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.

Well, it is a nanowire!

No nanowire growth has been done this week. Since there are two of my friends planning to use MBE with nitrogen-free chamber next week, the engineer decided to let growth chamber of MBE in the idle state or no nitrogen-related growth takes place for two-three days. Based on his experience, the decided recovery time is supposed to be sufficient enough to get nitrogen away from growth chamber, indicated from the pressure as low as 10E-10 or even lower. It means the growth chamber is in the ultra high vacuum state. The reason is because of my friends will grow in arsenic rich condition where the presence (even the small portion) of nitrogen is not desirable. Even with small portion of nitrogen, the growth sample will have diluted-nitride on it, a well-known method to alter energy band-gap of semiconductor.

No growth means characterization. I brought few of the grown samples to the scanning electron microscopy laboratory in the material department. I went training two weeks ago and did some two measurements where the results are not well-adjusted, according to my friends who has expertise in this area. In order to fix my weakness, I asked them, not once but twice to come with me but of course with different persons as I know they have their own work. Well, I think my efforts are fruitful in the sense of achieved better resolution image resulted from less distortion from astigmatism, focus and aperture optimization. Some tips and tricks were given to me , such as finding the beam location, the proper working distance (it is not in-lens type), in-plane and out-plane stage rotation.

I learned scanning-tunneling electron microscopy in Nanolab. It is bit different than the one in the material department where a transmission can be done in the same machine. Of course there is a dedicated machine to do transmission electron microscopy. As a result, such a small sample is required before it is loaded into the sample holder. I am impressed with this machine because it has way much faster venting and pumping time owing to the small chamber volume compared to the scanning electron microscopy in the material department. One of my friend always jokes that in order to get the machine in high vacuum, the user has to pay 75-100 NOK. In addition, scanning-tunneling electron microscopy in the Nanolab uses in-lens objective lens meaning the users do not have to bother much adjusting working distance as this parameter is the fixed value. With this type of lens, the resolution can go down to 0.5 nm but the drawback is located in the very limited sample size. Overall, both machine has similarities.

To use this wonderful machine, there is a theory, practical, self-training and examination part that I had to participate and pass. Generally, I did not have any difficulty in adapting with this machine. Just… The way of removing and inserting the sample holder is new for me. I have to rotate a bit, pull, rotate all the way, pull again, press “Air” button. The same method with opposite direction when I have to put it back. In one occasion of inserting the sample holder, after “Vac” button was press, I forgot to wait until the LED blink. I just force it to rotate and as a result, an alarm was beeping >_< I just realized my mistake after 5 minutes :p During this time, I was really afraid that I broke the machine 😄

I showed my scanning electron microscopy images to the one who assisted me growing the first 7 samples. I suggested that no nanowire was observed in these samples, even with the help of AlN buffer layer. Before meeting with him, I knew that he would be disappointed with these results. Surprisingly, he did not show that sign at all and said that those short things were actually the nanowire, especially his own growth! Well, those words are in fact what I needed for the efforts we have put in the first attempts of GaN nanowire growing. Though they were not properly forming as a nanowire due to its short length. He suggested that the next growth with higher nitrogen flow. I am thinking to grow with much longer time, probably 2-3 times than what I did in the last 7 samples.

As for the last growth, I used double Ga flux rate to confirm whether the nanowire will increase its height by double. It did not turn to the expectation. It had the same height approximately with the the others. Of course, as the Ga flux was doubled, the physical mechanism on GaN nanowire formation was slightly changed. For instance, the edge has more two dimension growth (see picture below) resulting in higher non-uniformity toward the center. What a bad sample unfortunately, but I learned something at least. Maybe a lower growth rate can be planned as well? Such extreme growth rate was done by Galopin et al, almost half of the assigned Ga flux rate I set.

Growth comparison between double (left) and normal Ga flux rate (right). Two dimensional film is dominating in the left picture
Growth comparison between double (left) and normal Ga flux rate (right). Two dimensional film is dominating in the left picture

Further GaN growth experience with MBE

Another two weeks, from the last week of July to the first week of August were scheduled with the attempt to grow nanowire structure of GaN material on the substrate of Si. There were five samples in total I could grow within these period. Considering the normal workload, the number of grown samples were quite normal.

I think I will not have any growth chance for the next 3-4 weeks ahead, since there are other person who also will use this MBE requiring different chamber condition. That is why the MBE system need to be stabilized for some time, 2-3 days before a cleaning and purifying stage are conducted from Nitrogen environmental condition.

