Light emitting diodes in the deep ultraviolet spectral range (DUV-LEDs) are of great interest for monitoring gases, pollutants in water as well as the in-vivo inactivation of multi-drug-resistant bacteria. This paper reviews advances in development of AlGaN-based DUV-LEDs, including the realization of low defect density AlN on sapphire. DUV-LEDs near 230 nm with output powers of more than 3 mW will be demonstrated and the root causes for the efficiency drop at shorter UV wavelength will be explored, including changes in the polarization of light emission, the role of point defects as well as carrier injection in AlGaN MQWs.
Recent advances in optimizing the efficiency and lifetime of far-UVC LEDs with emission wavelengths below 240 nm are presented. The design of the semiconductor heterostructure is considered as well as the chip layout. Cross-comparisons are used to draw general conclusions about degradation mechanisms in UV LEDs and to identify development strategies to minimize them. Furthermore, it is discussed which chip packaging is particularly suitable for a combination of far-UVC LEDs with spectral filters. Finally, far-UVC irradiation systems for skin-friendly irradiation of the human body are presented and their performance is illustrated with selected medical and biological data.
The development of efficient (In)AlGaN light emitting diodes (LEDs) in the ultraviolet B (UVB) spectral region (280nm-320nm) is essential due to their vast commercial potential. UVB LEDs are expected to not only replace traditional mercury lamps in applications such as curing of materials and phototherapy but also to establish new applications in the fields of plant growth and sensing. Although a lot of progress has been made on the performance of the UVB LEDs, the efficiency of the devices as well as the lifetime still needs to be improved. In this study the influence of the heterostructure design and package on the efficiency of UVB LEDs, grown by metalorganic vapor phase epitaxy on c-plane sapphire substrates, will be presented. Firstly, the performance of UVB AlGaN and InAlGaN multiple quantum well LEDs were studied and the influence of the material composition on the emission characteristics was analyzed. Secondly, the performance of LEDs with different electron blocking layer (EBL) designs and doping concentrations was compared. The highest internal quantum efficiency and emission power were obtained for LEDs with a gradient-like EBL, with decreasing aluminum content, because of the improved carrier injection. Additionally, the output power of the LEDs was found to increase with the p-doping level in the EBL. Finally, investigations on the influence of the metal contacts and insulator as well as the device packaging on the performance of UVB LEDs will be presented. Based on these optimizations, 315nm LEDs with output powers up to 10mW at 100mA were realized
UV-LEDs are of great interest for applications like disinfection, gas sensing, and phototherapy. The cost sensitive LEDs are commonly grown by MOVPE on transparent AlN/sapphire templates. The large thermal and lattice mismatch between AlN and sapphire generates a very high dislocation density (DD) and causes big challenges in strain management. The threading dislocation density should be reduced to the order of low 108cm-2 for high internal efficiency of the AlGaN based UV-LED structures. The TDD will be reduced mainly by dislocation annihilation during the growth of thick Al(Ga)N layers, which is a challenge in terms of strain management.
We present how in-situ reflectometry and curvature measurement (EpiCurveTT(at)LayTec) in commercial multiwafer growth reactors helps to optimize the growth processes concerning growth rates, surface roughening and avoidance of layer cracking on 2inch substrates and enhance the reproducibility of epitaxial growth. The growth of up to 3 μm thick planar AlN templates and up-to 10 μm thick AlN/sapphire templates by epitaxial lateral overgrowth of stripe patterned templates for UV-C LED structures will be highlighted. The implementation of different types of AlN/GaN superlattices for the subsequent growth of up to 5μm thick Al0.5Ga0.5N layer for UVB LED structures will be shown. Correlations to ex-situ measurements like X-ray diffraction and TEM analysis of defects in the LED structures will be shown.
Some challenges of in-situ control through very narrow viewports as in Close Coupled Showerhead reactors will be discussed as well as the influence of silicon doping on curvature and dislocation density in Al(Ga)N layers.
In this paper we report on the influence of the heterostructure design of (InAlGa)N-based UV-B LEDs grown by metalorganic vapor phase epitaxy on sapphire substrates on the degradation behavior of the device. Two types of LEDs with different heterostructure design, resulting in peak-wavelengths of about 290 nm and 310 nm, respectively, were stressed at a constant operation current of 100 mA and a heat sink temperature of 20°C. Electro-optical characterization of the LEDs over 1.000 h of operation shows two different degradation modes with respect to the change of the emission spectrum and leakage current. The first mode during the initial hours (290 nm LED: 0 h - 500 h, 310 nm LED: 0 h – 100 h) of operation is represented by a fast reduction of the quantum well (QW) luminescence, a constant or increasing parasitic luminescence between 310 nm and 450 nm and a fast increase of the reverse- and forward-bias leakage current. These changes are more pronounced (higher degradation rate) in the 290 nm LEDs and can therefore be attributed to the different heterostructure design. In contrast, the second degradation mode at longer operation times (290 nm LED: >500 h, 310 nm LED: >100 h) is marked by a slow reduction of both the QW and the parasitic luminescence, as well as a slow increase of the leakage current which are similar for both types of LEDs. Furthermore, the second mode is marked by a square-root time dependence of the QW luminescence intensity, indicating a diffusion process to be involved.
