# Electrically Driven Micro- and Nano-Scale Semiconductor Light Sources
## 5 Nanowires (NW)-Based Electrically Driven Light Sources  
Planar semiconductor growth technologies allow for the mass production of high-quality crystals with precisely controlled energy band structures. In addition, well-developed semiconductor microprocessing enables a robust and reproducible fabrication of electrically driven, active photonic devices. However, the sophisticated device structures associated with multiple junctions and bandgap engineering, and the high cost of device fabrication, make these light sources difficult to be integrated directly with other technologies such as silicon-based electronics, photonics, and micromechanical platforms. To address these issues for integrated applications, various active organic/inorganic materials and nanostructures have been transformed into devices, testing feasibility of achieving high-quality light generation [34; 35; 36; 37]. In particular, rapid advancement in synthesizing 1D nanomaterials with controlled sizes and tunable electrical and optical properties has accelerated the development of electrically driven light sources [36; 37]. Single-crystal semiconductor NWs exhibit several features that render them very attractive for optoelectronic devices. First, the semiconductors used for synthesizing NWs represent a high index of refraction in a broad range of optical frequencies. As a result, the single-NW functions as a stand-alone Fabry-Perot optical cavity and gain medium. Second, the high crystallinity ensures defect-free structures that guarantee both superior electrical transport and strong photon confinement. Third, the precise synthetic control allows for the rational design of electronic band structures, which enables the broadband tunability of central emission wavelengths. The first demonstration of an electrical injection single NW laser is shown in Figure 7[36]. Single-crystal \(n\)-type CdS NW optical cavities are registered onto \(p\)-Si electrodes defined in heavily \(p\)-doped planar substrates. This hybrid structure forms an \(n\)-CdS/\(p\)-Si hetero-junction, which provides a channel through which the carriers can flow. After the electrical isolation by Al\({}_{2}\)O\({}_{3}\) and \(n\)-type electrode metallization, the electrons and holes are injected along the entire length of the CdS NW. Bright-field optical microscope image shows a fully fabricated, single NW laser device. A strong EL emission at the NW end was observed at an injection current of 80 \(\mu\)A (below threshold). The device exhibited evident signatures of lasing oscillation (e.g., superlinear increase in output intensity vs. input current, abrupt linewidth reduction) at an injection current \(>\)200 \(\mu\)A. However, the lasing operation at levels substantially above threshold was limited due to the non-uniformity of carrier injection originating from the non-ideal \(n\)-Cd/\(p\)-Si junction, making it difficult to determine the clear threshold current. Tuning of the emission wavelength of single NWs in a broad range of optical frequencies has been successfully demonstrated by exploring advanced material growth technologies. Researchers at Harvard have reported various _n-i-p_ core/multishell single NW heterostructures [37]. Figure 8a schematically shows the triangular cross section of a single GaN-based heterostructure, comprising a _n_-GaN core and an In\({}_{\mathrm{x}}\)Ga\({}_{1-\mathrm{x}}\)N/GaN/_p_-AlGaN/_p_-GaN shell. Here, the In\({}_{\mathrm{x}}\)Ga\({}_{1-\mathrm{x}}\)N layer provides a tunable band gap well, allowing systematic redshift of the emission peak with an increasing In composition. In Figure 8b, the separately deposited metal contacts onto the _n_-GaN core and _p_-GaN shell forms a stable injection path for single NW LEDs. Multicolor emissions in the visible frequency range were observed with various In compositions (Figure 8c: purple (1%), blue (10%), green (25%), and yellow (35%)).