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!