Quantum-confined Stark Effect Electroabsorption Research Articles

30-Gb/s Optical Link Combining Heterogeneously Integrated III–V/Si Photonics With 32-nm CMOS Circuits

We present a silicon photonics optical link utilizing heterogeneously integrated photonic devices driven by low-power advanced 32-nm CMOS integrated circuits. The photonic components include a quantum-confined Stark effect electroabsorption modulator and an edge-coupled waveguide photodetector, both made of III-V material wafer bonded on silicon-on-insulator wafers. The photonic devices are wire bonded to the CMOS chips and mounted on a custom PCB card for testing. We demonstrate an error-free operation at data rates up to 30 Gb/s and transmission over 10 km at 25 Gb/s with no measured sensitivity penalty and a timing margin penalty of 0.2 UI.

(Invited) Light Emission from Highly-Strained Germanium for On-Chip Optical Interconnects

The relentless device scaling paradigm is being threatened by the limits of Cu-based interconnects including excessive power dissipation, insufficient communication bandwidth, and signal latency. Many of these limitations are caused by the increase in Cu resistivity as wire dimensions are scaled down [1]. To alleviate this performance bottleneck in integrated circuits (ICs), optical interconnects, which have already revolutionized long-haul communications, have recently gained much attention for on-chip applications.Ge has emerged as a viable candidate to augment Si for CMOS and optoelectronics. First, the lower effective mass and lower valley degeneracy of Ge could alleviate the problem by providing a higher source injection velocity, rendering higher drive current and smaller gate delay in MOSFETs [2]. Second, Ge has a larger absorption coefficient than Si in the telecommunications wavelength range (1.3-1.55 µm), which makes it attractive for integration of monolithic optical components for the use in optical interconnects. Third, lower processing temperatures allow 3-D integration of Ge on Si.Over the past decade many of the key Ge-based constituents of an on-chip optical interconnect system, such as high-performance photodetectors and modulators, have been demonstrated on a silicon-compatible platform. We have demonstrated p-i-n [3] and metal–semiconductor–metal (MSM) photodetectors [4] in Ge grown on Si with excellent quantum efficiency and responsivities at a wavelength of 1.55 µm. Dark current, a concern for Ge photodetectors due to its small bandgap, has been mitigated by using electrodes with asymmetric work functions in MSM detectors [4]. Researchers have also observed quantum-confined Stark effect electro-absorption in Ge quantum wells with SiGe barriers grown on Si substrates, and a Ge modulator based on this effect has been demonstrated on a Si substrate [5]. This effect is very promising for high-speed, low-power modulators fabricated compatibly with mainstream Si ICs [5].An efficient light source, on the other hand, remains particularly challenging: Si and Si-compatible materials such as Ge are not readily suitable for light emission because their band gaps are indirect. In Ge, however, the energy difference between the direct Γ valley and the indirect L valley is only 136 meV and this difference can be reduced further by introducing tensile strain [6]. It has been proposed to use tensile strain to make Ge’s band gap direct and thus suitable for light emission [7,8], however highly-strained Ge experimental device realization has thus far been limited to LEDs [9] rather than lasers.In this presentation, we focus on developing an efficient Si-compatible light emitter based on highly-strained Ge technology. Starting from theoretical calculations showing how tensile strain can improve the light emission efficiency of Ge [7,8], we present several approaches for enhancing light emission from highly strained Ge on a CMOS-compatible platform. We have developed methods to grow single crystal Ge films with low defect densities on Si [10]. First, we describe a thin film membrane technique in which a large residual stress in a tungsten layer as a stressor is used to induce a biaxial tensile strain in a Ge membrane, upon which we have fabricated optoelectronic devices [11]. Second, we introduce an approach to induce sufficiently large uniaxial strain to create a direct band gap in Ge wire using geometrical amplification of a small pre-existing strain induced during the heteroepitaxial growth of Ge on Si [12]. Lastly, we present a novel way to mimic double-heterostructure behavior within a single material, further enhancing light emission from Ge by capturing photo-generated carriers within a strain-induced potential well [13]. In summary we have demonstrated integration of Ge on Si for optoelectronic applications. This should alleviate a critical performance bottleneck in the continued scaling of integrated circuits to future technology nodes.

