Intermediate Growth Temperatures Research Articles

Investigation of thermally induced surface composition and morphology variation of magnetron sputtered Sb2Se3 absorber layer for thin film solar cells

One of the most promising semiconductor materials for the development of sustainable thin-film solar cell technology is antimony selenide (Sb2Se3). Its excellent optical and electrical properties have drawn attention lately for potential application in thin-film solar cells. In this study, Sb2Se3 films deposited using the direct current (DC) magnetron sputtering technique have been subjected to a post-annealing process without an extra selenium supply at temperatures between 150 and 450 °C. Without an extra selenium supply, the impact of post-annealing temperature on the surface composition as well as the physical properties of the fabricated films was investigated. The overall evaluations revealed that the post-annealing temperature is highly efficient in altering the physical properties of the Sb2Se3 absorber thin films. We further observed that the post-annealing process improved the crystallization and the heat treatment temperature quite affected preferential orientation. The surface morphology of films exhibited structural deformation at high post-annealing temperatures (> 350 °C). According to optical and electrical characterizations, respectively, the optical energy gap and the resistivity of Sb2Se3 films reduced with an increment in the post-annealing temperature. Based on the XPS result, the variation in temperature of post-annealing led to a change in the surface composition of the films. The findings on the growth of Sb2Se3 thin films indicate the existence of an intermediate growth temperature that permits the growth of Sb2Se3 films to be optimized. The study’s conclusions can serve as a guide to the growth of Sb2Se3 thin films with the desired crystallinity, surface morphology, and composition for thin film solar cell applications.

Phase Diagram for Twinning Superlattice Te-Doped GaAs Nanowires.

Twinning superlattices (TSLs) are a growing class of semiconductor structures proposed as a means of phonon and optical engineering in nanowires (NWs). In this work, we examine TSL formation in Te-doped GaAs NWs grown by a self-assisted vapor-liquid-solid mechanism (with a Ga droplet as the seed particle), using selective-area molecular beam epitaxy. In these NWs, the TSL structure is comprised of alternating zincblende twins, whose formation is promoted by the introduction of Te dopants. Using transmission electron microscopy, we investigated the crystal structure of NWs across various growth conditions (V/III flux ratio, temperature), finding periodic TSLs only at the low V/III flux ratio of 0.5 and intermediate growth temperatures of 492 to 537 °C. These results are explained by a kinetic growth model based on the diffusion flux feeding the Ga droplet.

An insight into growth transition in AlN epitaxial films produced by metal-organic chemical vapour deposition at different growth temperatures

This work demonstrates a clear picture growth transition of aluminium nitride (AlN) films from the three-dimensional (3D) to the two-dimensional (2D) regime on the sapphire substrate at various temperatures using metal-organic chemical vapour deposition (MOCVD) under low reactor pressure. The high deposition rate of large 3D AlN islands that isolated each other change to 2D growth mode with a smoother surface as temperature increases from 800 °C to 1340 °C. From x-ray diffraction measurement , the AlN (100), AlN (002), and AlN (101) planes exhibit strong peak monocrystalline AlN (002) films as the temperature increase. It found that the AlN film grew at 1100 °C in the Frank–van der Merwe or 2D growth mode exhibits the highest crystalline quality with the threading dislocation density around 2.21 × 10 9 cm −2 . In addition, the lattice vibrational parameters of the AlN films at 1100 °C shows the lowest phonon damping from IR spectra results. Thus, this study details the AlN epitaxial films growth transition, which is crucial for growing high crystalline quality AlN layer using the MOCVD technique. • AlN thin films transition from 3D to 2D as the growth temperatures increase at lower reactor pressure. • The smoothest surface was observed at a higher growth rate but not at a higher growth temperature. • The intermediate growth temperature produced the high-quality AlN thin films under Frank–van der Merwe or 2D growth mode. • Theoretical modelling of the infrared reflection spectrum reveals a stressed layer between AlN film and sapphire substrate.

Comprehensive Analysis of Metal Modulated Epitaxial GaN.

