What is SEM? How does it work? Major application and impact of acceleration voltages? difference with TEM

  

Introduction: 

SEM stands for Scanning Electron Microscopy, which is a type of electron microscopy used to obtain high-resolution images of materials and biological specimens at the nanoscale.

The basic working principle of SEM involves the use of a beam of high-energy electrons that are focused onto a sample using electromagnetic lenses. The electrons interact with the sample, causing the emission of secondary electrons and backscattered electrons, which are detected and used to generate an image of the surface of the sample. SEM images can provide high-resolution details about surface topography, morphology, and elemental composition. Its function of it is in Figure 1.

Figure 1: SEM working arrangement.

One of the major applications of SEM is in materials science, where it is used to study the microstructure of materials and to characterize their properties. SEM is also widely used in the biological sciences to study the structure and function of cells and tissues.

The acceleration voltage is an important parameter in SEM, as it affects the interaction between the electron beam and the sample. Increasing the acceleration voltage can result in higher-resolution images, as higher-energy electrons can penetrate deeper into the sample and generate more secondary and backscattered electrons. However, higher acceleration voltages can also cause damage to the sample and may affect the results of the imaging. The choice of acceleration voltage, therefore, depends on the nature of the sample and the specific application.

In addition to imaging, SEM can also be used for various other analytical techniques, such as energy-dispersive X-ray spectroscopy (EDS) and electron backscatter diffraction (EBSD), which can provide information about the elemental composition and crystallographic structure of the sample. The impact of acceleration voltage can also affect the performance of these analytical techniques.

 

What is TEM? What are the differences between SEM and TEM? How does sample preparation important?

TEM stands for Transmission Electron Microscopy, which is a type of electron microscopy used to obtain high-resolution images of the internal structure of materials and biological specimens at the nanoscale.

The basic working principle of TEM involves the transmission of a beam of high-energy electrons through a thin sample, which interacts with the sample, causing the electrons to scatter and diffract. The scattered electrons are then detected and used to generate an image of the internal structure of the sample. TEM images can provide detailed information about the crystal structure, morphology, and defects of materials and biological specimens.

The main difference between SEM and TEM is that SEM provides images of the surface of the sample, while TEM provides images of the internal structure of the sample. Another key difference is that SEM uses backscattered and secondary electrons to generate the image, while TEM uses transmitted electrons.

Sample preparation is extremely important in TEM, as the sample must be thin enough to allow the electrons to transmit through it. This typically involves cutting the sample into thin sections using a microtome, and then mounting the sections onto a thin support grid made of materials such as carbon or copper. The thickness of the sample section can vary depending on the application but is typically in the range of 50-200 nm.

In addition to thinning the sample, sample preparation for TEM can also involve additional steps such as staining, which is used to enhance the contrast between different components of the sample, and cryogenic freezing, which is used to preserve the sample in a near-native state.

Overall, both SEM and TEM are powerful tools for investigating the structure and properties of materials and biological specimens at the nanoscale, and the choice of which technique to use depends on the specific application and the type of information needed. However, sample preparation is crucial for obtaining high-quality images and accurate results in both techniques. Find more information here.

SPM i.e AFM and its Basic principle.

Introduction: 

SPM stands for Scanning Probe Microscopy, which is a family of techniques used to investigate the surface topography and properties of materials at the nanoscale. SPM includes several different techniques, such as Atomic Force Microscopy (AFM) and Scanning Tunneling Microscopy (STM).

The basic working principle of SPM involves a sharp probe or tip that is scanned over the surface of a sample, and the interaction between the tip and the sample surface is measured and used to generate an image of the surface topography. The interaction can be either repulsive or attractive, depending on the nature of the sample surface and the tip, and can be detected by measuring the deflection of the probe or the current flow between the probe and the sample.

One of the major applications of SPM is in the study of surfaces and interfaces in materials science, where it is used to investigate the structure, morphology, and properties of materials at the nanoscale. SPM is also used in the biological sciences to study the structure and function of biomolecules and cells.

