Figures 3(a)–3(c) show that temperature and pressure have the dominant impact on the -and the -planes, as far as determining the relative size and their surface roughness. Use of a high temperature has reduced the proportion of - to -plane observable compared with the facetted GaN core because the temperature increases the relative growth rate of the facet, and thus, this sample has only a small residual surface. Furthermore, there is a tendency for indium atoms to stick at the apices between planes, which create more irregular surfaces where the adatoms can be captured, leading to nonuniform-wavelength light emission.17 At 700°C and 300 mbar, the relative growth rates of the nonpolar planes are closer in value, causing a different - to -plane growth rate ratio. Also the indium desorption and diffusion length are both lower than at 750°C, making the incorporation of more indium atoms possible. As the indium atom diffusion length becomes comparable to the distance between binding sites on the sidewalls, there is a visible roughening of the sidewalls. In Ref. 37, it is indicated that a statistical roughening of the surfaces can be expected on the (0001) and (000-1) surfaces if the diffusion length becomes shorter than the mean distance between binding sites. Surface roughening encourages local fluctuations in the InN mole fraction on the nonpolar planes. Reduction of the growth pressure to 100 mbar, while keeping the temperature at 750°C, had the effect of reducing the InGaN growth rates of both the - and -planes, to an average rate of , as the increase in the size of the overall NR is smaller. The presence of residual -planes on this sample indicates that the relative growth rates of the - to -plane differ from the other sample grown at 750°C. Note the reduction in growth pressure tends to increase indium surface diffusion,38 because low pressure reduces the indium atom collision probability. In support of this, the roughness has diminished on the -plane in Fig. 3(c) compared to Fig. 3(b), while the -plane is still very rough even at low growth rate. The size of the -plane is determined by the initial GaN template, where high temperature and low pressure reduce the relative growth rate difference between the - and -planes. With the range of growth parameters available, the -plane grows as fast as or faster than the -plane, suggesting that finding a growth recipe to create a substantial -plane surface area would be difficult to achieve for a core–shell device.