Minority Carrier Lifetime Measurement in Germanium on Silicon Heterostructures for Optoelectronic Applications

Abstract

Demand for low cost and high density near infrared (NIR) detection has motivated the development and use of germanium on silicon (Ge/Si) heterostructures to extend the optoelectronic application of Si technology. Ge/Si structures are currently being considered for NIR p/n detectors that can be integrated with Si CMOS [1]. Various research demonstrations of integrated Ge/Si diodes with CMOS have been made including using sputtered poly-Ge to form Ge/Si photodiodes after the CMOS transistors were complete. Poly-crystalline germanium (poly-Ge) may be formed by various methods including the use of plasma enhanced chemical vapor deposition. In this work, the formation of poly-Ge/Si heterostructures by inductively coupled plasma enhanced chemical vapor deposition (ICP-CVD) is examined as an alternative method to integrate poly-Ge into a CMOS process flow. In this work, 25 nm poly-Ge on Si heterostructures are formed by either recrystallization of ICP-CVD hydrogenated amorphous germanium (α-Ge:H) or direct deposition of ICP-CVD poly-Ge. A rapid measure of the suitability for detectors of the different poly-Ge films is the minority carrier recombination lifetime, which can affect dark current, quantum efficiency and overall detector detectivity. Recombination lifetimes were measured, therefore, in α-Ge:H that was recrystallized using rapid thermal annealing between 400 – 1050 °C in nitrogen ambient. Lifetimes were measured using a non-contact inductively coupled photo-conductance setup and an effective surface recombination velocity is subsequently extracted for each Ge/Si heterostructure that describes the integrated recombination in the Ge layer and at the Ge/Si interface, which separates the Ge contribution from recombination in the bulk silicon and at the silicon surface. The effective recombination velocities for the 25 nm Ge layers are found to be ∼10³−10⁴ cm/s.