Ge—Si P-I-N photodiode with reduced dark current and fabrication method thereof

A photodiode comprising:

• a substrate layer;
• a bottom contact layer of a first semiconductor material and disposed on the substrate layer, the bottom contact layer doped to exhibit a first conductive type;
• an absorption layer of a second semiconductor material and disposed on the bottom contact layer, the absorption layer comprising:
• a bottom doped region doped to exhibit the first conductive type, wherein a doping profile of the bottom doped region is gradually doped with higher concentration at an interface between the bottom contact layer and the absorption layer, and with lower concentration away from the interface between the bottom contact layer and the absorption layer;
• an ion-implanted sidewall region doped to exhibit the first conductive type;
• and
• an intrinsic region unintentionally doped or lightly doped to function as a depletion region;
• a top contact layer doped to exhibit a second conductive type and disposed on the absorption layer;
• and
• an anti-reflection layer disposed on the top contact layer.