Low Substrate Temperature Research Articles

Microwave heating to form porous TiO2 layer on high-haze FTO film for dye-sensitized solar cell

A porous TiO2 layer was formed on a fluorine-doped tinoxide (FTO) film using a microwave heating technique, and applied to a dye-sensitized solar cell (DSSC). Under microwave irradiation, a self-heating process was induced in the FTO film to raise the substrate temperature up to 600 °C within a minute. After 12 min of microwave irradiation of the porous TiO2/FTO double layer, the internal resistances at the TiO2/FTO interface and the TiO2 interparticles were effectively reduced. By tuning the haze of the optical transmission through the FTO film, the photovoltaic performance was enhanced, and the conversion efficiency reached values as high as 7.2% at a haze of 31.5%, which was attributed to effective light harvesting within the cell. Unique ionic diffusion induced under microwave irradiation was detected at a lower substrate temperature, and the cell that formed at a substrate temperature of 350 °C exhibited a conversion efficiency of 6.5%, which is high for the sintering temperature. Although the photovoltaic performance was comparable to that of a general DSSC, a rapid sintering process that reduced the internal resistance within the cell was demonstrated using a microwave heating technique. Compared with a conventional electric furnace, the microwave heating technique can produce ceramics with high densities in a short time, and is an ideal tool for producing DSSCs.

High platinum utilization for proton exchange membrane fuel cells via low-temperature substrate sputtering on acid-treated carbon nanotube sheet

Low-temperature substrate sputtering on an acid-treated carbon nanotube (ATCNT) sheet result in high platinum (Pt) utilization. A thin catalyst layer (CL, ∼300 nm) with a high density of Pt and vacancies was formed on the ATCNT sheet (Pt-deposited ATCNT). It replaced the commercial carbon-supported Pt and microporous layers of the proton exchange membrane fuel cells. The substrate temperature (Tsub: 20, 5, −10, −30, and −50 ℃) contributed to crack formation in the porous Pt thin film and restrained the growth of Pt grain (average grain size from 8.60 to 3.97 nm). Below −10 ℃, the crack and grain size changed significantly. The increased crack and smaller grain size enlarged the Pt surface area while forming vacancies; this was confirmed via physical and electrochemical characterizations. When compared to Pt-deposited ATCNT (Tsub = 20 ℃, 47.61 m2 g-1Pt), the electrochemical surface area of Pt-deposited ATCNT (Tsub = -50 ℃, 72.06 m2 g-1Pt) increased by 51.35%. We present a high Pt mass-specific peak power density (5.93 kW g-1Pt), focusing on a strategy to increase Pt utilization.

Growing Cu2ZnSnS4 Thin Film at Low Temperature Using a New Improved Configuration of Close‐Distance CVD Technique

AbstractThis study presents a new possibility to deposit the polycrystalline Cu2ZnSnS4 (CZTS) thin film at low temperature using an improved close‐distance chemical vapor deposition reactor. Firstly, a theoretical study is conducted to predict the iodine pressure (PI2) and the substrate temperature (Ts) as the optimal conditions for the deposition of CZTS. The theoretical model is based on the minimization of the Gibbs energy of the Cu‐Zn‐Sn‐S‐I chemical system. An experimental study is realized, and the predicted optimal conditions to obtain quasi‐stoichiometric depositions are tested. The X‐ray diffraction, Raman spectroscopy, energy dispersive spectroscopy, scanning electron microscopy, and UV–vis–NIR spectroscopy are used to analyze the thin films elaborated at substrate temperatures of Ts = 400 and 550 °C for iodine pressure PI2 = 0.03 × 105 Pa. With the help of the thermodynamical calculations, the results show that it is possible to grow a good quality CZTS thin film at a low substrate temperature of 400 °C.

