OVERVIEW
Dilute Magnetic Semiconductors (DMS)
Spintronics, or spin electronics, is an emerging field that explores the spin and charge states of electrons By integrating electron spin with charge, spintronic systems enhance the performance of current electronic devices These systems are more energy-efficient, offering greater memory capacity and computing power while occupying less space compared to traditional electronics.
Conventional magnetic storage devices utilize passive magnetoresistive sensors and memory elements made from ferromagnetic 3d metal alloy electrodes The advancement of enormous magnetoresistance in (Fe/Cr)n multilayers and tunneling magnetoresistance has significantly contributed to their development However, creating practical magnetic semiconductors is challenging due to their ferromagnetic properties typically occurring at low Curie temperatures, often below 100 K To address this issue, researchers have developed diluted magnetic semiconductors (DMS) by introducing a small percentage of magnetic atoms into the structure of non-magnetic semiconductor materials.
DMS, or semimagnetic semiconductors, are unique materials where a small percentage of magnetic elements replace certain cations in a host semiconductor These semimagnetic semiconductors exhibit distinct properties that set them apart from traditional magnetic semiconductors, primarily due to a controlled dilution of the magnetic component.
In dilute magnetic semiconductors (DMS), the strong interaction between charge carriers and local magnetic moments gives rise to unique physical properties When an external magnetic field is applied, magnetic ions can influence the semiconductor band state, altering the arrangement of spins among the magnetic ions and impacting the spins of band electrons This magnetic enhancement of ions in semiconductors leads to significant magneto-optical effects, making DMS materials particularly intriguing for various applications.
Figure 1.1 Schematic showing a magnetic semiconductor (A), a non-magnetic semiconductor (B), and a diluted magnetic semiconductor (C) [4]
Metal transition doped compound semiconductors, particularly II-VI semiconductors, are utilized to create dilute magnetic semiconductors (DMS) like InMnAs and GaMnAs The substitution of trivalent metal ions with divalent metal ions in III-V semiconductors leads to hole formation and induces ferromagnetic order GaMnAs has been extensively selected for various spin-based device applications, such as spin-polarized light emitters and spin field-effect transistors, which display magnetic behavior at temperatures below room temperature (TC = 173 K) Consequently, these semiconductor compounds primarily operate at low temperatures (around 100 K), rendering II-VI DMS impractical for many electronic applications.
Achieving room-temperature ferromagnetism (RTFM) in diluted magnetic semiconductors (DMS) is essential for their application in electronic devices Recent studies have reported RTFM in various DMS classes, including wide bandgap III-V semiconductors like GaN and GaP, as well as group IV semiconductors such as Ge and Si Ferromagnetism in these materials is carrier-mediated, enabling the modification of magnetic properties through charge manipulation This has positioned oxide-based DMS as crucial materials for developing advanced electronic devices Additionally, many oxide-based DMSs are large bandgap semiconductors (> 3 eV), offering optoelectronic potential for next-generation spintronic applications Notably, the ferromagnetic properties of Co-doped TiO2 were first demonstrated by Matsumoto et al., marking a significant advancement in the field.
[11, 12], have gotten a lot of attention Table 1.1 summarizes the magnetic moments and
TC values published in the literature for these DMS-based thin films Following that, numerous scientists discovered the ferromagnetic properties of ZnO doped with transition metals at ambient temperature [13, 14]
Table 1.1 High-temperature oxide-based DMS (adapted from ref [15])
Recent research has focused on promising metal oxides, particularly Copper (II) oxide (CuO), due to their unique properties CuO is a semiconductor material that shows potential for DMS fabrication research and is recognized as a high critical temperature superconductor, with its superconductivity linked to Cu-O bondings Consequently, investigating the magnetic characteristics of nanosized CuO materials is crucial for advancing this field.
Copper oxide (CuO) doped with transition metals such as cobalt (Co), iron (Fe), and nickel (Ni) can demonstrate ferromagnetic order with a Curie temperature (TC) exceeding room temperature Research by Li et al indicates that the ferromagnetic interactions among randomly distributed iron ions within the antiferromagnetic CuO lattice lead to materials exhibiting significant magnetic properties at elevated temperatures.
