We investigate experimentally transport in gated microsctructures containing a band-inverted HgTe/Hg0.3Cd0.7Te quantum well. Measurements of nonlocal resistances using many contacts prove that in the depletion regime the current is carried by the edge channels, as expected for a two-dimensional topological insulator. However, high and nonquantized values of channel resistances show that the topological protection length (i.e., the distance on which the carriers in helical edge channels propagate without backscattering) is much shorter than the channel length, which is ∼100 μm. The weak temperature dependence of the resistance and the presence of temperature dependent reproducible quasiperiodic resistance fluctuations can be qualitatively explained by the presence of charge puddles in the well, to which the electrons from the edge channels are tunnel-coupled.

This paper reviews recent advances in the design and performance of our original InP- and GaAs-based plasmonic high-electron-mobility transistors (HEMTs) for ultrahighly-sensitive terahertz (THz) sensing and imaging. First, the fundamental theory of plasmonic THz detection is briefly described. Second, single-gate HEMTs with parasitic antennae are introduced as a basic core device structure, and their detection characteristics and sub-THz imaging potentialities are investigated. Third, dual-grating-gate (DGG)-HEMT structures are investigated for broadband highly sensitive detection of THz radiations, and the record sensitivity and the highly-sensitive THz imaging are demonstrated using the InP-based asymmetric DGG-HEMTs. Finally, the obtained results are summarized and future trends are addressed.

We report on non-resonant (broadband) and resonant detection of terahertz radiation using strained-Si modulation doped field effect transistors. The devices were excited at room temperature by two types of terahertz sources (an electronic source based on frequency multipliers at 0.292 THz and a pulsed parametric laser at 1.5 THz). In both cases, a non-resonant response with maxima around the threshold voltage was observed. Shubnikov-de Haas and photoresponse measurements were performed simultaneously and showed a phase-shift of pi/2 in good agreement with the theory, which demonstrates that the observed response is related to the plasma waves oscillation in the channel. The non-resonant features were used to demonstrate the capabilities of such devices in terahertz imaging. We also cooled our device down to 4.2 K to increase the quality factor and resonant detection was observed by using a tunable source of terahertz radiation.

The development of self-assembled nanostructure technologies has recently opened the way towards a wide class of semiconductor integrated devices, with progressively optimized performances and the potential for a widespread range of electronic and photonic applications. Here we report on the development of field effect transistors (FETs) based on semiconductor nanowires (NWs) as highly-sensitive room-temperature plasma-wave broadband terahertz (THz) detectors. The electromagnetic radiation at 0.3 THz is funneled onto a broadband bow-tie antenna, whose lobes are connected to the source and gate FET electrodes. The oscillating electric field experienced by the channel electrons, combined with the charge density modulation by the gate electrode, results in a source-drain signal rectification, which can be read as a DC signal output. We investigated the influence of Se-doping concentration of InAs NWs on the detection performances, reaching responsivity values higher than 100 V W-1, with noise-equivalent-power of similar to 10(-9) W Hz(-1/2). Transmission imaging experiments at 0.3 THz show the good reliability and sensitivity of the devices in a real practical application.

We report on a Terahertz magnetospectroscopy study of a set of five HgTe quantum wells of different thickness, from below to above the critical thickness d(c). In quantizing magnetic fields up to 11 T, both intraband and interband transitions have been observed. In samples with inverted band structures, we confirm the observation of the crossing avoiding of the zero-mode Landau levels at a critical value of the magnetic field. In samples with non-inverted band structures, close to the critical thickness, we report on the square root dependence of the intraband transition energy on the magnetic field, as expected in the single-particle model of massless Dirac fermions. The obtained results are compared with the allowed transition energies calculated using the 8 x 8 Kane model.

Silicon nanotransistors for Terahertz detection

Terahertz plasma oscillations in semiconductor nanostructures: basic physics and applications