This QWIP had a responsivity of 33.2 A/W and operated with a detectivity of 3.5 by 10<SUP>10</SUP> cmHz<SUP>1/2</SUP>W<SUP>-1</SUP> at a bias of 0.75 V and temperature of 80 K. This responsivity is the highest value reported for any QWIP in the (lambda) equals 8-9 micrometers range. Analysis is also presented explaining the dependence of the measured QWIP parameters to well doping density.

An electron transfer quantum well infrared photodetector (QWIP) consisting of repeating units of two coupled quantum wells (QWs) is capable of exhibiting a two color voltage dependent spectral response. However, significant electron transfer between the coupled QWs is required for spectral tuning, which may require the application of relatively high electric fields. Also, the band structure of coupled quantum wells is more complicated in comparison to a regular quantum well and, therefore, it is not always obvious if an electron transfer QWIP can be designed such that it meets specific performance characteristics. This paper presents a feasibility study of the electron transfer QWIP and its suitability for spectral tuning. Self consistent calculations have been performed of the bandstructure and the electric field that results from electron population within the quantum wells, from which the optical characteristics have been obtained. The band structure, spectral response, and the resonant final state energy locations have been compared with standard QWIPs. It is shown that spectral tuning in the long-wave infrared band can be achieved over a wide wavelength range of several microns while maintaining a relatively narrow spectral response FWHM. However, the total absorption strength is more limited in comparison to a standard QWIP, since the higher QW doping densities require much higher electric fields for electron transfer.

The AlInAs/InGaAs material system is promising for mid-wavelength infrared (MWIR) and multi-band quantum well infrared photodetectors (QWIPs) as a lattice-matched alternative to the strained AlGaAs/InGaAs system. In this paper, we report a large format (640 × 512) AlInAs/InGaAs QWIP focal plane array (FPA) with 4.9 µm cut-off wavelength and assess the performance of this material system for MWIR QWIP applications at both pixel and large format FPA level. We also experimentally demonstrate that the cut-off wavelength of AlInAs/InGaAs QWIPs can be tuned in a sufficiently large range in the MWIR atmospheric window by changing only the quantum well (QW) width at the lattice-matched composition. The cut-off wavelength is shifted from 4.15 to 4.9 µm when the QW width is decreased from 30 to 23 Å, in which case a very broad spectral response (Δλ/λp = 32%) with a reasonably high peak detectivity (5.5 × 1010 cm Hz1/2 W−1, f/1.5) is achievable. The AlInAs/InGaAs QWIP FPA yielded excellent sensitivity with a noise equivalent temperature difference as low as 22 mK and a background limited performance (BLIP) temperature as high as 115 K with f/1.5 aperture and 300 K background. The results clearly demonstrate the potential of this material system for completely lattice-matched dual- or multi-band QWIP FPAs for third-generation thermal imagers.

Chapter 3 Quantum Well Infrared Photodetector Physics and Novel Devices

Integrated HBT/QWIP structure for dual color imaging

Quantum Well Infrared Photodetectors (QWIPs) offer greater flexibility than usual extrinsically doped semiconductor IR detectors because the wavelength of the peak response and cutoff can be continuously tailored by varying layer thickness (well width), barrier composition (barrier height), and carrier density (well doping density). The GaAs/AlxGa1- xAs material system allows the quantum well parameters to be varied over a range wide enough to enable light detection at any wavelength range between 6 - 20 micrometer. The spectral band width of these detectors can be tuned from narrow ((Delta) (lambda) /(lambda) approximately 10%) to wide ((Delta) (lambda) /(lambda) approximately 40%), allowing various applications. Also, QWIP device parameters can be optimized to achieve extremely high performance at lower operating temperatures (approximately 40 K) for low background, long- wavelength, infrared applications in the strategic arena as well as in Astronomy. Furthermore, QWIPs offer low cost per pixel and highly uniform, large format, focal plane arrays (FPAs) mainly due to mature GaAs/AlGaAs growth and processing technologies.

