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The electro‐optical characteristics of deep ultraviolet light‐emitting diodes (DUV LEDs) emitting at 265 nm and grown on AlN/sapphire templates with different threading dislocation densities, i.e., high‐temperature annealed (HTA) AlN, epitaxially laterally overgrown (ELO) AlN, and HTA‐ELO AlN are analyzed. The external quantum efficiency of each individual device is separated into maximum radiative recombination efficiency, carrier injection efficiency, and light extraction efficiency. This is achieved by combining an ABC‐model‐based fit of the current‐dependent external quantum efficiency together with calibrated Monte Carlo ray‐tracing simulations. A maximum radiative recombination efficiency between 50% and 60% is estimated for DUV LEDs grown on ELO and HTA‐ELO AlN/sapphire, whereas the values for devices grown on HTA AlN/sapphire are around 45%. The extracted radiative recombination efficiency does not scale with the measured threading dislocation density (TDD), even when accounting for the inhomogeneous TDD in the AlN base layers. This discrepancy is attributed to the formation of dislocation half‐loops introduced by additional compressive strain caused by the HTA process and may result in the formation of additional nonradiative recombination centers in the AlGaN multi‐quantum well region. In addition, the carrier injection efficiency values ranging from 45% to 55% are determined for devices grown on all three templates. [ABSTRACT FROM AUTHOR]

Herein, the scope is to provide an overview on the current status of AlN/sapphire templates for ultraviolet B (UVB) and ultraviolet C (UVC) light‐emitting diodes (LEDs) with focus on the work done previously. Furthermore, approaches to improve the properties of such AlN/sapphire templates by the combination of high‐temperature annealing (HTA) and patterned AlN/sapphire interfaces are discussed. While the beneficial effect of HTA is demonstrated for UVC LEDs, the growth of relaxed AlGaN buffer layers on HTA AlN is a challenge. To achieve relaxed AlGaN with a low dislocation density, the applicability of HTA for AlGaN is investigated. [ABSTRACT FROM AUTHOR]

Herein, AlN growth by metalorganic vapor‐phase epitaxy on hole‐type nanopatterned sapphire substrates is investigated. Cracking occurs for an unexpectedly thin‐layer thickness, which is associated to altered nucleation conditions caused by the sapphire pattern. To overcome the obstacle of cracking and at the same time to decrease the threading dislocation density by an order of magnitude, high‐temperature annealing (HTA) of a 300 nm‐thick AlN starting layer is successfully introduced. By this method, 800 nm‐thick, fully coalesced and crack‐free AlN is grown on 2 in. nanopatterned sapphire wafers. The usability of such templates as basis for UVC light‐emitting diodes (LEDs) is furthermore proved by subsequent growth of an UVC‐LED heterostructure with single peak emission at 265 nm. Prerequisites for the enhancement of the light extraction efficiency by hole‐type nanopatterned sapphire substrates are discussed. [ABSTRACT FROM AUTHOR]

Michael Kneissl, TU Berlin (Germany) Jürgen Christen, University of Magdeburg (Germany) B Conference Chairs b Axel Hoffmann, TU Berlin (Germany) Bo Monemar, Linköping University (Sweden) B Honorary Chairs b Tim Wernicke, TU Berlin (Germany) B Conference Secretary b Ulrich Schwarz, TU Chemnitz (Germany) B Local Chair b Åsa Haglund, Chalmers University of Technology (Sweden) Matteo Meneghini, University of Padova (Italy) B Guest Editors and Publication Chairs b Two tutorial speakers (Andreas Waag, TU Braunschweig) and Chris Van de Walle (UCSB) opened the workshop, by discussing GaN-based micro-LEDs and point defects, respectively. The workshop organizers express their gratitude to the plenary and invited speakers, rump session chairs, panelists, and short-course presenters for their exceptional contributions to an engaging workshop. [Extracted from the article]

Smooth, uniform, and conductive AlxGa1−xN layers with high aluminum mole fractions of x > 0.7 are required as cladding layers for laser diodes emitting in the deep ultraviolet spectral region. In this paper, the growth of silicon‐doped AlGaN/AlGaN superlattices by metal‐organic vapor phase epitaxy is investigated and compared to bulk AlGaN layers. It is found that the superlattice approach enables the growth of AlGaN layers with improved lateral uniformity of composition and strain state. In order to reduce the surface roughness, growth interruptions between the superlattice layers are investigated. With increasing growth interruption time, the average aluminum content is increasing and the superlattice period thickness is decreasing. Scanning transmission electron microscopy investigations show that the growth interruptions lead to more abrupt interfaces and to the formation of one to two monolayer thin aluminum‐rich AlGaN layers at each interface. This also leads to a significantly smoother surface morphology. Low resistivities of 0.025 Ω cm are obtained for AlGaN:Si superlattices with average aluminum content of x = 0.8. The influence of the surface morphology of the AlGaN cladding layers on optically pumped laser heterostructures is investigated. By using a smooth AlGaN super­lattice, the lasing threshold decreases by a factor of 2 compared to lasers with bulk AlGaN cladding layers. [ABSTRACT FROM AUTHOR]

