Low Substrate Temperature Research Articles

Optical and electrical characterization of microwave power system chemical vapour deposited (MPS-CVD) graphene on Ni electroplated Cu foil at varying temperatures

Optical and electrical properties of graphene deposited on Ni electroplated Cu foil was studied by microwave power system chemical vapour deposition. Problems associated with non-uniformity using different substrates and the control of graphene properties instigated a new method for the preparation of graphene films. The growth process involved the use of Isopropanol for the formation of radicals at varying substrate temperatures and atmospheric pressure with microwave energy to form uniform solid films. Raman spectroscopy confirmed the quality of the films and designate single-layer graphene for 2D-band at low temperature while few-layer graphene for G-band at high temperature. Scanning electron microscope revealed changes in the morphology of graphene as the deposition parameters increases. The optical analysis revealed exponential decay of absorbance with spectra fringes in the visible region and transmittance of 92.8% and 93.7% which depends on the thickness uniformity of graphene. Hall Effect measurement showed n-type carrier conductivity with large positive magnetoresistance and high electrical conductivity due to the increased number of layers, low sheet resistance and high substrate temperature. The effect of varying temperature on graphene have been investigated, perhaps, low and intermediate substrate temperature yielded better optical properties while the high temperature yielded better electrical properties.

Investigation of p-type SnO films served as a potential hole-transporting material for highly efficient perovskite solar cells

As an essential component of perovskite solar cells (PSCs), the design of hole transport materials (HTMs) has always been a topic of interest in the photovoltaic community. In this work, p-type tin monoxide (SnO) films are grown by electron beam evaporation to investigate the influence of substrate temperature and deposition time on their properties. The film crystallization and preferred growth orientation of (001) showed prominent enhancement with the increase in substrate temperature. The prolongation of deposition time witnessed changes in grain morphology from nanoparticles to nanosheets. Hall measurements revealed that the film deposited at 200 °C showed the lowest resistivity, the maximum carrier concentration, and the lowest carrier mobility. Using these films as HTMs, the performance of PSCs based on caesium lead iodide perovskites (CsPbI3) was analyzed theoretically and experimentally, and high open-circuit voltage (VOC) of 1.16 V is obtained. With the increase of the substrate temperature, the power conversion efficiency of PSCs is decreased due to the decreased hole concentration. The results suggest that SnO film deposited at low substrate temperature holds promise for facilitating high-performance PSCs as HTMs.

Enhanced interfacial reaction of precursor and low temperature substrate in HfO2 atomic layer deposition with highly Ar diluted O2 plasma

The internal energy of metastable oxygen atoms in highly Ar diluted oxygen plasma was utilized in the initial stage of the atomic layer deposition of HfO2 on SiO2/Si(100) at low temperature of 150 °C. The highly Ar dilute oxygen plasma enhanced the oxidation of the incomplete chemisorption state of the precursor at low temperature, successfully formed Hf silicate interface, and decreased the impurity nitrogen atoms in the HfO2 film compared to the pure oxygen plasma ALD. Residual nitrogen atoms in the film were found to cause excessive precursor adsorption. The results of plasma emission spectroscopy and ion saturation current measurements show that the highly Ar-diluted O2 plasma can increase the O radical formation rate for ion fluxes at pressures above 100 Pa. The relatively high metastable oxygen atom irradiation is thought to be responsible for the removal of HfN bonds and enable ALD on low temperature substrates. Atomic force microscopy showed that the root mean square roughness in the high Ar dilution sample was 0.093 nm, indicating high flatness.

