| 1. |
- Bergmann, Michael Alexander, 1989, et al.
(författare)
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Increased Light Extraction of Thin-Film Flip-Chip UVB LEDs by Surface Texturing
- 2023
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Ingår i: ACS Photonics. - : American Chemical Society (ACS). - 2330-4022. ; 10:2, s. 368-373
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 2. |
- Bergmann, Michael Alexander, 1989, et al.
(författare)
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Thin-film flip-chip UVB LEDs realized by electrochemical etching
- 2022
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- We will give an overview of different concepts to increase the light extraction efficiency (LEE) of ultraviolet (UV) light-emitting diodes (LEDs) with a focus on thin-film flip-chip (TFFC) devices. Optical simulations show that a TFFC design can greatly improve the LEE with a transparent p-side, reflective contacts, and optimized surface roughening. We will demonstrate UVB-emitting TFFC LEDs based on our fabrication platform for AlGaN thin films with high aluminum content. The fabrication is compatible with a standard LED process and uses substrate removal based on selective electrochemical etching as the key enabling technology.
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| 3. |
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| 4. |
- Cardinali, G., et al.
(författare)
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Low-Threshold AlGaN-based UVB VCSELs enabled by post-growth cavity detuning
- 2022
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 121:10
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Tidskriftsartikel (refereegranskat)abstract
- The performance of vertical-cavity surface-emitting lasers (VCSELs) is strongly dependent on the spectral detuning between the gain peak and the resonance wavelength. Here, we use angle-resolved photoluminescence spectroscopy to investigate the emission properties of AlGaN-based VCSELs emitting in the ultraviolet-B spectral range with different detuning between the photoluminescence peak of the quantum-wells and the resonance wavelength. Accurate setting of the cavity length, and thereby the resonance wavelength, is accomplished by using doping-selective electrochemical etching of AlGaN sacrificial layers for substrate removal combined with deposition of dielectric spacer layers. By matching the resonance wavelength to the quantum-wells photoluminescence peak, a threshold power density of 0.4 MW/cm2 was achieved, and this was possible only for smooth etched surfaces with a root mean square roughness below 2 nm. These results demonstrate the importance of accurate cavity length control and surface smoothness to achieve low-Threshold AlGaN-based ultraviolet VCSELs.
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| 5. |
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| 6. |
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| 7. |
- Haglund, Åsa, 1976, et al.
(författare)
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Are blue and ultraviolet VCSELs a reality or just a dream?
- 2020
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Blue-emitting vertical-cavity surface-emitting lasers (VCSELs) with high optical output powers and decent threshold current densities have recently been demonstrated, but the power efficiency is still below 10%. We will give an overview of the challenges to realize high-performing devices and summarize state-of-the-art results, including our schemes for optically guided devices that are used in the best blue VCSELs today. In addition, we will present our method to simultaneously achieve high-reflectivity mirrors and good control over the cavity length for ultraviolet VCSELs. This has resulted in, to the best of our knowledge, the shortest VCSEL emission wavelength around 310 nm.
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| 8. |
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| 9. |
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| 10. |
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| 11. |
- Haglund, Åsa, 1976, et al.
(författare)
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Out of the blue: UV VCSELs
- 2022
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Ingår i: Proceedings of SPIE - The International Society for Optical Engineering. - 0277-786X .- 1996-756X. ; PC12001
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Konferensbidrag (refereegranskat)abstract
- In recent years, there has been tremendous improvement in the performance of blue-emitting vertical-cavity surface-emitting lasers (VCSELs) and they are now on the cusp of commercialization. We will summarize state-of-the-art results and outline the main challenges in extending the emission wavelength into the ultraviolet (UV). Our method to simultaneously achieve high-reflectivity mirrors and good cavity length control by selective electrochemical etching has been essential to demonstrate the world’s first UV-B VCSEL. The use of dielectric mirrors, where one material has a negative thermo-optical coefficient, counteracts the inherent red-shift of the resonance wavelength, enabling a temperature-stable emission.
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| 12. |
- Haglund, Åsa, 1976, et al.
