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 existing electronic devices These advanced systems are more energy-efficient and offer greater memory and computing capabilities while occupying less space compared to traditional electronics.
Conventional magnetic storage devices utilize passive magnetoresistive sensors and ferromagnetic 3d metal alloy electrodes The advancement of enormous magnetoresistance in (Fe/Cr)n multilayers, along with tunneling magnetoresistance, has significantly contributed to their development However, practical applications of magnetic semiconductors remain challenging due to their ferromagnetic properties typically manifesting at low Curie temperatures, often below 100 K To address this issue, researchers have created diluted magnetic semiconductors (DMS) by incorporating a small percentage of magnetic atoms into non-magnetic semiconductor structures.
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 compared to traditional magnetic semiconductors, primarily due to the controlled dilution of the magnetic component.
In diluted magnetic semiconductors (DMS), the strong interaction between charge carriers and local magnetic moments results in unique physical properties When an external magnetic field is applied, magnetic ions influence the semiconductor band state, altering the arrangement of spins in both the magnetic ions and band electrons This magnetic enhancement of ions within 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 II-VI semiconductors, such as InMnAs and GaMnAs, are utilized to create dilute magnetic semiconductors (DMS) The substitution of trivalent metal ions with divalent metal ions leads to hole formation, resulting in ferromagnetic order GaMnAs is particularly favored for various spin-based applications, including spin-polarized light emitters and spin field-effect transistors, which demonstrate magnetic behavior at temperatures below room temperature (TC = 173 K) However, these semiconductor compounds are limited to functioning at low temperatures (approximately 100 K), rendering II-VI DMS impractical for widespread electronic use.
Achieving room-temperature ferromagnetism (RTFM) in dilute magnetic semiconductors (DMS) is crucial for electronic device applications 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 behavior through charge manipulation This has positioned oxide-based DMS as essential materials for developing advanced electrical devices Notably, many oxide-based DMSs are large bandgap semiconductors (> 3 eV), which add an optoelectronic aspect to next-generation spintronic devices Recent investigations have highlighted the ferromagnetic properties of DMS oxides, including the significant discovery of Co-doped TiO2 material exhibiting RTFM by Matsumoto et al.
[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), which is a semiconductor with properties ideal for DMS fabrication CuO is recognized as a high critical temperature superconductor, with its superconductivity linked to Cu-O bondings Consequently, investigating the magnetic characteristics of nanoscale CuO materials is crucial.
CuO doped with transition metals such as Co, Fe, and Ni can demonstrate ferromagnetic order with Curie temperatures exceeding room temperature Research by Li et al indicates that the ferromagnetic interactions among randomly distributed Fe ions within the antiferromagnetic CuO matrix lead to materials exhibiting notable magnetic properties at Curie temperatures.
Research shows that Fe-doped CuO matrix samples exhibit ferromagnetic behavior at room temperature, as observed by Layek and Verma Additionally, Dolai et al demonstrated that Ni-doped CuO thin films, created through spin-coating deposition, display 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 various electronic products, including transistors and photovoltaic devices These semiconductor thin films, with thicknesses ranging from a few nanometers to hundreds of micrometers, are utilized across multiple applications, where the manufacturing technique directly influences their structural and physicochemical properties Due to their diverse characteristics, semiconductor thin films are emerging as ideal materials in the electronics industry By adjusting the fabrication methods and substrate parameters, it is feasible to produce single or polycrystalline structures on substrates with complex surface morphologies These films can be 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 single-crystal bulk materials, albeit influenced by surface characteristics These films play a crucial role in various optoelectronic applications, including heterojunction solar cells and light-emitting diodes.
