The influence of threading dislocations propagating through an AlGaN UVC LED

Abstract

During the epitaxy of AlGaN on sapphire for deep UV emitters, significant lattice mismatch leads to highly strained heterojunctions and the formation of threading dislocations. Combining cathodoluminescence, electron beam induced current and x-ray microanalysis reveal that dislocations with a screw component permeate through a state-of-the-art UVC LED heterostructure into the active region and perturb their local environment in each layer as growth progresses. In addition to acting as non-radiative recombination centers, these dislocations encourage high point defect densities and three-dimensional growth within their vicinity. We find that these point defects can add parasitic recombination pathways and compensate intentional dopants.

Core–Shell Nanorods as Ultraviolet Light-Emitting Diodes

“…Electrons and holes generated by the beam will be swept to opposite sides of the junction by any built-in electric field; if the p and n contacts are connected via an external circuit, this will result in a current flow analogous to the photovoltaic effect. The resultant electron-beam-induced current (EBIC) can be used as the image-forming signal for the SEM to map variations in field strength and carrier recombination rates …”

“…The resultant electron-beam-induced current (EBIC) can be used as the image-forming signal for the SEM to map variations in field strength and carrier recombination rates. 52 For electrical characterization on the single nanorod level, we created two contact schemes in the FIB, both sharing a common n contact, as illustrated in Figure 4a. The first p contact was formed by electron beam deposition of Pt on the tips of individual nanorods.…”

Defects, performance, and reliability in UVC LEDs

“…Unfortunately, the development of UVC LEDs is still in its early stages, and the efficiency and reliability of these devices are still limited, compared to the visible counterparts. A higher aluminum content can worsen the quality of the AlGaN epitaxial layers, and this can limit the internal quantum efficiency (IQE) of the devices [6][7][8]. In addition, doping of ultrawide bandgap semiconductors is not trivial: in the case of AlGaN, the high ionization energies of the n-and p-type dopants limit the free carrier densities, resulting in a low carrier injection efficiency (CIE) and asymmetric injection of electrons and holes towards the quantum wells.…”

Review—Reliability and Degradation Mechanisms of Deep UV AlGaN LEDs

“…The light-extraction efficiency of AlGaN UV LEDs grown on c-plane sapphire substrates is limited by the valence band structure of high Al content AlGaN, causing a shift in the polarization of light at short wavelength for c-plane device structures. [53][54][55][56][57][58][59][60] Emission at germicidal UVC wavelengths is primarily in the transverse electric (TE) polarization mode for a c-plane substrate, and thus perpendicular to the substrate surface. [61][62][63][64][65][66] At shorter wavelengths, the electron-hole recombination causes a change in photon polarization from having the E field perpendicular to the c-axis to having the emission being dominated by the transverse magnetic (TM) polarization mode, emitting parallel to the planar surface.…”

“…115 It is found that threading dislocations act as non-radiative recombination centers and induce point defects that can create parasitic recombination pathways and compensate intentional dopants. 53,54 As it is difficult to obtain high-quality bulk AlGaN single crystals, it is common to use AlN as a template, to reduce the dislocation density and strain in the LED structure. [62][63][64]67,69,80,92 Figure 3 shows a deep UV LED structure fabricated on an AlN template grown on a sapphire substrate.…”