Linear Mode Photon Counting Research Articles

Linear-Mode HgCdTe Avalanche Photodiodes for Photon-Counting Applications

An overview of recent improvements in our understanding of, and the maturity of, linear-mode photon counting with the HgCdTe electron-initiated avalanche photodiode is presented. In 2010 DRS fabricated an experimental 2 × 8 array with (64 μm)2 pixels which enabled, for the first time, linear-mode photon counting by use of the MWIR cutoff HgCdTe electron-initiated avalanche photodiode. The device had a high single-photon signal-to-noise ratio of 13.7, an excess noise factor of 1.3–1.4, a 7 ns minimum time between events, and a broad spectral response extending from 0.4 μm to 4.2 μm. DRS recently fabricated a new set of devices with improved yield and performance compared with the first device: the false event rate was reduced by a factor of almost 10 to 150 kHz, the photon detection efficiency was increased from 50% to >60%, and the APD gain was increased by a factor of 4 to over 1900.

Update on linear mode photon counting with the HgCdTe linear mode avalanche photodiode

The behavior of the gain-voltage characteristic of the mid-wavelength infrared cutoff HgCdTe linear mode avalanche photodiode (e-APD) is discussed both experimentally and theoretically as a function of the width of the multiplication region. Data are shown that demonstrate a strong dependence of the gain at a given bias voltage on the width of the n − gain region. Geometrical and fundamental theoretical models are exam- ined to explain this behavior. The geometrical model takes into account the gain-dependent optical fill factor of the cylindrical APD. The theoretical model is based on the ballistic ionization model being developed for the HgCdTe APD. It is concluded that the fundamental theoretical explanation is the dominant effect. A model is developed that combines both the geometrical and fundamental effects. The model also takes into account the effect of the varying multiplication width in the low bias region of the gain-voltage curve. It is concluded that the lower than expected gain seen in the first 2 × 8 HgCdTe linear mode photon counting APD arrays, and higher excess noise factor, was very likely due to the larger than typical multiplication region length in the photon count- ing APD pixel design. The implications of these effects on device photon counting performance are discussed.©

Linear mode photon counting with the noiseless gain HgCdTe e-avalanche photodiode

A linear mode photon counting focal plane array using HgCdTe mid-wave infrared (MWIR) cutoff electron initiated avalanche photodiodes (e-APDs) has been designed, fabricated, and characterized. The broad spectral range (0.4 to 4.3 μm) is unique among photon counters, making this a “first of its kind” system spanning the visible to the MWIR. The low excess noise [F(M)≈1] of the e-APDs allows for robust photon detection while operating at a stable linear avalanche gain in the range of 500–1000. The readout integrated circuit (ROIC) design included a very high gain-bandwidth product resistive transimpedance amplifier (3×1013 Ω-Hz) and a 4 ns output digital pulse width comparator. The ROIC had 16 high-bandwidth analogs and 16 low-voltage differential signaling digital outputs. The 2×8 array was integrated into an LN2 Dewar with a custom leadless chip carrier and daughter board design that preserved high-bandwidth analog and digital signal integrity. The 2×8 e-APD arrays were fabricated on 4.3 μm cutoff HgCdTe and operated at 84 K. The measured dark currents were approximately 1 pA at 13 V bias where the measured avalanche photodiode gain was 500. This translates to a predicted dark current induced dark count rate of less than 20 KHz. Single photon detection was achieved with a photon pulse signal-to-noise ratio of 13.7 above the amplifier noise floor. A photon detection efficiency of 50% was measured at a photon background limited false event rate of about 1 MHz. The measured jitter was in the range of 550–800 ps. The demonstrated minimum time between distinguishable events was less than 10 ns.

Issues in HgCdTe Research and Expected Progress in Infrared Detector Fabrication

The purpose of this paper is to review the trends in HgCdTe research, illustrating the discussed ideas with the latest results obtained at DEFIR (CEA-LETI and Sofradir joint laboratory). The beginning of this paper is devoted to an extended introduction to today’s issues concerning HgCdTe photodiode performance enhancement. In fact, very high-quality material is mandatory for ultrahigh performance at low temperature as well as for high noise operability at high operating temperature (HOT). Therefore, a strong effort has been carried out during the last few years for lattice-matched CdZnTe substrate and HgCdTe active layer growth improvement, leading to very large substrates and ultraflat liquid-phase epitaxy layers. The same analysis holds for diode process quality and passivation. Therefore, the photodiode process has been completely revisited in order to optimize HOT operability. Also necessary for the next generation of HOT devices, some significant progress has been made in small-pixel-pitch interconnection on silicon read-out circuits. Indeed, the first 10-μm focal-plane arrays (FPAs) have been successfully fabricated this year in the mid-wave infrared (IR) band. The latest avalanche photodiode (APD) realizations are also presented with 15-μm-pitch FPAs for passive imaging and 30-μm-pitch ultrafast arrays running at 1.5 kHz full frame rate. Linear-mode photon counting using APDs is also briefly discussed. Finally, the paper concludes on more complex structures for the third generation of IR detectors, discussing the latest achievements in dual-band FPA fabrication in various spectral bands.