Direct Observation of Al Migration Enhancement by Changing the Al and N Source Relative Position in the Molecular Beam Epitaxy of AlGaN Nanowires

Aluminum (Al)-containing nitrides, including AlN, AlScN, and AlGaN, are emerging material platforms for future-generation photonic, electronic, ferroelectric, and multifunctional semiconductor devices. Nonetheless, the low Al migration mobility in a nitrogen environment is a notorious hindrance to obtaining high-quality Al-containing nitride epilayers. This is particularly the case for the epitaxy of Al-containing nitrides with radio frequency (RF) plasma-assisted molecular beam epitaxy (MBE), due to the use of relatively low substrate temperatures. As such, various approaches have been employed in promoting Al migration such as metal-modulated epitaxy (MME). Despite the progress, the effect of the relative position of the Al and N sources on the Al migration mobility has remained unclear. In this study, we show that MBE-grown AlGaN nanowires can be an effective probe to examine the effect of the Al and N source relative position on the Al migration mobility and demonstrate clearly that Al adatoms appear to be more mobile from the Al source that is far away from the N source, compared to those from the Al source that is near the N source. This study therefore can potentially impact the development of emerging Al-containing semiconductor devices by MBE. Moreover, the demonstrated correlated Al source dependent morphological and optical properties of AlGaN nanowires can naturally help the development of AlGaN-nanowire-based UV light-emitting diodes (LEDs) and lasers.

Effect of Substrate Temperature on Variations in the Structural and Optical Properties of Cu2O Thin Films Deposited via RF Magnetron Sputtering

In the present study, Cu2O films were deposited on a glass substrate via RF (radio frequency) magnetron sputtering under substrate temperature conditions that ranged from room temperature (RT, 25 °C) to 400 °C. The structural, compositional, and optical properties of the Cu2O films were analyzed in relation to the experimental variables by applying various measurement methods. The substrate temperature was a crucial factor in shaping the structural, compositional, and optical properties of the Cu2O films that were synthesized via RF-magnetron sputtering. Our findings revealed that the Cu2O films exhibited a cubic structure, which was confirmed by XRD analysis. Specifically, the (111) and (200) planes showed different trends with respect to the substrate temperature. The intensity of the (111) peak increased at 250 °C, and above 300 °C, the preferred orientation of the (111) plane was maintained. The grain size, which was determined via FE-SEM, displayed a positive correlation with the substrate temperature. Additionally, XPS analysis revealed that the binding energy (BE) of the Cu2O film sputtered at 400 °C was similar to that which was previously reported. Notably, the as-grown Cu2O film demonstrated the highest transmittance (15.9%) in the visible region, which decreased with increasing substrate temperature. Furthermore, the energy band gap (Eg) of the Cu2O films remained constant (2.51 eV) at low substrate temperatures (25 °C to 200 °C) but exhibited a slight increase at higher temperatures, reaching 2.57 eV at 400 °C.

Synthesis of Polycrystalline Diamond Films in Microwave Plasma at Ultrahigh Concentrations of Methane

Polycrystalline diamond (PCD) films are usually grown by chemical vapor deposition (CVD) in hydrogen–methane mixtures. The synthesis conditions determine the structure and quality of the grown material. Here, we report the complex effect of the microwave plasma CVD conditions on the morphology, growth rate and phase composition of the resulting PCD films. Specifically, we focus on the factors of (i) increased methane concentrations (νc) that are varied over a wide range of 4%–100% (i.e., pure methane gas) and (ii) substrate temperatures (Ts) varied between 700–1050 °C. Using scanning electron microscopy, X-ray diffraction and Raman spectroscopy, we show that diamond growth is possible even at ultrahigh methane concentrations, including νc = 100%, which requires relatively low synthesis temperatures of Ts < 800 °C. In general, lower substrate temperatures tend to facilitate the formation of higher-quality PCD films; however, this comes at the cost of lower growth rates. The growth rate of PCD coatings has a non-linear trend: for samples grown at Ts = 800 °C, the growth rate increases from 0.6 µm/h at νc = 4% to 3.4 µm/h at νc = 20% and then falls to 0.6 µm/h at νc = 100%. This research is a step toward control over the nature of the CVD-grown PCD material, which is essential for the precise and flexible production of diamond for various applications.

