**Faebian Bastiman**

After the successful nucleation of NWs, optimisation can begin. The course to optimisation is identical, regardless of the quality of the NW in the first NW sample. Our first NW sample revealed some tell tale features that we would like to address (see Nanowires: First NW sample for more details).

To begin the optimisation, we must first state our desires: I would like (i) to minimise parasitic 2D growth, (ii) to grow straight (non-tapered) wires, (iii) to grow ZB (non-WZ) wires (iv) to grow long (>800 nm) NWs, (v) to grow narrow (10 – 100 nm diameter) NWs, (vi) to achieve a higher (preferably controllable) NW number density.

The variables we have to play with are fairly limited. We have (a) oxide conditioning temperature, (b) growth temperature, (c) III:V ratio and (d) the absolute Ga flux . These latter 3 represents the Archetypal MBE variable set. It gives us a 3D variable space to step through. For now we can ignore (d) and focus on the first 3 variables. Later, once (a) is established and fixed, we can eliminate it as a variable and concentrate on (d). Once (a), (b), (c) and (d) have been explored we can do more exotic things, like using a different III:V ratio during the nucleation c.f. steady state growth for example. For now we will keep it simple: We do this by fixing two of the three variables (e.g. (a) and (b)), and systematically varying the third (c) over a suitable range. Then we select the best results and fix (c) and (a), and vary (b) over a suitable range. An so on until we have mapped out a wide area and defined some upper and lower limits. Sadly the variables are not independent, particularly every (b) will have an optimal (c).

In order to begin we need to take a logical first step and analyse the result to see if we are travelling in the right direction. In this particular case the first NW sample resulted in NWs of an average 375 nm length. We hypothesised that the As was deficient and decided to increase the As by 33%, whilst keeping all other parameters the same.

The result of our experiment can be seen in Figure 1. In this sample the NWs are on average 500nm long – which is indeed 133% the length. We can conclude that As was deficient and hence can be confident in the taking the decision to increase the As flux further for the next sample. In fact let’s double the As flux compared to the first NW sample.You can find out what happened in the following article: Nanowires: Third NW sample.

a |
b |

Figure 1: SEM images of GaAs NWs grown self-seeded on Si(111) epiready substrate |

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