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The hole transport and the carrier distribution in AlGaN-based far-ultraviolett (UVC) light emitting diodes (LEDs) emitting around 233 nm was investigated. Temperature-dependent electroluminescence measurements on dual wavelength AlGaN multiple quantum well (MQW) LEDs show a strong shift in the spectral power distribution from 250 to 233 nm with decreasing temperature. Comparing experimental data with simulation shows that the hole mobility and the electron to hole mobility ratios have a significant influence on the carrier injection efficiency (CIE) and that the change in the spectral power distribution is originating from a change in the hole distribution in the MQWs. Poor hole injection and charge carrier confinement in the AlGaN MQW active region was identified as one of the main reasons for the low CIE in far-UVC LEDs. [ABSTRACT FROM AUTHOR]

In summary, this Special Topic provides an excellent overview and cross section of key challenges in the development of high-efficiency, high power, and long lifetime UV/DUV LEDs as well as the rapidly emerging area of UVB and UVC laser diodes. Recently, several groups have reported current-injection laser diodes in the UVB and UVC spectral region with emission wavelength as short as 272 nm.[[17]] Even though the demonstrated devices are still a long way from what is needed for practical systems, these breakthroughs illustrate that semiconductor UV/DUV laser diode technologies show great potential. Finally, Kondo I et al. i demonstrate high-power operation of AlGaN-based UVB laser diodes.[41] By improving the injection efficiency through modulation of the polarization charge at the interface between the p-side waveguide layer and the EBL, they were able to show pulsed laser diodes with a peak light output power of 150 mW at a lasing wavelength of 300 nm. Group III-nitride based light emitters in the ultraviolet (UV) and deep ultraviolet (DUV) spectral range are the fundamental building block for several key applications. [Extracted from the article]

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]

A concept for vertical‐cavity surface‐emitting lasers (VCSELs) is proposed and demonstrated to obtain a lasing wavelength with unprecedented temperature stability. The concept is based on incorporating a dielectric material with a negative thermo‐optic coefficient, dn/dT, in the distributed Bragg reflectors (DBRs) to compensate the positive dn/dT of the semiconductor cavity. In a short cavity, the optical field has a significant overlap with the DBRs, and the redshift of the lasing wavelength caused by the semiconductor cavity can be compensated by the negative dn/dT of the DBRs. Here, proof of this concept is presented for optically‐pumped VCSELs emitting at 310 nm, demonstrating a lasing wavelength that even blueshifts by less than 0.1 nm over an 80 °C range with a maximum slope of –3.4 pm K−1. This is to be compared with a redshift of 1–1.5 nm over the same temperature range reported for III‐nitride blue‐emitting VCSELs. Furthermore, this method can also be implemented in VCSELs with longer cavity lengths by including a dielectric layer between the semiconductor and the DBR. The approach used here to obtain a temperature‐stable lasing wavelength is generic and can therefore be applied to VCSELs in all material systems and lasing wavelengths. [ABSTRACT FROM AUTHOR]

Far-ultraviolet-C (far-UVC) light-emitting diodes (LEDs) with an emission wavelength of 234 nm with different polarization-doped AlGaN hole injection layers (HILs) are compared regarding their emission power, voltage, and leakage current. The influence of the thickness of the polarization-doped layer (PDL), an additional Mg doping of the PDL, as well as a combination of a PDL with a conventionally Mg-doped AlGaN HIL will be discussed. The different PDL thicknesses show nearly no influence on the emission power or voltage. However, the leakage current of the LEDs below the turn-on voltage decreases with an increasing thickness of the PDL. In contrast, an additional Mg doping of the PDL ([Mg] ∼ 1.5 × 1019 cm−3) results in a fivefold decrease in the emission power at an unchanged voltage and leakage current. Finally, a combination of a PDL and a conventionally Mg-doped AlGaN layer ([Mg] ∼ 1.5 × 1019 cm−3) as a HIL shows also a similar emission power and voltage compared to the single PDL, but the leakage current increases. Based on these optimizations, 234 nm LEDs were realized with a maximum external quantum efficiency of 1% at 20 mA, an emission power of 4.7 mW, and a voltage of 9.0 V at 100 mA. This shows that the polarization doping concept is well suited to realize far-UVC LEDs with improved performance compared to LEDs with a conventionally Mg-doped p-side. [ABSTRACT FROM AUTHOR]

