Oxygen is a ubiquitous impurity in AlN that influences its optical and electronic properties. A precise determination of its electronic levels is essential for device-specific material-engineering. In this study, we report on the experimental determination of the ionization energy of the substitutional oxygen donor O-N in sputter-deposited and subsequently high-temperature annealed AlN templates using temperature-dependent and time-resolved cathodoluminescence spectroscopy. The slow component of the oxygen-related defect luminescence, associated with donor-acceptor pair recombination and with decay times of hundreds of nanoseconds, exhibits distinct thermal quenching at temperatures above 250 K. By fitting this temperature dependence with a model based on the thermal emission of electrons from the donor level, we extract an ionization energy E-D = 371 +/- 77 meV for the oxygen donor in AlN. This experimental value is in excellent agreement with theoretical predictions for the shallow substitutional ON donor and clearly distinct from calculated energies for the deeper oxygen DX center. This work provides an important experimental insight into this fundamental parameter in AlN.

An isotype heterojunction n+-ZnO/n-Si photodetector is developed, showing adjustable wavelength-selective operation at self-powered conditions. Without an external bias voltage, the device can operate either as a broadband UV-vis-NIR or as a NIR-only photodetector, depending on the relative carrier concentrations of ZnO and silicon. In addition, the photodetector can be tuned to either broadband or NIR operation by the application of an external bias voltage, regardless of carrier concentrations. At negative bias, it demonstrates UV-vis-NIR photodetection, while at positive bias, NIR photodetection. Photovoltage and photocurrent measurements for pulsed illumination reveal a high-speed self-powered response, with rise and fall times <100 s across the UV-vis-NIR. The device can be engineered to reproduce undistorted pulsed light with frequencies as high as 1 kHz. Self-powered responsivity reaches approximate to 70 mA W-1, which becomes approximate to 4 A W-1 with an applied external bias.

Integrating molecules into semiconductor devices offers significant potential for complex nanoscale applications, particularly in the field of integrated optics thanks to the bright and stable photon emission of single molecular quantum emitters. For this purpose, phthalocyanine (Pc) molecules stand out with high chemical and thermal stability, as well as the ability to adjust their optical properties through chemical functionalization. However, it is an ongoing challenge to improve the emission efficiency in the crystalline phase, where fluorescence quenching through excitonic coupling becomes more prominent. To address this issue, we investigate the emission characteristics of Zinc Phthalocyanine (ZnPc) thin films, grown by organic molecular beam epitaxy (OMBE) on semiconductor surfaces. Our results demonstrate the influence of film thicknesses, ranging from multilayers to submonolayers (sub-ML), and stacking arrangements on the excitonic coupling. We were able to increase the emission intensity of ZnPc molecules in the solid phase significantly by integrating them into a co-evaporated matrix consisting of the alkane tetratetracontane (TTC). This approach allows us to reduce the excitonic coupling between the Pc molecules and presents a concept for the controlled synthesis of isolated ZnPc emitters, making them scalable for their use in optoelectronic devices. As a proof of concept for a hybrid device, we demonstrated efficient excitation of the molecules with a commercial GaN-based LED in the ultraviolet (UV) spectral range.

An isotype heterojunction n+-ZnO/n-Si photodetector is developed, showing adjustable wavelength-selective operation at self-powered conditions. Without an external bias voltage, the device can operate either as a broadband ultraviolet-visible-near-infrared (UV-vis-NIR) or as a NIR-only photodetector. This is achieved by careful engineering of the relative carrier concentrations of the constituting materials, i.e., ZnO and silicon, which in turn affects the built-in potential barrier of the n+-n junction. In addition to this self-powered behavior, the photodetector can be tuned to either broadband or NIR operation by the application of an external bias voltage, regardless of carrier concentrations. At negative bias, it demonstrates UV-vis-NIR photodetection, while at positive bias, NIR photodetection for any ratio of ZnO/Si carrier concentrations investigated here.

