Andreas Waag - Academia.edu (original) (raw)
Papers by Andreas Waag
Electronics
GaN-based light emitting diodes (LEDs) have been shown to effectively operate down to nanoscale d... more GaN-based light emitting diodes (LEDs) have been shown to effectively operate down to nanoscale dimensions, which allows further downscaling the chip-based LED display technology from micro- to nanoscale. This brings up the question of what resolution limit of the illumination pattern can be obtained. We show two different approaches to achieve individually switchable nano-LED arrays. We evaluated both designs in terms of near-field spot size and optical crosstalk between neighboring pixels by using finite difference time domain (FDTD) simulations. The numerical results were compared with the performance data from a fabricated nano-LED array. The outcome underlines the influence of geometry of the LED array and materials used in contact lines on the final illumination spot size and shape.
Springer Proceedings in Physics
2019 20th International Conference on Solid-State Sensors, Actuators and Microsystems & Eurosensors XXXIII (TRANSDUCERS & EUROSENSORS XXXIII)
Microsystems & Nanoengineering
The integration of gallium nitride (GaN) nanowire light-emitting diodes (nanoLEDs) on flexible su... more The integration of gallium nitride (GaN) nanowire light-emitting diodes (nanoLEDs) on flexible substrates offers opportunities for applications beyond rigid solid-state lighting (e.g., for wearable optoelectronics and bendable inorganic displays). Here, we report on a fast physical transfer route based on femtosecond laser lift-off (fs-LLO) to realize wafer-scale top–down GaN nanoLED arrays on unconventional platforms. Combined with photolithography and hybrid etching processes, we successfully transferred GaN blue nanoLEDs from a full two-inch sapphire substrate onto a flexible copper (Cu) foil with a high nanowire density (~107 wires/cm2), transfer yield (~99.5%), and reproducibility. Various nanoanalytical measurements were conducted to evaluate the performance and limitations of the fs-LLO technique as well as to gain insights into physical material properties such as strain relaxation and assess the maturity of the transfer process. This work could enable the easy recycling of ...
Zinc Oxide
ABSTRACT In ZnO, deep centres such as the transition metal ions Cu, Fe, Co, etc., deep acceptors ... more ABSTRACT In ZnO, deep centres such as the transition metal ions Cu, Fe, Co, etc., deep acceptors such as Li and Na as well as intrinsic defects such as the cation and anion vacancies are the origin of light emissions in various regions of the visible and infrared spectral range. One example of luminescence in the visible spectral range was already presented in Fig. 7.1. In Fig. 9.1 other examples of deep centre related emissions are given, which are often found in bulk and epitaxially grown ZnO or in nano rods. The presentation of optical properties of various deep centres is complemented by data from electron paramagnetic resonance (EPR). It proceeds from the green and yellow emission bands and their interpretation to transition metals. From the intrinsic defects, the oxygen vacancy is discussed since many years as one of the origins of a broad emission band in the green spectral range. Other intrinsic defects are frequently discussed in recent literature, but the discussion is still rather controversial and the experimental findings on which the various assignments are based are frequently much less elaborate compared, for example, to transition metals. Therefore, no individual section is devoted to these complexes but some information is given, for example, at the end of Sect 5.1.
Applied Physics Reviews
Gallium nitride (GaN) light-emitting-diode (LED) technology has been the revolution in modern lig... more Gallium nitride (GaN) light-emitting-diode (LED) technology has been the revolution in modern lighting. In the last decade, a huge global market of efficient, long-lasting, and ubiquitous white light sources has developed around the inception of the Nobel-prize-winning blue GaN LEDs. Today, GaN optoelectronics is developing beyond solid-state lighting, leading to new and innovative devices, e.g., for microdisplays, being the core technology for future augmented reality and visualization, as well as point light sources for optical excitation in communications, imaging, and sensing. This explosion of applications is driven by two main directions: the ability to produce very small GaN LEDs (micro-LEDs and nano-LEDs) with high efficiency and across large areas, in combination with the possibility to merge optoelectronic-grade GaN micro-LEDs with silicon microelectronics in a hybrid approach. GaN LED technology is now even spreading into the realm of display technology, which has been occupied by organic LEDs and liquid crystal displays for decades. In this review, the technological transition toward GaN micro-and nanodevices beyond lighting is discussed including an up-to-date overview on the state of the art.
