Fast, sensitive avalanche photodiodes (APDs) are required for applications such as high-speed data communications and light detection and ranging (LIDAR) systems. Unfortunately the InP and InAlAs used at the gain material in these APDs have similar electron and hole impact ionisation coefficients (α and β respectively) at high electric fields, giving rise to relatively high excess noise, limiting their sensitivity and gain bandwidth product (GBP) 1. Here, we report on extremely low excess noise in AlAs0.56Sb0.44 lattice matched to InP. The deduced β/α ratio as low as 0.005 in the avalanche region of 1550 nm is close the theoretical minimum and is significantly smaller than Silicon, with modelling suggesting that vertically illuminated APDs with a sensitivity of-25.7 dBm at a bit-error rate (BER) of 1×10-12 at 25 Gb/s at 1550 nm can be realised. The findings could yield a new breed of high performance receivers for applications in networking and sensing. The advantages of using InP based APDs at traditional telecommunication wavelengths of 1310 nm and 1550 nm as a way of increasing sensitivity and speed of communication networks is well documented 2,3,4. Such APDs utilise InGaAs lattice matched to InP substrates as the absorber and an InP or InAlAs gain (or multiplication) region in the Separate Absorption and Multiplication (SAM)-APD configuration. However due to the broadly similar β/α ratio in these materials, particularly at high fields, their sensitivity and GBP are limited, hampering their use as the bit rate increases beyond 10 Gb/s. The best results at 25 Gb/s to 50 Gb/s currently utilise ≤ 100 nm thick InAlAs multiplication regions and these provide a sensitivity of between-22.6 dBm to-10 dBm respectively at a BER of 1×10-125-7. There have been many attempts at improving the performance of APDs for telecommunications, for example utilising Ge/Si 8-10 , nanopillars 11 , AlInAsSb 12-14 or InAs 15 , however these face problems such as limited wavelength operation, often requiring difficult growth procedures, complex fabrication technologies or the use of more expensive substrates. In this letter we demonstrate that the InP lattice matched alloy system AlAs0.56Sb0.44 with its very small β/α ratio 16 leads to extremely low excess noise at room temperature, even at high gains, exceeding the performance of all materials lattice matched to InP and even silicon. This extremely small β/α ratio changes the paradigm whereby high speed APDs always use very thin avalanching structures to one where both high speed and sensitivity can be achieved with thicker avalanching structures. Three AlAs0.56Sb0.44 structures with avalanche region thicknesses of 1550 nm (P1), 660 nm (P2) and 1150 nm (P3) in a PIN configuration and two NIP structures with avalanche region thicknesses of 1550 nm (N1) and 660 nm (N2) were investigated. Fig. 1a shows a schematic diagram of P1 with

A series of strained GaAsBi/GaAs multiple quantum well diodes are characterised to assess the potential of GaAsBi for photovoltaic applications. The devices are compared with strained and strain-balanced InGaAs based devices. The dark currents of the GaAsBi based devices are around 20 times higher than those of the InGaAs based devices. The GaAsBi devices that have undergone significant strain relaxation have dark currents that are a further 10–20 times higher. Quantum efficiency measurements show the GaAsBi devices have a lower energy absorption edge and stronger absorption than the strained InGaAs devices. These measurements also indicate incomplete carrier extraction from the GaAsBi based devices at short circuit, despite the devices having a relatively low background doping. This is attributed to hole trapping within the quantum wells, due to the large valence band offset of GaAsBi

The performance of Al 0.52 In 0.48 P avalanche photodiodes was assessed as soft X-ray detectors at room temperature. The effect of the avalanche gain improved the energy resolution and an energy resolution (FWHM) of 682 eV is reported for 5.9 keV X-rays.

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In myopic eyes, pathological remodelling of collagen in the posterior sclera has mostly been observed ex vivo. Here we report the development of triple-input polarization-sensitive optical coherence tomography (OCT) for measuring posterior scleral birefringence. In guinea pigs and humans, the technique offers superior imaging sensitivities and accuracies than dual-input polarization-sensitive OCT. In 8-week-long studies with young guinea pigs, scleral birefringence was positively correlated with spherical equivalent refractive errors and predicted the onset of myopia. In a cross-sectional study involving adult individuals, scleral birefringence was associated with myopia status and negatively correlated with refractive errors. Triple-input polarization-sensitive OCT may help establish posterior scleral birefringence as a non-invasive biomarker for assessing the progression of myopia.

The experimentally determined impact ionization coefficients, α (β ), include intrinsically the presence of a dead-space, where carriers cannot impact ionize as they do not have sufficient energy. These, therefore, cannot be used by nonlocal ionization models, which require the enabled ionization coefficients, α * (β * ), which describe the ionization probability after the dead-space. A relatively simple relationship is shown to exist between α (β ) and α * (β * ), which requires only the knowledge of the carrier threshold energies. This allows α (β ), conventionally limited to the local model framework, to be used to give a very good prediction of the avalanche multiplication and excess noise for a wide range of device widths down to 0.05 μm, where the dead-space effect is significant. Parameterized values of α (β ) and the carrier threshold energies are listed for a range of commonly used III-V semiconductors lattice matched to GaAs and InP substrates, as well as Si and SiC.

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