Determination of Sapphire Off‐Cut and Its Influence on the Morphology and Local Defect Distribution in Epitaxially Laterally Overgrown AlN for Optically Pumped UVC Lasers

Herein, a systematic study of the morphology and local defect distribution in epitaxially laterally overgrown (ELO) AlN on c‐plane sapphire substrates with different off‐cut angles ranging from 0.08° to 0.23° is presented. Precise measurements of the off‐cut angle α, using a combination of optical alignment and X‐ray diffraction with an accuracy of ±5° for the off‐cut direction and ±0.015° for the off‐cut angle, are carried out. For ELO AlN growth, a transition from step flow growth at α  0.14° is observed. Furthermore, the terraces of the step‐bunched surface exhibit curved steps. An analysis of the local defect distribution by scanning transmission electron microscopy and a comparison with atomic force microscopy reveal a bunching of defects in line with the ELO pattern and a roughening of step edges in highly defective regions. In addition, a reduction in the threshold excitation power density for optically pumped ultraviolet‐C (UVC) lasers with smooth surface morphologies is observed.TU Berlin, Open-Access-Mittel - 201

Electrochemical etching of AlGaN for the realization of thin-film devices

Heterogeneously integrated AlGaN epitaxial layers will be essential for future optical and electrical devices like thin-film flip-chip ultraviolet (UV) light-emitting diodes, UV vertical-cavity surface-emitting lasers, and high-electron mobility transistors on efficient heat sinks. Such AlGaN-membranes will also enable flexible and micromechanical devices. However, to develop a method to separate the AlGaN-device membranes from the substrate has proven to be challenging, in particular, for high-quality device materials, which require the use of a lattice-matched AlGaN sacrificial layer. We demonstrate an electrochemical etching method by which it is possible to achieve complete lateral etching of an AlGaN sacrificial layer with up to 50% Al-content. The influence of etching voltage and the Al-content of the sacrificial layer on the etching process is investigated. The etched N-polar surface shows the same macroscopic topography as that of the as-grown epitaxial structure, and the root-mean square roughness is 3.5 nm for 1 \ub5m x 1 \ub5m scan areas. Separated device layers have a well-defined thickness and smooth etched surfaces. Transferred multi-quantum-well structures were fabricated and investigated by time-resolved photoluminescence measurements. The quantum wells showed no sign of degradation caused by the thin-film process

Thin-film flip-chip UVB LEDs realized by electrochemical etching

We demonstrate a thin-film flip-chip (TFFC) light-emitting diode (LED) emitting in the ultraviolet B (UVB) at 311 nm, where substrate removal has been achieved by electrochemical etching of a sacrificial Al0.37Ga0.63N layer. The electroluminescence spectrum of the TFFC LED corresponds well to the as-grown LED structure, showing no sign of degradation of structural and optical properties by electrochemical etching. This is achieved by a proper epitaxial design of the sacrificial layer and the etch stop layers in relation to the LED structure and the electrochemical etch conditions. Enabling a TFFC UV LED is an important step toward improving the light extraction efficiency that limits the power conversion efficiency in AlGaN-based LEDs

Spatial clustering of defect luminescence centers in Si-doped low resistivity Al0.82Ga0.18N

A series of Si-doped AlN-rich AlGaN layers with low resistivities was characterized by a combination of nanoscale imaging techniques. Utilizing the capability of scanning electron microscopy to reliably investigate the same sample area with different techniques, it was possible to determine the effect of doping concentration, defect distribution, and morphology on the luminescence properties of these layers. Cathodoluminescence shows that the dominant defect luminescence depends on the Si-doping concentration. For lower doped samples, the most intense peak was centered between 3.36 eV and 3.39 eV, while an additional, stronger peak appears at 3 eV for the highest doped sample. These peaks were attributed to the (VIII-ON)2− complex and the V3−III vacancy, respectively. Multimode imaging using cathodoluminescence, secondary electrons, electron channeling contrast, and atomic force microscopy demonstrates that the luminescence intensity of these peaks is not homogeneously distributed but shows a strong dependence on the topography and on the distribution of screw dislocations.DFG, 43659573, SFB 787: Halbleiter - Nanophotonik: Materialien, Modelle, BauelementeBMBF, 13N12587, Photonische Plattformtechnologie zur ultrasensitiven und hochspezifischen biochemischen Sensorik auf Basis neuartiger UV-LEDs (UltraSens

