High Substrate Temperature Research Articles

Optimizing the Structural, Electrical and Thermoelectric Properties of Antimony Telluride Thin Films Deposited on Aluminum Nitride-coated Stainless Steel Foil

In this study, we examined the thermoelectric (TE) properties of co-evaporated p-type antimony telluride (Sb2Te3) thin films on aluminum nitride (AlN)-coated stainless steel foil substrates. We investigated the influence of composition and substrate temperature on the thin-film microstructure and transport properties, by varying the tellurium (Te) concentration in the thin films as well as the substrate temperature during deposition (room temperature (RT) and 300 °C). Thin films prepared with an RT substrate were further annealed at 264 °C to obtain crystallized thin films with high phase purity. Columnar thin films with large grains and a standard multi-oriented crystal structure were obtained when thin films were deposited on substrates heated to 300 °C. Thin films deposited at RT and subsequently annealed at 264 °C had a dense, layered microstructure, with a preferential c-axis or (00 l) texture as the compositions approached phase stoichiometry. The temperature dependence of the thermoelectric properties was measured, and variations were interpreted in terms of the deviation from stoichiometry and the obtained microstructure. A maximum power factor (PF) of 0.87 mW/m ∙ K2 was obtained for off-stoichiometric 65.0 at% Te thin film, which was the highest among the samples deposited at high substrate temperatures. A higher PF of 1.0 mW/m ∙ K2 was found for off-stoichiometric thin films with 64.5 at% Te, which was deposited at RT and subsequently annealed. The improvement of thermoelectric power in films containing excess Te could be related to energy dependent carrier scattering at the Sb2Te3/Te interface.

The effect of Mn2Sb2 and Mn2Sb secondary phases on magnetism in (GaMn)Sb thin films.

In this work, a detailed study of structural, electrical and magnetic characterization of (GaMn)Sb diluted magnetic semiconductors (DMS) is presented. (GaMn)Sb thin films were grown by DC magnetron co-sputtering method as an innovative procedure to fabricate III-V DMS. The presence of unusual Mn2Sb2 and Mn2Sb secondary phases, induced by substrate temperature and deposition time, were revealed through XRD measurements. Magnetization measurements allow determining crossover between a paramagnetic-like to a ferromagnetic-like behavior controlled by secondary phases. It was found that both, the magnetic remanence and magnetic coercivity, increases with substrate temperature. Interestingly, the magnetic response is paramagnetic at lower deposition times and substrate temperatures, and XRD measurements suggest the absence of Mn2Sb and Mn2Sb2 in secondary phases. For longer deposition times or higher substrate temperature, XRD shows the presence of Mn2Sb2 and Mn2Sb phases and ferromagnetic-like behavior. The DC resistivity of our samples was characterized and the carrier density was determined by Hall measurements and, in contrast with the reported in other studies, found them to be a p-type semiconductor with carrier densities as big as one order of magnitude larger than reported values. From the ferromagnetic-like samples, evidence of an anomalous Hall-effect in the sample was found, with higher magnetic saturation and a anomalous Hall conductivity of 2380 S/cm. All the results point to a contribution of the secondary phases to the overall magnetic response of the samples used, and suggest the importance of studying the formation of secondary phases in the growth of DMS, especially, for the case of (GaMn)Sb where Mn ion can have multiple oxidation states.

Study on thermoelectric property optimization of mixed-phase bismuth telluride thin films deposited by co-evaporation process

The development of Bi2Te3 thin films has huge potential in the pursuit of efficient thermoelectric micro/nanodevices due to their high Seebeck coefficient, high electrical conductivity and low thermal conductivity. The optimization of experimental parameters of Bi-Te thin films produced by co-evaporation will be investigated in this study. Co-evaporation is a low cost, easy-to-control process which can be used for high throughput and is scalable. We found that an optimal Te/Bi ratio of 1.5 with good thermoelectric properties can be directly synthesized by Bi and Bi2Te3 co-evaporation. Compared to the conventional Bi/Te co-evaporation process, high temperature annealing or substrate heating is not necessary for the process mentioned in this paper, which is a desirable feature when using polymer-based substrates, organic/inorganic hybrid thermoelectric generators, and flexible devices since they have relatively low tolerance to heat. The optimized Bi2Te3 thin films, which are mixed phases of Bi2Te3, Bi3Te4 and Te, possess high carrier concentration (6.65 × 1020 cm−3), low electrical resistivity (3.17 × 10−3 Ωcm), and extremely low thermal conductivity (0.59 W/mK) at room temperature on a smooth surface (roughness <5.5 nm) and are achieved by adjusting the deposition rate of Bi and Bi2Te3. The correlation between the structures of mixed phases, electrical and thermal properties will be discussed in detail.

