All the way from NTNU – home, detour edition

I can’t keep these photos in my hard drive. I guess I can put in here :p

The first three photos are the lab where I mostly work. This molecular beam epitaxy (MBE) machine are considered as a new in here. We had the opening celebration last March. Official photos are available in this NTNU Flickr album.

MBE Lab 1

MBE Lab 1

MBE Lab 2
MBE Lab 2
MBE Lab 3
MBE Lab 3

 

Now I am out of the building. Time to roll out from university, back home. I believed it was Saturday, 25 July 2015. Normal distance I usually took from university to home is about 3.5 km with 25 min biking time. Since the weather was extremely good that day, I decided to take a little detour back home. The distance increased to 11.6 km and it took 45-60 min to bike back home.

NTNU main building
NTNU main building
View in front of NTNU main building
View in front of NTNU main building
Roundabout in IKEA Leangen
Roundabout in IKEA Leangen
Fjørd view from Brundalen 1
Fjørd view from Brundalen 1
Fjørd view from Brundalen 2
Fjørd view from Brundalen 2

 

This is the route I took. Just a raw footage.

Detour
Detour

My first GaN growth experience

After a year and four months waiting, last week I had a chance to actually grow GaN nanowires on Si substrate. Yep, it was my initial episode of my experimental work in this field.

As that time was my debut, actually and honestly, I did not expect something big to happen. I did not put any pressure on myself on getting “proper” nanowire structure. However, I put my best in choosing what parameter to choose with reference from Wierzbicka’s work.

“Growing GaN nanowire growth is more straightforward than GaAs nanowire.” That was a statement I received from my three friends who are much more experience than me. Such words encouraged and challanged me at the same time. I did not know which side I should take, but I choose to take both of them.

The growth of GaN nanowire needs two sources: Ga and N. Ga adatoms, in this case is the flow rate, can be controlled by determining the temperature in the Knudsen cell. N atoms in the other hand, are supplied in the form of plasma from the plasma source. The N atoms flow rate can be adjusted by setting forward power and N gas flow rate. Another important parameter is substrate temperature, where Si substrate sits during the growth. In this substrate, the reaction of Si substrate, Ga adatoms and N atoms interact with each other. Depending on the growth parameters, such growth can result either nanowire or thin film structure.

On Monday, the first day of growth, we made a slight mistake. After nitridation process, we did not notice any Ga adatoms were deposited on the nitrided silicon surface due to the too high substrate temperature. The observation of Ga adatoms are made possible thanks to the reflection high energy electron diffraction. The diffraction pattern were the same before and after 1 hour growth, which was diffused in the background pointing an amorphous surface. We decided to use the same sample for the second attempt.

Tuesday, the second day, our experiment was conducted at the same growth condition with the previous one, except lower substrate temperature. As a comparison to the earlier growth, after like 20 minutes of growth, I could notice that the diffraction pattern were slightly changed where a weak spot were aroused among the diffuse background. I was quite happy at that time, and decided to continue the growth up to another 1 hour and 40 minutes. After one hour, I came back to the lab and observed the diffraction pattern: a stronger spots arranging themselves as it is called as hexagonal or wurtzite crystal structure. Just before the growth was finished, I decided to take another diffraction pattern. I got more convinced that I will get nanowire structures, as Sobanska has proved in her work.

Wednesday, inspection was conducted using scanning electron microscope. Unfortunately, it did not show any nanowire along the surface we have investigated through. It just like random islands trying to coalescence with the nearest islands. Well, eventhough I did not expect too much from my first growth, I was dissappointed at that moment.

Thursday, after thinking what went wrong, I remembered that we set too much growth rate to get the desired structure (based on Calleja’s observation). It was not intentional though. We made miscalculation when considering the length of 450 nm for one hours from Wierzbicka’s work. It was for two hours.

Friday, the second growth using a new sample was conducted, of course with higher expectation. We decided to use the same growth condition, but altered only the growth rate of Ga adatoms. As a consquence of lower growth rate, the diffraction pattern did not show any interesting bright spot after even one hour growth. It was just… nothing compared with the higher growth rate. It was understandable though, but I was becoming really skeptical with my result. A slight changes on the diffraction pattern before and after growth was able to be noticed. Not bad.

