Band gap shrinkage induced by tensile strain is shown for Ge directly grown on Si substrate. In Ge-on-Si pin diodes, photons having energy lower than the direct band gap of bulk Ge were efficiently detected. According to photoreflectance measurement, this property is due to band gap shrinkage. The origin of the shrinkage is not the Franz–Keldysh effect but rather tensile strain. It is discussed that the generation of such a tensile strain can be ascribed to the difference of thermal expansion between Ge and Si. Advantages of this tensile Ge for application to photodiode are also discussed.

We demonstrate a 0.25% tensile strained Ge p-i-n photodetector on Si platform that effectively covers both C and L bands in telecommunications. The direct band edge of the Ge film has been pushed from 1550 to 1623 nm with 0.25% tensile strain, enabling effective photon detection in the whole L band. The responsivities of the device at 1310, 1550, and 1620 nm are 600, 520, and 100mA∕W under 0 V bias, which can be further improved to 980, 810, and 150mA∕W with antireflection coating based on calculations. Therefore, the device covers the whole wavelength range used in telecommunications. The responsivities at 1310 and 1550 nm are comparable to InGaAs photodetectors currently used in telecommunications. In the spectrum range of 1300–1650 nm, maximum responsivity was already achieved at 0 V bias because carrier transit time is much shorter than carrier recombination life time, leading to ∼100% collection efficiency even at 0 V bias. This is a desirable feature for low voltage operation. The absorption coefficients of 0.25% tensile strained Ge in the L band have been derived to be nearly an order of magnitude higher than bulk Ge. The presented device is compatible with conventional Si processing, which enables monolithic integration with Si circuitry.

We demonstrate a high-performance, tensile-strained Ge p-i-n photodetector on Si platform with an extended detection spectrum of 650–1605 nm and a 3 dB bandwidth of 8.5 GHz measured at λ=1040nm. The full bandwidth of the photodetector is achieved at a low reverse bias of 1 V, compatible with the low driving voltage requirements of Si ultralarge-scale integrated circuits. Due to the direct bandgap shrinkage induced by a 0.20% tensile strain in the Ge layer, the device covers the entire C band and a large part of the L band in telecommunications. The responsivities of the device at 850, 980, 1310, 1550, and 1605 nm are 0.55, 0.68, 0.87, 0.56, and 0.11A∕W, respectively, without antireflection coating. The internal quantum efficiency in the wavelength range of 650–1340 nm is over 90%. The entire device was fabricated using materials and processing that can be implemented in a standard Si complementary metal oxide semiconductor (CMOS) process flow. With high speed, a broad detection spectrum and compatibility with Si CMOS technology, this device is attractive for applications in both telecommunications and integrated optical interconnects.

Epitaxially grown Ge layers on Si substrate are shown to reveal an enhanced absorption of near-infrared light, which is effective for the photodiode application in Si-based photonics. Ge layers as thick as 1μm were grown on Si substrate by ultrahigh-vacuum chemical-vapor deposition with a low-temperature buffer layer technique. X-ray-diffraction measurements showed that the Ge layer possesses a tensile strain as large as 0.2%, which is generated during the cooling from the high growth temperature due to the thermal-expansion mismatch between Ge and Si. Photoreflectance measurements showed that the tensile strain reduces the direct band-gap energy to 0.77 eV (c.f. 0.80 eV for unstrained Ge), as expected from the theory. Reflecting the band-gap narrowing, photodiodes fabricated using the Ge layer revealed an enhanced absorption of near-infrared light with the photon energy below 0.80 eV, i.e., with the wavelength above 1.55μm. This property is effective to apply the photodiodes to the L band (1.56–1.62μm) in the optical communications as well as the C band (1.53–1.56μm). It is shown that the experimental absorption spectrum agrees with the theoretical one taking into account the splitting of light-hole and heavy-hole valence bands accompanied by the band-gap narrowing. Based on the calculation, the performance of the photodiode using the tensile-strained Ge is discussed.

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