L. Lever

Modulation of the absorption coefficient at 1.3 μm in Ge/SiGe multiple quantum well heterostructures on silicon

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 Si0.22Ge0.78 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.

Low-voltage broad-band electroabsorption from thin Ge/SiGe quantum wells epitaxially grown on silicon.

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.

Ge-on-Si Single-Photon Avalanche Diode Detectors: Design, Modeling, Fabrication, and Characterization at Wavelengths 1310 and 1550 nm

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 noise-equivalent 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.

Design of Ge–SiGe Quantum-Confined Stark Effect Electroabsorption Heterostructures for CMOS Compatible Photonics

We describe a combined 6 × 6 k · p and one-band effective mass modelling tool to calculate absorption spectra in Ge-SiGe multiple quantum well (MQW) heterostructures. We find good agreement with experimentally measured absorption spectra of Ge-SiGe MQW structures described previously in the literature, proving its predictive capability, and the simulation tool is used for the analysis and design of electroabsorption modulators. We employ strain-engineering in Ge-SiGe MQW systems to design structures for modulation at 1310 nm and 1550 nm.

Extended density-matrix model applied to silicon-based terahertz quantum cascade lasers

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 knowledge of electronic band structure, allowing its use in semiautomated 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 counterpropagating terms. We simulate the potential performance of bound-to-continuum Ge/SiGe QCLs and find that devices with 4\char21{}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.

Terahertz ambipolar dual-wavelength quantum cascade laser

Terahertz frequency quantum cascade lasers (THz QCLs) are compact solid-state sources of terahertz radiation that were first demonstrated in 2002. They have a broad range of potential applications ranging from gas sensing and non-destructive testing, through to security and medical imaging, with many polycrystalline compounds having distinct fingerprint spectra in the terahertz frequency range. In this article, we demonstrate an electrically-switchable dual-wavelength THz QCL which will enable spectroscopic information to be obtained within a THz QCL-based imaging system. The device uses the same active region for both emission wavelengths: in forward bias, the laser emits at 2.3 THz; in reverse bias, it emits at 4 THz. The corresponding threshold current densities are 490 A/cm(2) and 330 A/cm(2), respectively, with maximum operating temperatures of 98K and 120 K.

Si/SiGe quantum cascade superlattice designs for terahertz emission

Quantum cascade lasers (QCLs) are compact sources that have demonstrated high output powers at terahertz (THz) frequencies. To date, all THz QCLs have been realized in III-V materials. Results are presented from Si1−xGex quantum cascade superlattice designs emitting at around 3 THz which have been grown in two different chemical vapor deposition systems. The key to achieving successful electroluminescence at THz frequencies in a p-type system has been to strain the light-hole states to energies well above the radiative subband states. To accurately model the emission wavelengths, a 6-band k⋅p tool which includes the effects of nonabrupt heterointerfaces has been used to predict the characteristics of the emitters. X-ray diffraction and transmission electron microscopy have been used along with Fourier transform infrared spectroscopy to fully characterize the samples. A number of methods to improve the gain from the designs are suggested.

Material configurations for n-type silicon-based terahertz quantum cascade lasers

Silicon-based quantum cascade lasers (QCLs) offer the prospect of integrating coherent terahertz (THz) radiation sources with silicon microelectronics. Theoretical studies have proposed a variety of

Adiabatic mode coupling between SiGe photonic devices and SOI waveguides.

n

Theory and design of quantum cascade lasers in (111) n -type Si/SiGe

-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 semiclassical 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.