Post-deposition Thermal Annealing Research Articles

Effect of thermal annealing on physical properties of vacuum evaporated In2S3 buffer layer for eco-friendly photovoltaic applications

The present communication reports the effect of thermal annealing on the physical properties of In2S3 thin films for eco-friendly buffer layer photovoltaic applications. The thin films of thickness 150nm were deposited on glass and indium tin oxide (ITO) coated glass substrates employing thermal vacuum evaporation technique followed by post-deposition thermal annealing in air atmosphere within a low temperature range 150–450°C. These as-deposited and annealed films were subjected to the X-ray diffraction (XRD), UV–vis spectrophotometer, current–voltage tests and scanning electron microscopy (SEM) for structural, optical, electrical and surface morphological analysis respectively. The compositional analysis of as-deposited film is also carried out using energy dispersive spectroscopy (EDS). The XRD patterns reveal that the as-deposited and annealed films (≤300°C) have amorphous nature while films annealed at 450°C show tetragonal phase of β-In2S3 with preferred orientation (109) and polycrystalline in nature. The crystallographic parameters like lattice constant, inter-planner spacing, grain size, internal strain, dislocation density and number of crystallites per unit area are calculated for thermally annealed (450°C) thin films. The optical band gap was found in the range 2.84–3.04eV and observed to increase with annealing temperature. The current–voltage characteristics show that the as-deposited and annealed films exhibit linear ohmic behavior. The SEM studies show that the as-deposited and annealed films are uniform, homogeneous and free from crystal defects and voids. The grains in the thin films are similar in size and densely packed and observed to increase with thermal annealing. The experimental results reveal that the thermal annealing play significant role in the structural, optical, electrical and morphological properties of deposited In2S3 thin films and may be used as cadmium-free eco-friendly buffer layer for thin films solar cells applications.

Physical properties of chalcogenide Sn–Bi–S graded thin films annealed in argon

The development of cost-effective and non-toxic thin film materials is vital for fabrication of solar cells. We are presenting a combinatorial synthesis approach (CSA) for the deposition of chalcogenide Sn–Bi–S graded thin films by thermal evaporation. Post-deposition thermal annealing in the temperature range of 200–500 °C in an argon atmosphere has been carried out for the Sn–Bi–S thin films. The effect of annealing treatment and initial composition on the structural properties of the Sn–Bi–S graded thin films was studied by using energy-dispersive X-ray spectroscopy, X-ray diffraction (XRD), and Raman spectroscopy. XRD measurements showed that the thin films were grown in polycrystalline structure. Different microstructural parameters such as crystallite size, dislocation density, and microstrain were estimated after post-deposition thermal treatment and found annealing temperature dependent. From the transmission spectra the estimated optical band gap energy values were found in the range 1.27–1.43 eV for the (Sn/Bi) molar ratio of 2.18–0.67 in a typical sample annealed at 400 °C. Photoconductivity response was determined for incident light of wavelength 300–1100 nm and was observed to be annealing temperature and Sn/Bi molar ratio dependent. Photoconductivity was also noted to depend upon the Sn/Bi molar ratio with Sn-rich samples giving the strongest response. Sn-rich compositions also showed p-type conductivity over the temperature range of 350–400 °C. These findings show that the CSA has potential for the screening of high-quality Sn–Bi–S thin films.

Physical properties of vacuum evaporated CdTe thin films with post-deposition thermal annealing

This paper presents the physical properties of vacuum evaporated CdTe thin films with post-deposition thermal annealing. The thin films of thickness 500nm were grown on glass and indium tin oxide (ITO) coated glass substrates employing thermal vacuum evaporation technique followed by post-deposition thermal annealing at temperature 450°C. These films were subjected to the X-ray diffraction (XRD),UV-Vis spectrophotometer, source meter and atomic force microscopy (AFM) for structural, optical, electrical and surface morphological analysis respectively. The X-ray diffraction patterns reveal that the films have zinc-blende structure of single cubic phase with preferred orientation (111) and polycrystalline in nature. The crystallographic and optical parameters are calculated and discussed in brief. The optical band gap is found to be 1.62eV and 1.52eV for as-grown and annealed films respectively. The I–V characteristics show that the conductivity is decreased for annealed thin films. The AFM studies reveal that the surface roughness is observed to be increased for thermally annealed films.