A combined spectroscopic investigation of cobalt oxide films prepared by DC reactive sputtering

The structure and vibrational characteristics of cobalt monoxide and tetraoxide films, prepared by direct-current DC reactive sputtering, were investigated by different characterization techniques. According to X-ray diffraction analyses, the development of the Co3O4 phase takes place under low discharge power and high substrate temperature (80 W-623 K) conditions, whereas the CoO phase predominates when adopting high discharge power and low substrate temperature (240 W-473 K). Results of infrared absorption and Raman scattering spectroscopies indicate the presence of Co3O4 in all films, that are compatible with the optical transmittance spectra in the UV-Vis-NIR range − showing electronic transitions due to Co2+/3+ cations (related to the Co3O4 spinel structure) in the films deposited at 80 W-623 K and, to a lesser extent, at 240 W-473 K conditions. The investigations include Raman scattering measurements in the 83–623 K range. The results evidence a mixture of phases in the 240 W-473 K film, whose main Raman scattering peaks evolve irreversibly towards Co3O4 upon increasing measurement temperatures. Altogether, the present report provides deeper information about the cobalt oxide phases that shall be useful to improve the control of properties and production of cobalt oxide films by DC reactive sputtering.

The impact of multi-layered dielectrics on the electrical performance of ZnO thin-film transistors

The electrical performance of ZnO thin film transistors (TFTs) fabricated on three different dielectric layers is investigated using an inverted staggered bottom gate thin film transistor structure. This work demonstrated the potential of integrating high-k dielectrics in ZnO TFTs in a dielectric multi-layered stack using SiO2 as the support layer for all the hero-dielectrics layers. In order to achieve this, ZnO thin film semiconducting layers were deposited at room temperature by radio frequency (RF) sputtering method and with no post deposition heat treatment, making them suitable candidates for low cost large area electronics on low temperature substrates (such as plastics). The dielectric films investigated were aluminium nitride (AlN), silicon nitride (SiN) and silicon dioxide (SiO2). The effective mobility of SiN devices was about half that observed in AlN devices. However, the dielectric layers exhibited hysteresis of approximately 0 V, 2 V and 7 V for the SiO2, AlN and SiN respectively. In addition, SiO2 TFTs have lowest threshold voltage (VT) and turn-on voltage (Von). The capacitance-voltage measurement for these device show that the flatband and threshold voltage of SiN and AlN MOS capacitors shifted towards more negative voltages instead when compared with the single layer SiO2 devices.

Low-temperature synthesis of crystalline vanadium oxide films using oxygen plasmas

Vanadium oxide (VOx) compounds feature various polymorphs, including V2O5 and VO2, with attractive temperature-tunable optical and electrical properties. However, to achieve the desired material property, high-temperature post-deposition annealing of as-grown VOx films is mostly needed, limiting its use for low-temperature compatible substrates and processes. Herein, we report on the low-temperature hollow-cathode plasma-enhanced atomic layer deposition (ALD) of crystalline vanadium oxide thin films using tetrakis(ethylmethylamido)vanadium and oxygen plasma as a precursor and coreactant, respectively. To extract the impact of the type of plasma source, VOx samples were also synthesized in an inductively coupled plasma-enhanced ALD reactor. Moreover, we have incorporated in situ Ar-plasma and ex situ thermal annealing to investigate the tunability of VOx structural properties. Our findings confirm that both plasma-ALD techniques were able to synthesize as-grown polycrystalline V2O5 films at 150 °C. Postdeposition thermal annealing converted the as-grown V2O5 films into different crystalline VOx states: V2O3, V4O9, and VO2. The last one, VO2 is particularly interesting as a phase-change material, and the metal-insulator transition around 70 °C has been confirmed using temperature-dependent x-ray diffraction and resistivity measurements.

Electrical and optical properties of nanocrystalline ZnSnN2

The electron density of ZnSnN2 prepared in many experiments is as high as 1019 cm−3 and it is challenging to prepare ZnSnN2 with electron density of 1016 cm−3. Here it is shown that nanocrystalline ZnSnN2 with electron density of 1016 cm−3 can be deposited with sputtering an alloy target at relatively lower sputtering power and substrate temperature. The structural, electrical and optical properties of nanocrystalline ZnSnN2 are studied. The effect of annealing on the properties of nanocrystalline ZnSnN2 with electron density of 1016 cm−3 is also investigated. The results show that the electron density of ZnSnN2 remains to be 1016 cm−3 after being annealed at 300 °C.

