Low Noise Equivalent Power InAs Avalanche Photodiodes for Infrared Few-Photon Detection

Electron-only avalanche multiplication and low excess noise has previously been established in InAs avalanche photodiodes (APDs). However, there is currently a lack of experimental investigations into the noise and low photon detection capability of planar InAs APDs. Here, the noise equivalent power (NEP) of planar InAs APDs operating with a low-noise transimpedance amplifier is investigated for the first time. Our devices have a responsivity of 0.7 A/W and excellent linearity at the wavelength of 1550 nm. Using these APDs, a very low NEP of
$45 \text {fW/}\sqrt {\text {Hz}}$
is achieved at a gain of 54. Modeling of the NEP suggests that the excess factor is close to 1.6. This low NEP result is corroborated by the detection of weak optical pulses corresponding to < 70 photons per 1550-nm laser pulse. Our analysis suggests that this performance can be enhanced through the suppression of the background component of the photocurrent as well as reducing the unintentional doping in the devices.