Influence of the V/III ratio

GaN and the other III-nitrides have the specialty to be grown either under Ga-excess or N-excess unlike other III-V semi-conductors. However, the N-rich growth conditions are required in both approaches for the NWs to develop [19, 134]. Thus, it seems that theV/IIIratio is the most important growth parameter for the formation of GaN NWs by MBE. To study its influence on growth in more detail, both the Ga- and the N-rate were varied separately. NW growth rates were extracted from cross-sectional SEM im-ages as described in chapter 2 and compared to the nominal growth rate corresponding to the rate of the species in minority. In principle, the NW growth rate determined this way could be wrong if the nucleation time is very long as seen in the previous chapter.

However, at the growth temperature of 730^◦C the nucleation time is negligible in com-parison to the growth time (see chapter 3) for all samples except for conditione(lowest N-rate) for which the nucleation time lasts up to 10 min for the catalyst-assisted growth (sampleA0e) and already almost 30 min for the catalyst-free growth (Samples S1e and S0e). However, in these extreme cases, the loss of thickness was estimated from the QMS profiles to be on the one hand 6 nm (0.01 Å/s) and 40 nm (0.09 Å/s), and both values are within the range of uncertainty.

Influence of the N-rate

At first, the Ga-rate was kept constant while the N-rate was decreased down to a value close to but above stoichiometry (conditionsa,d, ande). Figure 4.12 presents the mor-phology of the corresponding samples. Under these conditions, the NW length

de-Figure 4.12:SEM images acquired at an angle of 90^◦ and 45^◦ (inset) to the surface normal of catalyst-assisted (first row) and catalyst-free GaN NWs on Si(111) and Si(001) (second and third rows, respectively) for various N-rates. Scale bar is 1µm.

creases with the N-rate for the growth on sapphire with Ni-catalyst as well as for the growth on Si substrates without any catalyst. In the case of self-induced growth at a V/IIIratio just above stoichiometry, Calleja et al.[129] already reported a low density of short nanocolumns emerging from a compact columnar layer. Similar results were observed for both approaches (see inset in Figure 4.12). The growth rate of this colum-nar layer, which is more compact for the growth on sapphire than on Si, matches the growth rate of sampleA0I corresponding to the supplied Ga-rate. In general such a bimodal morphology is clearly obtained for moderateV/III ratio but when the excess of N is increased the NW growth rate is enhanced for both approaches and the colum-nar layer is not noticeable anymore, but instead there are thicker and shorter columns between the NWs.

Figure 4.13:Growth rate of (a) catalyst-assisted (samplesA0a,A0d,A0e) and (b) catalyst-free GaN NWs (samplesS0a,S0d,S0e,S1a,S1d,S1e) as a function of the N-rate.

The growth rate of the NWs as a function of the N-rate is plotted for these experi-ments in Figure 4.13. For comparison, the N-rate and the Ga-rate are also reported in the graphs. In the case of growth on sapphire with Ni catalyst [Figure 4.13(a)], the NW growth rate matches closely the N-rate, similar to planar growth under Ga-rich conditions, instead of the present N-rich ones. This result can be fully explained by the growth model proposed in reference [19] that is described in detail in section 4.6.2.

However, on bare Si substrates 4.13(b) a different situation is observed and the results are very similar for both orientations of the Si substrates. ForV/III ratios higher than one, the average growth rate of the NWs is significantly lower than the N-rate but higher than the Ga-rate and only the few longest NWs grow at a rate approaching the N-rate. An interpretation of these observations is given in section 4.6.2 on the basis of the existing growth models [22, 23, 26, 27].

Influence of the Ga-rate

Next, the N-rate was kept constant and the Ga-rate was halved and doubled, respec-tively (conditionsa,f andg). SEM images of the corresponding samples are presented in Figure 4.14. In the case of growth on sapphire with Ni catalyst, for the lowest Ga-rate (sample A0f) the NW length is strongly reduced while for the higher Ga-rate (sample A0g) the NW length is almost unchanged in comparison to the medium Ga-rate (sample A0e). In addition, for the lowest Ga-rate planar growth is observed between the very short NWs (see inset) whereas for the highest growth rate coalescence-like feature typ-ical for lowV/IIIratio is forming a discontinuous base between the NWs. The growth rate of this base (0.9 Å.s) slightly exceeds the Ga-rate (0.8 Å.s). Note also that this base let the NWs root uncovered as if it will have formed after the NW nucleation. For the catalyst-free approach, no obvious difference is observed for the two different substrate, Si(001) and Si(111). The NW length increases with the Ga-rate and for conditiong, the longest NWs are as long as the catalyst-induced NWs.

The growth rate extracted from Figure 4.14 as a function of the Ga-rate is plotted in

Fig-Figure 4.14:SEM images acquired at an angle of 90^◦ and 45^◦ (inset) to the surface normal of catalyst-assisted (first row) and catalyst-free GaN NWs on Si(111) and Si(001) (second and third rows, respectively) for various Ga-rates. Scale bar is 1µm

ure 4.15. The N-rate and the Ga-rate are also reported on the graphs. For the catalyst-assisted NWs, the growth rate matches again closely the N-rate except for the lowest Ga-rate of conditionf. Once the Ga-rate is set over a critical value, NW growth occurs and saturates at the N-rate, even for the higher Ga-rate. This further confirms that the supplied N-rate imposes the NW growth rate. In the case of growth on bare Si sub-strates, the average growth rate is still significantly lower than the N-rate but increases with the Ga-rate for the investigated range. For growth under conditiong, the growth rate of the few longest NW reaches the N-rate. Therefore, we conclude that for the catalyst-free approach, the N-rate is the upper limit of the NW growth rate in agree-ment with the previous results of Songmuang et al. [23]. A possible explanation for this result is presented in section 4.6.2.

Figure 4.15:Growth rate of (a) catalyst-assisted (samplesA0a,A0f,A0g) and (b) catalyst-free GaN NWs (samplesS0a,S0f,S0g,S1a,S1f,S1g) as a function of the Ga-rate