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The UK National Quantum Technology Hub in Sensors and Metrology is one of four flagship initiatives in the UK National of Quantum Technology Program. As part of a 20-year vision it translates laboratory demonstrations to deployable practical devices, with game-changing miniaturized components and prototypes that transform the state-of-the-art for quantum sensors and metrology. It brings together experts from the Universities of Birmingham, Glasgow, Nottingham, Southampton, Strathclyde and Sussex, NPL and currently links to over 15 leading international academic institutions and over 70 companies to build the supply chains and routes to market needed to bring 10–1000x improvements in sensing applications. It seeks, and is open to, additional partners for new application development and creates a point of easy open access to the facilities and supply chains that it stimulates or nurtures.

Currently, up to 50% of the channel temperature in AlGaN/GaN electronic devices is due to the thermal-boundary resistance (TBR) associated with the nucleation layer (NL) needed between GaN and SiC substrates for high-quality heteroepitaxy. Using 3-D time-resolved Raman thermography, it is shown that modifying the NL used for GaN on SiC epitaxy from the metal-organic chemical vapor deposition (MOCVD)-grown standard AlN-NL to a hot-wall MOCVD-grown AlN-NL reduces NL TBR by 25%, resulting in ~10% reduction of the operating temperature of AlGaN/GaN HEMTs. Considering the exponential relationship between device lifetime and temperature, lower TBR NLs open new opportunities for improving the reliability of AlGaN/GaN devices.

We report on the development of time-resolved Raman thermography to measure transient temperatures in semiconductor devices with submicrometer spatial resolution. This new technique is illustrated for AlGaN/GaN HFETs and ungated devices grown on SiC and sapphire substrates. A temporal resolution of 200 ns is demonstrated. Temperature changes rapidly within sub-200 ns after switching the devices on or off, followed by a slower change in device temperature with a time constant of ~10 and ~140 mus for AlGaN/GaN devices grown on SiC and sapphire substrates, respectively. Heat diffusion into the device substrate is also demonstrated

Micro-machining of semiconductors is relevant to fabrication challenges within the semiconductor industry. For via holes for solar cells, laser drilling potentially avoids deep plasma etching which requires sophisticated equipment and corrosive, high purity gases. Other applications include backside loading of cold atoms into atom chips and ion traps for quantum physics research, for which holes through the semiconductor substrate are needed. Laser drilling, exploiting the melt ejection material removal mechanism, is used industrially for drilling hard to machine materials such as superalloys. Lasers of the kind used in this work typically form holes with diameters of 100’s of microns and depths of a few millimetres in metals. Laser drilling of semiconductors typically uses short pulses of UV or long wavelength IR to achieve holes as small as 50 microns. A combination of material processes occurs including laser absorption, heating, melting, vaporization with vapour and dust particle ejection and resolidification. An investigation using materials with different fundamental material parameters allows the suitability of any given laser for the processing of semiconductors to be determined. We report results on the characterization of via holes drilled using a 2000 W maximum power 1070 nm fibre laser with 1-20 ms pulses using single crystal silicon, gallium arsenide and sapphire. Holes were characterised in cross-section and plan view. Significantly, relatively long pulses were effective even for wide bandgap substrates which are nominally transparent at 1070 nm. Examination of drilled samples revealed holes had been successfully generated in all materials via melt ejection.

High-pressure photoluminescence (PL) experiments (at 9 K) are reported for GaAs 1 - x N x /GaAs quantum wells having N compositions (x = 0.0025, 0.004) in the dilute regime where the GaAs 1 - x N x alloy conduction band (CB)evolves rapidly by incorporation of N-pair states. Under increasing pressure, the PL spectra exhibit several new N-pair features that derive from CB-resonant states at 1 atm. Two of these features appear strongly at sub-band-gap energies for P ≥ 29 kbar in the x = 0.0025 sample, but are absent for all pressures in the x = 0.004 sample. Several competing PL assignments due to bound-exciton recombination at NN i pairs (i=1-4 is the anion separation) are considered in light of prior findings for N-doped (∼10 1 7 cm - 3 ) GaAs. The absence of certain PL features in the x = 0.004 sample shows that N-pair states mix into the CB-continuum via a selective process, and this selectivity offers an important test for band-structure calculations in dilute GaAs 1 - x N x alloys.