Study on laser annealing of niobium films deposited on copper for RF superconducting cavities

Niobium sputtered copper cavities were proposed as a kind of promising next generation superconducting cavities, but were still challenged by the Q-slope effect under high acceleration gradients. Current solutions focus on improving the film quality to make it more bulk-like, for which a higher substrate temperature is required. However, due to the limitation of the melting point of the copper substrate, both the deposition process and the post-annealing process cannot be performed at a high temperature. The laser annealing mentioned in this paper uses nanosecond pulsed laser as the heat source, and the local temperature field generated within the thickness scale of the niobium film can anneal the film without affecting copper substrate. Laser annealing system has been set up in Peking University, and experiments with niobium thin film on copper (Nb/Cu) samples have been carried out. Superconducting performance, surface topography and other properties of Nb/Cu samples before and after annealing are compared. Recrystallization of niobium films happened and various factors that may cause Q-slope have been suppressed according to the results. All these indicate the effectiveness of laser annealing and the possibility of being used in niobium sputtered copper cavities in the future.

Effect of melting point of grain boundary material on nanostructure and magnetic properties of L10 type FePt granular media

The effect of melting point (Tm) of various grain boundary materials (GBMs) on the magnetic properties and nanostructure of FePt granular films was investigated. According to the analysis for the granular films with a constant GBM volume of 30 vol% deposited at a high substrate temperature, the saturation magnetization, Ms, had a strong correlation with Tm. We newly proposed a degree of order evaluation from in-plane X-ray diffraction from the L10 FePt (110) and (220) planes. As the results, neither the degree of order nor the grain diameter had any significant correlation with the Tm of GBM. Based on these analyzes, it was found that an increase of the grain diameter increases the degree of order, which results in enhancement of the coercivity of the FePt granular films. And it was clarified that Ms strongly depends on the degree of phase separation related to the melting point of GBM.

Fabrication and growth of c-axis textured Nd2Fe14B thin films by high-rate sputtering

To apply Nd–Fe–B thin films for mass-produced heat-assisted magnetic recording media, we investigated the high-rate sputtering conditions required to obtain c-axis textured Nd2Fe14B thin films and analyzed the growth mechanism. Magnetization curves indicated that higher substrate temperatures and sputtering rates resulted in a higher degree of perpendicular magnetic anisotropy; a Nd–Fe–B layer deposited at a substrate temperature (Tsub_0) of 600 °C and a sputtering rate (Rsp) of 2.6 nm/s had the easy axis perpendicular to the film plane. The dependence of the magnetic properties on the sputtering rate was due to a decrease in the substrate temperature during sputtering; there was a threshold for obtaining a high squareness ratio. X-ray diffraction analysis and transmission electron microscopy (TEM) images showed that the c-axis textured Nd2Fe14B crystal phase was formed in the Nd–Fe–B layer deposited at Tsub_0 = 600 °C and Rsp = 2.6 nm/s, which resulted in the highly perpendicular magnetic anisotropy. In addition, the TEM images showed a layer of Nd2Fe14B with no obvious lattice fringes near the interface between the Nd–Fe–B layer and Mo underlayer, while the lattice fringes of the Nd2Fe14B phase were not parallel to this interface but gently curved along the Mo cap layer. We propose that the c-axis orientation was achieved by the rotation of the c plane, which has the lowest surface energy in the Nd2Fe14B phase, toward the vacuum-side surface during sputtering.