Polycrystalline thin films are characterized by their distinct grain-oriented plane arrangement, which sets them apart from bulk materials The grains in polycrystalline materials can exhibit a wide range of shapes and sizes These thin film semiconductors have led to numerous applications, including their use as gate electrodes in MOSFETs, thin-film transistors, and 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 arrangement, resulting in the absence of typical solid-state properties found in crystalline materials These unique characteristics make them 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 neighboring oxygen atoms, forming a deformed tetrahedron Detailed crystallographic parameters and interatomic distances to 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
CuO is an antiferromagnetic semiconductor characterized by an open d shell (3d^9) and is classified as a p-type semiconductor due to the presence of copper vacancies that act as acceptors within its lattice structure The electrical properties of pure CuO are significantly influenced by intrinsic defects, particularly copper and oxygen vacancies Among these, copper vacancies are the most common defects found in nonstoichiometric cupric oxide, primarily due 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 significantly influenced by structural changes such as phase transitions, doping, and grain boundary expansion, which affect the number of charge carriers and their mobility, directly impacting resistivity Research indicates that variations in annealing temperature also play a crucial role in altering electrical conductivity For instance, Saravanakannan et al demonstrated that increasing the annealing temperature from 523 K to 723 K results in a decrease in resistivity, highlighting the importance of thermal treatment in optimizing thin film performance.
The optical properties of thin films play a crucial role in the development of optoelectronic devices, making them a vital parameter in fabrication Bulk cupric oxides (CuO) have a direct narrow bandgap of 1.2 eV; however, the optical bandgap in CuO thin films can vary significantly based on the deposition methods and the factors influencing the quality of the thin films.
The bandgap energy of CuO films, typically ranging from 1.3 to 1.9 eV, can be influenced by several factors including doping concentration, thickness, grain size, and structural changes Research by Singh et al indicates that as the thickness of CuO films produced through the successive ionic layer adsorption and reaction (SILAR) method increases, the bandgap energy decreases.
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 garnered 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] n
Overview of deposition techniques
Thin films have gained significant interest due to their exceptional properties and applications across science, industry, and commerce This technology enables the creation of materials at nanoscale dimensions, making them ideal for 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, in 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, creating plasma with inert gases like argon or xenon, which are preferred for sputtering due to their non-reactive nature This process results in the formation of a thin film as ions bombard the substrate The sputtering system is illustrated in Figure 1.2.
Sputtering techniques are primarily categorized into two types: direct current (DC) and radio frequency (RF) DC sputtering is widely used for conductive materials like metals due to its ease of adjustment and low power consumption In contrast, RF sputtering is designed to address dielectric-insulating materials by pulsing the sputtering energy source, which helps neutralize the target surface and prevents the buildup of positive charge.
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 n
3) By adjusting the operating parameters and modifying the deposition time, the film thickness can be conveniently controlled
Ion beam sputtering has notable drawbacks, including the complexity and high cost of the required equipment, as well as the necessity for high-purity source materials Additionally, it demands specialized skills from users, and it is not effective for producing uniformly thick films over large areas.
RF sputtering is employed to deposit Ni-doped CuO thin films, which exhibit a polycrystalline structure with a predominant peak in the (111) direction Nickel doping significantly influences the crystal size and resistivity of the 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 demonstrate 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 various techniques, sol-gel and chemical vapor deposition (CVD) stand out for their effectiveness in creating high-quality films with minimal equipment requirements.
Sol-gel technology is a wet-chemical process that creates solid materials from small molecules through the agglomeration of micro particles or molecules in a solution (sols) Under specific conditions, these particles bond together via metal-oxygen-metal connections to form a gel network Once condensation is complete, sol particles develop into an inorganic gel network This bottom-up approach of the sol-gel technique offers advantages such as low energy consumption, cost-effectiveness, and precise control over chemical composition, making it particularly suitable for both laboratory and industrial applications.
A precursor solution is created through the sol-gel process, which is then used to produce thin films These coatings, consisting of thin layers of raw materials, can be functionalized and applied to various substrates including metal, glass, crystals, or ceramics to enhance their optoelectrical properties Thin films can be synthesized with thicknesses ranging from micrometers to nanometers, with dip coating and spin coating being the most common deposition methods for applying sol-gel solutions.
