Field Effect Transistors for Terahertz Detection: Physics and First Imaging Applications

Resonant frequencies of the two-dimensional plasma in FETs increase with the reduction of the channel dimensions and can reach the THz range for sub-micron gate lengths. Nonlinear properties of the electron plasma in the transistor channel can be used for the detection and mixing of THz frequencies. At cryogenic temperatures resonant and gate voltage tunable detection related to plasma waves resonances, is observed. At room temperature, when plasma oscillations are overdamped, the FET can operate as an efficient broadband THz detector. We present the main theoretical and experimental results on THz detection by FETs in the context of their possible application for THz imaging.Comment: 22 pages, 12 figures, review pape

Effect of High-Temperature Annealing on Graphene with Nickel Contacts

Graphene has shown great potential for ultra-high frequency electronics. However, using graphene in electronic devices creates a requirement for electrodes with low contact resistance. Thermal annealing is sometimes used to improve the performance of contact electrodes. However, high-temperature annealing may introduce additional doping or defects to graphene. Moreover, an extensive increase in temperature may damage electrodes by destroying the metal⁻graphene contact. In this work, we studied the effect of high-temperature annealing on graphene and nickel⁻graphene contacts. Annealing was done in the temperature range of 200⁻800 °C and the effect of the annealing temperature was observed by two and four-point probe resistance measurements and by Raman spectroscopy. We observed that the annealing of a graphene sample above 300 °C increased the level of doping, but did not always improve electrical contacts. Above 600 °C, the nickel⁻graphene contact started to degrade, while graphene survived even higher process temperatures

Optical Performance of laser-patterned high-resistivity Silicon Wafer in the frequency range of 0.1-4.7 THz

Direct laser ablation (DLA) is a mask-less technology used for the research and development of optical components of various materials [1]. The relevance of the DLA technology is verified demonstrating the functional optical components including multilevel phase Fresnel lenses on silicon and Soret zone plates developed on a free standing metal-foil [2]. In order to reduce the reflection losses, the anti-reflection structures on a back side of silicon wafer can be patterned by the same DLA technology as this has been proposed recently [3]. In this work we studied optical transmission of laser patterned high resistivity silicon wafers used for development of the diffractive optics in the frequency range of 0.1 – 4.7 THz. The samples were prepared on a 500 µm thick, both-sides polished, high resistivity silicon wafer varying the surrounding environment as well as the DLA parameters in order to modify the composition and roughness of the surface modified. Most of the samples were fabricated in an ambient air, while others were developed in an argon-rich atmosphere at the pressure of 1 atm and 2 atm. Stylus profiler and scanning electron microscope were employed to characterize the samples morphology. Optical performances were studied measuring with a Golay cell detector the transmittance of the THz beam of a quantum cascade laser (QCL) operating at 2.5, 3.1, and 4.7 THz. The dielectric constants dispersion for each sample was also obtained by a THz time domain spectroscopy (TDS). Dependence of transmittance on surface roughness at different THz frequency allowed us to identify the critical value Ra at which the transmittance dropped by 20%. For example data presented in Fig. 1 indicates that the critical Ra value at frequency 4.7 THz is of about 1.9 m. We will discuss a nonlinear dependence of the critical Ra value on the THz radiation frequency. The impact of silicon processing in an oxygen-free environment to the transmittance performance will also be demonstrated and discussed

Laser-processed diffractive lenses for the frequency range of 4.7 THz

The development of diffractive lenses for the upper terahertz (THz) frequency range above 1 THz was successfully demonstrated by employing a direct laser ablation (DLA) technology. Two types of samples such as the Soret Zone plate lens and the multi-level phase-correcting Fresnel lens were fabricated of a metal foil and crystalline silicon, respectively. The focusing performance along the optical axis of a 4.745 THz quantum cascade laser beam with respect to the positioning angle of the sample was studied by using a realtime microbolometric camera. A binary-phase profile sample demonstrated the values of the focusing gain and focused beam size up to 25 dB and 0.15 mm (2.4λ), respectively. The increase of the phase quantization level to eight led to higher (up to 29 dB) focusing gain values without a measurable increase of optical losses. All the samples were tolerant to misalignment as large as 10 deg of oblique incidence with a focusing power drop no larger than 10%. The results pave the way for new applications of industry-ready DLA technology in the entire THz range

Room temperature operation of AlGaN/GaN quantum well infrared photodetectors at a 3–4 µm wavelength range

Experimental results showing room temperature normal incidence mid-infrared detection by AlGaN/GaN quantum well infrared photodetectors are presented. Designed structures have intersubband transitions corresponding to wavelengths in the region of 3 and 4 µm, where strong absorption in a sapphire substrate dominates. The intersubband spectra, therefore, were characterized by electronic Raman scattering and infrared photocurrent spectroscopy. The absorption spectra agree well with theoretical predictions. Details of device fabrication are presented with sensitivity estimates for the devices