Film Growth Direction Research Articles

Optimizing thermoelectric transport through composite deposition of C-axis oriented interpolated multilayer bismuth films

Electron transport of low-dimensional materials is significantly influenced by both their orientation and grain size. Deposition energy can be used to tune grain size and orientation. Since the deposition energies of magnetron sputtering and thermal evaporation are different, multi-layer Bi films were prepared with different thicknesses of sputtering layer and evaporating layer. The c-axis oriented layer within a certain thickness can prevent the grain overgrowth along to the direction of film growth. This layer grown by thermal evaporation leads to the low-angle grain boundaries between grains. Carrier transport layer was introduced in this layer with low-angle grain boundaries. Importantly, it improves output power through increasing of ∆T.

Unravelling the development of preferred crystallographic orientation in dual-phase amorphous/nanocrystalline Zr-V thin films

Dual-phase amorphous/nanocrystalline alloys have been proven to present enhanced properties compared to their amorphous or nanocrystalline single-phase counterparts. These dual-phase materials are typically composed of nanocrystalline cores embedded in an amorphous matrix. In contrast, a new kind of dual-phase metallic films has recently been reported in which cone-shaped crystalline regions evolve as a result of competitive growth between the amorphous and crystalline phases with increasing film thickness. Here, we demonstrate that, unlike typical dual-phase alloys, Zr-V thin films exhibiting crystalline/amorphous competitive growth develop a preferred crystallographic orientation upon growth. Relying on pole figure observations, we uncover the development of a preferred orientation of the (110) crystallographic planes in the growth direction of the film with increasing film thickness. High-resolution transmission electron microscopy analysis reveals that the crystalline regions are formed by nano-branches which grow (110)-oriented. Based on this result, together with scanning and transmission electron microscopy observations, we associate the development of the thickness-dependent preferred orientation to the impingement between cone-shaped crystalline regions. This process triggers a selective growth mechanism that favors the selection of nano-branches growing near the direction of film growth. Our results and analyses provide a step forward in understanding the growth kinetics of dual-phase alloys presenting complex microstructures.

Low temperature crystallization of atomic-layer-deposited SrTiO3 films with an extremely low equivalent oxide thickness of sub-0.4 nm

Despite SrTiO3(STO) possessing a high dielectric constant, its application as a capacitor dielectric in dynamic random-access memory(DRAM) capacitors faces challenges due to the high-temperature annealing for crystallization, its compositional inhomogeneity, and the high leakage currents of STO films. To address these issues, we employ atomic layer deposition(ALD) of STO films onto Pt substrates at elevated temperatures(340–380 °C). The use of low-reactivity Pt electrodes effectively mitigates the initial growth of excess Sr, ensuring enhanced compositional uniformity along the film growth direction. Coupled with ALD at high temperatures, this approach facilitates the crystallization of STO films in the as-grown state, further enhancing the crystallinity with increasing film thickness. Subsequent low-temperature post-deposition annealing (PDA) at 400 and 500 °C achieves full crystallization. This process results in a remarkable increase in the dielectric constant, reaching approximately 150. Furthermore, the absence of microcracks after PDA, attributed to the formation of adequately dense films, contributes to substantially improved dielectric properties. Consequently, these STO films exhibit an exceptionally low equivalent oxide thickness of 0.34 nm coupled with an ultralow leakage current of 3.7 × 10−8 A/cm2 at an operation voltage of 0.8 V, promising for advancing DRAM capacitors. This study presents a pathway for the sustainable scaling of DRAMs, addressing challenges in ALD-grown STO films.

Tantalum-doped tin oxide thin films using hollow cathode gas flow sputtering technology

SnO2 and tantalum doped SnO2 (TTO) thin films were prepared using reactive hollow cathode gas flow sputtering (GFS) on glass substrates. An in-situ heating process under vacuum preceded the sputtering. The resistivity of the tin oxide films was reduced to a remarkable low of 2.02 × 10−3 Ω cm, with a carrier concentration of 2.55 × 1020 cm−3 and a mobility of 12.11 cm2V−1s−1. As the substrate temperature increased, the film resistivity decreased. Notably, at a substrate temperature of 270 °C, the effect of Ta doping on the film resistivity and carrier concentration was significantly stronger compared to higher temperatures. Elevating the substrate temperature and Ta doping resulted in a lower refractive index (n). This effect was consistently strong at higher temperatures, attributed to the higher film-free carrier concentration (4.54 × 1020 cm−3) compared to lower temperatures (2.35 × 1020 cm−3). The film's structure was characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and atomic force microscope (AFM). The preferred direction of film growth was discussed. The successful and reproducible fabrication of tin oxide films underscores the advantages of gas flow sputtering (GFS) technology. GFS offers stable operating conditions across various oxygen flow levels without requiring target oxidization control, as is required in magnetron sputtering when managing gas status and film quality.