Other major applications of SPM include:

1. Nanotechnology: SPM is an important tool in the development and characterization of nanoscale materials and devices, such as nanowires, nanoparticles, and quantum dots.
2. Surface analysis: SPM can be used to study the properties of surfaces and interfaces, such as adhesion, friction, and elasticity, which are crucial for understanding the behavior of materials in various applications such as lubrication, adhesion, and catalysis.
3. Material characterization: SPM can be used to measure various material properties, such as elastic modulus, hardness, and stiffness, which are important for understanding the mechanical behavior of materials and for the design of new materials.
4. Semiconductor device characterization: SPM can be used to investigate the properties of semiconductors, such as surface states, band structure, and electronic properties, which are important for the design and optimization of semiconductor devices.

Overall, SPM is a versatile and powerful tool for investigating the structure and properties of materials at the nanoscale and has applications in a wide range of fields, including materials science, biology, nanotechnology, and semiconductor device characterization.

 Atomic Force Microscopy:

The basic working principle of AFM involves a sharp probe or tip attached to a cantilever that is scanned over the surface of a sample. The interaction between the tip and the sample surface is measured and used to generate a high-resolution image of the surface topography. This interaction can be either repulsive or attractive, depending on the nature of the sample surface and the tip, and can be detected by measuring the deflection of the cantilever. Figure 1 shows the basic working principle diagram. 

Figure 1: detail of AFM operation diagram

AFM is widely used in various fields of research such as material science, chemistry, physics, biology, and medicine, due to its ability to provide high-resolution images of surfaces and its ability to measure properties such as elasticity, adhesion, and friction at the nanoscale. 

Some major applications of AFM include:

1. Characterization of surfaces and thin films: AFM can be used to obtain high-resolution images of surfaces and thin films, which is essential for understanding the properties and behavior of materials at the nanoscale.
2. Study of biological samples: AFM can be used to investigate the structure and mechanical properties of biological samples such as proteins, DNA, and cells, providing valuable insights into their function and behavior. 

3. 

   Figure 2: Bacteria colonies detection by AFM

   
   
   
4. Nanotechnology: AFM is an important tool in the development and characterization of nanoscale materials and devices, such as nanowires, nanoparticles, and quantum dots.
5. Surface analysis: AFM can be used to study the properties of surfaces and interfaces, such as adhesion, friction, and elasticity, which are crucial for understanding the behavior of materials in various applications such as lubrication, adhesion, and catalysis.
6. Material characterization: AFM can be used to measure various material properties, such as elastic modulus, hardness, and stiffness, which are important for understanding the mechanical behavior of materials and for the design of new materials. More help from here. 

What are crystal structures? How to determine crystal structure by x-ray diffraction?

Introduction:

Crystal is a periodic representation of atoms in 3D lattice space. There are seven crystal systems. They are cubic, tetragonal, hexagonal, rhombohedral, orthorhombic, monoclinic, and triclinic. Very often students and researchers get into trouble at the beginning of their research to determine crystal structures. They can not judge their findings properly. One of the tools to investigate the crystal of a crystal specimen is XRD.

2theta-Omega scanning:

When we carry out the XRD 2theta-Omega to study the epitaxial growth. After the measurement, we can easily relate the angle theta with the lattice spacing along the c-axis. Hence from the measured data, with the help of the lattice relation with the d-value, we can find the information about the lattice parameters and hence about the corresponding crystal structure. The detail of the lattice relation of the seven-crystal is shown in figure 1. 

Figure 1: Seven crystal system and their lattice relation

However, only 2theta-omega scanning can not give information about the detail of the crystal system. Figure 2 shows peaks from the planes parallel to the c-axis of the substrate. From this measurement, we can also calculate the lattice mismatch of the crystal system. Please check our primitive discussion.

Figure2. 2theta-omega scanning by symmetric x-ray diffraction technique

Along with the measurement, there is phi-scanning which gives information about the symmetry of the crystal. The symmetry in this case is mainly we call rotational symmetry.  Figure 3 shows the rotational symmetry of MnAs/InAs/GaAs epitaxial growth. Here, hexagonal MnAs (six-fold) with 3-fold rotational symmetry of InAs and GaAs. 

Figure 3: Phi-scanning to see the symmetry of the MnAs/InAs/GaAs hetero-structure.

Besides, we also carry out omega or theta scanning only to judge the displacement of the c-axis of the epitaxial film from the c-axis of the substrate. This gives information about the tilting of the grown film. Figure 4 shows omega scanning which reveals the relation between substrate and film distribution on the substrate along the c-axis. Tilting is usually visible in the case of large lattice mismatch growth i.e hetero-epitaxial growth. The Gaussian distribution theoretically also represents the distribution of the film c-axis with the substrate. 