We present UV-C LEDs emitting around 235 nm grown by MOVPE on ELO AlN/sapphire substrates. In order to account for the low conductivity of high Al content AlGaN layers and the associated high contact resistances, we designed an optimized compact LED geometry based on electro-thermal simulations of the current spreading. Experimental data (layer and contact resistances) are collected on test structures and used as input parameters for 3-D current spreading simulations. With resistances of the layers (n and p) approaching 0.1 Ωcm, the use of a segmented p-area with broad n-contact fingers (10 μm or more) that are close to the mesa edge (5 μm) help to maximize the emission power in the center of the structure. Based on this knowledge a series of compact LEDs of size 500 μm x 500 μm is designed and simulated. We get confirmation that the segmentation of the p-area is the most critical parameter to limit the non-uniformity introduced by the high n-sheet resistances. Up to 17% in emission power can be gained when the n-contacts are designed properly. LEDs with the optimum geometry were processed and measured. We get a good confirmation of our model concerning the distribution of the emission power. Both simulations and measurements show current crowding at the edge of the n-contact, however the power loss in the middle of the chip is higher than predicted.
The optical polarization of the in-plane emission of c-plane oriented (In)(Al)GaN multiple quantum well light emitting
diodes in the spectral range from 288 nm to 386 nm has been investigated by electroluminescence measurements. The
intensity of transverse-electric polarized light relative to the transverse-magnetic polarized light decreases with
decreasing emission wavelength. This effect is attributed to the different electronic band structures in the active region of
the light emitting diodes. A changing aluminum and indium mole fraction in the (In)(Al)GaN quantum wells results in a
rearrangement of the valence bands at the Γ-point of the Brillouin zone. For shorter wavelengths the crystal-field splitoff
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 increases. Moreover, the in-plane polarization is shown to depend on the
injection current. The correlation between the in-plane polarization and the injection current has been found to be
different for light emitting diodes with InGaN and (In)AlGaN multiple quantum wells. The results highlight that
polarization effects need to be considered when optimizing the light extraction from ultraviolet light emitting diodes in
the (In)AlGaN materials system.
Compact high power laser light sources emitting a single frequency with diffraction limited beams in continuous wave (CW) operation are required for many applications including frequency conversation.
We present a hybrid integrated package consisting of a distributed feedback ridge-waveguidemaster-oscillator power-
amplifier mounted on an AlN micro-optical bench with a CW output power of 4.5W at 976 nm having a beam propagation ratio of M2 < 2 (including second order moments). The longitudinal mode emission has a linewidth of λ < 10 pm. Due to thermal decoupling the wavelength shift is nearly independent of the amplifier
pump-current.
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.
Stimulated emission dynamics in InGaN-based multiple quantum wells (MQWs) is analyzed. The lasing threshold
measurements of the In0.09Ga0.91N/In0.02Ga0.98N MQWs revealed non-monotonous threshold dependence on the growth
temperature of the active MQW region. The optimal growth temperature range with the lowest stimulated emission
threshold (100 kW/cm2) in the active region was found to be 780 - 800°C. The influence of indium nano-clusters on
stimulated emission threshold is discussed. Optical gain in InGaN MQWs was measured using variable excitation stripe
length technique. The optical gain dependence on excitation stripe length and excitation power density was studied. The
onset of the gain saturation was observed on the high energy side of the stimulated emission peak. The onset exhibited
red-shift with increasing stripe length due to reduced electron-hole density caused by high optical transition rate.
