OVERVIEW
Dilute Magnetic Semiconductors (DMS)
Spintronics, or spin electronics, is an emerging field that explores the spin and charge states of electrons By harnessing both the spin and charge of electrons, spintronic systems enhance the performance of current electronic devices These systems are known for their lower power consumption, increased memory capacity, and superior computing power, all within a more compact design compared to traditional electronics.
Conventional magnetic storage devices have traditionally relied on passive magnetoresistive sensors and ferromagnetic 3D metal alloy electrodes However, the discovery of giant magnetoresistance in (Fe/Cr) n multilayers and tunneling magnetoresistance has significantly advanced their development Despite this progress, magnetic semiconductors have proven challenging to integrate into practical applications due to their low Curie temperatures, typically below 100 K To overcome this limitation, researchers have developed diluted magnetic semiconductors (DMS) by incorporating a small percentage of magnetic atoms into non-magnetic semiconductor materials, paving the way for new magnetic semiconductor materials.
DMS, or semimagnetic semiconductors, are materials where a small percentage of magnetic elements replace certain cations in a host semiconductor These semimagnetic semiconductors exhibit unique 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 interaction between charge carriers and local magnetic moments gives rise to distinct physical properties When an external magnetic field is applied, magnetic ions influence the semiconductor band state, altering the arrangement of spins in the magnetic ions and consequently affecting the spins of band electrons This magnetic enhancement of magnetic ions in semiconductors plays a crucial role in the material's behavior and 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, which induces ferromagnetic order GaMnAs has been widely selected for various spin-based device applications, such as spin-polarized light emitters and spin field-effect transistors, demonstrating magnetic behavior at temperatures below room temperature (T C = 173 K) However, these semiconductor compounds are only operational at low temperatures (around 100 K), limiting the practicality of II-VI DMS for electronic applications.
Achieving room-temperature ferromagnetism (RTFM) in dilute magnetic semiconductors (DMS) is crucial for electronic device applications Recent studies have highlighted 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 modification of magnetic properties through charge manipulation, which positions oxide-based DMS as essential for next-generation electrical devices Many oxide-based DMSs are large bandgap semiconductors (> 3 eV), adding an optoelectronic aspect to future spintronic technologies Notably, Co-doped TiO2 has demonstrated ferromagnetic properties at room temperature, as first reported by Matsumoto et al.
Recent studies have highlighted the magnetic moments and Curie temperature (T C) values of DMS-based thin films, as summarized in Table 1.1 Additionally, many researchers have identified the ferromagnetic properties of transition metal-doped ZnO at room temperature.
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), which is a semiconductor known for its unique properties that make it suitable for DMS fabrication CuO exhibits high critical temperature superconductivity, attributed to its Cu-O bonding Consequently, investigating the magnetic characteristics of nano-sized 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 exhibit ferromagnetic properties with a Curie temperature (T C) exceeding room temperature Research by Li et al indicated that ferromagnetic interactions occur among randomly distributed Fe ions at temperatures above 400 K Additionally, Layek and Verma found that all Fe-doped CuO samples displayed ferromagnetic behavior at room temperature Furthermore, Dolai et al demonstrated that Ni-doped CuO thin films, created through spin-coating deposition, exhibited temperature-independent magnetization between 20 K and 330 K.
Thin film semiconductors
Thin film semiconductors have experienced significant growth, leading to the commercialization of diverse electronic products, including transistors and photovoltaic devices These films, which vary in thickness from a few nanometers to hundreds of micrometers, are crucial in numerous applications, with their structural and physicochemical properties heavily influenced by manufacturing techniques Due to their versatile characteristics, semiconductor thin films are emerging as ideal materials in the electronics industry By adjusting fabrication methods and substrate parameters, it is possible to produce single or polycrystalline structures on complex surfaces, classifying these films as single-crystalline, polycrystalline, or amorphous.
