迈向硅芯片上锗光源的单片集成

Germanium (Ge) is a group-IV indirect band gap semiconductor, and therefore bulk Ge cannot emit light effiffifficiently. However, the direct band gap energy is close to the indirect one, and signifificant engineering efffforts are being made to convert Ge into an effiffifficient gain material monolithically integrated on a Si chip. In this article, we will review the engineering challenges of developing Ge light sources fabricated using nano-fabrication technologies compatible with Complementary MetalOxide-Semiconductor (CMOS) processes. In particular, we review recent progress in applying high-tensile strain to Ge to reduce the direct band gap. Another important technique is doping Ge with donor impurities to fifill the indirect band gap valleys in the conduction band. Realization of carrier confifinement structures and suitable optical cavities will be discussed. Finally, we will discuss possible applications of Ge light sources in potential photonics-electronics convergent systems.

Feynman precisely predicted the emergence of hand-held mobile computers with human face recognition systems  in 1959, when historical patents for Integrated Circuits (ICs) were fifiled by Kilby  and Noyce. Currently, more than 1 billion ComplementaryMetal-Oxide-Semiconductor (CMOS) Field-Effffect Transistors (FETs) are integrated in a Large-scale IC (LSI), and the semiconductor industry has been continuously scaling down the channel length of FETs in the last 4 decades towards sub-10-nm technology nodes. In addition to the electronic integration, various photonic devices have also been integrated on silicon (Si).

After the discovery of effiffifficient light emission from porous-Si by quantum confifinements , the performance of Si-based light sources has steadily improved over the years . In particular, a quantum effiffifficiency exceeding 23 % was achieved by passivating the surface using high-pressure water vapour annealing . These Si-based light sources would be useful for applications with wide wavelengths over a wide rang of visible to near-infrared (NIR). However, they are not compatible with Si photonic passive devices using a Si WG, since the emitted light from a quantum confifined Si nano-structure is blue-shifted compared with the band gap energy of Si (1.1 eV), and thus it will be strongly absorbed in a Si WG.

In this section, we will overview various technical approaches to develop Ge-based light sources using Si processes, and we will discuss technical details in the later sections. Figure 1 shows a schematic summary of various options to fabricate Ge-based light sources. Of course, there are many other process options, which are not covered in Fig. 1. The conventional wisdom for developing a new LD is to heavily rely only on epitaxial growth techniques . Whilst fully respecting the importance of the process developments in epitaxial growth, however, we feel that other routes towards the development of Ge-based light sources will also be important, since various nanofabrication technologies are available in Si processes in addition to epitaxial growth. However, the technical hurdle of converting an indirect band gap semiconductor to a direct band gap semiconductor is not easy to overcome, and the use of all available technologies in Si processes should be considered for Ge-based light sources.

Fig1

As we discussed above, n-type doping to fifill the L-valley certainly enhances the spontaneous light-emission probability, which was confifirmed by many groups and thus is well established. On the other hand, the impact of doping on the emergence of the optical gain is still controversial. In fact, the crystalline qualities of actual Ge samples depend on the process technologies used, and thus the carrier life-time can vary substantially, which would affffect the complex carrier dynamics upon pumping. This makes the impact of doping highly non-trivial . Moreover, some impurities are not activated upon doping, which would further reduce the carrier life-time, and the inhomogeneous strain profifile would further affffect the actual carrier profifile. At this moment, there is no consensus on how much we should dope Ge with n-type impurities to increase the gain value.

Nevertheless, n-type doping has been shown to improve the quantum effiffifficiency of LEDs. Here, we will discuss various technological options for n-type doping of Ge, as summarized in Table 3. In addition to the carrier concentration reported, the sheet resistance was also listed in Table 3, although the actual sheet resistance also depends on the depth of the impurity profifile.