Ga- and N-rich condition

The bottom-up method employing either MBE or MOVPE technique provides different possibilities, for growing the epilayer crystal which is decided by what growth condition is used. In this case, let us consider one of the III-V material: GaN crystal growth on our substrate.

To grow GaN, we need Ga and N atoms being supplied from the respective source, where Ga source cell and nitrogen gas or ammonia for MBE or TMGa and ammonia for MOCVD. In here, we can choose either the grown crystal is situated in Ga-rich or N-rich condition.

Depending on the what environmental that GaN crystal is grown, its structure is largely influenced by the majority of atoms dominating the growth of the crystal. It is well-established that Ga-rich condition prefers formation of GaN planar film-2D grow (for example a paper from Ramachandran C, et al from Carnegie Mellon University), while N-rich condition promotes GaN nanowire-3D grow (for example a paper from Callarco R and Marso M from Research Center Julich). There is an important point to be understood in here. What is actually Ga- and N-rich conditions?

I notice there are two important factors in determining how the growth is either Ga-rich or N-rich condition. First is the growth temperature and second is the flux contributing to the growth of GaN epilayer.

Growth temperature can be designed from the very beginning and we can directly assume what is actually the preferred condition is. Ga start desorbing at temperature around 600 C, both for GaAs (source) and GaN (source) system. Then Ga atoms situated at the growth above this range of temperature (higher than 600 C) will start to desorb from the surface of the substrate, and at the same time, only N atoms stay there. As a result, we have more N atoms and therefore this condition is referred as N-rich condition. For the growth conducted at the temperature below 600 C, Ga atoms will likely to stay in the surface of the substrate. This situation is called Ga-rich condition.

Determining flux is rather bit hard. One has to calibrate with the GaN planar film growth, where two condition has to be applied (Ga- and N-rich). To calculate Ga growth rate, the condition has to be in N-rich. Just imagine when the surface is saturated with N, then the only factor that governs the growth is limited Ga atoms due to the desorption. In the other hand, N growth rate can be calculated by using Ga-rich condition. Too much Ga atoms will saturate the substrate surface resulting Ga atoms can’t do much in controlling the structure growth. In fact, the limited presence of N will direct the growth of GaN. I learnt this from Heying et al (2000) and Koblmueller (2003). Next, one has to divide the ratio of Ga/N. If the value is more  than 1, it means it is Ga-rich condition, while value less than 1 is N-rich condition.

Growth of GaN nanowire itself is normally done at high temperature and Ga/N<1, implying the growth being done in N-rich condition. I encountered papers from Calleja’s group from Spain, where there is a possibility of having nanowire at Ga-rich condition (1, 2). Despite grown on high substrate temperature, Ga/N is more than 1. I am still looking the answer for this question. It might be that high growth temperature is more dominant?

This is approximately the same with other III-V semiconductor material, such as GaAs in Ga- or As-rich condition.

Front and back side of pyrometer

Pyrometer is basically a thermometer which can work in distance to measure the temperature of a surface. It is a touch-free thermometer.

The temperature measurement comes from the thermal radiation emitted from the measured surface, as it has its finger print in the range of spectrum.

Normally, the temperature measured in the growth chamber of MBE is measured by pyrometer, not the temperature set in the heater via computer. So much bias information will be given by that set temperature as many factors such as sample holder type, distance between coil and the sample and contamination affect the difference between the real temperature and the set temperature.

That is why pyrometer comes in handy. Beside we can measure the temperature directly on the surface, consideration of sample holder type can be neglected and distance between coil-sample is not something to be very worried about, since the measurement is actually done in the surface of the sample. The only thing need to be remembered is the contamination of the window where the pyrometer is looking at.

In the first setup, the pyrometer is looking at the sample surface directly with only heated viewport in between of them. The heated viewport is a window where the pyrometer observes the sample surface, and this window is heated to certain degree so that deposited metal can be evaporated as soon as possible, i.e. no metal accumulates in the viewport that can deviate temperature measurement.

First pyrometer setup
First pyrometer setup

In the second setup, I notice there is no such kind of controller that can increase temperature in viewport. I am actually do not know how they manage to clean the deposited metal. Maybe during opening schedule? Anyway, I made such a mistake when I tried to use the pyrometer. They have fixed pyrometer with a dial on the right hand side with a sign of “裏パイロ” and “表パイロ” that you can turn 360 deg. At the beginning, I did not really understand what “裏パイロ=back pyro” and “表パイロ=front pyro” actually mean because there is only one pyrometer in that growth chamber. During the past two weeks, sometimes I helped on recording the temperature by using this device, which basically just turning 180 deg to open the pyrometer and another 180 deg to close the pyrometer. Everything went well until last Wednesday.