Cold substrate method to prepare plasmonic Ag nanoparticle: deposition, characterization, application in solar cell

In this study, the surface plasmon effects of the Ag nanoparticle were investigated depending on the substrate temperature and coating time. Deposition procedure for the Ag coating was the vacuum deposition at low substrate temperature (< 300 K) instead of the commonly used the vacuum deposition at high substrate temperatures. The Ag thin films were deposited on n-type Si, glass and solar cell with safety glass substrates. The structural and optical characteristics of the Ag thin films prepared on Si and glass substrates were investigated. The Ag thin films had a polycrystalline structure with cubic phase. The (111) preferred orientation for 300 K substrate temperature was changed to (200) after 200 K substrate temperature. Homogeneous nano-sized Ag particles on Si were obtained at the 150–200 K temperature range. Optical measurements were performed for the Ag thin films prepared on glass substrates. According to reflectance measurements, plasmon resonance effect of the Ag nanoparticles was observed around 435–540 nm. The Ag nanoparticles prepared on solar cell at low substrate temperature increased the solar cell efficiency for all coating time because the nanoparticle size and shape were not changed significantly with the coating time. However, the Ag thin films prepared at high substrate temperature decreased device efficiency with the increasing coating time.

Understanding the thermal process during laser assisted ultra-high frequency induction deposition with wire feeding

This study proposes a novel metal deposition method referred to as laser assisted ultra-high frequency induction (UHF) deposition. In this method, the UHF induction heat is used as the main heat source to melt the deposited metal, and the laser heat acts as an auxiliary heat source that provides a high-temperature substrate surface for efficient fusion between the deposited metal and substrate. A numerical model coupled with electromagnetic and temperature fields is developed to understand the thermal process of laser assisted UHF induction deposition. The thermal process with different combination states of the two heat sources is numerically investigated to reveal the influence mechanism of the two heat sources on the penetration depth of the deposited layer. Results show that the UHF induction heat increases the penetration depth of the deposited layer by raising the temperature of the deposited metal, and laser heat leads to an increment in penetration depth by providing a high substrate surface temperature. Decreasing the distance between the laser beam and metal wire also increases the substrate surface temperature, thereby increasing the penetration depth. Criteria for characteristic temperatures Tpeak1, Tpeak2, and Tinterval are proposed based on thermal process analysis to control thermal process and prevent the deposited layer from having a large penetration depth. Deposition experiments reveal the process feasibility of the proposed deposition method and validate the numerical model. The performance evaluation of the deposited layers proves that controlling the thermal process is the key for ensuring the performance of the deposited layer. The numerical model and criteria for characteristic temperatures provide an efficient way for controlling the thermal process during deposition; thus, reasonable performance of the deposited layer can be ensured.

Incorporation of metal selenide thin films as the secondary absorber in the CdTe solar cells

The introduction of selenium in the CdTe solar cells has been responsible for high performance CdTe thin film solar cells in recent years. It is an approach to form CdSexTe1-x alloys by the interdiffusion using precursor CdSe layer during the CdTe high-temperature deposition process. Nevertheless, the compositionally gradient CdSexTe1-x formed by the diffusion makes it difficult to absorb long-wavelength photons adequately. So close-spaced sublimation deposited CdSexTe1-x interlayer was incorporated in the CdTe thin film solar cells. It is found that 600 nm CdSexTe1-x absorber is useful to increase long-wavelength photons absorption and extend the long-wavelength QE response. Meanwhile, the synergetic effects of the primary absorber deposition processing on CdTe solar cell performance were investigated. The substrate temperature of CdTe deposition has an obvious impact on the cell efficiency. The much higher substrate temperature can efficiently gain larger grain size, increase the crystal quality and promote the interdiffusion of different semiconductor layers. These improvements can efficiently decrease the carrier recombination to obtain a much higher fill factor and open-circuit voltage.

