Advances in GaN semiconductors for energy efficient solid state lighting

Abstract

Currently, GaN white LEDs exhibit luminous efficacy greater than 150 lm/Watt, and external quantum effiencies higher than 50% at low drive currents. However, current commercial LEDs grown on the c-plane of the wurtzite crystal suffer from the quantum confined Stark effect (QCSE) due to the large polarization-related electric fields. This effect causes band bending in the active region and results in the spatial separation of the electron and hole wave functions, thus lowering the radiative recombination rate and reducing the internal quantum efficiency (IQE). Moreover, the IQE is further reduced with increasing injection current or temperature by a phenomenon known as “efficiency droop”. Crystal orientations that have reduced polarization related effects, such as the semipolar and nonpolar orientations, enable the growth of thick QWs, which results in reduced carrier density in the wells. A review of the unique polarization anisotropy in GaN will be discussed for the different crystal orientations. Emphasis on nonpolar and semipolar LEDs will highlight high-power violet, blue and green emitters and considers the effects of indium incorporation and substrate miscut. In this work, we demonstrated a small-area (0.1 mm2) semipolar (2021) blue (447 nm) light-emitting diode (LED) with high light output power (LOP) and external quantum efficiency (EQE) by utilizing a single 12-nm-thick InGaN quantum well. Figure 1 showed the scanning transmission electron microscope (STEM) image of the active area of the LED. In addition, as we can see in Fig. 2, the LED had pulsed LOPs of 140, 253, 361, and 460 mW, and EQEs of 50.1, 45.3, 43.0, and 41.2 %, at current densities of 100, 200, 300, and 400 A/cm2, respectively. Low efficiency droop performance in LEDs also can be seen in the inset of Fig. 2. By lowering the 3-dimentional current density, a highest characteristic temperature of ∼ 900 K at a current density of 40 A/cm2 (see Fig. 3) and > 90% Hot/Cold factors in the current density region between 20 to 100 A/cm2 (see Fig. 4) were also demonstrated when increasing ambient temperature from 20 to 100 °C, owning to the reduction in carrier leakage out of the active region and Auger recombination.