Research has shown that Fe-doped CuO matrix samples display ferromagnetic behaviors at room temperature, as noted by Layek and Verma Additionally, Dolai et al revealed that Ni-doped CuO thin films, created through spin-coating deposition, exhibit temperature-independent magnetization between 20 K and 330 K.
Thin film semiconductors
Thin film semiconductors have experienced significant growth in recent years, leading to the commercialization of diverse electronic products such as transistors and photovoltaic devices These semiconductor thin films, which range in thickness from a few nanometers to hundreds of micrometers, are utilized in various applications, with their structural and physicochemical properties heavily influenced by the manufacturing techniques employed Due to their versatile characteristics, semiconductor thin films present a promising material for the electronics industry By adjusting fabrication methods, substrates, and other manufacturing parameters, it is possible to effectively produce single or polycrystalline structures on substrates with intricate surface morphologies These films are categorized into single-crystalline, polycrystalline, or amorphous types.
Single-crystalline thin films are typically deposited on compatible single-crystalline substrates, ensuring that their properties mirror those of bulk single-crystal materials, albeit with some influence from surface characteristics These films play a crucial role in various optoelectronic applications, including heterojunction solar cells and light-emitting diodes.
Polycrystalline thin films differ from bulk materials due to their grain-oriented plane arrangement, featuring grains of various shapes and sizes These thin films have numerous applications in technology, including serving as gate electrodes in MOSFETs, components in thin-film transistors, and in solar cells.
7 cell devices The polycrystalline films have structural flaws including grain boundaries and randomly oriented crystals
Amorphous thin films exhibit a uniform atomic order, which distinguishes them from crystalline materials by lacking typical solid-state properties These unique characteristics make amorphous thin films valuable in various applications, including metallurgy and semiconductor products.
Cupric oxide thin films
Cupric oxide, or copper (II) oxide (CuO), is one of the two copper oxide phases, the other being cuprous oxide (Cu2O) Characterized by its monoclinic lattice symmetry, CuO features a copper atom centrally located within a rectangle, surrounded by four nearest oxygen atoms that form a deformed tetrahedron Detailed crystallographic parameters and interatomic distances to the nearest neighbors are provided in Table 1.2.
Table 1.2 Crystallographic parameters (from ref [32])
Shortest distances d Cu – O 1.95 Å d O – O 2.62 Å d Cu – Cu 2.90 Å
Copper oxide (CuO) is an antiferromagnetic semiconductor characterized by an open d shell (3d^9) It functions as a p-type semiconductor due to the formation of copper vacancies, which act as acceptors within the CuO lattice The electrical properties of pure CuO are significantly influenced by intrinsic defects, primarily copper or oxygen vacancies Notably, copper vacancies are the most common defects found in nonstoichiometric cupric oxide, attributable to the instability of copper.
35] The mobility variation for the estimated free carriers concentration and the hole is
The electrical properties of thin films are influenced by structural changes resulting from phase transitions, doping, and grain boundary expansion, leading to variations in charge carrier density and mobility, which directly affect resistivity Research has demonstrated that annealing temperature significantly impacts electrical conductivity; specifically, Saravanakannan et al found that increasing the annealing temperature from 523 K to 723 K resulted in decreased resistivity.
The optical properties of thin films play a crucial role in the development of optoelectronic devices, making them an essential parameter in fabrication Bulk cupric oxides (CuO) exhibit a direct narrow bandgap of 1.2 eV; however, the optical bandgap in CuO thin films can vary significantly based on deposition methods and factors influencing thin film quality.
The bandgap energy of CuO films typically ranges from 1.3 to 1.9 eV Factors influencing this energy include doping concentration, film thickness, grain size, and structural changes Research by Singh et al indicates that the bandgap energy of CuO films produced through the successive ionic layer adsorption and reaction (SILAR) method decreases with increasing film thickness.