A water‐based synthesis for lanthanide‐doped NaGdF 4 nanoparticles is presented, yielding nanoparticles that emit robust and efficient luminescence when excited with an electron beam (cathodoluminescence). Scanning electron microscopy reveals that the as‐fabricated nanoparticles exhibit a spherical shape with diameters ranging from 100–200 nm, accompanied by a granular surface structure. Powder X‐ray diffraction confirms that the as‐prepared product particles are highly crystalline and possess a hexagonal‐phase pure structure with average crystal sizes of ≈5 nm for the (100) plane. The presence of sodium, gadolinium, fluorine, and europium/terbium dopants has been verified by energy dispersive X‐ray spectroscopy. The elemental ratios demonstrate a high degree of similarity to the stoichiometric ratios of the initial precursor solutions. Incorporating europium or thulium dopants leads to distinctive spectral characteristics in the cathodoluminescence spectra, manifesting in red or blue spectral emission lines, respectively. From a mixture of the differently doped nanoparticles, the type of doping could be clearly identified for individual particles, and sufficient stability is proven to enable multiple consecutive measurements. As the synthesized nanoparticles are moreover hydrophilic and compatible with aqueous media, they offer high promise for cathodoluminescence mapping of organic or biological samples, enabling precise labeling with distinct and stable spectral signatures.

High‐temperature annealing significantly improves the crystal quality of sputter‐deposited AlN templates, but at the same time introduces a high concentration of oxygen‐related point defects. The origin of this oxygen‐related defect luminescence is studied using time‐resolved and temperature‐dependent cathodoluminescence spectroscopy. Characterization of the defect luminescence reveals a complex multiexponential decay, with a fast component of about 2 ns and slow components of tens to hundreds of nanoseconds. The slow decay is attributed to a donor–acceptor‐pair transition between an O N donor and mainly the ( V Al ‐ O N ) 2− and ( V Al ‐2 O N ) 1− defect complexes as acceptors, whereas the fast component is ascribed to a free electron to acceptor transition involving the same type of acceptors. Furthermore, the impact of these oxygen‐related defects on the quality and transparency of the AlN templates is discussed.

Oxidative chemical vapor deposition (oCVD) converts coevaporated monomer and oxidant species into conductive polymer thin films with high surface conformity to the topology of the substrate. FeCl3 as the traditional oCVD oxidant, however, also has drawbacks, like the requirement of a postrinsing to remove nonreacted particles from the substrate surface or the need to install the crucible inside the deposition chamber, which leads to an increase in reactor volume, higher precursor consumption, and eventually higher cost. We present a self-designed compact oCVD reactor and demonstrate the synthesis of homogeneous oCVD PEDOT coatings on silicon oxide substrates of up to 5 cm in size without any postprocessing by employing the more volatile liquid SbCl5 oxidant and EDOT monomer chemistry. The growth rate, sheet resistance, and optical transmittance of as-deposited PEDOT layers are systematically investigated by screening the influence of various oCVD reactor parameters (substrate temperature, monomer temperature, deposition time, and nitrogen carrier gas flow). We use a photoresist-based lift-off process to obtain oCVD PEDOT stripes of 10 to 2000 mu m in width and integrate metal contact electrodes for subsequent transfer length method (TLM) characterization. Upon comparing TLM-derived conductivities of two different fabrication schemes, i.e., "PEDOT last" (similar to 312 S/cm) and "PEDOT first" (similar to 306 S/cm), with four-point probe measurements taken on nonpatterned PEDOT references (similar to 350 S/cm), we suggest that the micropatterning process has negligible influence on the electrical properties of oCVD PEDOT. Our TLM analysis also reveals that the patterned PEDOT/metal interfaces exhibit very low contact resistivities (rho(c) < 10(-2) Omega cm(2)), which in the best case (rho(c) = 7.110(-4) +/- 2.510(-4) Omega cm(2)) is about 1 order of magnitude below the current record reported for PEDOT:PSS. Our results demonstrate that the combination of a compact oCVD reactor with EDOT/SbCl5 chemistry and photolithography processing is a promising route toward microfabrication of highly integrated conductive polymer devices.

