Publisher Summary This chapter reviews the device physics, structures, characteristics, and performance parameters for a wide variety of n- and p-type QWIPs. Using intersubband transitions and bandgap engineering, single- and multicolor QWIPs with detection wavelengths ranging from 3.4 to 16 μm have been presented in this work. The basic device physics and the performance parameters of QWIPs have been described. Theoretical considerations include the calculations of the electronic states in quantum well using multilayer transfer matrix method (TMM), the physics of intersubband transitions, the effects of strain in the quantum well on the energy bandgap and band offset in the strained-layer QWIPs, and the performance parameters of QWIPs. Two p-type CSL-QWIPs using InGaAs/A1GaAs material system grown on GaAs substrates and a p-type TSL-InGaAs/InA1As QWIP grown on InP substrates have been presented. The advantages of using strained layer p-type QWIPs over the unstrained case have also been discussed. Several multicolor QWIPs for both MWIR and LWIR detection is also discussed. In comparing with HgCdTe LWIR detectors, QWIPs have the advantages of higher uniformity, higher reproducibility, and higher operability in large format FPAs with the potential of higher yields and lower costs due to the mature III-V semiconductor epitaxial growth (MBE) and processing technologies.

Quantum well infrared photodetectors

A high strain two-stack, two-color, InGaAs/AlGaAs and AlGaAs/GaAs quantum well infrared photodetector for midwavelength infrared (MWIR) and long wavelength infrared (LWIR) detection has been demonstrated. Each stack is designed to have detection in one of the two atmospheric windows, 3–5 μm and 8–12 μm, respectively. The MWIR stack has employed 35% of indium in the InGaAs well, which not only achieved peak wavelength at 4.3 μm, but also obtained very high peak responsivity of Rp=0.65 A/W, using 45° light coupling. Normal incidence without grating coupling also has high responsivity with 40%–50% in the MWIR stack and 35%–45% in the LWIR stack, respectively, compared with the 45° incidence. Despite the large in-plane compressive strain induced by the high indium concentration, the device is highly uniform and has very low dark current in the MWIR stack. The background limited temperature is 125 K for the MWIR stack with a cutoff wavelength λc=4.6 μm, and is 70 K for the LWIR stack with λc=10 μm.

A high strain two-stack two-color quantum well infrared photodetector

An infrared detector array includes a plurality of detector pixel structures, each having a plurality of coplanar sections responsive to different bands of infrared radiations. Each section of a pixel structure comprises a plurality of elongate quantum well infrared radiation absorbing photoconductor (QWIP) elements. The group of QWIP elements are spaced such that they comprise a diffraction grating for the received infrared radiation. Top and bottom longitudinal contacts are provided on opposite surfaces of the QWIP elongate elements to provide current flow transverse to the axis of the element to provide the required bias voltage. An infrared radiation reflector is provided to form an optical cavity for receiving infrared radiation. A plurality of detector pixel structures are combined to form a focal plane array. Each pixel structure section produces a signal that is transmitted through a conductive bump to a terminal of a read out integrated circuit. The group of signals from the detector pixel structures produces a multi-band (color) image corresponding to the received infrared radiation.

Multi-band infrared photodetector

Three-dimensional (3D) design and manufacturing enable flexible nanomembranes to deliver unique properties and applications in flexible electronics, photovoltaics, and photonics. We demonstrate that a quantum well (QW)–embedded nanomembrane in a rolled-up geometry facilitates a 3D QW infrared photodetector (QWIP) device with enhanced responsivity and detectivity. Circular geometry of nanomembrane rolls provides the light coupling route; thus, there are no external light coupling structures, which are normally necessary for QWIPs. This 3D QWIP device under tube-based light-trapping mode presents broadband enhancement of coupling efficiency and omnidirectional detection under a wide incident angle (±70°), offering a unique solution to high-performance focal plane array. The winding number of these rolled-up QWIPs provides well-tunable blackbody photocurrents and responsivity. 3D self-assembly of functional nanomembranes offers a new path for high conversion efficiency between light and electricity in photodetectors, solar cells, and light-emitting diodes.

https://advances.sciencemag.org/content/advances/2/8/e1600027.full.pdf

Self-rolling and light-trapping in flexible quantum well–embedded nanomembranes for wide-angle infrared photodetectors

InGaAs/AlGaAs quantum well structures have been shown to be versatile for infrared detection. By changing the material composition, one can tune the detection wavelength from 2 to 35 μm and beyond. However, there have been few systematic calculations on the absorption wavelength of these structures with respect to their structural parameters. In this work we have adopted the transfer-matrix method to calculate both their energy levels and the wave functions. From this calculation, the absorption and the responsivity spectra of the structures can be predicted. The theory agrees with the experimental result of the test structures. Supported by the experimental evidence, we applied the calculation to a general class of midwavelength detectors and thus established a useful guideline for the detector design in this wavelength range.

Detection wavelength of InGaAs/AlGaAs quantum wells and superlattices

A two‐color infrared detector using GaAs/AlGaAs and strained InGaAs/AlGaAs multiquantum wells is demonstrated. The response peak of the GaAs/AlGaAs quantum well is at 8 μm, and that of the InGaAs/AlGaAs quantum well is at 5.3 μm. The responsivity of the detector is 1 A/W at 8 μm and 0.27 A/W at 5.3 μm; these are the best values reported for a two‐color quantum well infrared detectors (QWIPs) with peak sensitivities in the spectral regions of 3–5.3 μm and 7.5–14 μm. Single‐colored 5.3 and 8 μm QWIPs were also fabricated to study the bias dependent behavior. This behavior can be explained using the concept of current continuity. Because of a higher electrical resistance, a high electric field domain is always formed first in the shorter wavelength quantum well stack.

Two-color infrared photodetector using GaAs/AlGaAs and strained InGaAs/AlGaAs multiquantum wells

Multiple quantum well infrared photodetectors (QWIPs) with bi‐periodic grating couplers have been demonstrated. The bi‐periodic grating consists of two gratings with different periods. The response linewidths of such QWIPs are found to be much wider than those of conventional grating coupled QWIPs. By using bi‐periodic gratings for coupling the radiation into QWIPs, the influence of variation of material and grating quality on the performance of QWIP can be largely eliminated.

Quantum well infrared photodetectors with bi‐periodic grating couplers

Asymmetric dark current versus voltage characteristics in quantum well infrared photodetectors have been studied. A model based on asymmetrical potential barriers was proposed. The asymmetrical potential barriers, which are most likely due to the accumulation of oxygen impurities at one of the interfaces, cause the asymmetrical I‐V characteristics. The height of the potential spike is found to increase with the Al content in the AlGaAs barriers. Calculations based on our model agree well with experimental results.

K. H. Chang

Papers

Two-color infrared photodetector using GaAs/AlGaAs and strained InGaAs/AlGaAs multiquantum wells

Quantum well infrared photodetectors with bi‐periodic grating couplers

Asymmetric dark current in quantum well infrared photodetectors