Growth was done in 2 hours and 30 minutes. Regarding the diffraction pattern, weak spots appeared and their intensities were nothing compared to the growth with higher growth rate.

In the evening, the same day, we did scanning electron microscopy measurement on this grown sample. Well, it was not a prefect nanowire structure found on the substrate surface, but at least a slightly nanowire-look alike-structure was there! It was better than the first growth at least.

The nanowire were standing very close to each other. I believed it was in the order of 5-10 nm with the closest vicinity. It was so densed and more uniform compared to the GaAs growth, from my point of view. Nevertheless, the growth was inhomogeneous across the wafer. The better nanowire structure was observed close to the edge and getting worse as it located in the center part. My friend thought it was because of the non-homogeneous heating from the sample holder. I just need to talk with the lab engineer.

Hmm… I wonder what to change in order to get more space between nanowire? I hope I can get a better one this week!

Summer in Trondheim, 25 July 2015

After weird summer for the past 2 weeks, finally the sun comes out today.

A good time to test my new Go Pro ;)

It is entry level 2014 edition though, but so far I am satisfied. I bought with 8 GB + vented helmet strap mount. I hope I can be more prepared documenting my life in here and onward.

I just… do not understand how Trondheim forget how to summer. Anyway, I want to this moment.

View from my office in the university. What a perfect day!
View from my office in the university. What a perfect day!

Igniting Nitrogen Plasma

This is my first note on stuff related with my research on molecular beam epitaxy or MBE for short, a technique I will be using for next 3-4 years ahead to make epitaxial grown gallium nitride nanowires on silicon.

Ok, where should I begin? To describe it in simple way, MBE takes place in really huge machine (compared to other techniques like metal organic chemical vapor deposition). One advantage of using this technique is the higher purity of the grown material compared to other technique or in fact, the highest purity quality that we can get. This fact is supported by the condition that need to be maintained inside the MBE machine itself, so-called high vacuum to ultra high vacuum pressure. As an illustration, we live in the atmospheric pressure, where this pressure is equal to 1.000-1.000.000.000.000 lower than atmospheric pressure. It implies the number of contaminants is getting less and less as the pressure goes to ultra high vacuum. Actually, ultra high vacuum provides “the straight pathway”, where intended gas can travel long enough reaching surface of the material without any collision with other gases.

Growing epitaxial layer on top of the substrate can be depicted easily with lego blocks. Imagine you have green big plate and you want to make something on top of it, say a house (Figure 1). The lego house is made of many block components and colors. It can be a 2D house or detailed house, depending the creativity and what passion one has. The same thing goes with MBE.

Figure 1. Green plate and some blocks can make simple house. With more creativity and passion, one can make the sophisticated house
Figure 1. Green plate and some blocks can make simple house. With more creativity and passion, one can make the sophisticated house

Now think like this:

Green big plate = substrate

Block components = gas

How the machine looks like, I will write another post for it. This method of growth, where you start from the scratch of substrate where there is nothing on top of it and putting one by one intended component on the substrate, we call it as bottom-up method. Another method is called top-down method, which is similar with carving a statue from single block of stone. Bottom-up method requires more time than top-down method, but guarantee better structural quality.

===

Back to the title. Honestly, this is my first time for me to be trusted with one component of MBE. Since my research is gallium nitride, it means that I need nitrogen to be introduced in the process later on, and the MBE in my professor’s lab is introducing nitrogen by using a plasma.

This end of June was the second opening of the MBE made by Veeco, namely GEN930. Opening in here was done because the some sources (for gases) were nearly finished and repairing electronics of the substrate holder. After opening was finished, then we baked (put lot of panel on it, basically cover the whole surface of the MBE system) it for nearly one week. The baking was done to reduce as much as possible unwanted particles (because of the opening: exposing some part to the atmospheric pressure) and make sure the pressure can go down low enough to ultra high vacuum.