CuO is an antiferromagnetic semiconductor with two Neel temperatures:
TN1 = 231 K and TN2 = 212 K [41] The magnetic structure of CuO is strongly antiferromagnetic, consisting of Cu–O parallel sheets
Copper oxide (CuO) has gained significant attention due to its unique surface chemistry, excellent recyclability, non-toxicity, high optical absorbance, and relatively low cost of raw materials To tailor CuO's properties for specific applications, various transition metals have been used as dopants in CuO nanostructures These transition metal ions facilitate electron excitation for photon absorption by creating new energy levels between the conduction and valence bands.
CuO structure has an impact on CuO solar cell efficiency [46] h
Overview of deposition techniques
Thin films have gained significant attention due to their remarkable features and applications across science, industry, and commerce This technology enables the creation of materials with nanoscale dimensions, making it ideal for the production of compact electronic and optoelectronic devices High-quality thin films are typically produced through two main deposition methods: physical and chemical deposition, each utilizing distinct types of precursors.
Table 1.3 Thin film deposition methods (adapted from ref [47])
1 Evaporation techniques a Vacuum thermal evaporation b Electron beam evaporation c Laser beam evaporation d Arc evaporation e Molecular beam epitaxy f Ion plating evaporation
4 Plating a Electroplating technique b Electroless deposition
5 Chemical vapor deposition (CVD) a Low pressure (LPCVD) b Plasma enhanced (PECVD) c Atomic layer deposition (ALD)
2 Sputtering techniques a Direct current sputtering
(DC sputtering) b Radio frequency sputtering
Doping transition metals, particularly nickel, into CuO thin films significantly influences their surface and physical properties, including structure and electro-optical characteristics Various fabrication techniques, such as sol-gel, successive ion layer adsorption and reaction (SILAR), and sputtering, have been employed to create Ni-doped CuO thin films.
Sputtering is a thin-film production technology utilized in a wide range of industries
A vacuum chamber is used to form the film A vacuum chamber is used to form the film
A voltage is applied between the cathode's target material and the anode's substrate, generating plasma with inert gases like argon or xenon These gases are preferred for sputtering because they do not react with the target material The process results in the formation of a thin film as ions bombard the anode or substrate, as illustrated in Figure 1.2.
The sputtering technique is categorized into two primary types: direct current (DC) and radio frequency (RF) DC sputtering is typically used for conductive materials like metals due to its ease of adjustment and low power consumption In contrast, RF sputtering is designed to replace or modulate the sputtering energy source, effectively neutralizing the target surface to prevent positive charge buildup, making it suitable for dielectric-insulating materials.
Figure 1.2 Schematic diagram of sputtering [47]
Sputtering technique has various advantages
1) On large substrates, sputtering can deposit thin films of uniform thickness
2) The thin film produced is impurity-free and of high quality h
3) By adjusting the operating parameters and modifying the deposition time, the film thickness can be conveniently controlled
Ion beam sputtering has several notable drawbacks, including the complexity and high cost of the required equipment, as well as the necessity for high-purity source materials Additionally, this technique demands users to possess specialized skills, and it is not effective for creating uniformly thick films over large areas.
RF sputtering is utilized to deposit Ni-doped CuO thin films, resulting in a polycrystalline structure with a prominent peak in the (111) direction Nickel doping significantly influences the crystal size and resistivity of CuO thin films, with the energy bandgap increasing from 1.62 to 1.76 eV as the concentration of Ni ions rises from 0 to 4.5 at.% All films exhibit p-type conduction, and the resistivity varies based on the nickel content.
Chemical deposition is a widely used technology in the semiconductor thin film industry due to its cost-effectiveness and ability to produce high-quality films Among the most popular techniques are sol-gel and chemical vapor deposition (CVD), both of which require minimal equipment while delivering exceptional results This section will focus on these two methods, highlighting their advantages in film production.
Sol-gel technology is a wet-chemical method used to synthesize solid materials from small molecules This technique involves the combination of micro particles or molecules in a solution (sols) that agglomerate under specific conditions, forming metal-oxygen-metal bonds to create a bonding network (gels) Once condensation is complete, sol particles develop into an inorganic network known as a gel The sol-gel method offers a bottom-up approach that is energy-efficient and cost-effective, allowing for precise control over chemical composition, making it highly suitable for both laboratory and industrial applications.
