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.

Theory, techniques for Schottky barrier diode evaluation

 Introduction:

In the previous discussions, I have clearly discussed the reason for forming Schottky contact. So, to evaluate the Schottky contact (SC), it is necessary to understand the relevant parameters those are governing the Schottky contact response. The parameters are Schottky barrier height (SBH), (Φ_B), ideality factor (η), effective Richardson’s constant (A**), doping concentration (N_D), and series resistance (R_s). The current (I)- voltage (V) characteristics are mainly for the thermionic emission model for a Schottky barrier diode (SBD). The SBD is very promising for fast switching, high current, low noise, and high-frequency applications. 

Theory of SBD:

The SC of SBD governing equation is shown in equation (1). Where, S Schottky contact area, T operating temperature in Kelvin, V applied voltage, K_B Boltzmann constant,  Richardson’s constant, A* material’s properties.  

The A* is influenced by the optical phonon scattering, quantum-mechanical reflection, electron tunneling over the barrier, inhomogeneous barrier distribution, and, metal contact properties. Therefore, taking these into consideration, A* becomes A**. Then equation (1) can be replaced by (2), where I_s reverse saturation current or leakage current and Φ_B0 Schottky barrier-height at zero bias.

The ideality factor is unity for the TE model. This measure deviation from the TE model of the Schottky contact. The ideality factor considers all the factors that affect the TE model. The factors come from carrier generation, field emission, recombination, thermionic field emission, image-force lowering (Δϕ_B) of the SBH, inhomogeneity of barrier, bias dependence of the SBH, edge leakage etc. For high-quality SC the ideality factor is within 1-1.3.  The image force lowering SBD can be expressed by equation (3), 

where, E_MS maximum electric field strength at the interface, this is more pronounced in reverse bias. The E_MS value is related to the bias voltage and can be expressed by equation (4). However, if the diode series resistance plays a crucial role when the current becomes too high. Then the total voltage drops across the diode and series resistance. The I-V relation is modified by the IRs drop. Then equation (2) becomes like (5) is known as Cheung’s method. 

SBD Evaluation: I-V and C-V methods

When we apply forward bias, and the bias V ≥ 3k_BT/q, then equation (2) can be represented by equation (6). Equation (6) can be written in linear form like (7). The linear fitting intercept gives a reverse saturation current, and the slope represents the ideality factor. Hence using the reverse saturation current relation shown in (2), we can estimate the SBH at zero bias with the known value of the Richardson constant. 

Along with the I-V relation, it is also necessary capacitance-voltage (C-V) relation to extracting the SC parameters. The reverse bias generates a depletion region which means the SBD behaves like a parallel plate capacitor. For an n-type semiconductor under the reverse bias condition, the C-V relation is shown in equation (8). C capacitance per unit area. The slope of 1/C² – V plot represents doping concentration N_D. Also, the intercept, Vi, of the plot related to built-in potential, V_bi. 

Then the zero bias SBH can be calculated by equation (9). where E_C-E_F is related to the effective carrier density (N_c) of the state and follows the relation in (10). 

The Nc strongly depends on the effective mass and operating temperature (equation (11)). 

Hence, from I-V and C-V, we can judge SBD applicability. You may find useful of SBD example in my previous explanation. If you need some help with your understanding, then feel free to comment or reach me.

Top 7 scholarships in European countries

European Government Scholarships for foreign students

Scholarships in Europe

British Chevening Scholarships (UK)

Chevening Scholarships are a global scholarship program by the UK government. They are awarded to outstanding scholars from Chevening-eligible countries. You can find here if your country is Chevening-eligible or not. A one-year Master’s degree is typically awarded by this scholarship. Full tuition fee coverage, a certain living allowance, airfare to the UK, and some additional grants.

DAAD Scholarships (Germany)

The German Academic Exchange Service (DAAD) provides scholarships to students from developing countries at German Universities. The students of the developing countries in Africa, Asia, Pacific Islands, Central, and South America, and Central and Eastern Europe are the main beneficiaries of the scholarship. The scholarship supports a variety of full or partial scholarships for the scholars.

Holland Scholarships

The Holland Scholarship is only for foreign students from outside the European Economic Area (EEA). Students who want to do their Bachelor’s or Master’s course at participating Universities in Holland are eligible for the scholarship. The scholarship money is up to € 5,000. The beneficiary will receive this scholarship money in the first year of your studies.

Swedish Institute Scholarships (Sweden)

The Swedish Institute scholarship is provided to the highly-qualified international students from developing countries who want to study at universities across Sweden. Scholarships are offered only for master’s level. Tuition fees, living expenses, some travel grants, and insurance are covered by the scholarship.

VLIR-UOS Scholarship (Belgium)

The students from 54 developing countries in Asia, Africa, and Latin America are eligible for VLIR-UOS Scholarship. One can get a scholarship either for an English Taught Training (ETT) or Master’s Program related to development at Universities in Belgium. The scholarships cover full tuition fees, accommodation allowance, travel allowance,  and other program-related allowances.

