Cross-sectional Transmission Electron Microscopy Research Articles

High-quality Ge epitaxial film based on dislocation trapping mechanism in patterned Si substrate

Threading dislocation density (TDD) in a Ge heteroepitaxial film on Si is reduced based on a dislocation trapping mechanism in a patterned substrate. An array of V-shaped grooves on the submicron scale is patterned in the [110] direction on a (001) Si substrate. Epitaxial growth of Ge with a thickness of 1 μm by CVD realizes a reasonable flat surface despite the non-flat starting surface. The TDD in Ge for a V-shaped groove pattern of 0.5 μm in width with an inter-groove distance of 0.3 μm is obtained as low as 4 × 107 cm–2, which is lower than about 6 × 107 cm–2 for the rectangular one with the same groove width and inter-groove distance and about 22 × 107 cm–2 for the unpatterned one. The reduction is attributed to the dislocation trapping at the groove regions, as observed by cross-sectional transmission electron microscopy.

Local strain distribution analysis in strained SiGe spintronics devices

This paper reports nanobeam X-ray diffraction (nanoXRD) measurement results for strained SiGe spintronics devices grown by MBE. A quantitative nanoXRD analysis verifies that in-plane strain is properly exerted on the SiGe spin channel layer in the device showing enhanced spin diffusion length, whereas overall strain relaxation and local change in crystallinity occur in the sample with unclear spin signals. Crystal defects such as dislocations and stacking faults found in cross-sectional transmission electron microscopy observations are correlated with the results of nanoXRD analysis and their influence on spin transport properties are discussed. The present results demonstrate the capability of the nanoXRD based method for quantitative nondestructive analysis of strain distribution and crystallinity in real device structures.

Boosted hydrophobic properties of leather surfaces by atmospheric pressure plasma brush reactor

In this article, we demonstrate the application of an atmospheric pressure plasma brush (APPB) reactor for improving the hydrophobic properties of leather surfaces treated with organo-silane precursors for the first time. A comprehensive investigation was conducted to analyze the influence of various process parameters, including plasma access current, treatment duration, nozzle height, and gas flow rate, on the observed outcomes effects. The dispersion of active particles in the APPB was scrutinized using an Optical Emission Spectrometer (OES), while the underlying reaction mechanism was elucidated through the integration of Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), cross-sectional Transmission Electron Microscope (TEM) and Scanning Electron Microscope (SEM) analysis. The findings unveiled that the resultant polysiloxane polymers bestowed a remarkable degree of hydrophobicity upon the leather, persisting steadfastly even following a 90-day aging period (WCA >140°). This APPB treatment combined with organosilane strategy may open a new design avenue for the development of surface treatments for intricate and temperature-sensitive substrates materials.

Influence of Ge to the formation of defects in epitaxial Mg2Sn1−x Ge x thermoelectric thin films

Defect formation in epitaxial Mg2Sn1–x Ge x thermoelectric (TE) thin films grown via MBE was studied. We examined the defect formations and structures using cross-sectional transmission electron microscopy and positron annihilation spectroscopy. The defect formation tends to be influenced by Ge incorporation into the Mg2Sn matrix phase of epitaxial thin films. Mg vacancies (V Mg) were identified as point defects, primarily concentrated in the film’s mid-layer. In films with higher Ge composition, stacking faults were observed. The concentration of vacancy-type point defects decreased as the Ge concentration increased. This implies that the vacancy atoms, which would have otherwise been created by increasing chemical pressure due to the higher Ge content, might have played a role in the formation of stacking faults. The high concentration of vacancy-type defects resulted in the lowest thermal conductivity, demonstrating their significance as effective phonon scattering centers in epitaxial TE films.

Green to deep-red emissive carbon dot formation by C+ ion implantation on nitrogen beam created self-masked nano-template

We report the formation of green to red emissive arrays of carbon dot on silicon-nitride nano-templates by successive implantation of nitrogen and carbon broad ion beams. The patterned nano-templates are formed by 14 keV N2 + ion-bombardment at grazing incident (70°) on Si. Subsequently, 5 keV C+ ions are implanted at the selective sites of the pyramidal nano-template by taking advantage of the self-masking effect. The nano-pyramidal pattern and the implanted carbon dots at the specific sites are confirmed by atomic force microscopy and cross sectional transmission electron microscopy measurements. The developed carbon dots (CDs) are mostly amorphous and consists of SiC and graphitic nitrogen (CN). G-band and D-band carbons are identified by Raman spectroscopy, while the presence of SiC and CN are detected by XPS measurements. A change of band-gap is observed for C-implanted templates by the UV–vis spectroscopy. Excitation wavelength-dependent photoemission from the dots is found in the green to red region. Maximum intense PL is observed in the green–orange region for excitation wavelength of 425 nm and a redshift of PL with decreasing intensity is observed with the increase of excitation wavelength. The observed photoluminescence is described in terms of the combined effects of quantum confinement, graphitic nitrogen and defect induced additional states formation in the carbon dots. The potential applications of CDs are also addressed.