I was thinking to grow two-three samples each day last week. But I guess it was impossible to do, since the growth itself took 3 hours added with another 2-3 hours calibrating the flux of the source, outgassing the sample and preparing the chamber in such way to favorably grow in suppressed contaminated environment which can destruct the growth of GaN. Moreover, I am not confident enough and officially not allowed to transfer substrate from one chamber to another, or else I mess up with the machine. Fallen substrate holder in any chamber is not really good idea, since the engineer has to vent the chamber, pump back to vacuum, bake the chamber to remove the contaminants-oxides and another source calibrations here and there. Believe it or not, this sequence will take around 3-4 weeks. That is why I do not want to mess up. Simple reason why I can’t grow more than one sample is because of the available substrate holder for the 2 inch wafer is only one. Well, at the same time I am relieved.

Normally, I start ramp down the load-lock chamber at 9 AM. While it is ramping down, I calibrated the required source fluxes to be met at certain number so that it equivalents to the growth rate I want. Well, It takes like one hour. Then my friend helped me to transfer the sample from load lock chamber to heater station located in buffer chamber. Another hour spent to let the sample heated to remove any possible contamination, until the pressure is low enough. Finally the sample is transferred from buffer chamber to growth chamber. Yet the growth can’t be started directly. The real outgassing is done in the growth chamber. Why is it real? Because this process undergoes very high temperature up to 900 °C. It can’t be done in the buffer chamber, as the highest temperature is limited up to 600 °C. It is taken to avoid As evaporation on the substrate holder which has been used previously with the As based sample or growth. The evaporated As atoms can make the viewport to be darkened, making the grower can’t really see and therefore disturbing the transfer process. While in the growth chamber, not much to be seen and it is ok to let the outgassing being done in the very high temperature.

Outgassing process itself takes time and once more, very low pressure or good vacuum pressure indicates less and less contamination exist on the surface of the sample. Once it is done, then the growth can be started. Generally, these steps are the same for all typical growth using MBE, no matter what substrate and kind of structure. One thing to remember is that the different sample may undergo different outgassing temperature. For instance GaAs substrate must not exceed than 650 °C to avoid evaporation of As even in the As chamber condition. Without As help, the temperature for GaAs must not exceed more than 400 °C. If it being done, the sample will be ruined even before the growth has not started.

Now what I did for the past two weeks actually can be divided into two different methods. First was directly grown on Si and the second part was helped by buffer layer of Al. The first growth was exactly similar with the previous weeks with the differences in the nitridation treatment and growth temperature. The reasons for changing these two growth conditions were to put more nitrogen incorporation in the nanowire structure and further increase the probability of getting nanowire structure instead of thin film. Well, after scanning electron microscopy measurement, I had a feeling of what the nanowire looks like, better than the last growth but the problem located on how short the grown nanostructure was. The growth time was almost two and half hours, but the height was less than 200 nm where I expected to be at least 400 nm.

Then at the second growth, as my friend suggested me that using Al deposition forming AlN after nitridation can increase the probability of getting nanowire structure, way much better than directly on bare Si. There are many reports using this method and one example is by Songmuang et al. After some discussion, in what sequence the Al and N shutter should be opened and the expectation of what formation will be occurred, finally the plan was written down. Few considerations have been taken to avoid and suppress the formation of SiN by our plan. The growth process was observed using reflective high energy electron diffraction where the formation of AlN and GaN can be witnessed.

The result was not as I was expecting. The grown structure was the same with the structure without AlN buffer layer: short grown nanostructure. I knew there was an issue on the inhomogeneous heating where I could find different structure from the edge to the center of the wafer. The first of two last growth experienced different substrate temperature during AlN formation and slightly higher growth temperature. For the last growth which I have not checked using scanning electron microscope, I increased the growth rate for Ga source. Why? I want to re-confirm my understanding whether I will be getting higher height of grown nanostructure. By using this growth rate, I may get planar thin film instead of nanowire structure. Well, my objective for this sample is not to get the nanowire structure, but to obtain different nanostructure and if it is proven, then I will be happy 😀

I will show the “giant” foot step mark I found in my sample during measurement using scanning electron microscope. I am not quite sure why there is suppressed growth on these area and I can find quite a lot structure similar like these actually. Many factors such as contamination or fracture can give rise to such kind of suppressed growth.

"Giant" foot step
“Giant” foot step