High Substrate Temperature Research Articles

Role of oxygen within end group substituents on film morphology and charge carrier transport in thiophene/phenylene small-molecule semiconductors

In this study, the end group polarity of (5,5′)-biphenyl-(2,2’)-bithiophenes (PTTPs) was systematically varied from alkyl (1) to alkoxy (2) with one oxygen atom to glycol (3) with two oxygen atoms while the overall length of the end groups is kept constant. Thin films of the three compounds were sublimated at different substrate temperatures and their morphology, crystallinity and charge carrier transport in field-effect transistors was investigated to draw structure-property relationships for the PTTP derivatives. For all three compounds, the effective charge carrier mobility is improved with higher substrate temperatures at which films with higher crystallinity and larger grains are formed. The effective mobility decreases with higher polarity of the end groups from alkyl to alkoxy and glycol. The reliability factor of the alkyl (1) and alkoxy (2) substituted PTTPs decreases with higher substrate temperature, but at the same time this value is enhanced for the glycol substituted molecules (3). The transistors of 3 prepared at higher substrate temperatures also show a reduced threshold voltage and smaller hysteresis in the transfer characteristics. These insights are important for the understanding of the impact of oxygen incorporation into side chain/end group substituents of organic semiconductors and their implementation in organic electrochemical transistors, thermoelectrics and photovoltaics.

Interdiffusion and formation of intermetallic compounds in high-temperature power electronics substrate joints fabricated by transient liquid phase bonding

Power electronics packages typically comprise a dielectric substrate bonded to a metal layer and attached to heat sinks using a low-thermal-conductivity solder. These multiple layers increase the effective thermal resistance of the package and are responsible for package failures under cyclic thermal loads. Bonding the aluminum nitride dielectric layer (AlN) directly to a low coefficient of thermal expansion (CTE) aluminum silicon carbide (AlSiC) cold plate using copper‑aluminum (Cu-Al) transient liquid phase (TLP) bonding has been shown to improve the mechanical reliability of the power electronic packages while making the package more compact by bringing the cooling solutions closer to the devices. This study aims to characterize the Cu-Al bonds formed during the TLP bonding of AlSiC with three different power electronics substrates: pure aluminum nitride (AlN), aluminum (DBA), and copper (DBC). The material compositions and microstructures of the bonds were analyzed using scanning electron microscopy (SEM), x-ray spectroscopy (EDS), confocal scanning acoustic microscopy (C-SAM), and x-ray diffraction (XRD). α-Al solid compound was identified as the dominant phase in AlN-AlSiC and Al-AlSiC, with a notable presence of SiC particles. In contrast, three intermetallic phases – θ-CuAl2, η-CuAl, and γ′- Cu9Al4 – were observed in the Cu-AlSiC bond. A computational solid-state diffusion model was developed to predict the intermetallic compounds (IMCs) formed during Cu-Al TLP bonding in each system, which supported the observation that an initial Cu volume fraction of 20 % in the material layers produced a final bond composition of >95 % Al with minimal IMC growth. However, increased Cu concentrations produced higher concentration gradients, leading to increased growth of IMCs, Kirkendall voids, and interstitial cracks.

Evaluation of a High-Temperature Pre-Heating System Design for a Large-Scale Additive Manufacturing System

Additive Manufacturing (AM) of titanium (Ti6Al4V) material using Selective Laser Melting (SLM) may generate significant residual stresses of a tensile nature, which can cause premature component failure. The Aeroswift platform is a large volume AM machine where a high-temperature substrate preheating system is used to mitigate high thermal gradients. The current machine platform is unable to achieve a target build-plate temperature of 600 °C. This study focuses on the analysis of the preheating system design to determine the cause of its inefficiency, and the experimental testing of key components such as the heater and insulation materials. A Finite Element Analysis (FEA) model shows the ceramic heater achieves a maximum temperature of 395 °C, while the substrates (build-plates) only attain 374 °C. Analysis showed that having several metal components in contact and inadequate insulation around the heater caused heat loss, resulting in the preheating system’s inefficiency. Additionally, experimental testing shows that the insulation material used was 44% efficient, and a simple insulated test setup was only able to obtain a maximum temperature of 548.8 °C on a 20 mm thick stainless steel 304 plate, which illustrated some of the challenges faced by the current pre-heating design. New design options have been developed and FEA analysis indicates that a reduction in heat loss through improved sub-component configurations can obtain 650 °C degrees above the substrate without changing the heating element power. The development and challenges associated with the large-scale preheating system for AM are discussed, giving an insight into improving its performance.