While metal modulated epitaxy (MME) has been shown useful for hyperdoping, where hole concentrations 40 times higher than other techniques have been demonstrated, and the ability to control phase separation in immiscible III-nitrides, the complexity of the dynamically changing surface conditions during the cyclic growth is poorly understood. While MME is capable of superb crystal quality, performing MME in an improper growth regime can result in defective material. These complications have made the transfer of MME knowledge challenging. This work provides a comprehensive study of the conditions necessary for achieving the benefits of MME while avoiding undesirable defects. The effects of growth temperature, Ga/N ratio, and excess Ga dose per MME growth cycle on the morphological, structural, electronic, and optical properties of unintentionally doped (UID) MME grown gallium nitride (GaN) have been investigated. Optimal structural and electrical quality were achieved for GaN films grown at ∼650 °C, at pre-bilayer Ga coverage and at the moderate droplet regime. However, high defect concentrations were observed at the lowest growth temperatures, and counter to traditional MBE, as the excess Ga dose transitioned from bilayer coverage to the low droplet regime. Optoelectronic properties were optimal for films grown at intermediate growth temperatures, an excess Ga dose condition just before the droplet formation, and, at a III/V ratio of 1.3. Optimization of growth temperatures, Ga/N ratios, and excess Ga dose results in a range of growth conditions achieving smooth surfaces, step-flow surface morphology, and high crystalline quality films with low threading dislocation densities, allowing researchers to utilize the extensive advantages of MME.

The temperature optima for tree seedling photosynthesis and growth depend on water inputs.

Understanding how tree growth is affected by rising temperature is a key to predicting the fate of forests in future warmer climates. Increasing temperature has direct effects on plant physiology, but there are also indirect effects of increased water limitation because evaporative demand increases with temperature in many systems. In this study, we experimentally resolved the direct and indirect effects of temperature on the response of growth and photosynthesis of the widely distributed species Eucalyptus tereticornis. We grew E. tereticornis in an array of six growth temperatures from 18 to 35.5°C, spanning the climatic distribution of the species, with two watering treatments: (a) water inputs increasing with temperature to match plant demand at all temperatures (Wincr ), isolating the direct effect of temperature; and (b) water inputs constant for all temperatures, matching demand for coolest grown plants (Wconst ), such that water limitation increased with growth temperature. We found that constant water inputs resulted in a reduction of temperature optima for both photosynthesis and growth by ~3°C compared to increasing water inputs. Water limitation particularly reduced the total amount of leaf area displayed at Topt and intermediate growth temperatures. The reduction in photosynthesis could be attributed to lower leaf water potential and consequent stomatal closure. The reduction in growth was a result of decreased photosynthesis, reduced total leaf area display and a reduction in specific leaf area. Water availability had no effect on the response of stem and root respiration to warming, but we observed lower leaf respiration rates under constant water inputs compared to increasing water inputs at higher growth temperatures. Overall, this study demonstrates that the indirect effect of increasing water limitation strongly modifies the potential response of tree growth to rising global temperatures.

Electrical Characterization of Amorphous Silicon MIS-Based Structures for HIT Solar Cell Applications.

A complete electrical characterization of hydrogenated amorphous silicon layers (a-Si:H) deposited on crystalline silicon (c-Si) substrates by electron cyclotron resonance chemical vapor deposition (ECR-CVD) was carried out. These structures are of interest for photovoltaic applications. Different growth temperatures between 30 and 200 °C were used. A rapid thermal annealing in forming gas atmosphere at 200 °C during 10 min was applied after the metallization process. The evolution of interfacial state density with the deposition temperature indicates a better interface passivation at higher growth temperatures. However, in these cases, an important contribution of slow states is detected as well. Thus, using intermediate growth temperatures (100–150 °C) might be the best choice.