A precursor solution is created through the sol-gel process and is used to form thin films, which are layers of raw materials applied to substrates like metal, glass, crystals, or ceramics to modify their optoelectrical properties These thin films can vary in thickness from micrometers to nanometers and are typically deposited using methods such as dip coating and spin coating.
Dipping and spinning techniques significantly influence thin film formation, with key parameters including spinning speed, surface tension, solution viscosity, and solvent evaporation rate Spin-coating, a method for creating a uniform film on a solid substrate, utilizes radial force to achieve a consistent thickness due to the strong interplay of radial and frictional forces related to solution viscosity This demonstrates that spin-coating provides distinct advantages over dip coating methods.
The CuO and Ni-doped CuO thin films were successfully produced as described by Baturay et al [48] through spin-coating method The XRD pattern indicates that all films h
The study revealed that the material exhibited a polycrystalline tenorite structure, with an increase in nanoparticles on the flat surface corresponding to higher doping levels, as evidenced by SEM images Electrical analysis using a Hall effect system confirmed that CuO is a p-type conductive substance Notably, with a 6% increase in Ni doping, the bandgap energy (E_g) slightly decreased from 2.03 eV to 1.96 eV, before rising again to 2.22 eV at 10% Ni doping.
In a study by Dolai et al [26], Superconducting Quantum Interference Device (SQUID) measurements were utilized to investigate the magnetic properties of nickel-doped copper oxide (CuO) thin films The findings revealed that the magnetization remained largely temperature-independent within the range of 20 K to 330 K Additionally, the research calculated the residual magnetization and coercive field in CuO-based films as a function of varying nickel doping levels.
1.4.2.2 Modified chemical bath deposition technique
Chemical bath deposition (CBD) is an effective technique for applying films to various substrates by immersing them in a precursor solution, allowing for precise control over temperature, pH, and concentration This process utilizes soluble salts like chlorides, nitrates, sulfates, or acetates, which react slowly in solution to form solid films on the substrate CBD is particularly advantageous for coating materials that cannot withstand high temperatures, such as polymers, and is capable of effectively covering complex surfaces, including powders, tubes, and porous structures, which are challenging to coat using traditional spray or vapor methods Additionally, CBD equipment is user-friendly and can be produced in large quantities for continuous processing.
Figure 1.4 Schematic of a chemical bath deposition [56]
The synthesis of multicomponent materials remains challenging due to the varying precipitation temperatures and pH levels of individual components Additionally, the CBD process is less efficient in converting precursor materials into deposits Heterogeneous nucleation can occur on the substrate surface in solution, leading to the formation of slow-growing granules that create films To address this issue, seed layers can be formed on pre-treated substrates, facilitating a more effective synthesis process.
Ha et al proposed a modified chemical bath deposition (CBD) system to achieve uniform Ni-doped CuO film formation This process involved applying a seed layer onto the substrate using the spin-coating method Following the crystallization of the film, the seeds were allowed to grow on the substrate by immersing it in a precursor solution The results indicated that increasing the Ni doping ratio to 5% led to a thicker nanorod structure, as evidenced by FE-SEM images However, when the Ni doping was raised to 20%, the growth of the nanorod structure was hindered, and the bandgap energy of the films increased significantly from 2.33 eV to 3.46 eV.
Spin-coating techniques
Spin-coating is an effective method for applying uniform films onto flat surfaces By utilizing the radial force and surface tension of a liquid, a consistent coating is achieved when a small volume of precursor solution is spread across the substrate This technique allows for the creation of thin films with thicknesses varying from just a few nanometers to several micrometers.
Spin coating is a widely used deposition technique known for its speed, cost-effectiveness, and simplicity in producing homogeneous samples This method excels in creating extremely uniform sample layers on various substrates Key factors influencing film thickness include spin speed and the viscosity of photoresists Additionally, spin coating does not necessitate stringent pressure, power, or manufacturing engineering conditions, making it a more accessible alternative to vacuum methods.
