A repository & source of cutting edge news about emerging terahertz technology, it's commercialization & innovations in THz devices, quality & process control, medical diagnostics, security, astronomy, communications, applications in graphene, metamaterials, CMOS, compressive sensing, 3d printing, and the Internet of Nanothings. NOTHING POSTED IS INVESTMENT ADVICE! REPOSTED COPYRIGHT IS FOR EDUCATIONAL USE.
Microbolometer is a radiation detector for infrared (IR) and terahertz (THz) waves. The temperature coefficient of resistance (TCR) of the thermistor is a vital factor, as the responsivity is proportional and noise equivalent power (NEP) is inversely proportional to it. The narrow-width effect on TCR and resistivity on two different substrates (SiO2/Si and SiNX/SiO2/Si) for platinum (Pt) and titanium (Ti) thermistor with various design width (DW) = 0.1–5 μm are investigated. Increased resistivity and reduced TCR of the devices with the decreased line width, is observed commonly for both metal and fitted with empirical formulae, which hold well for different substrates. It is evident from electron backscatter diffraction (EBSD) results showing reduced average grain size form Ti film to Ti nanowire (DW = 0.1 μm), that the reduced TCR is not dependent on crystal orientation or phase variation of material but can be correlated with reduced grain size due to reduction of width. The optimum value considering design requirement, thermistor of DW = 0.1 μm and 0.2 μm is used further for the fabrication of microbolometers. It is found that the device with DW = 0.1 μm of Ti thermistor has ∼1.5 times higher electrical responsivity (376 V/W) at maximum allowable current than that with DW = 0.2 μm (254 V/W), which is also ∼11 times higher than device with DW = 0.1 μm of Pt thermistor.
Terahertz (THz) imaging is a powerful tool in many applications such as security systems, materials testing and their identification and medical diagnostics. One of the limitations of these imaging systems at the moment is their physical size. Reducing the size is not only key to integrating the focusing optics and active components on to one chip, but also to making the system cheaper, more reliable and comfortable to use.
There are two 'elements' to the system that are being actively studied worldwide in order to reduce the size: more compact THz emission sources are being developed using quantum cascade lasers; and broadband and sensitive THz sensors are being developed using compact technology such as nanometric field effect transistors, Schottky diodes, microbolometers and bow-tie diodes. Less commonly studied are solutions to replace the bulky passive optical components such as parabolic or spherical mirrors from conventional imaging systems.
All in one
The team from Lithuania is focused on finding solutions to the challenges of THz imaging and spectroscopy, and one of their topics is the development of compact room-temperature operating sensors for real-time THz imaging cameras and sensor arrays for spectroscopic THz imaging.
The focusing features of the compact optics placed on the same chip with the THz sensor.
In their earlier research, they concentrated on the development of compact diffractive optics components for THz imaging systems.
"The first in this direction was the fabrication of free-standing zone plates and cross shape filter arrays, made from thin metal film used for focusing and frequency selection purposes," said Linas Minkevičius, lead author in this work. "The second step – filter array integration into the free standing zone plate – aimed to reduce the number of components and make the system more compact."
In the latest step, presented in this issue of Electronics Letters, the team show how they have further advanced the miniaturisation of THz imaging systems by integrating a room temperature InGaAs-based bow-tie THz diode, that has broadband operation up to 2.5 THz, with the secondary diffractive optics in a single chip.
"We have managed to attach the detector and thediffractive opticson separate sides of the semi-insulating semiconductor substrate," said Minkevičius. "This allowed us to simultaneously make the system more reliable and more compact while improving the sensing properties of InGaAs THz detector."
The measurements made with the detector at different angles of incidence showed an enhancement in the detected signal of an order of magnitude, which agreed with the team's numerical simulations, proving the effectiveness of the focusing performance of the zone plates.
Combined experience
Minkevičius believes that the key factors leading to the success of this work were collaboration, experience and technology.
"There are three important matters: first, the zone plate and detector design were selected by simulating the properties of the electric field distribution via the zone plate and substrate to the detector's apex using the 3D finite difference time domain method; secondly, fruitful collaboration with Prof. H. G. Roskos' group from Goethe University, Germany, and their experience in processing technology, let us overcome difficulties in the detector's manufacturing process; and finally, the most challenging issue was to arrange the zone plate focal spot and the detector active part at one geometrical point from both sides of substrate – this was achieved by employing advantages from our laser direct writing technique."
This solution is not only more compact, but also removes the problems arising from the need for precise optical alignment, as this strongly affects image quality and resolution. Another advantage is that this solution is not restricted to THz sensors of this type; it can be extended to other types of planar technology-based detectors as well.
More information: "On-chip integration of laser-ablated zone plates for detection enhancement of InGaAs bow-tie terahertz detectors" L. Minkevičius, et al. Electronics Letters, Volume 50, Issue 19, 11 September 2014, p. 1367 – 1369 DOI: 10.1049/el.2014.1893
Terahertz waves are utilized for a wide range of applications, from security, medical imaging to gas spectroscopy, etc. Previous investigations in the Teraherzt and Millimeter wave group at Chalmers, has shown that antenna-integrated Y Ba2Cu3O7 (YBCO) bolometer could serve as a potential detector for this range of the electromagnetic spectrum. The detector is composed of a 70 nm thick YBCO _lm with micron sized dimension and is deposited on a crystalline Al2O3 (sapphire) substrate. Phonons, the quasi-particles associated with the lattice vibrations, transport the heat from the _lm to the substrate, but are scattered in this process. This scattering is macroscopically represented by the thermal resistance. The thermal resistance is responsible for the response and speed of these bolometers. Two parameters are varied that we believe could affect this thermal resistance. The first parameter is the thickness of the CeO2 buffer layer. This layer is situated between the YBCO layer and the sapphire substrate. Its purpose is to provide a good lattice match and chemical isolation of the YBCO layer with respect to the substrate. The range studied is 10 - 50 nm. The second parameter is the deposition temperature during the deposition of the film using pulsed laser deposition, this parameter is known to affect the YBCO film quality, however no study has investigated its influence on the thermal properties of the detector. The range studied is 780◦C − 855◦C. The thermal resistance is experimentally studied by fabricating the bolometers with above mentioned parameters and measuring them using DC (IV, Resistance-Power) and RF techniques (voltage response versus modulation frequency). Results are analyzed and compared to reported measurements as well as with the Two Temperature model. Considerable variability is present for all devices, even when fabrication parameters are kept constant. The performance was not improved by either the buffer thickness or the temperature deposition in the studied range. The effective thermal resistance ex- tracted from the DC measurements is found to be situated between 0.1.10−3cm2K/W − 1.0.10−3cm2K/W (excluding outliers). These values are not in accordance with the RF-measurements. The study suggests that additional knowledge on the phenomena involved in the heat transport is required.
We report on noise and thermal conductance measurements taken in order to determine an upper bound on the performance of graphene as a terahertz photon detector. The main mechanism for sensitive terahertz detection in graphene is bolometric heating of the electron system. To study the properties of a device using this mechanism to detect terahertz photons, we perform Johnson noise thermometry measurements on graphene samples. These measurements probe the electron–phonon behavior of graphene on silicon dioxide at low temperatures. Because the electron–phonon coupling is weak in graphene, superconducting contacts with large gap are used to confine the hot electrons and prevent their out-diffusion. We use niobium nitride leads with a
Tc≈10 K to contact the graphene. We find these leads make good ohmic contact with very low contact resistance. Our measurements find an electron–phonon thermal conductance that depends quadratically on temperature above 4 K and is compatible with single terahertz photon detection.
