**5. In situ compound nanowire growth**

Compound nanowires grown inside TEM include insulator materials (Al2O3 [45]) and semiconductors (GaAs [49, 84, 99], GaN [79, 83, 100], GaP [46, 80], InAs [91] and PdSe [92]). VSS growth of compound nanowires in a TEM with a supply of precursors has not been reported so far; hence the discussion we have in this section is restricted to VLS growth of compound nanowires. In the cases where atomic resolution videos where obtained, ledge-flow was not instantaneous [49, 83, 84, 90]. The initial studies of MOCVD combined with in situ TEM were at very low precursor pressures compared to the typical 'ex situ' MOCVD; [83, 84] hence it was not sure if the gradual ledge-flow was representative of ex situ growths as well. The latest report was with orders of magnitude higher pressures than previous studies, but still the precursor pressures values were on the lower end of conventional ex situ MOCVD growth parameter regime [90]. If or not the ledge-flow of atomic layers is gradual in the entire range of growth parameters used in ex situ growths is yet to be investigated.

The gradual ledge-flow growth in compound nanowires, is in striking contrast to the VLS monoatomic nanowire growth. But this difference between elemental and compound nanowires is simple to understand. In elemental nanowire only one material species controls both nucleation and layer-growth events. For example, during Si nanowire growth with a Au catalyst the Si dissolving in the Au is the key factor. At typical growth temperatures of Si nanowire growth (400–600°C) the liquidus line where the Au-Si system is at equilibrium is with about 20–28% Si (depending on the growth temperature). A little extra Si is insufficient to supersaturate the system enough to trigger a nucleation event. The amount of excess Si that accumulates during the incubation time and triggers the nucleation of a layer could thus suffice to form an entire layer. However, in a compound nanowire case the miscibility of two different nanowire species within the catalyst could be decisive, in turn making the dynamics more complex. Species like Ga, In, Al and Zn alloys readily with Au while species like As, N, P and O are hardly soluble in Au [101]. In the case of Au-catalyzed GaAs growth, for example, theoretical calculations predicted that Ga mixes readily in Au but As has poor solubility in Au [102, 103]. Experimental studies of the catalyst composition was mostly done ex situ post growth until very recently.

### **6. Catalyst composition measured in situ**

Understanding the concentration of the different species during growth is key to understanding the growth dynamics. Parameters like surface and interface energies, vapor pressure and chemical potential of the catalyst are dependent on the catalyst composition [104–106]. The catalyst composition measured ex situ depends not just on the growth parameters, but also on the conditions used to terminate growth and cool down the sample (the ambient gas, ramp down rate etc.) [2, 44, 107]. Typically

**101**

**Figure 2.**

*In Situ TEM Studies of III-V Nanowire Growth Mechanism*

in MOCVD the temperature is decreased from the growth temperature down to either room temperature or an intermediate temperature in a group V (or group VI) precursor environment to prevent etching and surface roughening. At the initial part of this ramp down, where the temperature is still adequate to grow, the group III (or II) species already present in the catalyst reacts with the group V precursor to form an additional nanowire segment, in turn decreasing the concentration of the group III species in the catalyst [2, 44, 107]. Hence in situ measurement is key. Recently in situ measurement of the catalyst composition during the growth process was reported [89]. X-ray energy dispersive spectroscopy (XEDS) spectroscopy was used in situ to study the catalyst composition during Au-catalyzed GaAs growth performed inside an ETEM. Trimethylgallium (TMGa) and arsine (AsH3) were used as the precursors. The XEDS measurement was conducted in the TEM mode by condensing the beam to a small region and positioning it in the front part of the catalyst (like depicted in **Figure 2a**). Since the nanowire was growing, the sample stage was constantly repositioned so that the beam is all the time on the catalyst itself, and not hitting the nanowire part. The XEDS signal from Au, Ga and As was studied. The catalyst had a significant amount of Ga alloyed with the Au. The Ga % in the catalyst was found to increase with both temperature and the Ga precursor flux. **Figure 2b** shows the Ga % as function of the V/III ratio i.e. the ratio of the group V precursor to the group III precursor. These experiments were done in the 420–500°C temperature range. At these temperatures, the catalyst interaction with nanowire depends on the TMGa flow – (a) in the absence of a TMGa flow the catalyst particle can etch the nanowire (similar to what was reported by Tornberg *et al*. [108]); (b) at an intermediate TMGa flow there is neither growth nor etching; (c) at a slightly higher TMGa there is nanowire growth where the Ga % increases with increasing TMGa flow to a quasi-steady state and the catalyst bulges due to the additional Ga; (d) eventually there is a regime with truncated nanowire-catalyst interface and (e) finally at even higher TMGa the catalyst bulges and topples.