The long-term stability of ultraviolet (UV)-C light-emitting diodes (LEDs) is of major importance for many applications. To improve the understanding in this field, we analyzed the degradation of AlGaN-based UVC LEDs and modeled the variation of electrical characteristics by 2D simulations based on the results of deep-level optical spectroscopy (DLOS). The increase in the forward leakage current observed during ageing was ascribed an increase in trap-assisted tunneling. The analysis of the degradation kinetics suggests the role of a defect diffusion process, possibly involving impurities coming from the p-type layers. [ABSTRACT FROM AUTHOR]

The electroluminescence of UVB light-emitting diodes emitting at 310 nm before and after 1000 h of operation is studied in the temperature range from 20 to 340 K. Before operation, the external quantum efficiency (EQE) at 10 mA gradually increases with decreasing temperature from 0.8% at 340 K to 1.8% at 150 K and then levels off. This trend is attributed to a reduction of non-radiative recombination and finally the domination of radiative recombination at low temperatures. After 1000 h of operation, the EQE has dropped to 0.45% at 340 K with a maximum EQE of 1.4% at 80 K, followed by a drop for temperatures below 80 K. These findings suggest a stress-induced reduction of both the radiative recombination efficiency and the carrier injection efficiency. [ABSTRACT FROM AUTHOR]

The radiative recombination efficiency (RRE) of AlGaN quantum wells on epitaxially laterally overgrown AlN/sapphire templates was investigated by picosecond-laser–excited photoluminescence (PL) spectroscopy under selective and non-selective excitation conditions. The PL efficiency, which was deduced by excitation-power-dependent PL measurements at low temperature (LT) and room temperature (RT), was unity at LT under both excitation conditions; However, at RT, the PL efficiency under non-selective excitation conditions was lower than that under selective excitation conditions. Time-resolved PL measurements revealed that, under non-selective excitation conditions, additional carriers are provided from the surrounding layers to the quantum-well layers, especially at LT. Therefore, at RT, the PL efficiency does not correspond to the RRE under non-selective excitation conditions. We propose a model to explain carrier dynamics under the two excitation conditions showing that the PL efficiency equals the RRE under selective excitation conditions. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

In order to understand the electrical properties of V/Al/Ni/Au metal contacts to Si-doped Al0.75Ga0.25N layers, X-ray photoelectron spectroscopy analysis was performed on differently treated AlGaN:Si surfaces before metal deposition, and transmission electron microscopy was used to study the semiconductor-metal interface after contact annealing at 900 °C. Cl2 plasma etching of AlGaN increases the aluminum/nitrogen ratio at the surface, and Al oxide or oxynitride is always formed by any surface treatment applied after etching. After contact annealing, a complex interface structure including amorphous AlOx and different metal phases such as Al-Au-Ni, V-Al, and V2N were found. The electrical properties of the contacts were determined by thermionic emission and/ or thermionic field emission in the low voltage regime. Nearly ohmic contacts on AlGaN surfaces exposed to a Cl2 plasma were only obtained by annealing the sample at a temperature of 815 °C under N2/NH3 prior to metallization. By this treatment, the oxygen contamination on the surface could be minimized, resulting in a larger semiconductor area to be in direct contact with metal phases such as Al-rich Al-Au-Ni or V-Al and leading to a contact resistivity of 2.5 × 10-2 Ωcm2. This treatment can be used to significantly reduce the operating voltage of current deep ultraviolet light emitting diodes which will increase their wall plug efficiency and lower the thermal stress during their operation. [ABSTRACT FROM AUTHOR]

The dependence of the activation energy as well as the energetic levels of the neutral charge state and the DX center of the Si donor in AlxGa1-xN:Si samples on aluminum content and SiH4/III ratio were investigated by electron paramagnetic resonance (EPR) measurements, Van-der-Pauw resistivity measurements, and Hall-effect measurements. It was found by EPR measurements that the energy distance of the neutral charge state of the Si donor from the conduction band increases with increasing aluminum content from 61meV for x=0.82 to 106meV for x=0.89. Additionally, the formation of a DX center below the neutral charge state which is deepening from 6 meV for x=0.82 to 58meV for x=0.89 is observed. This results in a linearly increasing activation energy with increasing aluminum content from 67 meV for x=0.82 to 164meV for x=0.89. This is consistent with the activation energies as determined by Hall-effect measurements showing a linear increase from 24meV for x=0.85 to 211meV for x=0.96, as well as the activation energies as determined by Van-der-Pauw resistivity measurements. By varying the SiH4/III ratio we observed a formation of a minimum resistivity in accordance with the room temperature charge carrier density. However, no clear dependence of the activation energy was observed. EPR measurements of samples with a high SiH4/III ratio hint to an increased incorporation probability of a deep secondary donor species which might explain the increase in resistivity. [ABSTRACT FROM AUTHOR]