In confocal Raman microscopy, depth profiling is a key application that enables analysis of the structural and chemical composition and size of three-dimensional (3D) transparent objects. However, the precise interpretation of a probed sample's Raman depth profile measurement can be significantly affected by both its size and surrounding objects. This study provides a more comprehensive understanding of the observed optical effects at the interface between polymer spheres and different substrates. Ray- and wave-optical simulations support our results. We derive a correction factor that, depending on the instrumental configuration, allows us to determine the nominal dimensions of the scanned objects more accurately from Raman depth profiles. Our studies support the need for careful consideration when employing depth profiling in confocal Raman microscopy for nondestructive, quantitative tomography of 3D objects.

Herein, carbon‐implanted high‐temperature annealed (HTA) AlN layers are analyzed and donor–acceptor pair (DAP) transitions probably between the two most abundant impurities, carbon and oxygen, are identified. Both are regarded as the main, hard‐to‐avoid impurities in crystal growth. Oxygen is believed to lead to absorption in the deep UV below a wavelength of 250 nm. In contrast, carbon is the most likely candidate to be responsible for a distinct absorption band around 265 nm. This interpretation has recently been challenged. In this study, carbon‐implanted and HTA AlN layers with ion fluences above 8.1 × 10 15 cm −2 are analyzed using low‐temperature and time‐resolved cathodoluminescence spectroscopy. Due to the high concentration of oxygen inside the AlN, as a result of the HTA process, a DAP transition between a most likely carbon‐related acceptor and O N is observed. The measured temperature‐ and power‐dependent blueshift of the peak emission energy as well as the luminescence transients can be clearly explained by a continuous change from a DAP transition at low temperature to a free electron to acceptor transition with increasing temperature. The findings are supported by a configurational coordinate model that describes the measured behavior qualitatively.

The laser-induced ablation threshold of c-plane GaN films upon exposure to ultrashort laser pulses was investigated for different wavelengths from the IR to the UV range and pulse widths between 0.34 and 10 ps. The one-pulse ablation threshold ranges between 0.15 and 3 J/cm 2 and shows an increase with the wavelength and the pulse width, except for deep UV pulses. Based on a rate equation model, we attribute this behavior to the efficiency of seed carrier generation by interband absorption. In addition, the multi-pulse ablation threshold was analyzed. Accumulation effects are more prominent in case of IR than with UV pulses and are closely linked to damage precursors. By a thorough structural investigation, we demonstrate that threading dislocations, especially those with a screw component, significantly contribute to laser damage, since they provide a variety of dispersed states within the band gap.

An isotype heterojunction n+-ZnO/n-Si photodetector is developed, demonstrating wavelength-selective or broadband operation, depending on the applied bias voltage. Additionally, at self-powered (zero bias) operation, it distinguishes between UV, visible, and near IR (NIR) photons by polarity control of the photocurrent. The photodetector is developed by atomic layer deposition (ALD) of ZnO on n-Si, followed by electric contact deposition and annealing. Photoluminescence measurements reveal high optical quality and improved crystallinity of annealed ZnO on silicon. Photocurrent measurements as a function of illumination wavelength and bias voltage show small negative values in the UV-visible spectral range at zero and positive bias voltage and high positive values in the NIR spectral range. For these measurements, we consider the electric contact to ZnO as the anode and the electric contact to silicon as the cathode. At negative bias voltage, the device shows broadband operation with high photocurrent values across the UV-vis-NIR.

