# Electrically Driven Micro- and Nano-Scale Semiconductor Light Sources
## 3 Electrically Driven Photonic Crystal (PhC) Nanocavity Lasers
A rich radiative recombination occurred near the post, and most of the generated photons were successfully confined within the nanocavity. Single-mode lasing oscillation with an estimated \(Q\)-factor of \(>\)2500 was observed at a low threshold current of \(\sim\)260 \(\upmu\)A. In a series of follow-up studies from the same group, the cavity design was further improved with the help of an extensive mode analysis using the 3D finite-difference time-domain (FDTD) method and the controlled low-temperature wet-etching technique [23; 24; 25]. As a result, a significant reduction in the threshold current, down to \(\sim\)100 \(\upmu\)A, was successfully achieved. Taking advantage of the current injection scheme through a sub-micrometer-sized central post, even nanocavities smaller than 2D PhC nanocavities became a realistic candidate for an electrically pumped nanolaser. In the late 2000s, a 1D PhC structure, called a nanobeam, was extensively explored for ultrahigh-\(Q\) optical nanocavities with ultimately small physical and mode volumes [26; 27; 28]. These cavities strongly confine photons using PBG in the axial direction and TIR in both vertical and lateral directions, which often exhibit \(Q\)-factors \(>\) \(\sim\)10\({}^{6}\) and mode volumes \(<(\lambda/2n)^{3}\). Among the numerous designs of ultrahigh-\(Q\) nanobeam cavities, the L0 nanobeam cavity (i.e., zero missing air holes) with graded air-hole sizes was employed to demonstrate the smallest possible nanolaser [29]. Figure 4a shows the schematic of the cross-sectional view of the laser device that exhibits the carrier injection paths. In Figure 4b, the top and tilted SEM images of the fully fabricated device show the L0 nanobeam nanocavity consisting of eleven air holes. The physical volume of the device was surprisingly small, i.e., \(\sim\)4.6 \(\times\) 0.61 \(\times\) 0.28 \(\upmu\)m\({}^{3}\). The central posts supported the entire laser device. However, the lasing action was observed at a threshold current of \(\sim\)650 \(\upmu\)A at room temperature, which was not as low as expected considering the reduced physical size. The major factors that determined the threshold current were the large surface recombination losses, the weakened PBG confinement due to the limited number of air holes, as well as the increased thermal overload from the sub-micrometer-sized posts. Indeed, the L3 nanobeam cavity (i.e., three missing air holes) laser, with the enlarged post and the increased number of air holes, revealed that a larger size of the cavity structure helped in reducing the lasing threshold. The authors reported a measured threshold current of \(\sim\)5 \(\upmu\)A at room temperature, claiming a record threshold value among the electrically driven metal-free nanolasers at room temperature. Figure 3: Electrically driven 2D photonic crystal (PhC) nanocavity lasers. (**a**) Schematic of the cross section of a PhC nanocavity. The vertically _n-i-p_-doped semiconductor wafer is supported by dielectric material. The entire PhC structure is air-suspended except for the central cavity region, which is connected to the _p_-type substrate through the central post. (**b**) Tilted SEM image of electrically driven 2D InGaAsP-PhC/InP-post nanocavity laser. (**c**) SEM image of sample showing different regions with different air-hole sizes (top) and Charge-coupled device (CCD) image of lasing mode (bottom) [22]. Figure reproduced with permission from American Association for the Advancement of Science.