Effect of optimal rapid thermal alloying of ohmic contacts to microwave pseudomorphic HEMTs

Gallium Arsenide (GaAs) pseudomorphic High Electron Mobility Transistors (pHEMTs) are currently used as active devices in manufacturing Monolithic Microwave Integrated Circuits (MMICs) for microwave and millimetre wave applications like Transmit-Receive (T/R) modules in Radars and communication. Low contact resistances (Rc) of pHEMT devices are necessary to obtain low access resistances to the device channel, for enhancement of transconductance and cut-off frequency. Devices formed by optimal and suboptimal conditions of rapid thermal alloying by infra-red optical radiation are investigated using cross-section transmission electron microscopy (XTEM), backside secondary ion mass spectrometry (backside SIMS) and thermal imaging techniques to understand the role of optimal alloy formation and its role in enhancement of device parameters.

GeSn-on-GaAs with photoconductive carrier lifetime >400 ns: role of substrate orientation and atomistic simulation.

Group IV GeSn quantum material finds application in electronics and silicon-compatible photonics. Synthesizing these materials with low defect density and high carrier lifetime is a potential challenge due to lattice mismatch induced defects and tin segregation at higher growth temperature. Recent advancements in the growth of these GeSn materials on Si, Ge, GaAs, and with substrate orientations, demonstrated different properties using epitaxial and chemical deposition methods. This article addresses the effect of GaAs substrate orientation and misorientation on the materials' properties and carrier lifetimes in epitaxial Ge0.94Sn0.06 layers. With starting GaAs substrates of (100)/2°, (100)/6°, (110) and (111)A orientations, Ge0.94Sn0.06 epitaxial layers were grown with an intermediate Ge buffer layer by molecular beam epitaxy and analyzed by several analytical tools. X-ray analysis displayed good crystalline quality, and Raman spectroscopy measurements showed blue shifts in phonon wavenumber due to biaxial compressive strain in Ge0.94Sn0.06 epilayers. Cross-sectional transmission electron microscopy analysis confirmed the defect-free heterointerface of Ge0.94Sn0.06/Ge/GaAs heterostructure. Minority carrier lifetimes of the unintentionally doped n-type Ge0.94Sn0.06 epilayers displayed photoconductive carrier lifetimes of >400 ns on (100)/6°, 319 ns on (100)/2°, and 434 ns on (110) GaAs substrate at 1500 nm excitation wavelength. On the other hand, Ge0.94Sn0.06 layer showed poor carrier lifetime on (111)A GaAs substrate. The observed differences in carrier lifetimes were correlated with the formation energy of the Ge on (100)/6° and (100)/2° GaAs heterointerface using Stillinger-Weber interatomic potential model-based atomistic simulation with different heterointerfacial bonding by Synopsys QuantumATK tool. Total energy computation of 6280-atom Ge/GaAs supercell on (100)/6° leads to lower formation energy than (100)/2°, making it more thermodynamically stable. Hence, the growth of the GeSn/III-V material system using misoriented (100) substrates that are more thermodynamically stable will enhance the performances of optoelectronic devices.

