# Electrically Driven Micro- and Nano-Scale Semiconductor Light Sources
## 4 Lateral Injection PhC Nanocavity Lasers and Light-Emitting Diodes (LED) Modulators
Indeed, the reported threshold currents of the device were only 181 nA at 50 K and 287 nA at 150 K, claiming to be the lowest thresholds in any type of electrically pumped laser. The same device platform was exploited again to demonstrate ultrafast optical modulation (Figure 6b) [33]. Due to the insufficient QD gain at room temperature, the device no longer worked as a nanolaser, but operated as a nanoscale single-mode LED (nano-LED). In particular, the nano-LEDs benefitted from the sub-micrometer-sized lateral _p_-_i_-_n_ junction with an extremely small capacitance and exhibited the capability of ultra-fast electrical switching at a speed of several GHz. As a result, the nano-LED device showed a modulation speed of up to \(\sim\)10 GHz, which was an order of magnitude faster than that in the previous solutions. Figure 5: Lateral current injection in PhC nanocavity. Fabrication schematics of patterned ion implantation doping for the lateral _p_-_i_-_n_ junction structure. Spatially targeted implantation of Si and Mg ions on the GaAs slab including InAs QDs via silicon nitride patterning mask (left). Dopant activation through thermal treatment and aligned definition of PhC nanocavities to form a lateral _p_-_i_-_n_ structure using e-beam lithography and dry etching steps (middle). Contact metallization and oxidation of sacrificial layer underneath the PhC nanocavities (right) [31]. Figure reproduced with permission from American Institute of Physics. Figure 6: Lateral current injection PhC nanolasers and nano-LED modulators. (**a**) Schematic drawing of a laterally _p_-_i_-_n_-doped PhC nanocavity (left). Red and blue areas represent Be-doped (_p_-type) and Si-doped (_n_-type) regions, respectively. The PhC nanocavity is located in the narrow intrinsic region between the _p_- and _n_-type doped regions. White light image of the fabricated QD PhC nanocavity laser device (white-dotted box) with _p_- and _n_-contacts, respectively (middle). Far-field radiation image of the above threshold lasing action from the PhC nanocavity (right). The image is captured by an infrared (IR) camera at an applied current of 3 \(\upmu\)A at 50 K [32]. Figure reproduced with permission from Springer Nature Limited. (**b**) SEM image of electrically pumped GaAs PhC cavity LED (left). Ultrafast direct electrical amplitude modulation of PhC LED; intensity signal (middle) and electrical bit sequence signals (right) [33]. Figure reproduced with permission from Springer Nature Limited. Despite the many structural advantages, the lateral current injection scheme still has several limitations to be a practical solution. First, the implantation process causes inevitable damage to the semiconductor crystals and sacrifices the optical gain. Therefore, the process requires a precise control over the dopant density to strike a fine balance between sufficient carrier injection and reduction in optical gain. Second, the device fabrication includes multiple rounds of electron beam lithography with nanoscale precision and accuracy. In particular, the definition of the PhC pattern, with the nanocavity exactly aligned to the intrinsic region of the implanted wafer, is the most critical step that determines the successful formation of the junction. Third, the achieved lasing demonstrations at cryogenic temperatures have been limited. To be a practical light source in an integrated circuit, a laser device should operate at room temperature. This naturally requires a direct application of the scheme in typically used active materials at room temperature (e.g., QWs).