Cathodoluminescence Measurements Research Articles

Industrially Compatible Fabrication Process of Perovskite-Based Mini-Modules Coupling Sequential Slot-Die Coating and Chemical Bath Deposition.

To upscale the emerging perovskite photovoltaic technology to larger-size modules, industrially relevant deposition techniques need to be developed. In this work, the deposition of tin oxide used as an electron extraction layer is established using chemical bath deposition (CBD), a low-cost and solution-based fabrication process. Applying this simple low-temperature deposition method, highly homogeneous SnO2 films are obtained in a reproducible manner. Moreover, the perovskite layer is prepared by sequentially slot-die coating on top of the n-type contact. The symbiosis of these two industrially relevant deposition techniques allows for the growth of high-quality dense perovskite layers with large grains. The uniformity of the perovskite film is further confirmed by scanning electron microscopy (SEM)/scanning transmission electron microscopy (STEM) analysis coupled with energy dispersive X-ray spectroscopy (EDX) and cathodoluminescence measurements allowing us to probe the elemental composition at the nanoscale. Perovskite solar cells fabricated from CBD SnO2 and slot-die-coated perovskite show power conversion efficiencies up to 19.2%. Furthermore, mini-modules with an aperture area of 40 cm2 demonstrate efficiencies of 17% (18.1% on active area).

Effect of rapid thermal annealing on short period {CdO/ZnO}m SLs grown on m-Al2O3

Here, we report on the characterization of {CdO/ZnO}m superlattice structures (SLs) grown by plasma assisted molecular beam epitaxy. The properties of as-grown and annealed SLs deposited on m-oriented sapphire were investigated by secondary ion mass spectrometry (SIMS) and scanning electron microscopy (SEM) in cathodoluminescence (CL) and energy dispersive X-ray modes. The deformation of the crystallographic structure of SLs was observed after rapid thermal annealing at 900 °C in oxygen flow due to migration and segregation of Cd atoms. SIMS measurements revealed that the distributions of cadmium in the annealed samples depend on the thicknesses of the CdO and ZnO sublayers in the as-grown superlattice structures. Depth-resolved CL measurements showed that shifting of the near band edge emission peaks is closely related to the Cd profiles measured with SIMS.

Epitaxial growth of structure-tunable ZnO/ZnS core/shell nanowire arrays using HfO2 as the buffer layer.

Synthesis of high-quality ZnO/ZnS heterostructures with tunable phase and controlled structures is in high demand due to their adjustable band gap and efficient electron-hole pair separation. In this report, for the first time, remote heteroepitaxy of single-crystalline ZnO/ZnS core/shell nanowire arrays has been realized using amorphous HfO2 as the buffer layer. Zinc blende or wurtzite ZnS epilayer can be efficiently fabricated under the same thermal deposition condition by adjusting the buffer layer thickness, even among the same batch of products, respectively. Structural characterization reveals "(01-10)ZnOwz//(2-20)ZnSZB, [0001]ZnOWZ//[001]ZnSZB" and "(01-10)ZnOWZ//(01-10)ZnSWZ, [0002]ZnOWZ//[0002]ZnSWZ" epitaxial relationships between the core and the shell, respectively. The cathodoluminescence measurement demonstrates that the tuning of the optical properties can be accomplished by preparing a heterostructure with HfO2, in which a strong green emission increases at the expense of the quenching of UV emission. In addition, the core/shell heterostructure based Schottky diode exhibits an asymmetrical rectifying behavior and an outstanding photo-electronic switching-effect. We believe that the aforementioned results could provide fundamental insights for epitaxial growth of structure-tunable ZnO/ZnS heterostructures on the nanoscale. Furthermore, this promising route buffered by the high-k material can broaden the options for fabricating heterojunctions and promote their application in photoelectric nanodevices.

Desorption of excited H* atoms from free clusters Ar/CH4 and solid Ar doped with CH4

Desorption of excited hydrogen atoms was detected from both solid Ar doped with CH4 and free nanoclusters Ar/CH4 at irradiation with an electron beam. It was monitored by an emission of the Lyman-a line. Measurements of cathodoluminescence (CL) spectra in the VUV range were performed within the CH4 concentration limits 0.1–10% in the solid matrix. The CL of free clusters with an average size of 1200 atoms per cluster was detected from pure Ar cluster jet and from Ar clusters doped with 0.1% CH4. The mechanisms of desorption of electronically excited H* atoms from solids and clusters are proposed on the basis of an analysis of energy transfer pathways with the final stage of relaxation — population of the n = 3 state of hydrogen atoms.

