Influence of the temperature and V/III ratio on the nucleation pro-

monitoring of the Ga desorption rate in real time. Similarly, the influence of the growth parameters on this Ga desorption is further investigated, and three different tempera-tures, Ga- and N-rates were explored separately. However, to make a detailed study as in section 3.2.1 one would need to compare each experiment with the reference one of growth on a bare sapphire substrate. This was not done for all of these samples, and RHEED monitoring was not carried out either. Thus, it is not possible to detect any change of the Ni₁−xGa_x phases and determine the observation of stage 2. That is why this brief stage is ignored in the following and only stage 1 and 3 are considered.

The identification of these stages is indicated on the QMS profile in Figure 3.13(a). As before, stage 1 corresponds to the increase in the desorption of Ga for the duration t1 associated with the net amount of incorporated GaA1, while stage 3 is the decrease in the Ga desorption to which the durationt3and the net amountA3are associated. The steady state is indicated as stage 4. At this point it is though necessary to point out that A1as well asA3correspond to Ga incorporated both into the Ni islands and on the bare sapphire left exposed between the Ni islands. Note that for both nucleation stages, if the Ga incorporation rate does not change with the modified growth parameter, an increase of the stage duration should produce an increase of the Ga amount incorporated. Last, to understand these results, it is necessary to recall the investigation of Koblmuelleret al[46] on the nucleation of GaN on sapphire and SiC by QMS. They showed that the GaN nucleation under Ga-excess was extremely delayed and accompanied by a higher Ga reevaporation rate leading to an important GaN loss of thickness. This higher Ga reevaporation was related to the decomposition of unstable subcritical nuclei. Hence, similarly to these resultst3can be associated to the nucleation delay for the growth of GaN.

Influence of the temperature on the nucleation processes

Figures 3.14(a) and 3.14(b) illustrate the evolution oft1andt3as well asA1andA3in dependence on the substrate temperature. To this aim samplesA0a,A0bandA0cwere grown at 730^◦C, 755^◦C, and 780^◦C, respectively, under the same nominalV/IIIratio as previously. Their morphology is illustrated in Figure 4.16 in chapter 4 where it will also be commented. These graphs show that the durationst1of stage 1 andt3of stage 3, as well as the amountA1increase with the nucleation temperature only slightly between 730^◦C and 755^◦C but significantly between 755^◦C and 780^◦C, while the amountA3first increases between 730^◦C and 755^◦C and then drops at 780^◦C.

Figure 3.12:Cross sectional HRTEM images along[1¯210]_GaNof (a) sampleA0t1and (b)A0t2. In both cases, growth was terminated under Ga-rich conditions. Each image is followed by the calculated 2D Fourier transform of areas corresponding to (a) the sapphire, the left and the right particles, (b) GaN and the particle. Theβ-NiGa phase is unambiguously identified.

0 50 100 150 200

Figure 3.13:(a) The 3 stages of the nucleation as identified by QMS. Stage 1 corresponds to the incorpo-ration of a Ga amountA1on the substrate surface (Ni particles and bare surface) during the durationt1. No GaN is formed yet. Stage 3 corresponds to the nucleation of GaN with the delayt3and the amountA3of GaN formed. Stage 4 corresponds to the growth of GaN in the steady state. The QMS profile corresponds to sampleA0ggrown at 730^◦C with a Ga-rate of 0.8 Å/s and a N-rate of 2.0 Å/s. (b) Calibrated QMS profiles of Ga desorption during nucle-ation at 780^◦C on bare and covered by Ni islands sapphire substrates (samplesA0IIandA0c, respectively).

The increase in nucleation time can be explained by a higher Ga-reevaporation during both stages. In addition, A1 seems to vary linearly witht1although more statistic is required to evidence an exact dependence. In contrast,A3does not vary monotonously witht3indicating that the Ga incorporation rate changes with the temperature for stage 3. Thus the incorporation rate dependence on the growth temperature is different for each nucleation stage.

