Alternative Fabrication Method for High-Bandwidth Germanium Fin Photodiodes

Introduction

Germanium (Ge) photodiodes are essential components in silicon-based high-speed optoelectronics, and there has been a continuous drive to increase their bandwidth to meet the demands of ever-increasing data rates. This article presents an alternative fabrication method for Ge-fin photodiodes that aims to simplify the process, reduce the thermal budget, and facilitate their integration into Photonic Integrated Circuit (PIC) and Electronic-Photonic Integrated Circuit (ePIC) platforms.

Conventional Fabrication Challenges

Previous work by the authors demonstrated a Ge-fin photodiode construction scheme with a record-breaking 3-dB opto-electrical (OE) bandwidth of 265 GHz [1, 2]. However, this approach involved complex processes, such as separate silicon epitaxial deposition for the opposite Ge-fin side, filling trenches with SiO2, and planarization steps, which increased the process complexity and thermal budget.

Alternative Fabrication Approach

The alternative fabrication method presented in this work addresses these challenges by introducing the following key changes:

1. Simplified Ge-fin formation: The Ge-fin is realized by a single dry etching process followed by a single epitaxial deposition of undoped silicon (Si), eliminating the need for separate epitaxial depositions and planarization steps.

Figure 1: Schematic sketch of the simplified Ge-fin fabrication approach.

2. Reduced Si region thickness: The thickness of the Si regions sandwiching the Ge-fin is reduced from ~200 nm to ~50 nm to lower the thermal budget and enable steeper doping profiles through ion implantation.

3. Ion implantation for doping: The definition of the p- and n-regions is carried out by ion implantation into the Si layer, eliminating the need for separate epitaxial depositions and doping steps.

While these changes simplify the process, they also introduce potential drawbacks, such as limited control over dopant incorporation into the Ge and potential increases in resistivity due to shallow dopant profiles in the Si offshoots.

Results and Discussion

Despite the reintroduction of ion implantation, the fabricated Ge-fin photodiodes exhibited impressive 3-dB bandwidths exceeding 110 GHz, as shown in the normalized frequency response (Figure 2).

Figure 2: Normalized frequency response of Ge-fin photodiodes with nominal width of ~250 nm and length of 5 um.

The dark and photocurrent IV curves (Figure 3) demonstrate that the dark current behavior is not deteriorated by this novel fabrication scheme, and there is no indication of increased series resistance despite the reduced thickness of the Si offshoots and the absence of an annealing step after ion implantation.

Figure 3: Dark- and photocurrents of Ge-fin photodiodes, measured at room temperature.

The small capacitances of about 2 fF at a reverse bias of 2 V (Figure 4) suggest that the device frequency response is not limited by the RC time constant.

Figure 4: Capacitances extracted from S-parameter measurements of device Ge250_5.

The SEM cross-section image (Figure 5) shows a Ge-fin width of ~230 nm at the narrowest point, indicating that the frequency response is likely limited by the transit time constant. Considering drift velocity saturation, the estimated cutoff frequency for transit (or drift) yields ~124 GHz.

Figure 5: Cross-sectional STEM image of a germanium-fin photodiode (Ge250_5) with p- and n-Si contact regions realized by ion implantation.

While the internal responsivity of about 0.3 A/W is lower than previous work [2, 3], this is attributed to the nickel-silicide layer formed at the vertical Si sidewalls, which could be prevented by introducing spacers in future fabrication runs.

Conclusion

The authors have successfully demonstrated an alternative fabrication approach for Ge-fin photodiodes that reduces process complexity and thermal budget, facilitating their integration into PIC and ePIC platforms. Despite the reintroduction of ion implantation, the fabricated devices exhibit impressive 3-dB bandwidths exceeding 110 GHz, outperforming devices available in major silicon photonics platforms. Further enhancements in responsivity can be achieved by introducing sidewall spacers, although potential effects on resistance must be evaluated. This work paves the way for the broader adoption of high-bandwidth Ge-fin photodiodes in various applications.

References

[4] S. Lischke et al., "Ge Photodiode with -3 dB OE Bandwidth of 110 GHz for PIC and ePIC Platforms," 2020 IEDM, San Francisco, CA, USA, 2020, pp. 7.3.1-7.3.4, doi: 10.1109/IEDM13553.2020.9372033.

[5] S. Lischke et al., "Ultra-fast germanium photodiode with 3-dB bandwidth of 265 GHz," Nat. Photon. 15, 925–931 (2021)

[6] A. Peczek, et al., "Versatile Germanium Photodiodes with 3dB Bandwidths from 110 GHz to 265 GHz," ECS Trans. 109, 21–28 (2022).