CMOS-compatible graphene-silicon photodetectors

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Abstract

Graphene is an attractive material to realize ultrafast and broadband photodetectors (PDs) owing to its versatile electronic and optical properties such as high charge carrier mobilities and flat, broadband optical absorption spanning the technologically highly relevant spectral range from the near-UV to the short-wave IR. The co-integration of graphene with silicon technology in the back-end-of-line CMOS process allows the realization of a functional hybrid platform for optoelectronic applications that is suitable for large scale fabrication. This thesis presents careful investigations of the optoelectronic characteristics of graphenesilicon (Gr-Si) Schottky junction photodiodes. Gr-Si PDs with various device architectures and according fabrication processes were developed to study devices under optical excitation over a broad spectral range at varying optical powers from continuous wave to high speed excitation. The influence of the substrate and interface properties on the optoelectronic properties of Gr-Si PDs was determined using various characterization techniques such as current-voltage, capacitance-voltage and temperature-dependent measurements. The impact of interfacial oxide layer on optoelectronic characteristics and light detection capabilities of Gr-Si Schottky junction photodiodes was studied under a broad spectral illumination ranging from the near-UV to the short-/mid-infrared (thermal) wavelength regime. Results show that employment of a thin (âˆÂ¼2 nm) native oxide layer at the Gr-Si interface is beneficial to improve photodetection properties of such devices. The interfacial oxide layer enhances current rectification and photon detectivities by decreasing the leakage current and enhances the photovoltage responsivity. Fabrication processes were translated from standard bulk silicon substrate to silicon-oninsulator (SOI) substrates to realize Gr-Si PDs with a reduced active silicon layer thickness of âˆÂ¼10 µm. Decreasing the active silicon layer leads to a significant improvement in the response speed of the devices with only a minor decrease in responsivity compared to that of devices fabricated on bulk silicon substrates in the wavelength range λ = 800::1100 nm. Further, diode characteristics can be tuned by modifying the silicon topography. Results show that integration of micro-optical elements in the silicon surface through patterning enables control of the spectral response and angular dependence of the photodiodes. Based on the fabricated devices and their different architectures, important figures of merit (FOM) of Gr-Si PDs could be evaluated. Specifically, FOM such as responsivities, dark noise levels, noise equivalent power and detectivities were determined which allows comparing and evaluating the performance of Gr-Si PDs with alternative devices and materials. In summary, this thesis presents novel fabrication processes and device architectures for Gr-Si PDs, quantifies their performance, and evaluates the importance of the interfacial layer for device operation. It lays out possible routes for the development of more sensitive, faster, and CMOS-compatible Gr-Si PDs for free-space light detection ranging from the visual to short-wave infrared wavelength ranges.

Keyword(s)

CMOS; Graphene; Graphene photodetectors; Schottky junction; Schottky photodiodes

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