Single-crystalline thin films are typically deposited on compatible single-crystalline substrates, ensuring that their properties mirror those of bulk single-crystal materials However, these properties can be influenced by surface characteristics These thin films play a crucial role in various optoelectronic applications, including heterojunction solar cells and light-emitting diodes.
Polycrystalline thin films are characterized by their unique grain-oriented plane arrangement, differing from bulk materials These films consist of grains that vary in shape and size, which play a crucial role in various applications such as gate electrodes in MOSFETs, thin-film transistors, and solar cells However, polycrystalline films also exhibit structural imperfections, including grain boundaries and randomly oriented crystals, which can affect their performance.
Amorphous thin films, characterized by their uniformly ordered atoms, do not exhibit the typical solid-state properties seen in crystalline materials These unique films are 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 primary copper oxide phases, alongside cuprous oxide (Cu₂O) It features a monoclinic lattice symmetry where the copper atom is centrally located among four nearest oxygen atoms, forming a deformed tetrahedral structure For detailed crystallographic parameters and interatomic distances, refer to Table 1.2.
Table 1.2 Crystallographic parameters (from ref [32]).
Lattice parameter Monoclinic a = 4.6837 Å b = 3.4226 Å c = 5.1288 Å β = 99.54° Shortest distances d Cu – O 1.95 Å d O – O 2.62 Å d Cu – Cu 2.90 Å
CuO, an antiferromagnetic semiconductor with an open d shell (3d⁹), is classified as a p-type semiconductor due to the presence of copper vacancies acting as acceptors within its lattice The electrical properties of pure CuO are significantly influenced by intrinsic defects, such as copper and oxygen vacancies The instability of copper contributes to the prevalence of these copper vacancies.
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 indicates that annealing temperature significantly alters electrical conductivity; for instance, Saravanakannan et al demonstrated that increasing the annealing temperature from 523 K to 723 K results in decreased resistivity.
The optical properties of thin films play a crucial role in the development of optoelectronic devices, with the bandgap of bulk cupric oxides (CuO) typically measured at 1.2 eV However, the optical bandgap of CuO thin films can vary between 1.3 and 1.9 eV due to deposition methods and quality factors Key influences on the bandgap energy include doping concentration, film thickness, grain size, and structural changes Notably, research by Singh et al indicates that the bandgap energy of CuO films produced through successive ionic layer adsorption and reaction (SILAR) decreases as the thickness of the film increases.
CuO is an antiferromagnetic semiconductor with two Neel temperatures: T N1 231 K and T N2 = 212 K [41] The magnetic structure of CuO is strongly antiferromagnetic, consisting of Cu–O parallel sheets.
Copper oxide (CuO) has garnered significant attention due to its unique surface chemistry, excellent recyclability, non-toxicity, high optical absorbance, and relatively low cost of raw materials To enhance the properties of CuO 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 Additionally, the incorporation of transition metals into the CuO structure positively influences the efficiency of CuO solar cells.
Overview of deposition techniques
Thin films have gained significant attention due to their remarkable properties and applications in science, industry, and commerce This technology enables the creation of materials with nanoscale dimensions and thicknesses, making them ideal for compact electronic and optoelectronic devices High-quality thin films are commonly produced through two main deposition techniques: physical deposition and chemical deposition, each utilizing different types of precursors.
Table 1.3 Thin film deposition methods (adapted from ref [47]).
1 Evaporation techniques 1 Sol-gel techniques a Vacuum thermal evaporation 2 Chemical bath deposition b Electron beam evaporation 3 Spray pyrolysis technique c Laser beam evaporation 4 Plating d Arc evaporation a Electroplating technique e Molecular beam epitaxy b Electroless deposition f Ion plating evaporation 5 Chemical vapor deposition
2 Sputtering techniques (CVD) a Direct current sputtering a Low pressure (LPCVD)
(DC sputtering) b Plasma enhanced (PECVD) b Radio frequency sputtering c Atomic layer deposition (ALD) (RF 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, are employed to create Ni-doped CuO thin films.