Efficient Charge Transfers in Highly Conductive Copper Selenide Quantum Dot-Confined Catalysts for Robust Oxygen Evolution Reaction.

Defective quantum dots (QDs) are the emerging materials for catalysis by virtue of their atomic-scale size, high monodispersity, and ultra-high specific surface area. However, the dispersed nature of QDs fundamentally prohibits the efficient charge transfer in various catalytic processes. Here, we report efficient and robust electrocatalytic oxygen evolution based on defective and highly conductive copper selenide (CuSe) QDs confined in an amorphous carbon matrix with good uniformity (average diameter 4.25 nm) and a high areal density (1.8 × 1012 cm-2). The CuSe QD-confined catalysts with abundant selenide vacancies were prepared by using a pulsed laser deposition system benefitted by high substrate temperature and ultrahigh vacuum growth conditions, as evidenced by electron paramagnetic resonance characterizations. An ultra-low charge transfer resistance (about 7 Ω) determined by electrochemical impedance spectroscopy measurement indicates the efficient charge transfer of CuSe quantum-confined catalysts, which is guaranteed by its high conductivity (with a low resistivity of 2.33 μΩ·m), as revealed by electrical transport measurements. Our work provides a universal design scheme of the dispersed QD-based composite catalysts and demonstrates the CuSe QD-confined catalysts as an efficient and robust electrocatalyst for oxygen evolution reaction.

Processing, Microstructure, and Mechanical Properties of Laser Additive Manufactured Ti2AlNb-Based Alloy with Carbon, Boron, and Yttrium Microalloying

In this work, Ti-22Al-23Nb-0.8Mo-0.3Si-0.4C-0.1B-0.2Y (at. %) alloy powder was used to fabricate the Ti2AlNb-based alloy samples using Laser powder bed fusion (L-PBF) Additive Manufacturing with a high-temperature substrate preheating. L-PBF process parameters, including laser power, scan speed, hatching distance, and preheating temperature, allowing for obtaining fully dense (99.9% relative density) crack-free samples, were determined. The effects of substrate preheating temperature during the L-PBF process on microstructure, phase composition, and properties of the obtained Ti2AlNb-based alloy were investigated using X-ray diffraction, scanning electron microscopy, electron backscatter diffraction analysis, and microhardness testing. The results obtained for the material with C, B, and Y microalloying were compared to the Ti2AlNb-based alloy fabricated by L-PBF from the powder not alloyed with C, B, and Y. The results revealed that the microalloying reduced the number of solidification cracks; however, no significant microstructural changes were observed, and high-temperature substrate preheating was still necessary to suppress cold cracking of the alloy. The microstructure of the alloy varied from fully-β/B2, B2 + O, to fully-O depending on the preheating temperature. Effects of hot isostatic pressing and heat treatment conditions on microstructure and mechanical properties were investigated.

Pectin Derived from Hydrolysis of Ripe Kepok Kuning Banana Peel Powder Employing Crude Pectinases Produced by Aspergillus niger

Banana fruits consumption generates about 35% weight of peel waste containing approximately 10.61 to 24 w/w% of pectin. Hence, improper banana peel waste management may induce various environmental and health issues. The objectives of this work were to study the effect of substrate concentration, pH, temperature, and duration on the yield of pectin extracted from enzymatic hydrolysis of banana peel powder. In this work, the crude enzymes were obtained via submerged fermentation of Kepok Kuning banana peel powder utilising Aspergillus niger and directly used without prior purification. Pectin extraction from banana peel powder was performed through hydrolysis using crude pectinases at various substrate concentrations (0.033 to 0.123 g/mL), pH (4.0 to 6.0), and temperature (40 to 70°C) for 180 min. The increase of extraction parameters enhanced the pectin yield to a maximum value and then declined. High substrate concentration, temperature, pH, and monomeric pectin compounds formation at long hydrolysis duration were found to reduce enzyme activity. A recommended extraction condition is using 0.103 g/mL substrate concentration, pH 5.0, and 55 °C for 120 min to achieve 10.80% weight yield. Commercial implementations of the results can be worthwhile in solving the environmental problem and enhance the economic value of pectin-rich fruit peels and other agricultural wastes.