Increase of excitation power density resulted in the strong blue-shift of the optical gain spectra. The maximal optical gain
coefficient values of 200 cm-1 and 300 cm-1 were obtained for the samples with the lowest and the highest stimulated
emission thresholds, respectively. The calculated optical confinement factor (3.4 %) for the samples yielded the net gain
coefficient of about 5900 cm-1 and 8800 cm-1, respectively
MOVPE grown InGaN multiple-quantum-well (MQW) light emitting diodes (LEDs) on c-plane (0001) sapphire emitting at 375 nm with GaN, Al0.16Ga0.84N and InxAl0.16GaN-barrier layers were investigated in order to study the influence of the barrier composition on the light output characteristics of near UV devices. By substituting the GaN barrier layers with Al0.16Ga0.84N the output power increased 30-fold due to the increased band-offset between the In0.03Ga0.97N QWs and the barriers. The addition of 3.3% indium to the AlGaN barriers resulted in a reduction of the FWHM, and a 50-fold increase in light output power compared to LEDs with GaN barriers. Even though the band-offset and hence the carrier confinement for the InAlGaN barriers is smaller than in the case of AlGaN barriers, strain compensated In0.03Al0.16Ga0.79N barrier layers seem to be greatly beneficial for the external quantum efficiency of the near UV LEDs. The effect of an n-type Al0.23Ga0.77N hole-blocking-layer, which was inserted below the MQW stack to prevent hole carrier leakage from UV LED active region, on the light output was also investigated. By incorporating strain compensated In0.03Al0.16Ga0.79N barriers and an Al0.23Ga0.77N hole blocking layer we were able to realize 375 nm LEDs with an output of 1 mW (measured on-wafer) at 100 mA. Finally, the wavelength dependence of the light output from UV LEDs with InGaN MQWs emitting between 375 nm and 381 nm with peak output power of 4 mW at 200 mA for the longer wavelength devices is shown.
Experimental investigations on high-power broad area (BA) distributed-feedback (DFB) lasers emitting in the wavelength range around 808 nm are presented. An output power of 4.7 W at 20 °C with a differential quantum efficiency of 1.06 W/A is achieved with BA-DFB lasers having a stripe width of 100 μm and a cavity length of 3000 μm. The measured lateral far field angle is about 10° at a power of 3 W. The vertical far field angle is near 29°. The emission has a narrow spectral width of 0.06 nm (FWHM) at 3 W and 0.10 nm at 4 W. From mappings of the optical spectra a wavelength variation with output power of Δλ/ΔP = 0.44 nm/W and with injection current of 0.4 pm/mA can be deduced. At a temperature of 50°C a maximum output power of 2 W is measured. From the measurements a temperature coefficient of Δλ/ΔT = 0.075 nm/K is determined.
Experimental investigations on RW and BA DFB lasers emitting in the wavelength range between 760 and 790 nm are presented. The maximum output powers are 300 mW and 2.4 W for the RW and BA devices, respectively. The optical spectra of the RW DFB lasers show single mode emission with a side-mode suppression ratio of about 50 dB. The profile of the lateral far field reveals stable lasing of the fundamental lateral mode without any beam steering up to 250 mW power. The spectral linewidth of the RW devices is < 2 MHz and sufficiently small for spectroscopic applications (e.g. D2 line of rubidium vapor). The BA devices have a full l/e2 width of the spectrum of 0.08 nm at 0.5 W and 0.16 nm at 2 W.
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.
Tensile strained GaAsP quantum wells embedded in AlGaAs waveguide structures are used to realize high power, high brightness short wavelength tapered laser diodes. At 735nm these laser diodes show up to 3W nearly diffraction limited output power with a wall plug efficiency of about 40%. Single spectral mode behavior is observed at output power levels up to 1W. From aging test a high realiability with lifetime exceeding 5000 can be derived comparable to results obtained from broad area laser diodes with the same aperture width. There are only small changes of the beam quality during aging. In conclusion it is shown that well designed tapered laser are a step forward to high efficiency, diffraction limited light soruces in the Watt-range which can be easily fabricated in high volumes.
In contrast to Distributed Feedback DFB) lasers for wavelengths of about 1060nm for α-DFB lasers the fabrication of Bragg gratings using conventional wafer stepper lithography is possible, due to the necessary larger grating period. The use of a wafer stepper enables us to study especially the influence of slant angle of the fabricated gratings and stripe width on optical properties of the fabricated lasers on the same wafer independent from fluctuations of the vertical structure. Best beam quality was achieved for 15° and 13.5° tilted gratings with a small period of 594nm and 658nm respectively.
For optimised structures a nearly diffraction limited beam with an output power of more than 1W, a lateral far field divergence angle of θ= 0.3°, a beam quality factor M2 = 1.1 and 3.2 at 2.1W respectively, and a line width of 5.8 pm with 28dB side mode suppression rate was achieved.