The reconstruction of the three-dimensional (3D) morphology of polymeric microsphere layers based on confocal Raman microscopy was studied. Refraction of the Raman laser beam at the curved surface of the spheres broadens the focus volume inside the sphere. Compared to planar layers, the focus gets trapped inside the spheres such that the measured depth profiles are shifted and broadened. Additionally, the Raman signal of the underlying substrate is already observed for nominal focus positions above the microsphere layer. The results are successfully modeled with ray-optical simulations that allow for a clear understanding of the relevant mechanisms that lead to the generation of the Raman signals in the complex three-dimensional structures.

Light emitting diodes represent a key technology that can be found in many areas of everydays life. Therefore, the improvement of the efficiency of such structures offers a high economic and ecological potential. One approach is electrostatic screening of the quantum-confined Stark effect (QCSE) in polar III-V heterostructures by n-type doping in order to increase the oscillator strength of electronic transitions in quantum structures. In this study, we analyzed the cathodoluminescene (CL) spectra of different functional parts of individual AlN/GaN nanowire superlattices and studied their decay characteristics with sub-nanosecond time resolution. This allows us to extract information about strain and electric fields in such heterostructures with an overall spatial resolution <100 nm. The samples, which were investigated in a temperature range from 10 to 300 K by using time-integrated cathodoluminescence spectroscopy (TICL) and time-resolved cathodoluminescence spectroscopy (TRCL) consist of GaN bottom and top layer and a 40-fold stack of GaN nanodiscs, embedded in AlN barriers that were doped with Ge. We show, that the QCSE is reduced with increasing doping concentration due to a screening of the internal electric fields inside GaN nanodiscs, resulting in a reduction of the carrier lifetimes and a blue shift of the emitted light. Due to the small diameter of the electron excitation beam CL offers the possibility to individually analyze the different functional parts of the nanowires.

We develop ZnO/p-Si photodetectors by ALD deposition of ZnO thin films on laser- microstructured Si, which demonstrate high sensitivity and broadband operation (UV-Vis-NIR), due to increased specific surface area of the heterojunction and increased light absorption.

Production of high-aspect-ratio silicon (Si) nanowire-based anode for lithium ion batteries is challenging particularly in terms of controlling wire property and geometry to improve the battery performance. This report demonstrates tunable optimization of inductively coupled plasma reactive ion etching (ICP-RIE) at cryogenic temperature to fabricate vertically-aligned silicon nanowire array anodes with high verticality, controllable morphology, and good homogeneity. Three different materials [i.e., photoresist, chromium (Cr), and silicon dioxide (SiO2)] were employed as masks during the subsequent photolithography and cryogenic ICP-RIE processes to investigate their effects on the resulting nanowire structures. Silicon nanowire arrays with a high aspect ratio of up to 22 can be achieved by tuning several etching parameters [i.e., temperature, oxygen/sulfur hexafluoride (O2/SF6) gas mixture ratio, chamber pressure, plasma density, and ion energy]. Higher compressive stress was revealed for longer Si wires by means of Raman spectroscopy. Moreover, an anisotropy of lattice stress was found at the top and sidewall of Si nanowire, indicating compressive and tensile stresses, respectively. From electrochemical characterization, half-cell battery integrating ICP-RIE-based silicon nanowire anode exhibits a capacity of 0.25 mAh cm-2 with 16.67% capacity fading until 20 cycles, which has to be improved for application in future energy storage devices.

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 (~10 7 wires/cm 2 ), 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 native growth substrates and inspire the development of large-scale hybrid GaN nanowire optoelectronic devices by solely employing standard epitaxial LED wafers (i.e., customized LED wafers with additional embedded sacrificial materials and a complicated growth process are not required).

