In Si photonics, photodetectors (PDs) of a Ge epitaxial layer on Si have been widely used, which show a high photodetection efficiency, or a high responsivity in the O (1.260–1.360 µm) and C (1.530–1.565 µm) bands. However, an issue lies in a low responsivity in the longer wavelength range of L band (1.565–1.625 µm). In this regard, we have proposed a method using a Ge layer epitaxially grown on a Si-on-quartz (SOQ) wafer [1]. The tensile in-plane lattice strain, derived from the thermal expansion mismatch between the Ge layer and base substrate, is enhanced more than 0.3% in Ge on SOQ, despite ~0.2% on Si (or Si-on-insulator). The enhanced strain in Ge on SOQ extends the fundamental optical absorption edge to about 1.65 µm, leading to a potential increase in the L-band responsivity. In this study, a vertical pin PD is fabricated using a Ge layer on SOQ, showing an increased responsivity in the L band.

A bonded SQQ wafer was used as the starting substrate, fabricated by Shin-Etsu Chemical Co. Ltd. The top Si (001) layer was 200 nm in thickness, and was heavily doped p-type by B implantation. An undoped Ge epitaxial layer (500 nm) was grown by chemical vapor deposition using GeH₄ as the source gas, followed by a growth of Si cap layer (50 nm) using Si₂H₆. In the top region of Ge layer as well as in the Si cap layer, a heavily n-type doping was carried out by P implantation, forming a vertical pin junction. Finally, a Ti/TiN electrode was formed. Responsivity spectra were measured in the wavelength range of 1.46–1.64 µm at a normal incidence. A reference pin PD was also fabricated using a Ge layer on a standard bulk Si wafer. According to x-ray diffraction measurements, the Ge layer on SOQ showed a tensile strain of 0.33%, despite 0.14% for the reference Ge on Si.

Figure 1(a) shows typical current–voltage curves for a pin diode of Ge on SOQ measured under dark and under an illumination of 1.55-µm light (1.1 mW). The diode showed a good rectifying property under dark, while under the illumination, the current in the reverse bias significantly increased, showing a reasonable PD operation. Figure 1(b) shows typical responsivity spectra at a reverse bias of 1 V for PDs of Ge on SOQ and reference Ge on Si. Circles and lines correspond to experimental data points and theoretical curves, respectively. The responsivity for Ge on SOQ was larger in the measured wavelength range of 1.46–1.64 µm, including the L band. The increased responsivity is ascribed to the enhanced tensile strain. However, the increase in the L band was found to be suppressed by a high reflectance because of the multiple reflections at the interfaces. An optimized Si cap thickness as well as an anti-reflection coating with SiO₂ can increase the normal-incidence responsivity as high as 0.3 (0.1) A/W up to 1.60 (1.64) µm without an increase in the Ge thickness.

[1] Noguchi et al., J. Appl. Phys. 128, 133107 (2020).