Photovoltaic and impedance spectroscopy characterization of single-junction a-Si:H p–i–n solar cells deposited by simple shadow masking techniques using PECVD

Hydrogenated amorphous silicon p–i–n solar cells with a 1 × 1 cm2 active surface area were fabricated using shadow masks on the 20 × 20 cm2 glass substrate coated with a fluorine-doped tin oxide film. The intrinsic, n-type hydrogenated amorphous silicon (a-Si:H), and p-type a-SiC:H thin films were deposited using plasma-enhanced chemical vapor deposition at 13.56 MHz plasma excitation frequency and on 20 × 20 cm2 and SnO2:F covered glass substrates. Low rf-power densities (less than 0.1 W/cm2) and substrate temperatures (less than 190 °C) were used for this purpose. Raman spectra of the films are dominated by a broad peak around 480 cm−1 that is the characteristic of the amorphous silicon network for all the three types of films. Scanning electron microscopy measurements revealed that the surface of the a-Si:H films deposited on SnO2:F-coated glass substrates (Asahi-VU) replicates the texture of the SnO2:F film. Spectroscopic ellipsometry spectra were analyzed with the Tauc–Lorentz dispersion model, and the results revealed that the optical gap of the intrinsic a-Si:H films is on the order of 1.7 eV, while that of the a-SiC:H is on the order of 1.8 eV. These results were further confirmed by optical transmission measurements. The highest efficiency obtained for solar cells prepared with shadow masking under our condition is on the order of 8.83% with a Voc of 0.856 V, a short circuit current density of 15.6 mA/cm2, and a fill factor of 66.07%. The obtained efficiency is slightly lower than the record efficiency obtained in this family of cells (10.3%) prepared by laser scribing because the low short-circuit current slightly lowers the fill factor. Impedance spectroscopy measurements were performed on the cells in the dark in the frequency range of 1 kHz–100 kHz. The analysis of impedance either in the Nyquist diagram or in the Bode diagram suggests a lumped circuit consisting of resistance Rs in series with a parallel combination of resistance Rp and capacitance Cp that account for the p–i–n structure. The value of Rp changed with the applied DC bias. The value of the series resistance agrees with the value obtained from the current–voltage characteristics of the cell.

All-Inorganic Perovskite CsPb2Br5 Nanosheets for Photodetector Application Based on Rapid Growth in Aqueous Phase

All-inorganic cesium lead-halide perovskites exhibit great development prospect in optoelectronic devices owning to their stability and remarkable optoelectronic properties. Herein, we investigate the solution-processed synthesis of perovskite CsPb2Br5 nanosheets by using aqueous and ethanol as solvent. The result shows that the aqueous environment ensures the phase formation of CsPb2Br5, and that the supersaturated solution in ethanol boosts nucleation of the nanosheets. The substrates temperature is the key factor for the evolution of morphology and the variation of the thickness of CsPb2Br5 nanosheets. Lower substrate temperature (<35 °C) is conducive to the formation of evenly distributed nanosheets with less stacking. The spatial and time-resolved fluorescence spectra indicate the heterogeneity of the defect density and the recombination process in different nanosheet regions. The photodetector based on the prepared CsPb2Br5 nanosheet displays excellent switching current ratio (9×102), short rise and decay time (43 ms and 83 ms respectively) and good stability (75% of initial current after 90 days in air). In addition, the mechanical stability and flexibility of photodetector on the flexible substrate are investigates for bending 500 cycles.

Impact of arsenic species on self-assembly of triangular and hexagonal tensile-strained GaAs(111)A quantum dots

We use dimeric arsenic (As2) or tetrameric arsenic (As4) during molecular beam epitaxy to manipulate the structural and optical properties of GaAs(111)A tensile-strained quantum dots (TSQDs). Choice of arsenic species affects nucleation and growth behavior during TSQD self-assembly. Previously, epitaxial GaAs(111)A TSQDs have been grown with As4, producing TSQDs with a triangular base, and ‘A-step’ edges perpendicular to the three directions. We demonstrate that using As2 at low substrate temperature also results in triangular GaAs(111)A TSQDs, but with ‘B-step’ edges perpendicular to the three directions. We can therefore invert the crystallographic orientation of these triangular nanostructures, simply by switching between As4 and As2. At higher substrate temperatures, GaAs(111)A TSQDs grown under As2 develop with a hexagonal base. Compared with triangular dots, the higher symmetry of hexagonal TSQDs may reduce fine-structure splitting on this (111) surface, a requirement for robust photon entanglement. Regardless of shape, GaAs(111)A TSQDs grown under As2 exhibit superior optical quality.