Sputtering is a thin-film production technology used across various industries, involving the formation of films within a vacuum chamber This process requires a voltage between the cathode's target material and the anode's substrate material, with plasma generated using inert gases like argon or xenon These gases are preferred for sputtering because they do not react with the target material The resulting thin film is created when bombardment ions strike the substrate.
Sputtering techniques are primarily classified into two types: direct current (DC) and radio frequency (RF) DC sputtering is favored for conductive materials like metals due to its ease of adjustment and low power consumption In contrast, RF sputtering is designed to effectively neutralize the target surface, preventing a 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.
3) By adjusting the operating parameters and modifying the deposition time, the film thickness can be conveniently controlled.
Ion beam sputtering presents significant challenges, including the complexity and high cost of the necessary equipment, as well as the requirement for high-purity source materials Additionally, it demands users with specialized skills, and it is not effective for creating uniformly thick films over large areas.
RF sputtering is an effective method for depositing Ni-doped CuO thin films, as demonstrated by Aakib and colleagues These films exhibit 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 display p-type conduction, and their resistivity varies according to the amount of nickel incorporated.
Chemical deposition is a widely used technology in the semiconductor thin film industry, known for its cost-effectiveness and ability to produce high-quality films Among the various techniques, sol-gel and chemical vapor deposition (CVD) stand out due to their efficiency and minimal equipment requirements, making them ideal for generating superior thin films.
Sol-gel technology is a wet-chemical method that creates solid materials from small molecules through the agglomeration of micro particles or molecules in a solution, forming a bonding network of metal-oxygen-metal bonds This process culminates in the formation of an inorganic network known as a gel The bottom-up approach of the sol-gel technique offers benefits such as low energy consumption, cost-effectiveness, and precise control over chemical composition, making it highly practical for both laboratory and industrial applications.
A precursor solution is created through the sol-gel process and used to produce thin films, which are ultra-thin layers of materials that can be applied to substrates like metal, glass, crystals, or ceramics to enhance their optoelectrical properties These thin films can be synthesized in thicknesses ranging from micrometers to nanometers, with dip coating and spin coating being the most common techniques for depositing thin films from sol-gel solutions.
In thin film formation through dipping and spinning techniques, key parameters like spinning speed, surface tension, solution viscosity, and solvent evaporation rate play crucial roles Spin-coating, which utilizes radial force to create a homogeneous film on a solid substrate, benefits from the strong interplay of radial and frictional forces influenced by solution viscosity, resulting in a uniform thickness Consequently, spin-coating presents significant advantages over dip coating methods.
CuO and Ni-doped CuO thin films were successfully produced using the spin-coating method, revealing a polycrystalline tenorite structure through XRD analysis SEM images indicated an increase in nanoparticles on the surface with higher doping levels Electrical properties assessed via a Hall effect system confirmed that CuO is a p-type conductive material Notably, increasing Ni doping to 6% resulted in a slight reduction of the bandgap energy from 2.03 eV to 1.96 eV, followed by an increase to 2.22 eV at 10% Ni doping.
A study by Dolai et al [26] utilized superconducting quantum interference device (SQUID) measurements to investigate the magnetic properties of Ni-doped 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 the 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 depositing films onto substrates by immersing them in a precursor solution, allowing for precise control over temperature, pH, and concentration to facilitate film formation Utilizing salts with medium to high water solubilities, such as chlorides, nitrates, sulfates, or acetates, the method enables the slow chemical reaction that produces a solid film on the substrate CBD is particularly useful for coating materials like polymers that cannot withstand high temperatures and is capable of coating complex surfaces, including powders, tubes, and porous structures, which are challenging to coat using traditional spray or vapor deposition methods Additionally, CBD equipment is user-friendly and can be mass-produced for continuous processing.
