Epitaxial Graphene/n-Si Photodiode with Ultralow Dark Current and High Responsivity

Abstract

Graphene's exceptional carrier mobility and broadband absorption make it promising for ultrafast photodetection. However, its low optical absorption limits responsivity, while the absence of a bandgap results in high dark current, constraining the signal-to-noise ratio and efficiency. Although silicon (Si) photodetectors normally offer fabrication compatibility, their performance is severely hindered by interface trap states and optical shading. To overcome these limitations, we demonstrate an epitaxial graphene/n-Si heterojunction photodiode. This device utilizes graphene epitaxially grown on germanium integrated with a transferred Si thin film, eliminating polymer residues and interface defects common in transferred graphene. As a result, the fabricated photodetector achieves an ultralow dark current of 1.2 × 10^-9 A, a high responsivity of 1430 A/W, and self-powered operation at room temperature. This work provides a strategy for high-sensitivity and low-power photodetection and demonstrates the practical integration potential of graphene/Si heterostructures for advanced optoelectronics.

Keywords: graphene; heterojunction; n-type silicon; photodetector; self-powered operation.

Figure 1

Device structure and fabrication. (a) Illustration of the fabrication process of the graphene (Gr)/n-Si photodiode. (b) Optical image (scale bar: 50 μm) of the graphene/n-Si photodiode. (c) Cross-sectional schematic diagram of the device. (d) Current–voltage (I–V) curve of the device in the dark. The dark current values at 1 V and 3 V are given.

Figure 2

Photoresponse characterization of the device. (a) –V curves of the device under illumination at different wavelengths (405 nm, 516 nm, 638 nm, and 800 nm) and various light intensities. Time-resolved photoresponse of the device under zero bias upon illumination with (b) 405 nm, (c) 516 nm, (d) 638 nm, and (e) 800 nm lasers at varied light intensities. The detailed light intensity values for the step-like change in current are explicitly given in (c) for better understanding.

Figure 3

Performance evaluation of the device. (a) Photoresponsivity (R) as a function of incident light intensity under four laser wavelengths (405, 516, 638, and 800 nm). (b) Time-resolved photoresponse of the Gr/n-Si photodetector. Rise and fall times are defined as the time required to switch between 10% and 90% of the maximum photocurrent. (c) Density spectral density (S) as a function of frequency. (d) Specific detectivity (D*) as a function of incident light intensity under four laser wavelengths (405, 516, 638, and 800 nm).

Figure 4

Benchmarking and mechanism analysis of the device. (a) Benchmark comparison of Si-based photodetectors plotted as photoresponsivity (R) versus dark current ($I_{dark}$). The data of other reported Si-based photodiodes are cited from Mao 2022 [26], Wu 2023 [29], Li 2016 [32], Selvi 2018 [36], Zhang 2023 [38], Zeng 2015 [42], Liu 2019 [43], Feng 2021 [44], Song 2022 [45], Liu 2023 [46], and Song 2021 [47]. (b) Energy band diagrams of the graphene/n-Si heterojunction. E_C and E_V denote the conduction band and valence band of n-Si, E_F denotes the Fermi level, Φ_B denotes the Schottky barrier of the device. The red and blue arrows denote the transport paths of photogenerated electrons and holes, respectively. The black up-down arrow denotes the height of the Schottky barrier. The colorful crooked arrow illustrates the light illumination.