Graphene/silicon heterojunctions for optoelectronic applications
Riazimehr, Sarah; Lemme, Max Christian (Thesis advisor); Echtermeyer, Tim (Thesis advisor)
Aachen (2019, 2020)
Dissertation / PhD Thesis
Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2019
Graphene / silicon (G/Si) heterostructures are simple hybrid devices with potential for optoelectronic applications such as photodiodes, photodetectors and solar cells. Fundamental understanding of photocurrent generation in such hybrid devices is a prerequisite for designing them efficiently. This thesis presents an experimental investigation on the G/Si heterojunction junction photodetectors and proposes a simple design to improve their efficiency and performance.For this work, devices were fabricated by transferring large-area chemical vapor deposited (CVD) graphene onto prepatterned n- and p-type (Si) substrates, and their electrical and optical characteristics were investigated using several experimental techniques. The diodes fabricated on n-Si substrates showed much better rectification behavior compared to those fabricated on p-Si substrates in dark condition. Basic electrical parameters of the G/n-Si diodes, such as the Schottky barrier height (SBH), the ideality factor (n) and series resistance (RS) were obtained by analyzing the current-voltage (I-V) measurement data. Afterwards, photoconductivity of the devices was tested under white light illumination. The photodiodes displayed extraordinary sensitivity to changes in illumination. Furthermore, the local generation and distribution of photocurrent in G/n-Si photodiodes were investigated using scanning photocurrent microscopy (SPC) technique. It was observed that the G/silicon dioxide (SiO2)/Si (GIS) region, which is usually in parallel with the G/Si Schottky junction, has a significant contribution towards the measured photocurrent at reverse biases above a certain threshold voltage (Vth). This observation was explained with the formation of an inversion layer in Si underneath SiO2 using theoretical and simulation studies. Based on the findings, a concept of ‘generation area’ for genuine calculations of responsivity of graphene based photodiodes was put forth. Considering this concept, the spectral response (SR) of the diodes were measured using lock-in technique. G/n-Si photodiodes showed a substantial responsivity up to 270 mA/W within the silicon spectral range at reverse bias condition. SR measurements exhibited that Si mainly absorbs the light and graphene acts as a transparent conductive electrode.Employing these findings and invoking the ‘true’ active area, a simple interdigitated contact pattern was developed to fabricate and demonstrate highly sensitive G/n-Si heterojunction photodiodes. With this design, an external quantum efficiency (EQE) > 80% over a wide range of wavelengths from 380 to 930 nm was achieved. A maximum EQE of 98% and the highest responsivity of 635 mA/W were reached at a wave length of 850 nm, exceeding even commercial IR-enhanced Si PIN photodiodes. The achieved high EQE and responsivity were attributed to an enhancement in collection of charge carriers photogenerated in Si under the GIS regions of the devices. The experimental results were elucidated by numerical simulations of the devices. Moreover, a method for an accurate analyzing of the capacitance-voltage (C-V) measurement data of the G/Si Schottky diodes with embedded GIS capacitor was introduced.The device fabrication process used in this study is fully compatible with existing Si technology, which allows for immediate implementation in industrial applications. The findings of this this work may serve as guidelines for analyzing and taking the next step towards applications of G/Si photodiodes with graphene acting as a transparent electrode.
- Chair of Electronic Devices