As usual, I turned the dial 180 deg and my very first time looking at the target which is far off to the left of the sample. So, what I did was shifting the position of pyrometer to the right. After I did that, the recorded temperature was lower than the normal one in the previous growth (with the same set growth temperature). Then I asked help and they have fixed it for me. Again, after they left, I turned the dial 180 deg and observed the same thing again. Once more, I got very wrong temperature reading. Finally, one friend came to the lab and explained to me that the dial actually controls the side of the mirror where the pyrometer is looking at. The wrong side is “表パイロ” where lot of Ga and other metal contamination on it. That is why I got incorrect temperature value. In the other hand, “裏パイロ” has nice mirror surface with minimal surface contamination.

Second type of pyrometer setup, the correct one. The contamination on 表パイロ is illustrated with circles
Second type of pyrometer setup, the correct one. The contamination on 表パイロ is illustrated with circles

What a day. I almost ruined my friend’s data!

Isolation chamber for MBE

Since the plasma source has been installed in our MBE, honestly I did not really know what the significant of it. What actually was the reason of it sitting there?

The isolation chamber connects the plasma source and the growth chamber. It means there is one shutter controller for nitrogen plasma source and one gate valve that comes from the isolation chamber. Below is the simple illustration:

Growth chamber with isolation chamber
Growth chamber with isolation chamber

Now, how about the MBE where the plasma source is connected directly to growth chamber of MBE (without isolation chamber)? Simply, only shutter of the nitrogen plasma source that governs the nitrogen plasma flow to the sample surface. Here is the simplified sketch:

Growth chamber without isolation chamber
Growth chamber without isolation chamber

The impact of the isolation chamber can be noticed when one has to work with more than one sample in short transition time interval with the next sample. As the plasma is ignited and introduced into the growth chamber, the pressure in in increase remarkably, from 10E-9 to 10E-4 Torr. It is a common procedure that, to exchange the sample with another sample (in buffer chamber), pressure of the growth chamber must be recovered to its normal working vacuum level. This can be achieved either by closing the gate valve (for the system having isolation chamber) or turning off the nitrogen plasma completely (for the system without isolation chamber).

This is the main difference. Terminating nitrogen plasma and evacuating nitrogen, venting it out from the growth chamber takes much more time for the vacuum level back to ultra high level. According to my experience, normally 60 min or more. Meanwhile, for the MBE with isolation chamber, nitrogen plasma source (and hence the nitrogen gas) is still there but isolated from the growth chamber. This is where the isolation chamber plays its role. Compared to the former design, recovery time of the pressure to ultra high vacuum takes at most 6 times faster, barely 10 minutes on the MBE system with isolation chamber.

If the growth plan is finished that day, the evacuation of the MBE with isolation chamber takes the similar manner as the MBE without the isolation chamber. In other words, the isolation chamber increase the time efficiency in growing between samples.

In addition, surface nitridation process is unavoidable reaction taking place for the MBE without isolation chamber. Sometime, one prefers to suppress surface nitridation by depositing, for example Al on the surface of Si and nitridation can be done afterward. This fashion can only be done on the MBE with isolation chamber, as the Nitrogen plasma can be generated before introduced inside the growth chamber. For the MBE system connected directly with nitrogen plasma source, the ignition of the plasma happens inside the growth chamber, where the sample is sitting, resulting nitridation treatment on the surface of the substrate, for some time even without realizing it.

The only consequence I have not discovered is the physical impact of the nitrogen plasma source to the substrate as the distance gets longer. Update will come soon, I hope!

Another new day.. a new experience

Travelling for 20 hours from Trondheim to Tokyo, added with load of works I had done 6 hours before I woke up, continued with packing of what to bring for my long stay in Tokyo, really pushed my endurance to the limit. I had lack of sleep, less food resulting increase of temperature within my body. I did hope to not fall sick during this period and after I arrived at Tokyo.

This time I had to pass three different immigration processes. One was at Copenhagen airport since I was about to leave Europe to Asia, second was at Shanghai Pudong airport where the purpose was to take out my checked-in baggage (it was considered as leaving the airport) and then re-check it in again and the last one was in Narita airport, my last stop of flight. The immigration process (or queuing) was quite slow in Copenhagen, but when it was compared with Shanghai Pudong, the process in Copenhagen was relatively fast. Surprisingly, I did not expect too much in Narita. There were lot of foreigners on the plane flying us from Shanghai to Narita. I thought it would had gone in the same pace as it was in Shanghai Pudong airport.But the fact did not tell like that. I believed that the immigration process in Narita was the fastest I have ever encountered.