Low-threshold visible InP quantum dot and InGaP quantum well lasers grown by molecular beam epitaxy

III-V lasers based on self-assembled quantum dots (QDs) have attracted widespread interest due to their unique characteristics, including low threshold current density (Jth), low sensitivity to backreflections, and resistance to threading dislocations. While most work to date has focused on 1.3 μm InAs/GaAs QDs, InP QDs have also aroused interest in lasers emitting at visible wavelengths. Molecular beam epitaxy (MBE) enables the growth of high-density InP/AlGaInP QDs on exact (001)-oriented GaAs substrates but requires a relatively low substrate temperature of <500 °C. The low substrate temperature used for phosphide growth in MBE leads to degraded optical properties and makes post-growth annealing a crucial step to improve the optical quality. Here, we report the exceptional thermal stability of InP/AlGaInP QDs grown using MBE, with up to 50× improvement in room temperature photoluminescence intensity with the optimization of annealing temperature and time. We also demonstrate the room temperature pulsed operation of InP multiple quantum dot (MQD) lasers on GaAs (001) emitting close to 735 nm with Jth values of 499 A/cm2 after annealing, a factor of 6 lower than their as-grown counterparts and comparable to such devices grown by MOCVD. In0.6Ga0.4P single quantum well (SQW) lasers on GaAs (001) also exhibit a substantial reduction in Jth from 340 A/cm2 as-grown to 200 A/cm2 after annealing, emitting at 680 nm under pulsed operation conditions. This work shows that post-growth annealing is essential for realizing record-performance phosphide lasers on GaAs grown by MBE for applications in visible photonics.

Epitaxial growth and characterization of (001) [NiFe/M]20 (M = Cu, CuPt and Pt) superlattices

We present optimization of [(15Å) Ni80Fe20/(5Å) M]20 single crystal multilayers on (001) MgO substrates, with M being Cu, Cu50Pt50 and Pt. These superlattices were characterized by high resolution X-ray reflectivity (XRR) and diffraction (XRD) as well as polar mapping of important crystal planes. It is shown that cube on cube epitaxial relationship can be obtained when depositing at substrate temperature of 100 °C regardless of the lattice mismatch (5% and 14% for Cu and Pt, respectively). At lower substrate temperatures poly-crystalline multilayers were obtained while at higher substrate temperatures {111} planes appear at ∼10° off normal to the film plane. It is also shown that as the epitaxial strain increases, the easy magnetization axis rotates towards the direction that previously was assumed to be harder, i.e. from [110] to [100], and eventually further increase in the strain makes the magnetic hysteresis loops isotropic in the film plane. Higher epitaxial strain is also accompanied with increased coercivity values. Thus, the effect of epitaxial strain on the magnetocrystalline anisotropy is much larger than what was observed previously in similar, but polycrystalline samples with uniaxial anisotropy (Kateb et al. 2021).

High-performance printed electrode with rapid fabrication based on UV and IPL light processes without thermal treatment

We developed flexible and ultraviolet (UV)-curable electrodes with improved electrical conductivity using two light processes. Existing microparticle electrodes have poor durability. In this study, we improved electrode durability by facilitating UV light transmission using nanosilver particles and increased electrical conductivity using the photonic sintering process as a posttreatment process. The formulation of the UV-curable nanosilver paste developed this way had no problem in the cross-cut tape test and showed excellent pencil hardness (>3H). Furthermore, the electrical resistivity was 2.76 × 10−5 Ω·cm, and the resistance change rate was <1 % even after 50,000 times of repetitive tests with a 3-mm radius of curvature. When twelve electrode patterns with LED installation and bent were manufactured, we confirmed that there was no change in brightness. Finally, as polyethylene terephtalate, a low-temperature substrate, was not damaged even after the process was completed, the paste and process showed sufficient performance even in the low-temperature process.