In the processes of dip and spin coating, key factors like spinning speed, surface tension, solution viscosity, and solvent evaporation rate play crucial roles in the formation of thin films Spin-coating, which utilizes radial force to create a uniform film on a solid substrate, benefits from the strong interaction between radial and frictional forces influenced by solution viscosity As a result, spin-coated films achieve a consistent thickness across the substrate, demonstrating the advantages of spin coatings over dip coatings.
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 n
The study revealed that the material exhibited a polycrystalline tenorite structure, with an increase in nanoparticles on the surface corresponding to higher doping levels, as indicated by SEM imaging Electrical analysis using a Hall effect system confirmed that CuO is a p-type conductive material Notably, increasing Ni doping to 6% led to a slight reduction in bandgap energy (E_g) 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 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 across a range from 20 K to 330 K Additionally, the research calculated the residual magnetization and coercive field in CuO-based films, noting variations based on the amount of nickel doping.
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, which can be done multiple times By carefully controlling factors such as temperature, pH, and concentration, a solid phase forms and adheres to the substrate This method utilizes soluble salts like chlorides, nitrates, sulfates, or acetates as the deposition medium, enabling a slow chemical reaction that results in a solid film CBD is particularly advantageous for coating materials sensitive to high temperatures, including polymers, and is capable of coating complex surfaces like powders, tubes, and porous structures that are challenging to coat using traditional spray or vapor methods Additionally, CBD equipment is user-friendly, can be produced in large quantities, and supports 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 Heterogeneous nucleation on the substrate surface can occur in solution, leading to the formation of slow-growing granules that create films To address this issue, seed layers can be formed on a pre-treated substrate, enhancing the overall process.
Ha et al proposed a modified CBD system to achieve uniform Ni-doped CuO film formation, utilizing a seed layer applied via the spin-coating method Following crystallization, the seeds grew on the substrate by being immersed in a precursor solution Increasing the Ni doping ratio to 5% resulted in a thicker nanorod structure, as evidenced by FE-SEM images, while a further increase to 20% Ni doping slowed the formation of this structure Additionally, the bandgap energy of the films rose significantly from 2.33 eV to 3.46 eV with the 20% Ni doping.
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 thin films that can vary in thickness 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 homogeneous samples This technique excels in creating extremely uniform layers on various substrates, with film thickness influenced by factors such as spin speed and photoresist viscosity Additionally, unlike vacuum methods, spin coating does not necessitate stringent pressure, power, or engineering conditions, making it a more accessible option for sample production.
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 are essential in various electronic industries and nanotechnology applications, often integrated with other deposition methods in semiconductor manufacturing This technique is utilized to apply coatings of photoresists, insulators, organic semiconductors, and metal precursors on 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
The interaction between electron spins and charge carriers has led to the development of a promising multifunctional material, particularly in the realm of spin-based technology, which offers high storage density, low power consumption, and ease of operation Dilute magnetic semiconductors, such as ZnO, TiO2, SnO2, and GaN doped with transition metals, are key candidates for these applications However, limited research has focused on Ni-doped CuO materials This study aims to present a straightforward method for synthesizing Ni-doped CuO thin films at ambient temperature using the sol-gel technique, while also investigating their characteristic properties, especially the magnetic properties.
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 n
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 during the reaction process Detailed chemical information for the 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 Ni doping concentrations This mixture was stirred for 15 minutes before gradually adding MEA, maintaining a molar ratio of Cu²⁺ to MEA at 1:2, resulting in a blue solution Subsequently, the precursor solution was stirred and stabilized at 75°C for 60 minutes to facilitate the formation of a copper molecular network Finally, the solution was aged for 24 hours at ambient temperature to complete the process.
