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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. [ABSTRACT FROM AUTHOR]

The degradation behavior of ultraviolet-B light emitting diodes (UV-B LEDs) emitting near 310 nm has been investigated and a method to localize the degradation effects is presented. Measurements of the electro-optical characteristics of UV-B LEDs, during a 200 h constant-current degradation study, showed an initial fast decrease in the optical power accompanied by a decrease in the drive voltage and an increase in the capacitance. Furthermore, by using a specially designed contact geometry, it was possible to separate the degradation of the electrical properties of the p-layers and p-contacts from the degradation of the active region and n-side of the LED heterostructure. Our investigations show that the initial changes in capacitance and voltage can be attributed to changes in the p-side and at the p-contact of the LED, which can be explained by an activation of Mg dopants. [ABSTRACT FROM AUTHOR]

Laser gain is investigated for AlGaN wurtzite quantum-well structures emitting in the wavelength range from ~270 to 340 nm. The calculations show that gain properties vary notably with aluminum concentration in the quantum well.

The threshold current density of narrow (1.5 μm) ridge waveguide (In,Al)GaN based laser diodes is found to strongly depend on the ridge etch depth. By solving the complex-value two-dimensional waveguide equation, it is shown that, for shallow-ridge devices with a small built-in index step, the dependence of the modal gain on the material gain is strongly influenced by antiguiding effects. Taking into account the lateral extension of the material gain beyond the ridge as well as the optical mode absorption in the unpumped regions of the quantum wells, the observed differences in the threshold current density can be reproduced by simulations. [ABSTRACT FROM AUTHOR]

For the realization and the improvement of GaN-based optoelectronic devices (light emitting diodes and laser diodes) emitting from the ultraviolet to the red wavelength range GaInN quantum well structures with high internal quantum efficiency are of great importance. To determine parameters which affect the internal quantum efficiency, we have analyzed the emission intensity of GaInN quantum well structures with varied electron and hole wave function overlap by temperature and excitation power dependent and by time-resolved photoluminescence. The quantum confined Stark effect reduces the temperature dependent photoluminescence emission intensity for thick polar quantum wells at low temperature. But near room temperature, these thick polar GaInN quantum wells feature less relative intensity loss than thinner polar quantum wells. This behavior can partially be assigned to increased screening effects and higher quantum well barriers for thicker quantum wells. Additionally, excitation power dependent photoluminescence points to a transition from a radiative recombination based on excitons at 10 K to a bimolecular recombination at room temperature for thick c-plane quantum wells. This transition may also affect the intensity decrease by a changed carrier diffusivity. [ABSTRACT FROM AUTHOR]

Laser gain is investigated for AlGaN wurtzite quantum-well structures emitting in the wavelength range from ∼270 to 340 nm. The calculations show that gain properties vary notably with aluminum concentration in the quantum well. The TE gain dominates over the entire spectral range, although an enhancement of TM gain is observed for AlGaN quantum wells with the high aluminum mole fraction. The calculations also predict an increase in threshold current density for the shorter-wavelength lasers. [ABSTRACT FROM AUTHOR]

We demonstrate that the polarity of polar (0001), [formula] and semipolar [formula] and GaN thin layers can be determined by valence band X-ray photoemission spectroscopy (XPS). The polarity of the layers has been confirmed by wet etching and convergent beam electron diffraction. Unlike these two techniques, XPS is a non-destructive method and unaffected by surface oxidation or roughness. Different intensities of the valence band states in spectra recorded by using AlKα X-ray radiation are observed for N-polar and group-III-polar layers. The highest intensity of the valence band state at ≈3.5 eV for InN and ≈5.2 eV for GaN correlates with the group-III polarity, while the highest intensity at ≈6.7 eV for InN and ≈9.5 eV for GaN correlates with the N-polarity. The difference between the peaks for the group-III- and N-polar orientations was found to be statistically significant at the 0.05 significance level. The polarity of semipolar [formula] and GaN layers can be determined by recording valence band photoelectrons emitted along the [000 ± 1] direction. [ABSTRACT FROM AUTHOR]

We report on metal-organic vapor phase epitaxial growth of ([formula]) InN on ([formula]) GaN templates on m-plane ([formula]) sapphire substrates. The in-plane relationship of the ([formula]) InN samples is [[formula]]InN||[0001]sapphire and [[formula]]InN||[[formula]]sapphire, replicating the in-plane relationship of the ([formula]) GaN templates. The surface of the ([formula]) InN samples and the ([formula]) GaN templates shows an undulation along [[formula]]InN,GaN, which is attributed to anisotropic diffusion of indium/gallium atoms on the ([formula]) surfaces. The growth rate of the ([formula]) InN layers was 3-4 times lower compared to c-plane (0001) InN. High resolution transmission electron microscopy showed a relaxed interface between the ([formula]) InN layers and the ([formula]) GaN templates, consistent with x-ray diffraction results. Basal plane stacking faults were found in the ([formula]) GaN templates but they were terminated at the InN/([formula]) GaN interface due to the presence of misfit dislocations along the entire InN/GaN interface. The misfit dislocations were contributed to the fully relaxation and the tilts of the ([formula]) InN layers. X-ray photoelectron spectroscopy was used to determine the polarity of the grown ([formula]) InN sample, indicating an In-polar ([formula]) InN. The valence band maximum was determined to be at (1.7 ± 0.1) eV for the ([formula]) InN sample, comparable to In-polar c-plane InN. [ABSTRACT FROM AUTHOR]