9 research outputs found
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
Field effect transistors for terahertz imaging
International audienceResonant frequencies of the two-dimensional plasma in field effect transistors (FETs) increase with the reduction of the channel dimensions and can reach the terahertz (THz) range for micrometer and sub-micrometer channel lengths. Non linearity of the gated electron gas in the transistor channel can be used for the detection of THz radiation. The possibility of tuneable narrow band detection in sub-THz and THz range, related to plasma resonances, has been demonstrated for nanometre gate length transistors at cryogenic temperatures. At room temperatures the plasma oscillations are usually strongly damped, but field effect transistors can still operate as an efficient broadband detectors in the THz range. We present an overview of experimental results on THz detection by field effect transistors made of III-V and Si materials, The material issue is discussed and first room applications of FETs for imaging at frequencies above 1 THz are demonstrated
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