Isabelle Ribet-Mohamed

Ultimate performance of quantum well infrared photodetectors in the tunneling regime

Thanks to their wavelength diversity and to their excellent uniformity, Quantum well infrared photodetectors (QWIP) emerge as potential candidates for astronomical or defense applications in the very long wavelength infrared (VLWIR) spectral domain. However, these applications deal with very low backgrounds and are very stringent on dark current requirements. In this paper, we present the full electro-optical characterization of a 15 μm QWIP, with emphasis on the dark current measurements. Data exhibit striking features, such as a plateau regime in the I(V) curves at low temperature (4–25 K). We show that present theories fail to describe this phenomenon and establish the need for a fully microscopic approach.

Van Hove singularities in intersubband transitions in multiquantum well photodetectors

Photocurrent spectra of quantum well infrared photodetector (QWIP) devices have been studied over more than three orders of magnitude, revealing features which have been largely overlooked before. Electric field assisted tunneling and, more surprisingly, Van Hove singularities at the miniband edges are shown to play an important role in the low and high energy parts of the QWIP photocurrent spectra, respectively. The photoresponse of QWIPs away from their peak responsivity is found to be non-negligible (>1% in the 3–5μm for a 8–12μm detector), which has to be taken into consideration when optimizing multispectral devices.

Experimental study and modelization of oblique incidence effect on QWIP's spectral response

As far as calibrated radiometric imaging is concerned, a complete prediction of oblique incidence effect on the FPA pixels’ response is required. Since a light coupling scheme needs to be used in QWIP detectors, this effect is particularly complicated to understand. This article presents two complementary test benches which allow to quantify the effect of oblique incidence on cooled infrared detectors issued from different technologies. The first test bench performs measurements over a wide angular range with low background emission levels, but gives spectrally integrated measurements. The second one delivers spectrally resolved responses for incident angles lower than 30°. In order to validate both experimental concepts, we studied QWIPs equipped with 2D periodic gratings. Relatively large pixels (100x100μm2) were chosen to ease comparison with models. Calculations based on the modal expansion method reveal that diffraction off an infinite grating does not account very well for the observed spectral responses.

Effect of oblique incidence on the spectral response of quantum well infrared photodetectors

A full understanding of the effect of oblique incidence on the response of the pixels of a focal plane array is required for calibrated radiometric imaging. Measuring the angular response of infrared detectors requires an extremely complex experimental setup. This paper presents two complementary experimental setups that enable us to study the effect of the oblique incidence of light on the response of cooled infrared phototdetectors based on different technologies. The angular response measurement setup provides spectrally integrated measurements over a wide angular range (−80°–+80°) with a low background level and a low stray light. In contrast, the spectral response measurement setup provides the spectrally resolved response for incidence angles lower than 30°. Both experimental setups were used to characterize single-quantum well infrared photodetectors with crossed gratings. Relatively large pixel arrays (100×100μm2) were used to reduce finite-size effects. However, the comparison of our experimental...

Hyperspectral study of a 640x512 infrared focal plane array of QWIP technology

A test bench has been developed at ONERA in order to measure the spectral responses of infrared focal plane arrays (IRFPAs). This test bench can deliver hyperspectral cartographies with rather good resolutions (better than 16 cm-1) on large spectral ranges (from 1.3 μm to 20 μm). The principle of this test bench will be described. Using this technique, tests have been performed on a large format (640x512) IRFPA of quantum-well technology operating in the 8- to 10-μm spectral range. The prototype tested had several small defects that produce spectacular hyperspectral cartographies. To explain the hyperspectral structures observed across the array, an empirical model based on Fourier optics will be presented.

Dark current reduction in a long wavelength quantum well infrared photodetector operating at low temperature

Quantum Well Infrared Photodetector (QWIP) usually suffer from a too moderate quantum efficiency and too large dark current which is often announced as crippling for low flux applications. Despite this reputation we demonstrate the ability of QWIP for the low infrared photon flux detection. We present the characterization of a state of the art 14.5\mu m QWIP from Alcatel-Thales 3-5 Lab. We developed a predictive model of the performance of an infrared instrument for a given application. The considered scene is a Cryogenic Wind Tunnel (ETW), where a specific Si:Ga camera is currently used. Using this simulation tool we demonstrate the QWIP ability to image a low temperature scene in this scenario. QWIP detector is able to operate at 30K with a NETD as low as 130mK. In comparison to the current detector, the temperature of use is three times higher and the use of a QWIP based camera would allow a huge simplification of the optical part.