This paper presents a study on the incubation effect during fs-laser micromachining of gallium nitride films with three different wavelengths (1030, 515 and 343 nm) by varying the number of laser pulses applied per sample surface area and measuring the damage threshold fluence using the zero damage method. As we implemented the exponential defect accumulation model to the experimental incubation data, we determined the value of the incubation parameter of (0.02 +/- 0.01) for 1030 nm, (0.07 +/- 0.01) for 515 nm and (0.4 +/- 0.1) for 343 nm. This shows that for excitation in the green and infrared spectral region, GaN requires approximately 100 overlapping pulses in order for the incubation to take place while, for ultraviolet excitation, the incubation was achieved faster, with the overlap of about 10 pulses. Furthermore, we compared our data for the single pulse damage threshold with a theoretical model that takes into account multi photon and avalanche ionization. Our results indicate that at 343 nm and 515 nm, micromachining is dominated by multiphoton ionization, while at 1030 nm other effects, such as tunneling ionization, must also be contributing. Finally, we believe this paper brings relevant information on the fs-laser micro machining of GaN. (c) 2021 Elsevier B.V. All rights reserved.

The combination of inorganic semiconductors with organic thin films promises new strategies for the realization of complex hybrid optoelectronic devices. Oxidative chemical vapor deposition (oCVD) of conductive polymers offers a flexible and scalable path towards high-quality three-dimensional inorganic/organic optoelectronic structures. Here, hole-conductive poly(3,4-ethylenedioxythiophene) (PEDOT) grown by oxidative chemical vapor deposition is used to fabricate transparent and conformal wrap-around p-type contacts on three-dimensional microLEDs with large aspect ratios, a yet unsolved challenge in three-dimensional gallium nitride technology. The electrical characteristics of two-dimensional reference structures confirm the quasi-metallic state of the polymer, show high rectification ratios, and exhibit excellent thermal and temporal stability. We analyze the electroluminescence from a three-dimensional hybrid microrod/polymer LED array and demonstrate its improved optical properties compared with a purely inorganic microrod LED. The findings highlight a way towards the fabrication of hybrid three-dimensional optoelectronics on the sub-micron scale.

We report the synthesis and spectroscopic characteristics of two different sets of carbon dots (CDs) formed by hydrothermal reaction between citric acid and polyethylenimine (PEI) or 2,3-diaminopyridine (DAP). Although the formation of amide-based species and the presence of citrazinic acid type derivates assumed to be responsible for a blue emission is confirmed for both CDs by elemental analysis, infrared spectroscopy, and mass spectrometry, a higher abundance of sp(2)-hybridized nitrogen is observed for DAP-based CDs, which causes a red-shift of the n-pi* absorption band relative to the one of PEI-based CDs. These CD systems possess high photoluminescence quantum yields (QY) of similar to 40% and similar to 48% at neutral pH, demonstrating a possible tuning of the optical properties by the amine precursor. pH-Dependent spectroscopic studies revealed a drop in QY to < 9% (pH similar to 1) and < 21% (pH similar to 12) for both types of CDs under acidic and basic conditions. In contrast, significant differences in the pH-dependency of the n-pi* transitions are found for both CD types which are ascribed to different (de)protonation sequences of the CD-specific fluorophores and functional groups using zeta potential analysis.

Semiconductors thin films are the foundation of modern technology. While the nonlinear optical (NLO) properties of bulk semiconductors have been systematically studied in the last three decades, it is still a great challenge to obtain them for semiconductors thin films, as the high laser irradiance in NLO experiments tends to irreversibly damage the thin films. In addition, tuning the NLO response of semiconductor thin films by alloying and doping has not been explored yet. Here, we study the influence of the Aluminum content in AlxGa1-xN thin films and the n-type doping concentration in GaN thin films on their two-photon absorption (2PA) coefficients. For this, we investigate five different GaN-based thin films: an unintentionally doped one with no Al as a reference, two n-type doped GaN films with distinct concentrations of silicon impurities, and two AlxGa1-xN alloys with an aluminum content of 5.5% and 9.0%, respectively. The femtosecond 2PA spectra reveal that doping impurities reduce the non-linear coefficients (∼10%), while alloying with Al enhances the 2PA coefficient up to 30%. We use the model of Brandi and Araujo to determine Kane’s energy parameter related to the transition matrix element for each sample and compare them with recent theoretical studies based on the k·p theory where an excellent agreement is found.