(författare)
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Recent progress for blue VCSELs and challenges to move to UV
- 2020
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- We will summarize state-of-the-art results in III-nitride-based vertical-cavity surface-emitting lasers (VCSELs) and the main challenges in going from blue to ultraviolet-emitting devices. In particular we will highlight how to reduce threshold current densities by using optically guided cavities and our method to simultaneously achieve high-reflectivity mirrors and good control over the cavity length for AlGaN-based ultraviolet VCSELs. The latter has resulted in the shortest emission wavelength from a VCSEL at around 310 nm. This claim of lasing is supported by a nonlinear increase in output power versus pump power and a simultaneous beam width and spectral narrowing around threshold.
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| 13. |
- Haglund, Åsa, 1976, et al.
(författare)
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The quest for ultraviolet vertical-cavity surface-emitting lasers
- 2023
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Ingår i: 2023 Conference on Lasers and Electro-Optics Europe and European Quantum Electronics Conference, CLEO/Europe-EQEC 2023.
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Konferensbidrag (refereegranskat)abstract
- We daily rely upon vertical-cavity surface-emitting lasers (VCSELs) for facial recognition and data communication. These lasers are now experiencing exponential growth and serves in other applications as well such as oxygen monitoring in combustion processes and in anesthetized patients in hospitals and as a source of heating in industry in the form of a large-sized array. The large interest for this laser class is linked to its beneficial qualities such as low threshold current, circular-symmetric low-divergent output beam, high efficiency, compactness, and low fabrication cost due to on-wafer testing. Due to these advantages, there is a strong push to realize VCSELs in other wavelength regimes, beyond the commercially available infrared and red. This would open completely new markets such as flood lights, projectors, sterilization, and medical diagnosis and treatment.
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| 14. |
- Haglund, Åsa, 1976, et al.
(författare)
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Thin-film UV VCSELs and LEDs by electrochemical etching
- 2022
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Ingår i: Proceedings of SPIE - The International Society for Optical Engineering. - 0277-786X .- 1996-756X. ; PC12022
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Konferensbidrag (refereegranskat)abstract
- We will give an overview of state-of-the-art results and challenges to achieve high-performing III-nitride vertical-cavity surface-emitting lasers (VCSELs), with a particular focus on the requirements to push the emission wavelength into the ultraviolet (UV). Our method to simultaneously achieve high-reflectivity mirrors and good cavity length control by electrochemical etching enabled the world’s first UV-B VCSEL. The use of dielectric mirrors yielded lasers with a very temperature-stable emission wavelength thanks to the negative thermo-optic coefficient of the mirrors. We have used the same etch methodology to also lift-off fully processed LEDs from their growth substrate to improve the light extraction efficiency.
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| 15. |
- Haglund, Åsa, 1976, et al.
(författare)
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UV VCSELs – this is the way
- 2022
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Ingår i: 9th Workshop on Physics and Technology of Semiconductor Lasers.
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Konferensbidrag (refereegranskat)
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| 16. |
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| 17. |
- Hjort, Filip, 1991, et al.
(författare)
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A 310 nm Optically Pumped AlGaN Vertical-Cavity Surface-Emitting Laser
- 2021
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Ingår i: ACS Photonics. - : American Chemical Society (ACS). - 2330-4022. ; 8:1, s. 135-141
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Tidskriftsartikel (refereegranskat)abstract
- Ultraviolet light is essential for disinfection, fluorescence excitation, curing, and medical treatment. An ultraviolet light source with the small footprint and excellent optical characteristics of vertical-cavity surface-emitting lasers (VCSELs) may enable new applications in all these areas. Until now, there have only been a few demonstrations of ultraviolet-emitting VCSELs, mainly optically pumped, and all with low Al-content AlGaN cavities and emission near the bandgap of GaN (360 nm). Here, we demonstrate an optically pumped VCSEL emitting in the UVB spectrum (280-320 nm) at room temperature, having an Al0.60Ga0.40N cavity between two dielectric distributed Bragg reflectors. The double dielectric distributed Bragg reflector design was realized by substrate removal using electrochemical etching. Our method is further extendable to even shorter wavelengths, which would establish a technology that enables VCSEL emission from UVA (320-400 nm) to UVC (<280 nm).
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| 18. |
- Hjort, Filip, 1991, et al.
