A Short-Wavelength Infrared Photon Detector Based on Charge Injection

Vala Fathipour

doi:https://doi.org/10.21985/N24T2S

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A Short-Wavelength Infrared Photon Detector Based on Charge Injection

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Short-wave infrared (SWIR) photon detection has become an essential technology in the modern world. Sensitive SWIR detectors with low noise levels, and high signal-to-noise-ratios are highly desirable for a variety of applications including light detection and ranging, optical tomography, and astronomical imaging. As such many efforts in infrared detector research are directed toward improving the performance of the photon detectors operating in this wavelength range. Unfortunately, after decades of research, the existing technology candidates in the short-wave infrared band still suffer from inherent limitations, such as low quantum efficiency, high noise levels, or requirement of extremely low temperatures. To solve these problems, we have developed an electron-injection technique, based on the single-photon detection of the rod cells. The significance of this detection mechanism is that it can provide both high efficiency and high sensitivity at room temperature, a condition that is very difficult to achieve in conventional SWIR detectors. Electron-injection (EI) detector offers an overall system-level sensitivity enhancement due to a feedback stabilized internal avalanche-free gain. The amplification method is inherently low noise, and devices exhibit an excess noise of unity. The detector operates in linear-mode and requires bias voltage of a few volts. It is based on the mature InP material system and has a cutoff wavelength of 1700 nm. EI detectors operate by absorbing photons in a large volume, confining the photo-excited holes into a small sensor, and then amplifying the signal through electron-injection. EI detectors have been designed, fabricated, and tested during two generations of development and optimization cycles. The shortcomings of the first-generation devices were addressed in the second-generation detectors. Second-generation devices achieved high gain, high bandwidth, and low leakage current in a single structure at room temperature. Measurement on devices with an injector diameter of 10 μm and an absorber diameter of 30 μm showed high-speed response ~ 6 ns rise time, low jitter ~12 ps, high amplification of more than 1000 (at optical power levels larger than few nW), unity excess noise factor, low leakage current (amplified dark current ~10 nA), at bias voltage of –3 V, and at room temperature. Second-generation devices achieved two orders of magnitude reduction in the dark current and four orders of magnitude improvement in bandwidth compared with first-generation devices. The dark current density of the second-generation EI detector outperforms the state-of-the-art technology, the SWIR HgCdTe eAPD by more than an order of magnitude at all measured temperatures, from 300 K down to 160 K. A performance comparison with other SWIR detector technologies with internal amplification (including phototransistors and avalanche photodiodes) in terms of signal-to-noise ratio at low light level is demonstrated. Comprehensive analytical and simulation models are developed for the first time for the design and optimization of the electron-injection detectors and similar heterojunction phototransistors. Using these models, we illustrate the benefit of scaling of the injector diameter with respect to the trapping/absorbing layer diameter and confirm it by measurement results. The improvements in the second-generation devices have opened up applications for these detectors in medical field (optical coherence tomography), remote sensing (light detection and ranging), and astronomy (exoplanet detection). We demonstrated that the utilization of EI detectors in optical coherence tomography (OCT) greatly enhances system performance. The limitations of existing OCT photon detectors, including the nanowire single-photon detectors, have prevented achieving shot-noise-limit of sensing without using balanced detection scheme in swept source OCT (SS-OCT). Unfortunately balanced detection scheme increases OCT system size and cost as it adds complexity and requires additional components compared to a single detector operation. Using the electron-injection detector, we show for the first time achievement of the shot noise limited performance without using the balanced detection technique. Our system is significantly simpler, and can achieve the shot-noise-limited sensitivity of ~ -105 dB at more than 30 times lower reference laser power as the best-reported balanced detector results. As such, EI detectors allow weak light sources to be used for imaging weakly reflecting samples and can immediately address the demand for high-speed portable OCT imaging systems. Finally we investigated utilization of EI detectors in a Light Detection and Ranging (LiDAR) system. Among the different components of a LiDAR receiver system, the optical detector directly affects the instrument sensitivity performance. The EI detectors’ characteristics including the high gain, the unity excess noise, the good timing resolution, and the low bias voltage requirement, suggest that they are a good candidate for high-resolution flash LiDAR applications with millimeter scale depth resolution at longer ranges compared with p-i-n diodes. Based on our experimentally measured device characteristics, we compare the performance of the electron-injection detector with commercially available linear-mode InGaAs APD as well as a p-i-n diode using a theoretical model. Flash LiDAR images obtained by our model, show the electron-injection detector array achieves better resolution with higher signal-to-noise compared with both the InGaAs APD and the p-i-n array (of 100 x 100 element). We have designed a laboratory set up, with receiver optics aperture diameter of 3 mm that allows an EI detector (with 30 μm absorber diameter) to be used for LiDAR imaging with sub centimeter resolution.

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