Five-fold twin structures in sputter-deposited nickel alloy films

The deposition of a Ni-1at. % P film resulted in the facilitation of several quintuple twin junctions that give rise to a five-fold twin structure along the film's growth direction. This type of grain growth promoted a pentagonal pyramid surface topology. Upon increasing the deposition rate or the addition of cobalt to reduce the stacking fault energy, an increase in the areal density of such features was observed. Correlated atomic contrast imaging by electron microscopy with atom probe tomography revealed the preference of phosphorus and residual oxygen Cottrell atmospheres around a dislocation network that would be required to accommodate the misorientation strain from a five-fold twin structure filling space. The ability to tune these structures, over a large area of material, offers grain boundary engineering opportunities to readily create novel surface topologies that can act, for example, as an array of field emitters or textured surfaces.

Photovoltaics literature survey (no. 182)

Photovoltaics literature survey (no. 182)

Control of perovskite film crystallization and growth direction to target homogeneous monolithic structures

Getting performant organo-metal halide perovskite films for various application remains challenging. Here, we show the behavior of solvent and perovskite elements for four different perovskites families and nine different initial precursor solution systems in the case of the most popular preparation process which includes an anti-solvent dripping-assisted spin coating of a precursor solution and a subsequent thermal annealing. We show how the initial solution composition affects, first, the film formed by spin coating and anti-solvent dripping and, second, the processes occurring upon thermal annealing, including crystal domain evolution and the grain growth mechanism. We propose a universal typology which distinguishes three types for the growth direction of perovskite crystals: downward (Type I), upward (Type II) and lateral (Type III). The latter results in large, monolithic grains and we show that this mode must be targeted for the preparation of efficient perovskite light absorber thin films of solar cells.

Effects of substrate bias on the sputtering of high density (111)-nanotwinned Cu films on SiC chips

This article presents a study of the influence of the substrate bias on the microstructure, preferred orientation, and mechanical and electrical properties of nanotwinned Cu film. The formation of a nanotwinned structure and (111) surface orientation can be properly controlled by applied substrate bias. High density nanotwinned structures were introduced into Cu films sputtered on SiC substrates with over 90% of (111)-orientation at − 150 V. Densely packed Cu nanotwins were observed within the columnar grains stacked up on each other along the film growth direction, with an average twin spacing of 19.4 nm. The Cu films deposited on SiC substrate via bias sputtering had surface roughness of 8.6 to 15.8 nm. The resistivity of the copper nanotwinned films sputtered with various substrate biases varied. The optimal indentation, 2.3 GPa, was found in the nanotwinned Cu film sputtered with a bias voltage of − 150 V. The effects of Ar ion bombardment on microstructure, surface morphology and properties are further discussed.

Synthesis and structure of refractory high entropy alloy thin films based on the MoNbTaW system

To explore structure and properties of refractory high entropy alloy (HEA) thin films, targets with five different equimolar compositions based on the MoNbTaW system and alloyed with Ti, V, Cr, Mn or Hf were used for the synthesis of films by high power impulse magnetron sputtering. All HEA films showed a body-centered cubic structure and a dense, columnar morphology as revealed by X-ray diffraction and transmission electron microscopy, respectively. Alloying of the additional element affects the film stress and the mechanical properties. The overall compressive stress state present in the films was distributed inhomogeneously with an expected gradient along the film growth direction. Hardness and Young's modulus values ranging from 14 to 17 GPa and 230 to 295 GPa, respectively, were measured by nanoindentation.

High strength and ductility in a heterostructured nanotwinned Ni film

We report the synthesis and mechanical behavior of two nanotwinned (NT) Ni films with different distributions of twin widths along the film growth direction. The NT Ni films, deposited on Si (111) substrates with an Ag buffer layer, are composed of two crystallographic variants with (111) out-of-plane orientation, which results in the formation of twin boundaries at their interfaces. The uniaxial stress-strain response of the freestanding NT Ni films was measured using MEMS tensile testing stages. While both films exhibited high strength, the film with a larger mean and broader distribution of twin widths exhibited significant strain hardening as well, which led to uniform elongation of ∼10%. These results suggest that by controlling the twin width distribution it is possible to realize a synergistic combination of strength and ductility in NT metal films.

