A high temperature, elemental boron evaporation source has been used for the study of the boron doping behaviour in Si as a function of growth temperature and doping level. Significant profile smearing of boron at doping levels below 5×1018cm−3 is observed. Profile smearing is more severe in higher doped samples for growth temperatures above 600°C at a growth rate of 0.28 nms−1. This is interpreted as arising from the formation of a surface phase of boron at higher doping levels. The marked improvement in profile abruptness at low temperatures suggests significant benefits associated with the use of an elemental boron source for the growth of high resolution Si/Si1−xGex device structures.

We demonstrate the growth, by MBE, of high sheet density B delta layers in both Si and SiGe epitaxial layers. Double Crystal X-Ray Diffraction is shown to be a non-destructive method of characterising the width of very narrow (0.3 nm) delta layers and the sheet density of the activated B. The ability of delta layers to withstand high temperature anneals is considered and it is found that a 750 °C anneal for 1 hour broadens the delta layer to beyond the width required for carrier confinement.

In this paper we address the problem of producing SiGe buffer layers of acceptable quality for the growth of symmetrically strained SiGe structures. Initially we consider SiGe layers grown to well beyond the metastable critical thickness and examine the degree of residual strain both as - grown and post anneal. The defect levels in metastable SiGe layers following high temperature anneal were also studied. A buffer layer was grown consisting of stacked metastable SiGe layers each of which is annealed in situ prior to the growth of the next layer and terminating with a 0.45 SiGe alloy. This produces nearly fully relaxed 1.15pim thick structures with threading dislocation densities of 4 × 106cm−2. Limited area growth on Si suggests that elastically relaxed material free of both threading and misfit dislocations can be produced.

Delta doping in MBE has attracted considerable attention since its inception in 1980 [1], as a vehicle for studies of 2D phenomena and for its potential applications in, for example, FET devices [2] and I.R. detectors [3]. Boron delta doping in silicon is a recent development [4]. In previous papers we have reported on the growth, TEM, CV profiling, SIMS profiling and XRD characterisation of B delta layers [4,5,6]. It is of interest to know if the present growth method [4] produces a dopant sheet a monolayer wide or, in any event, what is the true width. Previous techniques [4,5,6] have yielded values between 0.4nm and 3nm for the width. Schottky barrier tunnelling spectroscopy provides further information on this question, but is also of interest for a study of the electronic structure of the delta layer. It has been used previously for a study of delta doping in III-V materials [7] and also for Sb delta doping in Si [8,9], but there have so far been no reports of its application to Si:B delta layers.

The performance of many Si/SiGe devices, particularly those involving modulation doped quantum wells, will depend on the quality of the matrix and doping interfaces involved. These may be adversely affected by profile smearing of Ge and the dopant. A study of boron incorporation in SiGe, as a function of substrate temperature and Ge fraction, shows a marked difference in profile smearing for boron in Si and in the SiGe alloy. This is shown to be associated with a reduction in the temperature for transition from equilibrium to kinetically limited accumulation in the alloy.

A dedicated, grower-friendly MBE computer control system is described, and its performance in demonstrated in growth of high resolution doped SiGe structures.

We describe the optical characterisation of two silicon cold-electron bolometers each consisting of a small (\(32\,\times \,14~\mathrm {\upmu m}\)) island of degenerately doped silicon with superconducting aluminium contacts. Radiation is coupled into the silicon absorber with a twin-slot antenna designed to couple to 160-GHz radiation through a silicon lens. The first device has a highly doped silicon absorber, the second has a highly doped strained-silicon absorber. Using a novel method of cross-correlating the outputs from two parallel amplifiers, we measure noise-equivalent powers of \(3.0 \times 10^{-16}\) and \(6.6 \times 10^{-17}~\mathrm {W\,Hz^{{-1}/{2}}}\) for the control and strained device, respectively, when observing radiation from a 77-K source. In the case of the strained device, the noise-equivalent power is limited by the photon noise.