RF/DC Magnetron Sputtering Deposition of Thin Layers for Solar Cell Fabrication

Thin film Cu(In,Ga)Se2 (CIGS)-based solar cells with relatively high efficiency and low material usage might become a promising alternative for crystalline silicon technology. The most challenging task nowadays is to decrease the PV module fabrication costs by application of easily scalable industrial process. One of the possible solutions is the usage of magnetron sputtering system for deposition of all structures applied in CIGS-based photovoltaic device. The main object of these studies was fabrication and characterization of thin films deposited by sputtering technique. Structural and electrical properties of the sputtered films were analyzed using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), X-ray Powder Diffraction (XRD), and four-point probe resistivity measurements. The presented findings revealed technological parameters for which sheet resistance of molybdenum (Mo) back contact decreased up to 0.3 Ω/□ and to even 0.08 Ω/□ in case of aluminum layer. EDS analysis provided evidence for the appropriate stoichiometry of CIGS absorber (with CGI and GGI equal to 0.96 and 0.2, respectively). XRD characterization confirmed high-quality chalcopyrite polycrystalline structure of Cu(In,Ga)Se2 film fabricated at relatively low substrate temperature of 400 °C. Characteristic XRD peaks of hexagonal-oriented structures of sputtered CdS and i-ZnO layers were noticed.

Effect of molecular beam epitaxy growth conditions on phase separation in wide-bandgap InAlAsSb lattice-matched to InP

InxAl1-xAs1-ySby is the only III-V material lattice-matched to InP with a direct bandgap energy range as large as ~1.45–1.80 eV, making it interesting for multiple optoelectronic applications. However, inherent material challenges in this immature quaternary alloy have thus far precluded the growth of InxAl1-xAs1-ySby with device-quality properties at the InP lattice constant. We have investigated how molecular beam epitaxy growth conditions affect the photoluminescence intensity, spectrum, temperature- and power-dependence, as well as the surface morphology of In0.23Al0.77As0.75Sb0.25, which is lattice-matched to InP with bandgap energy of ~1.68 eV as measured by ellipsometry. We find that reducing the adatom mobility, via a lower substrate temperature and higher group-V overpressure, diminishes the tendency for phase separation, but also quenches the photoluminescence intensity. These findings provide a step toward identifying an optimal growth window for realizing high-quality, wide-bandgap InxAl1-xAs1-ySby lattice-matched to InP.

Effect of Low Substrate Temperature on the Magnetic Properties and Domain Structure of Fe₇₀Ga₃₀ Thin Films

We report on the effect of low substrate temperature on the correlation between structure, magnetic properties, and micromagnetic behavior of Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">70</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">30</sub> thin films. The enhanced grain size and the decrease in the number of pinning centers are in correlation with the enhanced saturation magnetization and domain size (~0.2-0.5 μm), respectively. Estimated effective magnetic anisotropy energy (K <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">eff</sub> ) is found to increase with substrate temperature from 7.5 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup> J/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> (room temperature) to 13.6 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup> J/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> (350 °C). The coercivity variation from the object-oriented micromagnetic framework (OOMMF) simulation is in line with respect to experimental values. Spin structure from simulation also indicates multiple-domain configuration when there is a magnetization reversal.

Photonic Curing of Copper Ink Films on Liquid Crystal Polymer Substrate

AbstractThe application of intense pulsed light (IPL) to printed copper nanoparticle (CuNP) films enables rapid curing on low temperature substrates in ambient conditions. In this work, we printed CuNP ink on liquid crystal polymer (LCP; Vectra A resin) and then cured the films using a high energy density light pulse. High-resolution SEM images of the cured films revealed that the CuNPs on LCP were fused together. Optimal curing parameters were a 5 ms pulse, 75% duty cycle, and an energy density range of 4.2–5.2 J⋅cm-2. Sheet resistance, Rs, values as low as ~0.1 Ω⋅sq-1were obtained. The LCP substrate took on a yellowed appearance after the application of five pulses and exhibited a surface free energy increase. A filter that blocked wavelengths &lt;450 nm was placed over the printed copper film on LCP. As expected, the presence of the filter decreased the total energy density and produced a cured film with high Rs; however, when the energy density was increased in the presence of the filter, the Rs remained high (0.95 Ω⋅sq-1). This preliminary work indicates that additional studies are required not only to understand low thermal budget curing on LCP, but also to elucidate the properties of substrates that enable low Rs.

