A low dark current InGaAs/InP p-i-n photodiode with covered mesa structure

A new InGaAs p-i-n photodiode with a covered mesa (CM) structure having extremely low dark current characteristics and high yields has been developed. The device consists of only two epitaxial layers: n^--InP and n^--InGaAs, continuously grown on an n⁺-InP substrate by liquid-phase epitaxy. The InGaAs layer is chemically etched to be a tapered shape in order to make the fabrication process simple, as compared with a conventional mesa diode. The Zn diffusion to form a p-n junction is carried out without a diffusion mask such as Si3N4or SiO2, which induces damage due to the thermal stress. The tapered-shape InGaAs layer is covered with the Zn-diffused layer because a surface p-n junction occurring in an InGaAs region is leaky. Therefore, the surface p-n junction of the photodiode appears in the n^--InP layer, which has a bandgap about two times wider than the InGaAs. Finally, the passivation of the surface p-n junction is carried out with a Si3N4film formed by a plasma-assisted chemical vapor deposition. We have successfully achieved an extremely low dark current of 20 pA at a reverse bias voltage of 10 V and a high yield of 80 percent, by adopting the CM structure and the simple fabrication process.