Dual-doped CdSe:Cu:O films grown by sputtering using CdSe–CuO composite targets

The effects of the simultaneous incorporation of Cu and O in CdSe films grown by sputtering are presented. The Cu and O contents varied between 1 and 5 at.% in films deposited at 150, 200, 250 and 300 °C. Concentrations of 2, 3, 4 and 5 at.% of CuO in the target promoted the formation of copper selenide clusters immersed within the CdSe:Cu:O host. Energy considerations (enthalpy of formation and bond dissociation energy) were used to discuss the absence of copper oxide and the formation of copper selenide aggregates, as well as the film thickness dependence on the concentration of CuO in the target. The band gap of the films ranged from 1.21 to 2.07 eV, depending upon growth conditions. Significant below-band-gap absorption was observed which was ascribed to the copper selenide micro and nano clusters. Good crystalline quality of the films, for high substrate temperatures, was evidenced through the appearance of overtones of the vibrational longitudinal optic modes detected by Raman micro spectroscopy. It was determined that the electronic properties, optical transmission and electrical conductivity depended on the chemical composition and crystalline structure. This characteristic is relevant because through copper and oxygen co-doping is possible to control these technologically important physical properties of CdSe in a simple and reliable manner.

Highly responsive tellurium-hyperdoped black silicon photodiode with single-crystalline and uniform surface microstructure.

Femtosecond laser hyperdoped silicon, also known as the black silicon (BS), has a large number of defects and damages, which results in unstable and undesirable optical and electronic properties in photonics platform and optoelectronic integrated circuits (OEICs). We propose a novel method that elevates the substrate temperature during the femtosecond laser irradiation and fabricates tellurium (Te) hyperdoped BS photodiodes with high responsivity and low dark current. At 700 K, uniform microstructures with single crystalline were formed in the hyperdoped layer. The velocity of cooling and resolidification is considered as an important role in the formation of a high-quality crystal after irradiation by the femtosecond laser. Because of the high crystallinity and the Te hyperdoping, a photodiode made from BS processed at 700 K has a maximum responsivity of 120.6 A/W at 1120 nm, which is far beyond the previously reported Te-doped silicon photodetectors. In particular, the responsivity of the BS photodiode at 1300 nm and 1550 nm is 43.9 mA/W and 56.8 mA/W with low noise, respectively, which is valuable for optical communication and interconnection. Our result proves that hyperdoping at a high substrate temperature has great potential for femtosecond-laser-induced semiconductor modification, especially for the fabrication of photodetectors in the silicon-based photonic integration circuits.

Optoelectronic properties and anisotropic stress of Mo:ZnO thin films deposited on flexible substrates by radio frequency magnetron sputtering.

This research investigated the optoelectronic properties and anisotropic stress of Mo-doped ZnO (MZO) films, which were deposited on polyethylene terephthalate and polycarbonate flexible substrates with radio frequency magnetron sputtering. The optical properties, x-ray diffraction (XRD) spectra, Hall effect measurements, and self-made phase-shift shadow moiré interferometer readings were utilized to evaluate the performances of the MZO films. Based on the results, the transmittance and (002) peak size of the XRD spectra decreased when the substrate temperature increased. However, this took place especially when the oxygen flow was on the increase. Also, carrier mobility, carrier concentration, and anisotropic stresses increased at higher substrate temperatures, but this was not the case when the oxygen flow increased. The energy gap (Eg) of the MZO films showed a blueshift with an increase in the substrate temperatures, but this rather changed to a redshift when the oxygen flow was observed to be on the rise.

The effects of substrate temperatures on the electrical properties of CaZrO3 thin films prepared by RF magnetron sputtering

The radio frequency magnetron sputtering technology (RFMS) was employed to deposit perovskite structure orthogonal phase CaZrO3 thin films on Pt/Ti/SiO2/Si substrates. The effects of substrate temperatures on structure and electrical properties of these films were investigated in detail. The CaZrO3 thin films were systematically characterized by means of X-ray diffraction (XRD), Scanning electron microscope (SEM), Multi-frequency LCR meter (HP4294A) and Radiant Precision Workstation to study the phase structure, cross-section morphology, dielectric and ferroelectric properties at different substrate temperatures. The result indicates that these films can withstand 80 V DC Bias voltage and have excellent stability of frequency, voltage and temperature. The CaZrO3 thin film prepared at 550 °C turned out to be mainly orthorhombic CaZrO3 phase with high permittivity, low dielectric loss, extremely low leakage current (at 1 MHz, the dielectric constant is 39.42, the dielectric loss is 0.00455, the quality factor is 220 and the leakage current density is 9.11 × 10−7A/cm2 at 80 V applied voltage.). This work demonstrates that higher substrate temperature can boost the formation of orthorhombic CaZrO3 phase and the CaZrO3 thin film prepared by RF magnetron sputtering is a very promising paraelectric material in the application of thin film capacitor.