p- and n-type InxGa1-xAs-InP quantum wells are suitable for multi-color infrared photodetector applications in atmospheric windows due to improved barrier quality and carrier-transport properties. We apply the k ·p method to study the energy band structures and optical transition properties, which show that the peak response wavelengths of p- and n-type InxGa1-xAs-InP quantum well infrared photodetectors (QWIPs) are determined not only by the energy distance from the ground sublevels in the quantum well to the energy band edges of extended states, but also by the characteristics of the extended states. The optical phonon scattering process converts the broad absorption spectrum of the p-QWIP from 0 to 16 μm into a short-wavelength spectrum centered at 4.5 μm. The transport of electrons in the extended states of the n-QWIP is characterized by running wave boundary conditions, resulting in a theoretically optimal absorption rate by a 8-nm-thick In0.53Ga0.47As quantum well. Moreover, a conduction-band offset of 0.5 for an InxGa1-xAs-InP (x=0.53) heterostructure gives the best data fitting of theoretical and experimental response peaks, whereas 0.55 is generally recommended in the literature.

Summary form only given. Recent numerical work (Ershov et al, 1997; Thibaudeau et al, 1996) has shown that at low operating temperatures or large incident photon fluxes, carriers deplete from the quantum wells near the emitter contact in a quantum well infrared photodetector (QWIP). This work finds a physical model (with closed form analytical expressions) which explains the recent numerical work on carrier depletion in QWIPs. The physical model found in this work is computationally much less intensive than the full numerical model, but retains the essential physics. As an example, we have considered device designs with the same periodic structure (the same compositions and layer widths) throughout the QWIP. In our physical model, the incident radiative flux was fixed, while the device behavior was studied for a varying applied bias. The current was seen to rise linearly with the applied bias in the different operating regimes, but with a different differential resistance in each operating regime. The physics behind this device characteristic was studied, and results are summarized here.

The asymmetry in the characteristics of GaAs/AlGaAs quantum well infrared photodetectors

Detection wavelength tuning of quantum well infrared photodetector (QWIP) has been carried out by using impurity free vacancy disordering (IFVD) technique with SiO/sub 2/ capping layer. The detection wavelength of QWIP was changed from 8.2/spl mu/m to 10/spl mu/m due to bandgap change by intermixing of constituent atoms in multiple quantum well (MQW) absorption region in Fig.1. The red-shift in detection wavelength can be explained by gap narrowing between confined state and continuum state in conduction band, which is a consequence of blue-shift in MQW bandgap after intermixing in Fig.2. The bandgap of the intermixed MQW structure was theoretically investigated. The Al-Ga inter-diffusion, which induces the red-shift of detection wavelength from 8.2/spl mu/m to 10/spl mu/m, was calculated to be 7/spl Aring/ at 850/spl deg/C.

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.

The GaAs/AlGaAs based quantum well IR photodetectors (QWIPs) afford greater flexibility than the usual extrinsically doped semiconductor IR detectors because the wavelength of the peak response and cutoff can be continuously tailored over any wavelength between 6-20 micrometers . The spectral band width of these detectors can be tuned from narrow to wide allowing various applications. Also, QWIP offers multi-color IR cameras which is capable of simultaneously acquiring images in different IR bands. Each pixel of such array consists of vertically stacked, independently readable, QWIP detectors sensitive in different narrow IR bands. In this article, we discuss the result of a 10-16 micrometers large format broadband QWIP focal plane array IR bands. In this article, we discuss the results of a 10-16 micrometers large format broadband QWIP focal plane array (FPA). The size of the FPA is 640 X 512 and its pixel pitch is 25 microns. The highest operating temperature of the FPA is 45K, and it was determined by the charge storage capacity and the other features of the particular readout multiplexer used in this demonstration. Excellent imagery, with a noise equivalent differential temperature of 55 mK has been achieved. In addition, we will discus the developments and results of the 640 X 512 dual-band QWIP FPA.

Long wavelength (7.5–10 μm) quantum well infrared photodetectors (QWIPs) based on AIGaAs/GaAs quantum wells (QWs) have been shown to exhibit high peak detectivities D* =1.1010–9.1010 cm Hz1/2 W-1 at 80 K and close to background limited operation1–3. Due to the well established GaAs material and processing technology QWIPs are viable candidates for large, low cost LWIR (8–12 μm) focal plane arrays (FPAs)4–5.