Effect of thermal annealing on the optical and electrical properties of boron doped a-SiOx:H for thin-film silicon solar cell applications

The p-type layer in a p-i-n thin-film solar cell plays a crucial role in determining efficiency. The requirements for p-type layer films are high optical band gap (Eg), narrow valence band tail to minimize optical absorption, high dark conductivity, and low activation energy to reduce the parasitic series resistance of the solar cell. We investigated the effects of temperature during film growth and post-deposition thermal annealing on the optical and electronic properties of p-type amorphous silicon oxide films (p-a-SiOx:H) for thin-film silicon solar cell applications. The activation energy of thermally annealed p-a-SiOx:H film prepared at low substrate temperature decreased from 0.72eV to 0.56eV with similar Eg. Our improvements are explained in the changed ratio of conjugation with the three- and four-fold coordinated boron atoms by the shift of the B (1s) X-ray photoelectron spectrum. Taking into account the reversible electrical change by thermal annealing while maintaining high optical properties, we propose necessary process-procedure conditions for obtaining high photovoltaic performance in thin-film-Si solar cells with high-quality p-a-SiOx:H. We carried out device modeling of p-i-n junction amorphous silicon solar cells by employing a thermal annealing effect on p-type a-SiOx:H layer, using an advanced semiconductor analysis simulator. Due to reduced Ea with high Eg of p-type a-SiOx:H layer after thermal annealing, the solar cell performance of the open circuit voltage, fill factor, and conversion efficiency improved by 11.1%, 60.42%, and 53.75%, respectively.

Variation of the nanostructural feature of nc-SiC:H thin films with post-deposition thermal annealing

Amorphous silicon carbide (a-SiC) thin films were prepared by plasma enhanced chemical vapor deposition. The films were post-deposition annealed in a N2-H2 atmosphere at temperatures ranging from 700 to 1300°C. As the annealing temperature was increased from 1000 to 1300°C, nanocrystalline silicon carbide (nc-SiC) formed and the mean crystallite size varied from~2 to~5nm. The thermal energy at high annealing temperatures broke the Si-H and C-H bonds, and rearranged the amorphous network to generate local crystalline states, resulting in the formation of nc-SiC. The photoluminescence (PL) peaks varied in the wavelength range of~425 to~470nm with annealing temperature. The optical band gap of the films has been associated to the maximum PL peak position and estimated to range from~2.92 to~2.64eV.

Influence of thermal annealing on microstructural, morphological, optical properties and surface electronic structure of copper oxide thin films

In this study, effect of the post-deposition thermal annealing on copper oxide thin films has been systemically investigated. The copper oxide thin films were chemically deposited on glass substrates by spin-coating. Samples were annealed in air at atmospheric pressure and at different temperatures ranging from 200 to 600°C. The microstructural, morphological, optical properties and surface electronic structure of the thin films have been studied by diagnostic techniques such as X-ray diffraction (XRD), Raman spectroscopy, ultraviolet–visible (UV–VIS) absorption spectroscopy, field emission scanning electron microscopy (FESEM), atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS). The thickness of the films was about 520 nm. Crystallinity and grain size was found to improve with annealing temperature. The optical bandgap of the samples was found to be in between 1.93 and 2.08 eV. Cupric oxide (CuO), cuprous oxide (Cu2O) and copper hydroxide (Cu(OH)2) phases were observed on the surface of as-deposited and 600 °C annealed thin films and relative concentrations of these three phases were found to depend on annealing temperature. A complete characterization reported herein allowed us to better understand the surface properties of copper oxide thin films which could then be used as active layers in optoelectronic devices such as solar cells and photodetectors.

High-Temperature Rubbing: A Versatile Method to Align π-Conjugated Polymers without Alignment Substrate

Mechanical rubbing of polymer films has been widely used in the liquid crystal display industry to prepare oriented alignment layers of polyimides. We show that this fast orientation method can be successfully applied to a large palette of different π-conjugated systems, i.e., p- and n-type semiconducting homopolymers and alternating copolymers. Transmission electron microscopy, grazing incidence X-ray diffraction and UV–vis absorption spectroscopy reveal that both, the temperature of the films during rubbing and the molecular weight of the polymer strongly influence the level of orientation. For polythiophenes and polyfluorenes, the dependence of the orientational order parameter on the rubbing temperature (Trub) was determined. A strong increase of alignment with Trub is explained by the progressive alignment of higher molecular weight fractions at higher Trub. The disordering of alkyl side chains allows the hairy-rod shaped macromolecules to disentangle and align during rubbing. In addition, for certain conjugated polymers, the in-plane orientation, crystallinity, and polymorphism of the rubbed films can be substantially improved/modified by postdeposition thermal or solvent vapor annealing. This high level of orientation results in highly anisotropic optical and electronic properties (UV–vis absorption, fluorescence, charge transport).