Controlling the Formation Process of Methylammonium‐Free Halide Perovskite Films for a Homogeneous Incorporation of Alkali Metal Cations Beneficial to Solar Cell Performance

AbstractIncorporating multiple cations of the 1A alkali metal column of the periodic table (K+/Rb+/Cs+) to prepare perovskite films is promising for boosting photovoltaic properties but requires a uniform distribution. The effects of NH4Cl additives and alkali metal cations (K+/Rb+/Cs+) on the one‐step formation process of methylammonium‐free, formamidinium‐based, iodide perovskite films are analyzed in a step‐by‐step manner. NH4Cl improves the solubility of PbI2 in solution by forming an intermediate and then favors the perovskite phase formation. Moreover, during the annealing process, this additive is shown to increase grain size, to improve crystallinity and to suppress PbI2 formation. K at low concentration is always homogeneously distributed across the film thickness. On the other hand, Cs is more concentrated at the surface and Rb in the depths of pristine films. With NH4Cl additives, these two alkali metals are more homogeneously distributed because NH4Cl slows down the movement of Cs+ and Rb+, it changes the growth direction of the perovskite film, making the overall crystallization quality improved and the distribution more uniform. It results in perovskite films with large monolithic grains. Combined with a perovskite film surface treatment with n‐propylammonium iodide, a high stabilized power conversion efficiency of 22.04% is reached.

Study on the Performance Impact of Introducing an InN Buffer Layer at Various Deposition Temperatures on InN Film Grown by ECR-PEMOCVD on Free-Standing Diamond Substrate

In this study, InN films are grown at a relatively low temperature by electron cyclotron resonance plasma-enhanced metal organic chemical vapor deposition (ECR-PEMOCVD) on free-standing diamond substrates. Due to the high lattice mismatch rate between InN film and the free-standing diamond substrate, the function of a buffer layer is to build a bridge between the substrate and film to reduce the lattice mismatch between them. Therefore, here, we study the performance impact of introducing an InN buffer layer at various deposition temperatures and explore the optimal buffer layer deposition temperature used to grow relatively high-quality InN films. The experimental results show that when an InN buffer layer is introduced at a deposition temperature of 100 °C, the growth direction of the InN film is perpendicular to the substrate with a high c-axis preferred orientation, the roughness of the surface is minimal, and the particle sizes are consistent with growth in the same direction. Additionally, the carrier mobility is highest, and the carrier concentration is lowest compared with other conditions.

Variety of Ordered Patterns in Donor-Acceptor Polymer Semiconductor Films Crystallized from Solution.

A huge challenge is to control the nucleation of crystallites/aggregates in the solution during polymer film formation to generate desired structures. In this work, we investigate crystallization of P(NDI2OD-T2), a donor–acceptor polymer semiconductor, with controlled solution flow along the contact line between the drying film and solution through a seesaw-like pivoting of samples during polymer drying. By controlling the pivoting frequency/amplitude, various types of line patterns can be observed: (I) an array of fishbone-like stripes oriented in the film-growth direction; (II) the pinning–depinning of contact line (PDCL)-mechanism-defined patterned wires along the contact line; and (III) periodic twined crystalline line pattern oriented in the direction of the contact line. The rich variety of pattern formation observed is attributed to the distinctiveness of the donor–acceptor conjugated polymer structure. The result measured from thin-film transistors made of the generated films/structures showed that the charge mobility of P(NDI2OD-T2) does not change much with the film morphology, which supports recent controversy over the charge-transportation mechanism of some donor–acceptor polymer semiconductors.

Highly epitaxial YBa2Cu3O7−δ films grown on gradient La2−x Gd x Zr2O7-buffered NiW-RABiTS using all sol–gel process