Temperature dependence of cathodoluminescence emission in irradiated Si-doped β-Ga2O3

Temperature dependent continuous and time-resolved cathodoluminescence measurements were employed to understand the luminescence from Si-doped β-Ga2O3 prior to irradiation and after 10 MeV proton and 18 MeV alpha-particle irradiation. The shape and location of the luminescence components [ultraviolet luminescence (UVL′) at 3.63 eV, UVL at 3.3 eV, and blue-luminescence at 2.96 eV] obtained from Gaussian decomposition did not change in either width or peak location, indicating that new radiation-induced trap-levels were non-radiative in nature between the 4.5 and 310 K temperature range. Activation energies, associated with thermal quenching of UVL′ and UVL bands, show temperature dependence, suggesting ionization of shallow Si-donors and a thermally activated non-radiative process.

Characterization of deep interface states in SiO2/B-doped diamond using the transient photocapacitance method

We fabricated a diamond metal oxide semiconductor structure with a silicon dioxide (SiO2) film as the gate dielectric on B-doped diamond grown by a high-power-density microwave-plasma chemical vapor deposition method. The SiO2/B-doped diamond interface was characterized using a transient photocapacitance technique. The concentration of B in the B-doped diamond film was estimated to be 4.0 × 1017 cm−3, based on the intensity ratio of the exciton emissions observed in the cathodoluminescence measurements at approximately 80 K. Acceptor-type defects could be observed at approximately 1.28 and 1.82 eV above the valence-band edge. The observed defect at approximately 1.82 eV above the valence-band edge is expected to be an interface state of the SiO2/B-doped diamond.

Luminescence Characteristics in Hexagonal and Cubic‐Phase GaN on Micropatterned Si(100) Substrate

The luminescence characteristics of the hexagonal GaN (h‐GaN) and cubic GaN (c‐GaN) on micropatterned Si(100) substrates are explored. Microstripes of InGaN/GaN multiple quantum wells in the cubic and hexagonal phases are grown on V‐grooved Si(100) substrate. The crystal phases are identified by X‐ray diffraction and selective area electron diffraction, which shows the top surface is c‐GaN(001) phase at the center and h‐GaN(101) at the side regions. Then, the energy dispersive X‐ray spectra of the indium content demonstrate the indium content in cubic InGaN/GaN multiple quantum wells (c‐MQWs) is higher than that in hexagonal InGaN/GaN MQWs (h‐MQWs). In addition, photoluminescence and cathodoluminescence measurements reveal that a cubic InGaN/GaN quantum well has produced longer wavelengths. The c‐MQWs could incorporate higher indium content and realize longer wavelength emissions, which has great potential to realize red light emitting diode.

Improved minority carrier lifetime in p-type GaN segments prepared by vacancy-guided redistribution of Mg

To accelerate the development of GaN power devices, reproducible fabrication of p-type GaN (p-GaN) segments by ion-implantation (I/I) that enables selective-area doping is preferred. In this Letter, the results of time-resolved photoluminescence (PL) and spatially resolved cathodoluminescence (CL) measurements on p-GaN fabricated by I/I are presented. The p-GaN samples of a 220-nm-deep box-shaped Mg concentration profile ([Mg] = 3×1018 cm−3) were fabricated by sequential I/I of Mg and N followed by atmospheric-pressure post-implantation annealing (PIA). During PIA, Mg redistribution was guided by N-I/I-induced vacancies. The intensities of PL peaks originating from Mg acceptors were increased by increasing the PIA temperature (Ta), indicating progressive activation of Mg and simultaneous decrease in the concentration of nonradiative recombination centers (NRCs). As a result, a record-long room-temperature PL lifetime for the near-band edge emission of Ga-polar Mg-implanted p-GaN (1.4 ps) was obtained when Ta = 1300 °C. Because the species of major vacancy-type defects in the present sample was determined as vacancy clusters comprised of few Ga-vacancies (VGa) and N-vacancies (VN) such as (VGaVN)2–3, the NRC concentration is roughly estimated at around 1017 cm−3 provided that (VGaVN)2–3 are major NRCs with the electron-capture-coefficient of 7×10−6 cm3 s−1. Cross-sectional CL linescan images revealed certain activation of Mg from the surface to the leading edge of the box-shaped [Mg] profile, while NRCs remained at the depths right below the box-shaped profile. The results support advantages of using the vacancy-guided Mg redistribution technique for controlling the activated [Mg] profile and realizing p-type Ga-polar GaN:Mg segments by I/I.