Cold Spray Metallization of Hybrid Thermoplastic - Thermoset Fiber Reinforced Composite

Surface metallization is amongst the recent trends in the polymer and polymer matrix composites (PMCs) research industries to improve the electrical and thermal properties and exploit the subsequent utilization in the aerospace sector. Specifically, polymer matrix composites have been subjected to the limitations in form of high temperature exposure and substrate deterioration. The present study encompasses a new strategy in the manufacturing and metallization process. The first stage in the manufacturing of hybrid thermoplastic-thermoset composite was the hot compaction which comprised of primary preform preparation enabling the partial impregnation of the thermoplastic resin through the fabric reinforcement layer. The subsequent stage entailed the preform vacuum bagging and conducting catalyzed thermoset resin impregnation. The vacuum resin infusion step included a cocuring cycle to generate a fiber reinforced composite comprising of thermoplastic and impregnated thermoset resin with improved adhesion. Resin flow front movement was analyzed during the resin infusion process. Composite metallization was achieved through cold spray (CS). CS process parameters influence on the coating quality and characterization of laminates through microstructural analysis and results have been reported. The hybrid composite with thermoset resin through thickness and in-plane impregnation was achieved with the intact adherent thermoplastic layer after the curing cycle. In the CS metallization, the effective operative window of stand-off distances (SoD) and temperature has been determined.

Structure and defects-related optical properties of highly (002)-oriented zinc oxide thin films

We report on the highly (002)-oriented ZnO thin films deposited on glass substrates at different substrate temperatures using the recently developed pulsed laser deposition (PLD) facility at our institution. Effects of the substrate temperature on the structural, compositional, morphological, and optical properties of the ZnO thin films are studied. The XRD studies revealed the deposition of high-quality ZnO thin films with preferred hexagonal wurtzite structure along the (002) plane above 200 °C substrate temperatures. An increase in the crystallite size estimated using the Scherrer formula from 9.58 nm to 13.25 nm of the deposited thin films reflects the enhancement of the crystallinity with the increasing substrate temperature. Scanning electron microscope (SEM) micrographs revealed a smooth surface morphology with small grains for the thin film deposited at room temperature. The thin films deposited at high substrate temperatures showed agglomeration of nanoparticles resulting in nano rod-like structures. The optical band gap energy was found to be increased with an increase in the crystallinity of the thin films. In the photoluminescence (PL) spectra; near band edge (NBE) emissions were observed in all of the deposited thin films. The deep-level emissions of ZnO thin films were found to be highly dependent on the substrate temperature during film deposition. The thin films grown at 500 °C showed the visible emission in blue, green, and yellow-orange regions, which corresponded to zinc vacancy (VZn), oxygen vacancy (VO), and oxygen interstitial (Oi) defect levels, respectively. These crystal defects in the deposited ZnO thin films are responsible for the deep level emission linked to the visible emission.

Propagation of dislocations in diamond (111) homoepitaxial layer

Dislocations in a diamond (111) homoepitaxial layer were investigated in terms of the line directions and types. After a thick homoepitaxial layer was grown on a {111} high-pressure high-temperature (HPHT) substrate using chemical vapor deposition (CVD), the Raman spectra of diamond were taken using a confocal setup to obtain the spatial distribution of the peak positions of the diamond Raman line. Dislocations in the CVD layer and HPHT substrate were detected as local stress variation in the peak position images. The line directions of dislocations were, respectively, [112] for the HPHT substrate and [011] for the CVD layer. Local stress variation was in the [1¯1¯2] direction in both the CVD layer and the HPHT substrate. The Burgers vector deduced from the local stress-variation direction indicated the dislocation structures, respectively, as an edge type for [112] dislocations in the HPHT substrate and as a 60° mixed type for [011] dislocations in the CVD layer. Because both dislocations had the same slip plane, the line direction and dislocation type changed on the same slip plane at the CVD layer/HPHT substrate interface, when dislocation in the HPHT substrate reached the CVD layer.