High-quality thin film growth of the weak topological insulator BiSe on Si (111) substrates via pulsed laser deposition

In this work, we report the growth of high-quality BiSe thin films deposited on Si (111) substrates at different temperatures via pulsed laser deposition. We observe poor sample quality at a low substrate temperature (Tsub=175°C), and as the substrate temperature increases, the crystallinity of the samples increases. At a substrate temperature, Tsub=250°C, BiSe Raman modes (modes centered around 97.6 and 112.9 cm−1) start to emerge with less intensity and evolve with the increase in the substrate temperature and at Tsub=325°C closely match with that of single crystals. These modes correspond to the vibrations of Se-atoms from the Bi2Se3 quintuple layers and Bi-atoms from the Bi-bilayer. By carefully investigating the structural properties and the Raman modes of BiSe thin films at each substrate temperature, we provide an optimal condition to grow high-quality thin films of BiSe by pulsed laser deposition.

Low-temperature ion beam sputtered optical coatings.

In thin film deposition processes, the lower limit of the deposition temperature is determined by the used coating technology and the duration of the coating process and is usually higher than room temperature. Hence, the processing of thermally sensitive materials and the adjustability of thin film morphology are limited. In consequence, for factual low-temperature deposition processes, an active cooling of the substrate is required. The effect of low substrate temperature on thin film properties during ion beam sputtering was investigated. The S i O 2 and T a 2 O 5 films grown at 0°C show a trend of lower optical losses and higher laser induced damage threshold (LIDT) compared to 100°C.

Hydrothermal performance analysis of microchannel heat sink with embedded module with ribs and pin-fins

A new microchannel heat sink with embedded modules with ribs and pin-fins is proposed as an effective solution to realize microchannel heat dissipation within low-temperature co-fired ceramic substrates. Enhance heat dissipation with the following strategies: 1. microchannel cooling within the low-temperature co-fired ceramic substrate to shorten the heat transfer path; 2. embedding embedded modules in the substrate to reduce the overall thermal resistance; 3. creating ribs and pin-fins on the embedded modules to enhance convective heat transfer. The impacts of relative rib height, relative pin-fin height, and relative number of auxiliary channels on the heat transfer performance, temperature uniformity, and pressure drop were investigated. The performance of the proposed design has been compared to the performance of similar designs like straight rectangular microchannels, microchannels with pin-fins embedded modules, and microchannels with ribs embedded modules. Under the condition of Reynolds number 458, the heat transfer performance is improved by 63.91%, the temperature uniformity is improved by 67.65% and the pressure drop is increased by only 10.24%. The heat transfer performance of the microchannel heat sink with embedded modules with ribs and pin-fins can be effectively improved by adjusting the height of the pin-fins and the ribs height on the embedded module.

Thin-film (Al)BCN materials synthesized by sequential precursor pulses to mimic atomic layer deposition

This work brings novel insights into the existing knowledge on the deposition of films containing boron (B), carbon (C), nitrogen (N), and aluminum (Al). The (Al)BCN films are obtained at low substrate temperatures (TS) of 250–400 °C from triethylborane, ammonia (NH3), and trimethylaluminum. For BCN films, a nearly similar elemental composition of B0.42C0.41N0.15O0.02, with 1–2 at. % variations, is observed for substantial ranges of TS and NH3-exposure time. This can indicate a similar growth mechanism and/or formation of a single-phase material. While excluding precursor underdosing, a remarkable dependence of growth rate per cycle (GPC) on total gas pressure (Ptot) is observed. The GPC approaches near saturation regime for Ptot between 1 and 10 mbar, for TS = 330–375 °C, which might support the occurrence of a surface-adduct assisted pathway. The level of GPC saturation is influenced by TS. For a wide range of process conditions, N-share in the films slightly varies between 12 and 16 at. %. C-share only changes between 40 and 42 at. %. The attempt to increase N-share by dissociating NH3 into NH2 radicals by hot wire assistance remarkably shows the opposite effect, i.e., a decrease in the N-share from 15 to 6 at. %. This is accompanied by a corresponding increase in the B- and, in particular, C-shares, suggesting that the removal of carbon can occur via the incorporation of nitrogen. For AlBCN films, changing Ptot has a strong effect on their elemental composition. At Ptot = 10 mbar, Al-deficient films grow, whereas a Ptot of 0.2 mbar leads to mainly AlN-containing films with some inclusions of BN.