Dominant defects and carrier transport in single crystalline cuprous oxide: A new attribution of optical transitions

Electronic properties of single crystal (111) Cu2O wafers have been investigated using a number of complementary techniques. Secondary ion mass spectrometry has shown significant presence of hydrogen and nitrogen. Cathodoluminescence measurements reveal strong near-band emission indicating the good electronic quality of the wafers. Two deep emission lines are observed at 1.3 and 1.7 eV. Temperature-dependent Hall effect measurements reveal electronic levels at around EV + 0.16 eV, EV + 0.22 eV, and ∼EV + 0.4 eV, where EV is the valence band edge. The discussion on the identity of the electronic centers calls for a revision of the traditional assignments of the 1.3-eV and 1.7-eV lines in order to take into account independent theoretical predictions. The temperature dependence of carrier mobility shows that the mechanism limiting the mobility can be described by scattering on neutral and ionized defect centers.

Dislocation and indium droplet related emission inhomogeneities in InGaN LEDs

This report classifies emission inhomogeneities that manifest in InGaN quantum well blue light-emitting diodes grown by plasma-assisted molecular beam epitaxy on free-standing GaN substrates. By a combination of spatially resolved electroluminescence and cathodoluminescence measurements, atomic force microscopy, scanning electron microscopy and hot wet potassium hydroxide etching, the identified inhomogeneities are found to fall in four categories. Labeled here as type I through IV, they are distinguishable by their size, density, energy, intensity, radiative and electronic characteristics and chemical etch pits which correlates them with dislocations. Type I exhibits a blueshift of about 120 meV for the InGaN quantum well emission attributed to a perturbation of the active region, which is related to indium droplets that form on the surface in the metal-rich InGaN growth condition. Specifically, we attribute the blueshift to a decreased growth rate of and indium incorporation in the InGaN quantum wells underneath the droplet which is postulated to be the result of reduced incorporated N species due to increased N2 formation. The location of droplets are correlated with mixed type dislocations for type I defects. Types II through IV are due to screw dislocations, edge dislocations, and dislocation bunching, respectively, and form dark spots due to leakage current and nonradiative recombination.

Catalyst-assisted heteroepitaxial strategy for highly ordered β-Ga2O3 nanoarrays and their optical property investigation

In this work, we demonstrate the growth of highly ordered β-Ga2O3 nanoarrays with (001) preferred growth plane for the first time through a facile heteroepitaxial strategy using metal Ga and c-sapphire as Ga precursor and monocrystalline substrate. The (001) preferred growth plane means that the β-Ga2O3 nanowires grow along the normal direction of the (001) plane. The β-Ga2O3 nanoarrays along (001) preferential plane exhibit inclined six equivalent directions that correspond to the six crystallographic symmetry of (0001) α-Al2O3. High-resolution transmission electron microscopy analyses confirm the good crystallinity and the existence of unusual epitaxial relationship of {310} β-Ga2O3 ǁ (0001) α-Al2O3 and <001> β-Ga2O3 or <132> β-Ga2O3 ǁ [100] α-Al2O3. UV–vis and cathodoluminescence measurements reveal the wide band gap of 4.8 eV and the strong UV-blue luminescence (300–500 nm) centered at ∼388 nm. Finally, the luminescence mechanism is further investigated with the assistance of x-ray photoelectron spectroscopy. The heteroepitaxial strategy of highly ordered β-Ga2O3 nanoarrays in this work will undoubtedly pave a solid way toward the fundamental research and the applications of Ga2O3 nanodevices in optoelectronic, gas sensor, photocatalyst and next-generation power electronics.

Near-infrared emission from spatially indirect excitons in type II ZnTe/CdSe/(Zn,Mg)Te core/double-shell nanowires

ZnTe/CdSe/(Zn, Mg)Te core/double-shell nanowires are grown by molecular beam epitaxy by employing the vapor–liquid–solid growth mechanism assisted with gold catalysts. A photoluminescence study of these structures reveals the presence of an optical emission in the near infrared. We assign this emission to the spatially indirect exciton recombination at the ZnTe/CdSe type II interface. This conclusion is confirmed by the observation of a significant blue-shift of the emission energy with an increasing excitation fluence induced by the electron–hole separation at the interface. Cathodoluminescence measurements reveal that the optical emission in the near infrared originates from nanowires and not from two-dimensional residual deposits between them. Moreover, it is demonstrated that the emission energy in the near infrared depends on the average CdSe shell thickness and the average Mg concentration within the (Zn, Mg)Te shell. The main mechanism responsible for these changes is associated with the strain induced by the (Zn, Mg)Te shell in the entire core/shell nanowire heterostructure.

