While avalanche photodiodes (APDs), which achieve high sensitivity through impact ionization, are the standard for semiconductor-based low-light photodetectors, they suffer from some drawbacks, including high excess noise, a limited gain-bandwidth product, and high operation voltages of about 20 to 200 V. Scientists at the University of California, San Diego (La Jolla, CA) have come up with an alternative based on the phonon (sound quanta)-based cycling excitation process (CEP, first reported by the same group in 2015) for amplification—devices using CEP have a quantum noise limit potentially 30 times lower than those based on impact ionization. Knowing that CEP was based on Auger excitation of localized states, the researchers had an epiphany: amorphous silicon (Si), with its disorder and thus abundant localized states, would be a good active material for a CEP-based low-light detector.
And indeed it is. Introducing 5% carbon into the amorphous Si, the researchers fabricated a detector with a 30-nm-thick CEP layer sandwiched between a top transparent contact and a Si substrate. With a 30-μm-diameter active area, the CEP-based detector has a photocurrent at zero bias of 17 nA and an extrapolated dark current of about 0.5 pA. Characterized with light at a 405 nm wavelength, the device has a gain of 2000 for a 4 V bias, with the frequency response having a 3 dB cutoff at about 1.5 GHz. The excess noise factor (ENF) for the CEP-based device was no more than about 2 for gains up to the maximum of on the order of 2000, in contrast to an ENF of up to 40 under the same conditions for an APD. The gain-bandwidth product of the new carbon-doped detector is about 2.25 THz. Reference: L. Yan et al., Appl. Phys. Lett. (2017);http://dx.doi.org/10.1063/1.5001170.