We compare the optical properties and device performance of unpackaged InGaN/GaN multiplequantum-well light-emitting diodes (LEDs) emitting at ~430 nm grown simultaneously on a high-cost small-size bulk semipolar (112 2) GaN substrate (Bulk-GaN) and a low-cost large-size (1122) GaN template created on patterned (1012) r-plane sapphire substrate (PSS-GaN). The Bulk-GaN substrate has the threading dislocation density (TDD) of ~105 cm-2-106cm-2 and basal-plane stacking fault (BSF) density of 0 cm-1, while the PSS-GaN substrate has the TDD of ~2 x 108 cm-2 and BSF density of ~1 x 10³ cm-1. Despite an enhanced light extraction efficiency, the LED grown on PSS-GaN has two-times lower internal quantum efficiency than the LED grown on Bulk-GaN as determined by photoluminescence measurements. The LED grown on PSS-GaN substrate also has about two-times lower output power compared to the LED grown on Bulk-GaN substrate. This lower output power was attributed to the higher TDD and BSF density. [ABSTRACT FROM AUTHOR]

Laser diodes were examined for small changes in gain and distributed loss during life testing.

The polarization fields in wurtzite group III-nitrides strongly influence the optical properties of InAlGaN-based light emitters, e.g., the electron and hole wave function overlap in quantum wells. In this paper, we propose a new approach to determine these fields by capacitance-voltage measurements (CVM). Sheet charges generated by a change of the microscopic polarization at heterointerfaces influence the charge distribution in PIN junctions and therefore the depletion width and the capacitance. We show that it is possible to determine the strength and direction of the internal fields by comparing the depletion widths of two PIN junctions, one influenced by internal polarization fields and one without as a reference. For comparison, we conducted coupled Poisson/carrier transport simulations on the CVM of the polarization-influenced sample. We also demonstrate the feasibility and limits of the method by determining the fields in GaN/InGaN and GaN/AlGaN double heterostructures on (0001) c-plane grown by metal organic vapor phase epitaxy and compare both evaluation methods. The method yields (–0.50 ± 0.07) MV/cm for In0.08Ga0.92N/GaN, (0.90 ± 0.13) MV/cm for Al0.18Ga0.82N/GaN, and (2.0 6 0.3) MV/cm for Al0.31Ga0.69N/GaN heterostructures. [ABSTRACT FROM AUTHOR]

The light extraction efficiency (LEE), external quantum efficiency (EQE), and current–voltage characteristics of deep ultraviolet light emitting diodes (DUV-LEDs) with different aluminum mole fractions in the p-AlGaN layers have been investigated. Optimizing the p-AlGaN layer composition requires a tradeoff between reducing the absorption losses and limiting the increases in the p-contact resistance and operation voltage. AlGaN multiple quantum well LEDs emitting around 263 nm with different AlGaN:Mg short period super lattices (p-SPSL) ranging from x = 33% (UV-absorbing) to x = 68% (UV-transparent) average aluminum mole fraction have been explored. DUV-LEDs with different p-contact metals and UV-reflectivities have been characterized by electroluminescence measurements and analyzed by ray-tracing simulations. The comparison shows an increased operating voltage and a five-fold increase of the on-wafer EQE with a maximum value of 3.0% for DUV-LEDs with UV-transparent p-SPSL (x = 68%) and UV-reflective indium contacts in comparison to LEDs with a UV-absorbing p-SPSL (x = 33%). Ray-tracing simulations show that the increase in EQE can be partially ascribed to a 2.5-fold improved LEE in combination with a two-fold increase in internal quantum efficiency. [ABSTRACT FROM AUTHOR]

The possibility of a hybrid semiconductor laser modulator of analog optical fiber systems, especially the effectiveness of an electro-optical modulator using the Franz-Keldysh effect has been shown. Experimental results obtained from the GaAs-AlGaAs system showed the signal-to-noise ratio is small enough for analog optical transmission, as in broad-band multichannel fiber networks.

