Fundamental and Applied Problems of Terahertz Devices and Technologies

The following sections are included:

• Preface
• Contents

Compact and fast detectors, for imaging and wireless communication applications, require efficient rectification of electromagnetic radiation with frequencies approaching 1 THz and modulation bandwidth up to a few tens of GHz. This can be obtained only by using a mature technology allowing monolithic integration of detectors with low-noise amplifiers. One of the best candidates is indium phosphide bipolar transistor (InP HBT) technology. In this work, we report on room temperature high sensitivity terahertz detection by InP double-heterojunction bipolar transistors (DHBTs) operating in a large frequency range (0.25–3.1 THz). The performances of the DHBTs as terahertz sensors for communications were evaluated showing the modulation bandwidth of investigated DHBTs close to 10 GHz.

THz response of AlGaAs/InGaAs/GaAs HEMT structure has been investigated. The structure consists of the serpentine chain of series connected HEMTs. The source of one is the drain for the subsequent transistor. Experiments have been showed THz response peculiarities of such structures and enhanced noise equivalent power.

TeraFETs are THz power detectors based on field-effect transistors (FETs) integrated with antennas. The first part of this paper discusses the design of Si CMOS TeraFETs leading to an optimized noise-equivalent power close to the room-temperature limit. The impact of the choice of the gate width and gate length, the role of the parasitic effects associated with the technology node, and the conjugate matching of antenna and FET impedance – which is possible over narrow THz frequency bands because of the frequency dependence of the channel impedance resulting from plasmonic effects – are highlighted. Taking these aspects into account, we implement narrow-band detectors of two different designs. Using a 90-nm and a 65-nm CMOS technology, we reach a room-temperature cross-sectional NEP of 10 pW/√Hz at 0.63 THz. We then explore the optimization of AlGaN/GaN TeraFETs equipped with broadband antennae. A room-temperature optical NEP of 26 pW/√Hz is achieved around 0.5 THz despite the fact that the existence of pronounced ungated regions leads to a significant hot-electron thermoelectric DC voltage reducing the rectified signal. AlGaN/GaN TeraFETs become competitive and they have the added advantage that they are extraordinarily robust against electrostatic shock even without inclusion of protection diodes into the design.

A CMOS cascode amplifier, biased near the threshold voltage of a MOSFET, for terahertz direct detection is proposed. A CMOS terahertz imaging circuit (size: 250 × 180 μm) is designed and fabricated on the basis of low-cost 180-nm CMOS process technology. The imaging circuit consists of a microstrip patch antenna, an impedance-matching circuit, and a direct detector. It achieves a responsivity of 51.9 kV/W at 0.915 THz and a noise equivalent power (NEP) of 358 pW/Hz^½ at a modulation frequency of 31 Hz. NEP is estimated to be reduced to 42 pW/Hz^½ at 100 kHz. These results suggest that cost-efficient terahertz imaging is possible in the near future.

We predict the instability of plasma waves excited by a DC current in the field-effect transistors (FETs) with a partly gated channel. The excitation of plasma waves is due to amplified reflection from the boundary of the gated and ungated regions. The boundary also supports the turbulent edge modes whose increment strongly depends on the ratio of the carrier densities in the two FET regions.

In 2003, we demonstrated a non-destructive terahertz spectroscopic imaging of illicit drugs hidden in envelopes using a widely tunable THz-wave parametric source, though its dynamic range at that time was less than four orders. Recently, we have realized more than nine orders of dynamic range with an evolved injection seeded THz-wave spectrometer. Now we can detect drugs under much thicker obstacles than before. In this report, we introduce the result of THz spectroscopic imaging of chemicals under thick envelopes and related topics; Enhanced tuning range of is-TPG up to 5 THz and >90 dB dynamic range THz spectroscopic system.