Optimization of the substrate temperature of narrow bandgap CIS solar cells by three stage coevaporation process

In this paper, three-stage co-evaporation method was used to prepare narrow bandgap CuInSe2 (CIS) thin-film absorber. The effect of the substrate temperature on the performance of CIS solar cells was investigated. The substrate temperatures of the CIS films varied from 420 °C to 580 °C. The microstructure structure, crystallinity, morphology, and optoelectrical performances of the CIS solar cell were investigated by various characterization methods. We found that the substrate temperature played a great role in the properties of the CIS thin films and solar cell devices: that was the CIS solar cell deposited at low temperature showed poor crystallinity, fine grain size and low efficiency, while CIS solar cell deposited very high temperature also showed high efficiency. However, high substrate temperature can lead to the Indium loss at the surface of the CIS films, although the bulk composition of CIS films did vary much at different substrate, resulting in FF (fill factor) reduce.

Fabrication of ordered arrays of GeSn nanodots using anodic aluminum oxide as a template

A method to grow GeSn nanodots has been developed by magnetron sputtering using anodic aluminum oxide as a template. With a high substrate temperature and a high deposition rate, flattened hill-like GeSn nanodots with high Sn content have been successfully formed directly on Ge(001) and Si(001) substrates. The GeSn nanodots are polycrystalline on Si and monocrystalline on Ge without Sn segregation. High-resolution transmission electron microscopy observations revealed that GeSn nanodots formed on Ge had a perfect interface without misfit dislocations.

Excellent properties of Ga‐doped ZnO film as an alternative transparent electrode for thin‐film solar cells

AbstractOptical and electrical properties of transparent electrode are directly related to the photoelectric conversion efficiency of thin‐film solar cells. For this reason, Ga and Al‐doped ZnO (GZO and AZO) transparent conducting films were fabricated on float glass through the magnetic sputtering technique. Compared with AZO films, GZO films show a higher figure of merit (FoM) value indicating their outstanding optical and electrical properties. The smaller difference of ionic radius between Ga3+ and Zn2+ than Al3+ and Zn2+ contributes to high carrier concentration and electron mobility of GZO films. In addition, it has been shown that GZO films can be stable at high substrate temperatures. After being annealed at 550°C in N2 atmosphere, the FoM value of GZO films is twice as much as that of FTO films, indicating that GZO can be applied as the front contact material not only in CIGS thin‐film solar cells, but also in CdTe thin‐film solar cells.

C-axis texture of high-rate-sputtered Nd2Fe14B thin films: independence of sheet texture of Mo underlayer

In this study, we investigated the effects of the sheet texture of the Mo underlayer on the c-axis texture of the high-rate-sputtered Nd2Fe14B phase. The magnetic properties revealed that the squareness of the Nd–Fe–B magnetic layer did not depend on the sheet texture of the Mo underlayer but on the substrate temperature: large squareness was obtained when the magnetic layer was sputtered at high substrate temperature above 500 °C. In-plane X-ray diffraction measurements indicated that the intensity of diffraction from the Nd2Fe14B (410) plane did not depend on the sheet texture of the Mo underlayer but on the substrate temperature: high intensity was obtained when the magnetic layer was sputtered at above 500 °C, which was consistent with magnetic properties. These results support that the c-axis texture of the high-rate-sputtered magnetic layer was driven by the c plane with the lowest surface energy among the crystal planes in the Nd2Fe14B phase.

Coaxial Ceramic Direct Ink Writing on Heterogenous and Rough Surfaces: Investigation of Core-Shell Interactions.

