Tim Kolbe

Advances in group III-nitride-based deep UV light-emitting diode technology

The field of AlGaInN ultraviolet UV light-emitting diodes (LEDs) is reviewed, with a summary of the state-of-the-art in device performance and enumeration of applications. Performance-limiting factors for high-efficiency UV LEDs are identified and recent advances in the development of deep UV emitters are presented.

Application of GaN-based ultraviolet-C light emitting diodes – UV LEDs – for water disinfection

GaN-based ultraviolet-C (UV-C) light emitting diodes (LEDs) are of great interest for water disinfection. They offer significant advantages compared to conventional mercury lamps due to their compact form factor, low power requirements, high efficiency, non-toxicity, and overall robustness. However, despite the significant progress in the performance of semiconductor based UV LEDs that has been achieved in recent years, these devices still suffer from low emission power and relatively short lifetimes. Even the best UV LEDs exhibit external quantum efficiencies of only 1-2%. The objective of this study was to investigate the suitability of GaN-based UV LEDs for water disinfection. The investigation included the evaluation of the performance characteristics of UV LEDs at different operating conditions as well as the design of a UV LED module in view of the requirements for water treatment applications. Bioanalytical testing was conducted using Bacillus subtilis spores as test organism and UV LED modules with emission wavelengths of 269 nm and 282 nm. The results demonstrate the functionality of the developed UV LED disinfection modules. GaN-based UV LEDs effectively inactivated B. subtilis spores during static and flow-through tests applying varying water qualities. The 269 nm LEDs reached a higher level of inactivation than the 282 nm LEDs for the same applied fluence. The lower inactivation achieved by the 282 nm LEDs was compensated by their higher photon flux. First flow-through tests indicate a linear correlation between inactivation and fluence, demonstrating a well designed flow-through reactor. With improved light output and reduced costs, GaN-based UV LEDs can provide a promising alternative for decentralised and mobile water disinfection systems.

Optical polarization characteristics of ultraviolet (In)(Al)GaN multiple quantum well light emitting diodes

The polarization of the in-plane electroluminescence of (0001) orientated (In)(Al)GaN multiple quantum well light emitting diodes in the ultraviolet-A and ultraviolet-B spectral range has been investigated. The intensity for transverse-electric polarized light relative to the transverse-magnetic polarized light decreases with decreasing emission wavelength. This effect is attributed to rearrangement of the valence bands at the Γ-point of the Brillouin zone with changing aluminum and indium mole fractions in the (In)(Al)GaN quantum wells. For shorter wavelength the crystal-field split-off hole band moves closer to the conduction band relative to the heavy and light hole bands and as a consequence the transverse-magnetic polarized emission becomes more dominant for deep ultraviolet light emitting diodes.

Effect of strain and barrier composition on the polarization of light emission from AlGaN/AlN quantum wells

For AlGaN-based multi-quantum-well light emitters grown on c-plane substrates there is a tendency for the polarization of the emitted light to switch from transverse electric (TE) polarization to transverse magnetic (TM) polarization as the wavelength decreases. This transition depends on various factors that include the strain in the quantum well. Experimental results are presented that illustrate the phenomenon for nitride light emitting diodes (LEDs) grown on sapphire and on bulk AlN. Model calculations are presented which quantify the dependence of the TE/TM switch on the quantum well strain and the Al composition in the barriers surrounding the well.

Effect of the barrier composition on the polarization fields in near UV InGaN light emitting diodes

The electroluminescence from near ultraviolet (UV) light emitting diodes containing InGaN multiple quantum wells (MQWs) with GaN, AlGaN, and InAlGaN barriers was investigated. Based on band-structure calculations the observed wavelength shift in the peak emission with increasing injection current is attributed to the screening of the polarization fields and to band gap renormalization. InGaN MQWs with almost zero net polarization have been realized. No blueshift in the emission spectra of these devices was observed over the entire current range.

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.

Effect of temperature and strain on the optical polarization of (In)(Al)GaN ultraviolet light emitting diodes

The temperature and strain dependence of the polarization of the in-plane electroluminescence of (0001) orientated (In)(Al)GaN multiple quantum well light emitting diodes in the ultraviolet spectral range has been investigated. For light emitting diodes with emission wavelength shorter than 300 nm the transversal-electric polarized emission intensity increases relative to the transversal-magnetic emission with increasing temperature, whereas it decreases for ultraviolet light emitting diodes with longer emission wavelength. This effect can be attributed to occupation of deeper valence bands with increasing temperature. In addition, strain also strongly influence the in-plane light polarization of near ultraviolet light emitting diodes. The transversal-magnetic polarized emission becomes more dominant with decreasing in-plane tensile strain of the InGaN/(In)(Al)GaN multiple quantum well active region.

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.

MOVPE growth for UV-LEDs

Challenges for the MOVPE growth of LED heterostructures for emission in the UV-A and UV-B spectral range are discussed. Special attention is given to the effects of strain in the In(Al)GaN active region as well as in the complete layer stack. Here in-situ monitoring of wafer bowing is shown to be an important tool for optimization of the growth sequence. We will compare different buffer layer technologies, in particular GaN/sapphire for LEDs emitting at 380 nm and AlN/AlGaN buffer for shorter wavelength LEDs. By increasing the aluminum content in the InAlGaN multiplequantum- well active region and by optimizing the composition and doping profile of the electron blocking layers UV LEDs with emission wavelength between 380 nm and 318 nm are demonstrated.