F. Gouider

The cyclotron resonance in Heterostructures with the InSb/AlInSb quantum wells

The absorption of two-dimensional electrons in InSb-based quantum wells in the quantized magnetic fields in the terahertz spectral region are studied. A p-Ge-based cyclotron laser was used as the radiation source. The effective mass of carriers at the Fermi level equal to 0.0219m0 (m0 is the mass of a free electron) is determined from the cyclotron resonance spectra. It is shown that the electron spectrum is described by the Kane model in a wide range of magnetic fields. An anomalously pronounced splitting of the cyclotron resonance line not associated with the nonparabolicity of the conduction band of InAs is observed in low magnetic fields, which can be attributed to the effect of the spin-orbit interaction.

Mechanism of terahertz photoconductivity in semimetallic HgTe/CdHgTe quantum wells

Terahertz photoconductivity in magnetic fields in semimetallic HgTe/CdHgTe quantum wells has been studied. The main contribution to photoconductivity comes from a signal that appears as a result of electron-gas heating. It is shown that, with the cyclotron resonance conditions satisfied, the photoconductivity signal is composed of cyclotron-resonance and bolometric components. However, in this case too, the bolometric contribution predominates.

The detection of terahertz waves by semimetallic and by semiconducting materials

We present a survey of photoresponse (PR) measurements of various devices containing quantum wells (QWs) of HgTe of various widths dQW and of InSb. By varying dQW for HgTe, the material properties of the QW material change from semiconducting to semimetallic as dQW is increased above a value of about 6nm. We have studied the PR of devices made from CdxHg1−xTe/HgTe/CdxHg1−xTe wafers with values of the QW width in the range of 6 nm ≤ dQW ≤ 21 nm. Only for samples with semimetallic HgTe QWs, a measurable PR could be detected. However, our investigations of samples made from AlxIn1−xSb/InSb/AlxIn1−xSb wafers gave evidence that a measurable PR also can appear from devices with a semiconducting QW. Both cyclotron-resonant (CR) and nonresonant (bolometric, BO) interaction mechanisms can contribute to the PR signal. Whereas the CR contribution is dominant in AlxIn1−xSb/InSb/AlxIn1−xSb samples, in CdxHg1−xTe/HgTe/CdxHg1−xTe samples the behavior is more complex. In a sample with dQW = 8 nm, the PR is clearly dominated by the BO contribution. In the PR of another sample of dQW = 12 nm, both contributions (BO and CR) are present. The sample of dQW = 21 nm shows a PR with not clearly separable BO and CR contributions.

THz detectors with HgTe and InSb quantum wells

Terahertz-frequencies in semiconductor quantum wells are of interest due to the opportunity for making devices that operate at THz frequencies. In this study we present magnetotransport and magnetooptical data obtained in the magnetic field range 0 < B < 7T at QH detectors patterned as Corbino rings on AlInSb/InSb/AlInSb and HgCdTe/HgTe/HgCdTe quantum well wafers.

Magneto-infrared study of electron-hole system in strained semimetallic HgTe quantum wells

Magneto infrared absorption measurements have been performed on HgTe/CdHgTe quantum wells with different thicknesses grown on (013) GaAs substrate. Cyclotron resonance effective masses, inter-Landau-level transition energies and their dependence on magnetic field are measured. The measured intersubband energies are in good agreement with the theoretically calculated values [1]. Strong spin-orbit interaction is responsible for cyclotron resonance splitting in asymmetric quantum wells. We demonstrate that the increase of the quantum well thickness leads to a semimetallic state, allowing for simultaneous observation of holes and electron transitions.

Edge‐Channel Photo‐Effect In Asymmetric Two‐Dimensional Systems

We report on the photo‐effect in an asymmetric two dimensional electron system where edge currents are induced by homogeneous terahertz (THz) radiation in tilted magnetic field without any external bias. We discuss the features of the observed effect and its mechanism taking into account properties of edge states in magnetic fields. The process is understood as the quantum Hall effect associated with electron‐hole pairs generated in Landau levels by THz radiation under CR conditions.