In this work, coaxial conductor-ceramic direct ink writing enables the printing of sensitive or encapsulated materials onto heterogeneous and rough substrates. While encasing the core fluid within a stiff ceramic shell, continuity may be maintained, even while printing onto conventionally challenging substrates. Here, we report the development of a coaxial ceramic direct ink writing suite and explore coflow interrelationships based on microfluidic principles. A coaxial nozzle is designed to facilitate the coextrusion of an alumina shell, whereas indium-tin-oxide inks constitute the core. In this manner, a core-shell ceramic element may be printed onto rough substrates for future high-temperature applications. Colloidal inks are engineered to provide the required rheological and sintering performance. Moreover, flow simulations in conjunction with microfluidic coflow principles are used to explore the coaxial printing processing space, thus controlling the core-shell architectures. Physical modeling is further used to analyze core deformations and eccentricity. Simulations are validated experimentally, and the analyses are used to deposit coaxial ceramic features onto heterogeneous, high-temperature ceramic substrates.

Deposition temperature dependence of thermo-spin and magneto-thermoelectric conversion in Co2MnGa films on Y3Fe5O12 and Gd3Ga5O12

We have characterized Co2MnGa (CMG) Heusler alloy films grown on Y3Fe5O12 (YIG) and Gd3Ga5O12 (GGG) substrates at different deposition temperatures and investigated thermo-spin and magneto-thermoelectric conversion properties by means of a lock-in thermography technique. X-ray diffraction, magnetization, and electrical transport measurements show that the deposition at high substrate temperatures induces the crystallized structures of CMG, while the resistivity of the CMG films on YIG (GGG) prepared at and above 500 °C (550 °C) becomes too high to measure the thermo-spin and magneto-thermoelectric effects due to large roughness, highlighting the difficulty of fabricating highly ordered continuous CMG films on garnet structures. Our lock-in thermography measurements show that the deposition at high substrate temperatures results in an increase in the current-induced temperature change for CMG/GGG and a decrease in that for CMG/YIG. The former indicates the enhancement of the anomalous Ettingshausen effect in CMG through crystallization. The latter can be explained by the superposition of the anomalous Ettingshausen effect and the spin Peltier effect induced by the positive (negative) charge-to-spin conversion for the amorphous (crystallized) CMG films. These results provide a hint to construct spin-caloritronic devices based on Heusler alloys.

Enhanced photosensitive of Schottky diodes using SrO interfaced layer in MIS structure for optoelectronic applications

In this study, the Schottky diodes (SDs) based on an interfacial layer of strontium oxide (SrO) thin film were fabricated. Thin films (TFs) were coated on glass and silicon substrates by low-cost spray pyrolysis coating technique with varying substrate temperatures of 350, 400, 450, and 500°C. Structural, surface morphology, optical, and electrical characteristics of SrO TFs were investigated. In particular, the I–V characteristics of Cu/SrO/n-Si diodes in dark and light excitations were analyzed. The maximum barrier height (ΦB) for the diode fabricated at 500°C under Xenon lamp light irradiation was observed at 0.82 eV. Also, near ideal ideality factor (n) of the diode parameters, it was found at highest substrate temperature at 500°C. The results show that diodes are more fitting for the improvement of good quality photodiodes as well photodetector applications.

Growth-Promoting Mechanism of Bismuth-Doped Cu(In,Ga)Se2 Solar Cells Fabricated at 400 °C.

The classical high-temperature synthesis process of Cu(In,Ga)Se2 (CIGS) solar cells limits their applications on high-temperature intolerant substrates. In this study, a novel low-temperature (400 °C) fabrication strategy of CIGS solar cells is reported using the bismuth (Bi)-doping method, and its growth-promoting mechanism is systematically studied. Different concentrations of Bi are incorporated into pure chalcopyrite quaternary target sputtered-CIGS films by controlling the thickness of the Bi layer. Bi induces considerable grain growth improvement, and an average of approximately 3% absolute efficiency enhancement is achieved for Bi-doped solar cells in comparison with the Bi-free samples. Solar cells doped with a 50 nm Bi layer yield the highest efficiency of 13.04% (without any antireflective coating) using the low-temperature technology. The copper-bismuth-selenium compounds (Cu-Bi-Se, mainly Cu1.6Bi4.8Se8) are crucial in improving the crystallinity of absorbers during the annealing process. These Bi-containing compounds are conclusively observed at the grain boundaries and top and bottom interfaces of CIGS films. The growth promotion is found to be associated with the superior diffusion capacity of Cu-Bi-Se compounds in CIGS films, and these liquid compounds function as carriers to facilitate crystallization. Bi atoms do not enter the CIGS lattices, and the band gaps (Eg) of absorbers remain unchanged. Bi doping reduces the number of CIGS grain boundaries and increases the copper vacancy content in CIGS films, thereby boosting the carrier concentrations. Cu-Bi-Se compounds in grain boundaries significantly enhance the conductivity of grain boundaries and serve as channels for carrier transport. The valence band, Fermi energy level (EF), and conduction band of Bi-doped CIGS films all move downward. This band shift strengthens the band bending of the CdS/CIGS heterojunction and eventually improves the open circuit voltage (Voc) of solar cells. An effective doping method and a novel mechanism can facilitate the low-temperature preparation of CIGS solar cells.