Enlarged growth window for plasmonic silicon-doped InAs using a bismuth surfactant

Semiconductors such as InAs with high dopant concentrations have a variety of applications, including as components of mid-infrared optoelectronic devices. Unfortunately, growth of these materials by molecular beam epitaxy is challenging, requiring high growth rates and low growth temperatures. We show that the use of a bismuth surfactant improves silicon incorporation into InAs while simultaneously reducing the optical scattering rate, increasing the carrier mobility, reducing surface roughness, and enabling growth at higher substrate temperatures and slower growth rates. We explain our findings using microscopic theories of dopant segregation and defect formation in III-V materials.

Epitaxial Ge thin film Growth on Si Using a Cost-Effective Process in Simplified CVD Reactor

Germanium on silicon has the potential to enable growth of high quality group IV semiconductors for the field of silicon photonics by being used as virtual buffers. In addition Ge layers can be used in the fabrication of passive components such as waveguides and resonators. A cost-effective technology for the heteroepitaxial growth of Ge on Si will be highly beneficial. In this work, a simplified chemical vapor deposition reactor was assembled in-house to deposit Ge films. Epitaxial Ge film growth on Si (100) substrates was accomplished without requiring ultra-high vacuum conditions or a high temperature substrate pre-deposition bake thereby minimizing the economic and thermal budget of the process. Films were deposited using digermane (Ge2H6) precursor in a single step at a process temperature of 350 °C and chamber pressures of 1–10 Torr. Films were achieved at high chamber background pressures (>10−6 Torr) by implementing a thorough ex situ substrate cleaning process and optimizing times for wafer loading, chamber pumping and initiation of film deposition. Transmission electron microscopy and X-ray diffraction were used to confirm the epitaxial growth and crystalline quality of the obtained films. Optical properties of the films were characterized using Raman spectroscopy, Photoluminescence and Infra-red spectroscopy.

Complex Mare Deposits Revealed by CE-2 CELMS Data in Mare Nubium

The study of mare basalts in microwave range will be of great significance to better understand the basaltic units with complex surface contaminations, particularly in Mare Nubium. In this article, the China Chang'E-2 Lunar Microwave Sounder data and the generated normalized brightness temperature (nTB) and brightness temperature difference (dTB) maps are employed to evaluate the basaltic units in Mare Nubium. The results are as follows. First, two regions with hot anomaly are found, which is likely brought by the high substrate temperature. The anomaly in central Nubium is extensive in range and high in intensity. Second, the nTB and dTB performances can pursue the mare deposits contaminated by the impact ejecta, which reveals special mare basalt with the thin layer in the southeastern part of Mare Nubium. Third, the nTB and dTB maps indicate the complexity of the mare deposits in Mare Nubium. Fourth, the cold dTB anomaly in the western part is proposed as the floor deposits produced during the formation of the Nubium basin. These unusual findings will be of fundamental significance to better understand the basaltic volcanism and the thermal evolution of the Moon.

Tuning the Microstructure of the Pt Layers Grown on Al &lt;sub&gt;2&lt;/sub&gt;O &lt;sub&gt;3&lt;/sub&gt; (0001) by Different Sputtering Methods

High-quality epitaxial Pt _lms are usually grown by molecular beam evaporation (MBE) techniques, where the deposited atoms reach the substrate with typical thermal energies. To obtain a good epitaxial growth, the substrate is kept at elevated temperatures ranging between few hundred to thousand degrees. While the epitaxial quality improves at higher substrate temperatures, above a critical temperature Volmer-Weber growth mode starts and causes a rough _lm morphology. Here, we use a new type of facing target cathode (FTC) to grow Pt onto Al2O3 (0001) substrates. In contrast to conventional sputtering sources, FTC sources provide adatoms with lower kinetic energies, but higher growth rates compared to MBE. In this study, the crystal structure of Pt films is compared for different substrate temperatures. Using FTC, at Pt films with low strain and a morphology that is either nanocrystalline or highly epitaxial could be grown through proper choice of substrate temperature.