Adhesive strength and micromechanics of wood bonded at low temperature

Load-bearing structural timber requires durable and reliable adhesive bonding. Hence manufacture is determined by strict regulations for instance regarding wood temperature specifications. This research investigates bonding at significantly lower substrate temperatures providing sufficient time for cure to ascertain whether appropriate bond strength can be achieved. Scarf joints were produced at different wood temperatures with melamine-urea-formaldehyde (MUF). Resin and hardener were applied on separate sides. Adhesive strength was verified by macroscopic testing of the bond strength and microscopic examination of the properties of the adhesive. Resulting tensile shear strength varied marginally for different temperatures even after water immersion. Penetration of the adhesive into the wood in fiber direction was deeper on hardener than on resin application side due to differences in viscosity with no dependence on temperature. Nanoindentation visualized adhesive penetration into the cell wall with increased hardness and reduced modulus of elasticity from unfilled cells to adhesive filled cells independent of temperature. The results indicate least influence of the substrate temperature at the time of adhesive application on the macro- and microscopic level permitting bonding at low temperatures, given that enough time for cure at sufficient temperature is allowed.

Stabilization of complex orthorhombic o-Cr3C2 thin films under high energetic growth conditions: Experiments and calculations

We report on the deposition and characterization of hard and wear resistant orthorhombic Cr3C2 thin films. Films are deposited using a novel linear filtered cathodic arc (LFCA) at high energetic conditions and relatively low substrate temperatures (<200 °C). The deposited films have amorphous (a-Cr3C2), nanocrystalline (nc-Cr3C2) and orthorhombic (o-Cr3C2) structure by carefully tuning the growth process via the ion current density and substrate bias voltage. The o-Cr3C2 films presented a moderate stress (−5.3 GPa), low coefficient of friction, low nanowear and high hardness values (33.4 GPa) exceeding in almost 12 GPa the reported hardness values for bulk Cr3C2 (21.5 GPa). Here we show that the generation and relief of compressive stresses in the Cr3C2 films can be controlled through the energy delivered during the growth. The effect of stresses generated by ion bombardment on the mechanical properties of the films was comprehensively studied utilizing ab-initio calculations and compared with experimental results. The hardness increment with compressive stress cannot be assigned only to changes in the electronic structure, but also due to specific microstructural features and chemical bonding of the films. The possibility to deposit the o-Cr3C2 protective films at relatively low temperature on temperature-sensitive substrates shows its potential for industrial applications.

Pulsed very high-frequency plasma-enhanced chemical vapor deposition of silicon films for low-temperature (120 °C) thin film transistors

With the aim of improving silicon (Si) film quality deposited by atmospheric-pressure (AP) plasma-enhanced chemical vapor deposition at a low substrate temperature of 120 °C, we examine the difference between continuous wave and pulsed plasmas for deposition characteristics depending on H2 and SiH4 flow rates and inpur VHF power. A specifically designed parallel-plate-type electrode system is used to generate the AP plasma, which is excited by 150 MHz very high-frequency (VHF) electric power. Hydrogen and monosilane diluted with helium are used as the process gases. The electrode system enables us to form turbulence-free one-dimensional gas flow in the plasma zone, thereby ensuring a deposition process without contamination of the substrate surface with dust particles. The electrical property of the deposited Si layers was evaluated by fabricating bottom-gate thin film transistors (TFTs). The performance of the bottom-gate TFTs revealed that the a-Si TFTs, i.e. those channel layers prepared with a pulse duty cycle of 10%, exhibit reasonably high field effect mobilities of around 1 cm2 · V–1 · s–1, suggesting that the pulse modulation of input VHF power is highly effective in achieving a plasma chemistry suitable for high-quality Si growths at low temperatures.