We present a study of a cold-electron bolometer operating at 350 mK with a twin-slot antenna coupling radiation at 160 GHz. The detector's absorbing element consists of degenerately-doped strained silicon and has Schottky contacts to superconducting aluminium leads. These contacts allow for direct electron cooling of the absorber to below the phonon temperature, enabling the cold-electron bolometer to achieve much faster time constants (τ <; 1 μs) compared to conventional bolometric detectors while not sacrificing sensitivity. We measure both the dark and optically-loaded noise of the detector via a novel method of cross-correlating the outputs of two amplifiers in order to measure noise below the amplifier noise level. From this we measure the photon-noise limited noise-equivalent power of the detector to be 6.6 × 10-17 W Hz-1/2 when observing a 77-Kelvin source.

This paper addresses a high-performance electron-tunneling cooler based on a novel heavy-fermion/insulator/superconductor junction for millikelvin cooling applications. We show that the cooling performance of an electronic tunneling refrigerator could be significantly improved using a heavy-fermion metal to replace the normal metal in a conventional normal metal/insulator/superconductor junction. The calculation, based on typical parameters, indicates that, for a bath temperature of 300 mK, the minimum cooling temperature of an electron tunneling refrigerator is reduced from around 170 mK to below 50 mK if a heavy-fermion metal is employed in place of the normal metal. The improved cooling is attributed to an enhancement in electron tunneling due to the existence of a resonant density of states at the Fermi level.

We demonstrate electron cooling in silicon using platinum silicide as a superconductor contact to selectively remove the highest energy electrons. The superconducting critical temperature of bulk PtSi is reduced from around 1 K to 0.79 K using a thin film (10 nm) of PtSi, which enhances cooling performance at lower temperatures and enables electron cooling to be demonstrated from 100 mK to 50 mK.

The control of electronic and thermal transport through material interfaces is crucial for numerous micro and nanoelectronics applications and quantum devices. Here we report on the engineering of the electro-thermal properties of semiconductor-superconductor (Sm-S) electronic cooler junctions by a nanoscale insulating tunnel barrier introduced between the Sm and S electrodes. Unexpectedly, such an interface barrier does not increase the junction resistance but strongly reduces the detrimental sub-gap leakage current. These features are key to achieving high cooling power tunnel junction refrigerators, and we demonstrate unparalleled performance in silicon-based Sm-S electron cooler devices with orders of magnitudes improvement in the cooling power in comparison to previous works. By adapting the junctions in strain-engineered silicon coolers we also demonstrate efficient electron temperature reduction from 300 mK to below 100 mK. Investigations on junctions with different interface quality indicate that the previously unexplained sub-gap leakage current is strongly influenced by the Sm-S interface states. These states often dictate the junction electrical resistance through the well-known Fermi level pinning effect and, therefore, superconductivity could be generally used to probe and optimize metal-semiconductor contact behaviour.

We describe optical characterisation of a Strained Silicon Cold Electron Bolometer (CEB), operating on a $350~\mathrm{mK}$ stage, designed for absorption of millimetre-wave radiation. The silicon Cold Electron Bolometer utilises Schottky contacts between a superconductor and an n++ doped silicon island to detect changes in the temperature of the charge carriers in the silicon, due to variations in absorbed radiation. By using strained silicon as the absorber, we decrease the electron-phonon coupling in the device and increase the responsivity to incoming power. The strained silicon absorber is coupled to a planar aluminium twin-slot antenna designed to couple to $160~\mathrm{GHz}$ and that serves as the superconducting contacts. From the measured optical responsivity and spectral response, we calculate a maximum optical efficiency of $50~\%$ for radiation coupled into the device by the planar antenna and an overall noise equivalent power (NEP), referred to absorbed optical power, of $1.1 \times 10^{-16}~\mathrm{\mbox{W Hz}^{-1/2}}$ when the detector is observing a $300~\mathrm{K}$ source through a $4~\mathrm{K}$ throughput limiting aperture. Even though this optical system is not optimised we measure a system noise equivalent temperature difference (NETD) of $6~\mathrm{\mbox{mK Hz}^{-1/2}}$. We measure the noise of the device using a cross-correlation of time stream data measured simultaneously with two junction field-effect transistor (JFET) amplifiers, with a base correlated noise level of $300~\mathrm{\mbox{pV Hz}^{-1/2}}$ and find that the total noise is consistent with a combination of photon noise, current shot noise and electron-phonon thermal noise.