Thermal annealing effect on ZnO surface acoustic wave-based ultraviolet light sensors on glass substrates

Surface acoustic wave (SAW) based ultraviolet (UV) light sensors have a high sensitivity and have been extensively studied and explored for application. However, all of them were made of piezoelectric (PE) bulk materials or PE thin films on crystalline substrates such as Si and sapphire. This paper reports the fabrication of ZnO thin film SAW UV-light sensors on glass substrates and the effect of post-deposition thermal annealing on the sensing performance. It was found that annealing at temperatures higher than 300 °C can improve the properties of ZnO films and the sensing performance of the UV-sensors remarkably. When the ZnO film annealed at 400 °C was used for sensors, the UV light induced resonant frequency shift increased more than 20 times with the response speed reduced to less than 2.4 s, much better than those made on ZnO films with lower temperature annealing.

Phase Stabilization of Al:HfO2 Grown on InxGa1–xAs Substrates (x = 0, 0.15, 0.53) via Trimethylaluminum-Based Atomic Layer Deposition

Al:HfO2 is grown on III-V compound substrates by atomic layer deposition after an in situ trimethylaluminum-based preconditioning treatment of the III-V surface. After post-deposition rapid thermal annealing at 700 °C, the cubic/tetragonal crystalline phase is stabilized and the chemical composition of the stack is preserved. The observed structural evolution of Al:HfO2 on preconditioned III-V substrates shows that the in-diffusion of semiconductor species from the substrate through the oxide is inhibited. Al-induced stabilization of the Al:HfO2 crystal polymorphs up to 700 °C can be used as a permittivity booster methodology with possibly important implications in the stack scaling issues of high-mobility III-V based logic applications.

Optical phonons in nanostructured thin films composed by zincblende zinc selenide quantum dots in strong size-quantization regime: Competition between phonon confinement and strain-related effects

Raman scattering in combination with optical spectroscopy and structural studies by X-ray diffraction was employed to investigate the phonon confinement and strain-induced effects in 3D assemblies of variable-size zincblende ZnSe quantum dots close packed in thin film form. Nanostructured thin films were synthesized by colloidal chemical approach, while tuning of the nanocrystal size was enabled by post-deposition thermal annealing treatment. In-depth insights into the factors governing the observed trends of the position and half-width of the 1LO band as a function of the average QD size were gained. The overall shifts in the position of 1LO band were found to result from an intricate compromise between the influence of phonon confinement and lattice strain-induced effects. Both contributions were quantitatively and exactly modeled. Accurate assignments of the bands due to surface optical (SO) modes as well as of the theoretically forbidden transverse optical (TO) modes were provided, on the basis of reliable physical models (such as the dielectric continuum model of Ruppin and Englman). The size-dependence of the ratio of intensities of the TO and LO modes was studied and discussed as well. Relaxation time characterizing the phonon decay processes in as-deposited samples was found to be approximately 0.38ps, while upon post-deposition annealing already at 200°C it increases to about 0.50ps. Both of these values are, however, significantly smaller than those characteristic for a macrocrystalline ZnSe sample.

High-power pulsed plasma deposition of hematite photoanode for PEC water splitting

The fabrication of crystalline α-Fe2O3 hematite thin films by means of novel high power impulse magnetron sputtering (HiPIMS) and high power hollow cathode plasma jets system is reported. The coatings are based on low temperature pulsed-plasma reactive sputtering. These methods were compared with a more common method of medium frequency (MF) pulsed reactive DC magnetron sputtering. Although both high power methods yielded crystalline structure of the films already during the depositions, the films had to be thermally treated at elevated temperature in order to improve their physical (crystallinity) and electronic properties. The deposition methods used and the effect of the post deposition thermal annealing were judged on the basis of physical properties such as crystalline structure, optical absorption, surface topography, electronic properties, and electrical behavior. The functional properties were investigated under simulated photoelectrochemical water splitting conditions. Despite the revealed hematite phase of the as-deposited films, these were almost photoelectrochemically inactive. The annealing improved crystal structure of the deposited films and increased their dark conductivity. Furthermore the annealing initiated the diffusion of tin atoms from FTO (fluorine doped tin oxide) substrate into the film increasing its extrinsic conductivity. These improvements lead to higher photoefficiency.

