Electrically Injected AlGaN Nanowire Deep Ultraviolet Lasers

We report on the demonstration of AlGaN nanowire lasers in the ultraviolet (UV)-B and UV-C bands. The AlGaN nanowires were grown directly on Si substrate and were characterized by the presence of quantum-dot-like nanostructures. At room temperature, the lasing threshold was measured to be ~30 μA. Semiconductor light sources that can operate efficiently in the ultraviolet (UV)-B (280–315 nm) and UV-C (200–280 nm) bands are important for a broad range of applications, including water purification, disinfection, and medical diagnostics. To date, it has remained challenging to achieve electrically injected AlGaN quantum well lasers operating in these wavelength ranges, due to the presence of large densities of dislocations and the inefficient p-type conduction. In this context, we have performed a detailed investigation of the molecular beam epitaxial (MBE) growth and characterization Al-rich AlGaN nanowire heterostructures on Si substrate. We have demonstrated electrically injected AlGaN nanowire lasers at ~289 nm, with a very small threshold current of 30 μA at room temperature. AlGaN nanowire heterostructures were grown directly on Si substrate by plasma-assisted MBE under nitrogen rich conditions. Shown in Fig. 1(a), the nanowires are vertically aligned on the Si substrate along the c-axis and exhibit a high degree of size and height uniformity. By varying the growth conditions, the optical bandgap can be continuously varied in the deep UV spectral range [1]. Such AlGaN nanowire heterostructures exhibit extremely high (>80%) luminescence efficiency, shown in Fig. 1(b) [2]. From detailed scanning transmission electron microscopy (STEM) studies, strong atomic-scale compositional modulations were observed in AlGaN nanowires. The Ga signal map and the concurrently acquired annular dark-field signal are shown in Fig. 1(c). A distinct local enrichment in the Ga signal is evident. The Ga-rich AlGaN regions had sizes varying from single atomic plane to ~2 nm along the growth direction and lateral sizes varying from ~2 to 10 nm. Such atomic-scale compositional fluctuations possessed quantum-dot-like structural characteristics within the nanowires [2]. The AlGaN nanowire laser heterostructures consist of GaN: Si (250 nm), AlGaN: Si (100 nm), AlGaN (100 nm), AlGaN: Mg (100 nm), and GaN: Mg (10 nm) layers, shown in Fig. 2(a). Such vertically aligned, randomly distributed AlGaN nanowires can lead to strong photon confinement, due to Anderson localization of light [2]–[4]. Detailed studies have further shown that high Q optical cavities (λ ~290 nm) can be obtained in spontaneously formed AlGaN nanowire arrays with average diameters of ~65 nm and filling factor of ~30%. Figure 2(b) shows the simulated electrical field distribution (λ~290 nm) in such AlGaN nanowire arrays. It is seen that strong light confinement was achieved, due to the recurrent, multiple scattering process. Another important consideration is the optical confinement along the vertical direction of AlGaN nanowires, which was made possible by the inversely tapered nanowire morphology, i.e., increasing nanowire diameter along the growth direction. Shown in Fig. 2(c) the strong optical confinement in the AlGaN active region is evident, and any optical loss through the underlying GaN and Si and the top p-GaN and metal layer is seen to be very small. The fabrication of electrically injected AlGaN nanowire lasers involved the use of standard photolithography and contact metallization techniques. Figure 3(a) shows the lasing spectra measured at room temperature. At low injection current, only a very broad emission spectrum is seen. A sharp peak at 289 nm emerges with increasing current. The light-current (L-I) characteristics are shown in Fig. 3(b), which exhibit a distinct threshold of 30 μA. The inset shows the L-I curve in a logarithmic scale. A clear S-shape, corresponding to linear spontaneous emission, superlinear amplified spontaneous emission, and linear lasing emission, is observed, providing an unambiguous evidence for lasing. The spectrallinewidth also shows a sharp reduction near the threshold, further confirming the achievement of lasing. Moreover, we have demonstrated electrically pumped lasers operating at 262 nm, shown in Fig. 3(c) [3]. The demonstration of electrically injected lasers operating below 240 nm is in progress and will be reported.