At first week of July, we turned off the baking mode and removed the panel. After connecting all necessary cables, the source outgassing part was done. Once again, the outgassing is required to make sure that the contaminants are minimized to the lowest point. Afterward, dedicated baking for the nitrogen line source was carried out, but no big panel was required. We simply cover it with conductive heating element. After another two days, the baking was finished and it was removed. We need two weeks to do all of these work. Finally, the last step must be done before getting the MBE ready to work: source calibration.

Before doing calibration, my professor would like to test the plasma source, since there was a change in aperture plate (nozzle). The reason to replace with the new one was the attempt to get high brightness mode of nitrogen plasma. This mode is required to grow gallium nitride efficiently. Honestly, I have not tried the growth with low brightness mode of nitrogen plasma source. Probably it would not be appearing as I wanted.

However, there were two main problems which we encountered. First, as we turned on the chiller (for cooling the plasma source), my friend noticed a leakage on input and output of the hose, from the autotuner to the plasma source. Not only leakage in that part, but there was noticed as well on the OD stainless pipe in the plasma source. We discovered the caused which was deformed O-rings, which we forgot to take them out before baking of nitrogen line source. After I got the required O-rings from workshop near my home (great exercise though ^^), we managed to install them and run the chiller with the cooling system not being leaked.

Now, the second problem. We got the plasma at the first time (mediated with argon gas owing to its lower ionization energy than nitrogen gas) and after decreasing the power, suddenly the plasma was off and no plasma was observed since that time. It took about 5 minutes for us to recognize a burnt smell on the autotuner unit and the worse thing was the accumulated heat on the surface near the RF shield. My friend was afraid that he broke the autotuner, but discovered afterward that the smell actually came from the remnant of the leakage liquid that had happened before. The heat was found to be high inside the RF shield itself.

The day ended as my friend sent an e-mail to Veeco engineer, describing the situation. Next day, my friend took a day off and I was the only one left in the lab. The Veeco engineer replied the message with several questions, meaning that I was the responsible person to answer those questions. I was not really familiar with the things like required flow rate, cooling flow direction and RF reflected power, so I needed a bit of time to adapt with it. It took me almost 2 hours to understand the questions, compose the answer and send it back to him.

After I had replied the e-mail, the lab engineer came to the lab and I forwarded the message from my friend. He suggested to clean the copper electrode inside the RF shield, since it was oxidized during nitrogen line baking. As a note oxidized copper is insulating. He used sand paper to remove the oxide from copper electrode. He also explained another electrode which connects to the copper is made from silver and oxidized silver is conductive, so he need to clean the copper electrode only. After putting everything back, we tried to ignite the plasma, but we failed. Still, there was abnormal heat in the same place.

Returning back to my office, I received an e-mail from the Veeco engineer. We talked a bit over the phone and he asked me to send a graph of the forward power, tuning cap and load cap over time.

The day after, I did as he requested. Based on the user manual, I tried to re-ignite the plasma and oberved the important parameter of the graph. Afterward, I sent the exported data of the information, for the two days measurement duration. It took no more than 1 hour for him to analyze the graph! That was such fast response, especially during lunch time. He called me once more and suggested to me that the chiller temperature need to be lowered. He also sent me the procedure to do regarding the difficulty in igniting the plasma.

Well, I was bit skeptic with the new procedural document. In that time, my friend who took day off the day before, coming to my office and encouraging me back to do the new step. I was surprised as the first time and so did my friend, that the required flow rate of the nitrogen was really high, 4 times higher than the highest flow rate we used in the previous experiments. We were afraid of having much high pressure as a result of massive nitrogen flow rate in the plasma source chamber. However, the document said the limit that should be kept an eye for, so we did that and it was fine actually, the pressure still in the safe zone.

After about one minute introducing such high flow rate, I stood close to the autotuner and heard a motor moved. I did not know what it mean though, but I saw a slight RF reflected  power changed. I went to the computer and observed the graph. There was a slight change in tuning and load cap over time. Without any further second thought, I peeked the viewport and voila! The nitrogen plasma source was successfully ignited! You can see how the nitrogen plasma looks like in Figure 2.

