We report on the resonant, voltage tunable emission of terahertz radiation ͑0.4 -1.0 THz͒ from a gated two-dimensional electron gas in a 60 nm InGaAs high electron mobility transistor. The emission is interpreted as resulting from a current driven plasma instability leading to oscillations in the transistor channel ͑Dyakonov-Shur instability͒.Plasma waves in a gated two-dimensional electron gas have a linear dispersion law, similar to that of sound waves. The transistor channel acts as a resonator cavity for plasma waves that can reach THz frequencies for a sufficiently short ͑nanometer-sized͒ field effect transistor. 1 As was predicted in Ref. 2, when a current flows through a field effect transistor, the steady state can become unstable against the generation of plasma waves ͑Dyakonov-Shur instability͒ leading to the emission of an electromagnetic radiation at plasma wave frequencies. The emission is predicted to have thresholdlike behavior. It is expected to appear abruptly after the device current exceeds a certain threshold value for which the increment of the plasma wave amplitude exceeds losses related to electron collisions with impurities and/or lattice vibrations.The excitation of plasma waves in a field effect transistor channel can be also used for the detection of terahertz radiation. 3 Recent reports demonstrated a resonant 4 detection in GaAs-based high electron mobility transistors ͑HEMTs͒ and in gated double quantum well heterostructures. 5 This is the first report of resonant THz emission by plasma generation. The terahertz emission ͑0.4 -1.0 THz͒ was obtained by using an InGaAs HEMT with a 60-nm-long gate. We show that the results can be interpreted assuming that the emission is caused by the current driven plasma instability leading to terahertz oscillations in the channel through Dyakonov-Shur instability.Lattice-matched InGaAs/AlInAs HEMTs grown by molecular beam epitaxy on an InP substrate were used in this study. The active layers consisted of a 200 nm In 0.52 Al 0.48 As buffer, a 20 nm In 0.53 Ga 0.47 As channel, a 5-nm-thick undoped In 0.52 Al 0.48 As spacer, a silicon planar doping layer of 5ϫ10 12 cm Ϫ2 , a 12-nm-thick In 0.52 Al 0.48 As barrier layer, and, finally, a 10-nm-silicon-doped In 0.53 Ga 0.47 As cap layer. Details of the technological process are given elsewhere. 6 The gate length was 60 nm, and the drain-source separation was 1.3 m. An InP-based HEMT was chosen for its high InGaAs channel mobility and high sheet carrier density.Output and transfer characteristics are shown in Fig. 1. The low field, linear output region is marked by the dotted line. The deviation of the I d (U sd ) curve from linear behavior indicates the beginning of the saturation region. The arrow indicates the emission threshold voltage, U sd ϳ200 mV at I d ϳ4.5 mA. The horizontal dashed line shows the level of the current saturation (I d ϳ4.8 mA). The I d (U sd ) characteristic shows an unstable behavior for U sd higher than 300 mV. This well-known phenomenon is related to a self-excitation a͒ Also at

Combining Scanning Gate Microscopy (SGM) experiments and simulations, we demonstrate low temperature imaging of electron probability density |Ψ| 2 (x, y) in embedded mesoscopic quantum rings (QRs). The tip-induced conductance modulations share the same temperature dependence as the Aharonov-Bohm effect, indicating that they originate from electron wavefunction interferences. Simulations of both |Ψ| 2 (x, y) and SGM conductance maps reproduce the main experimental observations and link fringes in SGM images to |Ψ| 2 (x, y). Thanks to the scanning tunnelling microscope (STM), remarkable precision has been achieved in the local scale imaging of surface electron systems. Only a few years after the STM invention, electron interferences could be visualized in real space inside artificially confined surface structures, the "quantum corrals" [1]. However, since they rely on the measurement of a current between a tip and the sample, STM techniques are useless when the system of interest is buried under an insulating layer, as in two-dimensional electron gases (2DEGs) confined in semiconductor heterostructures. To circumvent the obstacle, a new method was developed: the Scanning Gate Microscopy (SGM). SGM consists in mapping the conductance of the system as the polarized tip, acting as a flying nano-gate, scans at a constant distance above the 2DEG. SGM gave many valuable insights into the physics of quantum point contacts (QPCs) [7].[In some cases, the mechanism of SGM image formation is readily understandable. For example, in the vicinity of a QPC [2], coherent electron flow is imaged due to multiple reflections and interferences of electrons bouncing between the QPC and the tip-induced depleted region. In comparison, the situation seems more complex when the tip scans directly over an open mesoscopic billiard [6]: the tip perturbation extends over the whole system of interest, so that all semi-classical trajectories are modified. The mechanisms that link conductance maps to the properties of unperturbed electrons still need to be clarified. Recently, we showed that SGM images in the vicinity of a QR allow direct observation of iso-phase lines for electrons in an electrostatic Aharonov-Bohm (AB) experiment [8].In this Letter, we discuss SGM images obtained as the tip scans directly over coherent quantum rings (QRs). Experimentally, we find that the amplitude of conductance modulations shares a common temperature dependence with the Aharonov-Bohm effect, a direct evidence that SGM probes the quantum nature of electrons. On the other hand, we perform quantum mechanical simulations of SGM experiments. First, the amplitude of conductance fringes is found to evolve linearly at low perturbation amplitude, both in experiments and simulations. Second, we observe a direct correspondence between simulated SGM data and simulations of the electron probability density |Ψ| 2 (x, y, E F ). We deduce that, in this linear regime, SGM reliably maps |Ψ| 2 (x, y, E F ) in coherent QRs.We fabricated two QRs, samples R1 and R2, from an InGa...

