The design, modeling, fabrication, and characterization of single-photon avalanche diode detectors with an epitaxial Ge absorption region grown directly on Si are presented. At 100 K, a single-photon detection efficiency of 4% at 1310 nm wavelength was measured with a dark count rate of ∼6 megacounts/s, resulting in the lowest reported noiseequivalent power for a Ge-on-Si single-photon avalanche diode detector (1 × 10 −14 WHz −1/2 ). The first report of 1550 nm wavelength detection efficiency measurements with such a device is presented. A jitter of 300 ps was measured, and preliminary tests on after-pulsing showed only a small increase (a factor of 2) in the normalized dark count rate when the gating frequency was increased from 1 kHz to 1 MHz. These initial results suggest that optimized devices integrated on Si substrates could potentially provide performance comparable to or better than that of many commercially available discrete technologies.Index Terms-Detector, germanium on silicon, single-photon avalanche diode, single-photon counting.

We demonstrate electroabsorption contrast greater than 5 dB over the entire telecommunication S- and C-bands with only 1V drive using a new Ge/SiGe QW epitaxy design approach; further, this is demonstrated with the thinnest Ge/SiGe epitaxy to date, using a virtual substrate only 320-nm-thick. We use an eigenmode expansion method to model the optical coupling between SOI waveguides and both vertically and butt-coupled Ge/SiGe devices, and show that this reduction in thickness is expected to lead to a significant improvement in the insertion loss of waveguide-integrated devices.

Silicon-based terahertz quantum cascade lasers (QCLs) offer potential advantages over existing III-V devices. Although coherent electron transport effects are known to be important in QCLs, they have never been considered in Si-based device designs. We describe a density matrix transport model that is designed to be more general than those in previous studies and to require less a priori knowlege of electronic bandstructure, allowing its use in semi-automated design procedures. The basis of the model includes all states involved in interperiod transport, and our steady-state solution extends beyond the rotating-wave approximation by including DC and counter-propagating terms. We simulate the potential performance of bound-to-continuum Ge/SiGe QCLs and find that devices with 4-5-nm-thick barriers give the highest simulated optical gain. We also examine the effects of interdiffusion between Ge and SiGe layers; we show that if it is taken into account in the design, interdiffusion lengths of up to 1.5 nm do not significantly affect the simulated device performance.

Silicon-based quantum cascade lasers (QCLs) offer the prospect of integrating coherent THz radiation sources with silicon microelectronics. Theoretical studies have proposed a variety of n-type SiGe-based heterostructures as design candidates, however the optimal material configuration remains unclear. In this work, an optimization algorithm is used to design equivalent THz QCLs in three recently-proposed configurations [(001) Ge/GeSi, (001) Si/SiGe and (111) Si/SiGe], with emission frequencies of 3 and 4 THz. A systematic comparison of the electronic and optical properties is presented. A semi-classical electron transport simulation is used to model the charge carrier dynamics and calculate the peak gain, the corresponding current density and the maximum operating temperature. It is shown that (001) Ge/GeSi structures yield the best simulated performance at both emission frequencies.

We report modulation of the absorption coefficient at 1:3 μm in Ge/SiGe multiple quantum well heterostructures on silicon via the quantum-confined Stark effect. Strain engineering was exploited to increase the direct optical bandgap in the Ge quantum wells. We grew 9 nm-thick Ge quantum wells on a relaxed Si 0:22 Ge 0:78 buffer and a contrast in the absorption coefficient of a factor of greater than 3.2 was achieved in the spectral range 1290-1315 nm. © 2011 Optical Society of America OCIS codes: 230.4110, 230.4205, 250.4110, 250.5590, 260.6580. Existing silicon Mach-Zehnder modulators that exploit the carrier dispersion effect are typically either large and dissipate considerable amounts of power [1] or require the use of resonant cavities, which have temperature stabilization issues and are very sensitive to fabrication tolerances. The development of silicon-based electroabsorption modulators (EAMs) is desirable for emerging silicon photonics applications, including optical network interconnects and fibre-to-the-home, because such devices can have a small footprint, low power consumption, and good temperature stability. Several optical fiber telecommunications systems exploit the spectral 'window' around 1:3 μm, which corresponds to zero dispersion in standard single-mode fibers. In particular, some passive optical network (PON) architectures use 1:3 μm radiation for upstream signals [2]. Therefore, it is desirable to fabricate optical modulators that can operate at this wavelength. Ge/SiGe multiple quantum well (MQW) heterostructures can be epitaxially grown on silicon wafers using a relaxed buffer layer (see, e.g., [3]), where the alternating layers should be strain-balanced to the buffer layer so that no net strain accumulates in the MQW stack. Previous studies of the quantum-confined Stark effect (QCSE) in Ge/SiGe MQW structures have reported buffers layers with a Ge fraction of 90% or more [4][5][6][7]. Here, we describe absorption spectra for a strain-balanced stack of ten 9 nm thick Ge quantum wells and eleven 7 nm thick Si 0:4 Ge 0:6 barriers grown on a relaxed Si 0:22 Ge 0:78 buffer. The large compressive strain in the Ge quantum wells results in an increase in the direct bandgap compared with relaxed Ge, which results in a blue-shift of the absorption edge, and proper choice of the layer widths and compositions allows us to control the absorption edge of the structure [8].The MQW heterostructures were grown using reduced pressure chemical vapor deposition (RP-CVD) on a relaxed Si 0:22 Ge 0:78 buffer. The buffer was grown using reverse linear grading (RLG) from a relaxed Ge seed layer, which was grown on an Si substrate [9]. Further details of the epitaxial growth can be found in [10]. Circular mesa devices of 80 μm diameter were defined using optical lithography and reactive-ion etching and a Ti/Al metal stack was deposited and sintered at 400°C for 30 minutes to form electrical contacts. A schematic diagram of the cross section of the devices is shown in Fig. 1.Absorption spectra were inferred f...