III-Nitride Self-Assembled Nanowire Light Emitting Diodes and Lasers on (001) Silicon
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Substantial research is being devoted to the development of III-nitride light emitting diodes (LEDs) and lasers. Current commercially available white LEDs are nearly all phosphor-converted, but these have some serious disadvantages. Planar quantum well (QW) devices on foreign substrates exhibit large threading dislocation densities, strong strain induced polarization field, and In-rich nanoclusters resulting in poor electron-hole wavefunction overlap, large emission peak shift with injection, and large efficiency drop at high injection currents in LEDs and large threshold current densities in lasers. As an alternative, self-assembled InGaN disks-in-nanowire (DNW) based devices with superior optical properties are demonstrated. Due to their large surface to volume ratio, the growth optimized and surface passivated DNWs on (001) silicon are relatively free of extended defects and have smaller polarization field resulting in higher radiative efficiencies. Blue-, green- and red-emitting DNW LEDs, with optimized nanowire densities, are demonstrated with reduced efficiency droop and smaller peak shift with injection. Phosphor-free white nanowire LEDs are realized by incorporating InGaN/GaN disks with different color emissions in the active region. The first ever monolithic edge-emitting electrically pumped green and red nanowire lasers on (001) silicon are demonstrated using DNWs as the gain media and are characterized by low threshold current densities of 1.76-2.88 kA/cm2, small peak shifts of 11-14.8 nm, large T0 of 234 K and large differential gain of 3×10-17 cm-2. Dynamic measurements performed on these lasers yield a maximum small signal modulation bandwidth of 5.8 GHz, extremely low value of chirp (0.8 …) and a near-zero linewidth enhancement factor at the peak emission wavelength.