Calculation of the characteristics of coplanar resonators for kinetic inductance detectors

Photon detectors based on the change of kinetic inductance of a thin superconducting film have a number of applications, particularly in astronomy, owing to their high sensitivity and ease of integration into large arrays. Here we discuss in detail the analysis of kinetic inductance detectors that use thin film microwave coplanar resonators. Photon absorption decreases the electron pair density, increasing the magnetic penetration depth ?, which causes a decrease in the resonant frequency (or phase) of an irradiated resonator. To quantify this effect, we first compute the resonator current distribution, from which the ?-dependent parameters (such as kinetic inductance) are calculated. Optimum responsivity for phase measurement is achieved by using the thinnest film with the narrowest center conductor width at the lowest possible temperature. However, the responsivity is compromised by extrinsic microwave losses, in particular due to residual surface resistance, which is likely to be significant in the thinnest films

Antenna-coupled direct detector for millimetre and submillimetre astronomy based on 2D electron gas in semiconducting heterostructure

The energy resolution of a detector is related to the figure of merit NEP×√τ which is proportional to the heatcapacity of the detector. Hot electron (cold electron) devices have much lower heat capacity than bolometers withsilicon nitride based thermal isolation. Traditional hot electron bolometers (HEB) require sub-micron fabricationfor use at submm wavelengths and it is difficult to simultaneously couple radiation and read out these devices.The 2D electron gas (2DEG) in a semiconductor heterojunction effectively acts as a metal film with a thicknessof a few angstroms and a tunable density and electron mobility. We describe a HEB that uses a 2DEG as anabsorber and present simulations of optical coupling schemes for this type of detector including an antennacoupled to a coplanar waveguide with distributed 2DEG absorbers

Lumped element kinetic inductance detectors

Kinetic Inductance Detectors (KIDs) provide a promising solution to the problem of producing large format arrays of ultra sensitive detectors for astronomy. Traditionally KIDs have been constructed from superconducting quarter-wave resonant elements capacitively coupled to a co-planar feed line [1]. Photon detection is achieved by measuring the change in quasi-particle density caused by the splitting of Cooper pairs in the superconducting resonant element. This change in quasi-particle density alters the kinetic inductance, and hence the resonant frequency of the resonant element. This arrangement requires the quasi-particles generated by photon absorption to be concentrated at positions of high current density in the resonator. This is usually achieved through antenna coupling or quasi-particle trapping. For these detectors to work at wavelengths shorter than around 500 μm where antenna coupling can introduce a significant loss of efficiency, then a direct absorption method needs to be considered. One solution to this problem is the Lumped Element KID (LEKID), which shows no current variation along its length and can be arranged into a photon absorbing area coupled to free space and therefore requiring no antennas or quasi-particle trapping. This paper outlines the relevant microwave theory of a LEKID, along with theoretical and measured performance for these devices

Photoemission studies of the surfactant-aided growth of Ge on Te-terminated Si(100)

The interactions of Ge adatoms with a Si(100) surface terminated by an ordered layer of Te have been studied in detail using XPS, SXPS, STM and LEED. It has been demonstrated that the Te layer has a surfactant action on the growth mode of the Ge in that the two dimensional growth regime is extended to at least 200 Å and the Te is seen to segregate to the growing Ge surface. The surface reconstruction of the Ge layer changes from (1 × 1) in the initial stages to (2 × 2) as growth proceeds and the surface population of Te is reduced. SXPS line shape analysis has indicated that the initial stages of Ge incorporation are characterised by the formation of small islands above those surface Si sites not fully coordinated with Te. Continued growth of such islands is, however, restricted due to their high surface free energy with respect to the surrounding Te-terminated areas. Ge atoms therefore site-exchange with Te atoms in bridge sites, thus becoming incorporated onto the Si lattice and displacing the Te to bridge sites on the growing surface. In this manner islanding is prevented and two-dimensional growth continues beyond the critical thickness. No evidence is seen for any significant incorporation of the Te within the growing Ge layer

Water-gated organic nanowire transistors

We gated both p-type, and n-type, organic nanowire (NW) films with an aqueous electric double layer (EDL) in thin-film transistor (TFT) architectures. For p-type NWs, we used poly(3-hexylthiophene) (P3HT) NWs grown via two different routes. Both can be gated with water, resulting in TFTs with threshold lower than for conventionally cast P3HT films under the same gating conditions. However, TFT drain currents are lower for NWs than for conventional P3HT films, which agrees with similar observations for ‘dry’ gated TFTs. For n-type NWs, we have grown ‘nanobelts’ of poly(benzimidazobenzophenanthroline) (BBL) by a solvent/non-solvent mixing route with later displacement of the solvent, and dispersion in a non-solvent. Water-gating such films initially failed to give an observable drain current. However, BBL nanobelts can be gated with the aprotic solvent acetonitrile, giving high n-type drain currents, which are further increased by adding salt. Remarkably, after first gating BBL NW films with acetonitrile, they can then be gated by water, giving very high drain currents. This behaviour is transient on a timescale of minutes. We believe this observation is caused by a thin protective acetonitrile film remaining on the nanobelt surface