Figure 2. The presence of nitrogen plasma is indicated by the orange color
Figure 2. The presence of nitrogen plasma is indicated by the orange color

I can’t describe how happy I was that time. Probably it can be described like an adventurer who finds a treasure after several days wandering in the desert. I let the nitrogen plasma on for one hour, to make it stable. At the beginning, I feared of the unusual accumulated heat in the RF shield. I monitored the heat in every 5 minutes. Surprisingly, the RF shield went warm at stable temperature. A second happiness :)

I stayed in the lab for another two hours, just to make sure that I can re-ignite the nitrogen plasma again and again and again and again… Well, several times. The nitrogen plasma was actually reproducible and I can play a bit with the forward current and flow rate from the set point, running from high brightness mode to low brightness mode.

That Friday evening was one of the best Friday I have ever had in my life :))

 

Crystallography (II) – Unit Cell, continuation

Basically there are two distinctively types of unit cell in constructing the lattice: primitive and non-primitive unit cells. First, let us take a look a unit cell on the Figure 1a, which is the example of primitive unit cell. If we look closely, the total part of the lattice points (from each corners) which contributes in forming the primitive unit cell is one, as illustrated from Figure 1b.

 

Figure 1a. Primitive unit cell
Figure 1a. Primitive unit cell

 

 

Figure 1b. In total, there is one lattice point in the primitive unit cell
Figure 1b. In total, there is one lattice point in the primitive unit cell

 

Let us now consider unit cell in Figure 2a. We notice there is another one lattice point inside the unit cell and it sums to be two lattice points (Figure 2b) in building this unit cells. Generally, for the type of unit cell which possess more than one lattice points is called non-primitive unit cell. Therefore, we can distinguish whether the unit cell is primitive or non-primitive by counting the number of lattice point contributing to one unit cell, regardless the shape of that unit cell.

 

Figure 2a. Non-primitive unit cell
Figure 2a. Non-primitive unit cell

 

Figure 2b. In total, there are more than one lattice point (in this example, there are two lattice point) in the non-primitive unit cell
Figure 2b. In total, there are more than one lattice point (in this example, there are two lattice point) in the non-primitive unit cell

Determining the lattice whether it is composed of primitive or non-primitive unit cell can be also have the same criteria as it has described earlier: The symmetry of unit cell -> having higher symmetry is more likely to be real unit cell, and The volume of unit cell -> having smaller volume is more likely to be real unit cell. To understand this, let us consider lattice point in the Figure 3a. Next, we can draw parallelograms in this lattice to form possible unit cells, shown in Figure 3b.

 

Figure 3a. The lattice
Figure 3a. The lattice

 

Figure 3b. Possible unit cell from various parallelograms
Figure 3b. Possible unit cell from various parallelograms

As we can see from Figure 3b, at least there are three possible parallelograms drawn on the available lattice point. The rectangular (green) is chosen as the unit cell because it has the smallest volume while at the same time possesses the most symmetry compared to other hexagonal and parallelogram (red). The symmetry in here includes rotation and reflection symmetry. This result implies that it is not always the smallest unit cell chosen as the unit cell, but it must also contain largest possible symmetry. Therefore, the unit cell composing this lattice is non-primitive.

Let’s say we have different arrangement of lattice, as it is illustrated in Figure 4a, which is more compact than lattice point in the Figure 3a. As usual, we try to find possible parallelogram for this unit cell 4b. We found out the unit cell for this type lattice arrangement is hexagonal unit cell (red), for it is having 6-fold symmetry rotation and 6 symmetry reflection. The rectangular one does not longer fulfill the criteria as unit cell, not as capable as the hexagonal one. Since this unit cell has two lattice points in it, we can say that hexagonal unit cell in this lattice arrangement is non-primitive.

 

Figure 4a. The lattice
Figure 4a. The lattice

 

 

Figure 4b. Possible unit cell from various parallelograms
Figure 4b. Possible unit cell from various parallelograms

Now I think we are done with 2D and have enough understanding to move further to 3D lattice with its 3D unit cell.