Study of post annealing effects on structural and optical properties of sol-gel derived ZnO thin films grown on n-Si substrate

Zinc oxide (ZnO) thin films were grown on n-type silicon (100) substrates by sol- gel spin coating technique. The prepared thin films were annealed in the presence of Ar at three different temperatures (at 450°C, 550°C and 650°C) to study the impact of annealing temperature on the structural and optical properties of the ZnO thin films. The structural, surface morphology and optical properties of the thin film were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM) and photoluminescence (PL) measurements respectively. The grown ZnO thin films are polycrystalline in nature with wurtzite hexagonal structure as evident from the XRD and SEM analyses. It further indicates that the crystalline size increases with increasing annealing temperature. The post annealing is also found to influence the optical properties in the terms of band gap energy of the ZnO thin films. The optical energy band gap was found to be decreased from 3.205 to 3.13eV as the annealing temperature is increased from 450°C to 650°C. However, our results concerning the growth of ZnO thin films on Si substrates suggest that there is an intermediate growth temperature allowing for the optimization of the ZnO film growth. The results of the study can be used as a guideline for growing ZnO thin films on n-Si substrates with a homogenous surface morphology, high surface to volume ratio and desired particle size, which are suited for optoelectronic/ gas sensing applications.

Comparison study of N‐ and In‐polar {0001} InN layers grown by MOVPE

AbstractWe compare InN layers grown directly on c ‐plane (0001) sapphire and on (0001) GaN templates using metal‐organic vapor phase epitaxy. InN grown on nitridated c ‐plane sapphire showed N‐polarity, while InN grown on c ‐plane GaN templates showed In‐polarity. N‐polar and In‐polar InN layers showed different surface morphology and crystallinity. N‐polar InN was smoothest when grown at higher growth temperatures, while In‐polar InN was smoothest at intermediate growth temperatures. Growth mode of the N‐polar InN on sapphire is Frank‐Van der Merwe, while growth mode of the In‐polar InN on GaN templates is Volmer‐Weber. Electrical properties were similar for both layers. Lowtemperature photoluminescence of In‐polar InN layers is shifted to higher energies most likely due to biaxial compressive stresses. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Study of intermediate states in shape transition of ZnO nanostructures from nanoparticles to nanorods

ZnO nanostructures synthesized by chemical solution method were observed to exhibit a shape transition from nanoparticle to nanorod when the growth temperature was varied from 130 to 200 °C. At the intermediate growth temperatures (150–170 °C), nanoparticles start getting attached to form nanorods and exhibit enhanced strain and defects which seems to be due to the presence of interface between attached nanoparticles at these growth temperatures. The difference between valence band maxima and Fermi energy level at intermediate growth temperatures get increases which has been attributed to the enhancement of defects.

The effects of Mn concentration on structural and magnetic properties of Ge1−xMnx diluted magnetic semiconductors

Reflexion high-energy electron diffraction (RHEED), transmission electron microscopy (TEM) along with physical property measurement system (PPMS) were used to investigate the growth kinetics of Ge1−xMnx diluted magnetic semiconductors (DMS) grown on Ge(001) by means of molecular beam epitaxy (MBE). At a given intermediate growth temperature of 130 °C, we have identified the formation of successive heterogeneous phases when increasing the Mn concentration from 1 to 14 %: DMS phase containing nanosized Mn-rich clusters for x below 2%, DMS phase containing high Curie temperature (TC) nanocolumns for x ranging from 5 to 6 %, DMS phase in which GeMn nanocolumns and Mn5Ge3 clusters coexist and then finally DMS containing mainly Mn5Ge3 clusters at Mn concentration higher than 12%. Our results confirm that the low solubility of Mn in Ge is the main origin of the formation of heterogeneous phases and provide evidence that it is extremely difficult to form a homogenous GeMn DMS even for Mn concentrations being below 2%. We also demonstrate that high-TC nanocolumns and Mn5Ge3 clusters are competing processes and the process window corresponding to the stabilisation of high-TC nanocolumns remains extremely tight.