Fabrication and Electrical Characterization of GaAs/GaN Junctions

GaAs/GaN heterojunctions are likely to be an ideal platform for high-frequency/high-output-power electron devices because of excellent electrical properties of GaAs and GaN as well as the similarity in their coefficients of thermal expansion. We have to note, however, that heteroepitaxial growth of GaAs/GaN junctions is quite difficult because of difference in crystal structures (in case of GaAs/wurtzite GaN junctions) and large mismatch in lattice constants (GaAs/zincblende GaN junctions). In this work we fabricate p+-GaAs/n-GaN heterojunctions by bonding GaAs and GaN epi wafers to each other using surface-activated-bonding technologies [1,2], selectively etching off the GaAs substrate used in the epitaxial growth, making ohmic contacts on the exposed surface of GaAs layer, and forming GaAs mesas. Ohmic contacts on the backside of GaN epi wafer are formed before bonding. The junctions are annealed at 400 ℃ in contact formation on GaAs surfaces. The temperature is much lower in comparison with the temperature in a wafer-fusion process of GaAs and GaN (750 ℃), which were reportedly applied for fabricating group-III arsenide/nitride heterojunction bipolar transistors [3]. Observation using cross sectional transmission electron microscope (TEM) of as-bonded and 400-℃ annealed GaAs/GaN interfaces shows that no voids are formed at the GaN/GaAs interfaces by the 400-℃ annealing. We also fabricate n+-GaAs/n-GaN heterojunctions for comparison.We measure the capacitance-voltage characteristics and current-voltage characteristics for reverse-bias voltages of the p+-GaAs/n-GaN and n+-GaAs/n-GaN junctions and find that the characteristics of the two junctions are close to each other, which suggests that the band profiles of GaN layers in the p+-GaAs/n-GaN and n+-GaAs/n-GaN junctions are almost the same, i.e., the Fermi-level pinning occurs at the GaAs/GaN interfaces. More importantly, the reverse-bias characteristics are measured up to -60 V. The reverse-bias voltage corresponds to an electric field of as high as ~2 MV/cm, which is comparable to the breakdown field of GaN (3-4 MV/cm) [4]. We also excite minority electrons in the p+-GaAs layer using a 488-nm Ar laser and successfully observe the photo current due to the transport of minority electrons across the reverse-biased GaAs/GaN interfaces.Acknowledgement: TEM samples were fabricated under the Inter-University Cooperative Research in IMR of Tohoku University. Epi wafers used in the work were provided from Sciocs (present Sumitomo Chemical) Co., Ltd.[1] H. Takagi, et al. Appl. Phys. Lett. 68, 2222 (1996). [2] S. Yamajo, et al., Jpn. J. Appl. Phys. 57, 02BE02 (2018). [3] C. Lian, et al. Appl. Phys. Lett. 91, 063502 (2007). [4] T. Maeda, et al., IEEE Electron Device Lett., 43, 96 (2022).

Oxide-Free SiC-SiC Direct Wafer Bonding and Its Characterization

There are different requirements for the production process and the final product of SiC-SiC wafer bonding. The manufacturing of devices that are sensitive to high temperature processing – due to broadened doping profiles and induced thermal stresses – requires room temperature bonding with high bond strength, while for electrical devices, it is mandatory that the bonding interface with a thin amorphous layer is oxide-free.[1] Reduced complexity of processes is also an important point for the final production. Hence, the goal of this work was to perform and characterize direct bonding of SiC-SiC without any added/deposited bonding layer. This type of bonded SiC wafers can be used for power electronics such as for the fabrication of traction inverter for automotive applications, DC/DC converter, on board charger or charging station.[2]For the wafer bonding processes standard 100 mm 4H SiC wafers were bonded with their Si-terminated faces in order to fabricate oxide-free bonds. The wafer bonding process was performed using the EVG ComBond® system: first the native oxides from both wafer surfaces were removed using an ion beam sputtering process, followed by the transfer of both wafers to the bonding process station operated in ultra-high vacuum (UHV) to significantly retard the oxidation process. Finally, the bonding process was performed at room temperature (RT).The goal of this study was to demonstrate the feasibility of the bonding process and to gain insight on the surface chemistry after activation. The investigations covered three areas: incoming inspection of the original wafer, characterization of activated single wafer and analysis of bonded wafer pairs. The focus was on compositional, chemical, mechanical and morphological analysis of the surfaces and of the bonded interfaces. In the case of single wafers, the focus of the incoming inspection was on whether the wafers fulfill the requirements of wafer bonding, and on the characterization of activated wafers to measure the surface modifications. Atomic force microscopy (AFM), white light interferometry (WLI) and spectroscopic ellipsometry (SE) were used to determine the surface roughness, the wafer topography and the surface layer structure, respectively. All three parameters are essential for successful RT SiC-SiC wafer bonding. The change of the surface chemistry was investigated by angle resolved x-ray photoelectron spectroscopy (AR-XPS). The quality of the bonded wafers and the bonding energy were verified using scanning acoustic microscopy (SAM) measurements (Fig. 1) as well as the Maszara blade test. Furthermore, cross-section transmission electron microscopy (X-TEM) showed a bonding interface with a thin amorphous layer and no noticeable additional oxygen containing layer (Fig. 2). In order to gain quantitative elemental distributions of oxygen and argon, energy dispersive x-ray spectroscopy (EDXS) was applied.The work reported is a demonstration of the capability of the different characterization methods regarding SiC-SiC wafer bonding. Future work will focus on the investigation of the bonded interface characteristics in the function of the bonding process parameters and the annealing conditions.[1] F. Mu, M. Fujino, T. Suga, Y. Takahashi, H. Nakazawa and K. Iguchi, "Wafer bonding of SiC-SiC and SiC-Si by modified surface activated bonding method" 2015 International Conference on Electronics Packaging and iMAPS All Asia Conference (ICEP-IAAC), Kyoto, Japan, 2015, pp. 542-545, doi: 10.1109/ICEP-IAAC.2015.7111073.[2] Tsunenobu Kimoto, "Material science and device physics in SiC technology for high-voltage power devices" Jpn. J. Appl. Phys. 54 040103 (2015), doi: 10.7567/JJAP.54.040103. Figure 1