The basic principle of the spin-coating method is described in Figure 1.6
Figure 1.6 The stages of thin film formation using spin-coating process [60] This process can be divided into 4 main stages:
Stage I – Deposition: The solution is dropped onto the surface of substrates, typically using a pipette Due to centrifugal motion, an amount of sol will be uniformly dispersed over the surface
Stage II – Spin-up: The coater is rotated at the desired rotation speed according to the installed program A large amount of solution will be expelled from the substrate The fluid is spread evenly over the substrate when the drag is equal to the acceleration of rotation
Stage III – Spin-off: The excess precursor solution flows to the edge and leaves as droplets because the centrifugal force is greater than the viscous forces
Stage IV – Evaporation: In this stage, evaporation plays a major mechanism of thinning The solvent evaporation rate will depend on its volatility, vapor pressure, and ambient temperatures
Spin coatings play a crucial role in various electronic industries and nanotechnology applications, often working in conjunction with other deposition methods in the semiconductor sector This technique is essential for applying coatings such as photoresists, insulators, organic semiconductors, and metal precursors to surfaces.
18 and metal oxides In short, spin coatings are ubiquitous in the nanotechnology and semiconductor research and development fields as well as in the industrial fields.
Motivation and the objectives of the studies
Recent advancements in electron spin and charge interactions have led to the development of a multifunctional material with significant potential for spin-based technology, enabling high storage density, cost-effectiveness, low power consumption, and ease of operation Among the leading candidates for this technology are dilute magnetic semiconductors, with extensive research focused on materials such as ZnO, TiO2, SnO2, and GaN doped with transition metals However, limited studies have explored the properties of Ni-doped CuO materials This research aims to present a straightforward method for synthesizing Ni-doped CuO thin films at ambient temperature via the sol-gel technique and to examine their characteristic properties, particularly their magnetic attributes.
Hence this research comprises of several objectives:
- To synthesize a homogeneous precursor solution from Ni and Cu elements using the sol-gel process
- To develop Ni-doped CuO thin films with various Ni doping levels by applying spin-coating
- To investigate material structural, morphological, optical, electrical, and magnetic characteristics of CuO-based thin films h
FILM DEPOSITION AND CHARACTERIZATION
Synthesis of precursors
This thesis focuses on cupric oxide (CuO) as the host semiconductor, with Nickel (Ni) serving as the transition metal dopant The preparation involved combining Copper (II) acetate monohydrate and nickel acetate tetrahydrate solutions in ethanol, using MEA as a stabilizer for the reaction Detailed chemical information for sample preparation is provided in Table 2.1.
Table 2.1 List of chemical compounds
Copper(II) acetate monohydrate - 199.65 Cu(CH3COO)2.H2O
Nickel acetate tetra hydrate - 248.86 Ni(CH3COO)2.4H2O
In the initial phase, Cu(CH3COO)2·H2O and Ni(CH3COO)2·4H2O were dissolved in absolute ethanol at varying concentrations of Ni doping, as detailed in Table 2.2 The mixture was stirred for 15 minutes, followed by the gradual addition of MEA while maintaining a molar ratio of 1:2 between Cu²⁺ and MEA After this period, the solution exhibited a blue color Subsequently, the precursor solution was stirred and heated to 75°C for approximately 60 minutes to facilitate the formation of a copper molecular network Finally, the solution was aged at ambient temperature for 24 hours, as illustrated in Figure 2.1.
Figure 2.1 Precursor solution preparation process
Table 2.2 Mass of chemicals used to manufacture precursors with difference Ni doping concentrations
No sample Ni doping concentration (wt.%)
Thin film deposition
Sol-gel coatings are thin films applied from a liquid solution onto solid substrates For optimal results, surfaces must be clean and free from dust and particles, ensuring uniform wetting with the sol-gel solution Contaminated surfaces can lead to issues such as open resistors or localized high resistance.
The glass and ITO substrates underwent a thorough cleaning process using an ultrasonic bath with acetone and absolute ethanol for 10 minutes each to remove dust and organic residues Following this, the substrates were rinsed with distilled water to eliminate any remaining solvents and then air-dried To enhance surface roughness and improve adhesion for the liquid coating, the ITO substrates were treated with a plasma method Simultaneously, the glass substrates were etched with a 2% aqueous solution of hydrofluoric acid for 60 seconds, then rinsed with ethanol and distilled water to facilitate the redistribution of electrons from Si-O bonding, resulting in the formation of O-H and Si-FH groups.