This chapter discusses the cooling of a platform, which requires the electronic coolers to extract heat by coupling to phonons within the platform material. Major results obtained within the nanofunction NoE on the development of nanomodulated magnetic materials and the investigation of their main properties are also presented. The cooling power of the devices becomes paramount, as opposed to the base temperature that could be reached, and must exceed heat leaks into the platform from the surroundings. This indirect cooling is desirable for systems where electrical isolation from the refrigeration elements is required, such as in quantum information applications or superconducting transition edge sensors (TESs). Thick porous Si layers on the Si wafer constitute alternative structures that could replace the rather fragile silicon nitride membranes for use as thermal isolation platforms. The structure and morphology of porous Si determines its electrical and thermal conductivity.

A thin, flat, and single crystal germanium membrane would be an ideal platform on which to mount sensors or integrate photonic and electronic devices, using standard silicon processing technology. We present a fabrication technique compatible with integrated-circuit wafer scale processing to produce membranes of thickness between 60 nm and 800 nm, with large areas of up to 3.5 mm2. We show how the optical properties change with thickness, including appearance of Fabry-Pérot type interference in thin membranes. The membranes have low Q-factors, which allow the platforms to counteract distortion during agitation and movement. Finally, we report on the physical characteristics showing sub-nm roughness and a homogenous strain profile throughout the freestanding layer, making the single crystal Ge membrane an excellent platform for further epitaxial growth or deposition of materials.

A thin, flat and single crystal membrane on which to mount sensors is generally required for integration with electronics through standard silicon processing technology. We present an approach to producing single crystal membranes of germanium with in-built tensile strain, which serves to keep the membrane flat and ripple free, and demonstrate a 600nm thick, free-standing 1mm2 Ge membrane. We convert the fabrication technique into an integrated-circuit compatible wafer scale process to produce 60nm thin membranes with large areas of 3.5mm2. The single crystal Ge membrane provides an excellent platform for further epitaxial growth or deposition of materials.

The hole-phonon energy loss rate in silicon is measured at phonon temperatures ranging from 300 mK to 700 mK. We demonstrate that it is approximately an order of magnitude higher than the corresponding electron-phonon energy loss rate over an identical temperature range. (C) 2014 Elsevier Ltd. All rights reserved.

The impact of the O2 content in SF6-O2 gas mixtures on the etch rate and sidewall profile of silicon (Si), germanium (Ge), and phosphorous doped germanium (Ge:P) in reactive ion etching has been studied. The characteristics of etch rate and sidewall profile are greatly affected by the O2 content. Below 50% of O2 content, a large variation in Ge etch rates is found compared to that of Si, but for O2 content above 50% the etch rates follow relatively the same trend. Lightly doped Ge shows the highest etch rate at a O2 concentration up to 20%. Sidewall angles range from a minimum of 80° to a maximum of 166°, with O2 concentration of 20% yielding perfect anisotropic mesa etch. Also at this O2 concentration, reasonable Si/Ge selectivity is possible. These observations indicate that by adjusting the O2 concentration, precision plasma etching of Si, Ge, and Ge:P is possible.

A silicon-superconductor tunnel junction is capable of cooling electrons from a temperature of 300 mK to 150 mK and below when a current is passed through it and may also be used as the thermometer in a silicon “cold electron bolometer”. Recent work on these novel devices is described here.