# Electrically Driven Micro- and Nano-Scale Semiconductor Light Sources
## 2 Current Injection Disk-Type Microcavity Lasers
Figure 1c shows a scanning electron microscope (SEM) image of the fabricated laser device exhibiting the InGaAs/InGaAsP microdisk, supported by the InP posts [12]. Figure 1: Current injection microdisk lasers: (**a**) Schematics of the key fabrication steps for an injection microdisk structure; electron-beam lithographic definition of circular metal masks on a vertically \(p\)-\(i\)-\(n\)-doped semiconductor wafer where the active medium is included in the middle (left); formation of circular mesa with tilted sidewalls after applying chemically assisted dry etching (middle); and final microdisk structure with asymmetric \(n\)- and \(p\)-type posts after the wet etching process. (**b**) Schematic of InGaAsP microdisk injection laser. The microdisk is supported by top and bottom InP posts connected to the top Ti/Au contact and bottom \(p\)-type InP substrate [13]. Figure reproduced with permission from the Institute of Electrical and Electronics Engineers. (**c**) SEM image of the fabricated injection microdisk laser [12]. Figure reproduced with permission from the Institute of Electrical and Electronics Engineers. Semiconductor micropillars with embedded QDs have also proven to be very attractive structures for microscale injection light sources. In particular, when additional distributed Bragg reflector (DBR) structures are introduced to the micropillar, resembling minimized vertical-cavity surface emitting lasers, it serves as a high-\(Q\) microcavity with a more efficient collection of the emitted light and a better heat transfer compared to planar microdisk injection lasers [16; 17]. Figure 2a illustrates a schematic view of the electrically pumped micropillar laser structure. Figure 2b shows the SEM image of the fabricated device before metallization [17]. The device comprises a \(p\)-doped upper DBR, an intrinsic one-\(\lambda\)-thick GaAs cavity layer, and an \(n\)-doped lower DBR. Here, the upper and lower DBRs are composed of multiple pairs of GaAs/AlAs layers. The QDs are placed at the center of the intrinsic layer. A key step in the fabrication that makes this structure distinguishable from other planar injection microlasers is the planarization using benzocyclobutene (BCB), a photosensitive polymer frequently used as a low-\(k\) dielectric in microelectronics packaging and interconnect applications [18]. This dielectric planarization serves as both a stable physical ground, on which the upper contact can be located, and an efficient electrical isolation layer. In addition, the low-\(k\) nature minimizes the undesirable optical perturbation to the micropillar cavities. Using the BCB layer, the electrical current can be injected through the upper \(p\)-contact and generate electron-hole pairs in the QD layer. Since the first experimental observation of the pulsed-injection lasing, a series of intensive efforts have been made to demonstrate more practical injection microcavity lasers to achieve the best performance. For example, research efforts in reducing the size of the microdisk cavity down to a single-mode condition allowed for a significant decrease in threshold current from sub-mA to a few hundred \(\upmu\)A [12]. A rational design based on the systematic theoretical analysis of scattering loss, by post and carrier diffusion in the active medium, enabled the CW lasing operation at room temperature [13; 19]. Moreover, an improved dry etching technique helped to form vertical sidewalls in the microcavity, as well as symmetric top and bottom posts, which led to a further reduction in the lasing threshold [14]. Micropillar cavity lasers benefited from BCB planarization and the optimized ring-shaped upper contact scheme, facilitating the demonstration of strong lasing oscillations associated with lasing threshold currents as low as few \(\upmu\)A at cryogenic temperature [16].