Effects of wall hydrophobicity on the thermohydraulic performance of the microchannels with nanofluids

Three-dimensional heat transfer of nanofluids in microchannels with hydrophilic, hydrophobic and, superhydrophobic surfaces were studied using numerical simulation. For this purpose, Navier Stokes and energy equations were solved using the finite volume method. The flow Reynolds numbers were in the range of 40–500. Water/Al2O3 and Water/TiO2 were considered as coolant fluids. By applying hydrophobic surfaces, the flow pressure drop decreased, especially for the case of superhydrophobic walls. Microchannel with hydrophobic walls required 12% less pumping power than those with hydrophilic walls. The superhydrophobic wall caused a 66% reduction in the required pumping power. The use of nanofluids as a coolant improved the thermal performance. The results showed that the simultaneous use of the hydrophobic surface and the addition of nanoparticles could increase the performance evaluation criterion to 1.75 at Re = 400 and caused the lowest substrate temperatures. In addition, for the flows with Re > 150, applying the superhydrophobic walls was more effective than the usage of nanofluids as coolants with conventional walls.

High-performance thermochromic VO2-based coatings with a low transition temperature deposited on glass by a scalable technique

We report on high-performance thermochromic ZrO2/V0.982W0.018O2/ZrO2 coatings with a low transition temperature prepared on glass by a low-temperature scalable deposition technique. The V0.982W0.018O2 layers were deposited by a controlled high-power impulse magnetron sputtering of V target, combined with a simultaneous pulsed DC magnetron sputtering of W target to reduce the transition temperature to 20–21 °C, at a low substrate surface temperature of 330 °C in an argon–oxygen gas mixture. ZrO2 antireflection layers both below and above the thermochromic V0.982W0.018O2 layers were deposited at a low substrate temperature (< 100 °C). A coating design utilizing a second-order interference in the ZrO2 layers was applied to increase both the luminous transmittance (Tlum) and the modulation of the solar transmittance (ΔTsol). The ZrO2/V0.982W0.018O2/ZrO2 coatings exhibit Tlum up to 60% at ΔTsol close to 6% for a V0.982W0.018O2 thickness of 45 nm, and Tlum up to 50% at ΔTsol above 10% for a V0.982W0.018O2 thickness of 69 nm.

Wetting Properties of Transparent Anatase/Rutile Mixed Phase Glancing Angle Magnetron Sputtered Nano-TiO2 Films.

Transparent polycrystalline TiO2 thin films have been deposited on unheated glass substrates using RF reactive magnetron sputtering. Depositions were carried out at different glancing angles and with different total gas mixture pressures. The variation of these parameters affected the crystal phase composition and the surface morphology. Depending on the glancing angle and the pressure, rutile, mixed anatase/ rutile and pure anatase were deposited at low substrate temperature. Both hydrophilic and hydrophobic TiO2 were obtained, exhibiting fast photoconversion to superhydrophilic upon UV irradiation. The effect of the materials physicochemical properties on the wettability and rate of the UV induced superhydrophilicity is evaluated.

Plasmonic nitridation of SiO2/Si(100) surface covered with gold nanoparticles via nitrogen plasma-produced radicals and light

In this work, the optical response of gold nanoparticles was used for radical-induced nitridation of a SiO2/Si(100) surface. High-quality SiON thin films were successfully formed via radicals and light from a low-temperature, low-pressure nitrogen inductively coupled plasma at low substrate temperatures &amp;lt;200 °C. The SiO2 surface was covered with gold nanoparticles with an average diameter of 5.4 nm and irradiated with light and nitrogen radicals produced using a remote plasma. The combination of light, gold nanoparticles, and radicals including low-energy ions resulted in a conversion of the Si–O bond to Si–N, forming a nitrogen-rich SiON film. The SiON thin film (equivalent oxide thickness of 3 nm) formed at a low temperature and had a small leakage current (3 × 10–5 A cm–2) that was comparable to a thermal oxide. It could be inferred that hot electrons supplied by surface plasmon resonance, which is unique to the gold nanoparticles, or photoemission by ultraviolet rays promoted the reaction between the nitrogen radicals and the substrate surface.