(författare)
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Advances in ultraviolet-emitting vertical-cavity surface-emitting lasers
- 2021
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Ingår i: Proceedings of SPIE - The International Society for Optical Engineering. - : SPIE. - 0277-786X .- 1996-756X. ; 11686
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Konferensbidrag (refereegranskat)abstract
- We will give an overview of the progress in ultraviolet-emitting vertical-cavity surface-emitting lasers (VCSELs) and their potential applications in areas such as disinfection and medical therapy. This includes our demonstration of the shortest wavelength VCSEL, emitting at 310 nm under optical pumping, and a detailed analysis of its filamentary lasing characteristics. The UVB-emitting AlGaN-based VCSEL was realized by substrate removal using electrochemical etching, enabling the use of two high-reflectivity dielectric distributed Bragg reflectors. The potential of using this or alternative methods to push the emission to shorter wavelengths will be examined as well as concepts to realize electrically injected devices.
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| 19. |
- Persson, Lars, 1996, et al.
(författare)
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Athermalization of the Lasing Wavelength in Vertical-Cavity Surface-Emitting Lasers
- 2023
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Ingår i: Laser and Photonics Reviews. - 1863-8899 .- 1863-8880. ; 17:8
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Tidskriftsartikel (refereegranskat)abstract
- 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 nm over an 80 °C range with a maximum slope of –3.4 pm K−1. This is to be compared with a redshift of 1–1.5 nm 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 wavelengths.
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| 20. |
- Torres, Estrella, 1995, et al.
(författare)
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Ultraviolet-B Resonant-Cavity Light-Emitting Diodes with Tunnel Junctions and Dielectric Mirrors
- 2024
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Ingår i: ACS Photonics. - 2330-4022. ; In Press
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Tidskriftsartikel (refereegranskat)abstract
- We demonstrate the first electrically injected AlGaN-based ultraviolet-B resonant-cavity light-emitting diode (RCLED). The devices feature dielectric SiO2/HfO2 distributed Bragg reflectors enabled by tunnel junctions (TJs) for lateral current spreading. A highly doped n++-AlGaN/n++-GaN/p++-AlGaN TJ and a top n-AlGaN current spreading layer are used as transparent contacts, resulting in a good current spreading up to an active region mesa diameter of 120 μm. To access the N-face side of the device, the substrate is removed by electrochemically etching a sacrificial n-AlGaN layer, leading to a smooth underetched surface without evident parasitic etching in the n- and n++-doped layers of the device. The RCLEDs show a narrow emission spectrum with a full width at half-maximum (FWHM) of 4.3 nm compared to 9.4 nm for an ordinary LED and a more directional emission pattern with an angular FWHM of 52° for the resonance at 310 nm in comparison to ∼126° for an LED. Additionally, the RCLEDs show a much more stable emission spectrum with temperature with a red-shift of the electroluminescence peak of about ∼18 pm/K and a negligible change of the FWHM compared to LEDs, which shift ∼30 pm/K and show spectrum broadening with temperature. The demonstration of those devices, where a highly reflective mirror is spatially separated from an ohmic metal contact, opens up a new design space to potentially increase the poor light extraction efficiency in UV LEDs and is an important step toward electrically injected UV vertical-cavity surface-emitting lasers.
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| 21. |
- Apaydin, Dogukan, 1991, et al.
(författare)
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Optically Pumped UVC Photonic Crystal Surface-Emitting Laser
- 2023
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Photonic crystal surface-emitting lasers (PCSELs) are a new type of semiconductor lasers that offer a high optical output power while maintaining single-mode operation and a low divergence angle. Such devices rely upon the in-plane optical feedback from a two-dimensional photonic crystal and feature out-of-plane emission of the modes with zero group velocity at the photonic band edges by diffraction. Since the demonstration of the first PCSEL [1], the concept has been implemented in standard semiconductor materials with the demonstration of highperforming infrared and blue-emitting lasers [2, 3]. Extending the laser operation to shorter emission wavelengths would be a major breakthrough as such lasers would be of high interest for disinfection, material processing, curing, and medical treatments.
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| 22. |
- Apaydin, Dogukan, 1991, et al.
