Lateral collection doping superlattice photodiodes

Ultralow-capacitance, lateral collection photodiodes with high quantum efficiency have been fabricated in the GaAs system by utilizing a nearly depleted doping superlattice as the active region of a photodetector. The devices are fabricated from symmetric n-p-n-p-... doping superlattices with 1500-Å-thick layers doped to 2×10 ¹⁷ cm^-3. The total thickness of the active region is 1 μm. Contact to the individual n and p layers of the devices is effected by ion implanting the sidewalls of a mesa with Si and Be, respectively. The devices exhibit capacitances in the range of 50 fF at zero bias, decreasing abruptly to 4 fF at a reverse bias of 20 V. Beyond this bias the device behaves as a lateral p-i-n device, and the capacitance decreases slowly with further reverse bias. The reverse leakage current in such a large area device is less than 2×10^-5 A/cm². The forward I-V characteristics exhibit nonideal behavior. The devices exhibit high quantum efficiency at all reverse biases, but are limited above 20 V by the optical thickness of the active region. A strong enhancement of the absorption below the bandgap with reverse bias is attributed to the Franz-Keldysh effect. A shift of the band edge absorption of 0.036 eV has been observed, and a peak in the below band edge photoresponse is observed at biases above 5 V, exceeding the near band edge response at a bias of 15 V. The speed of these devices is expected to be limited by the lateral transit time between contacts, and the geometry of the contacts provides no obscuration of the optical input