# Electrically Driven Micro- and Nano-Scale Semiconductor Light Sources
## 2 Current Injection Disk-Type Microcavity Lasers  
In previous demonstrations of various micro-scale injection light sources, conventional epitaxial semiconductor wafers were widely used. These wafers comprise vertical \(p\)-type/intrinsic/\(n\)-type (\(p\)-\(i\)-\(n\)) layered structures, where the intrinsic layer often includes an active medium (i.e., multiple quantum wells (MQWs) or quantum dots (QDs)) that allows a rich radiative recombination of the injected carriers. The semiconductor crystals typically used are InGaAs/InGaAsP compressive-strained MQW structures grown on an InP substrate, shown in Table 1[11; 12; 13; 14; 15]. The highly doped top and bottom InP layers (impurity concentration \(>10^{18}\) cm\({}^{-3}\)) help for efficient carrier injection from metal to semiconductor. In addition, the 1.2 umthick InP layers can be selectively wet-etched, which provides the intrinsic InGaAs/InGaAsP MQWs with high-index contrast. The semiconductor crystal exhibits a central emission peak at \(\sim\)1.55 um, which covers the telecommunication C-band. \begin{table}
\begin{tabular}{c c c} \hline
**Material** & **Thickness (um)** & **Impurity Concentration (cm\({}^{-3}\))** \\ \hline InGaAsP & 0.3 & \(p=3\times 10^{18}\) \\ InP & 1.2 & \(p=3\times 10^{18}\) \\ InGaAsP & 0.11 & 0.0 \\ InGaAs & 0.01 & 0.0 \\ InGaAs & 0.01 & 0.0 \\ InGaAsP & 0.0015 & 0.0 \\ InGaAs & 0.01 & 0.0 \\ InGaAsP & 0.0015 & 0.0 \\ InGaAs & 0.01 & 0.0 \\ InGaAsP & 0.11 & 0.0 \\ InP & 1.2 & \(n=2\times 10^{18}\) \\ InP & Substrate & \(n=3\times 10^{18}\) \\ \hlineThe microcavity structures are then introduced to the crystals so that an efficient spatial confinement of carriers and photons can be simultaneously achieved in a small physical volume. Among the various microcavity designs, some of the key advantages of disk-type microcavities with a central post structure have attracted intense interest. A high-index semiconductor microdisk, with a diameter of a few micrometers, enables strong photon confinement inside the cavity, which supports various whispering gallery modes (WGMs) with high-quality (high-\(Q\)) factors. The central post sustains the entire microdisk structure and enables high air\(-\)semiconductor boundaries along the disk edge where the WGMs are tightly confined. In addition, the post provides a robust vertical carrier injection pathway as well as a stable channel for heat escape. Moreover, the simple injection scheme requires only a few key fabrication steps, enabling an easy realization of injection laser devices. Figure 1a shows the key fabrication steps of current-injection microdisk cavity lasers. Circular metallic masks are lithographically defined on a vertically \(p\)-\(i\)-\(n\)-doped semiconductor wafer that contains an active medium in its intrinsic layer (left). Circular mesa structures with tilted sidewalls are formed after the conventional dry etching process (middle). A brief thermal treatment can be taken into account to make a good ohmic contact. The \(p\)-type top and \(n\)-type bottom layers are then selectively wet-etched, which results in a vertical micro-architecture consisting of a complex multileveled top-post/middle-microdisk/bottom-post structure. Here, due to the angled sidewalls from the dry etching process, the selective wet etching often gives rise to asymmetric top and bottom posts in size. Figure 1b illustrates a schematic overview of the laser device with a layered structure and current flow [13]. A sharp metallic probe directly injects pulsed or continuous-wave (CW) currents into the top metal electrode, which is connected to a highly \(p\)-doped contact layer. Carriers are injected into the active layer from the posts. The injected carriers diffuse from the center of the microdisk to the edge, where high-\(Q\) WGMs travel and repeat the total reflection at the semiconductor\(-\)air boundaries. When the WGMs acquire enough optical gain from the diffused carriers, and subsequent radiative recombination, strong lasing oscillations occur.