Figure 4: Omega scanning to judge the tilt of the film c-axis with respect to the substrate 

Please check the measurement configuration in the earlier discussion.

What is metal-semiconductor contact? Physics behind it.

 Introduction:

In semiconductor technology, there are two types of contact one is rectifying(Schottky), and another is ohmic contact. So, the type of contact depends on the difference in the work function. The Schottky contact has applications in semiconductor physics and Ohmic contact has an influence on the performance of the device specifically power devices.

Physics behind metal-semiconductor contact

The physics behind the contact nature mainly depends on the doping concentration (Nd) of the semiconductor, temperature (T), and carrier transport mechanism at the metal-semiconductor interface (M-S). There are three carrier transport mechanisms that exist in the M-S interface: Firstly, in the case of low doping concentration, the thermionic emission (TE) model dominates where the temperature plays the main role for carrier transport over barrier height between M-S. Secondly, in the case of intermediate doping concentration thermionic field emission dominates (TFE), and the tunnel barrier also makes an additional contribution. Thirdly, the high doping reduces the tunnel barrier more than the field emission dominates the carrier transport. The following figures explain the phenomenon in detail. In Ohmic contact the thermionic dominant which means non-rectifying M-S contact with negligible resistance which means no disturbance for the current-voltage (I-V) characteristics. 

The contact area mainly determined the total contact resistance, Rc(Ω). It is mainly dependent on the contact area, contact geometry, and obviously the quality of the interface. The quality of ohmic contact is determined by the specific contact resistance ρc (Ω cm² ) and is independent of geometry. ρc depends on Schottky barrier height (SBH), semiconductor doping, quality of the interface, semiconductor surface, metal deposition chamber, etc.

On the other hand, the rectifying M-S contact with asymmetrical and nonlinear (I-V) behaviors is said to be a Schottky contact. This is activated if the metal work function is higher than that of the semiconductor electron affinity. Usually, SBH is varied by surface states, metal-induced gap states, defects, and a thin interfacial layer. The interfacial layer having atomic layer thickness allows tunneling of charge carriers. The potential drops across the interfacial layer result in the lowering of SBH. The surface sometimes acts like an acceptor i.e neutral when empty and negative when occupied and the opposite is true. These acceptors are distributed within a forbidden gap resulting in control of the fermi level. The SBD may also be lowered by the image force called the Schottky effect. The image force is nothing but coulombic force from interface electron to far away positive charge.

To learn more about structural characterization please visit this page.

What is lattice mismatch in hetero-structural growth?

Introduction

The hetero-structural growth is the growth of A material on B material having different lattice constants. The different lattice constant results in structural deformation which leads to defects in the grown A material. The discrepancy of the A material quality (in terms of crystal perfection) can be easily understood by the lattice mismatch percentage. Lattice mismatch is a very important terminology to understand hetero-structural growth. In hetero-structural growth, the mismatch percentages play an important role to decide the performance of the proposed structure. The researcher can judge their expectation with the reality of the mismatch. This is very popular in device physics to judge defects or defect-free growth (strain film less defect), structural deformation for a new role(tetragonal to ferroelectric orthorhombic phase change leading to new properties of the material in new structure, etc.). 

How to calculate lattice mismatch?

The mismatch is nothing but a mismatched strain in the film against the substrate is used. For example, if we consider Si substrate and HfZrO₂ film of different crystal structures as shown in table I. Then the mismatch can be defined in terms of percentage as follow:

                            Lattice mismatch strain= [a(film)-a(substrate)]/a[substrate]  -----------(1)

Table I: Si and HfZrO2 (HZO) lattice constant

Using equation (1), we can have the lattice mismatch strain in the film. Table II shows the mismatch percentages between cubic Si and HfZrO2 corresponding to their lattice constants executed from the relation. 

Table II: Lattice mismatch percentage between Si and HZO

Using equation (1), we can have the lattice mismatch strain in the film. Table II shows the mismatch percentages between cubic Si and HfZrO2 corresponding to their lattice constants executed from the relation. The sign of the strain can be negative or positioned. The negative sign indicates compressive stress which means the lattice constant of the film is smaller than the substrate. More elaboratively, the compressive stress exists in the film (film lattice smaller than substrate), therefore tensile stress is necessary to accommodate the film lattice with the substrate lattice in the in-plane direction (figure 1). 

