Strong quantum-confined Stark effect in germanium quantum-well structures on silicon

Abstract

Silicon is the dominant semiconductor for electronics, but there is now a growing need to integrate such components with optoelectronics for telecommunications and computer interconnections. Silicon-based optical modulators have recently been successfully demonstrated; but because the light modulation mechanisms in silicon are relatively weak, long (for example, several millimetres) devices or sophisticated high-quality-factor resonators have been necessary. Thin quantum-well structures made from III-V semiconductors such as GaAs, InP and their alloys exhibit the much stronger quantum-confined Stark effect (QCSE) mechanism, which allows modulator structures with only micrometres of optical path length. Such III-V materials are unfortunately difficult to integrate with silicon electronic devices. Germanium is routinely integrated with silicon in electronics, but previous silicon-germanium structures have also not shown strong modulation effects. Here we report the discovery of the QCSE, at room temperature, in thin germanium quantum-well structures grown on silicon. The QCSE here has strengths comparable to that in III-V materials. Its clarity and strength are particularly surprising because germanium is an indirect gap semiconductor; such semiconductors often display much weaker optical effects than direct gap materials (such as the III-V materials typically used for optoelectronics). This discovery is very promising for small, high-speed, low-power optical output devices fully compatible with silicon electronics manufacture.

Indirect absorption in germanium quantum wells

“…6 Since then, Ge-based FKE modulation has achieved 12.5 Gb/s modulation, a 3 dB bandwidth of 30 GHz with only 100 fJ/bit of energy consumption in an integrated Si waveguide architecture. 7 In 2005, QCSE was first demonstrated in Ge/SiGe quantum wells 8,9 with initial results showing the possibility of modulation. 10,11 While it has not yet been tested under digital modulation to such high speeds, 12,13 power consumption has already met 2022 off-chip energy targets of <20 fJ/bit through tight integration with a Si waveguide.…”

Giant Gating Tunability of Optical Refractive Index in Transition Metal Dichalcogenide Monolayers

“…change in optical absorption with the presence of electric fields in the vertical direction, [7][8]15 but most of these works are in the infrared or even lower energy ranges, the tuning efficiency is often small (no more than few percent), or the structure is too bulky for integration with CMOS circuits.…”

Design of low bias voltage Ge/SiGe multiple quantum wells electro-absorption modulator at 1550 nm

“…13 Owing to the quantum-confined Stark effect (QCSE), 14 the absorption spectra of Ge/SiGe quantum well have a red shift with electric field applied across the structure, first demonstrated in 2005. 15 Previous researches adopted thin quantum well with thick barrier structure and produced large compressive strain in the Ge well which shifts the absorption edge to shorter wavelength. To neutralize this effect, bias voltage of 2-5 V is required to perform at 1550 nm.…”

“…To neutralize this effect, bias voltage of 2-5 V is required to perform at 1550 nm. 10,11,13,15,16 Thus the modulator is not convenient to integrate with electronic logic circuits. Moreover, the thick barrier reduces the overlap between optical field and effective absorption area in a waveguide configuration, as the electrons and holes are confined in the quantum wells.…”