(Invited) Formation and Characterization of Fe-Silicide Nanodots for Optoelectronic Application

Light emission from group-IV based nanostructures has been attracting much attention in the field of Si-based photonics because of its potential to combine photonic processing with electronic processing on a single chip [1]. Among various Fe-silicides, semiconducting β-FeSi2 shows light emission in the near-infrared region [2, 3] and is one of promising materials in optoelectronic application from viewpoints of tuning emission energy and improving luminous efficiency by means of quantized structures. In this work, we focus on low temperature formation of β-FeSi2 nanodots (NDs) over ~1011 cm-2 in areal density on SiO2 and the characterization of light emission properties of β–FeSi2 NDs.After conventional wet-chemical cleaning steps of p-type Si(100) substrates, a ~300-nm-thick SiO2 layer was grown at 1000°C in dry O2 and followed by dipping shortly in a dilute HF solution to make uniform surface termination with Si-OH bonds. After that, Fe films in the thickness range from 1.0 to 3.0 nm were formed on the OH-terminate SiO2 layer by electron beam evaporation without any extra heating. And then, the Fe–films were exposed to remote plasma of pure H2 (H2-RP) at 400°C to form highly-dense Fe-NDs [4]. Subsequently, so-prepared Fe-NDs were exposed to pure SiH4 at 400°C while controlling the amount of SiH4 exposure by duration and pressure.AFM images taken after exposure of Fe-films to H2-RP confirm the high-density formation of well-defined NDs. In the cases of H2-RP exposure of 1.3-nm-thick and 3.0-nm-thick Fe-films, the areal dot densities as high as ~1.1 × 1011 and ~1.3 × 1011 cm−2 were obtained, respectively, in which the average dot-heights of self-assembled Fe-NDs were determined to be ~5.3 and ~10.9 nm, respectively, by fitting a log-normal function to each measured size distribution. Notice that AFM images taken after SiH4 exposure show almost no change in the areal dot density while a slight increase in the average dot height. From cross-sectional transmission electron microscope images, we have also verified the formation of distorted-spherical shape NDs with a lattice spacing of ~0.18 nm associated with β-FeSi2. And the result is well consistent with the chemical composition of NDs after SiH4 exposure as evaluated by x-ray photoelectron spectroscopy. In addition, with 976 nm light excitation using a semiconductor laser, the NDs after the SiH4 exposure show stable photoluminescence (PL) signals in the energy region from ~0.7 to ~0.9 eV even at room temperature. On the other hand, no PL signals were detected from the NDs before the SiH4 exposure and after Ar anneal at 400°C. Also, when the size of the Fe-silicide NDs was increased from ~5.3 to ~10.9 nm in average dot height by increasing the initial Fe-film thickness, a distinct red-shift in PL was clearly observed. Thus, the observed PL can be interpreted in terms of radiative recombination of photogenerated carriers through quantized states of β-FeSi2 NDs. These results indicate that SiH4 exposure to Fe-NDs formed on SiO2 is a very promising technique for the low-temperature fabrication of luminescent FeSi2 NDs.AcknowledgementThis work was supported in part by Grant-in-Aid for Scientific Research (A) 21H04559 of MEXT Japan, and the Cooperative Research Project Program of the Research Institute of Electrical Communication, Tohoku University.

