Strained germanium nanowire optoelectronic devices for photonic-integrated circuits

Abstract

Strained germanium nanowires have recently become an important material of choice for silicon-compatible optoelectronic devices. While the indirect bandgap nature of germanium had long been problematic both in light absorption and emission, recent successful demonstrations of bandstructure engineering by elastic strain have opened up the possibility of achieving direct bandgap in germanium, paving the way towards the realization of various high-performance optical devices integrated on a silicon platform. In particular, the latest demonstration of a low-threshold optically pumped laser in a highly strained germanium nanowire is expected to vitalize the field of silicon photonics further. Here, we review recent advances and challenges in strained germanium nanowires for optoelectronic applications such as photodetectors and lasers. We firstly introduce the theoretical foundation behind strained germanium nanowire optoelectronics. And several practical approaches that have been proposed to apply tensile strain in germanium nanowires are further discussed. Then we address the latest progress in the developments of strained germanium nanowire optoelectronic devices. Finally, we discuss the implications of these experimental achievements and the future outlook in this promising research field.

The theoretical direct-band-gap optical gain of Germanium nanowires

“…Finally, nanowires can be easily integrated in photonic-integrated circuits because of the small size, as reported in Ref. 1 . Parallel to the experimental work, the theoretical work on Ge nanowires were also carried out.…”

“…As we know, Gemanium (Ge) is a special kind of semiconductor material compared with Silicon (Si). Because on the one hand, it has small carrier effective mass, indicating that the large carrier mobilities in the devices made of Ge material, and on the other hand, a small direct band gap is about 0.80 eV at the minimum Γ-vally, which is 136 meV larger than the indirect band gap at the minimum L-valley 1 . The lower energy in the L-valley makes the applications of Ge-based optical devices in laser fields a big obstacle, because most electrons will fill the indirect L-valley firstly and only a small amount of electrons can leak into the direct Γ-valley, thus the low luminescent efficiency will be encountered.…”

“…Besides the quantum wells, a number of other papers are devoted to studying the properties of Ge-related nanowires. For example, in experiments, Ge nanowires were reported to be synthesized successfully in many groups 1,[18][19][20] , and especially, the excellent size control of the diameters of Ge nanowires below 10 nm were able to be realized 19,21,22 . In comparison with the bulk structures, there are many advantages for the use of nanowires or quantum wires in the field of optoelectronics.…”

Second-harmonic generation in germanium-on-insulator from visible to telecom wavelengths

“…In contrast, for Ge, there is only a theoretical prediction of the mid-infrared (MIR) SHG in a strained Ge waveguide 16 . Since Ge is already on the list of materials in the Si photonics foundry and has recently gained prominence for on-chip lasers [17][18][19][20][21][22] and MIR waveguides 23,24 . It is desirable to enable and enhance second-order nonlinear optical processes in Ge to widen the functionality of this important material.…”

The Theoretical Optical Gain of Ge1−xSnx Nanowires

“…The indirect‐band‐gap nature of semiconductors Si and Ge make their applications harmful in the filed of lasers and light emitting diodes (LEDs), although they have been reported for realizing optoelectronic devices . However, compared with Si, the studies on Ge are more interesting due to the small energy difference in minimum bandgap, namely about 136 meV, between the indirect L ‐valley and direct Γ‐valley in Ge material . Therefore, a lot of research efforts, including applying the tensile strain and doping other elements in Ge material, were devoted to eliminate such small energy difference and obtain the direct band gap.…”