In order to realize UVB LEDs with high wall‐plug efficiencies, the light extraction efficiency from the LED heterostructure must be maximized and operating voltages reduced. In this study, we investigate the effect of the GaN:Mg contact layer thickness on the light‐output and current‐voltage characteristics of UVB LEDs. AlGaN‐based LED heterostructures, that are fully transparent for UVB emission except for the GaN:Mg contact layer are grown by metal organic vapor phase epitaxy on c‐plane sapphire substrates. From transfer line measurements, it is found that the p‐contact resistivity increases rapidly with decreasing GaN:Mg thickness and exhibits a pronounced Schottky behavior for layer thicknesses below 40 nm. At the same time, the emission power increases from 0.1 to 1.5 mW at 20 mA with decreasing GaN:Mg thickness. Ray tracing simulations of the light extraction efficiency of the UVB LEDs show that absorption in the GaN:Mg layer leads to lower emission powers for thicker GaN:Mg layers. Furthermore, with increasing GaN:Mg thickness additional losses occur due to a decrease of the internal quantum efficiency. The electro‐optical and the structural properties of the devices show that a 40 nm thick GaN:Mg contact layer is the best compromise due to the low p‐contact resistivity (0.01 Ω cm2) and at the same time still sufficient UVB‐transmission resulting in UVB LEDs with external quantum efficiencies of more than 1% and a wall plug efficiency of 0.4% (at 20 mA), measured on‐wafer. [ABSTRACT FROM AUTHOR]

InGaN quantum dots (QDs) were grown on GaN templates in the Stranski-Krastanov growth mode at 675 °C via metalorganic vapor phase epitaxy (MOVPE). Typically, QDs with overall density of 109 cm−2, an average diameter 50 nm and average height of 7 nm were obtained. A systemic investigation on the influence of group III and group V partial pressure on the size and distribution of the InGaN QDs is reported. A higher growth rate and/or higher V/III ratio increases the mean height and the density of QDs. The distribution of the dots is found to scale with growth rate and the V/III ratio. This is attributed to a reduction in the surface diffusion of adatoms as the results of increase in partial pressure of the precursor in the reactor. [ABSTRACT FROM AUTHOR]

The effect of the active region growth temperature (TMQW) on the external quantum efficiency (EQE) of AlGaN‐based far‐ultraviolet‐C light‐emitting diodes (far‐UVC LEDs) emitting near 235 nm is investigated. AlGaN multi‐quantum well (MQW) active regions are grown at temperatures between 850 and 1100 °C by metal‐organic vapor‐phase epitaxy, while special care is taken to keep aluminum mole fractions and thicknesses constant for all MQWs. Temperature‐ and excitation‐power‐dependent photoluminescence spectroscopy reveal a more than tenfold increase of the radiative recombination efficiency (RRE) when the growth temperature increases from 850 to 1020–1060 °C. The output powers for mounted far‐UVC LEDs at 0.2 A increase from 0.5 mW for TMQW of 900 °C to 2.5 mW for TMQW of 1020 °C, corresponding to an increase in EQE from 0.04% to 0.23% at 0.2 A. However, lifetime measurements reveal a reduction of the L70 lifetime from 400 to 1 h when TMQW increases from 900 to 1060 °C. In this investigation, it is shown that optimizing the growth conditions provides a promising approach to further increase the RRE and EQE and lifetime of far‐UVC LEDs. [ABSTRACT FROM AUTHOR]

Herein, a systematic study of the morphology and local defect distribution in epitaxially laterally overgrown (ELO) AlN on c‐plane sapphire substrates with different off‐cut angles ranging from 0.08° to 0.23° is presented. Precise measurements of the off‐cut angle α, using a combination of optical alignment and X‐ray diffraction with an accuracy of ±5° for the off‐cut direction and ±0.015° for the off‐cut angle, are carried out. For ELO AlN growth, a transition from step flow growth at α < 0.14° with height undulations on the surface to step bunching with step heights up to 20 nm for α > 0.14° is observed. Furthermore, the terraces of the step‐bunched surface exhibit curved steps. An analysis of the local defect distribution by scanning transmission electron microscopy and a comparison with atomic force microscopy reveal a bunching of defects in line with the ELO pattern and a roughening of step edges in highly defective regions. In addition, a reduction in the threshold excitation power density for optically pumped ultraviolet‐C (UVC) lasers with smooth surface morphologies is observed. [ABSTRACT FROM AUTHOR]

Herein, the lasing threshold and gain characteristics of ultraviolet‐C optically pumped edge‐emitting lasers with thick single‐quantum‐well (SQW) active regions are investigated by the variable‐stripe length method. Positive net modal gain is observed in lasers with AlGaN‐based SQWs with thicknesses up to 12 nm. The lasers show a reduction of the threshold power density with increasing SQW thickness, with the lowest threshold of 1.3 MW cm−2 achieved for a 9 nm SQW laser. The high gain and low threshold in lasers with thick quantum wells are attributed to lasing from excited states, where the polarization fields are screened by the carriers in the fundamental state and the barriers, thus recovering larger electron and hole wavefunctions overlap. These findings are supported by k · p simulations, and for a 9 nm SQW, the calculation predicts a contribution of the fundamental transition to the gain of 75% for non‐resonant optical excitation and below 1% for resonant optical or electrical excitation. [ABSTRACT FROM AUTHOR]