Fluoride ions effectively break siloxane (Si-O-Si) bonds, necessitating careful adjustment of cleaning concentration and time based on glass composition to prevent excessive corrosion This process enhances the contact angle by removing oxides and exposing hydrogen on the surface Following this, distilled water is used to cleanse the substrates, eliminating small molecules that may have leached into the slightly porous glass After rinsing, it is crucial to dry the glass quickly, as prolonged exposure to water can lead to the absorption and redeposition of contaminants from the air.
2.2.2 Deposition of Ni-doped CuO thin films
After the surface cleaning, Ni-doped CuO thin films were coated on cleaned glass and ITO substrates utilizing a spin-coating system
Step 1: The precursor solution is dripped onto a spinning substrate with a rotation speed of 1500 rpm for 40 seconds A surplus amount of fluid is expelled on the substrate's surface
Step 2: The sample was heated to 90°C for 3 minutes by using a hot plate To achieve the desired thickness of the CuO thin film, the coating and drying process was done three times
Step 3: After the final coating, the films were annealed in the air at 550°C for 30 minutes to form a crystalline film h
Figure 2.2 Spin-coating system (a), annealing furnace (b).
Film characterization
The study examined the structural, surface morphological, electrical, optical, and magnetic properties of Ni-doped CuO thin films after deposition The crystal structure was analyzed using an X-ray diffractometer (XRD, Bruker, D2 phase) with Cu-Kα radiation (λ = 1.54 cm -1) Surface morphology was assessed through scanning electron microscopy (SEM, JEOL JSM - IT100, 20 kV) Additionally, the optical characteristics of the thin films were evaluated using a UV-Vis spectrophotometer (UV 2450).
The sheet resistance of thin films was measured using a four-probe system, while electrochemical impedance spectroscopy (EIS) was employed to assess their electrical properties EIS measurements utilized a three-electrode setup in a 0.5 M Na2SO4 electrolyte, comprising a working electrode (WE), reference electrode (RE), and counter electrode (CE) The working electrode consisted of a Ni-CuO thin film on an ITO substrate, with silver chloride (AgCl/Ag) and platinum electrodes serving as the reference and counter electrodes, respectively Additionally, the magnetic properties of films deposited on glass substrates were evaluated using a vibrating sample magnetometer (VSM).
The X-ray diffraction (XRD) method is a strong instrument for determining crystal structures and unit cell sizes in great detail X-rays are electromagnetic radiation with h
23 wavelengths 1Å (1Å = 10 -10 meters), which is close to the size of atoms, so X-rays are useful for exploring inside crystals
Furthermore, the Einstein equation states that the energy of X-rays is inversely proportional to their wavelength:
Where E is energy, h is Planck’s constant, 6.62517 x 10 -27 erg sec, υ is frequency, c is velocity of light = 2.99793 x 10 10 cm/sec, λ is wavelength
X-rays have higher energy than visible light because their wavelength is shorter (Figure 2.3) Therefore, X-rays are more effective in penetrating matter than visible light The diffraction of x-rays can determine the lattice parameters, crystallite size, strain, and dislocation density of polycrystalline materials of powders, thin films samples
The technique of XRD is ideal for thin film analysis for two reasons:
(1) X-ray diffraction techniques are non-destructive material
(2) The wavelengths of X-rays are close to the atomic distances in the matter, so this device is used as structural probes h
Figure 2.4 Diffraction of X-rays by a crystal
The diffracted wave, illustrated in Figure 2.4, occurs when X-rays interact with atomic planes, leading to interference and the formation of a diffracted beam A portion of this beam is also reflected by the electrons in the atoms Measurements of the diffractometer reflections were conducted within a 20 to 80° range at room temperature The relationship between the incident X-ray angle and diffraction is defined by Bragg's Law, represented by the equation nλ = 2d sinθ (2.2), where λ denotes the X-ray wavelength, d signifies the inter-planar spacing, n is an integer indicating the peak order (n = 1, 2, 3,…), and θ represents the incidence angle.