Further reading: http://www.doitpoms.ac.uk/tlplib/crystallography3/unit_cell.php

Crystallography (II) – Unit Cell

So we have talked about lattice in the previous part. One should know that lattice is actually composed of the smallest possible regular (repetitive) array of unit cells, which is defined as the smallest building blocks of the lattice. First, let us inspect what the unit cell of the crystal structure from Figure 6 is:

Figure 1. Identification of unit cell from crystal structure in Figure 6 from https://andreaslm.wordpress.com/2015/06/21/crystallography-i-basis-and-lattice/
Figure 1. Identification of unit cell from crystal structure in Figure 6 from https://andreaslm.wordpress.com/2015/06/21/crystallography-i-basis-and-lattice/

Based on the repetition pattern of the arranged atoms in the lattice, we find that the unit cell in this crystal structure is hexagonal unit cell. Remember that repetition of the smallest possible pattern plays role here.

To elaborate more, how the unit cell build the lattice, let us assume we have unit cell formed as it is illustrated in the Figure 2a As we stack this unit cell, in the sense of arranging the unit cell with the closest neighboring unit cell and so on, eventually the lattice is produced, as we can see from Figure 2b.

Figure 2a. Unit cell
Figure 2a. Unit cell
Figure 2b. Lattice
Figure 2b. Lattice

Ok, now let us dive to the real example, a NaCl crystal structure, Figure 3. The gold atoms are Na and green atoms are Cl. There are three unit cells identified in this crystal structure and from our understanding of the relation between between unit cell and lattice, there must be only one unit cell composing the lattice. It means that we need to examine the possible options out of three unit cells and eliminate the other two.

Figure 3. NaCl crystal structure with its possible identified unit cells. Source: http://minerva.mlib.cnr.it/mod/book/view.php?id=269&chapterid=77
Figure 3. NaCl crystal structure with its possible identified unit cells. Source: http://minerva.mlib.cnr.it/mod/book/view.php?id=269&chapterid=77

All the three unit cells have the same possibility to arrange the NaCl crystal structure by put the respective unit cell one to each other. To determine which is the real unit cell, we need to consider:

  1. The symmetry of unit cell –> having higher symmetry is more likely to be real unit cell.
  2. The volume of unit cell –> having smaller volume is more likely to be real unit cell.

Basically, there are two different shapes of unit cell in here: A being parallelogram, B and C being square. The unit cell A has 2-fold axis of rotation, meaning that if you pick one corner and rotate it 360° (either clockwise or counterclockwise), it only fit every 180° rotation to its corresponding shape (Figure 4a). The unit cell B and C has 4-fold axis of rotation, as it firs perfectly when it is rotated 90° (Figure 4b and 4c).

Figure 4a. 2-fold rotation of parallelogram, unit cell A
Figure 4a. 2-fold rotation of parallelogram, unit cell A
Figure 4b. 4-fold rotation of square, unit cell B
Figure 4b. 4-fold rotation of square, unit cell B
Figure 4c. 4-fold rotation of square, unit cell C
Figure 4c. 4-fold rotation of square, unit cell C

We also discover that according the volume of the unit cell (remember, lattice is composed of the smallest possible pattern), the rank of compactness is unit cell C being the most dense with minimum space between atoms followed by unit cell A and unit cell B for having the widest empty space among the atoms.

Based on the two established criteria, we can conclude that unit cell C is the real unit cell composing lattice and thus the crystal structure of NaCl.

The figures and concept I made in here are based on these sources:

http://departments.kings.edu/chemlab/animation/untolat.html

http://minerva.mlib.cnr.it/mod/book/view.php?id=269&chapterid=77

Crystallography (I) – Basis and Lattice

Finally, I can make a note for myself as a continuation from last post. I hope you will find this useful.

———-

Have you ever wondered how atom arrangement in solid? Then welcome! If you interest with this, I recommend you to study further in crystallography, a branch of physics investigating the arrangement of atoms in the crystalline solids. With the aid of transmission electron miscoscopy (TEM), the possibility of having resolution up to nanometer scale, hence seeing the real atom is realized, and even down to Angstrom scale with the help of aberration-corrected technique in the TEM. An example of how atoms look like:

