Thesis (MScEng (Electrical and Electronic Engineering))--Univerity of Stellenbosch, 2011. / ENGLISH ABSTRACT: The design and simulation of hot electron bolometer sensors for the detection of millimeter and terahertz wave radiation is presented. These devices can be used for the formation of images or for spectroscopy in these frequency ranges. Many molecules resonate and have absorption spectra over these ranges, allowing for the identi cation, non-destructively and at a distance, of the constitution of many di erent materials. The penetrating ability of the radiation makes mm-wave and THz wave detectors ideal for security imaging. The design and simulation of these devices predicts performance as mixers and as interferometers. Manufacturing processes used while fabricating these thin lm devices are outlined and experimental results are reported. / AFRIKAANSE OPSOMMING: Hierdie tesis bespreek die ontwerp en simulasie van warm-elektron bolometers vir die waarneming van millimetergolf- en terahertz-straling. Sodanige toestelle kan in bogenoemde frekwensiebereike gebruik word vir beeldvorming of spektroskopie. Aangesien heelwat molekules binne hierdie bande resoneer, kan die samestelling van 'n verskeidenheid materiale oor 'n afstand en op 'n nie-vernietigende wyse gedenti seer word. Die deurdringingsvermo van millimetergolf- en terahertz-straling maak sulke detektors ideaal vir beeldvorming in veiligheidstoepassings. Met die ontwerp en simulasie van hierdie toestelle word gewys wat hul werkverrigting as mengers en interferometers kan wees. Die prosesse wat gebruik is om hierdie dun lmtoestelle mee te vervaardig word ook bespreek en eksperimentele resultate word aangebied.

Thanks to high Tc of 40 K, high Jc of > 10^7 A.cm^-2, and no weak link behavior across the grain boundary in MgB2 material. This highest Tc among all conventional BCS superconductors, and better material properties of MgB2 compared to high Tc cuprate superconductors makes this material attractive for many applications including, but not limited to, power cables, Josephson junction based electronic devices, SRF cavities, THz sensors and single photon counters. Ultrathin superconducting films are a key element in various detectors utilized in remote sensing over a large part of the entire electromagnetic spectrum. The superconducting hot electron bolometer (HEB) mixer is a crucial detector for high-resolution spectroscopy at THz frequencies. The state-of-the-art NbN phonon-cooled HEB mixers have a relatively narrow (IF) bandwidth ~ 3- 4 GHz as a direct result of the poor acoustic transparency of the film-substrate interface and low sound velocity in NbN reducing the phonon escape time in the film. Alternatively, MgB2 displays a very short τe-ph ~ ps. The phonon escape time is also short due to the high sound velocity in the material (~ 7 Km.s^-2) thus giving rise to a broader IF bandwidth. Also, smaller magnetic penetration depth (λ ≈ 40 nm) of MgB2 makes material of choices for single photon detector application. The response time of an SNSPD is proportional to the square of its magnetic penetration depth λ. Therefore, MgB2 may potentially operate 10-fold faster than the NbN (λ =200 nm) based SNSPD. In this work, I present my effort to fabricate high quality ultrathin superconducting MgB2 films on 6H-SiC (0001) substrates, and study their superconducting and electronic properties. C- epitaxial 10 nm showed Tc of above 36 K, while residual resistivity of up to 26 μΩ.cm was achieved. Critical currents of more than 6 × 10^6 A · cm^−2 at 20 K have been measured for the films with thicknesses iv ranging from 10 to 100 nm. Fishtail structures have been observed in the magnetic field dependence of the critical current density for the thinnest of these films, indicating the presence of defects, which act as vortex pinning centers. From the magnetic field dependence, an average distance between adjacent pinning centers of 35 nm has been obtained for the thinnest films. Ultrathin film as thin as 1.8 nm (6 unit cells) can be achieved by Hybrid Physical-Chemical Vapor Deposition (HPCVD) followed by low angle Ar ion milling. These post processed films exhibit better superconducting properties compared to directly grown films. The 1.8 nm, showed Tc > 28 K and Jc > 10^6 A/cm^2 4 K. The surface roughness of the films was significantly improved and the suppression of Tc from the bulk value is much slower in milled films than in as-grown films. These results show the great potential of these ultrathin films for superconducting devices and present a possibility to explore superconductivity in MgB2 at the 2D limit. Finally, I measured the upper critical field of MgB2 films of various thickness and extracted their thickness dependent in-plane intraband diffusivities by using Gurevich model developed for two-band MgB2 superconductor in dirty limit. Results showed that π band diffusivity (Dπ) decreases rapidly from 71.12 cm^2/s for 100 nm film to 4.6 cm^2/s for 5 nm film where as �� band diffusivity (����) decreases much slower from 2.8 cm^2/s for 100 nm film to 0.8 cm^2/s for 5 nm film. This larger Dπ than ���� indicates the cleaner π band. / Physics

Nous avons développé un récepteur hétérodyne terahertz (THz) compact et ultra-sensible à base de laser à cascade quantique (QCL) comme oscillateur local et de bolomètre à électron chaud (HEB) comme mélangeur. Le récepteur est basé sur un nouveau concept pour le couplage quasi-optique entre l'oscillateur local et le mélangeur ce qui a permis de ne pas utiliser de lame semi-réfléchissante pour la superposition du signal provenant du QCL et du signal à détecter. Le mélangeur utilisé est un HEB en nitrure de niobium avec une antenne planaire formée d’une double hélice log-spiral. Le HEB est monté sur la partie plane d’une lentille convexe en silicium. L’oscillateur local est un QCL que nous avons développé avec un système de contre-réaction répartie du troisième ordre avec une faible dissipation thermique, un faisceau peu divergent et un fonctionnement mono-mode à la fréquence cible de 2.7 THz. Le couplage entre l’oscillateur local et le mélangeur HEB a également été amélioré en couplant le QCL avec une fibre creuse en diélectrique ce qui a permis d’améliorer la directivité du faisceau laser à 55 dBi. Grâce aux précédents résultats, nous avons obtenu un récepteur THz hétérodyne compact qui présente une sensibilité proche de l’état de l’art à 2.7 THz. / We demonstrate an ultra-compact Terahertz (THz) heterodyne detec- tion system based on a quantum cas- cade laser (QCL) as local oscillator and a hot electron bolometer (HEB) for the mixing. It relies on a new opti- cal coupling scheme where the local oscillator signal is coupled through the air side of the planar HEB an- tenna, while the signal to be de- tected is coupled to the HEB through the lens. This technique allows us to suppress the beam splitter usu- ally employed for heterodyne mea- surements. The mixer is a Niobium Nitride HEB with a log-spiral planar antenna on silicon and mounted on the back of a plano-convex silicon lens. We have developed a low power consumption and low beam di- vergence 3rd-order distributed feed- back laser with single mode emis- sion at the target frequency of 2.7 THz to be used as local oscillator for the heterodyne receiver. The cou- pling between the QC laser and the the HEB has been further optimized, using a dielectric hollow waveguide that reliably increases the laser beam directivity up to 55 dBi. Upon the high beam quality, sufficient output power in a single mode at the tar- geted frequency and low power dissi- pation of our local oscillator, we have build an ultra compact THz hetero- dyne receiver with sensitivity close to the state of the art at 2.7 THz.