Advanced Energy Materials
Sensors
Continuous cell culture monitoring as a way of investigating growth, proliferation, and kinetics ... more Continuous cell culture monitoring as a way of investigating growth, proliferation, and kinetics of biological experiments is in high demand. However, commercially available solutions are typically expensive and large in size. Digital inline-holographic microscopes (DIHM) can provide a cost-effective alternative to conventional microscopes, bridging the gap towards live-cell culture imaging. In this work, a DIHM is built from inexpensive components and applied to different cell cultures. The images are reconstructed by computational methods and the data are analyzed with particle detection and tracking methods. Counting of cells as well as movement tracking of living cells is demonstrated, showing the feasibility of using a field-portable DIHM for basic cell culture investigation and bringing about the potential to deeply understand cell motility.
IEEE Transactions on Electron Devices
Proceedings
A silicon photonic microresonator comprising two curved vertical grating couplers and a single su... more A silicon photonic microresonator comprising two curved vertical grating couplers and a single suspended Si nanowaveguide (NWG) is developed to investigate the giant enhanced Brillouin scattering in subwavelength NWGs caused by photon-phonon interaction. Finite element modelling based on COMSOL Multiphysics is conducted to optimize the critical device parameters (e.g., waveguide width, height, and length). As the smallest structures that need to be resolved are down to ~15 nm in size, electron-beam nanolithography is employed. In this case, dosage tests are carried out to minimize proximity charging effects during the nanopatterning of the silicon-on-insulator (SOI) surface, resulting in appropriate adaptive current area dosage distributions for the periodic gratings, couplers peripheral areas, and NWG, respectively. Furthermore, an enhanced inductively coupled plasma dry reactive ion etching (ICP-DRIE) process at a cryogenic temperature is used to realize smooth vertical sidewalls....
Proceedings
UV-light emitting diodes (395–278 nm) were used to investigate the gas sensing attributes of plan... more UV-light emitting diodes (395–278 nm) were used to investigate the gas sensing attributes of planar and nanostructured ZnO/AlN thin films on Si substrate towards NO2 at room temperature. A significant increased sensitivity ((Rg − Ra)/Ra = 65.3 ppm NO2 in air) and a strong reduction in recovery time (Trec = 14 min) were already observed for the planar ZnO/AlN thin films under UV-B (305 nm) irradiation compared to the other UV wavelengths, while the device showed no obvious response in dark. By enlarging the surface-to-volume ratio of the sensors (i.e., creating nanostructured ZnO/AlN thin films), an increased response time is expected to be observed.
Proceedings
We developed an integrated platform to build up conductometric sensors with controlled illuminati... more We developed an integrated platform to build up conductometric sensors with controlled illumination. Our device contains a miniaturized indium gallium nitride (InGaN) LED as a light source, and a set of interdigitated electrodes (IDEs) in close contact with the LED. The sensor material is later deposited on top of the IDE, to monitor its resistance. In this configuration, all the light emitted by the LED is collected by the sensor material, leading to a very efficient photoexcitation. We demonstrate the effectiveness of the approach building a photoactivated gas sensor based on ZnO operating with as little as 100 μW.
Proceedings
Cell registration by artificial neural networks (ANNs) in combination with principal component an... more Cell registration by artificial neural networks (ANNs) in combination with principal component analysis (PCA) has been demonstrated for cell images acquired by light emitting diode (LED)-based compact holographic microscopy. In this approach, principal component analysis was used to find the feature values from cells and background, which would be subsequently employed as neural inputs into the artificial neural networks. Image datasets were acquired from multiple cell cultures using a lensless microscope, where the reference data was generated by a manually analyzed recording. To evaluate the developed automatic cell counter, the trained system was assessed on different data sets to detect immortalized mouse astrocytes, exhibiting a detection accuracy of ~81% compared with manual analysis. The results show that the feature values from principal component analysis and feature learning by artificial neural networks are able to provide an automatic approach on the cell detection and r...