Figure 1: Lattice mismatch in-plane lead strain in the film

What is the significance of the mismatch sign?

Fundamentally, the strain is nothing but a ratio of the change in length, radius, area, or volume with respect to its original value and stress in the amount of force applied to or exerted by a body to its surrounding. Please review basic mechanics to understand the detail of the strain and stress phenomenon.

 The opposite is the case for a positive sign. The positive sign of the mismatches indicates tensile stress exists in the film. This tensile stress may lead to the structural change of the film which results in new phenomena in the grown film. Please find the lattice calculation here.

Crystal parameters extraction from mix structure of cubic and hexagonal MnAs/InAs/GaAs(111)B heterostructure system.

 X-ray diffraction (XRD)

    Crystal structures are very important for device technology. Specifically, single-crystal or epitaxial grown material systems play a key role to develop device technology on the right track. So, studying crystal structures of novel materials systems is quite important. It is well known that the crystal structure of solid is widely examined by x-ray diffraction (XRD) techniques. The wavelength of the x-ray is around 0.1 to 10 Å. The material structure study is carried out based on the interference principle of x-ray with the materials system. Through the interaction, we can extract crystal parameters. The interference obeying a famous law defined by Bragg is known as Bragg law. We consider a simple crystal structure where crystal parameters (a=b=c) are connected with miller indices (hkl) and interplanar spacing d. It is seen that a monochromatic x-ray beam has a coherent radiation incident on a crystal as shown in the schematic figure 1. 

Figure 1: Diffraction relation examines.

The atomic lattice which constitutes diffraction is from parallel planes in symmetric measurement. For the simple cubic system, the d and a, and hkl are related by the following relations,

Similarly for a hexagonal unit cell d is connected as follows. 

Bragg's law derivation

The diffraction peaks are visible from constructive interference of diffracted beams from parallel planes. It is necessary to have an in-phase of diffracted beams after leaving the crystal planes. We labeled beams 1 and 2 in figure 1. Path difference between the two diffracted beams (EF+FG) will be equal to an integral number (n) of the wavelength of the incident x-ray.

EF+FG= nλ (3)

EF=FG then, Sinɵ=EF/d

Therefore, EF=dsinɵ, then equation (3) becomes

nλ=2dsinɵ (4)

It is known as Bragg’s law. This explains the angular relation of diffracted x-ray with their wavelength. For first-order diffraction, (n=1) Bragg’s law becomes,

λ=2dsinɵ (5)

At the diffraction conditions incident angle ω is equal to the diffraction angle ɵ. Through this measurement, we only can have information about interplanar distance and hence lattice constants. The basic XRD measurement setup schematic is shown in figure 2.

Figure 2: Schematic of symmetric XRD measurement 

Example of lattice parameter extraction

Now, we consider MnAs/InAs/GaAs heterostructure where MnAs is a hexagonal ferromagnetic metal, InAs and GaAs are cubic compound semiconductors. The structure is very important for spintronic device applications. Anyway, we have carried out XRD symmetric diffraction, where incident angle ω and diffraction angle ɵ maintain the same angle during the 2ɵ/ω measurement. Figure 3 shows the measured XRD result. From figure 3, we see diffraction peaks from different planes from respective materials. Hence with angle position information and corresponding Miller indices (hkl), we can extract the lattice parameters using equations (1), (2), and (5). The corresponding lattice parameters of cubic GaAs and InAs are 5.65 and 6.05 Å respectively. The lattice constant of hexagonal MnAs is around 5.71 Å. 

Figure 3: Symmetric ω/2ɵ XRD results of MnAs/InAs/GaAs(111)B

However, for the early rising researcher, this kind of basic study is quite important to develop themselves in science and technology. The above discussion is somehow different from a usual discussion about crystal structure study.  I believe that my little effort will be helpful to understand the mix-crystal structure study. Therefore, I would like to invite people who are really interested in studying crystallography. You may also have an interest in TLM devices, ferromagnetic phase transition-related problem solving, or Schottky diode evaluation for power device applications. These are available in my blog also please check here.