Influence of ZnO Cap Layer Morphology on the Electrical Properties and Thermal Stability of Al‐Doped ZnO Films

Herein, ZnO cap layers are prepared by chemical vapor deposition on Al‐doped ZnO (AZO) films and demonstrate a reduction in the electrical resistivity of the films. When prepared at 600 °C, a continuous ZnO cap layer is formed and leads to an increase in a Hall mobility from 22 to 37 cm2 V−1s−1, resulting in a resistivity of 5.1 × 10−4 Ω cm, which is superior to those of nanoparticle and nanorod morphologies formed at lower and higher substrate temperatures, respectively. Furthermore, the continuous ZnO cap layers successfully prevent decreases in the carrier concentration and Hall mobility during annealing at the temperatures of up to 600 °C in air, resulting in a figure of merit (FOM) of 1.6 × 10−2 Ω−1, which is approximately one order of magnitude better than those of uncapped films annealed in Ar. The improvement is due to the cap layer having proper morphology to provide sufficient protection for restructuring of the AZO grain boundaries, thereby reducing the defect density and sacrificing its structural order to suppress Zn desorption in AZO and environmental oxygen migration into AZO during annealing. Using ZnO as a cap layer also reduces the possibility of introducing unexpected band offset at the interface due to extrinsic elements.

Al-Doping of ZnO Thin Films Deposited by Spray Pyrolysis

Undoped and Al-doped zinc oxide (AZO) layers, greatly transparent and with high mobility, have been prepared at low substrate temperature using a chemical reactive spray technique. The effect of aluminum incorporation in the zinc oxide (ZnO) lattice has been characterized by means of X-ray powder diffraction, Raman spectroscopy, electrical and optical measurements. AZO layers reveal a hexagonal wurtzite structure whose lattice parameter decreases with increasing Al content and whose crystal quality decreases for Al content higher than 2%. Low resistivity AZO films have been found for 3% Al content. Additionally, AZO layers exhibit a blue shift of the optical gap with the increase of Al content that is attributed to the Burstein–Moss effect due to the increase of the charge carrier concentration. A density of free electron greater than 2.0 × 1019 cm–3 is obtained for the AZO thin films with 5% Al. Our results encourage the use of AZO films deposited at a low temperature as electrodes and optical windows in photovoltaic devices.

Effect on SmBa2Cu3Oy films of lattice strain induced by BaHfO3 nanorods

BaMO3 (BMO, M=Zr, Sn, Hf, etc.) is well known to undergo self-organisation into many nanorods in a REBCO film grown by vapour phase epitaxy. However, REBCO experiences strain resulting from the BMO nanorods and this strain causes the superconducting properties to deteriorate. In this study, we prepared SmBCO films containing 16 vol% BaHfO3 (BHO) using a pulsed laser deposition method. Deposition was accomplished by using a low-temperature growth (LTG) technique to grow the films even at low substrate temperatures. By decreasing the substrate temperature, the number density of the BHO nanorods, which is estimated from the critical current density in magnetic fields and the irreversibility line, increased and their diameter decreased. The lattice strain exerted upon SmBCO and BHO could conveniently be evaluated by conventional X-ray diffraction owing to the substantial BHO content. This evaluation indicated that, as the substrate temperature decreased, the tensile strain exerted upon SmBCO decreased, whereas the compressive strain experienced by BHO increased. These results indicate that the effect of stress exercised by the narrow nanorods on REBCO is small. Our findings suggest LTG to be an effective technique to prepare REBCO films with a high number density of BMO nanorods owing to the small amount of stress induced by these nanorods.