Assuming that during stage 1 no GaN has formed yet, for each stage different mecha-nisms would be in competition. During stage 1 only the mechamecha-nisms of Ga incorpora-tion into the Ni seeds and Ga desorpincorpora-tion would occur, while for stage 3 GaN nucleaincorpora-tion and GaN decomposition would also to be considered. Thus the difference in Ga incor-poration rate between both stages could be related to GaN growth and decomposition that would affect only stage 3. The assumption that no GaN forms during stage 1 is in agreement with the experiment done at 730^◦C. Moreover, for higher growth tem-perature, during stage 1 the Ga supplied is still fully incorporated into the Ni seeds in contrast to nucleation on bare sapphire where it almost fully desorbs as seen in Figure 3.13(b) presenting the QMS profiles of nucleation on bare sapphire and sapphire cov-ered with Ni seeds at 780^◦C. In addition, the GaN nucleation time on bare sapphire also increases and less than 0.3 ML would have nucleated during stage 1 on the bare sapphire exposed between the Ni seeds. Last, the decrease in Ga incorporationA3 re-lated to GaN decomposition during stage 3 observed above 755^◦C would be consistent with experimental results on GaN decomposition [48]. These experiments showed that the rate of GaN decomposition exponentially increases above 750^◦C under active N-excess whereas under Ga-rich conditions or vacuum, decomposition sets in already for temperatures as low as 700^◦C [36].

740 760 780 20

40 60

Stag

Growth Tem perature [°C]

t1

0.5 1.0 1.5 2.0

30 60 300 400 500

t1 t2 (c)

Stageduration[s]

N-rate [Å/s]

0.2 0.4 0.6

0 100 200 300 400 (e)

t1 t2

Stageduration[s]

Ga-rate [Å/s]

Figure 3.14:Duration of the nucleation stages and corresponding amount of incorporated Ga as a function of (a), (b) the growth temperature (SamplesA0a,A0b,A0c), (c), (d) the N-rate (SamplesA0a, A0d,A0e) and (e), (f) the Ga-rate (SamplesA0a,A0f,A0g). The straight lines are guides to the eye.

The effect of the Ga desorption and thermal decomposition of GaN is directly observ-able in the QMS profile of sample A0cgrown at 780^◦C for which the reference exper-iment of nucleation on bare sapphire (sample A0II) was also carried out. The corre-sponding Ga-desorption profiles are shown in Figure 3.13(b). In both cases, growth on bare sapphire and growth on sapphire covered by Ni islands, the duration of the whole nucleation process and the corresponding loss of thickness are significantly increased in comparison to the growth at 730^◦C (see Figure 3.1 for comparison). Moreover, in the steady state corresponding to stage 4, the Ga-desorption rate is different from zero.

This means that the growth rate is effectively lower than the supplied Ga-rate and that Ga desorption and GaN decomposition are not negligible effects. Also, the decompo-sition rate might be slightly different between samplesA0candA0II. Possible reasons are that facets of different orientation are involved and that the effective surface of the NW sampleA0cis larger.

Interesting enough is that during stage 1 the Ga desorption from the sample covered with the Ni islands (sampleA0c) is still reduced in comparison to the one from bare sap-phire (sampleA0II). The duration of this stage lastst1=58 s which is almost the double in comparison to one measured during nucleation at 730^◦C. The desorbed amount of Ga during the initial stage is in GaN equivalent thickness 4 Å for sample A0cand 22 Å for sampleA0II. Still assuming a fractionS=70% of the sapphire surface not cov-ered by the Ni particles for sampleA0c, by the same calculation as in section 3.2.1, a Ga concentration ranging from 65 % to 82 % is obtained. This range is higher than the one deduced at 730^◦C and would correspond in the binary NiGa phase diagram [145, 147]

to the Ga-rich portion where for this temperature theβ’-Ga₃Ni₂phase is in equilibrium with the liquid one as illustrated in Figure 3.4 by the left-hand side shaded box. Hence in this case it is expected that the VLS mechanism probably sets in. However, it should be noted that the extraction of the composition from QMS profiles is not very exact and that the Ga concentration ranges for samples A0a andA0calmost overlap. Also it is questionable to what extent bulk phase diagrams can be applied to nanoscale particles [106]. In any case, the richer Ga composition of the Ni islands for higher temperature observed here is consistent with the results of Venugopalanet al[150] and Aurongzeb et al[166]. These groups reported an increasing dissolution of Ga into Ni with temper-ature in the range of 600^◦C to 900^◦C. A RHEED analysis as done in section 3.2.2 would be needed to assertain if effectively another phase than NiGa is observed before GaN growth sets in and whether it is solid or liquid.

Influence of theV/III ratio on the nucleation processes

Figures 3.14(c) and 3.14(d) illustrate the evolution oft1andt3as well asA1andA3in dependence on the N-rate. To this aim the samplesA0a,A0dandA0eare now consid-ered. These samples were grown at 730^◦C with a N-rate of 2.0 Å/s, 1.1 Å/s and 0.5 Å/s respectively, but with the same Ga-rate of 0.4 Å/s. Their morphology is illustrated in Figure 4.12 in chapter 4 where it will also be commented in detail.