Next-generation optical-communications systems require on-wafer integration of active and passive opto-electronic components to increase operating speed and reduce packaging costs. Increased coupling efficiencies between semiconductor waveguides and optical fibers are of particular interest. A simple and cost-effective method of fabricating a mode-size converter monolithically integrated with a semiconductor waveguide is presented. An on-wafer mode-size converter reduces the number of interfaces in an opto-electronic circuit and improves the coupling efficiency between semiconductor waveguide and optical fiber. Vertically tapered epilayers are deposited in a single epitaxial growth run using shadow-masked growth by chemical-beam epitaxy, avoiding complex and expensive processing and regrowth stages. Waveguides that taper vertically and horizontally over /spl sim/1 mm for gradual expansion of the mode size are demonstrated. Waveguide loss measurements showed that there was negligible loss across the tapered regions. A loss of

Photonic integrated circuits based on III-V semiconductor polarization-maintaining waveguides were designed and fabricated for the first time for application in a compact cold-atom gravimeter1,2 at an operational wavelength of 780 nm. Compared with optical fiber-based components, semiconductor waveguides achieve very compact guiding of optical signals for both passive functions, such as splitting and recombining, and for active functions, such as switching or modulation. Quantum sensors, which have enhanced sensitivity to a physical parameter as a result of their quantum nature, can be made from quantum gases of ultra-cold atoms. A cloud of ultra-cold atoms may start to exhibit quantum-mechanical properties when it is trapped and cooled using laser cooling in a magneto-optical trap, to reach milli-Kelvin temperatures. The work presented here focuses on the design and fabrication of optical devices for a quantum sensor to measure the acceleration of gravity precisely and accurately. In this case the cloud of ultra-cold atoms consists of rubidium (87Rb) atoms and the sensor exploits the hyperfine structure of the D1 transition, from an outer electronic state of 5 2S ½ to 5 2P3/2 which has an energy of 1.589 eV or 780.241 nm. The short wavelength of operation of the devices dictated stringent requirements on the Molecular Beam Epitaxy (MBE) and device fabrication in terms of anisotropy and smoothness of plasma etch processes, cross-wafer uniformities and alignment tolerances. Initial measurements of the optical loss of the polarization-maintaining waveguide, assuming Fresnel reflection losses only at the facets, suggested a loss of 8 dB cm-1, a loss coefficient, α, of 1.9 (±0.3) cm-1.

Nitrogen incorporation in the growth of epitaxial GaNxAs(1−x) by chemical beam epitaxy (CBE) using 1,1-dimethylhydrazine (DMHy) was compared, for the first time, when using each of 2 sources of arsenic. Each of cracked arsine (AsH3) and uncracked tris(dimethylamino) arsine were used with triethylgallium as Group III source, at a growth temperature of 490(±5)°C. Different incorporations were found at high flow pressures of DMHy in the two cases. Nitrogen compositions of fully-strained GaNxAs(1−x) epilayers from 0.01% to 4.2% were measured by X-ray diffractometry (XRD) on the assumption that Vegard's law applies. Higher nitrogen compositions, up to 7(±2)%, were measured by cross-sectional high resolution electron microscopy (HREM) to yield structural information and a measurement of the lattice parameter. Collation of the secondary ion mass spectrometry (SIMS), XRD and HREM data showed that there is good agreement between the nitrogen composition inferred from the lattice parameter and the chemical content, suggesting that the nitrogen is incorporated substitutionally in GaNxAs(1−x) grown under these conditions by CBE. Using trimethylindium and ethyldimethylaminealane with AsH3 and TEGa, the nitrogen incorporations into both 15% InGaAs and, for the first time, into 30% AlGaAs were also measured. Distinctly different nitrogen incorporation efficiencies were observed for the three alloys, particularly at high nitrogen percentages. Unintentional hydrogen incorporation was measured by SIMS for layers grown with AsH3 and found to be at around 10% of the nitrogen level for all three alloys studied.

An integration concept which involves the selective deposition of quantum wells in spatially selected areas of a wafer and in the middle of a waveguide structure is presented for the first time. The selective deposition is achieved by the in situ insertion and removal of a shadow mask during the growth process. The shadow-mask concept, and data on alignment and edge definition are presented. Experimental results on selectively grown quantum wells show that the shadow mask has no effect on the non-radiative carrier lifetime even when only a single quantum well is selectively deposited. The performance of laser diodes fabricated using this novel integration technique is compared with broad area devices grown without a shadow mask.

Time-resolved Raman thermography, with a temporal resolution of , was used to study the thermal dynamics of AlGaN/GaN electronic devices (high-electron mobility transistors and ungated devices). Heat diffusion from the device active region into the substrate and within the devices was studied. Delays in the thermal response with respect to the electrical pulse were determined at different locations in the devices. Quasi-adiabatic heating of the AlGaN/GaN devices is illustrated within the first of device operation. The temperature of devices on SiC was found to reach of the dc temperature when operated with -long electrical pulses.

