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Detectors
Published in C. R. Kitchin, Astrophysical Techniques, 2020
Photoconductive detectors that do not require the electrons to be excited all the way to the conduction band have been made using alternating layers of gallium arsenide (GaAs) and indium gallium arsenide phosphide (InGaAsP) or aluminium gallium arsenide (AlGaAs), each layer being only ten or so atoms thick. The detectors are known as quantum well infrared photodetectors (QWIPs). The lower energy required to excite the electron gives the devices a wavelength sensitivity ranging from 1 to 12 μm. The sensitivity region is quite narrow and can be tuned by changing the proportions of the elements. Recently, the National Aeronautics and Space Administration (NASA) has produced a broadband 1k × 1k QWIP with sensitivity from 8 to 12 µm by combining more than a hundred different layers ranging from 10 to 700 atoms thick. Quantum dot infrared photodetectors (QDIPs) have recently been developed wherein the ‘well’ is replaced by a ‘dot’, i.e., a region that is confined in all spatial directions. It remains to be seen if QDIPs have any advantages for astronomy over QWIPs.
The Basic Physics of Photoconductive Quantum Well Infrared Detectors
Published in Manijeh Razeghi, Long Wavelength Infrared Detectors, 2020
QWIP materials are normally grown by molecular beam epitaxy (MBE). The GaAs-AlGaAs MBE technology is mature enough for the fabrication of QWIPs [41]. Devices are made by standard GaAs microfabrication techniques. The operation of a photoconductive QWIP is easily summarized using the bandedge profile shown in Fig. 1. Upon application of a finite bias, incident photons excite electrons out of the quantum wells creating a photocurrent. The wells are doped to provide a finite population of electrons in the ground state subbands. The most widely studied materials system is GaAs-AlxGa1−x As, where x is the Al alloy fraction. The conduction band is commonly used. For this system, the conduction band offset (barrier height) as a function of x is given approximately by [22] ΔEC≈(0.7−0.8)×x(eV),forx<0.5.
Quantum Well Infrared Photodetectors
Published in Joachim Piprek, Handbook of Optoelectronic Device Modeling and Simulation, 2017
A QWIP uses a multiple quantum well (MQW) structure to detect light (Levine et al., 1987). This structure is realized by growing alternate material layers of different bandgaps on a suitable substrate. The MQW creates a series of energy subbands, E1, E2, …, in each QW unit. The QWs are doped n-type with a doping density ND so that there are free electrons in the ground subband E1 with a Fermi energy EF. The band structure of a typical QWIP made of AlxGa1–xAs/InyGa1–yAs is shown in Figure 37.1. In Figure 37.1, x is the aluminum mole fraction, y is the indium mole fraction, W is the well width, and B is the barrier thickness. An inter-subband absorption brings an electron from E1 to En where the conductivity across the layers is higher. A photocurrent is thus created.
Performance Improvement of Quantum Well Infrared Photodetectors Through Dark Current Reduction Factor
Published in IETE Journal of Research, 2023
Mohamed S. El_Tokhy, Elsayed H. Ali, Sergey P. Polyutov
A multiple QW structure consists of the QWIP under consideration, sandwiched between the barriers of the emitter and collector, followed by heavily doped contact layers. The QW structure is composed of thin doped narrow-gap QW separated by thick wide gap barriers that are undoped [20–22]. The QWIP model takes into account the excitation of electrons form bound states in QW into continuum states, the injection of electrons from the emitter through the intense barrier, the capture of electrons into QW, the transport of electrons through the QW structure, and their escape to the collector in the self-consistent electric field. The barriers between all QWs, the barriers between the extreme QW and the collector layer, are believed to be similar. The barrier separating the contact layer of the emitter and the first QW (top-most barrier or emitter barrier) can vary in height from the interwell barriers. The thickness of the QWs is slightly lower than that of the barriers . In such a way that QW has a single bound state, the thickness and depth of QWs are selected when the first excited level lies near the top of the barriers. It is believed that, due to tunneling, the electron injection from the emitter contacts the QW structures. When the QWIP is not illuminated, thermionic emission is known to be the main mechanism of electron escape from the QWs.