(Invited) Metal-Organic Chemical Vapor Deposition of WS2 on Patterned Substrate

Transition metal dichalcogenides (TMDs) have attracted much attention in recent years due to their excellent electrical properties and unique nature. Tungsten disulfide (WS2) , as a representative TMD, is a typical two-dimensional stacking material with moderately wide band gap (single layer: 2.03 eV, bulk: 1.32 eV) and, therefore, has led to active application research in various fields. In particular, large-area, few-layer WS2 is expected to have high potential as a channel material for MOSFETs in the next-generation LSIs because of relatively high carrier mobility even in the extremely thin film. Recently, we have succeeded in fabricating continuous WS2 thin films on Si substrates at low temperatures using newly developed organometallic precursor [1]. In this study, we further report the film characteristics and our attempts to deposit films on Fin structures in order to evaluate their applicability to the next-generation channel materials, which should have complicated 3-dimesional structure.In the experiment, n-BuNC-W(CO)5 was synthesized for WS2 deposition. This novel precursor is a liquid above 35 °C with stable vapor supply. It is also expected to interact with the substrate efficiently due to its polarity in the molecule. The vapor pressure is 1.2 torr at 115 °C, which is an appropriate for a CVD precursor. As a sulfur precursor, (t-C4H9)2S2, which is non-toxic in contrast to H2S, was adopted. WS2 thin films were grown by MOCVD in a cold-wall reactor, using the organic compounds introduced above as precursors for W and S, respectively, on flat and patterned silicon (001) substrates at 300 to 350 °C for 5 to 15 minutes. The deposited films were evaluated by X-ray photoelectron spectroscopy (XPS), UV-Raman spectroscopy (excitation wavelength: 355 nm), and transmission electron microscopy (TEM).Figure 1 summarizes the evaluation results of the WS2 film deposited on the flat substrate. W 4f XPS spectrum (Fig. 1 (a)) show that the film is mostly composed of 2H with a possible slight amount of 1T/1T' structure. In conjunction with S 2p spectrum (Fig. 1 (b)), it can be calculated using the relative sensitivity coefficient that the composition ratio of W to S is approximately 2.1, which is close to the stoichiometric ratio. Moreover, it was confirmed the film was stable more than 6 month in the room atmosphere. In the Raman spectrum shown in Fig. 1(c), two peaks appeared at approximately 350 cm-1 and 420 cm-1 corresponding to in-plane and out-of-plane vibrations, respectively, which are consisted of 2H-WS2 specific peaks of A1g (418 cm-1) and E1 2g (351 cm-1). Here, E1 2g might be broadened by the presence of another smaller mode called 2LA(M) (second-order longitudinal elastic mode), which may be attributed to the various distortions in the grown WS2 thin film. These results are also consistent with previous reports. This Raman results indicating the 2H structure of the film is also consistent with the XPS results as well as the previously reported literatures. Fig. 1(d) shows bright-field TEM images. It can be observed that the film continuously covers the substrate, with no voids larger than micrometer scale. The higher magnification image also shows approximately 10 layers of WS2 film grown with a layered structure parallel to the surface. The WS2 layers are stacked with a face spacing of approximately 0.7 nm, whose distance correlates well with the theoretical tungsten-tungsten interlayer distance of 0.62 nm for the bulk material. The domain size can be estimated to be roughly 50-100 nm.Figure 2 shows cross-sectional TEM images of WS2 thin films deposited on Si Fin structures. It can be observed that a continuous layered film is formed along the 3D Fin structure, and the layered structure is parallel to the surface all over the entire Fin structure. However, the number of layers differs depending on the position, with fewer layers on the sidewalls than those on the top surface, and even fewer layers toward the bottom. This may be due to the insufficient supply of precursor deep in the Fin structure. To improve the uniformity and controllability of layer number, further optimization of the deposition conditions such as precursor supply and deposition temperature, including atomic layer deposition technique, is desired.This research was supported by the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT) under the project JPJ011438 for the creation of a center for the creation of next-generation X-nics semiconductors.[1] C. Kirito, K. Yamazaki, Y. Hibino, Y. Hashimoto, H. Machida, M. Ishikawa, H. Sudoh, H. Wakabayashi, R. Yokogawa and A. Ogura, ECS Trans. 104, 3 (2021). Figure 1

Solid and Liquid State Dewetting of Thin Silicon Films Sandwiched between Two Silicon Carbide Substrates