Metalorganic compounds used as precursors for epitaxial growth by metalorganic vapor phase epitaxy (MOVPE) and chemical beam epitaxy (CBE), may be contaminated with oxygen-containing impurities. These impurities are a particular problem in the precursor purification process when they, or their adducts, are of similar volatility to the precursor. We report that improvements in precursor purity, in this case trimethylindium (TMIn), may be quantitatively assessed through growth at low temperatures by CBE and subsequent two-stage testing. Indium-containing III–V semiconductor test structures were characterized first using secondary ion mass spectrometry (SIMS) and second by photoluminescence (PL) lifetime measurements which sensitively probe the presence of nonradiative centers. A factor of 4 improvement in PL lifetime was found for Epipure™ TMIn as compared with conventional adduct-purified TMIn. The Epipure™ grade of TMIn was used for 980 nm lasers. State-of-the-art threshold currents (162 A cm−2) and low i...

Indium antimonide (InSb) has the highest electron mobility and saturation velocity of any conventional semiconductor, giving potential for a range of analogue and digital ultra-high speed, low power dissipation applications. N-channel quantum well FETs have been fabricated with current gain cut-off frequency (f T ) of more than 250 GHz and power gain cut-off frequency (f max ) of 500 GHz. Outline designs confirm the potential for multi-stage low noise amplifiers operating at more than 200 GHz, for applications such as integrated passive millimetre wave imaging.

Single pole double throw (SPDT) switches are becoming more and more key components in phased-array radar transmit/receive modules. An SPDT switch must be able to handle the output power of a high power amplifier and must provide enough isolation to protect the low noise amplifier in the receive chain when the T/R module is transmitting. Therefore gallium nitride technology seems to become a key technology for high power SPDT switch design. The technology shows good performance on microwave frequencies and is able to handle high power. An X-band SPDT switch, with a linear power handling of over 25 W, has been designed, measured and evaluated. The circuit is designed in the coplanar waveguide AlGaN/GaN technology established at QinetiQ.

This paper employs, for the first time, RF waveform engineering to monitor device degradation over an RF "burn in" period. Measured RF current and voltage waveforms are used to monitor the degradation effects seen in GaN HFET transistors during large signal CW RF stress testing. The technique provides extra information on device performance compared with standard RF performance measures, demonstrating clearly where on the output IV plane the degradation is occurring and allowing device designers advanced insight into the degradation mechanisms limiting RF performance.

This paper uses measured waveforms to demonstrate how to optimise GaN HFET PA designs in order to achieve high power and high efficiency. Efficiency values of 80% were achieved at a power density of 3 Wmm-4. The design procedure shows how waveform engineering, i.e. a combination of RF current and voltage waveform measurements, bias control and active harmonic load-pull, allow maximum performance to be achieved. The significant role of the device output capacitance in GaN designs that utilise large voltage swings is also explained, and a simple method for limiting the effect of Cds 5 is presented.

GaN/AlGaN HEMTs can suffer from short channel effects as a result of insufficient buffer doping. The paper show that controlled iron doping of the GaN buffer during MOVPE growth can suppress all short-channel effects in 0.25mum gate length devices. The authors show that optimised iron doping has no effect on the RF output power or on the knee walkout (current-slump), but significantly improves the power added efficiency

Detailed time-domain IV waveforms at RF frequencies are employed for characterisation of AlGaN/GaN HFETs in order to steer and advance device development. The IV time-domain data is used to isolate the separate effects of pinch-off and knee-walkout behaviour in limiting device performance. Furthermore, the waveform measurements which are obtained with a previously unseen level of detail, allowed the direct extraction of optimum device operating conditions.

This chapter introduces dilute nitride semi conductors as these potential semiconductor device applications stem from the decrease in the fundamental band gap and the associated increase in the effective mass of the electrons. The studies of quaternaries, such as epitaxial GaNAsSb are more common than those of ternary dilute nitrides. The compositional analysis of these alloys is complex, especially given that nitrogen content is challenging to measure quantitatively, moreover, it is difficult to incorporate sufficient nitrogen in additional lattice sites. Therefore, the dilute nitride epi-layer thicknesses employed in dilute nitride devices are limited by critical thickness considerations. The published achievements and future potential for progress in the growth of dilute nitride alloy use the Chemical Beam Epitaxy (CBE) technique and indeed it reproduces responsive control of the semi-conductor fluxes that are essential in the growth of dilute nitride devices. It is clear that significant progress has been made, in spite of the much smaller number of research groups involved in the development of CBE compared with those involved in MBE/MOVPE.

Component alignment slots and low attenuation hollow waveguides formed in silicon substrates by standard DRIE etching techniques provide the optical analogue of the electronic printed circuit board. In conjunction with conventional pick-and-place equipment this new hybrid integration technology will facilitate low cost mass production of compact, rugged, electro-optic microsystems for military platforms. In addition to the integration of discrete components (lasers, modulators, detectors, wave-plates etc) monolithic components are also feasible. These include: (i) the analogues of free space components - plane and curved mirrors, polarization splitters, diffraction gratings, MEMS devices for beam steering etc, and, (ii) hollow waveguide components - tapers for mode matching, proximity couplers, waveguide attenuators, and, a range of multimode interference (MMI) devices. MMI devices are attractive in that they offer a wide range of optical functionality from simple geometric structures, these include intensity and wavelength splitters and mixers. In this context the military efficacy of the technology is highlighted in the demonstration of a novel form of waveguide spectrometer for a new generation of hyperspectral sensors.