Elevating Leidenfrost temperature by orderly droplet stream impingement boiling

Impinging high frequency microscale droplet stream on a high temperature substrate is an effective method to realize point heat sink. The boiling patterns and the heat transfer characteristics of droplet impingements were investigated based on the captured images and the measured heat transfer. In the experiments, six different hydrodynamic patterns could be identified, base on which the phase diagram was drawn. The boundaries of different boiling patterns varied with the flowrate as well as the frequency of the droplet stream. The substrate temperature threshold for the transition from the contact boiling regime to the levitation transition boiling regime was influenced by flowrate apparently, but it was independent of droplet frequency. However, both the hydrodynamic patterns and the heat transfer measurements showed that the Leidenfrost temperature was elevated by increasing the flowrate or the droplet frequency. Compared to the droplet stream with random size and spacing, the Leidenfrost temperature could be increased by impinging orderly droplet stream without using more coolant. The reason was inferred that periodical collisions of droplet train released vapor in an efficient manner.

Investigating the structures and residual stress of Cux(FeAlCr)100−x film on Ni substrate using molecular dynamics

This investigation explores the formation of a Cux(FeAlCr)100−x film on a Ni(001) substrate performed by molecular dynamics simulation. The effects of compositions, incident angle, and temperature of the substrate on the intermixing, surface morphology, structure composition and residual stress are studied. The surface roughness of the film increases as the Cu ratio decreases from 55% to 10%. Besides, the roughness of the film surface achieves the maximum value at 30°. However, the surface roughness of the film decreases at a higher substrate temperature. The intermixing between Cu-Fe-Al-Cr atoms and Ni atoms happens at the interfacial zone. The intermixing rate is affected by the composition and incident angle in the coating process. The intermixing rate rises when rising the temperature of the substrate. Interestingly, during the coating process, the crystal structure of the high entropy alloys film is formed. The crystalline structure ratio of the Cux(FeAlCr)100−x film slightly increases when the composition deviation decreases. In comparison, it reduces with the higher temperature of the Ni sample. The average biaxial stress and average normal stress of the Cu25Fe25Al25Cr25 film only have minor changes when increasing the incident angle. However, the average biaxial stress of the Cu25Fe25Al25Cr25 film increases as raising the temperature.

Low-Temperature Direct Growth of Few-Layer Hexagonal Boron Nitride on Catalyst-Free Sapphire Substrates.

Wide-band-gap layered semiconductor hexagonal boron nitride (h-BN) is attracting intense interest due to its unique optoelectronic properties and versatile applications in deep ultraviolet optoelectronic and two-dimensional electronic devices. However, it is still a great challenge to directly grow high-quality h-BN on dielectric substrates, and an extremely high substrate temperature or annealing is usually required. In this work, high-quality few-layer h-BN is directly grown on sapphire substrates via ion beam sputtering deposition at a relatively low temperature of 700 °C by introducing NH3 into the growth chamber. Such low growth temperature is attributed to the presence of abundant active N species, originating from the decomposition of NH3 under ion beam irradiation. To further tailor the properties of h-BN, carbon was introduced into the h-BN layer by simultaneously introducing CH4 and NH3 during the growth process, indicating the wide applicability of this approach. Moreover, a deep ultraviolet (DUV) photodetector is also fabricated from a C-doped h-BN layer and exhibits superior performance compared with an intrinsic h-BN device.