Ge/SiGe quantum confined Stark effect electro-absorption modulation with low voltage swing at λ = 1550 nm.

Low-voltage swing (≤1.0 V) high-contrast ratio (6 dB) electro-absorption modulation covering 1460 to 1560 nm wavelength has been demonstrated using Ge/SiGe quantum confined Stark effect (QCSE) diodes grown on a silicon substrate. The heterolayers for the devices were designed using an 8-band k.p Poisson-Schrödinger solver which demonstrated excellent agreement with the experimental results. Modelling and experimental results demonstrate that by changing the quantum well width of the device, low power Ge/SiGe QCSE modulators can be designed to cover the S- and C-telecommunications bands.

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).

Quantum-Confined Stark Effect in Ge/SiGe Quantum Wells on Si

In this paper, we present observations of quantum confinement and quantum-confined Stark effect electroabsorption in Ge quantum wells with SiGe barriers grown on Si substrates. Though Ge is an indirect gap semiconductor, the resulting effects are at least as clear and strong as seen in typical III-V quantum well structures at similar wavelengths. We also designed and fabricated a coplanar high-speed modulator, and demonstrated modulation at 10 GHz and a 3.125-GHz eye diagram for 30-¿m-sized modulators.

Controlled disordering of compressively strained InGaAsP multiple quantum wells under SiO:P encapsulant and application to laser-modulator integration

We investigated the potentiality of a phosphorus-doped silicon oxide (SiO:P) carrier-free disordering source for applications in photonic devices integration schemes. This is accomplished in three successive steps by employing an InGaAsP/InGaAsP structure with compressively strained wells and lattice-matched barriers designed for operation around ∼1.55 μm. First of all, we showed that the SiO:P encapsulant offers a good control over a wide range of disorder (blue shifts as high as ∼150 meV). Later on, the high optical quality of the disordered regions is demonstrated by detecting 300 K excitonic features in moderately blue-shifted (∼40 meV) samples. And, finally, a first attempt of its application in integration technology is made by realizing a monolithic composite of a distributed feedback laser and a quantum-confined stark effect electroabsorption modulator operating around 1.54 μm.

Comparative analysis of the performance limits of Franz-Keldysh effect and quantum-confined Stark effect electroabsorption waveguide modulators

The author systematically analyses and compares the performance limits of quantum-confined Stark effect (QCSE) and Franz-Keldysh effect (FKE) electroabsorption waveguide modulators, in terms of insertion loss, contrast ratio, drive power and bandwidth (or bit-rate). The author first derives the universal material figures of merit for electroabsorption modulators which form the basis for comparison. In addition to the magnitude of electroabsorption Delta alpha , the critical material parameters are Delta alpha / alpha0 and Delta alpha /F2 where alpha0 is the onstate residual absorption, and F is the applied electric field. The author proposes a waveguide design which will meet the insertion loss and contrast ratio requirements while minimising the available power/bandwidth (Pac/ Delta f) ratio. The author shows that, while satisfying the same insertion loss requirement, a QCSE modulator employing the optimum quantum well structure can have, in principle, an order of magnitude better performance in terms of Pac/ Delta f than one based on FKE. Correspondingly, the Delta alpha /F2 is an order of magnitude larger in QCSE than in FKE, principally because of the much larger absorption change that is possible with QCSE.

Multiple quantum well reflection modulator

We demonstrated a quantum-confined Stark effect electroabsorption modulator consisting of quantum wells of AlGaAs and GaAs on an epitaxial multilayer dielectric mirror, all grown by molecular beam epitaxy. The resulting reflection modulator avoids problems of substrate absorption, and has relatively high contrast ratio (up to ∼8:1 with peak reflectivity of 25% at 853 nm) because the light passes twice through the quantum wells. Reflection modulators are of interest for bidirectional communication systems, in parallel arrays of optical switching and processing devices and for optical interconnects. For the latter there exists the possibility of this device grown on the same substrate alongside a GaAs integrated circuit or even on Si substrates.