François-Régis Jasnot

Space charge mediated negative differential resistance in terahertz quantum well detectors

In terahertz quantum well infrared photodetectors, a built-in-charge-mediated regime transition of the electronic transport is thoroughly investigated. The very strong current discontinuity and negative differential resistivity behavior are explained in terms of band structure reorganizations. The analysis of bias versus current measurements reveals that the transition occurs when the first two wells of the structure become partially drained, and the second well enters the ionized regime before the first one. Both many-body effects and a careful model of the contact have to be considered to account for these features. The source of the built-in charge is identified as intersubband impact ionization. The regime transition is one of its few experimental proofs, and provides an original approach to investigate hot electron kinetics in multi-quantum-well structures.

Direct surface cyclotron resonance terahertz emission from a quantum cascade structure

A strong magnetic field applied along the growth direction of a semiconductor quantum well gives rise to a spectrum of discrete energy states, the Landau levels. By combining quantum engineering of a quantum cascade structure with a static magnetic field, we can selectively inject electrons into the excited Landau level of a quantum well and realize a tunable surface emitting device based on cyclotron emission. By applying the appropriate magnetic field between 0 and 12 T, we demonstrate emission from a single device over a wide range of frequencies (1-2 THz and 3-5 THz).

Intersubband impact ionization in THz QWIPs: shaping band structure reorganizations to design novel detectors

Electronic transport in AlGaAs/GaAs THz Quantum Wells Intersubband Photodetectors (QWIPs) exhibits two different regimes separated by huge discontinuities (up to five orders of magnitude) in the resistivity. They are interpreted in terms of band structure reorganizations triggered by intersubband impact ionization. We will analyze and model their in uence on the electronic transport. The magnitude of the transport modifications is explained by the small transition energy and the sharpness of the electrons distribution at stake in THz QWIPs. Measurements under magnetic field or temperature show that the broadening of the electron distribution damps the effects of impact ionization. Some experimental features of the electronic transport of shorter wavelength detectors are then reproduced. The use of intersubband impact ionization in THz QWIPs to design high gain and fast novel detectors is discussed.

Magnetotransport in quantum cascade detectors: analyzing the current under illumination.

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.

Magnetotransport in very long wave infrared quantum cascade detectors: Analyzing the current with and without illumination

Current measurements of current have been performed on a very long wave infrared quantum cascade detector under magnetic field under both dark and light conditions. The analysis of dark current as a function of temperature highlights three regimes of transport. Under illumination, the model developed is in agreement with the oscillatory component of the experimental magnetophotocurrent. It allows to identify the key points controlling the electronic transport: crucial role of extraction, location of ionized impurities and scattering mechanisms involved in the structure. This work is valuable for the future conception of high-performance quantum cascade detectors in the infrared range.

Description of transport mechanisms in a very long wave infrared quantum cascade detector under strong magnetic field

Measurements of current have been performed on a very long wave infrared quantum cascade detector under strong magnetic field applied parallel to the growth axis, both under dark and light conditions. The analysis of dark current as a function of temperature highlights three regimes of transport involving the different energy levels of the structure. For photocurrent analysis, we developed a model based on a rate equation approach taking into account all the electronic levels of the structure. This model is in agreement with the oscillatory component of the experimental magnetophotocurrent. It allows to identify the key points controlling the electronic transport such as extraction from the upper level of the optically active quantum well, location of ionized impurities, and scattering mechanisms involved in the structure. This work is valuable for the future conception of high-performance quantum cascade detectors in infrared and far infrared range.

Cyclotron emission in a THz quantum cascade structure

A terahertz quantum cascade structure has been designed to allow population relaxation between crossing Landau levels from different subbands under perpendicular magnetic field. Emission spectra have been performed and show expected intersubband luminescence as well as radiative inter‐Landau level emission (cyclotron emission) through the surface of the sample. Interface roughness is the mechanism responsible for the population relaxation at resonance.

THz inter-Landau level emission in a quantum cascade structure

A THz quantum cascade structure has been designed to allow population relaxation between crossing Landau levels from different subbands under perpendicular magnetic field. Electroluminescence measurements show intersubband luminescence as well as inter-Landau level emission (cyclotron emission).

Impact ionization in THz QWIPs

Recently phase transitions in THz Quantum Wells Inter-subband Photodetectors (QWIP) were observed. Very large current discontinuities - up to five orders of magnitude - were obtained, at low temperature [1]. Quantum well impact ionization shifts the structure from a resistive “down” state, where the current flows through inter-well quantum tunneling to a highly conductive “up” state. In this paper, thanks to the analysis of the bias versus current “s” curve, an accurate investigation of impact ionization processes in quantum wells is analyzed. The use of these phenomena for high sensitivity quantum well THz detection will be presented.