Structure Analyses of Room Temperature Deposited AlOx Passivation Films for Crystalline Silicon Solar Cells

We studied the structure of ozone-based atomic layer deposited aluminium oxide (AlOx) films as a passivation layer for p-type crystalline silicon (c-Si) solar cells and focused on the differences in the structure by the production conditions of AlOx films. Carbon (C)-related groups such as methyl, hydroxyl, and carboxyl groups which originate from the aluminium source, trimethylaluminium, were only found in the AlOx film deposited at room temperature (RT-sample). By post-deposition thermal annealing (PDA), the C-related groups were desorbed from the film and a part of their space remained as voids. The C-related groups were not found in the films deposited at 200 or 300 °C (heated-samples) since they were desorbed during the deposition. Even though C-related groups did not exist in the both RT- and heated-samples after PDA, the structure of the AlOx film of the RT-sample was different from that of the heated-sample.

A model for low temperature interface passivation between amorphous and crystalline silicon

Excellent passivation of the crystalline surface is known to occur following post-deposition thermal annealing of intrinsic hydrogenated amorphous silicon thin-film layers deposited by plasma-enhanced chemical vapour deposition. The hydrogen primarily responsible for passivating dangling bonds at the crystalline silicon surface has often been singularly linked to a bulk diffusion mechanism within the thin-film layer. In this work, the origins and the mechanism by which hydrogen passivation occurs are more accurately identified by way of an interface-diffusion model, which operates independent of the a-Si:H bulk. This first-principles approach achieved good agreement with experimental results, describing a linear relationship between the average diffusion lengths and anneals temperature. Similarly, the time hydrogen spends between shallow-trap states is shown to decrease rapidly with increases in temperature circuitously related to probabilistic displacement distances. The interface reconfiguration model proposed in this work demonstrates the importance of interface states and identifies the misconception surrounding hydrogen passivation of the c-Si surface.

SiO2/TiO2 distributed Bragg reflector near 1.5 μm fabricated by e-beam evaporation

The authors report on the fabrication and characterization of SiO2/TiO2 distributed Bragg reflector (DBR) mirrors operating at the eye safe and optical communication wavelength window, λ = 1.5 μm. Our experimental results demonstrated that SiO2/TiO2 DBR mirrors with reflectivity exceeding 95% at λ = 1.5 μm can be achieved using e-beam evaporation in conjunction with postdeposition thermal annealing process in ambient air. It was found that the postdeposition annealing process transformed the crystal structure of the as-deposited TixOy to TiO2, leading to a significant reduction in optical absorption. Erbium doped III-nitride semiconductors incorporating DBR mirrors at 1.5 μm emission may open up many novel applications, including infrared emitters, optical amplifiers, and high power infrared lasers.

SrTiO<sub>3</sub> Thin Films Deposition Using Pulsed Laser Deposition Technique

Strontium titanate (SrTiO3) thin film has been deposited on Si (100) substrate using pulsed laser deposition technique. Film deposition was carried out at low temperature (150°C) by maintained the pressure at 10-4 Torr. Nanometer-thick SrTiO3 film on Si substrate was characterized using SEM, AFM, XRD, and Raman Spectroscopy. SEM and AFM images show that SrTiO3 film has growth on Si substrate uniformly. Raman and XRD spectroscopy also support the growth of SrTiO3 film on Si substrate. Furthermore, to investigate the effect of post-deposition thermal annealing, the samples were annealed up to 900°C. Thermal stability of SrTiO3/Si structure was studied by mean XRD spectra. The X-Ray Diffraction pattern indicates the crystallinity improvement through atomic arrangements during thermal annealing process.