(författare)
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UVC photonic crystal surface-emitting lasers with low-divergent far-fields
- 2023
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Photonic crystal surface-emitting lasers (PCSEL) emitting in the ultraviolet (UV) C spectral range are exciting devices due to their low divergence and single-mode emission capable of high output powers as already demonstrated in the infrared [1] and blue spectral range [2]. This is due to their unique design, which incorporates a photonic crystal leading to a large optical gain area. PCSELs are based upon in-plane feedback from the photonic crystal and out-of-plane emission by the diffraction of the modes with zero group velocity at the photonic band edges. We recently demonstrated, to the best of our knowledge, the first UVC PCSEL with an emission at 279 nm. The device structure consists of 3 x 2 nm AlGaN quantum wells (QW) in a 60 nm Al0.70Ga0.30N waveguide and AlN cladding layers. The 140x140 μm large photonic crystal is dry etched into the top AlN cladding layer with a hexagonal lattice consisting of circular holes with a lattice constant of 140 nm and an etch depth of 65 nm, leaving 65 nm between the bottom of the photonic crystals and the first quantum well. Lasing in these PCSELs was achieved by resonant pumping of the QWs by a 266 nm pulsed laser with a spot size of 82 μm at room temperature. The devices exhibit threshold pump power densities from 25 down to 13 MW/cm2 showing a spectral narrowing down to 25 pm. Far-field patterns and band structures were investigated for a range of filling factors (fraction of the surface that is etched) between 10% to 26%, and the far-fields contain emission bands that were not yet reported in PCSELs at longer wavelengths. Changing the filling factor affects the photonic crystal band structure and thereby the optical mode at the Γ-point that will reach threshold first. This feature enables us to intentionally select the lasing mode with the desired far-field pattern. By a proper choice of filling factor, the intensity in the angular emission bands is diminished, resulting in a far-field with a narrow beam divergence of <1°.
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| 23. |
- Bergmann, Michael Alexander, 1989, et al.
(författare)
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Electrochemical etching of AlGaN for the realization of thin-film devices
- 2019
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 115:18, s. 182103-
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Tidskriftsartikel (refereegranskat)abstract
- 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 µm x 1 µm 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.
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| 24. |
- Bergmann, Michael Alexander, 1989, et al.
(författare)
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Thin-film flip-chip UVB LEDs realized by electrochemical etching
- 2020
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 116:12, s. 121101-
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 25. |
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| 26. |
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| 27. |
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| 28. |
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| 29. |
- Kneissl, Michael, et al.
(författare)
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Nitride Semiconductors
- 2023
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Ingår i: Physica Status Solidi (A) Applications and Materials Science. - : WILEY-V C H VERLAG GMBH. - 1862-6319 .- 1862-6300. ; 220:16
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)
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| 30. |
- Kneissl, Michael, et al.
(författare)
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Nitride Semiconductors
- 2023
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Ingår i: Physica Status Solidi (B): Basic Research. - : WILEY-V C H VERLAG GMBH. - 1521-3951 .- 0370-1972. ; 260:8
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)
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| 31. |
- Mehnke, Frank, et al.
(författare)
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Electronic properties of Si-doped AlxGa1-xN with aluminum mole fractions above 80%
- 2016
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Ingår i: Journal of Applied Physics. - : AMER INST PHYSICS. - 0021-8979 .- 1089-7550. ; 120:14
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Tidskriftsartikel (refereegranskat)abstract
- The dependence of the activation energy as well as the energetic levels of the neutral charge state and the DX center of the Si donor in AlxGa1-xN: Si samples on aluminum content and SiH4/III ratio were investigated by electron paramagnetic resonance (EPR) measurements, Van-der-Pauw resistivity measurements, and Hall-effect measurements. It was found by EPR measurements that the energy distance of the neutral charge state of the Si donor from the conduction band increases with increasing aluminum content from 61 meV for x = 0.82 to 106 meV for x = 0.89. Additionally, the formation of a DX center below the neutral charge state which is deepening from 6 meV for x = 0.82 to 58 meV for x = 0.89 is observed. This results in a linearly increasing activation energy with increasing aluminum content from 67 meV for x = 0.82 to 164 meV for x = 0.89. This is consistent with the activation energies as determined by Hall-effect measurements showing a linear increase from 24 meV for x = 0.85 to 211 meV for x = 0.96, as well as the activation energies as determined by Van-der-Pauw resistivity measurements. By varying the SiH4/III ratio we observed a formation of a minimum resistivity in accordance with the room temperature charge carrier density. However, no clear dependence of the activation energy was observed. EPR measurements of samples with a high SiH4/III ratio hint to an increased incorporation probability of a deep secondary donor species which might explain the increase in resistivity. Published by AIP Publishing.
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| 32. |
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| 33. |
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