| dc.description.abstract | Detecting single photons with high efficiency and timing resolution opens up new possibilities for various technologies such as light detection and ranging (LIDAR), long-distance secure communication, singlet oxygen detection for dose monitoring in cancer treatment, optical coherence tomography (OCT) for eye imaging and quantum computing. For instance, the image quality for analysis of biological tissue or precision of LIDAR systems can be significantly improved with quantum sensing technologies. OCT imaging systems can be improved for higher resolution mapping of human retina leading to early detection of blinding diseases. Detecting single photons at a high bandwidth can also enable high speed quantum communication technologies by improving the detection speed.
Currently, the two leading technologies for single-photon detection are superconducting nanowires and semiconductor-based avalanche photodiodes. Superconducting nanowires have excellent detection efficiencies ($>$90\%) and precise timing resolution ($<$50 ps); however, they require cryogenic temperatures, bulky compressors and pumps to operate (typically $<$4K), which results in a serious limitation for practical and portable applications. In contrast, semiconductor-based single-photon avalanche diode (SPAD) technology exists for portable applications, nonetheless, the high efficiency is achieved for a small wavelength range and at the cost of timing resolution (200-500 ps).
This work offers a novel approach to improve the quantum efficiency of semiconductor-based avalanche photodiodes by taking advantage of the remarkable optical and electrical properties of semiconductor nanowires. Embedding a photodiode architecture in nanowire arrays shows great promise for improving device performance, leading the way towards next generation quantum detectors. In this work, we designed a near-unity absorber from arrays of InGaAs nanowires with a truncated cone shape using finite-difference time-domain (FDTD) simulations. We optimized the nanowire taper angle and geometry of the array to achieve an average absorptance of 97\% from 400 nm to 1650 nm. Next, we developed and optimized a nanofabrication recipe to realize highly ordered truncated cone shaped InGaAs nanowire arrays. Fabricated InGaAs nanowire arrays exhibited near-unity absorption (average of 93\%) for an unprecedented wavelength range from 900 nm to 1500 nm when the nanowire shape and geometry is optimized. Fabricated nanowire arrays with a 350 nm top diameter, 880 nm bottom diameter and 900 nm pitch resulted in the best absorptance spectrum.
Previously developed InP based nanowire quantum sensors exhibited single photon sensitivity and a fast response with 0.6 ns rise time and only 17 ps timing jitter at room temperature. In this thesis, our aim was to extend the single photon sensitivity towards the near infrared and short-wave infrared wavelengths by using InGaAs as an active material. Our novel nanowire pin photodiodes based on InGaAs displayed remarkable improvement in quantum efficiency (from a typical value of 40\% to 287\% at 900 nm), including the wavelength band called the “valley of death” (800-1000 nm) where OCT imaging systems operate for enhanced sensitivity and axial resolution for eye imaging. Near-unity absorption combined with a gain mechanism in the nanowires was found to be the driving factor behind this high performance. We also built nanowire-based avalanche photodiodes to introduce a high gain mechanism that is able to detect single photons efficiently in the short-wave infrared. A separate absorption, charge and multiplication avalanche photodiode design based on InGaAs and InAlAs in individual nanowires to avoid band-to-band tunneling in devices made with low bandgap semiconductors only. The devices exhibited an avalanche gain of 40 showing promise towards detecting single photons.
The thesis describes the approach to design nanowire arrays to achieve near-unity absorption, optimization and creation of process recipes to realize nanowire arrays and devices and electrical and optical characterization of fabricated nanowire arrays demonstrating high quantum efficiency with avalanche gain. | en |
