Low pressure epitaxial growth, fabrication and characterizion of Ge-on-Si photodiodes

Abstract

In order to facilitate the integration of photonic systems onto an electronic chip, near infrared photodiodes utilizing novel materials such as germanium must be monolithically integrated onto the Si CMOS platform. Such near-infrared photodiodes can be utilized for a plethora of applications such as optoelectronic ADCs, optical interconnects, photonic integrated circuits, and near infrared cameras. In this work, the major focus is on investigating processes utilizing a Low Pressure Chemical Vapor Deposition (LPCVD) Applied Materials Epi CenturaTM system to deposit germanium onto silicon substrates (Ge-on-Si). A growth space is identified to deposit blanket and selective epitaxial 1 to 3 rim-thick Ge-on-Si films via a two-step process. These deposited Ge-on-Si films have a low root-mean-square surface roughness (below 2 nm) and a moderate threading dislocation density (- 107 cm-2) after an annealing process. Utilizing these Ge-on-Si films, vertically illuminated Ge-on-Si pin photodiodes are fabricated in a CMOS compatible process. The best photodiodes fabricated in this work have low dark current values (below 10 mA/cm2), high responsivity (- 0.45 A/W at 1.55 pim wavelengths) and 3-dB frequency response in the gigahertz range.

(cont.) Due to the importance of the photodiode reverse bias leakage current for circuit applications, the reverse bias leakage current is investigated and characterized in detail for various Ge-on-Si pin photodiodes. Trap assisted tunneling was found to be the dominant reverse bias leakage mechanism. These Ge-on-Si films show great promise for leveraging the integration of photonic devices onto the Very Large Scale Integration (VLSI) platform, and once there is improved reproducibility in the fabrication process, specifically the passivation of germanium surface states, the promise of these Ge-on-Si films can be fully realized.