Magnetotransport in quantum cascade detectors: analyzing the current under illumination
© Jasnot et al; licensee Springer. 2011
Received: 16 July 2010
Accepted: 9 March 2011
Published: 9 March 2011
Photocurrent measurements have been performed on a quantum cascade detector structure under strong magnetic field applied parallel to the growth axis. The photocurrent shows oscillations as a function of B. In order to describe that behavior, we have developed a rate equation model. The interpretation of the experimental data supports the idea that an elastic scattering contribution plays a central role in the behavior of those structures. We present a calculation of electron lifetime versus magnetic field which suggests that impurities scattering in the active region is the limiting factor. These experiments lead to a better understanding of these complex structures and give key parameters to optimize them further.
The quantum cascade detector (QCD)  recently proposed and realized in both the mid-infrared  and the THz [3, 4] range is a photovoltaic version of the quantum well infrared photodetector . Their band structure are designed as quantum cascade lasers without any applied bias voltage [1, 3]. QCD are totally passive systems and show a response only to photon excitation. As such, the QCD structure is designed to generate an electronic displacement under illumination through a cascade of quantum levels without the need of an applied bias voltage.
In a semiconductor quantum well structure, magnetic field applied along the growth direction breaks the 2D in-plane continuum into discrete Landau levels (LLs). This experimental technique has been used to evaluate the different contributions of scattering mechanism in complex quantum cascade structures [5–9].
We present in this article experimental photocurrent measurements under magnetic field applied along the growth direction. We develop a simple model of transport under illumination in a QCD. Through a comparison between experimental and calculation results, we evidence the mechanism limiting the response of the QCD.
Experimental setup and sample
QCDs are mounted inside an insert at the center of a superconducting coil where a magnetic field B up to 16 T can be applied parallel to the growth axis. Light is emitted by a globar source from an FTIR spectrometer and guided to the sample. The experiment consists in measuring the current under illumination (I light) without any applied voltage at 80 K while the magnetic field is swept from 0 to 16 T.
The parameters α and N down are, respectively, the absorption factor and sheet density of |down〉 and are constant. The subscribe c stands for the whole cascade. The quantum efficiency QE is the ratio of the lifetime τup-down divided by τup-down + τup-c and corresponds to the fraction of electrons on the level |up〉 that contributes to the photocurrent. In our model we suppose that any incident photon generates an absorption between the levels |down〉 and |up〉.
Calculated scattering rates in s-1.
LO phonon emission
7.0 × 1011
7.2 × 1011
6.0 × 1011
8.6 × 1012
1.8 × 1013
5.2 × 1013
The oscillating behavior at high magnetic field (B > 9T) is a result of the electronic transfer from |up〉 to |down〉. This transfer leads to minima in the current which fit well with and QE. The long period oscillating behavior of as a function of B enhances the peak at B = 14 T in QE in agreement with experimental data. QE, which describes the performance of the detector, is oscillating between 74 and 85% under B. By extrapolating, at B = 0T, QE is equal to 75%, a value that should be increased to improve the detector performance. An optimized structure should take these results into account by shifting the ionized impurities from the active region, where they are enhancing , to a position where they would only enhance . The series of peak at B < 9T corresponds to a characteristic energy of 37 meV. This energy is attributed to transitions in the cascade involving an elastic scattering mechanism.
In conclusion, we observe oscillations of the photocurrent in a mid-infrared QCD as a function of B. These oscillations are due to electron-ionized impurities scattering. This mechanism is dominant in this structure because impurities are located in the active region. In order to improve further this efficiency, we suggest to shift the impurities in another location of the structure in order to minimize .
The Laboratoire Pierre Aigrain is a " Unité Mixte de Recherche" between École Normale Supérieure, the CNRS, the University Paris 6 and the University Paris 7.
quantum cascade detector.
This study has been supported by a grant of the Agence Nationale pour la Recherche (ANR).
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