The crystal structure of the mineral cordierite taken from approximately 200 Angstrom thick using high resolution TEM. Source: A. Putnis, Introduction to Mineral Sciences, Cambridge University Press, 1992 - frontispiece, available from http://www.doitpoms.ac.uk/tlplib/crystallography3/intro.php
Figure 1. The crystal structure of the mineral cordierite taken from approximately 200 Angstrom thick using high resolution TEM. Source: A. Putnis, Introduction to Mineral Sciences, Cambridge University Press, 1992 – frontispiece, available from http://www.doitpoms.ac.uk/tlplib/crystallography3/intro.php

The atoms are represented as a white spots arranged in 6-fold rings, while the black area corresponds to hollow channels through the structure. The distance between the black spots is approximately 9.7 Angstrom. If we take only one 6-fold rings, compare it with its vicinity first, and observe throughly the whole figure, do you notice something? Yes, It looks like all the atoms have the same size and they repeat themselves in regular manner, where, in this case in x-and y-direction. Of course, when it comes to bulk structure, the arrangement of atoms will have the same behaviour in x-, y- and z.

Here we deal with two important concept. First concept is what we call translational symmetry, defined as the invariance of a system under any translation, as it is illustrated in Figure 2. With this characteristics, the size of the atoms will not change as it translates from one place to another.

The shape of the circle is maintained as it experiences translation symmetry from point A to point B
Figure 2. The shape of the circle is maintained as it experiences translation symmetry from point A to point B

Generally, the number of atom which exist in here is not limited by how many atoms and whether it is identical or different atoms, as it is depicted in Figure 3. It can be only one atom (Cu), two identical atoms (Si), two different atoms (GaAs or GaN) or up to \textit{n} number of atoms (protein crystals). No matter how many atoms exist in here, as long as it follows the translational symmetry, this/these what composes/compose basis.

The number of atom and its size is maintained as it experiences translation symmetry from point A to point B
Figure 3. The number of atom and its size is maintained as it experiences translation symmetry from point A to point B

The second part is what we called as lattice. In mathematical point of view, lattice is actually specific coordinates in space. The reason why these atoms are arranged regularly is because of they sit on the lattice. By observing the lattice points, we can identify what pattern these atoms are forming. Once more, the arrangement of atoms in lattice is required to be a repeatable. So, how to recognize the lattice then? This flash animations of Lattice Point Game is good starting point. Here I put my own result (selected lattice is highlighted with yellow color) in Figure 4

Identifying brick pattern
Figure 4. Identifying brick pattern, taken from http://www.doitpoms.ac.uk/tlplib/crystallography3/lattice.php

In the same way, we can identify the lattice pattern of Figure 1, and in the Figure 5a we recognize it has triangular pattern. Furthermore, as we observe other than recognized lattice pattern carefully, we notice that there is another lattice pattern. Figure 5b shows two different arrangements of lattice, one with triangular with black dots on it (red) and one without the black dots (green). Each of them forms different orientation of triangle.

Identifying lattice pattern
Figure 5a. Identifying lattice pattern, taken from Figure 1. Source: A. Putnis, Introduction to Mineral Sciences, Cambridge University Press, 1992 – frontispiece, available from from http://www.doitpoms.ac.uk/tlplib/crystallography3/lattice.php
Two lattice patterns
Figure 5b. Two lattice patterns, taken from Figure 1. Source: A. Putnis, Introduction to Mineral Sciences, Cambridge University Press, 1992 – frontispiece, available from from http://www.doitpoms.ac.uk/tlplib/crystallography3/lattice.php

Now, by putting basis (atom/atoms) on lattice, the crystal structure is formed. As we can see from Figure 6, the crystal structure will have same size of atoms everywhere (fulfilling criteria of the translational symmetry) and they are arranged periodicaly (lattice).

Lattice + Basis=Crystal structure

Figure 6. Combination of basis and lattice forms crystal structure
Figure 6. Combination of basis and lattice forms crystal structure

That is all for the first part. The concept introduced in here is heavily influenced by these resources:

http://www.doitpoms.ac.uk/tlplib/crystallography3/lattice.php

http://www.physics.iisc.ernet.in/~aveek_bid/PH208/Lecture%202%20Crystal%20lattice.pdf

http://www.tf.uni-kiel.de/matwis/amat/def_en/kap_1/basics/b1_3_1.html