Silicon carbide (SiC) is a very promising material for power electronics thanks to its wide band gap and high thermal conductivity. Stacking of SiC substrates of different crystalline quality and/or doping levels through wafer bonding is of particular interest for applications. Although a direct SiC/SiC bonding may result in an unstable interface, a layer of intermediate material playing a role of a “glue” can be sandwiched between the SiC substrates to ensure their good attachment. However, the processing of the power electronics devices at a high temperature may degrade the properties of this material which evolution under high temperature annealing must be studied in advance.In this work, we exploited the possibility to use silicon (Si) as a bonding material. We studied the structure evolution of a nanometer-thick polycrystalline Si layer sandwiched between two monocrystalline SiC substrates with a high temperature annealing. We focused our attention on understanding the impact of as-bonded Si layer thickness and the temperature of annealing on the eventual Si layer morphology. Tests were conducted for Si films of as-bonded thickness of 8 nm, 20 nm and 40 nm at temperatures ranging from 950°C to 1400°C, i.e. below Si melting temperature of 1414°C, and above it up to 1700°C. The experiments were realized using transmission electron microscopy (TEM), scanning electron microscopy (SEM) and high-resolution X-ray diffraction techniques. Cross-sectional TEM images were used to describe the structure and morphology of the interface. SEM was used to observe the inner surface of one of the SiC substrates once the other was removed by a cleavage. X-ray measurements were carried out to determine Si film thickness and texture as well as the strain in Si and SiC.We show that annealing at temperatures ranging from 950°C to 1400°C results in pitting, i.e. creation of voids, at the Si/SiC interfaces which are observed for any studied as-bonded thickness of a Si layer. We attribute this effect to a solid state dewetting of Si from SiC. This dewetting effect evolves into complete piercing of the silicon film and formation of cylindrical voids within the Si layer which grow in diameter with annealing temperature. We also observe a thickening of the silicon layer. The driving force for a solid state dewetting will be discussed in terms of SiC/Si interface energy, vacancies precipitation and Si atoms diffusion.When annealed to relatively higher temperatures allowing for Si melting followed by its solidification, we put into evidence the formation of step bunching on the internal surfaces of both SiC substrates. Regarding Si, it is concentrated along the SiC steps. The mechanism of the reconstruction of SiC internal surfaces resulting in the creation of step bunching will be discussed in relation to Si liquid state dewetting, carbon atoms dissolution, recondensation and migration within liquid Si.The effect of as-bonded silicon film thickness on the Si dewetting process is not pronounced after annealing at 950°C: all samples contain pits of a similar size and density. When the annealing temperature is slightly increased above 1000°C, the pits in the Si films of a lower initial thickness transform into cylindrical voids piercing the layers. The thicker layers contain both pits and cylindrical voids. At a high enough temperature of 1400°C only cylindrical voids were observed in all samples. The thicker layers present the lowest densities of voids.For the annealing at temperatures higher than 1400°C, the as-bonded thickness of a Si layer had a direct impact on the height of the steps formed on the internal surfaces of the SiC substrates. The eventual steps were higher the thicker the Si layer was used for bonding.The role of the diffusion and interaction of Si and C atoms and vacancies during annealing will be discussed in relation to the dynamics of solid and liquid state Si dewetting, size and density of pits and cylindrical voids and eventual Si layer morphology.Figure 1, on the left, SEM images showing the evolution of Si solid state dewetting with annealing temperature, from pitting to the formation of cylindrical voids. On the right, SEM and cross-sectional TEM images showing the step bunching on the SiC internal surfaces and the Si accumulation at the steps obtained after Si liquid state dewetting. Figure 1

Experimental study and modeling of extreme ultraviolet 4000 lines/mm diffraction gratings coated with periodic and aperiodic Al/Mo/SiC multilayers.

Multilayer coated diffraction gratings are crucial components for extreme ultraviolet (EUV) applications such as spectroscopy or spectro-imaging. However, for high groove density, the smoothening of the grating surface profile with multilayer deposition remains a limitation that requires further investigation. In this paper, we report on the design, characterization, and modeling of 4000 lines/mm diffraction gratings coated with periodic and aperiodic Al/Mo/SiC multilayers for EUV radiation. Two types of gratings with different groove depths are compared. Multilayer coatings were designed using a genetic algorithm to maximize the first-order diffraction efficiency in the 17-21 and 19-23nm wavelength ranges at normal incidence. Periodic and aperiodic multilayers with different numbers of layers were deposited by magnetron sputtering on the two types of fused silica gratings, and the grating groove profile evolution was measured by atomic force microscopy and cross-section transmission electron microscopy. The first-order diffraction efficiency was measured in the EUV at 5° incidence using monochromatic synchrotron radiation and modeled using the rigorous coupled-wave analysis method. The simulation models refined by using the Debye-Waller factor to account for the multilayer interfacial roughness show good agreement with experimental data. The results reported in this study will allow for designing efficient EUV multilayer gratings for high-resolution spectro-imaging instruments.