Modulation transfer function measurements on a MWIR T2SL focal plane array in IDDCA configuration

Type-II InAs/GaSb superlattice (T2SL) has recently matured into a commercially available technology addressing both MWIR and LWIR spectral domains. As the prerequisites such as Quantum Efficiency (QE) and dark current were met, more advanced figures of merits related to the ElectroOptic (EO) system as a whole can now be studied in order to position this technology. In this paper, we focus on modulation transfer function (MTF) measurements. Knowing the MTF of a detector is indeed of primary importance for the EO system designers, since spatial filtering affects the system range. We realized MTF measurements on a 320x256 MWIR T2SL FPA provided by IRnova, using a Continuously Self Imaging Grating (CSIG). The advantage of this experimental configuration is that no high performance projection optics is required. Indeed, the CSIG exploits the self-imaging property (known as Talbot effect) to project a pattern with known spatial frequencies on the photodetector. Such MTF measurements have never been done in Integrated Detector Dewar Cooler Assembly (IDDCA) configuration, so we had to study the effect of the vibrations induced by the cryocooler. Vibrations indeed affect the MTF measurement in the same way electrical diffusion would do. Using three accelerometers we optimized our experimental setup and extracted MTF measurements with reduced vibrations. The pixel size is 26μm for a pitch of 30μm.

Comparison of the electro-optical performances of MWIR InAs/GaSb superlattice pin photodiode and FPA with asymmetrical designs

We first present an electro-optical characterization of the radiometric performances of a type-II InAs/GaSb superlattice (T2SL) pin photodiode operating in the mid-wavelength infrared domain. This photodiode was grown with an InAs-rich structure. We focused our attention on quantum efficiency and responsivity: quantum efficiency of mono-pixel device reaches 23% at λ = 2.1 μm for 1 μm thick SL structure and 77K operating temperature. Then we measured the angular response of this photodiode: the response of the photodiode doesn’t depend on the angle of incidence of the flux. We also report the QE of 2μm-thick InAs-rich T2SL pin 320×256 pixels focal plane array, which reaches 61% at λ = 2.6 μm.

Modulation transfer function measurement of microbolometer focal plane array by Lloyd's mirror method

Today, both military and civilian applications require miniaturized and cheap optical systems. One way to achieve this trend consists in decreasing the pixel pitch of focal plane arrays (FPA). In order to evaluate the performance of the overall optical systems, it is necessary to measure the modulation transfer function (MTF) of these pixels. However, small pixels lead to higher cut-off frequencies and therefore, original MTF measurements that are able to extract frequencies up to these high cut-off frequencies, are needed. In this paper, we will present a way to extract 1D MTF at high frequencies by projecting fringes on the FPA. The device uses a Lloyd mirror placed near and perpendicular to the focal plane array. Consequently, an interference pattern of fringes can be projected on the detector. By varying the angle of incidence of the light beam, we can tune the period of the interference fringes and, thus, explore a wide range of spatial frequencies, and mainly around the cut-off frequency of the pixel which is one of the most interesting area. Illustration of this method will be applied to a 640×480 microbolometer focal plane array with a pixel pitch of 17µm in the LWIR spectral region.

Dark current and noise measurements in InAs/GaSb superlattice detectors

Infrared detectors based on InAs/GaSb superlattices (SL) have recently emerged as a promising technology for high performance infrared (IR) imaging systems. In this paper, we present the results of dark current and noise measurements realized on MWIR superlattice single detectors. The SL structure was made of 8 InAs monolayers (MLs) and 8 GaSb MLs, for a total thickness of 2μm. This structure exhibits a cut-off wavelength of 4.8μm at 77K. An original chemical etching solution was designed to obtain smooth mesa sidewalls, followed by a simple passivation technique. Dark current measurements were carried out to prove the good quality of both the etching and the passivation steps. The measured R0A product reaches the state-of-the-art values at 80K. Noise measurements were also performed under dark conditions. The detectors under test proved to be Schottky-limited on a range of bias voltage of 200mV typically, which confirms the very good quality of the technological process.