Jens Rass

Indium incorporation and emission wavelength of polar, nonpolar and semipolar InGaN quantum wells

InGaN quantum wells were grown by metal organic vapor-phase epitaxy on polar (0 0 0 1), nonpolar (1 0  0) and on semipolar (1 0  2), (1 1  2), (1 0  1) as well as (2 0  1) oriented GaN substrates. The room-temperature photoluminescence (PL) and electroluminescence (EL) emission energies for quantum wells grown on different crystal orientations show large variations of up to 600 meV. The following order of the emission energy was found throughout the entire range of growth temperatures: (1 0  1) < (1 1  2) = (0 0 0 1) < (2 0  1) < (1 0  0) = (1 0  2). In order to differentiate between the effects of strain, quantum-confined stark effect (QCSE) and indium incorporation the experimental data were compared to k.p theory-based calculations for differently oriented InGaN QWs. The major contribution to the shift between (1 0  0) and (0 0 0 1) InGaN quantum wells can be attributed to the QCSE. The redshift between (1 0  0) and the semipolar (1 0  2) and (2 0  1) QWs can be attributed to shear and anisotropic strain affecting the valence band structure. Finally, for (1 1  2) and (1 0  1) the emission energy shift could be attributed to a significantly higher indium incorporation efficiency.

III-Nitride Ultraviolet Emitters

This chapter provides a brief introduction to group III-nitride ultraviolet light emitting diode (LED) technologies and an overview of a number of key application areas for UV-LEDs. It covers the state of the art of UV-LEDs as well a survey of novel approaches for the development of high performance UV light emitters.

Performance Characteristics of UV-C AlGaN-Based Lasers Grown on Sapphire and Bulk AlN Substrates

The performance characteristics of optically pumped laser heterostructures emitting in the UV-C spectral range between 272 and 279 nm are investigated. The laser heterostructures were grown by metal-organic vapor phase epitaxy on (0001) planar AlN/sapphire, epitaxially laterally overgrown (ELO) AlN/sapphire, and bulk AlN substrates with threading dislocation densities ranging from 2×1010 to 104 cm-2. We found that the defect density strongly affects the laser performance. The lowest pulse threshold energy density of 50 mJ/cm2 under resonant optical pumping condition was obtained for an AlGaN multiple quantum well laser grown pseudomorphically on low defect density bulk AlN substrate. Lasing was also observed for AlGaN MQW heterostructures grown on ELO AlN/sapphire templates. The laser emission in all lasers was TE polarized. However, no lasing was observed for heterostructures grown on high defect density AlN/sapphire.

Efficient charge carrier injection into sub-250 nm AlGaN multiple quantum well light emitting diodes

The design and Mg-doping profile of AlN/Al0.7Ga0.3N electron blocking heterostructures (EBH) for AlGaN multiple quantum well (MQW) light emitting diodes (LEDs) emitting below 250 nm was investigated. By inserting an AlN electron blocking layer (EBL) into the EBH, we were able to increase the quantum well emission power and significantly reduce long wavelength parasitic luminescence. Furthermore, electron leakage was suppressed by optimizing the thickness of the AlN EBL while still maintaining sufficient hole injection. Ultraviolet (UV)-C LEDs with very low parasitic luminescence (7% of total emission power) and external quantum efficiencies of 0.19% at 246 nm have been realized. This concept was applied to AlGaN MQW LEDs emitting between 235 nm and 263 nm with external quantum efficiencies ranging from 0.002% to 0.93%. After processing, we were able to demonstrate an UV-C LED emitting at 234 nm with 14.5 μW integrated optical output power and an external quantum efficiency of 0.012% at 18.2 A/cm2.