Vanadium doping using VCl 4 source during the chloro-carbon epitaxial growth of 4H-SiC

Semi-insulating 4H-SiC epitaxial layers were produced by chloro-carbon epitaxial growth at an intermediate growth temperature of 1450 °C and at high growth temperatures of 1600 °C, enabling growth rates of 6 and in excess of 60 μm/h, respectively. Vanadium tetrachloride was used as the source of vanadium doping. In epitaxial layers otherwise dominated by nitrogen donors, vanadium-acceptor compensation mechanism was achieved, providing resistivities in excess of 10 5 Ω cm in fully compensated epilayers. Partial compensation enabled control of n-type doping in a wide range, down to around 10 14 cm −3 in epilayers with N 2 donor concentration of 1×10 15 cm −3. The domination of deep levels of vanadium was confirmed by photoluminescence spectroscopy. The limits for vanadium concentration consistent with degradation-free epilayer morphology were established to be around 1–1.5×10 17 and slightly less than 1×10 17 cm −3 for the growth at 1450 and 1600 °C, respectively.

Challenges of introducing quantitative elementary reactions in multiscale models of thin film deposition

The implementation of detailed surface kinetic mechanisms describing the thin film growth dynamics into models of chemical vapor deposition (CVD) reactors has been a challenge for many years. In this article we review the literature concerning the study of the dynamics of the Si(100)2 × 1 surface and introduce a multiscale model that captures the main features of its reactivity. The model combines the results of ab initio calculations with an atomistic description of the Si surface, obtained using a 3D-kinetic Monte Carlo (KMC) model that explicitly accounts for the 2 × 1 surface reconstruction and the formation and diffusion of Si dimers on a hydrogenated surface. At the atomistic scale, we determined pre-exponential factors and activation energies of hydrogen desorption reactions proceeding through the 2H, 3H, and 4H mechanisms. The calculated kinetic constants were embedded in the KMC model and used to simulate literature TPD experimental data. The simulations were used to fit the activation energies of hydrogen desorption reactions, which showed that DFT calculations performed with B3LYP functionals are likely to overestimate hydrogen desorption energies by up to 9 kcal mol−1, which was confirmed by successive ab initio calculations. Two examples of the solution of the KMC model in conjunction with a reactor scale model are provided, in which the coupling was performed adopting both a hierarchic and a two-way coupling strategy. We found that in the plasma deposition of nanocrystalline silicon performed at low substrate temperatures the growth proceeds through a layer-by-layer mechanism on a surface almost completely covered by hydrogen. The application of the same model to the simulation of the thermal CVD of Si showed that at intermediate growth temperatures, when the hydrogen surface concentration is high, a new hydrogen desorption mechanism, in which Si adatoms play an important role, is active. Length scales encountered in multiscale modeling of thin films deposition.

Effect of Nucleation and Growth Kinetics on the Electrical and Optical Properties of Undoped ZnO Films

Physical properties of ZnO thin films are of great importance for their potential application. In this paper, we reported the electrical and optical properties of undoped ZnO films with a high dependence on the nucleation and growth kinetics. These ZnO films were deposited on glass substrates within the temperature range of 250−350 °C by pulsed-spray evaporation chemical vapor deposition (PSE-CVD). X-ray diffraction (XRD) shows that ZnO films grow in the c-axis orientated hexagonal structure. The inspection of the surface morphology with scanning electron micrographs (SEM) shows a substantial effect of the growth temperature, which was directly correlated with the nucleation kinetics. A comprehensive nucleation and growth process was also proposed for the interpretation of changeable film morphology and corresponding electrical conductivity. The films deposited at intermediate growth temperatures of 310−340 °C show compact structures and consequent high electrical conductivity, which was still improved wi...

