13 mW operation of a 295–310 nm AlGaN UV-B LED with a p-AlGaN transparent contact layer for real world applications

Abstract

Smart, high-power ultraviolet (UV)-B light-emitting diode (LED) light sources are demanded for both medical and agricultural applications, including vitamin D3 production in human skin (294-304 nm), immunotherapy (310 nm), cancer therapy (295-310 nm) and enriching phytochemicals in plants (310 nm). To achieve this, we have combined graded stacks of AlGaN buffer layer (BL), AlGaN multi quantum wells (MQWs) with high internal quantum efficiency (IQE), a transparent p-AlGaN contact layer, and a highlyreflective p-type electrode for the fabrication of a UV-B LED. By optimizing the growth conditions, we demonstrated an output power of 7.1 mW for a 310 nm UV-B LED under bare-wafer conditions using a highly reflective Ni/Mg p-electrode. We also demonstrated a high IQE of 47% for UV-B emission from UV-B LED at 295 nm, by using a graded n-AlGaN BL. The light-extraction efficiency (LEE) was increased by introducing both a highly-transparent p-AlGaN and a highly reflective Ni/Mg p-electrode. As a result, we achieved an EQE of 4.4% at a dc drive current of 20 mA under CW-operation at RT and a maximum output power of 13 mW for a 295 nm UV-B LED for medical applications.

Epitaxial Growth and Characterization of AlInN-Based Core-Shell Nanowire Light Emitting Diodes Operating in the Ultraviolet Spectrum

“…The AlInN LEDs have excellent current-voltage characteristic with low resistance measured at room temperature, shown in Figure 4(a). The leakage current was found to be very small which is about 1 µA at -8 V. Turn on voltage of these UV nanowire LEDs is ~ 5 V which is significantly lower compared to current thin-film AlGaN LEDs at similar wavelength range 44,45 and is also better/comparable to that of currently reported AlGaN UV nanowire LEDs 43,46,47 . Figure 4 This observation agrees well with the simulation results in which the TM polarized emission is more than two orders of magnitude strong than TE polarized light, shown in Figure 5(b).…”

The Impact of AlN Templates on Strain Relaxation Mechanisms during the MOVPE Growth of UVB‐LED Structures

“…Here one has to take into account the lower lattice constants and the lower TDD of the HTA‐AlN (in comparison to MOVPE‐grown AlN). A possible way to reduce the strong compressive strain in the nonpseudomorphic AlGaN would be the change of the growth mode to a 3D mode as it was shown for the development of low TDD AlN templates with increased AlN layer thickness [ 20,31–34 ] and for UVB‐LED structures [ 10,35 ] as well as recently for the deposition of Al 0.55 Ga 0.45 N layers for UVB laser structures. [ 9 ] However, here the surface needs to be smoothened before the MQW is grown, which again is a challenge.…”

“…This high mismatch as well as the not perfect alignment between the AlN nuclei formed at the start of heteroepitaxial growth generally causes threading dislocation densities (TDDs) much higher than 10 9 cm −2 . Since the group of Miyake [ 5–7 ] showed the possibility of decreasing the TDD of AlN/sapphire templates down to 2 × 10 8 cm −2 by high temperature annealing (HTA), many groups tried to grow UV‐LED structures on such low dislocation density templates and found this to be challenging, [ 8–10 ] especially for LED structures emitting in the UVB region (UVB‐LEDs). Here the Al‐mole fraction of the (In)AlGaN layers of the active region is below 40% making pseudomorphic growth on AlN templates impossible due to the high compressive strain.…”

Nanopatterned sapphire substrates in deep-UV LEDs: is there an optical benefit?

“…A mirror reflector with R = 0.8 has been taken at the rear of the device. This could be in the form of highly reflective Ni/Mg [52] or Rh electrodes [25]. Table 2: The integrated transmittance for the data presented in figure 5 The fully planar device shows a high transmittance at low emission angles, mainly limited by the 0.8 reflectance of the rear reflector.…”

“…While nitride based DUV-LEDs typically have a highly absorbing p-side above the MQWs, efforts towards a transparent p-side have been reported in the literature [40,52]. In this section we analyze the optical benefit of NPSS on the LEE from the AlN layer into the sapphire substrate, considering a transparent p-side above the MQWs and a reflective p-contact.…”