Epitaxial synthesis of unintentionally doped p-type SnO (001) via suboxide molecular beam epitaxy

By employing a mixed SnO2 + Sn source, we demonstrate suboxide molecular beam epitaxy (S-MBE) growth of phase-pure single-crystalline metastable SnO (001) thin films on Y-stabilized ZrO2 (001) substrates at a growth rate of ∼1.0 nm/min without the need for additional oxygen. These films grow epitaxially across a wide substrate temperature range from 150 to 450 °C. Hence, we present an alternative pathway to overcome the limitations of high Sn or SnO2 cell temperatures and narrow growth windows encountered in previous MBE growth of metastable SnO. In situ laser reflectometry and line-of-sight quadrupole mass spectrometry were used to investigate the rate of SnO desorption as a function of substrate temperature. While SnO ad-molecule desorption at TS = 450 °C was growth-rate limiting, the SnO films did not desorb at this temperature after growth in vacuum. The SnO (001) thin films are transparent and unintentionally p-type doped, with hole concentrations and mobilities in the range of 0.9–6.0 × 1018 cm−3 and 2.0–5.5 cm2 V−1 s−1, respectively. These p-type SnO films obtained at low substrate temperatures are promising for back-end-of-line (BEOL) compatible applications and for integration with n-type oxides in pn heterojunctions and field-effect transistors.

Micro-topographies formed by erosion can drive seedling emergence by rebuilding micro-habitats on weathered waste dumps in northeastern China

Restoring vegetation on waste dumps is imperative because soil erosion heavily impacts these areas and creates erosion micro-topographies, including bare slopes, rills, ephemeral gullies, and deposit bodies. These micro-topographies may affect seedling emergence by forming special micro-habitats, although the mechanism is vague. This study determined the substrate chemical (i.e., pH and electrical conductivity) and physical (i.e., capillary porosity, capillary water content, and mechanical composition) properties of different micro-topographies. In addition, the temporal dynamics of micro-climates (i.e., air temperature and air humidity), substrate hydrothermal conditions (i.e., substrate temperature and water content), and seedling emergence were investigated. Redundancy analysis (RDA), random forest (RF), and receiver operating characteristic (ROC) curve analysis were then used to identify the main factors affecting seedling emergence and clarify the relationships among the environmental conditions. Our results demonstrate that seedling densities in the rill, ephemeral gully, and deposit body were 1.78 times, 3.42 times, and 3.97 times higher than those on the bare slope, respectively. More species were found in the rill, ephemeral gully, and deposit body (Artemisia annua, Salsola collina, Setaria viridis, and Tribulus terrestris) than on the bare slope (Salsola collina). The main factors affecting seedling emergence were air humidity, substrate temperature, and substrate water content. The mechanical composition may have affected substrate water content during the initial stage of seedling emergence and substrate temperature during the entire period. We demonstrate that the ephemeral gully and deposit body may provide micro-habitats with a lower substrate temperature and higher substrate water content, which are favorable to seedling emergence, thus guiding vegetation restoration on waste dumps or other disturbed areas.

Structure and characteristic studies of ZrAlNiCu films prepared by magnetron sputtering

This study explores the influence of process parameters on the structural, mechanical, and electrical properties of ZrAlNiCu alloy and nitride film obtained by magnetron sputtering of a single Zr51Al11Ni4Cu34 target. Selected variables affecting the process are working pressure, substrate temperature, and nitrogen partial pressure. Almost all films are amorphous except for nitride films deposited at moderate nitrogen partial pressure, which consist of the FCC-ZrN nanophase. A low working pressure and high substrate temperature result in refined boundary interfaces, thus increasing their hardness and electrical conductivity. Nitride films have higher hardness and lower electrical conductivity than alloy films because of their strong ionic metal-N bonding. Thus, the formation of FCC-ZrN nanocrystals is conducive to the further improvement of hardness.