The overall purpose of this work is to develop a reliable and low-cost technique for fabrication of coated conductors (CCs) on the flexible substrates, and to understand the effects of oriented growth and microstructure on the superconducting performance of CCs in-depth. The investigations of gradient La2−x Gd x Zr2O7 buffer architecture and YBa2Cu3O7−δ (YBCO) film on Ni-5at%W rolling-assisted biaxially textured substrate using all sol–gel process are reported. Combining x-ray diffraction and transmission electron microscopy (TEM) analysis, it is revealed that the gradient buffer architecture with increased lattice constant along the direction of film growth has an excellent c-axis orientation, while the inversed gradient architecture with decreased lattice constant shows the deteriorated orientation, indicating that the highly epitaxial growth mechanism most likely related to the interfacial lattice matching degree. The detailed analysis of YBCO films grown on gradient La2−x Gd x Zr2O7 under different deposition parameters, and the influence of microstructure on electrical properties are systematically discussed using x-ray diffraction, rocking curve test, scanning electron microscopy and TEM observations. Finally, the excellent biaxial texture YBCO film on gradient La2−x Gd x Zr2O7 buffer architecture is prepared at a proper heat-treatment temperature of 780 °C, and the full width at half maximum of ϕ-scan and ω-scan are 5.9° and 6.4°, respectively. The critical transition temperature T c,onset of the as-prepared YBCO film is 90 K. With elevating the temperatures, the appearance of a large number of weak-link and amorphous regions, being indicative of the microstructural collapse, is mainly responsible for the reduced superconductivity.

Влияние кислорода и паров воды на электрические свойства наногранулированных композитов (Co-=SUB=-40-=/SUB=-Fe-=SUB=-40-=/SUB=-B-=SUB=-20-=/SUB=-)-=SUB=-x-=/SUB=-(LiNbO-=SUB=-3-=/SUB=-)-=SUB=-100-x-=/SUB=-

Influence of oxygen and water vapor in the chamber during the deposition of thin-film nanocomposites (Co40Fe40B20)x(LiNbO3)100-x on the electrical properties of the heterogeneous system has been studied. A significant increase in the resistivity of (Co40Fe40B20)x(LiNbO3)100-x nanocomposites with an increase in the partial pressure of reactive gases (oxygen and water vapor) has been established. A significant shift of the percolation threshold towards higher metal phase concentration in the film plane and perpendicular to the film was found during the synthesis of composites with the addition of reactive gases, which is associated with an increase in the volume concentration of the dielectric phase. It is revealed that the percolation threshold when measured in the geometry perpendicular to the plane of the film corresponds to a significantly lower concentration of atoms of the Co40Fe40B20 alloy than in the case of measurements in the plane of the film, which is associated with the elongated shape of the metallic granules in the direction of film growth and the effects of Coulomb blockade suppression by a high transverse electric field.

Thickness-dependent electronic transport induced by in situ transformation of point defects in MBE-grown Bi2Te3 thin films

Interactions among various film growth parameters, such as the substrate temperature (Tsub), film thickness (d), and composition, play a crucial role in controlling the type and density of the intrinsic point defects. In turn, the point defects modulate and control electronic transport properties of Bi2Te3 films. We have grown n-type Bi2Te3 films with different d by molecular beam epitaxy at different Tsub. The formation of point defects was analyzed by a combined use of angle-resolved photoelectron spectroscopy (ARPES) and electronic transport measurements. Two important findings were made: (i) the negatively charged vacancies, VTe··, initially the dominant intrinsic defects, transform gradually during the growth process into positively charged anti-site defects, BiTe′, driven by thermal annealing from a continuously heated substrate; and (ii) from the film's surface into the inner strata of the film, the density of VTe·· decreases while the density of BiTe′ increases, leading to a gradient of vacancies and anti-site defects along the film growth direction. As a result, the electron density in Bi2Te3 films decreases monotonically with increasing d. Moreover, elevating Tsub leads to a more significant in situ annealing effect and an eventual onset of intrinsic excitations that deteriorates electronic transport properties. The thinnest Bi2Te3 film (16 nm) grown at Tsub = 245 °C has the highest electron concentration of 2.03 × 1020 cm−3 and also the maximum room temperature power factor of 1.6 mW m−1 K−2 of all grown epitaxial films. The new insights regarding the defect formation and transformation pave the way for further optimization of electronic transport properties of n-type Bi2Te3-based films.

Columnar structure growth of Mn-doped ZnO (MZO) thin films by radio frequency co-sputtering and studies on films properties

ABSTRACT The present investigation deals with the columnar structured growth of Mn doped ZnO (MZO) thin films by radio frequency sputtering technique. The X-ray diffraction and the processed electron diffraction pattern confirmed the hexagonal wurtzite structure and the film growth direction along (101) plane. The structure of grown film was compared with the pristine ZnO film and the JCPDS card No. 00-36-1451. The microstructures of the pristine ZnO and MZO films were studied by scanning electron microscopy and atomic force microscopy. The columnar structure growth of MZO was confirmed by transmission electron microscopy using TEM cross-section sample. The presence of constituent elements and their concentrations in the films were recorded by energy dispersive X-ray diffraction spectroscopy. The optical transmittance, absorption edge and band gap of the material were studied by UV-Visible transmission spectroscopy. The field dependent magnetization at room-temperature was analysed by using vibrating sample magnetometer and magneto-optic Kerr effect.