Eiffel Excellence Scholarship Program (France)

The Eiffel scholarship program is provided by the Ministry of Foreign Affairs. This is used as a tool to attract the best international students for master's and Ph.D. programs to French institutions. This scholarship, however, does not cover tuition fees. The beneficiary will have a monthly allowance, travel costs, and program-related allowances.

Swiss Government Excellence Scholarships (Switzerland)

The Swiss Government Excellence Scholarships (SGES) provide scholarships to graduates from around 180 countries around the world with the opportunity to pursue doctoral or postdoctoral research in any field at one of the recognized educational institutions in Switzerland. The scholarship covers a monthly stipend as well as tuition fee coverage, health insurance, lodging allowance, and other allowances as per requirement.

You may also like to read about the top five most demanding Japanese scholarships here.

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.

5 best scholarship in Japan for international students

Scholarships in Japan for international students 

In terms of technology and education, there is no doubt that Japan has progressed its best. This is why, the country  is consistently  attracting international students from all over the world. There are five types of higher education institutions to be admitted. Undergraduate schools, graduate schools, professional training colleges, colleges of technology, and junior colleges are those. These institutions are open for both local and international students.

The tuition fee at most Japanese universities are comparatively lower than other developed countries like the U.K. Also, there are several scholarships and tuition reduction programs available. This makes the study ever cheaper here. Are you looking forward to study in Japan but can’t afford the fees? Look at these most popular scholarships in Japan.

Best Scholarships for International Students in Japan

1. MEXT Scholarships

You can find the Scholarship Link here. 

Scholarship Allowance: Full tuition fee coverage, accommodation, and other allowances like health insurance and taxes

Degree offered: Bachelor’s, Master’s and PhD

MEXT is offered by Japan’s Ministry of Education, Culture, Sports, Science, and Technology. This is a fully funded scholarship. One can typically have around 180000 Japanese yen per month. This helps the international students who are straggling financially during their study program. It’s available to both undergraduate and postgraduates at any Japanese university.

The MEXT authority announces the application process typically in April of every year. You can then follow the guidelines posted on the official scholarship website. You may notice that there are several MEXT scholarships. But the most populars are Young Leader’s Program and the Japanese Government Scholarship.

2. Scholarship of Japan Educational Exchanges and Services (JEES) 

You can find the Scholarship Link here

Scholarship Allowance: Full Tuition fee coverage

Degree offered: Bachelor’s, Master’s and PhD

JEES offers a variety of scholarships for international students living in Japan. The scholarship can be  based on either their field of study or country of origin. They are available to students in all levels of studies, starting from junior colleges to doctoral degrees. The program has collaboration with private companies or persons who are willing to support students.

Depending on the eligibility one can apply to one of the five different categories of JEES scholarship. But the most popular ones include Japanese language learning program. Other scholarships are Asian students and students of selected fields of study. You can go through their official website to check which one suits you.

3. Waseda University Japanese Government Scholarship

The Scholarship Link is for your smooth navigation. 

Scholarship Allowance: Full Tuition fee coverage and accommodation allowance 

Degree offered: Masters and Ph.D.

To enhance the International Priority Graduate Program (PGP) Japan government has selected some universities within the country. Waseda University is one of those. The university offers a chance to every international student applying for a graduate program. One thing worth noting is that, this is part of the MEXT financial aid programs.

The Graduate School of IPS is responsible for students assessment. Every single application at IPS Waseda will be assessed. If someone passes the test, IPS forwards his information to Japanese Government. The selected students will then receive the award. Make sure you visit the official scholarship page for more information.

4. Japan Foundation of UNU

Scholarship Link is here for you.

Scholarship Allowance: Tuition fee waiver and up to $1100 allowances

Degree offered: Ph.D.

As an international student, you have a chance of doing a doctoral degree in Sustainability Science in Japan with fully covered tuition fee by JFUNU. You are automatically considered as a candidate when you submit the initial application for the course. If you are selected, you’ll receive monthly allowances of 145,000 yen as well as waived tuition fee. These allowances will only be valid for 3 years.

You must be from a developing country to be eligible for this scholarship. But if you are pursuing a second Ph.D. in the same university then you will not be eligible for the scholarship.

5. KUAS Undergraduate Scholarship

Scholarship Link is here for you.

Scholarship Allowance: Partial or full tuition fee allowance 

Degree offered: Bachelor’s 

KUAS also offers a institution-based scholarship programs to the undergraduate students. KUAS-E and Super KUAS-E are the two main types of scholarships. Depending on one’s academic performances one can get up to 100% tuition fee waiver. In addition, the students also receive $110,000 yearly allowance.

If you want to learn more, Please visit the official scholarship page.

You can also find my recent blog post here

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.