Lewis Fleming

Diamond-Like-Carbon (DLC) durable anti-reflection optical coatings for the longwave infrared (LWIR 8-12 m) spectral band have been deposited on germanium substrates using two different methods- pulsed DC sputtered hydrogenated amorphous carbon (a-C:H) and Radio-Frequency (13.56 MHz) Plasma Enhanced Chemical Vapour Deposition (RF-PECVD). The work describes characterisation of the DLC coated germanium from the two deposition methods including stress, hardness, Young’s modulus, Raman, X-ray photoelectron spectroscopy, scanning electron 22 microscopy, surface free energy, optical bandgap, surface roughness and electron paramagnetic 23 resonance. Characterisation results have been used to optimise the pulsed DC sputtered hy 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 drogenated carbon to achieve same or improved DLC performance compared with RF-PECVD, specifically in relation to long wave infrared (waveband 8 to 12 m) optical, durability and en vironmental performance. Direct comparison of the DLC material properties of the sputtered and RF-PECVD methods demonstrates that sputtered films of optimised hydrogen flow rates are of comparable hardness, durability and environmental performance to RF-PECVD, both deposition methods passing MIL-C-48497A durability & environmental requirements and MIL C-675C (para 4.5.9) salt spray. Moreover, optimised DLC antireflection coatings deposited by both methods pass 10,000 wipes of the severe abrasion TS1888 para 5.4.3 sand slurry wiper test. A key advantage of sputter deposition compared to RF-PECVD is ability to combine sputtered pulsed DC a-C:H with sputtered germanium as a durable multilayer antireflection coating. The resulting a-C:H/Ge multilayer designs can be deposited in a single vacuum chamber and coating run, resulting in higher transmission (increase of 1.3% aerage transmission) and lower reflection (decrease of 1.2% average reflection) over the 8-12 m waveband compared to traditional single layer RF-PECVD&sputterdeposited designs without compromising durability and environmen tal performance. The improved process control of sputtered a-C:H DLC is described, enabling optimal control of its material properties compared to RF-PECVD, namely stress, hardness and optical absorption, primarily through the control of hydrogen gas flow introduced during depo sition. A further advantage of sputter deposited DLC is lower deposition temperature (typically 70C) high throughput process compared to RF-PECVD (typically 120C), enabling sputter de position of DLC onto temperature sensitive substrates such as chalcogenides, typically requiring deposition temperatures ≤100C

This work is focussed on sputtered lead cadmium selenide (Pb _1−x Cd _x Se) thin films as the active material within high-speed room temperature mid-infrared photovoltaic (PV) detectors for use in non-dispersive infrared gas sensors. Variation of the Cd content provides a means of controlling material bandgap and hence wavelength response. Traditional methods of Pb _1−x Cd _x Se deposition are thermal evaporation or chemical bath techniques which do not provide required stable deposition for accurate control of Cd content and hence wavelength response. This paper describes pulsed direct current (DC) sputter deposition of Pb _1−x Cd _x Se, from a sintered pre-mixed sputter target as an alternative deposition technique. Sputtered Pb _1−x Cd _x Se material characterisation is described identifying optimum Cd content as x = 0.1 for maximised response at a wavelength of 3.3 μm, matching use as a detector for methane gas detection. Work has focussed on characterising pulsed DC sputter deposition of the 3.3 μm optimised Pb _0.9 Cd _0.1 Se (n-type) and post deposition oxygen doping by thermal diffusion to create p-type Pb _0.9 Cd _0.1 Se: O thereby maximising PV detector photo-response at a specific wavelength of 3.3 μm. PV detector design, associated photolithographic mask designs/ lift-off process sequence, sputter deposition of Pb _1−x Cd _x Se and Ti interdigitated electrodes for PV detector fabrication is described. The resulting 1 × 1 mm ² PV detector device exhibited a peak detectivity D* of 1 × 10 ¹⁰ , 4 × 10 ⁹ and 3 × 10 ⁹ cm∙Hz ^1/2 ∙W ⁻¹ at 233 K, 298 K and 313 K respectively under 500 K blackbody illumination.

Here, we discuss the application of a very-narrow bandpass mid-IR optical filter designed to capture the Q-branch of the methane ν₃ absorption band, providing high-sensitivity requirements for NDIR sensing in Digital Dairy Farming applications.