Synthesis of model Al-Al2O3 multilayer systems with monolayer oxide thickness control by circumventing native oxidation

A hybrid thin film deposition system was built by combining physical vapor deposition (PVD) with atomic layer deposition (ALD) without breaking high vacuum, which was achieved by a chamber-in-chamber design. The vacuum is better than 1 × 10−5 Pa during the entire sample transfer process between the ALD and PVD chambers. The Al-Al2O3 multilayers synthesized by the PVD-ALD integrated system consist of ALD Al2O3 sublayers between pure PVD Al sublayers. The Al2O3 sublayers, with incremental thickness of 1 nm from 1 to 10 nm, effectively interrupted the grain growth of the ~250 nm thick Al sublayers. Native surface oxide formation on the PVD deposited pure Al sublayers was circumvented by keeping the substrate in high vacuum between processing steps. The pure Al sublayers are constituted of equiaxed grains with no epitaxial or texture relationship with the neighboring layers. The Al grain boundaries are parallel to the film growth direction and extend the height of each Al sublayer, to form a brick-and-mortar type microstructure. In this system, ALD allows precise control of the thickness of each ultrathin Al2O3 layer, to thicknesses below the native surface oxide thickness of pure Al, and avoids further natural oxidation through the low-oxygen environment of the PVD-ALD system.

Architecting a Gradient Multilayered Structure to Concurrently Strengthen and Toughen Ti/Mo Nanoscale Multilayers

Most nanoscale metallic multilayers (NMMs) exhibit ultra-high hardness but limited toughness. To toughen Ti/Mo NMMs, a gradient multilayered (GM) structure was architected via manipulating individual layer thickness (h) gradient distribution along the film growth direction. The resulting GM structure significantly raises the toughness of Ti/Mo NMMs without reducing their peak hardness. High back stress developed from the plastic incompatibilities stemmed from aggravated microstructural heterogeneity is primarily responsible for the unprecedented synergy of hardness and toughness. Our findings provide a promising approach to concurrently strengthen and toughen NMMs.

Self-assembling behavior and interface structure in vertically aligned nanocomposite (Pr0.5Ba0.5MnO3)1-x:(CeO2)x films on (001) (La,Sr)(Al,Ta)O3 substrates

Heteroepitaxial oxide-based nanocomposite films possessing a variety of functional properties have attracted tremendous research interest. Here, self-assembled vertically aligned nanocomposite (Pr0.5Ba0.5MnO3)1-x:(CeO2)x (x = 0.2 and 0.5) films have been successfully grown on single-crystalline (001) (La,Sr)(Al,Ta)O3 substrates by the pulsed laser deposition technique. Self-assembling behavior of the nanocomposite films and atomic-scale interface structure between Pr0.5Ba0.5MnO3 matrix and CeO2 nanopillars have been investigated by advanced electron microscopy techniques. Two different orientation relationships, (001)[100]Pr0.5Ba0.5MnO3//(001)[1-10]CeO2 and (001)[100]Pr0.5Ba0.5MnO3//(110)[1-10]CeO2, form between Pr0.5Ba0.5MnO3 and CeO2 in the (Pr0.5Ba0.5MnO3)0.8:(CeO2)0.2 film along the film growth direction, which is essentially different from vertically aligned nanocomposite (Pr0.5Ba0.5MnO3)0.5:(CeO2)0.5 films having only (001)[100]Pr0.5Ba0.5MnO3//(001)[1-10]CeO2 orientation relationship. Both coherent and semi-coherent Pr0.5Ba0.5MnO3/CeO2 interface appear in the films. In contrast to semi-coherent interface with regular distribution of interfacial dislocations, interface reconstruction occurs at the coherent Pr0.5Ba0.5MnO3/CeO2 interface. Our findings indicate that epitaxial strain imposed by the concentration of CeO2 in the nanocomposite films affects the self-assembling behavior of the vertically aligned nanocomposite (Pr0.5Ba0.5MnO3)1-x:(CeO2)x films.