Transparent and low resistive nanostructured laser ablated tungsten oxide thin films by nitrogen doping: II. Substrate temperature

Nitrogen incorporated tungsten oxide thin films are deposited onto fused quartz substrates at various substrate temperatures (Ts) in nitrogen ambient (pN2) of 12 Pa by pulsed laser deposition. The structural, optical and electrical properties of the deposited films are found to depend on the substrate temperature/nitrogen doping concentration. Compositional analysis by energy dispersive x-ray spectra confirms the incorporation of nitrogen into the films, with maximum nitrogen incorporation for films deposited at Ts = 973 K. X-ray diffraction analysis reveals an orthorhombic crystalline phase for the WO3 : N films deposited at Ts = 300, 873 and 973 K, with a nanocrystalline structure for the films prepared at intermediate growth temperatures. Morphology investigation of WO3 : N films in relation to substrate temperature/nitrogen doping is done by scanning electron microscopy and atomic force microscopy. Vibrational properties of the WO3 : N films are measured using micro-Raman spectroscopy. Bandgap of WO3 : N films decreases from 3.31 ± 0.04 to 2.85 ± 0.03 eV and room temperature resistivity decreases from 1.77 × 103 to 4.3 × 10−2 Ω m with change in substrate temperature/nitrogen content. Analysis of optical and electrical properties reveals that the incorporated nitrogen acts as an electronic dopant in WO3. Narrow bandgap, low dc resistivity at room temperature and average transmittance in the visible range, observed for the films deposited at higher substrate temperatures (873 and 973 K), make very interesting prospects for technological transfer, especially as novel solar cell materials.

Surface activation pre-treatments for NCD films grown by HFCVD

The control of the nucleation density is essential for the production of ultra-thin, continuous and well adhered nanocrystalline diamond coatings. Surface pre-treatments such as abrasive scratching with diamond powder or bias enhanced nucleation are commonly used methods. In this work, surface activation by a pre-growth step is done using the hot-filament chemical vapour deposition (HFCVD) technique prior to seeding with diamond. The Si surface is modified during exposure to typical CVD diamond growth conditions. After ultrasonication with suspension of diamond in ethanol or n-hexane, an amorphous carbon layer is revealed at the surface of the Si substrates. The densest NCD films were obtained for the surface activation step done using a lower temperature and poorer methane mixture due to the improved seeding allowed by the amorphous carbon layer. Intermediate growth temperature of 750 °C resulted in high nucleation density while lower temperatures decrease it and higher ones produce a DLC nanostructured layer along with moderate NCD nucleation density.

Control of GaAs nanowire morphology and crystal structure

The morphology and crystal structure of Au-seeded GaAs nanowires (NWs) grownby molecular beam epitaxy were investigated as a function of the temperature,V/III flux ratio, and Ga flux. Low and intermediate growth temperatures of 400 and500 °C resulted in a strongly tapered morphology, with stacking faults occurring at an average rate of0.1 nm−1. NWs with uniform diameter and the occurrence of crystal defectsreduced by more than an order of magnitude were achieved at600 °C,a V/III flux ratio of 2.3, and a Ga impingement rate on the surface of0.07 nm s−1. Comparison of nanowire densities on the various post-growth surfaces suggests a possibleincubation time between the moment the Ga shutter is opened and when nanowire growthis initiated. Increasing the flux ratio favored uniform sidewall growth, making the processsuitable for the fabrication of core–shell structures.

Structure and magnetism of self-organizedGe1−xMnxnanocolumns onGe(001)

We report on the structural and magnetic properties of thin ${\mathrm{Ge}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}$ films grown by molecular beam epitaxy (MBE) on $\mathrm{Ge}(001)$ substrates at temperatures $({T}_{g})$ ranging from 80 to $200\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$, with average Mn content between 1% and 11%. Their crystalline structure, morphology, and composition have been investigated by transmission electron microscopy (TEM), electron energy loss spectroscopy, and x-ray diffraction. In the whole range of growth temperatures and Mn concentrations, we observed the formation of manganese-rich nanostructures embedded in a nearly pure germanium matrix. The growth temperature mostly determines the structural properties of Mn-rich nanostructures. For low growth temperatures (below $120\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$), we evidenced a two-dimensional spinodal decomposition resulting in the formation of vertical one-dimensional nanostructures (nanocolumns). Moreover, we show in this paper the influence of growth parameters (${T}_{g}$ and Mn content) on this decomposition, i.e., on the nanocolumn size and density. For temperatures higher than $180\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$, we observed the formation of ${\mathrm{Ge}}_{3}{\mathrm{Mn}}_{5}$ clusters. For intermediate growth temperatures, nanocolumns and nanoclusters coexist. Combining high-resolution TEM and superconducting quantum interference device magnetometry, we could evidence at least four different magnetic phases in ${\mathrm{Ge}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}$ films: (i) paramagnetic diluted Mn atoms in the germanium matrix, (ii) superparamagnetic and ferromagnetic low-${T}_{C}$ nanocolumns $(120\ensuremath{\le}{T}_{C}\ensuremath{\le}170\phantom{\rule{0.3em}{0ex}}\mathrm{K})$, (iii) high-${T}_{C}$ nanocolumns $({T}_{C}\ensuremath{\ge}400\phantom{\rule{0.3em}{0ex}}\mathrm{K})$, and (iv) ${\mathrm{Ge}}_{3}{\mathrm{Mn}}_{5}$ clusters.