Detection of THz radiation with devices made from wafers with HgTe and InSb quantum wells

In this study we present measurements of the Terahertz (THz) photoconductivity of 2D electron system realized at HgTe∕HgCdTe and AlInSb∕InSb∕AlInSb quantum wells (QWs) in Corbino geometry (inner and outer radius: 500 μm and 1500 μm) with different mobilities and electron densities. To characterize the devices, the Shubnikov‐de Haas (SdH) effect up to magnetic fields B of 7T and current‐voltage (I‐V) characteristics at various magnetic fields were measured. The THz radiation is provided by a p‐Ge laser which operates with a magnetic field and a high voltage for the electrical pumping. The stimulated emission is caused by transistions between Landau levels of light holes [1]. The laser is tunable in the range between 1.7 to 2.5 THz (corresponding to wavelengths between 120 to 180 μm or energies of 7 to 12 meV). The laser is pulsed with a pulse rate of 1 Hz and pulse lengths of about 1 μs with low switching times (about 20 ns). The monochromatic THz radiaton is tranferred to our samples via a 0.32m long brass waveguide immersed in liquid Helium. The detection of a change in the conductivity of the sample due to absorption of THz‐radiation (photoresponse) requires a low‐noise circuit. For the Corbino‐shaped samples the photoresponse (PR) is measured via a resistor RV of 1 kΩ. The signal is transferred via in a high‐frequency cable and detected with a digital oscilloscope.

THz photoresponse of quantum Hall devices based on HgTe‐Quantum wells

This study concerns the experimental investigation of the Terahertz ‐(THz‐) photoresponse in systems under quantum‐Hall‐(QH‐) conditions. These investigations are interesting regarding a potential application of QH‐systems as fast and spectrally sensitive THz‐detectors. The measurements of the THz‐photoresponse (PR) of devices with HgTe quantum wells (QWs) embedded in CdHgTe barriers are aimed at obtaining photosignals at smaller magnetic fields in comparison to detectors made of GaAs/AlGaAs wafers. This can be realized by changing the electron density (application of a gate electrode). The QWs have a thickness of dQW between 7 nm and 12 nm, so that the material HgTe of the QW possesses a semimetallic band structure. We found a cyclotron mass of about mc = 0.026 m0 for our samples from cyclotron resonance measurements (also approximately determined from our PR). As this cyclotron mass is by about a factor 3 smaller than the one of electrons in GaAs, the same Landau level splitting is reached at about 1/3 ...

THz Photoresponse and Magnetotransport of detectors made of HgCdTe/HgTe quantum well structures

In addition to space- and time-resolved measurements of the electrical generation of excited electrons in quantum Hall (QH) devices the dissipation in these devices induced by Terahertz (THz) laser impulses is investigated. For these tasks a THz laser system (p-Ge-Laser [1]) is applied. This laser uses transitions between Landau levels of light holes and emits laser impulses with an adjustable duration from 300 ns to some μs, tunably in the wavelength range 120μm < λ < 180μm. The THz laser radiation is used in order to stimulate charge carriers optically over the Landau-gap. The response of the sample to the laser impulses is in 2D-samples in the QH regime measured in order to receive data of the relaxation of the charge carriers. In this presentation we present photoconduction measurements of the HgTe/HgCdTe-(MCT) – quantum well devices in Hall bar, Corbino as well as combined Corbino- Hall bar geometry in the QH- regime. The materiel system HgTe/HgCdTe is characterized by a small effective mass (compared to GaAs [2], in our case 0.023m0) and accordingly smaller magnetic fields for the appropriate cyclotron energy.

PHOTO-INDUCED QUANTUM HALL CURRENTS IN 2D ELECTRON SYSTEMS

A novel photocurrent effect induced by Terahertz (THz) radiation in two-dimensional (2D) electron systems is reported. It was found that when the system is tuned to electron cyclotron resonance conditions, THz radiation induces a visible signal related to currents flowing in the plane of 2D electron systems without any external bias. This effect is employed as a technique for the study of the integer Quantum Hall samples in the absence of external bias. Our data indicate that at the integer ν the photocurrents are circulating currents in the dissipationless quantum Hall regime.