Temperature Dependence of Dark Spot Diameters in GaN and AlGaN

Threading dislocations in c‐plane (Al,Ga)N layers are surrounded by areas with reduced light generation efficiency, called “dark spots.” These areas are observable in luminescence measurements with spatial resolution in the submicrometer range. Dark spots reduce the internal quantum efficiency in single layers and light‐emitting devices. In cathodoluminescence measurements, the diameter of dark spots (full width at half maximum [FWHM]) is observed to be 200–250 nm for GaN. It decreases by 30–60% for AlxGa1−xN with x ≈ 0.5. Furthermore, the dark spot diameter increases with increasing temperature from 83 to 300 K in AlGaN, whereas it decreases in GaN. Emission energy mappings around dark spots become less smooth and show sharper features on submicrometer scales at low temperature for AlGaN and, on the contrary, at high temperature for GaN. It is concluded that charge carrier localization dominates the temperature dependence of dark spot diameters and of the emission energy distribution around threading dislocations in AlGaN, whereas the temperature‐dependent excitation volume in cathodoluminescence and charge carrier diffusion limited by phonon scattering are the dominant effects in GaN. Consequently, with increasing temperature, nonradiative recombination related to threading dislocations extends to wider regions in AlGaN, whereas it becomes spatially limited in GaN.

Understanding the Role of Grain Boundaries on Charge‐Carrier and Ion Transport in Cs2AgBiBr6 Thin Films

AbstractHalide double perovskites have gained significant attention, owing to their composition of low‐toxicity elements, stability in air, and recent demonstrations of long charge‐carrier lifetimes that can exceed 1 µs. In particular, Cs2AgBiBr6 is the subject of many investigations in photovoltaic devices. However, the efficiencies of solar cells based on this double perovskite are still far from the theoretical efficiency limit of the material. Here, the role of grain size on the optoelectronic properties of Cs2AgBiBr6 thin films is investigated. It is shown through cathodoluminescence measurements that grain boundaries are the dominant nonradiative recombination sites. It also demonstrates through field‐effect transistor and temperature‐dependent transient current measurements that grain boundaries act as the main channels for ion transport. Interestingly, a positive correlation between carrier mobility and temperature is found, which resembles the hopping mechanism often seen in organic semiconductors. These findings explain the discrepancy between the long diffusion lengths &gt;1 µm found in Cs2AgBiBr6 single crystals versus the limited performance achieved in their thin film counterparts. This work shows that mitigating the impact of grain boundaries will be critical for these double perovskite thin films to reach the performance achievable based on their intrinsic single‐crystal properties.

Time-resolved cathodoluminescence in an ultrafast transmission electron microscope

Ultrafast transmission electron microscopy (UTEM) combines sub-picosecond time-resolution with the versatility of TEM spectroscopies. It allows us to study the ultrafast materials' response using complementary techniques. However, until now, time-resolved cathodoluminescence was unavailable in UTEM. In this paper, we report time-resolved cathodoluminescence measurements in an ultrafast transmission electron microscope. We mapped the spatial variations of the emission dynamics from nano-diamonds with a high density of NV centers with a 12 nm spatial resolution and sub-nanosecond temporal resolution. This development will allow us to study the emission dynamics from quantum emitters with a unique spatiotemporal resolution and benefit from the wealth of complementary signals provided by transmission electron microscopes. It will further expand the possibilities of ultrafast transmission electron microscopes, paving the way to the investigation of the quantum aspects of an electron/sample interaction.

Comparison of surface and subsurface damage of mosaic single-crystal diamond substrate processed by mechanical and plasma-assisted polishing

The surface and subsurface damage of a mosaic single-crystal diamond (SCD) (100) substrate processed by mechanical polishing (MP) and plasma-assisted polishing (PAP) were studied. Scanning white-light interferometer (SWLI) (84-μm square) observation for different local positions showed that the surface damage such as scratches and deep pits were observed on the mosaic SCD substrate after MP, while such damage was completely removed after PAP. Scanning cathodoluminescence (CL) measurement of band-A emission for different local positions showed that scratches with high luminescence intensity attributed to subsurface damage were observed on the mosaic SCD substrate after MP, while such specific luminescence was not observed after PAP. Micro-Raman spectroscopy measurement showed that peak position shift of diamond Raman lines attributed to residual stress and full width at half maximum (FWHM) broadening attributed to destruction of the crystalline structure were observed on the mosaic SCD substrate after MP, while such a peak position shift and FWHM broadening were not observed after PAP.