Figure 1.4 Schematic of a chemical bath deposition [56].
The synthesis of multicomponent materials remains challenging due to the differing precipitation temperatures and pH levels of individual components Additionally, the CBD process is less efficient in converting precursor materials into deposits In solution, heterogeneous nucleation on the substrate surface can lead to the formation of slow-growing granules that create films To overcome this issue, seed layers can be formed on a pre-treated substrate.
Spin-coating techniques
Spin-coating is a method used to apply uniform films onto flat surfaces by utilizing the radial force and surface tension of a liquid When a small amount of precursor solution is placed on the substrate, it spreads evenly, resulting in a consistent coating The thickness of the resulting thin films can vary significantly, ranging from just a few nanometers to several micrometers.
The spin coating method is a widely used deposition technique known for its speed, cost-effectiveness, and simplicity in producing uniform samples It excels in creating highly consistent film layers on various substrates, with film thickness influenced by factors like spin speed and photoresist viscosity Additionally, spin coating does not necessitate stringent pressure, power, or engineering conditions, making it more accessible than 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, particularly in the development of metal oxides Their widespread use in semiconductor research and industrial sectors highlights their importance in advancing technology.
Motivation and the objectives of the studies
The interaction of electron spins and charge carriers has led to the development of a promising multifunctional material, enabling the creation of spin-based devices with high storage density, low power consumption, and cost-effectiveness Dilute magnetic semiconductors, such as ZnO, TiO2, SnO2, and GaN doped with transition metals, are prime candidates for these applications However, research on Ni-doped CuO materials remains limited This study aims to provide a straightforward method for synthesizing Ni-doped CuO thin films at ambient temperature using the sol-gel technique, while also investigating 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.
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 synthesis involved combining Copper (II) acetate monohydrate and nickel acetate tetrahydrate solutions in ethanol, using MEA as a stabilizer Detailed chemical information for the sample preparation is provided in Table 2.1.
Table 2.1 List of chemical compounds.
Chemical name Acronym weight Molecular formula
Copper(II) acetate monohydrate - 199.65 Cu(CH 3 COO) 2 H 2 O
Nickel acetate tetra hydrate - 248.86 Ni(CH 3 COO) 2 4H 2 O
To prepare the precursor solution, Cu(CH₃COO)₂·H₂O and Ni(CH₃COO)₂·4H₂O were dissolved in absolute ethanol at varying Ni doping concentrations The mixture was stirred for 15 minutes, after which MEA was gradually added, maintaining a molar ratio of Cu²⁺ to MEA at 1:2 Following this, the solution turned blue, and it was further stirred and stabilized at 75°C for approximately 60 minutes to facilitate the formation of a network of copper molecules.
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 of dust to ensure 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 acetone and absolute ethanol in an ultrasonic bath for 10 minutes each, effectively removing 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 the adhesion of the liquid coating, the ITO substrates were treated with a plasma method Simultaneously, the glass substrates were etched with a 2% aqueous hydrofluoric acid solution for 60 seconds, followed by rinsing with ethanol and distilled water to facilitate the redistribution of electrons from Si-O bonding, resulting in the formation of O-H groups and Si-FH groups.
Fluoride ions effectively break siloxane (Si-O-Si) bonds, necessitating adjustments in cleaning concentration and time based on the glass composition to ensure effective cleaning without excessive corrosion This process enhances the contact angle by removing oxides and exposing hydrogen on the surface Following this, distilled water is utilized to cleanse the substrates, eliminating small molecules that may have leached into the slightly porous glass It is crucial to dry the glass quickly, as prolonged exposure to ambient air can lead to the re-deposition of contaminants on the surface.
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. a) b)Figure 2.2 Spin-coating system (a), annealing furnace (b).