Electronics
GaN-based light emitting diodes (LEDs) have been shown to effectively operate down to nanoscale d... more GaN-based light emitting diodes (LEDs) have been shown to effectively operate down to nanoscale dimensions, which allows further downscaling the chip-based LED display technology from micro- to nanoscale. This brings up the question of what resolution limit of the illumination pattern can be obtained. We show two different approaches to achieve individually switchable nano-LED arrays. We evaluated both designs in terms of near-field spot size and optical crosstalk between neighboring pixels by using finite difference time domain (FDTD) simulations. The numerical results were compared with the performance data from a fabricated nano-LED array. The outcome underlines the influence of geometry of the LED array and materials used in contact lines on the final illumination spot size and shape.
Springer Proceedings in Physics
2019 20th International Conference on Solid-State Sensors, Actuators and Microsystems & Eurosensors XXXIII (TRANSDUCERS & EUROSENSORS XXXIII)
Microsystems & Nanoengineering
The integration of gallium nitride (GaN) nanowire light-emitting diodes (nanoLEDs) on flexible su... more The integration of gallium nitride (GaN) nanowire light-emitting diodes (nanoLEDs) on flexible substrates offers opportunities for applications beyond rigid solid-state lighting (e.g., for wearable optoelectronics and bendable inorganic displays). Here, we report on a fast physical transfer route based on femtosecond laser lift-off (fs-LLO) to realize wafer-scale top–down GaN nanoLED arrays on unconventional platforms. Combined with photolithography and hybrid etching processes, we successfully transferred GaN blue nanoLEDs from a full two-inch sapphire substrate onto a flexible copper (Cu) foil with a high nanowire density (~107 wires/cm2), transfer yield (~99.5%), and reproducibility. Various nanoanalytical measurements were conducted to evaluate the performance and limitations of the fs-LLO technique as well as to gain insights into physical material properties such as strain relaxation and assess the maturity of the transfer process. This work could enable the easy recycling of ...
Zinc Oxide
ABSTRACT In ZnO, deep centres such as the transition metal ions Cu, Fe, Co, etc., deep acceptors ... more ABSTRACT In ZnO, deep centres such as the transition metal ions Cu, Fe, Co, etc., deep acceptors such as Li and Na as well as intrinsic defects such as the cation and anion vacancies are the origin of light emissions in various regions of the visible and infrared spectral range. One example of luminescence in the visible spectral range was already presented in Fig. 7.1. In Fig. 9.1 other examples of deep centre related emissions are given, which are often found in bulk and epitaxially grown ZnO or in nano rods. The presentation of optical properties of various deep centres is complemented by data from electron paramagnetic resonance (EPR). It proceeds from the green and yellow emission bands and their interpretation to transition metals. From the intrinsic defects, the oxygen vacancy is discussed since many years as one of the origins of a broad emission band in the green spectral range. Other intrinsic defects are frequently discussed in recent literature, but the discussion is still rather controversial and the experimental findings on which the various assignments are based are frequently much less elaborate compared, for example, to transition metals. Therefore, no individual section is devoted to these complexes but some information is given, for example, at the end of Sect 5.1.
Applied Physics Reviews
Gallium nitride (GaN) light-emitting-diode (LED) technology has been the revolution in modern lig... more Gallium nitride (GaN) light-emitting-diode (LED) technology has been the revolution in modern lighting. In the last decade, a huge global market of efficient, long-lasting, and ubiquitous white light sources has developed around the inception of the Nobel-prize-winning blue GaN LEDs. Today, GaN optoelectronics is developing beyond solid-state lighting, leading to new and innovative devices, e.g., for microdisplays, being the core technology for future augmented reality and visualization, as well as point light sources for optical excitation in communications, imaging, and sensing. This explosion of applications is driven by two main directions: the ability to produce very small GaN LEDs (micro-LEDs and nano-LEDs) with high efficiency and across large areas, in combination with the possibility to merge optoelectronic-grade GaN micro-LEDs with silicon microelectronics in a hybrid approach. GaN LED technology is now even spreading into the realm of display technology, which has been occupied by organic LEDs and liquid crystal displays for decades. In this review, the technological transition toward GaN micro-and nanodevices beyond lighting is discussed including an up-to-date overview on the state of the art.
