David T. Danielson

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.

Surface-energy-driven dewetting theory of silicon-on-insulator agglomeration

The thermal agglomeration of ultrathin (<30nm) single crystal silicon-on-insulator (SOI) films is a morphological evolution phenomenon with practical and scientific importance. This materials phenomenon represents both a critical process limitation for the fabrication of advanced ultrathin SOI-based semiconductor devices as well as a scientifically interesting morphological evolution problem. Investigations to date have attributed this phenomenon to a stress-induced morphological instability. In this paper, we demonstrate that SOI agglomeration is a surface-energy-driven dewetting phenomenon. Specifically, we propose that agglomeration occurs via a two-step surface-energy-driven mechanism consisting of (1) defect-mediated film void nucleation and (2) surface-diffusion-limited film dewetting via capillary edge and generalized Rayleigh instabilities. We show that this theory can explain all of the key experimental observations from the SOI agglomeration literature, including the locations of agglomeration 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...

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.

Monolithic Si-based technology for optical receiver circuits

Optical communications networks must be terminated by receiver circuitry capable of converting an optical circuit to an electrical one. While current III-V technology is capable of delivering high performance, it is costly and difficult to integrate with low-cost Si based technologies. In order to overcome these barriers, we are pursuing a Si-compatible technology for integrated photodetectors. Ge, monolithically integrated with Si, offers a low-cost, high-performance materials system for photodetector integration with existing Si technology. In this paper we discuss the performance requirements and figures of merit for integrated photodetectors. We then discuss the materials issues associated with the integration of Ge on Si and show that high quality Ge films can be grown directly on Si, despite the 4% lattice mismatch. By cyclic annealing after growth, the dislocation density can be reduced to 2.3x107 cm-2, and diodes fabricated on these films show a responsivity of 300 mA/W at 1300 nm without an AR coating. Finally, we discuss the integration of waveguides with photodetectors and propose an integration scheme we believe will be capable of delivering high-performance integrated photoreceivers on a Si platform.

Electronic Photonic Integrated Circuits for High Speed, High Resolution, Analog to Digital Conversion

Progress in developing high speed ADCs occurs rather slowly - at a resolution increase of 1.8 bits per decade. This slow progress is mostly caused by the inherent jitter in electronic sampling - currently on the order of 250 femtoseconds in the most advanced CMOS circuitry. Advances in femtosecond lasers and laser stabilization have led to the development of sources of ultrafast optical pulse trains that show jitter on the level of a few femtoseconds over the time spans of typical sampling windows and can be made even smaller. The MIT-GHOST (GigaHertz High Resolution Optical Sampling Technology) Project funded under DARPAs Electronic Photonic Integrated Circuit (EPIC) Program is trying to harness the low noise properties of femtosecond laser sources to overcome the electronic bottleneck inherently present in pure electronic sampling systems. Within this program researchers from MIT Lincoln Laboratory and MIT Campus develop integrated optical components and optically enhanced electronic sampling circuits that enable the fabrication of an electronic-photonic A/D converter chip that surpasses currently available technology in speed and resolution and opens up a technology development roadmap for ADCs. This talk will give an overview on the planned activities within this program and the current status on some key devices such as wavelength-tunable filter banks, high-speed modulators, Ge photodetectors, miniature femtosecond-pulse lasers and advanced sampling techniques that are compatible with standard CMOS processing.

High performance Ge p-i-n photodetectors on Si

We demonstrate a high performance Ge p-i-n photodetector on Si platform with an extended detection spectrum of 650-1605 nm, a 3 dB bandwidth of 8.5 GHz, and a responsivity of 0.68 A/W, measured at /spl lambda/ = 1040 nm.