The only difference that I felt was how active the immigration staffs were. Ok, I was relieved. Out of immigration, I took my checked-in baggage, another worries came over my head: choosing train from the Narita airport all the way to the place where I am planning to stay. Ok ok, I got the information about the train I had to take from and, but still, it was piece of papers. It was an enormous feeling when I saw ticket machines standing here and there which languages I knew very little. After taking out money from ATM, since the exchange rate is much better than money changer, I went to the counter to buy train ticket. I was planning to get passmo with my name registered on it, but I cancelled it because I did not have my Japanese mobile number. I had a bit of struggle to understand the staff whom I asked from but fortunately, I finally got my ticket :)

Anyhow, my Norwegian phone number was useless in here and fortunately enough get Wifi coverage to send my confirmation arrival and the approximate time of my train to reach there to the staff of that shared house. The problem was, I was already inside the train station located in the basement of Narita airport, which the signal was very low.

Now, catching the train. I was completely no idea how to read the direction for transiting between stations. In every station, I always asked somebody to make sure that I took the correct train. I told myself not to fall asleep at the moment where I was waiting for the upcoming station. Finally, I arrived at the last transit station, however I had to go out from that station because of different train company operating that line. Going out itself was a problem. I did not know that I had to insert my ticket to the check-out machine, but instead putting it in the fare adjustment machine lol.

Finally, out of the station. After 15 min or so looking for the designated station, by the helped of one guy (he was distributing free tissue with an advertisement in it, which I thought night club), finally I found the direction. To reach there, I had to take my luggage, lifted it up to the fourth floors..

I need a new ticket for this one. I did not know where the “English” option in that particular ticket machine. Bit panic since I was late already from the planned time, I looked toward to 3 old ladies (about their 50’s) and asked their help with stupid hand gesture. I did not know what kind of face I showed to them, explaining in broken Japanese and simple English did not guarantee them to catch my message, but they tried their best to understand and help me. I asked whether I was in the correct train station and which ticket should I buy. To my surprise, they gave me 100 Yen to buy 200 Yen ticket (another 100 Yen was from me). I actually had 500 Yen that time, but they were not pretty sure whether it gave the change. One of them said it was a “puresento=present”. We had short chat, like where I come from, what I will be doing in Tokyo and where the university is. The funny thing was, I replied some time in Norwegian, like “nei” or “takk” lol. Once again, they explained to me the number of stations that I would be passing before reaching my destination. Just to make things more clear (in term of language), they told me as well their station where they had to stop together with the number of stations they would be passing. “Wakaru? wakaru” “Do you understand? do you understand?”

In the end, they said, “ganbattene”, meaning “please keep your spirit”.

I was really thanksful for that moment. Then I said thank you to them, and continued to check-in counter. The train finally was able to bring me to the place I am living in here, now. The first day I arrived in Japan was really interesting. I wonder what will encounter me tomorrow?

Trial run lab course

Aside of conducting my research, I have a duty work as a lab assistant for one course work per semester. At the beginning I thought my involvement in this activity will bother me a lot. In fact, I kind of enjoy it.

A pastor in Baptist Church, to whom I usually update what I have been doing in the university, my current general research status, told me once, “that is great, now you have a chance to take a break. Reading a bit, planning for the next experiment and execute it confidently”. For some reason, his words relieve my stress.

What I am looking forward this semester is assisting student doing their coursework. Of course I have legal reason to get out from my office or else I have to sit the whole day in there, most likely with wandering mind if I can’t focus reading. Being with the students also interests me a lot. I can interact with them, showing them something, teaching them what I have learnt before them. I know at some point that they will surpass me and no surprise from my side, as they are the best mind that Norway has at the current moment.

Lab course will be officially started next Monday. I have schedule on Monday, at 8 AM. So, in order to make the lab session fruitful and lively with the student, I and other three lab assistants have conducted trial and run on two first module. We did it yesterday for lithography part and today for scanning electron microscopy (SEM)-energy dispersive x-ray spectroscopy (EDS) part.

Most of the steps I went through for this labwork are the same with the labwork this spring. In here, the teaching assistant basically will show them the basic guidance how to do proper work, and then licensed will be given to them once they pass. After that, they will do their own work, based on the objectives that we have set for them. This labwork will train them to be an independent researcher! Cool!

For the trial and run, since we were not able to find the mask specifically designed for this course, we used the one from the labwork last semester. The rest are exactly the same. It took place in Student lab in Nanolab. We did scribbing, then it is continued to cleaning+dehydration, coating, soft baking, exposing, developing and inspecting the sample; These steps are typical for conventional photolithography. Oh yes, we skipped post exposure bake and hard bake. If my memory serves correctly, we did not do post exposure bake and hard bake due to the datasheet for the negative photoresist we used, does not require these steps.

Some mistakes were done during this lab session. In the scribbing part, we spent quite some time to split the 2-inch Si wafer properly with 111 direction.

