M. Weyers

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.

Pulse repetition rate up to 92 GHz or pulse duration shorter than 110 fs from a mode-locked semiconductor disk laser

A semiconductor disk laser based on an InGaAs/AlGaAs quantum-well gain medium was mode-locked by a fast semiconductor saturable absorber mirror. By high-order harmonic mode-locking a 92 GHz pulse train was obtained with a pulse duration of <200 fs. In order to achieve fundamental mode-locking, too strong saturation of the semiconductor elements had to be avoided. In a single-pulse regime, pulses shorter than 110 fs were generated at a wavelength of 1030 nm.

12 W continuous-wave diode lasers at 1120 nm with InGaAs quantum wells

Highly strained InGaAs quantum wells were grown by metalorganic vapor-phase epitaxy. By lowering the growth temperature to 530 °C, a maximum photoluminescence wavelength of 1192 nm was achieved. High-power diode lasers with a maximum lasing wavelength of 1175 nm were fabricated. A continuous-wave output power of 12 W at a heat-sink temperature of 25 °C was obtained at a lasing wavelength of 1120 nm.

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.

MOVPE growth of highly strained InGaAs/GaAs quantum wells

The indium incorporation into strained InGaAs quantum wells grown on GaAs substrate by metalorganic vapourphase epitaxy is found to be reduced in comparison to relaxed layers. Additionally, the indium uptake into strained QWs is limited to approximately 30% InAs at 650°C. Excessive trimethyl indium supply in the vapour phase leads to a drop of the In-content of the QW and to a reduced total In-content in the whole structure. Only a small amount of the excess indium is incorporated into InAs-rich clusters observed as dark-spot defects and into a graded interfacial layer. A model for this behaviour based on the enhanced In-reevaporation from In-rich areas is presented.

Enhancement of light extraction in ultraviolet light-emitting diodes using nanopixel contact design with Al reflector

We report on a nanopixel contact design for nitride-based ultraviolet light-emitting diodes to enhance light extraction. The structure consists of arrays of Pd ohmic contact pixels and an overlying Al reflector layer. Based on this design a twofold increase in the light output, compared to large area Pd square contacts is demonstrated. Theoretical calculations and experiments reveal that a nanopixel spacing of 1 μm or less is required to enable current overlap in the region between the nanopixels due to current spreading in the p-GaN layer and to ensure current injection into the entire active region. Light emitted in the region between the nanopixels will be reflected by the Al layer enhancing the light output. The dependence of the light extraction on the nanopixel size and spacing is investigated.

Design and realization of high-power DFB lasers

The development of high-power GaAs-based ridge wave guide distributed feedback lasers is described. The lasers emit between 760 nm and 980 nm either in TM or TE polarization. Over a large current range, the lasers exhibit stable operation in a single transversal and longitudinal mode. A maximum continuous-wave output power of about 400 mW, a spectral linewidth below 1 MHz and a side mode suppression ratio greater than 50 dB have been demonstrated at room temperature. The distributed feedback is provided by first or second order gratings, formed in an InGaP/GaAsP/InGaP multilayer structure embedded into the p-AlGaAs cladding layer. Applications of such wavelength stabilized devices in non-linear frequency conversion, spectroscopy and for excitation of atomic transitions are discussed.

High gain ultraviolet photodetectors based on AlGaN/GaN heterostructures for optical switching

We report on the optoelectronic properties of Al0.25Ga0.75N/GaN-based ultraviolet (UV) photodetectors for the application as a high current, high gain optical switch. Due to an internal gain mechanism combined with the high conductivity of the two-dimensional electron gas at the heterostructure interface, photocurrents in the milliampere-range were obtained with UV illumination. By employing a mesa structure design with meander geometry very low dark currents below 50 nA up to a bias voltage of 100 V were achieved. Optical switching with an on/off-current-ratio of five orders of magnitude was demonstrated. The response time was determined to be 6 ms and persistent photoconductivity was observed. The photodetector is visible-blind with a cut-off wavelength of 365 nm according to the band gap energy of the GaN absorption layer. A high responsivity with a maximum of 70 A/mW at 312 nm and 100 V bias voltage was demonstrated.

Heat transfer and mass transport in a multiwafer MOVPE reactor: modelling and experimental studies

Abstract An improved detailed model for the calculation of the temperature distribution in a multiwafer Planetary Reactor™ has been developed. The temperature field of the reactor has been calculated in dependence of the reactor parameters for (Al,Ga)As growth as well as on the kind and the thickness of the wall and susceptor deposits. The amount of parasitic wall deposits can be minimized by a proper tuning of the reactor temperature distribution. Calculated GaAs growth rate profiles on 3 inch wafers show a strong dependence on the temperature field in the reactor and the amount of parasitic deposits. These predicted relationships have been used to optimize the reactor temperature distribution in order to minimize parasitic wall depositions. By this procedure a growth rate uniformity of