Alexandru Nedelcu

Quantum cascade detectors for very long wave infrared detection

A high responsivity GaAs/AlGaAs quantum cascade detector is demonstrated at a wavelength of 15 μm. The quantum design is optimized for negative bias operation, so that the capture of photoexcited electrons back to the fundamental level is minimized. The detectivity of the detector presented here reaches 1.1×1012 Jones at 25 K for an applied bias of −0.6 V.

Impact ionization processes in quantum well infrared photodetector structures

The importance of impact ionization processes on the electronic transport in quantum well infrared photodetectors (QWIPs) is experimentally and theoretically investigated. Dark and optical current analysis in AlxGa1−xAs/GaAs-QWIP structures in which the first barrier thickness or the number of wells is varied leads to the conclusion that, under normal QWIP bias conditions, impact ionization processes occur only in the first two periods and not in the entire structure.

Quantum well infrared photodetectors: present and future

A review of the III-V Lab activities in the field of quantum well infrared photodetectors (QWIPs) is presented. We discuss the specific advantages of this type of detector and present the production facilities and status. A large section is dedicated to broadband QWIPs for space applications and to QWIPs on InP for mid-wavelength infrared detection. We review the progress of QWIP technology for the next generation (dual band, polarimetric, and multispectral) of thermal imagers. Finally, the state-of-the-art of very long wavelength QWIPs is discussed.

High contrast polarization sensitive quantum well infrared photodetectors

The responsivity of polarization sensitive quantum well infrared photodetectors with small pixel size (down to 20μm) is investigated. It is shown that pixels suitable for integration into very large focal plane arrays (1000×1000) can discriminate the polarization of the incoming signal. A responsivity contrast higher than 60% is obtained although the grating size is reduced to only six periods. The quantum efficiency and polarimetric capabilities can be both optimized with the same grating parameters.

Single color and dual band QWIP production results

Since 1997, Sofradir has been working with Thales Research and Technologies (TRT) to develop and implement Quantum Well Infrared Photodetectors (QWIP) as an alternative and complementary offer with Mercury Cadmium Telluride (MCT) Long Wave (LW) detectors, to provide large LW staring arrays. Thanks to the low dark current technology developed by TRT, the QWIP detectors can be worked at FPA temperature above 73K, enabling the development of new compact IR cameras thanks to the use of compact microcoolers, and today, Sofradir is entering production with these highly compact QWIP components. For the Long Wave applications, SOFRADIR offers the European TV/4 format with the VEGA-LW detector (25μm pitch 384×288 IDDCA) and the full TV format with the SIRIUS-LW detector (20μm pitch 640×512 IDDCA). The first one is under production for several hundreds of units, to equip the Catherine-XP thermal imager from Thales. The second one has been initially developed for the Catherine-MP high resolution (SXGA) thermal imager and is ready for production. Both detectors present highly uniform performances and sharp images with NETD in the 50mK range when working around 75K at video frame rate. The TV/4 VEGA detector is also offered as a demonstrator for the Mid Wave applications, with a QWIP array adapted to this waveband. In the same time, a dual band MW-LW similar array is developed with spatial coherence, and is currently under demonstration. The performances of these four QWIP detectors are reviewed in this paper.

Quantum transport in weakly coupled superlattices at low temperature

We report on the study of the electrical current flowing in weakly coupled superlattice (SL) structures under an applied electric field at very low temperature, i.e. in the tunneling regime. This low temperature transport is characterized by an extremely low tunneling probability between adjacent wells. Experimentally, I(V) curves at low temperature display a striking feature, i.e a plateau or null differential conductance. A theoretical model based on the evaluation of scattering rates is developed in order to understand this behaviour, exploring the different scattering mechanisms in AlGaAs alloys. The dominant interaction in usual experimental conditions such as ours is found to be the electron-ionized donors scattering. The existence of the plateau in the I(V) characteristics is physically explained by a competition between the electric field localization of the Wannier-Stark electron states in the weakly coupled quantum wells and the electric field assisted tunneling between adjacent wells. The influence of the doping concentration and profile as well as the presence of impurities inside the barrier are discussed.

QWIP products and building blocks for high-performance systems

Standard GaAs/AlGaAs Quantum Well Infrared Photodetectors (QWIP) are coming out from the laboratory. In this presentation we demonstrate that production and research cannot be dissociated in order to make the new generation of thermal imagers benefit as fast as possible from the building blocks developed by researchers. Since 2002, the THALES Group has been manufacturing sensitive arrays using QWIP technology based on AsGa techniques through THALES Research and Technology Laboratory. This QWIP technology, integrated in IDDCA built by Sofradir, allows the realization of large staring arrays for Thermal Imagers (TI) working in the IR band III (8-12 μm). A review of the current QWIP products, offered by Sofradir, is presented. In the past researchers claimed many advantages of QWIPs. Uniformity was one of these and was the key parameter for the production start. Another advantage widely claimed also for QWIPs was the so-called band-gap engineering, allowing the custom design of quantum structure to fulfill the requirements of specific applications like very long wavelength or multispectral detection. In this presentation, we present the performances for Middle Wavelength InfraRed (MWIR) detection and demonstrate the ability of QWIP to cover the two spectral ranges (3-5 μm and 8-20 μm). At last but not least, the versatility of the GaAs processing appeared for QWIPs as an important gift. This assumption was well founded. We give here some results achieved on building blocks for two color QWIP pixels. We also report the expected performances of focal plane arrays we are currently developing with the CEA-LETI-SLIR.

State of the art of quantum cascade photodetectors

The Quantum Cascade Detector (QCD) is a multiple quantum well photodetector working at low bias or zero bias. It has a zero dark current occurring at 0V, together with a high photovoltaic photoresponse, since the QCD does not need any applied field to improve the collection of electrons. QCDs have been tested at various wavelengths, from short wavelengths (1.5 microns) up to THz waves, through the entire infrared spectrum (middle and long wavelengths). Theory of transport in QCD is now well established, and leads to accurate calculations of current and noise in QCDs, with a very good agreement with experimental results. Latest results and state of the art of performances of QCDs are presented.

Spectral cross-talk in dual-band quantum well infrared detectors

We propose a general definition of the spectral cross-talk in dual-band infrared (IR) photodetectors, based on the common information carried by the spectral channels. This definition includes detector characteristics as well as scene characteristics and can be applied to any real configuration. We use it to evaluate narrowband and wideband quantum well infrared photodetector structures and set up their interest for dual-band imaging. The spectral cross-talk is negligible for interband IR (3–5μm∕8–12μm) applications and can be minimized for intraband IR (8–12μm) applications by properly tailoring the responsivity peaks.

Double barrier strained quantum well infrared photodetectors for the 3–5μm atmospheric window

We present a detailed study of double barrier strained Al0.35Ga0.65As/AlAs/GaAs/In0.2Ga0.8As quantum well infrared photodetectors on GaAs substrate. Measurements were made on four different well widths active layers and on several mesa pixels with different optical coupling structures. We obtained responses peaked in the spectral range 3.6–4.6 μm. Based on the experimental results, we show that in the background limited regime, the impact ionization is the restrictive transport process for midwave detectors. We also demonstrate a 4 μm structure with the high background limited detectivity of 2×1011 Jones at 77 K and 9×1010 Jones at 110 K (2π field of view, 300 K background).