An overview of main results concerning THz detection related to plasma nonlinearities in nanometer field effect transistors is presented. In particular the physical limits of the responsivity, speed and the dynamic range of these detectors are discussed. As a conclusion, we will present applications of the FET THz detectors for construction of focal plane arrays. These arrays, together with in purpose developed diffractive 3D printed optics lead to construction of the demonstrators of the fast postal security imagers and nondestructive industrial quality control systems. We will show also first results of FET based imaging that uses for contrast not only usual THz radiation amplitude, but also the degree of its circular polarization. Sub-THz high resolution gas spectroscopy is shown to be a powerful means to diagnose various diseases via exhaled breath analysis.

In this work we calculate and analyze modal gain for terahertz lasers based on HgCdTe heterostructures with quantum wells (QWs) taking into account the symmetry-enforced light hole-heavy hole mixing at the quantum well interfaces. We have found that modal gain for a structure with 5 HgTe QWs of the 5.2 nm width can be 33 cm⁻¹ at 9 THz.

This paper reviews the plasmonic effects in graphene THz photodetectors (PD) and light emitters (LE). It is demonstrated that the devices based on double graphene-layer (DGL) or multiple graphene-layer structures with the graphene layers separated by thin tunnel barrier layers have advantages over the single graphene-layer (SGL) devices. In DGLs, this advantage is due to the photon-assisted resonant tunneling when the band offset of the graphene layers is aligned to the THz photon energy. The resonant emission or absorption of the THz radiation is enhanced by the cooperative resonant excitation of the graphene plasmons leading to an extremely high gain and/or responsivity in the graphene THz device structures.

We report on room temperature non-resonant detection of terahertz radiation using strained Silicon MODFETs with nanoscale gate lengths. The devices were excited at room temperature by an electronic source at 150 and 300 GHz. A maximum intensity of the photoresponse signal was observed around the threshold voltage. Results from numerical simulations based on synopsys TCAD are in agreement with experimental ones. The NEP and Responsivity were calculated from the photoreponse signal obtained experimentally. Those values are competitive with the commercial ones. A maximum of photoresponse was obtained (for all devices) when the polarization of the incident terahertz radiations was in parallel with the fingers of the gate pads. For applications, the device was used as a sensor within a terahertz imaging system and its ability for inspection of hidden objects was demonstrated.

Charge-sensitive infrared phototransistor (CSIP) is a highly sensitive semiconductor terahertz (THz) detector with single-photon sensitivity. Due to the excitation mechanism via intersubband transition in a quantum well, the CSIP requires careful design of the photo-coupler and proper light illumination method to achieve high quantum efficiency. We have improved the quantum efficiency by introducing the radiation from the backside of the CSIP substrate, which leads to efficient surface plasmon excitation in the photo-coupler.

We have designed GaAs/Al_0.15Ga_0.85As multilayer heterostructure (MH) with diagonal transitions and optimized oscillator strength – 0.425. We investigate the dependence of the MH energy band profile on the electric field and thermal properties of terahertz quantum cascade lasers (THz QCL) under different operation conditions. Furthermore, we develop a technique for the fabrication of THz QCL with double metal waveguide via low-temperature In-Au wafer bonding followed by substrate removal. Inductively coupled plasma reactive ion etching in BCl₃/Ar at 15:15 sccm has been used to obtain ridge structure of various widths with vertical sidewalls.

We have proposed and investigated In_yGa_1-yAs photoconductor grown by molecular-beam epitaxy on low-temperature step-graded metamorphic buffer. It exhibits superior bandwidth up to 6 THz and provides optical-to-terahertz conversion efficiency up to ∼ 10⁻⁵ for rather low optical fluence ∼ 40 μJ/cm². The intensity of THz generation for the given structure is two orders higher than for low-temperature grown GaAs due substantial contribution of photo-Dember effect.

We numerically analyze the system based on the essentially non-oscillatory shock capturing scheme in order to characterize the Dyakonov-Shur (DS) instability in a gated two-dimensional electron gas system (2DES). The predictions of the linearized model are examined for a 2DES sandwiched by the top and back metallic gates. By solving Poisson equation self-consistently, the dispersive properties of plasma wave are properly estimated. Special attention is paid to the impact of dispersion to nonlinear dynamics of plasma-wave oscillation. A single-gated 2DES is also investigated for demonstrating the DS instability in practical devices.

The following section is included:

• Author Index