# Electrically Driven Micro- and Nano-Scale Semiconductor Light Sources
## 4 Lateral Injection PhC Nanocavity Lasers and Light-Emitting Diodes (LED) Modulators  
Despite the successful demonstrations of the electrically driven lasing operation in some PhC nanocavities, the vertical carrier injection scheme through the _p-i-n_ (or _n-i-p_)-doped PhC slab with a thickness of a few hundred nanometers still has limitations. For example, the central post structure requires complicated fabrication, including a precise, time-controlled undercut step [22; 23; 24; 25; 29]. In addition, the cavity design for the current injection is limited because the size and position of the air holes critically determines the formation of the sub-micrometer-sized post. This diminishes one of the key advantages of using PhCs: controlling the laser beam quality by cavity engineering. Moreover, the central post is located right underneath the nanocavity, so the leakage of carriers and photons is unavoidable, which can be a major factor for the increased threshold and thermal overload in lasing action. On the other hand, the vertical carrier injection scheme in nanocavities without the central post often suffered from the limited current spreading due to thin conductive layers in the PhC membrane, which gave rise to an inefficient cavity-coupled electroluminescence (EL) [30]. The researchers at Stanford proposed an alternative scheme to address these issues and electrically pumped arbitrary PhC nanocavities. [PERSON] et al. (2010) developed a technique to form a lateral _p-i-n_ junction in a 2D PhC nanocavity by employing the ion implantation technique, a standard method of doping in the electronics industry [31]. Figure 5 describes the important fabrication steps of the lateral current injection PhC nanocavity. First, a nitride mask is lithographically defined on top of the GaAs membrane, where a high-density single InAs QD layer is included in the middle. Second, Si and Mg ions are separately implanted on the targeted areas. These dopants are subsequently capped by a nitride layer and activated by high-temperature annealing. Third, a PhC pattern is defined in a resist and transferred into the GaAs membrane by dry etching. To form a lateral _p-i-n_ junction, the upper and lower sections of the PhC pattern are precisely aligned to the upper _p_- and lower _n_-doped regions, while the nanocavity is aligned to the middle intrinsic region. Finally, the _n_- and _p_-type contacts are metallized after the AlGaAs sacrificial layer is oxidized. With this technique, the authors successfully injected electrical current through the nanocavity and observed a cavity EL peak at low temperature, but failed to observe the lasing action due to the insufficient optical gain needed for amplified stimulated emission. Figure 4: Electrically driven 1D PhC nanocavity lasers. (**a**) Schematic illustration of an electrically driven 1D nanobeam cavity. Carriers are injected from the top electrode and bottom post to the central nanocavity region. (**b**) Fabricated nanobeam nanolaser with an extremely small device footprint (top and bottom). Sub-micron post structures support the cavity structure. (**c**) Measured electroluminescence (EL) image (top) and finite-difference time-domain (FDTD)-calculated electric filed intensity distribution (bottom) [29]. Figure reproduced with permission from Springer Nature Limited. Additional improvements in doping density, dopant material, number of QD layers, and cavity fabrication allow for a successful demonstration of an ultralow threshold lasing action from a 2D QD PhC nanocavity [32]. A schematic diagram in Figure 6a shows the upper Be-doped (_p_-type), middle undoped (intrinsic), and lower Si-doped (_n_-type) PhC regions, forming a lateral _p_-_i_-_n_ junction on the sub-micron scale. A bright-field IR camera image displays a PhC pattern between the _p_- and _n_-contacts. Strong lasing oscillation at a pump current of only 3 \(\upmu\)A demonstrated an ultralow-threshold current nanolaser.