Douglas D. Cannon

Strain-induced band gap shrinkage in Ge grown on Si substrate

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.

High-performance, tensile-strained Ge p-i-n photodetectors on a Si platform

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 ...

Tensile strained Ge p-i-n photodetectors on Si platform for C and L band telecommunications

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 coeffic...

Tensile strained epitaxial Ge films on Si(100) substrates with potential application in L-band telecommunications

Tensile strained epitaxial Ge films were grown on Si(100) substrates by ultra-high vacuum chemical vapor deposition. The tensile strain was induced by the thermal expansion coefficient mismatch between Si and Ge during the cooling process from elevated growth temperatures, which induces narrowing of the Ge direct band gap, EgΓ, and pushes the absorption spectrum of Ge toward longer wavelengths. The EgΓ versus strain relation was measured experimentally by photoreflectance and x-ray diffraction, and the result agrees well with calculations by deformation potential theory. With an in-plane tensile strain of 0.21%, the EgΓ of the Ge film grown at 800 °C decreased from 32 meV to 0.768 eV compared with 0.80 for bulk Ge, and corresponded to an absorption edge at 1610 nm. The broadened absorption spectrum of tensile strained Ge makes it promising as a Si-compatible photodector material for L-band (1560–1620 nm) optical communications.

High-speed resonant cavity enhanced Ge photodetectors on reflecting Si substrates for 1550-nm operation

We have designed and fabricated high-speed resonant cavity enhanced germanium (Ge) Schottky photodetectors on a silicon-on-insulator substrate. These back-illuminated detectors have demonstrated 3-dB bandwidths of more than 12 GHz at 3-V reverse bias and a peak quantum efficiency of 59% (R=0.73 A/W) at the resonant wavelength of /spl sim/1540 nm. Time domain measurements of our Ge photodetectors with diameters of up to 48 /spl mu/m show transit-time limited impulse responses corresponding to bandwidths of at least 6.7 GHz, making these detectors compatible with 10-Gb/s data communication systems.

Strain-induced enhancement of near-infrared absorption in Ge epitaxial layers grown on Si substrate

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) i...

Large electro-optic effect in tensile strained Ge-on-Si films

We report the observation of an enhanced electro-optic effect in the weakly absorbing regime for tensile strained Ge epitaxial films. With Deltan/F=260 pm/V and Deltaalpha/alpha-3 the material has significant potential for field-induced phase or electro-absorption modulator devices

Silicidation-induced band gap shrinkage in Ge epitaxial films on Si

Ge epitaxial films on Si grown at high temperatures show a shrinkage in the direct band gap EgΓ as a result of the tensile strain accumulated during the cooling process after growth, making it a promising candidate for effective photon detection in L-band telecommunications. However, because of strain relaxation at temperatures >750 °C, only about 0.20% tensile strain can be accumulated at most. This leads to a direct band gap of 0.773 eV, corresponding to 1605 nm and is not enough to cover the whole L band (1561–1620 nm). In this letter, we report the strain enhancement in epitaxial Ge films induced by the formation of C54TiSi2 on the backside of the Si wafers. The backside C54-TiSi2 layer not only forms a good electric contact, but also increases the tensile strain of the Ge film on the front side from 0.20% to 0.24% and a further direct band gap shrinkage from 0.773 to 0.765 eV, corresponding to 1620 nm, which covers the whole L band. Since the silicidation process is compatible with Si complementary m...

Resonant cavity enhanced Ge photodetectors for 1550 nm operation on reflecting Si substrates

We have fabricated and characterized the first resonant cavity-enhanced germanium photodetectors on double silicon-on-insulator substrates (Ge-DSOI) for operation around the 1550-nm communication wavelength and have demonstrated over four-fold improvement in quantum efficiency compared to its single-pass counterpart. The DSOI substrate is fabricated using an ion-cut process and optimized for high reflectivity (>90%) in the 1300-1600-nm wavelength range, whereas the Ge layer is grown using a novel two-step ultra-high vacuum/chemical vapor deposition direct epitaxial growth technique. We have simulated a Ge-DSOI photodetector optimized for operation at 1550 nm, exhibiting a quantum efficiency of 76% at 1550 nm given a Ge layer thickness of only 860 nm as a result of both strain-induced and resonant cavity enhancement. For this Ge thickness, we estimate a transit time-limited 3-dB bandwidth of approximately 25 GHz.

Germanium-rich silicon-germanium films epitaxially grown by ultrahigh vacuum chemical-vapor deposition directly on silicon substrates

We have grown device quality germanium-rich silicon-germanium films on silicon substrates using a two-step ultrahigh vacuum chemical-vapor deposition growth process. The films have thermally induced tensile strain, resulting in a direct band gap reduction of ∼30meV, in agreement with what we observe for similarly grown pure germanium films. Our data suggest that alloying of silicon increases the band gap reduction with strain at the high germanium end of the composition range. Annealing of the films allows for reduction in the dislocation density to 2×107∕cm2, comparable to what we achieve in pure germanium films and showing that alloying small amounts of silicon does not inhibit dislocation motion. p-i-n diodes fabricated from these films using a silicon compatible process exhibit reverse leakage currents of ∼10mA∕cm2 at 0.5V reverse bias. The responsivity of a Si0.04.8Ge0.952 diode was measured at 0.23A∕W at 1280nm, demonstrating the high quality of these epitaxial films.