The crystallites size was calculated from Debye-Scherer’s equation formula, based on the width of the diffraction peak and the diffraction angle of θ:
Where β is the FWHM (full width at half maximum) of diffraction peaks, θ is the incidence angle, λ is the X-ray wavelength, and D is the crystallite size h
Scanning Electron Microscopy (SEM) utilizes a focused, high-energy electron beam to generate numerous electrons that interact with the surface of solid specimens By analyzing the signals collected from these materials, SEM effectively reveals the surface morphology and chemical composition of the samples under investigation.
The signals utilized in electron microscopy include secondary electrons (SE), backscattered electrons (BSE), diffracted backscattered electrons (DBSE), X-rays, and cathode-luminescence (CL) SE is instrumental in revealing the surface morphology of the sample, while BSE provides compositional contrasts in multiphase materials Additionally, DBSE is used to analyze the crystalline structure and orientations within the sample X-rays, generated by the electron beam, are specific to each element, making them essential for elemental analysis.
Figure 2.5 Schematic diagram of a Scanning Electron Microscope [62] (a), SEM machine in this study (b)
The electron beam is first generated by the electron gun and accelerated in an electric field Then, the electron beam is passed through a magnetic lens system to h
A narrow electron beam is generated by the system, with its direction adjustable through coils positioned above the objective lens, which create a magnetic field that influences the electron flow The signals produced by this interaction are subsequently detected by appropriate detectors.
The surface characteristics of Ni-doped CuO thin films were analyzed using a scanning electron microscope (SEM, JEOL JSM - IT100) at the Nanotechnology Laboratory of Vietnam Japan University (VJU), VNU, as illustrated in Figure 2.5b.
UV-Vis spectroscopy is a quantitative method used to assess the optical transmittance and absorbance of various materials by comparing the light passing through a sample to that of a standard This versatile technique can analyze a diverse range of samples, including liquids, solids, and thin films, across a photon wavelength spectrum of 190 nm to 1100 nm.
The experimental measurements of transmittance (T%) and absorbance (Abs) were utilized to calculate the optical band gap energy Additionally, the optical band gap and absorption coefficient were determined using the provided expression.
(1) The absorption coefficient (α) was derived from the Beer-Lambert law in the light absorption spectral region, using the following expression [63]
Where A is the absorbance, d is the film thickness, T is the transmittance, and α is the absorption coefficient
(2) The optical bandgap energy E g was calculated using the absorption coefficient’s (α) dependence on incident photon energy (hν):
Where A is an energy-independent constant, α the material's absorption co- effective, and m is a constant with values of 1/2, 1/3, 2, and 3 depending on direct h
27 allowed, direct forbidden, indirect allowed, and indirect forbidden transitions, respectively [64]
The value of m for CuO is 1/2, indicating a direct permitted transition To determine the energy gap (E_g), a graph of (αhν)^m versus hν is plotted The results reveal that the plots of (αhν)^2 as a function of hν exhibit linear behavior for Ni-doped CuO films Consequently, E_g is obtained by extrapolating the linear portion of the spectrum to the point where αhν equals zero.
Sheet resistance, also known as surface resistivity, quantifies how easily charge can flow across uniform thin films By measuring the sheet's resistance, one can ascertain the electrical properties of the material The four-probe method is the most widely used technique for this evaluation, involving four collinear probes in contact with the material's surface In this approach, current is applied through the outer probes, while the voltage is measured at the inner probes, allowing for accurate determination of the sample's resistivity A schematic representation of the four-probe setup is illustrated in Figure 2.6.
Figure 2.6 Schematic of four-point probe configuration
The voltage at probe 2 (V2) is generated from the current flowing between probe 1 and probe 4, which is given by:
The voltage at probe 3 is:
The voltage difference between the two probes 2 and 3 is V = V2-V3, the resistivity can be determined using the equation as:
When the probe spacing is equal (s1 = s2 = s3 = s4 = s), the resistivity from equation (2.8) becomes:
For a very thin layer (thickness t