The degradation of the electrical and optical properties of (InAlGa)N-based multiple quantum well light emitting diodes (LEDs) emitting near 308 nm under different stress conditions has been studied. LEDs with different emission areas were operated at room temperature and at constant current densities of 75 A/cm2, 150 A/cm2, and 225 A/cm2. In addition, the heat sink temperature was varied between 15 °C and 80 °C. Two main modes for the reduction of the optical power were found, which dominate at different times of operation: (1) Within the first 100 h, a fast drop of the optical power is observed scaling exponentially with the temperature and having an activation energy of about 0.13 eV. The drop in optical power is accompanied by changes of the currentvoltage (I-V) characteristic. (2) For operation times beyond 100 h, the optical power decreases slowly which can be reasonably described by a square root time dependence. Here, the degradation rate depends on the current density, rather than the current. Again, the rate of optical power reduction of the second mode depends exponentially on the temperature with an activation energy of about 0.21 eV. The drop in the optical power is accompanied by an increased reverse-bias leakage current. [ABSTRACT FROM AUTHOR]

The effects of the heterostructure design on the injection efficiency and external quantum efficiency of ultraviolet (UV)-B light emitting diodes (LEDs) have been investigated. It was found that the functionality of the Alx Ga1-x N:Mg electron blocking layer is strongly influenced by its aluminum mole fraction x and its magnesium doping profile. By comparing LED electroluminescence, quantum well photoluminescence, and simulations of LED heterostructure, we were able to differentiate the contributions of injection efficiency and internal quantum efficiency to the external quantum efficiency of UV LEDs. For the optimized heterostructure using an Al0.7Ga0.3N:Mg electron blocking layer with a Mg to group III ratio of 4% in the gas phase the electron leakage currents are suppressed without blocking the injection of holes into the multiple quantum well active region. Flip chip mounted LED chips have been processed achieving a maximum output power of 3.5 mW at 290 mA and a peak external quantum efficiency of 0.54% at 30 mA. [ABSTRACT FROM AUTHOR]

Light emitting diode structures emitting in the ultraviolet spectral range are investigated. The samples exhibit defect luminescence bands. Synchrotron-based photoluminescence excitation spectroscopy of the complicated multi-layer stacks is employed to assign the origin of the observed defect luminescence to certain layers. In the case of quantum well structures emitting at 320 and 290 nm, the n-type contact AlGaN:Si layer is found to be the origin of defect luminescence bands between 2.65 and 2.8 eV. For 230 nm emitters without such n-type contact layer, the origin of a defect double structure at 2.8 and 3.6 eV can be assigned to the quantum wells. [ABSTRACT FROM AUTHOR]

The impact of hydrogen impurities in the semiconductor heterostructure of (InAlGa)N-based laser diodes on the stability of the device's operating voltage is investigated. Diodes emitting at a wavelength of around 400 nm with different hydrogen concentrations and different p-contact metals, here Pt and Pd, were operated under constant current stress. After specific operation times, current–voltage measurements from the p- to the n-side and between two adjacent p-contacts were performed. During continuous-wave (cw) operation, three effects can be observed, namely (a) an increase of the operating voltage originating from degradation of the p-contact, (b) an increase of the voltage due to degradation of the heterostructure, and (c) a reduction of the voltage caused by a reduction of the series resistance. Degradation of the p-contact only occurs when Pt is used as the p-contact metal. In particular, Pd as the p-contact metal leads to stable p-contacts, regardless of the hydrogen concentration. Operating-voltage variations related to changes in the heterostructure may be attributed to the migration of residual hydrogen during operation. A reduction of the hydrogen level to a ratio of 20 between magnesium and hydrogen atoms, in combination with Pd p-contacts, led to stable forward voltages during cw operation for up to 100 h. [ABSTRACT FROM AUTHOR]

In group-III-nitride heterostructures with semipolar or nonpolar crystal orientation, anisotropic lattice and thermal mismatch with the buffer or substrate lead to a complex distortion of the unit cells, e.g., by shearing of the lattice. This makes an accurate determination of lattice parameters, composition, and strain state under assumption of the hexagonal symmetry impossible. In this work, we present a procedure to accurately determine the lattice constants, strain state, and composition of semipolar heterostructures using high resolution X-ray diffraction. An analysis of the unit cell distortion shows that four independent lattice parameters are sufficient to describe this distortion. Assuming only small deviations from an ideal hexagonal structure, a linear expression for the interplanar distances dhkl is derived. It is used to determine the lattice parameters from high resolution X-ray diffraction 2[variant_greek_theta]-ω-scans of multiple on- and off-axis reflections via a weighted least-square fit. The strain and composition of ternary alloys are then evaluated by transforming the elastic parameters (using Hooke's law) from the natural crystal-fixed coordinate system to a layer-based system, given by the in-plane directions and the growth direction. We illustrate our procedure taking an example of (11[formula]2) AlκGa1-κN epilayers with Al-contents over the entire composition range. We separately identify the in-plane and out-of-plane strains and discuss origins for the observed anisotropy. [ABSTRACT FROM AUTHOR]