Tech Briefs

Applications include quantum key distribution for the financial industry and photon-starved optical communications needs.

This design constitutes a self-resetting (gain quenching), room-temperature operational semiconductor single-photon-sensitive detector that is sensitive to telecommunications optical wavelengths and is scalable to large areas (millimeter diameter) with high bandwidth and efficiencies.

The schematic diagram of the InGaAs-InAsAs SPAD Layer Structure (a) and (b) the Concept of Operation for the structure showing the self-quenching and self-recovery processes.
The device can detect single photons at a 1,550-nm wavelength at a gain of 1 × 106. Unlike conventional single photon avalanche detectors (SPADs), where gain is an extremely sensitive function to the bias voltage, the multiplication gain of this device is stable at 1 × 106 over a wide range of bias from 30.2 to 30.9 V. Here, the multiplication gain is defined as the total number of charge carriers contained in one output pulse that is triggered by the absorption of a single photon. The statistics of magnitude of output signals also shows that the device has a very narrow pulse height distribution, which demonstrates a greatly suppressed gain fluctuation. From the histograms of both pulse height and pulse charge, the equivalent gain variance (excess noise) is between 1.001 and 1.007 at a gain of 1 × 106. With these advantages, the device holds promise function as a PMT-like photon counter at a 1,550-nm wavelength.

The epitaxial layer structure of the device allows photons to be absorbed in the InGaAs layer, generating electron/ hole (e-h) pairs. Driven by an electrical field in InGaAs, electrons are collected at the anode while holes reach the multiplication region (InAlAs p-i-n structure) and trigger the avalanche process. As a result, a large number of e-h pairs are created, and the holes move toward the cathode. Holes created by the avalanche process gain large kinetic energy through the electric field, and are considered “hot”. These hot holes are cooled as they travel across a p-InAlAs low field region, and are eventually blocked by energy barriers formed by the InGaAsP/InAlAs heterojunctions.

The composition of the InGaAsP alloy was chosen to have an 80 meV valance band offset with InAlAs, which is high enough to hinder the transport of the already cooled holes. Being stopped by the energy barrier, holes are accumulated at the junctions to shield the electric field, resulting in a decrease of the electric field in the multiplication region. Because the impact ionization rate is extremely sensitive to the magnitude of the electric field, the field-screening effect drastically reduces the impact ionization rate and quenches the output signals.

After the avalanche pulse signal is self-quenched, the accumulated holes at the InGaAsP/InAlAs interface escape the energy barrier through thermal excitation and tunneling and finally leave the device. The device is thus reset and ready for subsequent photon detection. This recovery time is controlled by the height of the energy barrier and the hole-cooling rate.

This work was done by Kai Zhao and Yu-Hwa Lo of the University of California San Diego and William Farr of Caltech for NASA’s Jet Propulsion Laboratory. For more information, download the Technical Support Package (free white paper) at under the Physical Sciences category. NPO-45801

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