Advanced Energy Materials
Sensors
Continuous cell culture monitoring as a way of investigating growth, proliferation, and kinetics ... more Continuous cell culture monitoring as a way of investigating growth, proliferation, and kinetics of biological experiments is in high demand. However, commercially available solutions are typically expensive and large in size. Digital inline-holographic microscopes (DIHM) can provide a cost-effective alternative to conventional microscopes, bridging the gap towards live-cell culture imaging. In this work, a DIHM is built from inexpensive components and applied to different cell cultures. The images are reconstructed by computational methods and the data are analyzed with particle detection and tracking methods. Counting of cells as well as movement tracking of living cells is demonstrated, showing the feasibility of using a field-portable DIHM for basic cell culture investigation and bringing about the potential to deeply understand cell motility.
IEEE Transactions on Electron Devices
Proceedings
A silicon photonic microresonator comprising two curved vertical grating couplers and a single su... more A silicon photonic microresonator comprising two curved vertical grating couplers and a single suspended Si nanowaveguide (NWG) is developed to investigate the giant enhanced Brillouin scattering in subwavelength NWGs caused by photon-phonon interaction. Finite element modelling based on COMSOL Multiphysics is conducted to optimize the critical device parameters (e.g., waveguide width, height, and length). As the smallest structures that need to be resolved are down to ~15 nm in size, electron-beam nanolithography is employed. In this case, dosage tests are carried out to minimize proximity charging effects during the nanopatterning of the silicon-on-insulator (SOI) surface, resulting in appropriate adaptive current area dosage distributions for the periodic gratings, couplers peripheral areas, and NWG, respectively. Furthermore, an enhanced inductively coupled plasma dry reactive ion etching (ICP-DRIE) process at a cryogenic temperature is used to realize smooth vertical sidewalls....
Proceedings
UV-light emitting diodes (395–278 nm) were used to investigate the gas sensing attributes of plan... more UV-light emitting diodes (395–278 nm) were used to investigate the gas sensing attributes of planar and nanostructured ZnO/AlN thin films on Si substrate towards NO2 at room temperature. A significant increased sensitivity ((Rg − Ra)/Ra = 65.3 ppm NO2 in air) and a strong reduction in recovery time (Trec = 14 min) were already observed for the planar ZnO/AlN thin films under UV-B (305 nm) irradiation compared to the other UV wavelengths, while the device showed no obvious response in dark. By enlarging the surface-to-volume ratio of the sensors (i.e., creating nanostructured ZnO/AlN thin films), an increased response time is expected to be observed.
Proceedings
We developed an integrated platform to build up conductometric sensors with controlled illuminati... more We developed an integrated platform to build up conductometric sensors with controlled illumination. Our device contains a miniaturized indium gallium nitride (InGaN) LED as a light source, and a set of interdigitated electrodes (IDEs) in close contact with the LED. The sensor material is later deposited on top of the IDE, to monitor its resistance. In this configuration, all the light emitted by the LED is collected by the sensor material, leading to a very efficient photoexcitation. We demonstrate the effectiveness of the approach building a photoactivated gas sensor based on ZnO operating with as little as 100 μW.
Proceedings
Cell registration by artificial neural networks (ANNs) in combination with principal component an... more Cell registration by artificial neural networks (ANNs) in combination with principal component analysis (PCA) has been demonstrated for cell images acquired by light emitting diode (LED)-based compact holographic microscopy. In this approach, principal component analysis was used to find the feature values from cells and background, which would be subsequently employed as neural inputs into the artificial neural networks. Image datasets were acquired from multiple cell cultures using a lensless microscope, where the reference data was generated by a manually analyzed recording. To evaluate the developed automatic cell counter, the trained system was assessed on different data sets to detect immortalized mouse astrocytes, exhibiting a detection accuracy of ~81% compared with manual analysis. The results show that the feature values from principal component analysis and feature learning by artificial neural networks are able to provide an automatic approach on the cell detection and r...