This work presents a study of piezoelectric zinc oxide (ZnO) thin films deposited by a novel post‐reactive sputtering method. The process utilizes a rotating drum with DC magnetron sputtering deposition onto substrates with subsequent DC plasma‐assisted oxidation of the deposited metal to metal oxide. The paper analyzes the influence of plasmaassisted magnetron sputtering (PA‐MS) deposition parameters (O₂ plasma source power, O₂ flow, and Ar flow) on the morphological, structural, optical, and piezoelectric properties of ZnO thin films. Design of experiments has been utilized to evaluate the role of these parameters on the growth rate (r_g) and the properties of resulting films. Results indicate a predominant influence of the plasma power on the r_g over other parameters. Among the eight tested samples, three of them show high crystal quality with high intensity (0001) diffraction peak, characteristic of the wurtzite crystalline structure of ZnO, and one of them exhibits piezoelectric coefficient values of ≈11pC N⁻¹. That sample corresponding to a ZnO film deposited at the lowest r_g of 0.075 nm s⁻¹, confirmed the key role of the deposition parameters on the piezoelectric response of films, and demonstrated PA‐MS as a promising technique to produce high‐quality piezoelectric thin films.

This work describes band-gap engineering of PbCdSe thin films for their use as light emitters in methane gas sensors. Pb _0.9 Cd _0.1 Se thin films were synthesized by pulsed direct current (DC) magnetron sputtering. Optical characterisation of films demonstrated successful emission of light at 3.32 μm. Post-sensitization (PS) - in highly reactive oxygen and iodine environment - was also analysed. Design of Experiments (DoE) was used to optimize photoluminescence (PL) of PbCdSe films as a function of PS conditions. The studies demonstrated a high influence of PS temperature on PL properties. The thickness of the films was also demonstrated to have a significant effect on the enhancement of PL. The analysis of the morphology revealed that recrystallisation of the material was key for the emission of light, probing its applicability as mid-IR light source in non-dispersive IR gas sensors.

This work details the triboelectric characteristics of diamond‐like carbon (DLC) film where a proportioned sp³:sp² bond ratio is engineered through a patented pulsed DC magnetron sputtering process to achieve a durable commercial energy harvesting material. A triboelectric nanogenerator (TENG) is fabricated by creating the triboelectric interface between DLC and PTFE. The presence and synchronization of σ – σ and σ – π bonds between DLC‐PFTE contact surface amplify the electronic cloud overlap between their atoms leading to an enhancement of the triboelectric surface charge density. The inherent hardness and reduced friction achieved through DLC and PTFE respectively prevent the mass transfer, and consequent power loss upon consecutive mechanical contact and achieves a stable electric power output of 141 mW m⁻². The DLC durability achieved with PTFE in TENG demonstrates its significant potential as low frequency (1 – 10 Hz) energy harvesting devices and self‐/low‐power electronic devices and sensors. The paper uniquely contributes to a better understanding of the triboelectrification mechanism by insightfully detailing the band‐to‐band transition of electrons between the PTFE and DLC tribo‐interface, as well as discussing gap and frequency limitation of the tribo‐pair on the triboelectric charge yield, storage, transfer, and on the friction layer electric field.

Fabrication of linear variable filter designs are demonstrated using a high throughput drum with linear sputter targets and static graded uniformity masks. Optical performances are characterised and functionality tested in 3D printed miniature spectrometers.

The authors wish to modify the funding section as follows to this paper

In this work, performance improvements are described for a low-power consumption non-dispersive infrared (NDIR) methane (CH₄) gas sensor using customised optical thin film bandpass filters (BPFs) centered at 3300 nm. BPFs shape the spectral characteristics of the combined mid-infrared III–V based light emitting diode (LED)/photodiode (PD) light source/detector optopair, enhancing the NDIR CH₄ sensor performance. The BPFs, deposited using a novel microwave plasma-assisted pulsed DC sputter deposition process, provide room temperature deposition directly onto the temperature-sensitive PD heterostructure. BPFs comprise germanium (Ge) and niobium pentoxide (Nb₂O₅) alternating high and low refractive index layers, respectively. Two different optical filter designs are progressed with BPF bandwidths (BWs) of 160 and 300 nm. A comparison of the modelled and measured NDIR sensor performance is described, highlighting the maximised signal-to-noise ratio (SNR) and the minimised cross-talk performance benefits. The BPF spectral stability for various environmental temperature and humidity conditions is demonstrated.