Aoife M. Moloney

Simulated monolithically integrated single photon counter

Simulations of a Geiger-mode avalanche photodiode (GMAP) monolithically integrated with its corresponding active quench and reset circuit (AQRC) are presented. The AQRC design is simulated in Agilents ADS simulator using BSIM3 MOSFET models acquired from Europractices 0.6 /spl mu/m CMOS process. The novelty of the design ensures a minimum dead-time of 13 ns between adjacent incident photons. This corresponds to a maximum achievable saturated count-rate of 77 Mcounts/sec, not previously reported.

Novel black silicon PIN photodiodes

The many advantages of silicon such as low cost, abundancy and a level of maturity that allows for very large scale integration, means that silicon is the most commonly used semiconductor in microelectronics and optoelectronic devices. Silicon, however, has one disadvantage, this being that it is unable to absorb light greater than 1100 nm. The two primary telecommunications wavelengths, 1300 nm and 1550 nm, can therefore not be detected. An interesting method used to extend silicons wavelength range is the formation of black silicon on the silicon surface. Black silicon is formed when gases that are passed over the silicon react and etch the silicon surface, forming a dark spiky pattern. When light is shone on such a pattern, it repeatedly bounces back and forth between the spikes thus reducing surface reflection and trapping the light. This reduced reflectance and light trapping increases the sensitivity of the silicon to long wavelengths and makes it viable for use in a wide range of commercial devices such as infrared detectors and solar cells. This paper presents novel black silicon PIN photodiodes of various sizes (25 mm2, 4 mm2 and 1 mm2). The diodes have been extensively characterized at wafer level, with breakdown voltage, dark current, shunt resistance, threshold voltage and junction capacitance measurements being made. Extensive responsivity measurements were also performed and it was established that the black silicon surface resulted in responsivity increases of greater than 50 % at long wavelengths (≈ 1100 nm).

Investigation of the bandwidth of Geiger–mode avalanche photodiodes

Considerable interest currently exists in the use of plastic optical fibre (POF) for short distances data communications. Attenuation in POF is reduced at 650 nm compared to longer wavelength light and hence silicon based photoreceivers are ideal candidates for use with POF. The difficulty with the development of a CMOS photoreceiver, however, is the realisation of a high speed CMOS photodiode. This paper presents CMOS compatible, shallow junction Geiger-mode avalanche photodiodes (GMAPs) and investigates their bandwidth at 650 nm. Various sized GMAPs (500 μm and 250 μm diameter GMAPs with 20 μm cathode-anode overlaps and 20 μm diameter GMAPs with 3 μm, 4 μm and 5 μm overlaps) were mounted on PCBs. The anodes and cathodes were wirebonded to ground and 50Ohm transmission lines respectively. Impulse response measurements were made for each diode over a range of bias voltages, using a 650 nm picosecond pulsed laser diode. The bandwidths of each diode were calculated from the measured impulse responses and plots of bandwidth versus reverse bias were made. The results indicate very high speed operation is possible (> 1 GHz (20 μm diameter diode)), even for large detectors (> 250 MHz (500 μm diameter diode)).

Optimisation of a Geiger mode avalanche photodiode imaging pixel based on a hybrid bulk SOI CMOS process

Single photon detection has a wide variety of scientific and industrial applications including optical time domain reflectometry, astronomy, spectroscopy, defect monitoring of Complementary Metal Oxide Semiconductor (CMOS) circuits, fluorescence lifetime measurement and imaging. In imaging applications, the dead time is the time during which the detector is inhibited after a photon has been detected. This is a limiting factor on the dynamic range of the pixel. The rate of photon detection will saturate if the dead time is too large. Time constants generated by Metal Oxide Semiconductor (MOS) transistor bulk and sidewall capacitances adversely affect the dead time of pixels developed in conventional CMOS technology. In this paper, a novel imaging pixel configuration based on a Geiger Mode Avalanche Photodiode (GMAP) and fabricated using a dedicated hybrid bulk Silicon On Insulator (SOI) CMOS process is presented. The GMAP is fabricated in the bulk layer and the CMOS circuitry is implemented in the upper SOI layers. As a result, bulk and sidewall capacitance effects are significantly reduced. As both the diode and the CMOS transistors are on the same wafer there is a reduction in pixel area and an additional reduction in the parasitic capacitance effects. This leads to a significant improvement in pixel performance. Pixels incorporating 5 micron and 10 micron diameter GMAPs have been simulated. The circuits were optimised with a view to maximising the photon count rate. Results show a significant improvement in the dead time with values of 14 nanoseconds and 15 nanoseconds being observed for the 5 micron and 10 micron GMAPs respectively.

Investigating the effects of package capacitance on the Frequency Response of Shallow Junction Avalanche Photodiodes

Shallow junction silicon avalanche photodiodes developed for photon-counting applications exhibit a multiplication gain of several hundred when operated near breakdown. The small size and relatively large gain of these devices identifies them as potential candidates for short-haul optical networking at 650nm. Of importance is the frequency response of these devices and in particular the limitations on achievable bandwidth placed by the packaging of the diodes. This work investigates the effect package capacitance has on the frequency response of Geiger Mode Avalanche Photodiodes (GMAP) when compared to micro-stripline mounted devices. Impulse response measurements are made of the diode using a pulsed laser diode at a wavelength of 650 nm which provides pulses with full-width at half maximum (FWHM) of 70 ps typical and 200 ps maximum. A Fast Fourier Transform (FFT) is applied to the measured pulse to convert it to the frequency domain and de-embed the response of the test fixture and cable assembly. The electrical parameters of the packaged and micro-stripline mounted devices are compared in the frequency domain to see the effect of the package capacitance on the frequency response. Different device geometries are explored to identify suitable candidates for short-haul plastic optical fibre communications.

A novel flip-chipped integrated single photon detection platform for proteomics

Novel integrated sensors will be required for future detection platforms for the measurement of fluorescence and luminescence. The current trend towards integration of optical detectors and the broad advances in optical emitting dyes and proteins will be combined in robust, low-cost, point-of-use, diagnostic equipment. To this end we are experimenting with an integrated optical hybrid sensing device which will combine a flip-chipped, array of solid-state single photon counting detectors with surface mount passive quench circuits on a conventional glass substrate. This flip-chipped arrangement both 1) increases the speed of response of the detector and 2) increases the robustness and ease of integration and reduces single photon detector handling requirements. The potential of integrated solid-state photon detectors will be demonstrated for the real-time quantitative detection of luciferase, a light emitting protein expression reporter molecule. A 15μm solid-state Geiger-mode avalanche photodiode (APD) operating in single photon counting mode will be compared with a standard photomultiplier tube (PMT) for luciferase luminescence detection. Detection levels of 2×106 and 1×107 enzyme molecules will be demonstrated for PMT and Geiger-mode APD respectively. The size of the Geiger-mode APD active area will be shown to be the limiting factor in luciferase signal detection for non-integrated applications. A simple geometric model will show that detection limits of 1×104 are achievable in integrated sensing platforms using room temperature operated single photon counting detectors.