# Electrically Driven Micro- and Nano-Scale Semiconductor Light Sources
## 7 Metal-Waveguide-Integrated Nano-LEDs for Surface Plasmon Polariton (SPP) Sources
Furthermore, when the _p_- and _n_-type metal contacts are defined for the nanomaterial, they should minimally disturb the plasmonic waveguide structure, which is not always straightforward in many nanodevices. 8 Integration of Electrically Driven Light Sources with Silicon-On-Insulator (SOI) Photonic Circuits  
Based on the electrical excitation schemes, we have reviewed various micro- and nano-scale optical and/or plasmonic light sources at a single device level. The research on these devices naturally indicates the next logical step: coupling to, or integration with, other optical components. Indeed, one can find a truly practical usefulness of these devices only if they are efficiently integrated with other passive components, and only if they properly serve as robust light sources in a photonic circuit that is designed to complete a desired function. To this end, finding a good passive photonic platform that eases the integration with low optical loss becomes critical. Among others, silicon-on-insulator (SOI) has been considered as one of the most promising platforms [44; 45; 46; 47]. The high index of refraction can tightly confine the light, resulting in low optical leakage. In addition, silicon is transparent at telecommunication wavelengths. Importantly, the SOI platform can share the mature technological advances of the complementary metal oxide semiconductor (CMOS)  
Figure 10: Injection plasmonic light source based on NW LEDs. (**a**) Electrically pumped NW light source coupled to plasmonic waveguides (left). Synthesized GaAs NW with axial _p-n_ junction emits light that is coupled to plasmonic modes and re-emitted at the waveguide terminations (right) [42]. Figure reproduced with permission from American Chemical Society. (**b**) Top-down fabricated GaAs NW LEDs coupled to double-strip plasmonic waveguide that is gradually tapered at the end (left). Light coupled to plasmonic modes propagates through the waveguide and is focused into a sub-micron-sized light spot (right) [43]. Figure reproduced with permission from American Chemical Society. industry. In particular, this CMOS compatibility can guarantee large-scale fabrication processing, and consequently, electronic -photonic integration. The heterogeneous integration shown in Figure 11 has been a successful solution [47]. The integration takes several key fabrication steps. First, current-injectable InP-based microdisk lasers were separately fabricated in a thin epilayer, while passive SOI waveguide circuits were fabricated using COMS processes. Second, after depositing SiO\({}_{2}\), the SOI wafer was planarized by chemical-mechanical polishing until the targeted thickness of the deposited SiO\({}_{2}\) was reached. Third, the laser devices in the thin epilayer were vertically aligned to, and integrated with, the waveguides in the SOI wafer through the wafer-bonding technique. In 2007, [PERSON] et al. used this method, and reported the successful integration of electrically pumped microdisk lasers onto SOI waveguide circuits [47]. Figure 11a shows the top and cross-sectional views of optical and SEM images of the integrated device, revealing the details of layer configurations. In this device, light is primarily confined in the InP-based microdisk cavity, while a portion of it is evanescently coupled to the SOI waveguides. The microdisk laser with a threshold current of 0.5 mA was operated under the CW condition at room temperature. From the output power spectrum at the end of the SOI waveguide, the estimated slope efficiency just above the threshold was \(\sim\)30 uW/mA, assuming a fiber coupler efficiency and propagation loss. These results represented the actual usefulness and practicality of the electrically driven microdisk lasers in a photonic circuit. For more practical and power-efficient applications, researchers have applied this integration method to even smaller optical cavities, such as PhC nanocavities [48; 49; 50]. [PERSON] et al. (2017) reported a highly power-efficient hybrid InP-based nanolaser diode (Figure 12) [50].