Structure and luminescence properties of Eu3+ and Dy3+ implanted GaN films

X-Ray diffraction (XRD), Raman scattering , Cathodoluminescence measurements (CL) and Photoluminescence (PL) were employed to characterize the structure and optical properties of Dysprosium (Dy) and Europium (Eu) implanted GaN films grown by MOCVD. For GaN: Dy 3+ samples, Dy 3+ implantation leads to strain and radiation damage increasing with Dy 3+ implanted fluence . From the CL spectra, the characteristic transitions from the excited 4 F 9/2 level of Dy 3+ to 6 H 15/2 (485 nm), 6 H 13/2 (583 nm) and 6 H 11/2 (671 nm) were observed. For Dy 3+ and Eu 3+ co-doped GaN, considering the doping effects of Dy 3+ , we proposed that there might exist a resonance energy transfer process from Eu 3+ ions 7 F 0 → 5 D 2 to Dy 3+ ions 4 F 9/2 → 6 H 15/2 , which is dominated by the electric dipole-dipole interaction. • Eu 3+ , Dy 3+ co-implanted GaN crystalline thin films were prepared for the first time. • Energy transfer processes between Eu 3+ and Dy 3+ were proposed. • Dy implantation leads to strain and radiation damage increasing with Dy dose.

Employing Cathodoluminescence for Nanothermometry and Thermal Transport Measurements in Semiconductor Nanowires.

Thermal properties have an outsized impact on efficiency and sensitivity of devices with nanoscale structures, such as in integrated electronic circuits. A number of thermal conductivity measurements for semiconductor nanostructures exist, but are hindered by the diffraction limit of light, the need for transducer layers, the slow scan rate of probes, ultrathin sample requirements, or extensive fabrication. Here, we overcome these limitations by extracting nanoscale temperature maps from measurements of bandgap cathodoluminescence in GaN nanowires of <300 nm diameter with spatial resolution limited by the electron cascade. We use this thermometry method in three ways to determine the thermal conductivities of the nanowires in the range of 19–68 W/m·K, well below that of bulk GaN. The electron beam acts simultaneously as a temperature probe and as a controlled delta-function-like heat source to measure thermal conductivities using steady-state methods, and we introduce a frequency-domain method using pulsed electron beam excitation. The different thermal conductivity measurements we explore agree within error in uniformly doped wires. We show feasible methods for rapid, in situ, high-resolution thermal property measurements of integrated circuits and semiconductor nanodevices and enable electron-beam-based nanoscale phonon transport studies.

Coloration changes in natural ruby induced by oxygen ion implants correlated with cathodoluminescence data

Ruby is a mineral in the corundum (Al2O3) family and it is remarkably valuable as a gemstone. From the physics point of view, the ruby coloration depends on the balance of trace or chromophore impurities and defect complexes in the Al2O3 lattice. Shifting this balance into right direction is of paramount importance for the ruby value as the gemstone and ion implantation can be applied for this purpose. Thus, in the present study, we explored the influence of high dose oxygen ion implants on the natural ruby coloration. By performing quantitative colorimetry analysis, we detected prominent coloration changes unambiguously attributed to the effect of the implants, even though the implanted oxygen profile was located at the near surface region of the sample. Moreover, applying near surface sensitive cathodoluminescence measurements we observed spectacular changes in the spectra correlated with the implants. Importantly, the trends observed in the colorimetry and cathodoluminescence as a function of the implantation parameters were consistent. As such, the present paper provides both the scientific perspective and quantitative interpretations potentially paving the way for the ion beams applications in the gemstone industry.

Femtosecond Laser-Pulse-Induced Surface Cleavage of Zinc Oxide Substrate.

The induction of surface cleavage along the crystalline structure of a zinc oxide substrate (plane orientation: 0001) by femtosecond laser pulses (wavelength: 1030 nm) has been reported; a scanning electron microscope image of the one-pulse (pulse energy: 6–60 μJ) irradiated surface shows very clear marks from broken hexagons. This cleavage process differs from the general laser-induced melt process observed on the surfaces of narrower-bandgap semiconductors and other metal materials. This phenomenon is discussed using a multi-photon absorption model, and the pulse-energy dependence of the cleavage depth (less than 3 μm) is quantitatively analyzed. Laser-induced cleavage is found not to occur under multi-pulse irradiation; when more than four pulses are irradiated upon the same spot, the general laser-induced melt process becomes dominant. This cleavage–melt shift is considered to be caused by the enhancement of absorption due to the initial pulses, which is supported by our measurement of cathodoluminescence.