Reaction mechanism of the Me3AuPMe3-H2 plasma-enhanced ALD process.

The reaction mechanism of the recently reported Me3AuPMe3-H2 plasma gold ALD process was investigated using in situ characterization techniques in a pump-type ALD system. In situ RAIRS and in vacuo XPS measurements confirm that the CH3 and PMe3 ligands remain on the gold surface after chemisorption of the precursor, causing self-limiting adsorption. Remaining surface groups are removed by the H2 plasma in the form of CH4 and likely as PHxMey groups, allowing chemisorption of new precursor molecules during the next exposure. The decomposition behaviour of the Me3AuPMe3 precursor on a Au surface is also presented and linked to the stability of the precursor ligands that govern the self-limiting growth during ALD. Desorption of the CH3 ligands occurs at all substrate temperatures during evacuation to high vacuum, occurring faster at higher temperatures. The PMe3 ligand is found to be less stable on a gold surface at higher substrate temperatures and is accompanied by an increase in precusor decomposition on a gold surface, indicating that the temperature dependent stability of the precursor ligands is an important factor to ensure self-limiting precursor adsorption during ALD. Remarkably, precursor decomposition does not occur on a SiO2 surface, in situ transmission absorption infrared experiments indicate that nucleation on a SiO2 surface occurs on Si-OH groups. Finally, we comment on the use of different co-reactants during PE-ALD of Au and we report on different PE-ALD growth with the reported O2 plasma and H2O process in pump-type versus flow-type ALD systems.

Structural and optical properties of Al-doped ZnO thin films produced by magnetron sputtering

Experimental and theoretical investigations of the structure and optical properties of Al-doped ZnO (AZO) thin films produced by magnetron sputtering under different values of electric current were conducted. The XRD results confirm the formation of the AZO thin films with hexagonal wurtzite structure, with preferential orientation along the crystallographic plane (002), direction c. The increment of electric current allowed an increase in average crystallite size. The FE-SEM and AFM images analyses of the AZO films revealed the occurrence of nucleation on the substrate surface that formed films with granular and rough structure. The higher substrate temperature caused by the higher value of electric current had influence on the grain size and thickness (ranging from 974 to 1500 nm) of the formed thin films. Due to the high absorption of free carriers, the optical transmittance of the AZO films was acceptable for the visible spectrum and limited to the near infrared region. The energy band gap values for both AZO films, measured from the optical transmission spectra, were ideal for semiconductor applications. The ab initio calculations using DFT and the method LSDA + U along with the correction of Hubbard were successfully applied to investigate the structural and optical effects. The band structures of the pure ZnO and ZnO:Al, calculated in this work, presented Eg values close to the experimental results. Therefore, these results imply that our methods are reliable and that the calculations are in accordance with the experimental results.

Experimental investigation of evaporation dynamic of sessile droplets in pure vapor environment with low pressures

In order to understand the evaporation dynamic of sessile droplet in pure vapor environment, this paper presented a series of experimental observations on the evolution of thermal patterns and the variation of evaporation rate during water and ethanol droplet evaporation at low pressures. The contact radius of droplet is fixed at 2.5 mm and the height varies from 0.4 to 1.6 mm. Results show that the surface temperature distributions for both water and ethanol are uniform at a high pressure ratio. When the pressure is reduced, a ring-like thermal pattern appears on the water droplet surface, while the gear-like pattern and the cellular flow happen on the ethanol droplet surface. Interestingly, the gear-like pattern is static when the substrate temperature is low, while it will oscillate at a high substrate temperature. Meanwhile, the cellular flow is affected by the droplet height. The evaporation is promoted by reducing the pressure. Moreover, due to the decrease of the evaporation surface area and the increase of surface temperature, the evaporation rate decreases first and then increases with the decrease of the droplet height.