For these samples, the durations t1andt3as well as the amounts of Ga incorporated A1andA3are very similar. When the N-rate is decreased from 2.0 Å/s to 1.1 Å/s, the

composition. This result was interpreted in term of ternary phase diagram for which two pseudobinary sections were evidenced between GaSb, Ga and Au so that it was concluded that growth occurs along both these sections. Note that the ternary phase diagram of the Ni-Ga-N system shown in appendix 5.2 also present similarly for the temperature range considered two pseudobinary sections with tie lines between GaN, NiGa and Ga3Ni2 and that for larger N-ratet1andA1are almost unchanged indicat-ing a stable Ga incorporationA1into the Ni-seeds before GaN nucleation. However, a similar study by XEDS would be needed before such conclusion could be drawn.

The increase of Ga incorporationA3observed for a lower N-rate is apparently in con-tradiction with the lower decomposition rate of GaN occurring for higher active N-rich excess reported by Fernández et al.[48]. However, as the N-rate is decreased, t3 in-creases faster thanA3. Hence, the mean Ga-incorporation rate estimated form the ratio A3/t3during stage 3 effectively decreases when the N-rate is decreased in agreement with [48]. The rise of the Ga incorporation during stage 3 suggests an additional ef-fect. The morphology of sampleA0ereveals the presence of few short NWs embedded in a planar rough layer of GaN. This contrasts with the morphology of samplesA0a andA0cthat corresponds to thin NWs. For a small N-rate, i.e. smallerV/IIIratio, this observation indicates that the Ni islands are competing with the sapphire surface ex-posed between the islands for the Ga-adatom incorporation and further GaN growth.

In this case, it is highly likely that the NW nucleation is limited by the low N-rate, as will be discussed in more detail in section 4.4.1. Ga adatoms still incorporate preferen-tially into the Ni islands, which is indicated by negligible Ga desorption at the start of growth. However, if the Ga in the Ni islands is not consumed fast enough by the active N, the further arriving Ga atoms accumulate outside the particles, and planar growth also occurs leading to an increase ofA3.

Last, the effect of the Ga-rate on the nucleation was also investigated. To this aim the Ga-desorption of samplesA0a,A0f andA0ggrown with a Ga-rate of 0.4 Å/s, 0.2 Å/s and 0.8 Å/s at a temperature of 730^◦C and with a N-rate of 2.0 Å/s was monitored.

Their morphology is illustrated in Figure 4.14 in chapter 4.

In Figure 3.14(e),t1decreases when the amount of supplied Ga is increased. However, in Figure 3.14(f), the variation ofA1is barely noticeable. This result indicates that at fixed temperature and N-rate, the supplied Ga-rate limits the Ga incorporation rate into the Ni seeds but has no influence on the amount of Ga incorporated into Ni before GaN nucleation.

For the second stage a different dependence is ascertained. t3is nearly the same for

Figure 3.15:The 3 stages of the nucleation at 730^◦C of GaN NWs in the catalyst-assisted approach. In stage 1 (top-left), Ga is preferentially incorporated into the Ni seeds. At the beginning of the nucleation, the seeds have thefccstructure with orientations that are twinned. The Ni₃GaL1₂ structure also most probably appears during this stage, due to Ga incorporation leading to a very similar diffraction pattern. In stage 2 (top-right), the transformation of the seed crystal structure induced by Ga accumulation occurs. Two different seed structures appear whereas the parent phase vanishes. The first one is thebccNiGa structure and the second one corre-sponds to another Ni-Ga related compound of hexagonal structure. In stage 3 (bottom), GaN finally nucleates under the seeds. The compositions corresponding to the Ni-Ga and Ni-Ga-N phase diagrams are given here as an indication but may differ from the exact composition during the nucleation.

Ga-rates lower or equal to 0.4 Å/s while the Ga incorporationA3is much lower at the lowest Ga-rate corresponding to sample A0f. This indicates that the nucleation rate is largely reduced for the lowest Ga-rate. For the highest Ga-rate corresponding to sample A0g, the duration of the nucleationt3is three times faster than for the lower Ga rates whereas the incorporated amount of GaA3decrease to one third in comparison to the amount obtained for the Ga-rate of 0.4 Å/s of sampleA0a. This observation shows that the increase in Ga-rate also increases the incorporation rate as would be expected for the growth under N-rich conditions.