SrTiO<sub>3</sub> Thin Films Deposition Using Pulsed Laser Deposition Technique

Strontium titanate (SrTiO3) thin film has been deposited on Si (100) substrate using pulsed laser deposition technique. Film deposition was carried out at low temperature (150°C) by maintained the pressure at 10-4 Torr. Nanometer-thick SrTiO3 film on Si substrate was characterized using SEM, AFM, XRD, and Raman Spectroscopy. SEM and AFM images show that SrTiO3 film has growth on Si substrate uniformly. Raman and XRD spectroscopy also support the growth of SrTiO3 film on Si substrate. Furthermore, to investigate the effect of post-deposition thermal annealing, the samples were annealed up to 900°C. Thermal stability of SrTiO3/Si structure was studied by mean XRD spectra. The X-Ray Diffraction pattern indicates the crystallinity improvement through atomic arrangements during thermal annealing process.

Optical investigations of the effect of solvent and thermal annealing on the optoelectronic properties of Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) films

Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is one of the most promising conducting polymers that can be used as transparent electrode or as buffer layer for organic electronic devices. However, when used as an electrode, its conductivity has to be optimized either by the addition of solvents or by post-deposition processing. In this work, we investigate the effect of the addition of the polar solvent dimethylsulfoxide (DMSO) to an aqueous PEDOT:PSS solution on its optical and electrical properties by the implementation of the Drude model for the analysis of the measured pseudo-dielectric function by Spectroscopic Ellipsometry from the near infrared to the visible–far ultraviolet spectral range. The results show that the addition of DMSO increases significantly the film conductivity, which reaches a maximum value at an optimum DMSO concentration as it has confirmed by experimentally measured conductivity values. The post-deposition thermal annealing has been found to have a smaller effect on the film conductivity.

Enhanced charge storage characteristics of nickel nanocrystals embedded flash memory structures

Nickel nanocrystals (NCs) embedded flash memory structure with SiO2 tunnelling and pulsed laser deposition grown Al2O3 blocking oxide is reported here. The post-deposition thermal annealing of an ultrathin (∼5 nm) nickel film evaporated on thermally grown SiO2 tunnelling barrier has been performed to form the nickel NCs. The formation of tiny nickel NCs is confirmed from the high-resolution transmission electron microscope micrographs. The electrical capacitance–voltage (C–V) characteristics of the optimised 1000°C, 5 min annealed sample shows a large memory window of 20 V at ±20 V sweeping voltage. The charge storage properties are found to be significantly improved as compared to control samples. The frequency dependent C–V measurements indicate the dominant charge trapping in Ni NCs, making the memory structure attractive for use in future nanoscale high-performance applications.

Electroluminescence Transition From Visible- to Ultraviolet-Dominant Mode in $\hbox{n-Mn}_{0.04}\hbox{Zn}_{0.96}\hbox{O/i-ZnGa}_{2}\hbox{O}_{4}\hbox{/} \hbox{n-GaN}$ Structure With Highly Ultraviolet Detection Performance

We report an n-Mn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.04</sub> Zn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.96</sub> O(MZO) /i-ZnGa <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> / n-GaN structure both as a light-emitting diode and as an ultraviolet (UV) photodetector at forward and reverse biases, respectively. The n-MZO films were prepared on n-GaN coated sapphire substrates, followed by postdeposition thermal annealing treatment at 700 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> C, and an i-ZnGa <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> interface layer was formed at the MZO/GaN interface after the annealing treatment. We found that the electroluminescence characteristics showed a transition from visible- to UV-dominant mode when the deposition temperature of the MZO film is from 100 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> C to 300 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> C. For the UV detection performance, the devices based on the low-temperature deposition (100 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> C) of the MZO film showed the lowest dark current and the biggest ratio of photocurrent to dark current with a selectivity detectivity for 365 nm around light.

Improvement in carrier transport properties by mild thermal annealing of PbS quantum dot solar cells

We studied the effect of post-deposition thermal annealing in the preparation of PbS quantum dot (QD) solar cells. We find an optimal annealing temperature that improves the power conversion efficiency by a factor of 1.5 for different sized QDs with bandgaps of 1.65 and 1.27 eV. We examined the onset of the photocurrent response and correlated that with domain grain growth and find that annealing the PbS QD array at 120 °C causes little change in the PbS QD size, bandgap, and open-circuit voltage and yet leads to an increase in the carrier transport as realized by an improved current response. We also find a decrease in the activation energy of a shallow trap, which also likely contributes to the improvement in the solar cell efficiency.