KEYWORDS: Semiconductor lasers, Waveguides, Quantum wells, Aluminum, Cladding, Resonators, Lab on a chip, Near field optics, Gallium, Broad area laser diodes
In this paper we report on Al-free InGaAs/InGaAsP/InGaP broad area laser diodes emitting at 950 nm and on 810 nm- laser diodes with Al-free GaAsP quantum wells in AlGaAs waveguides. 2 mm long diode lasers show a high wall plug efficiency above 50% at output powers of about 3 W. The beam characteristics of these diode lasers benefit from small confinement factors. Results depending on stripe width and resonator length are given.
Tensile-strained GaAsP quantum wells (QWs) embedded in AlGaAs waveguide and cladding layers are an alternative approach for the wavelength range 700 - 800 nm. We will present a detailed experimental and theoretical study of the dependence of the threshold current on the thickness and the strain of the QW for 800 nm. The optimum thickness of the GaAsP QW for a minimum threshold current density is about 14 nm and is thus much larger than for compressively strained QWs. Higher characteristic temperatures T0 can be obtained with even thicker QWs. In order to achieve high optical output powers and good fiber coupling efficiencies, we used broad waveguides with weak optical confinement and small far field divergence. We prepared two structures with 1 micrometers thick Al0.65Ga0.35As (structure A) and 2 micrometers thick Al0.45Ga0.55As (structure B) waveguides, respectively. For structure B, the thickness of the Al0.70Ga0.30As cladding layers must be carefully optimized in order to suppress higher-order transverse modes. Whereas structure B yields a higher maximum cw output power of AR/HR coated broad-area devices, structure A shows a better high-temperature behavior. Aging tests performed at 2 W (100 micrometers stripe width) and 25 degree(s)C suggest a very good reliability of these devices.
We report device properties and results of lifetime tests for Al-free InGaAs/InGaAsP/InGaP broad-area (BA) laser diodes, emitting at 950 nm. The epitaxial layers were grown by metal organic vapor phase epitaxy (MOVPE). The mounted diode lasers have a high wallplug efficiency around 60%, for a resonator length of 2 mm, and about 50% for 4 mm long devices due to low threshold current densities of jth equals 110 . . . 140 A/cm2, high slope efficiencies of 75% and the typical low series resistance of the Al-free material. The lasers were mounted on copper heatsinks, episide-down as well as episide- up. Lifetime tests were performed with a facet load of 15 mW/micrometers at temperatures between 25 degrees Celsius and 70 degrees Celsius and with a facet load of 20 mW/micrometers at 25 degrees Celsius. All diodes survived 3000 h with degradation rates lower than 6 X 10-5h-1 at 50 degrees Celsius and 1 X 10-4h-1 at 70 degrees Celsius as well as 2000 h with a low degradation rates of 2 X 10-5h-1 at 20 mW/micrometer. As far we know, the results belong to the best ones reported until now for Al-free BA laser diodes.
Goetz Erbert, Frank Bugge, Arne Knauer, Juergen Maege, Andrea Oster, Juergen Sebastian, R. Staske, A. Thies, Hans Wenzel, Marcus Weyers, Guenther Traenkle
In this paper, we present results on diode lasers in the wavelength range between 715 nm and 840 nm with Al-free QWs which are embedded in a high-quality AlGaAs LOC broadened waveguide structure with low optical loss and a small vertical far field divergence. The laser structures were grown by LP- MOVPE. We studied tensile-strained GaAsP-QWs as well as compressively strained InGaAsP-QWs with strain compensating barriers. For lasers with GaAsP QWs, the lowest transparency current densities of about 130 A/cm2 were obtained in the wavelength range between 750 nm and 800 nm. Very low transparency current densities were achieved with InGaAsP-QWs at wavelengths above 800 nm. At 810 nm, high output powers (100 micrometer aperture) of about 7 W was achieved with both types of QWs from devices mounted epi up. However, with respect to high temperature operation and reliability tensile- strained GaAsP QWs seem to be the better choice, especially for the wavelength range below 760 nm.
Broad-area stripe laser diodes based on GaAs and Al-free spacer and Al-free spacer and waveguide layers were studied and compared to conventional AlGaAs-laser diodes. The structures were grown by low pressure MOVPE (metal organic vapor phase epitaxy). For the active region InGaAs-single quantum wells with an emission wavelength of 915 nm were used. Threshold current density, internal loss and internal efficiency are comparable for the three structures under study. The use of a quaternary spacer layer instead of GaAs improves the performance for laser diodes with a low divergence ((Theta) perpendicular equals 20 degree(s) FWHM). Using a non-optimized facet coating procedure about 1.2 W cw output power at a vertical divergence of (Theta) perpendicular equals 26.5 degree(s) are obtained from a 50 micrometers wide stripe laser with Al-free waveguides.
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