Elizabeth H. Edwards

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.

Simple Electroabsorption Calculator for Designing 1310 nm and 1550 nm Modulators Using Germanium Quantum Wells

With germanium showing significant promise in the design of electroabsorption modulators for full complementary metal oxide semiconductor integration, we present a simple electroabsorption calculator for Ge/SiGe quantum wells. To simulate the quantum-confined Stark effect electroabsorption profile, this simple quantum well electroabsorption calculator (SQWEAC) uses the tunneling resonance method, 2-D Sommerfeld enhancement, the variational method and an indirect absorption model. SQWEAC simulations are compared with experimental data to validate the model before presenting optoelectronic modulator designs for the important communication bands of 1310 nm and 1550 nm. These designs predict operation with very low energy per bit ( <; 30×fJ/bit).

Indirect absorption in germanium quantum wells

Germanium has become a promising material for creating CMOS-compatible optoelectronic devices, such as modulators and detectors employing the Franz-Keldysh effect (FKE) or the quantum-confined Stark effect(QCSE), which meet strict energy and density requirements for future interconnects. To improve Ge-based modulator design, it is important to understand the contributions to the insertion loss (IL). With indirect absorption being the primary component of IL, we have experimentally determined the strength of this loss and compared it with theoretical models. For the first time, we have used the more sensitive photocurrent measurements for determining the effective absorption coefficient in our Ge/SiGe quantum well material employing QCSE. This measurement technique enables measurement of the absorption coefficient over four orders of magnitude. We find good agreement between our thin Gequantum wells and the bulk material parameters and theoretical models. Similar to bulk Ge, we find that the 27.7 meV LA phonon is dominant in these quantum confined structures and that the electroabsorption profile can be predicted using the model presented by Frova, Phys. Rev., 145 (1966).

Electrically controlled modulation in a photonic crystal nanocavity

We describe a compact modulator based on a photonic crystal nanocavity whose resonance is electrically controlled through an integrated p-i-n junction. The sub-micron size of the nanocavity promises very low capacitance, high bandwidth, and efficient on-chip integration in optical interconnects.

Investigation of Limits to the Optical Performance of Asymmetric Fabry-Perot Electroabsorption Modulators

We have investigated the suitability of surface-normal asymmetric Fabry-Perot electroabsorption modulators for short-distance optical interconnections between silicon chips. These modulators should be made as small as possible to minimize device capacitance; however, size-dependent optical properties impose constraints on the dimensions. We have thus performed simulations that demonstrate how the optical performance of the modulators depends on both the spot size of the incident beam and the dimensions of the device. We also discuss the tolerance to nonidealities such as surface roughness and beam misalignment. The particular modulators considered here are structures based upon the quantum-confined Stark effect in Ge/GeSi quantum wells. We present device designs that have predicted extinction ratios greater than 7 dB and switching energies as low as 10 fF/bit, which suggests that these silicon-compatible devices can enable high interconnect bandwidths without the need for wavelength division multiplexing.

Ge/SiGe asymmetric Fabry-Perot quantum well electroabsorption modulators

We demonstrate vertical-incidence electroabsorption modulators for free-space optical interconnects. The devices operate via the quantum-confined Stark effect in Ge/SiGe quantum wells grown on silicon substrates by reduced pressure chemical vapor deposition. The strong electroabsorption contrast enables use of a moderate-Q asymmetric Fabry-Perot resonant cavity, formed using a film transfer process, which allows for operation over a wide optical bandwidth without thermal tuning. Extinction ratios of 3.4 dB and 2.5 dB are obtained for 3 V and 1.5 V drive swings, respectively, with insertion loss less than 4.5 dB. For 60 ?m diameter devices, large signal modulation is demonstrated at 2 Gbps, and a 3 dB modulation bandwidth of 3.5 GHz is observed. These devices show promise for high-speed, low-energy operation given further miniaturization.

Surface-Normal Ge/SiGe Asymmetric Fabry–Perot Optical Modulators Fabricated on Silicon Substrates

We demonstrate the first vertical-incidence Ge/SiGe quantum well reflection modulators fabricated entirely on standard silicon substrates. These modulators could help enable massively parallel, free-space optical interconnects to silicon chips. An asymmetric Fabry-Perot resonant cavity is formed around the quantum well region by alkaline etching the backside of the Si substrate to leave suspended SiGe membranes, upon which high-index-contrast Bragg mirrors are deposited. Electroabsorption and electrorefraction both contribute to the reflectance modulation. The devices exhibit greater than 10 dB extinction ratio with low insertion loss of 1.3 dB. High-speed modulation with a 3 dB bandwidth of 4 GHz is demonstrated. The moderate-Q cavity (Q ~ 600) yields an operating bandwidth of more than 1 nm and permits operation without active thermal stabilization.

Si-Ge surface-normal asymmetric Fabry-Perot quantum-confined stark effect electroabsorption modulator

The strong electroabsorption modulation possible in Ge/SiGe quantum wells promises efficient, CMOS-compatible integrated optical modulators. Using an asymmetric Fabry-Perot design, we demonstrate the first surface-normal semiconductor modulator structure grown on silicon.

Ge quantum well resonator modulators

The strong electroabsorption modulation possible in Ge/SiGe quantum wells promises efficient, CMOS-compatible integrated optical modulators. We demonstrate surface-normal asymmetric Fabry-Perot and microdisk resonator modulators employing Ge quantum wells grown on silicon.

Spot size effects in asymmetric fabry-perot electroabsorption modulators

Simulations of an asymmetric Fabry-Perot modulator reveal degradation of the contrast ratio as the incident spot size is decreased. The minimum practical spot size imposes constraints on the modulator dimensions and hence device capacitance.