We daily rely upon vertical-cavity surface-emitting lasers (VCSELs) for facial recognition and data communication. These lasers are now experiencing exponential growth and serves in other applications as well such as oxygen monitoring in combustion processes and in anesthetized patients in hospitals and as a source of heating in industry in the form of a large-sized array. The large interest for this laser class is linked to its beneficial qualities such as low threshold current, circular-symmetric low-divergent output beam, high efficiency, compactness, and low fabrication cost due to on-wafer testing. Due to these advantages, there is a strong push to realize VCSELs in other wavelength regimes, beyond the commercially available infrared and red. This would open completely new markets such as flood lights, projectors, sterilization, and medical diagnosis and treatment.
This presentation will provide an overview of the state-of-the-art in the development of AlGaN-based far-UVC-LED technologies. We will explore origins for the observed decline in the external quantum efficiency (EQE) with decreasing emission wavelength and present different approaches to improve the RRE, CIE, and LEE of UV light emitters. We will also discuss design aspects for far-UVC irradiation systems and provide an outlook of future prospects of far-UVC-LED device technology as well as the potential for a wider use of far-UVC sources in applications like room air disinfection.
Due to changes in the spontaneous and piezoelectric polarization, AlGaN/GaN heterostructures exhibit strong polarization fields at heterointerfaces. For quantum wells, the polarization fields lead to a strong band bending and a redshift of the emission wavelength, known as quantum-confined Stark effect. In this paper the polarization fields of thin AlGaN layers in a GaN matrix were determined by evaluating the changes in the depletion region width in comparison to a reference sample without heterostructure using capacitance–voltage-measurements. The polarization fields for Al0.09Ga0.91N (0.6 ± 0.7 MV cm−1), Al0.26Ga0.74N (2.3 ± 0.6 MV cm−1), Al0.34Ga0.66N (3.1 ± 0.6 MV cm−1), Al0.41Ga0.59N (4.0 ± 0.7 MV cm−1) and Al0.47Ga0.53N (5.0 ± 0.8 MV cm−1) heterostructures were determined. The results of the field strength and field direction of all samples are in excellent agreement with values predicted by theory and a capacitance–voltage based Poisson-carrier transport simulation approach giving experimental evidence for a nonlinear increasing polarization field with Al-concentration.
Trademark litigants need to be increasingly careful about how they conduct and present survey evidence in court following a key ruling in the 1993 Supreme Court case Daubert v. Merrell Dow Pharmaceuticals, IP owners were warned at a session on Monday. In its ruling in Daubert, the Court said that federal trial judges should be the “gatekeepers” of scientific evidence and assess whether the testimony of expert witnesses is both “relevant” and “reliable.” The ruling has had a big impact on trials. In the 13 and a half years before Daubert only four surveys were excluded during trademark litigation, said Shari Seidman Diamond of the Northwestern University School of Law. In the 13 and a half years since, that figure has risen to 25. Panelists at the session Survey Blues ‐ Litigation Survey Errors Causing Exclusion or Reduced Weight considered some of the reasons why survey evidence is now more likely to be thrown out of court. Jerre B. Swann of Kilpatrick Stockton said the blame had to be shared by lawyers, survey experts, and the courts. He argued that lawyers and judges often lack understanding about surveys and statistics. And he suggested that survey experts had to shoulder some of the blame. “They appear to have accepted a ‘hired gun’ attitude to their work,” he said. This makes their findings more vulnerable to criticisms about methodology raised by the other party.
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 cm 2 ) 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.
The degradation behavior of AlGaN multiple-quantum well (MQW) deep-ultraviolet light emitting diodes emitting near 233 nm stressed at a constant current of 100 mA over 1000 h of operation has been investigated. A strong reduction of the MQW luminescence in the first 250 h and nearly stable MQW emission power over the remaining operation time is found. Moreover, the emission powers of the MQW peak as well as the emission power of parasitic peaks centered at around 266 and 403 nm are changing in a comparable manner, indicating a common degradation process. Furthermore, the leakage current below the turn-on voltage increases with increasing operation time and the change of the emission power as a function of drive current is stronger in the low current regime compared to higher currents. All effects noted up to now show the strongest impact in the first 250 h of operation. In contrast, the emission power of an additional parasitic emission band centered around 500 nm increases continuously over time. Taken altogether these observations can be explained by the following physical degradation mechanisms: the concentration of charged point defects acting as non-radiative recombination centers in the AlGaN active region increases. In addition, the concentration of point defects acting as centers for trap-assisted radiative recombination outside the active region increases.
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.
Recent progress in the development of AlGaN-based materials and high-efficiency ultraviolet light emitting diodes (UV-LEDs) will be reviewed. The impact of UV-LEDs on applications in environmental sensing, life sciences, and medical diagnostics will be illustrated.
The impact of the operation parameters current and temperature on the degradation of AlGaN-based 233 nm far-ultraviolet-C LEDs is investigated. The observed effects can be divided into two groups: First, a rapid reduction in the optical power to about 50%–30% of the initial value during the first ∼100 h of operation, which is accompanied by an increase in the current below the diffusion voltage from 0.3 to about 1 μA, and a reduction in the hydrogen concentration in the p-side close to the active region. The second group is represented by a gradual reduction of the optical power, which runs in parallel to the effects in the first group and dominates for operation times ≥100 h. The reduction of the optical power is due to a decrease in the slope of the optical power–current characteristic. All effects are accelerated at increased stress currents and current densities—the reduction in the optical power at low (∼20 mA) and high measuring current (∼80 mA) scales with the current to the power of three. For example, after 250 h of operation, the relative optical power at a measuring current of 20 mA has decreased to about 40% when the LED was operated at a stress current of 20 mA and to <10% for a stress current of 100 mA. Furthermore, temperature has no significant impact on the reduction of the optical power during operation, i.e., the relative optical power reduced to about 25% after 250 h both when the LEDs were operated at 20 °C and when they were operated at 75 °C.
High temperature annealed AlN/sapphire templates exhibit a reduced in-plane lattice constant compared to conventional non-annealed AlN/sapphire grown by metalorganic vapor phase epitaxy (MOVPE). This leads to additional lattice mismatch between the template and the AlGaN-based ultraviolet-C light emitting diode (UVC LED) heterostructure grown on these templates. This mismatch introduces additional compressive strain in AlGaN quantum wells resulting in enhanced transverse electric polarization of the quantum well emission at wavelengths below 235 nm compared to layer structures deposited on conventional MOVPE-grown AlN templates, which exhibit mainly transverse magnetic polarized emission. In addition, high temperature annealed AlN/sapphire templates also feature reduced defect densities leading to reduced non-radiative recombination. Based on these two factors, i.e., better outcoupling efficiency of the transverse electric polarized light and an enhanced internal quantum efficiency, the performance characteristic of far-UVC LEDs emitting at 231 nm was further improved with a cw optical output power of 3.5 mW at 150 mA.
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