Improved injection efficiency in 290 nm light emitting diodes with Al(Ga)N electron blocking heterostructure

The effect of different Al(Ga)N electron blocking heterostructures (EBH) on the emission spectra and light output power of 290 nm light emitting diodes (LEDs) has been investigated. The carrier injection and internal quantum efficiency of the LEDs is simulated and compared to electroluminescence measurements. The highest light output power has been found for LEDs with an Mg-doped AlN/Al0.7Ga0.3N EBH with an AlN layer thickness >3 nm. The output power of these LEDs was 8.5-times higher compared to LEDs without EBH. This effect is attributed to an improved carrier injection and confinement which prevents electron leakage into the p-doped region of the LED with a simultaneously enhanced hole injection into the active region.

On the optical polarization properties of semipolar InGaN quantum wells

Polarized photoluminescence of strained quantum wells grown on c–plane, semipolar (101−2), (112−2), (101−1), (202−1) planes, and nonpolar GaN substrates was studied experimentally and in theory. The observed optical polarization switching between the substrate orientations (101−2) and (112−2) is in accordance with our general model of polarization switching, based on a k→·p→ model of arbitrary substrate orientation. Spectrally resolved measurements of the polarization degree stemming from (101−2) samples show that the maximum of the polarization degree is red–shifted with respect to the maximum of the photoluminescence intensity. We ascribe this effect to an increased polarization of the transitions for higher indium content.

High-power UV-B LEDs with long lifetime

UV light emitters in the UV-B spectral range between 280 nm and 320 nm are of great interest for applications such as phototherapy, gas sensing, plant growth lighting, and UV curing. In this paper we present high power UV-B LEDs grown by MOVPE on sapphire substrates. By optimizing the heterostructure design, growth parameters and processing technologies, significant progress was achieved with respect to internal efficiency, injection efficiency and light extraction. LED chips emitting at 310 nm with maximum output powers of up to 18 mW have been realized. Lifetime measurements show approximately 20% decrease in emission power after 1,000 operating hours at 100 mA and 5 mW output power and less than 30% after 3,500 hours of operation, thus indicating an L50 lifetime beyond 10,000 hours.

Strongly transverse-electric-polarized emission from deep ultraviolet AlGaN quantum well light emitting diodes

The optical polarization of emission from ultraviolet (UV) light emitting diodes (LEDs) based on (0001)-oriented AlxGa1−xN multiple quantum wells (MQWs) has been studied by simulations and electroluminescence measurements. With increasing aluminum mole fraction in the quantum well x, the in-plane intensity of transverse-electric (TE) polarized light decreases relative to that of the transverse-magnetic polarized light, attributed to a reordering of the valence bands in AlxGa1−xN. Using k ⋅ p theoretical model calculations, the AlGaN MQW active region design has been optimized, yielding increased TE polarization and thus higher extraction efficiency for bottom-emitting LEDs in the deep UV spectral range. Using (i) narrow quantum wells, (ii) barriers with high aluminum mole fractions, and (iii) compressive growth on patterned aluminum nitride sapphire templates, strongly TE-polarized emission was observed at wavelengths as short as 239 nm.

Low absorption loss p-AlGaN superlattice cladding layer for current-injection deep ultraviolet laser diodes

Current injection into AlGaN-based laser diode structures with high aluminum mole fractions for deep ultraviolet emission is investigated. The electrical characteristics of laser diode structures with different p-AlGaN short period superlattice (SPSL) cladding layers with various aluminum mole fractions are compared. The heterostructures contain all elements that are needed for a current-injection laser diode including cladding and waveguide layers as well as an AlGaN quantum well active region emitting near 270 nm. We found that with increasing aluminum content in the p-AlGaN cladding, the diode turn-on voltage increases, while the series resistance slightly decreases. By introducing an SPSL instead of bulk layers, the operating voltage is significantly reduced. A gain guided broad area laser diode structure with transparent p-Al0.70Ga0.30N waveguide layers and a transparent p-cladding with an average aluminum content of 81% was designed for strong confinement of the transverse optical mode and low optical losses. Using an optimized SPSL, this diode could sustain current densities of more than 4.5 kA/cm2.

Degradation of (InAlGa)N-based UV-B light emitting diodes stressed by current and temperature

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 current-voltage (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. A...