The authors report on detection of terahertz radiation by high electron mobility nanometer InGaAs∕AlInAs transistors. The photovoltaic type of response was observed at the 1.8–3.1THz frequency range, which is far above the cutoff frequency of the transistors. The experiments were performed in the temperature range from 10to80K. The resonant response was observed and was found to be tunable by the gate voltage. The resonances were interpreted as plasma wave excitations in the gated two-dimensional electron gas. The minimum noise equivalent power was estimated, showing possible application of these transistors in sensing of terahertz radiation.

Terahertz emission from InGaAs/ InAlAs lattice-matched high electron mobility transistors was observed. The emission appears in a threshold-like manner when the applied drain-to-source voltage U DS is larger than a threshold value U TH. The spectrum of the emitted signal consists of two maxima. The spectral position of the lower-frequency maximum ͑around 1 THz͒ is sensitive to U DS and U GS , while that of the higher frequency one ͑around 5 THz͒ is not. The lower-frequency maximum is interpreted as resulting from the Dyakonov-Shur instability of the gated two-dimensional electron fluid, while the higher frequency is supposed to result from current-driven plasma instability in the ungated part of the channel. The experimental results are confirmed by and discussed within Monte Carlo calculations of the high-frequency current noise spectra.

T raditionally, the understanding of quantum transport, coherent and ballistic 1 , relies on the measurement of macroscopic properties such as the conductance. Although powerful when coupled to statistical theories, this approach cannot provide a detailed image of 'how electrons behave down there'. Ideally, understanding transport at the nanoscale would require tracking each electron inside the nanodevice. Significant progress towards this goal was obtained by combining scanning probe microscopy with transport measurements 2-7 . Some studies even showed signatures of quantum transport in the surroundings of nanostructures 4-6 . Here, scanning probe microscopy is used to probe electron propagation inside an open quantum ring exhibiting the archetype of electronwave interference phenomena: the Aharonov-Bohm effect 8 . Conductance maps recorded while scanning the biased tip of a cryogenic atomic force microscope above the quantum ring show that the propagation of electrons, both coherent and ballistic, can be investigated in situ, and can even be controlled by tuning the potential felt by electrons at the nanoscale.An open quantum ring (QR) in the coherent regime of transport is a good example of an interferometer: its conductance peaks when electron waves interfere constructively at the output contact and decreases to a minimum for destructive interferences. Varying either the magnetic flux encircled by the QR or the electrostatic potential in one arm allows the interference to be tuned. This gives rise to the well-known magnetic 9 and electrostatic 10,11 Aharonov-Bohm (AB) oscillations. Although these effects have been studied extensively through transport measurements, those techniques lack the spatial resolution necessary to probe interferences in the interior of QRs. In this work, we perturb the propagation of electrons through a QR with an atomic force microscope (AFM) tip. We therefore take advantage of both the imaging capabilities of the AFM and the high sensitivity of the conductance measurement to electron phase changes.A three-dimensional image of the QR used here, as measured by our AFM in the conventional topographical mode, is shown in Fig. 1a. The QR is fabricated from an InGaAs/InAlAs heterostructure hosting a two-dimensional electron system (2DES) with a sheet density of 2 × 10 16 m −2 , buried 25 nm below the sample surface 12 . Electron-beam lithography and wet etching were used to pattern the QR and interconnections. At the experimental temperature (4.2 K), the QR is smaller than the intrinsic electron mean free path measured in the 2DES (l μ = 2.3 μm). Transport is thus in the ballistic regime, with electrons travelling along 'billiardball'-like trajectories. Moreover, the observation of periodic AB oscillations (Fig. 1b, inset) in the magnetoconductance of our QR attests that transport is also in the coherent regime 13 . The periodicity of these oscillations is found to be 26 mT, consistent with the average radius of circular electron trajectories in the QR: r = 220 nm.The metallized tip of t...

Room-temperature generation of terahertz radiation in nanometer gate length InAlAs∕InGaAs and AlGaN∕GaN high-mobility transistors is reported. A well-defined source-drain voltage threshold for the emission exists, which depends on the gate bias. Spectral analysis of the emitted radiation is presented. The highest emission power emitted from a single device reached 0.1μW.

By using a semi-classical two-dimensional Monte Carlo simulation, simple devices (T-branch junctions (TBJs) and rectifying diodes) based on AlInAs/InGaAs ballistic channels are analysed. Initially, the model is validated by means of Hall-effect measurements of mobility and electron concentration in long (diffusive) channels. Then, quasi-ballistic transport at room temperature is confirmed in a 100 nm channel. Our simulations qualitatively reproduce the experimental results of electric potential measured in a TBJ appearing as a result of electron ballistic transport, and in close relation with the presence of space charge inside the structure. As examples of devices exploiting the ballistic transport of electrons, preliminary simulations of a multiplexor/demultiplexor and a rectifying diode are presented, demonstrating their capability for terahertz operation.

By using a semi-classical two-dimensional (2-D) Monte Carlo simulation, simple ballistic devices based on AlInAs/InGaAs channels are analyzed. Our simulations qualitatively reproduce the experimental results in T-and Y-branch junctions as well as in a ballistic rectifier appearing as a result of electron ballistic transport. We show that a quantum description of electron transport is not essential for the physical explanation of these results since phase coherence plays no significant role. On the contrary, its origin can be purely classical: the presence of classical electron transport and space charge inside the structures.