# Electrically Driven Micro- and Nano-Scale Semiconductor Light Sources
## 3 Electrically Driven Photonic Crystal (PhC) Nanocavity Lasers  
The development of miniaturized semiconductor lasers, with high beam quality and low threshold with minimum thermal overload, is one of the essential goals for the realization of compact integrated photonic circuits [20]. For more than a decade, PhCs, an artificial structure whose refractive index is  
Figure 2: Current injection micropillar lasers. (**a**) Schematic of the electrically driven quantum dot (QD)-micropillar laser structure (left). The cavity layer where QDs are located is sandwiched by top and bottom distributed Bragg reflector structures. Vertical micropillar structures are planarized with benzocyclobutene (BCB), which electrically isolates the top and bottom contacts. (**b**) Scanning electron microscope (SEM) image of InGaAs QD micropillar before definition of metallic \(p\)-contact [17]. Figure reproduced with permission from American Institute of Physics. periodically modulated with sub-micrometer periodicity, have served as the most powerful platform to achieve this goal [21]. In particular, a two-dimensional (2D) semiconductor PhC slab structure has attracted tremendous interest, owing to its critical advantages. First, it can be readily realized by the conventional semiconductor crystal growth and fabrication technologies. The introduction of a sacrificial layer to the semiconductor wafer and definition of an array of air holes bring forth the air-suspended slab structure with periodic air-semiconductor index modulation. Second, light can be tightly confined inside the slab by total internal reflection (TIR) in the vertical direction and photonic band gap (PBG) in the lateral direction. Third, there is considerable room for designing various PhCs and manipulating the behaviors of the confined light. These pronounced features allow for the realization of wavelength-scale optical nanocavities that support resonant optical modes with a high-\(Q\) factor and small mode volume. Consequently, the PhC nanocavity lasers exhibit an ultrasmall device footprint and operate with a very low threshold. In addition, cavity engineering provides access to control important beam qualities (e.g., polarization state and spatial emission pattern) as well as device functionalities. Considerable efforts have been made to put forth various cavity designs, characterize lasing properties, reduce various optical losses, and optimize device performance. However, the lasing operation from wavelength-scale optical cavities by direct electrical pumping has remained challenging. Because of such a small physical footprint, constructing a robust pathway through which the carriers can flow is a limitation. In addition, the confinement of carriers and photons should be achieved simultaneously. The injected carriers should be recombined in the vicinity of the nanocavity without seriously degrading the optical properties of resonant modes. All of these require extra consideration in the cavity design. [PERSON] et al. (2004) reported an electrically driven 2D PhC nanocavity laser that overcame the abovementioned limitations [22; 23; 24]. Figure 3 shows a schematic illustration and a tilted SEM image of the electrically driven single-cell PhC nanocavity. The 2D PhC pattern in a triangular-lattice fashion was introduced to an air-suspended and vertically \(n\)-\(i\)-\(p\)-doped InGaAsP slab containing an MQW structure. The nanocavity was a single air hole. Here, the authors judiciously modified the single-cell cavity by reducing the nearest air holes and slightly pushing them outward. This modification enlarged the cavity area and helped the InP sacrificial layer to survive after time-controlled wet-etching, which enables the introduction of a sub-micrometer central post underneath the cavity. The electrons and holes were supplied by the \(n\)-type electrical contact, defined at a distance from the PhC pattern and the \(p\)-doped InP post, respectively.