We report high-pressure photoluminescence (PL) experiments (to $P=62\mathrm{kbar}$ at 9 K) on ${\mathrm{GaAs}}_{1\ensuremath{-}x}{\mathrm{N}}_{x}/\mathrm{GaAs}$ quantum wells (QWs) having N compositions $(x=0.0025,0.004)$ in the dilute regime where the ${\mathrm{GaAs}}_{1\ensuremath{-}x}{\mathrm{N}}_{x}$ alloy conduction band (CB) evolves rapidly via delocalization of N-pair (cluster) states. Under increasing applied pressure, we observe low-energy broadening of the emission spectra, an increase in the Stokes shift of PL peaks relative to the QW absorption edge, and several new N-pair PL features that derive from CB-resonant states at 1 atm. Two of the latter features (assigned to ${\mathrm{NN}}_{3}$ replica) appear strongly in the $x=0.0025$ sample at energies below the QW absorption edge for $Pg~29\mathrm{kbar},$ but are completely absent in the $x=0.004$ sample---an effect that has not been seen previously in ${\mathrm{GaAs}}_{1\ensuremath{-}x}{\mathrm{N}}_{x}$ alloys to our best knowledge. The trends for broadening and increase in Stokes shift under pressure are accounted for using a model of the recombination kinetics that considers competing fluctuation and N-pair states. The absence of the ${\mathrm{NN}}_{3}$ features in the $x=0.004$ sample provides evidence that N-pair states incorporate into the CB continuum via an energy- and/or state-selective delocalization process. The observed selectivity in the narrow composition range $0.0025l~xl~0.004,$ while bound states and other resonant states closer to the CB edge remain unaffected, offers an important test for band-structure calculations in ${\mathrm{GaAs}}_{1\ensuremath{-}x}{\mathrm{N}}_{x}$ dilute alloys. Selective delocalization of resonant N-pair states is difficult to explain within an impurity-band model, but it is qualitatively consistent with recent theoretical studies of CB formation in ${\mathrm{GaAs}}_{1\ensuremath{-}x}{\mathrm{N}}_{x}$ dilute alloys that use a full-hybridization approach to treat the incorporation of N-pair (cluster) states.

There is significant commercial interest in 1.3 /spl mu/m dilute nitride-based laser devices. Such devices rely on low concentrations of nonradiative centres in the active region in order to achieve the predicted high performances. Unintentional oxygen contamination in alkyl precursors used in the epitaxy is a source of nonradiative centres. An investigation of the Secondary Ion Mass Spectrometry (SIMS) relative sensitivity factor (RSF) for oxygen in Al/sub 0.3/Ga/sub 0.7/N/sub y/As/sub (1-y)/ was made in order to calibrate and compare background oxygen impurities in 3 different precursor batches of dimethylhydrazine (DMHy). A DMHy batch was identified which gave background oxygen concentrations in Al-containing layers below the SIMS detection limit (2-8E16 cm/sup -3/) with 0.2% nitrogen content. Therefore, providing that suitable growth conditions are chosen such that nonradiative centres other than oxygen do not form, this DMHy precursor batch is expected to give In/sub x/Ga/sub (1-x)/N/sub 0.002/As/sub 0.998/ quantum wells with good luminescence properties.

A breakthrough in cold atom quantum technology is hindered by a bottleneck in the supporting technologies. We discuss a cold atom technology platform developed within the European iSense project, aiming at a gravimeter as demonstrator.

This paper describes how dc-radio-frequency (RF) dispersion manifests itself in AlGaN/GaN heterojunction field-effect transistors when the devices are driven into different RF load impedances. The localized nature of the dispersion in the I-V plane, which is confined to the "knee" region, is observed in both RF waveform and pulsed I-V measurements. The effect is fully reproduced using 2-D physical modeling. The difference in dispersive behaviors has been attributed to the geometry of a trap-induced virtual-gate region and the resulting carrier velocity saturation being overcome by punchthrough effects under high electric fields. This paper describes how dc-radio-frequency (RF) dispersion manifests itself in AlGaN/GaN heterojunction field-effect transistors when the devices are driven into different RF load impedances. The localized nature of the dispersion in the I-V plane, which is confined to the "knee" region, is observed in both RF waveform and pulsed I-V measurements. The effect is fully reproduced using 2-D physical modeling. The difference in dispersive behaviors has been attributed to the geometry of a trap-induced virtual-gate region and the resulting carrier velocity saturation being overcome by punchthrough effects under high electric fields.