Picture 2-inch Si (111) wafer and the scribber direction
Picture 2-inch Si (111) wafer and the scribber direction

It is not easy :p

Errors during the photolithography steps were unavoidable: fallen sample, flipped sample with applied photoresist on it. But overall, it was fun :D

We spent like three hours in total to do the step. The main problem was the produced samples (three in total) were not developed completely even after waiting time for about 10 minutes. The reason can be the expired photoresist or expired developer. Since we will not measure electrical properties of our sample, spots laying around on the channel are not problem. Nevertheless, I will do strict examination so that they have such kind of habit when they are working indepently. Ah, the geometry that we are aiming is Hall bar.

Picture Hall bar
Picture Hall bar

The processed samples were then brought to SEM for geometry inspection, focused especially on the cross section where the undercut pattern is checked. Once they were imaged, the last characterization was done using EDS. This technique can tell us what kind of elements are building up in specific area of the imaged sample. With EDS, we can get information whether we have correct deposited materials on the sample… or not.

Honestly, we had a struggle during elemental mapping using EDS technique. Theoretically, no matter what the size of the analysis area is, it should give what it should be. We had elements which were not supposed to be there, aside of other elements which were successfully mapped correctly. Let’s say we have image as illustrated like this:

Illustration with rectangular and analysis area
Illustration with rectangular and analysis area

Based on the image, we have metal on top of the GaAs sample. The metal itself should be consisted of, for example: Ge/Ti/Pt/Au where Au being the thickest part (200 nm) while Ge, Ti and Pt are around 20 nm, each. The EDS show expected Ga and As mapping where its concentration are more intense (indicated with more words of “Ga” and “As”) in the outside of the metal contact.

Illustration with rectangular and analysis area, Ga and As
Illustration with rectangular and analysis area, Ga and As

In the metal contact itself, we had Pt and Au elemental mapping at the correct position.

Illustration with rectangular and analysis area, Pt and Au
Illustration with rectangular and analysis area, Pt and Au

Unexpected results we got from the two most bottom layer of the metal: Ge and Ti, where they appeared not only in the metal contact, but outside as well. Even more, it looks like the concentration of Ge was lot stronger in the outside area than in the contact.

Illustration with rectangular and analysis area, Ge and Ti
Illustration with rectangular and analysis area, Ge and Ti

After discussion with friend, a suggestion come to use smaller analysis area. Beside larger spot size of the incoming electron, the reason is the scattering of the x-ray signal passing through other parts of the sample, illustrated as rough sketch below (size is not scaled).

SEM signal + scattering
SEM signal + scattering

The results were much better than the previous one with larger analysis area. Each of the registered elements sit in the place where they should be sitting. Especially for Ge, more intense signal came from it with smaller analysis area. The only drawback was that, we cannot image the transition border between the metal and substrate at the same time. If we had done this, the same result would have appeared like the one before. The only thing that can be done were imaging one part standing close to each other, and makes an analysis of them separately.

Illustration with rectangular and smaller analysis area, Ge/Ti/Pt/Au and Ga/As
Illustration with rectangular and smaller analysis area, Ge/Ti/Pt/Au and Ga/As

I am not sure whether this is a correct approach or not. Any suggestion and thought are welcome!

Random thought in the past two weeks

I have had a pause from GaN growth since two weeks ago. It is actually good time to pull back myself from the hectic of growth ritual, where I can read, observe other group works and at the same time (try to) design experimental procedure what to do next. Surprisingly, it is not easy as I though before. To find out what is wrong, what is missing, something to be fixed and improved are tough work. Such kind of works have exhausted my mind to some sense. I tried different ways to implement what so-called active reading of those papers and it turns out to be a little bit better and effective, at least.

One or two questions are popped out in my mind while reading a paper. Sometimes they even do not appear at all. “This is a perfect paper!”, as it has been cited by many other authors. However, the questions which I can find after “digging” diligently throughout the paper, are not superb or wow. I can say that active reading is not easy at all. I may need more and more hours to sharpen this skill.

Most of the time, I have a wonder how is it possible for most academicians and intellectuals that they can easily point out something that is not “right”, just few minutes after they have read a paper or listened to a presentation. Just within some time, a new idea that I have not thought about it, a paper has been published. A friend joke about how these group of people have a dedicated “print” button on their PC for publishing their research data. The critical thinking and active reading which are normally attributed as something that hard enough to be grasped (at least for me), are they transformed into their instinct? Just like how human perceive hunger and without any further thought, he/she will look after food.

Such a disappointment of not having ability like them, has aroused many times for these past two weeks. No, I think even more than that. Such an inevitable circumstance since I have decided myself to start living my life in this way. Not easy path to live in, but I hope I will have such kind of instinct like that one day.