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, the surface 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 were thoroughly cleaned using an ultrasonic bath with acetone and absolute ethanol for 10 minutes each to remove dust and organic residues Following this, they were rinsed with distilled water to eliminate any solvent traces and air-dried The ITO substrates underwent plasma treatment to enhance surface roughness and improve liquid coating adherence, while the glass substrates were etched in a 2% hydrofluoric acid solution for 60 seconds This etching process was also 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 and Si-FH groups.
Fluoride ions effectively break siloxane (Si-O-Si) bonds, necessitating careful adjustment of cleaning concentration and time based on the glass composition to avoid excessive corrosion This process enhances the contact angle by removing oxides and exposing hydrogen on the surface After cleaning, distilled water is used to eliminate small molecules that may have leached into the slightly porous glass It is crucial to dry the glass quickly, as prolonged exposure to 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 n
Figure 2.2 Spin-coating system (a), annealing furnace (b).
Film characterization
The structural, surface morphological, electrical, optical, and magnetic properties of Ni-doped CuO thin films were thoroughly investigated post-deposition The crystal structure was analyzed using X-ray diffraction (XRD) with Cu-Kα radiation (λ = 1.54 cm -1) Surface morphology was examined through scanning electron microscopy (SEM, JEOL JSM - IT100, 20 kV) Additionally, the optical characteristics of the thin films were assessed using a UV-Vis spectrophotometer (UV 2450).
The sheet resistance of the thin films was assessed using a four-probe measuring system, while electrochemical impedance spectroscopy (EIS) was employed to evaluate their electrical properties EIS measurements were conducted with a three-electrode system in a 0.5 M Na2SO4 electrolyte, comprising a working electrode (Ni-CuO thin film on ITO substrate), a reference electrode (AgCl/Ag), and a counter electrode (platinum) Additionally, the magnetic properties of the films deposited on glass substrates were analyzed 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 n
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 n
Figure 2.4 Diffraction of X-rays by a crystal
The diffracted wave, as 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 in the atoms The diffractometer measurements were taken within a 20 to 80° range at room temperature Bragg's Law, expressed in equation (2.2), defines the relationship between the angle of the incident X-ray and the resulting diffraction, where nλ = 2d sinθ, with λ representing the X-ray wavelength, d the inter-planar spacing, n an integer indicating the peak order, and θ 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 n
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 determines 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 is utilized to reveal the surface morphology of the sample, while BSE provides compositional contrasts in multiphase materials The crystalline structure and orientation of crystals are analyzed using DBSE, and X-rays, produced by the electron beam, facilitate elemental analysis by identifying specific elements within the sample.
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 n
The electron beam is converted into a narrow stream, with its direction adjustable via coils positioned above the objective lens, creating a magnetic field that influences the electron flow This setup allows for accurate detection of signals by the appropriate detectors.
The surface of Ni-doped CuO thin films was examined using a scanning electron microscope (SEM, JEOL JSM - IT100) at the Nanotechnology Laboratory of Vietnam Japan University (VJU), VNU, as shown in Figure 2.5b.
UV-Vis spectroscopy is a quantitative method used to measure the optical transmittance and absorbance of various materials By comparing the light that passes through a sample to that of a standard sample, this technique effectively analyzes 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):
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 n
27 allowed, direct forbidden, indirect allowed, and indirect forbidden transitions, respectively [64]
The value of m is established as 1/2, indicating that CuO exhibits a direct permitted transition The band gap energy (E g) is derived by plotting (𝛼ℎ𝜈) 𝑚 against hν, revealing that the plots of (𝛼ℎ𝜈)² as a function of hν are linear for Ni-doped CuO films Consequently, E g is determined by extrapolating the linear portion of the spectrum to where 𝛼ℎ𝜈 equals zero.
Sheet resistance, also known as surface resistivity, quantifies how well a charge can move across uniform thin films By assessing the sheet's resistivity through resistance measurements, one can determine the material's electrical properties The most widely used technique for measuring sheet resistance is the four-probe method, which employs four collinear probes in contact with the material's surface In this method, current flows through the outer probes, allowing for the calculation of resistivity based on voltage readings from the inner probes, as 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