Effect of electron blocking layer doping and composition on the performance of 310 nm light emitting diodes

The effects of composition and p-doping profile of the AlGaN:Mg electron blocking layer (EBL) in 310 nm ultraviolet B (UV-B) light emitting diodes (LEDs) have been investigated. The carrier injection and internal quantum efficiency of the LEDs were simulated and compared to electroluminescence measurements. The light output power depends strongly on the temporal biscyclopentadienylmagnesium (Cp 2 Mg) carrier gas flow profile during growth as well as on the aluminum profile of the AlGaN:Mg EBL. The highest emission power has been found for an EBL with the highest Cp 2 Mg carrier gas flow and a gradually decreasing aluminum content in direction to the p-side of the LED. This effect is attributed to an improved carrier injection and confinement that prevents electron leakage into the p-doped region of the LED with a simultaneously enhanced carrier injection into the active region

Increased Light Extraction of Thin-Film Flip-Chip UVB LEDs by Surface Texturing

Ultraviolet light-emitting diodes (LEDs) suffer from a low wall-plug efficiency, which is to a large extent limited by the poor light extraction efficiency (LEE). A thin-film flip-chip (TFFC) design with a roughened N-polar AlGaN surface can substantially improve this. We here demonstrate an enabling technology to realize TFFC LEDs emitting in the UVB range (280-320 nm), which includes standard LED processing in combination with electrochemical etching to remove the substrate. The integration of the electrochemical etching is achieved by epitaxial sacrificial and etch block layers in combination with encapsulation of the LED. The LEE was enhanced by around 25% when the N-polar AlGaN side of the TFFC LEDs was chemically roughened, reaching an external quantum efficiency of 2.25%. By further optimizing the surface structure, our ray-tracing simulations predict a higher LEE from the TFFC LEDs than flip-chip LEDs and a resulting higher wall-plug efficiency

Athermalization of the Lasing Wavelength in Vertical-Cavity Surface-Emitting Lasers

A concept for vertical-cavity surface-emitting lasers (VCSELs) is proposed and demonstrated to obtain a lasing wavelength with unprecedented temperature stability. The concept is based on incorporating a dielectric material with a negative thermo-optic coefficient, dn/dT, in the distributed Bragg reflectors (DBRs) to compensate the positive dn/dT of the semiconductor cavity. In a short cavity, the optical field has a significant overlap with the DBRs, and the redshift of the lasing wavelength caused by the semiconductor cavity can be compensated by the negative dn/dT of the DBRs. Here, proof of this concept is presented for optically-pumped VCSELs emitting at 310 nm, demonstrating a lasing wavelength that even blueshifts by less than 0.1\ua0nm over an 80 \ub0C range with a maximum slope of –3.4\ua0pm K−1. This is to be compared with a redshift of 1–1.5\ua0nm over the same temperature range reported for III-nitride blue-emitting VCSELs. Furthermore, this method can also be implemented in VCSELs with longer cavity lengths by including a dielectric layer between the semiconductor and the DBR. The approach used here to obtain a temperature-stable lasing wavelength is generic and can therefore be applied to VCSELs in all material systems and lasing\ua0wavelengths

Influence of light absorption on the performance characteristics of UV LEDs with emission between 239 and 217 nm

The development of ultraviolet AlGaN multiple quantum well (MQW) light emitting diodes (LEDs) in the wavelength range between 239 and 217 nm is presented. The effects of aluminum composition in the MQW active region and of the underlying AlxGa1−xN:Si current spreading layer on the emission characteristics and operating voltages are investigated. A strong reduction in output power is observed with decreasing emission wavelength which is partly attributed to light absorption within the underlying AlxGa1−xN:Si. Additionally, a reduced carrier injection efficiency is identified as the root cause for the reduced emission power with decreasing emission wavelength. Emission powers at a dc current of 20 mA between 310 and 0.15 μW have been achieved for LEDs emitting between 239 and 217 nm. The maximum light output in pulsed mode operation of these LEDs ranged between 4.6 mW and 3.6 μW, respectively.DFG, 43659573, SFB 787: Halbleiter - Nanophotonik: Materialien, Modelle, Bauelement