MBE growth of SnTe films on GaAs substrates with ZnTe buffer layers

SnTe (100) single-domain thin films have been previously grown on GaAs substrates by molecular beam epitaxy but obtaining excellent crystallinity can be difficult. In this paper, ZnTe buffer layers are introduced to improve the crystallinity of SnTe films. The effects of the molecular beam flux ratio (JTe/JSn) and growth temperature (Tg) on the crystallinity are investigated to achieve high-quality layers. The surface morphology and electrical properties of the SnTe thin films are also evaluated in addition to the conventional X-ray diffraction (XRD)measurements. The crystallographic properties of the grown samples are shown to be significantly improved by introducing a ZnTe buffer layer. XRD (θ–2θ) full width at half maximum of the lowest value obtained so far is about 0.14°. It is also found that lowering JTe/JSn to 0.6 results in a reduced growth rate, and an increased growth temperature (Tg = 220 °C) promotes the migration of atoms at the growth front. Both effects contribute to improvement in the crystallographic quality, including the layer surface smoothness. The cross-sectional transmission electron microscopy observation of the sample indicates the abrupt interface formation with different lattice structures. It is also found that low growth temperatures (below 200 °C) are not favorable for the SnTe layer growth, which is confirmed by the surface morphology.

Defects distribution and evolution in selected-area helium ion implanted 4H–SiC

An experimental demonstration of the generation of VSi, CSiVC, and NCVSi color centers in 4H–SiC using helium ion microscope irradiation of 5 × 5 μm2 areas with subsequent annealing treatment is presented. With the aid of transmission electron microscopy (TEM), photoluminescence (PL) and cathodoluminescence (CL) spectroscopy and Raman stress analysis, the evolution and distribution of color centers were thoroughly investigated. Cross-sectional TEM revealed the presence of helium bubbles in the center of the regions implanted with high doses which account for the observed quench of PL emission in the center of the implanted regions in both PL and CL measurements. PL spectra from the virgin, implanted and annealed samples proved the appearance of VSi after implantation and the transformation from VSi to CSiVC and NCVSi centers after annealing at 1000 °C. Moreover, with the increase of the implantation dose, the area of NCVSi centers increases whereas that of CSiVC decreases, which provide the first evidence of the competitive relationship between the formation of CSiVC and NCVSi defects. The comparison between stress distribution and CSiVC defect distribution illustrated that CSiVC centers predominantly distributed around the surface rupture region after thermal annealing where significant stress repair occurred. The results show that focused helium ion implantation holds promise for the precise coupling of VSi, CSiVC and NCVSi centers in predefined locations in integrated photonics applications.

Characterization of irradiation-induced dislocation loops and helium bubbles via electron channeling contrast imaging

Characterizing irradiation-induced defects is essential to understand damage mechanisms and develop irradiation-resistant materials. Prevailing strategies rely on transmission electron microscopy (TEM) observations which require rigorous prerequisite to prepare high-quality foils, making it difficult for rapid screening of irradiation-resistant materials. Here, we propose that the electron channeling contrast imaging (ECCI) is expected to provide a novel strategy for characterizing surface irradiation defects of ion-irradiated materials. The comparison between ECCI and conventional cross-sectional TEM for the observation of dislocation loops and helium (He) bubbles is analyzed in a common FeCoNiCr high-entropy alloy. The results indicate that the ECCI images of irradiation-induced defects are optimal at the extra-high tension of 20 kV, with a limit resolution of ∼1.5 nm for visible defects. The irradiation-induced dislocation loops show sharper contrast than bubbles in ECCI images, and the acceptable misorientation was within 4° of sample tilt. These findings offer a promising solution to the challenges for rapid assessment of irradiation damage in ion-irradiated materials.

Fine structure of the tracks of several-tens-MeV Au ions in YBa2Cu3O7-x studied by cross-sectional transmission electron microscopy

We studied the morphology of the tracks (irradiation damage) of 44, 66, and 84 MeV Au ions in YBa2Cu3O7-x (YBCO) thin films using cross-sectional transmission electron microscopy (TEM). The electronic energy loss (Se = 11.6, 16.7, and 20.1 keV/nm) ranged near the onset of ion-track formation previously determined by plan-view TEM and critical-temperature measurements (about 13 keV/nm). The morphology changed from 1D arrays of large spheres of about 5 to 10 nm diameter in 44-MeV irradiation to their elongated variant (spheroid) that were statistically pierced by thin-lines of about 1 to 2 nm and 3 to 5 nm diameter in 66 and 84 MeV, respectively. In the same series of Au-irradiated YBCO films, we observed a sudden increase in critical current density at self-field and 77 K between 44- and 55-MeV irradiation, in agreement with the appearance of the thin-line damage in TEM (between 44- and 66-MeV).