Conductimetry and impedance spectroscopy study of low pressure metal organic chemical vapor deposition TiNxOy films as a function of the growth temperature: a percolation approach

Titanium oxinitride thin films have been grown by low pressure metal organic chemical vapor deposition (LP-MOCVD) using titanium isopropoxide, Ti(OCH(CH3)2)4 (TIP) and NH3 precursors in a growth temperature range from 450 to 750°C on sapphire substrates. The electrical behaviour of these films was studied between 400 and 173K, revealing three different behaviours, ranking from a hopping conductivity (450–500°C) to a conducting one (700–750°C), with a dual behaviour for the intermediate growth temperatures. Moreover, at room temperature, both conductimetry and impedance spectroscopy highlighted a percolation behaviour, interpreted in terms of continuum percolation. The effective media theory equations led to the usual percolation parameters (s, t, Φc) and the difference between the values thus obtained and the expected ones was explained in terms of anisotropic percolation occurring in the columnar film structure.

Influence of growth temperature on emission efficiency of InGaN/GaN multiple quantum wells

AbstractA comparative study, using time-resolved and CW photoluminescence spectroscopy, of MOVPE grown InGaN/GaN multiple quantum wells deposited on HVPE GaN/Sapphire at different growth temperatures was undertaken. It was found that the PL linewidth increased and the peak emission energy decreased as the growth temperature was reduced. Moreover, the sample grown at an intermediate growth temperature exhibited total integrated luminescence intensity much greater than the samples grown at higher or lower growth temperatures. A phenomenological carrier recombination dynamics model based on the competition of quantum well-like radative recombination, spatially localized radiative recombination in potential minima and non-radiative recombination through defects is presented to provide an explanation of the observed emission dynamics and efficiency. In this model, the emission efficiency is determined by the relative area of defects and the number density of localized states in the potential minima, both of which are influenced by the growth temperature. Furthermore, the photon energy dependent lifetimes are well fitted with this model by assuming a Gaussian shape localized states distribution. The localized potential minima are consistent with nanoscale indium rich regions due to indium aggregation.

Trigger factor and DnaK cooperate in folding of newly synthesized proteins.

The role of molecular chaperones in assisting the folding of newly synthesized proteins in the cytosol is poorly understood. In Escherichia coli, GroEL assists folding of only a minority of proteins and the Hsp70 homologue DnaK is not essential for protein folding or cell viability at intermediate growth temperatures. The major protein associated with nascent polypeptides is ribosome-bound trigger factor, which displays chaperone and prolyl isomerase activities in vitro. Here we show that delta tig::kan mutants lacking trigger factor have no defects in growth or protein folding. However, combined delta tig::kan and delta dnaK mutations cause synthetic lethality. Depletion of DnaK in the delta tig::kan mutant results in massive aggregation of cytosolic proteins. In delta tig::kan cells, an increased amount of newly synthesized proteins associated transiently with DnaK. These findings show in vivo activity for a ribosome-associated chaperone, trigger factor, in general protein folding, and functional cooperation of this protein with a cytosolic Hsp70. Trigger factor and DnaK cooperate to promote proper folding of a variety of E. coli proteins, but neither is essential for folding and viability at intermediate growth temperatures.