Enhancing Responsivity with the Franz-Keldysh Effect in Ge Strip Photodetectors

Introduction

Germanium (Ge) is a key material for near-infrared photodetectors in silicon (Si) photonics due to its suitable bandgap for detecting telecom wavelengths. This tutorial explores the use of the Franz-Keldysh (FK) effect to significantly enhance the responsivity of lateral pin photodetectors made from Ge strips on Si, not just in the C and L bands, but also the U band.

Device Structure

The photodetectors studied have a strip-loaded structure of Ge grown on a silicon-on-insulator (SOI) pedestal, as shown in the schematic top view and transmission electron microscope (TEM) cross-section in Figure 1.

Figure 1: A schematic top view of a lateral pin PD of a Ge strip and a typical cross-sectional TEM image for the Ge bottom width of 0.8 μm.

A lateral pin junction is formed in the SOI layer by phosphorus and boron implantation. A Si channel waveguide connects to the pedestal, with a selectively grown Ge strip on top (0.2 μm thick, 40 μm long). The Ge strip has a trapezoidal cross-section with {113} faceted sidewalls. Additional phosphorus/boron implants dope the Ge sidewalls, an oxide over-cladding is deposited, and Al/Ti electrodes are formed.

The strip width is a key parameter, with widths of 0.5 μm, 0.8 μm and 1.2 μm studied. Narrower strips relax the built-in tensile strain, widening the direct bandgap and blue-shifting the absorption edge.

Experimental Results

Figure 2 shows typical current-voltage (I-V) curves for a 0.8 μm wide Ge photodetector at 30°C, under dark and 1.55 μm illumination conditions. The diode exhibits good rectifying behavior in the dark, while photocurrent is clearly generated under illumination, demonstrating photodetection.

Figure 2: Typical I-V characteristics at 30°C for Ge PDs with the strip width of 0.8 μm.

The responsivity spectra in Figure 3 highlight the blue-shift of the absorption edge as the Ge strip width decreases from 1.2 μm to 0.8 μm due to strain relaxation. For the 0.8 μm strip, the responsivity drops below 0.1 A/W in the L and U bands (1.565-1.675 μm).

Figure 3: Typical responsivity spectra at 0 V for the Ge strip widths of 0.8 and 1.2 μm at 30°C.

However, applying a high reverse bias voltage activates the Franz-Keldysh effect, dramatically enhancing the photodetector responsivity as shown in Figure 4 for the 0.8 μm strip width.

Figure 4: Typical responsivity spectra at reverse voltages of 0-5 V for a Ge PD with 0.8 μm strip width at 30°C.

The FK effect boosts responsivity not just in the C and L bands, but also the U band up to 1.675 μm, compensating for the blue-shift. At 5V reverse bias, avalanche multiplication further increases responsivity across all wavelength bands.

This remarkable enhancement from the FK effect, combined with avalanche multiplication at high fields, enables high responsivity operation across a broad wavelength range for optical communication and sensing applications.

Conclusion

Lateral pin photodetectors based on Ge strips integrated on Si exhibit excellent high-speed performance. While narrower Ge strips suffer from a blue-shifted absorption edge, applying a high reverse bias activates the Franz-Keldysh effect. This substantially recovers and enhances the responsivity not just in the C and L bands, but also the U band up to 1.675 μm. Further avalanche multiplication at the highest bias provides a broadband responsivity boost across all wavelengths studied.

This unique approach leveraging the Franz-Keldysh effect enables high-performance Ge-on-Si photodetectors to operate efficiently across an extremely wide wavelength range, proving their versatility for diverse applications in optical communications and sensing.

Reference

[1] S. Kaneko, J. A. Piedra-Lorenzana, K. Yamane, J. Fujikata, and Y. Ishikawa, "Franz-Keldysh Effect in Lateral pin Photodetectors of Ge Strip on Si at C-, L- and U-Band Wavelengths," Department of Electrical and Electronic Information Engineering, Toyohashi University of Technology, Toyohashi, Japan, and Institute of Post-LED Photonics, Tokushima University, Tokushima, Japan, 2024.