Improving vertical GaN p–n diode performance with room temperature defect mitigation

Defect mitigation of electronic devices is conventionally achieved using thermal annealing. To mobilize the defects, very high temperatures are necessary. Since thermal diffusion is random in nature, the process may take a prolonged period of time. In contrast, we demonstrate a room temperature annealing technique that takes only a few seconds. The fundamental mechanism is defect mobilization by atomic scale mechanical force originating from very high current density but low duty cycle electrical pulses. The high-energy electrons lose their momentum upon collision with the defects, yet the low duty cycle suppresses any heat accumulation to keep the temperature ambient. For a 7 × 105 A cm−2 pulsed current, we report an approximately 26% reduction in specific on-resistance, a 50% increase of the rectification ratio with a lower ideality factor, and reverse leakage current for as-fabricated vertical geometry GaN p–n diodes. We characterize the microscopic defect density of the devices before and after the room temperature processing to explain the improvement in the electrical characteristics. Raman analysis reveals an improvement in the crystallinity of the GaN layer and an approximately 40% relaxation of any post-fabrication residual strain compared to the as-received sample. Cross-sectional transmission electron microscopy (TEM) images and geometric phase analysis results of high-resolution TEM images further confirm the effectiveness of the proposed room temperature annealing technique to mitigate defects in the device. No detrimental effect, such as diffusion and/or segregation of elements, is observed as a result of applying a high-density pulsed current, as confirmed by energy dispersive x-ray spectroscopy mapping.

2D Hybrid Perovskites Employing an Organic Cation Paired with a Neutral Molecule.

Two-dimensional (2D) hybrid perovskites harness the chemical and structural versatility of organic compounds. Here, we explore 2D perovskites that incorporate both a first organic component, a primary ammonium cation, and a second neutral organic module. Through the experimental examination of 42 organic pairs with a range of functional groups and organic backbones, we identify five crystallization scenarios that occur upon mixing. Only one leads to the cointercalation of the organic modules with distinct and extended interlayer spacing, which is observed with the aid of X-ray diffraction (XRD) pattern analysis combined with cross-sectional transmission electron microscopy (TEM) and elemental analysis. We present a picture in which complementary pairs, capable of forming intermolecular bonds, cocrystallize with multiple structural arrangements. These arrangements are a function of the ratio of organic content, annealing temperature, and substrate surface characteristics. We highlight how noncovalent bonds, particularly hydrogen and halogen bonding, enable the influence over the organic sublattice in hybrid halide perovskites.

Elucidating the role of stress-induced martensite on the tensile behaviour of metastable β Ti-10V-2Fe-3Al alloy

Stress-induced martensitic transformation (SIMT) during loading in metastable alloy systems result in strength-ductility enhancement via transformation-induced plasticity (TRIP). The present study aims to investigate the influence of stress-induced martensites (SIM) on the tensile behaviour of metastable β Ti-10 V-2Fe-3Al (Ti-1023) alloy. The solutionized, and thermo-mechanically processed (i.e., cold rolling and annealing at 820 °C for 5 to 30 min) condition of Ti-1023, with single β phase microstructure, has been considered. The β grain size of solutionized Ti-1023 was ~625 μm, while the thermo-mechanically processed Ti-1023 was in the range of ~48 μm to ~106 μm as function of annealing time. The room temperature tensile behaviour of considered samples show SIMT, which results in strength enhancement (~7.5%), along with an increase in the trigger stress (~30%) as a function of β grain size (i.e., ~48 μm to ~625 μm). However, the % strain to failure show a significant decrease. A detailed microstructural characterization indicates the relation of SIM formation capability with β grain size. For finer β grain size, SIM displays a pronounced embrittling effect on the Ti-1023 tensile behaviour. Fracture surfaces show distinct regions with lath morphologies, which are further confirmed by surface as well as cross-sectional transmission electron microscopy (TEM) of the tensile fractured surfaces, illustrating the embrittling effects induced by SIM. In comparison, the dislocation-induced plasticity regions in the surrounding remnant β matrix result in dimple morphology. Thus, the coupling of enhanced martensite transformability, with reduced grain size, and the eventual decrease in martensitic laths spacing (i.e., from ~48 μm to ~7 μm), increase the density of martensite/matrix interfaces and the embrittling action. This results in pronounced dislocation pileups and formation of multiple martensitic variants facilitating premature failure by micro-void nucleation (at the martensite/matrix interface), growth and coalescence mechanism. The observations suggest that grain boundary engineering be optimized in order to fully exploit SIM's potential in enhancing the strength-ductility relation in Ti-1023 alloy.