Nanowires: Third NW sample

Faebian Bastiman

After comparing the results from the first and second NW samples, the decision was made to further increase the As flux. The SEM results (Figure 1) indicate that the NWs have indeed doubled in length compared to the first sample. A good indication that the As flux is still slightly deficient. The only way to test this is to further increase the As flux and see if the wires continue to lengthen, whilst maintaining the same diameter. A shorter, broader wire could constitute more NW volume (and hence more GaAs going into the NW). Total GaAs growth is a combination of NW length, area and 2D parasitic growth. Great care must therefore be taken to evaluate the III:V deposition rates.





Figure 1: SEM image of 3rd NW sample

The 3 samples thus far have something in common. They all have significant parasitic growth. Since we have established that the As flux was too low, we can now assume that the growth temperature is also too low. When one increases the growth temperature, one must also increase the V:III ratio in order to balance As loss. Rather than continue to plod along toward the correct conditions we are going to take a logical leap of faith. We will increase the growth temperature by 40°C AND once more double the As flux. Why do we feel justified doing this? We are justified because…

  1. We suspect we are growing too low in temperature
  2. The As flux must be increased whenever the growth temperature is increased
  3. Even if we fail to grow NWs we gain a data point, a boundary condition. We find a point at which NWs DO NOT form. This is very valuable at the moment, perhaps even more valuable than continuing to produce NWs.

The other possibility for the large amounts of parasitic growth is that the oxide is too thick. To address this we will need to try annealing the substrate at higher temperatures. The images also show a significant number of tilted NWs. This is also a good indication that the oxide is too thick. However, being systematic is also sometimes time consuming. You cannot change to many variables at once or your results will not be closely related enough for you to logically plan your next step. You can of course randomly stab at the problem until you obtain the result you want, however in so doing you display all the scientific prowess of a trained monkey. Let’s try to be clever about this. If we can establish a good algorithm to optimize our samples that is applicable to ALL MBE optimization problems, we can significantly reduce our workload and feel confident that we can expand into ANY and EVERY material combination. We reach the level of experts and more importantly we actually know what we are doing! You can see how our logical leap of faith turned in the following article: Nanowires: 4th NW sample.

2 thoughts on “Nanowires: Third NW sample

  1. Pingback: Nanowires: Second NW sample | Dr. Faebian Bastiman

  2. Pingback: Nanowires: Fourth NW sample | Dr. Faebian Bastiman

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