Film characterization
The thin films were investigated for structural, surface morphological, electrical, optical, and magnetic properties after they were deposited An X-ray diffractometer
The crystal structure of Ni-doped CuO thin films was analyzed using X-ray diffraction (XRD) with Bruker D2 phase and Cu-Kα radiation (λ = 1.54 Å) Surface morphology was investigated through scanning electron microscopy (SEM, JEOL JSM-IT100, 20 kV), while the optical properties of the thin films were assessed 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) assessed their electrical properties The EIS measurements utilized a three-electrode setup in a 0.5 M Na2SO4 electrolyte, comprising a Ni-CuO thin film as the working electrode 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 the films on glass substrates were evaluated using a vibrating sample magnetometer (VSM, Lake Shore).
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 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
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 Additionally, some of the beam is reflected by the electrons within the atoms Measurements of the diffractometer reflections were conducted within a temperature range of 20 to 80° The relationship between the incident X-ray angle and diffraction is governed by Bragg's Law, expressed in the equation nλ = 2d sinθ, where λ represents the X-ray wavelength, d is the inter-planar spacing, n is an integer denoting the peak order (n = 1, 2, 3,…), and θ is 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.
Scanning Electron Microscopy (SEM) utilizes a focused high-energy electron beam to produce numerous electrons that engage with the surface of solid specimens By analyzing the signals obtained from these materials, researchers can determine both the surface morphology and chemical composition of the samples.
The signals generated during electron microscopy include secondary electrons (SE), backscattered electrons (BSE), diffracted backscattered electrons (DBSE), X-rays, and visible light (cathodoluminescence - CL) SE and BSE are essential for imaging samples, with SE revealing surface morphology and BSE providing compositional contrasts in multiphase materials The use of DBSE allows for the examination of the crystalline structure and orientations within the sample Additionally, X-rays, produced by the electron beam, are utilized for elemental analysis, as they are characteristic of each element present in the sample.
A Scanning Electron Microscope (SEM) utilizes a schematic diagram to illustrate its components, including the conversion of an electron beam into a narrow stream The direction of this beam is modulated by coils positioned above the objective lens, which create a magnetic field that influences the electron flow Subsequently, the appropriate detectors capture the resulting signals, as depicted in Figure 2.5a.
The surface of Ni-doped CuO thin film was analyzed using a scanning electron microscope (SEM, JEOL JSM - IT100) at the Nanotechnology Laboratory of Vietnam Japan University (VJU), VNU, as depicted 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 transmitted through a sample to that of a standard, this technique effectively measures a diverse range of samples, including liquids, solids, and thin films, across a photon wavelength spectrum of 190 nm to 1100 nm.
The experimental transmittance (T%) and absorbance (Abs) values 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):
In the context of energy transitions, A represents an energy-independent constant, while α denotes the material's absorption coefficient The value of m varies based on the type of transition, taking on values of 1/2 for direct allowed transitions, 1/3 for direct forbidden transitions, and 2 or 3 for indirect allowed and indirect forbidden transitions, respectively.
The value of m for CuO is 1/2, indicating a direct permitted transition The energy gap (E g) is calculated by plotting (αhν)² against hν, revealing linear functions for Ni-doped CuO films E g is then determined by extrapolating the linear portion of the spectrum to hν = 0.
Sheet resistance, also known as surface resistivity, quantifies how easily charge can flow over uniform thin films By measuring the sheet's resistivity, one can assess the electrical properties of the material The most widely used technique for this evaluation is the four-probe method, which involves four co-linear probes in contact with the surface of the material In this approach, a current is applied through the outer probes, allowing for the determination of resistivity by measuring the voltage across the inner probes A schematic representation of this setup is illustrated in Figure 2.6.
Figure 2.6 Schematic of four-point probe configuration.
The voltage at probe 2 (V 2 ) 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 = V 2 -V 3 , the resistivity can be determined using the equation as:
When the probe spacing is equal (s 1 = s 2 = s 3 = s 4 = s), the resistivity from equation
For a very thin layer (thickness t