The As signal in the EDX spectra was too low to be conclusively attributed to be arising from the catalyst and was suspected to be due to scattered signal from the nanowire [89]. The As content was however estimated by an indirect method calculating phase diagrams or liquidus lines for different As % and comparing the Ga % in these calculations to the measured Ga % value. The estimated minimum As % in the catalyst was ~0.01%. For the nanowire dimensions used, this would be

*In situ catalyst composition measurement. (a) TEM image of a nanowire. The catalyst composition was measured in situ by XEDS by condensing the beam in the front of the catalyst. (b) the Ga % in the catalyst is plotted as a function of the V/III ratio. The As % measured in the catalyst was negligible. (a) and (b) are* 

*adapted from Maliakkal* et al. *2019 [89] with permission as per creative commons license.*

*DOI: http://dx.doi.org/10.5772/intechopen.95690*

#### *In Situ TEM Studies of III-V Nanowire Growth Mechanism DOI: http://dx.doi.org/10.5772/intechopen.95690*

*Nanowires - Recent Progress*

**5. In situ compound nanowire growth**

fully grown.

yet to be investigated.

growth until very recently.

**6. Catalyst composition measured in situ**

for forming a complete bilayer, in turn making the ledge-flow process slow [74, 78]. The limited solubility of the nanowire species inside the solid catalyst offers the opportunity to grow compositionally abrupt axial heterostructure [74]. Another interesting aspect about VSS growth of elemental nanowires is that there can be two or more ledges growing simultaneously [73, 78, 82]. This also is in contrast to VLS growth of elemental nanowires were a second ledge starts only after the first is

Compound nanowires grown inside TEM include insulator materials (Al2O3 [45]) and semiconductors (GaAs [49, 84, 99], GaN [79, 83, 100], GaP [46, 80], InAs [91] and PdSe [92]). VSS growth of compound nanowires in a TEM with a supply of precursors has not been reported so far; hence the discussion we have in this section is restricted to VLS growth of compound nanowires. In the cases where atomic resolution videos where obtained, ledge-flow was not instantaneous [49, 83, 84, 90]. The initial studies of MOCVD combined with in situ TEM were at very low precursor pressures compared to the typical 'ex situ' MOCVD; [83, 84] hence it was not sure if the gradual ledge-flow was representative of ex situ growths as well. The latest report was with orders of magnitude higher pressures than previous studies, but still the precursor pressures values were on the lower end of conventional ex situ MOCVD growth parameter regime [90]. If or not the ledge-flow of atomic layers is gradual in the entire range of growth parameters used in ex situ growths is

The gradual ledge-flow growth in compound nanowires, is in striking contrast to the VLS monoatomic nanowire growth. But this difference between elemental and compound nanowires is simple to understand. In elemental nanowire only one material species controls both nucleation and layer-growth events. For example, during Si nanowire growth with a Au catalyst the Si dissolving in the Au is the key factor. At typical growth temperatures of Si nanowire growth (400–600°C) the liquidus line where the Au-Si system is at equilibrium is with about 20–28% Si (depending on the growth temperature). A little extra Si is insufficient to supersaturate the system enough to trigger a nucleation event. The amount of excess Si that accumulates during the incubation time and triggers the nucleation of a layer could thus suffice to form an entire layer. However, in a compound nanowire case the miscibility of two different nanowire species within the catalyst could be decisive, in turn making the dynamics more complex. Species like Ga, In, Al and Zn alloys readily with Au while species like As, N, P and O are hardly soluble in Au [101]. In the case of Au-catalyzed GaAs growth, for example, theoretical calculations predicted that Ga mixes readily in Au but As has poor solubility in Au [102, 103]. Experimental studies of the catalyst composition was mostly done ex situ post

Understanding the concentration of the different species during growth is key to understanding the growth dynamics. Parameters like surface and interface energies, vapor pressure and chemical potential of the catalyst are dependent on the catalyst composition [104–106]. The catalyst composition measured ex situ depends not just on the growth parameters, but also on the conditions used to terminate growth and cool down the sample (the ambient gas, ramp down rate etc.) [2, 44, 107]. Typically

**100**

in MOCVD the temperature is decreased from the growth temperature down to either room temperature or an intermediate temperature in a group V (or group VI) precursor environment to prevent etching and surface roughening. At the initial part of this ramp down, where the temperature is still adequate to grow, the group III (or II) species already present in the catalyst reacts with the group V precursor to form an additional nanowire segment, in turn decreasing the concentration of the group III species in the catalyst [2, 44, 107]. Hence in situ measurement is key.

Recently in situ measurement of the catalyst composition during the growth process was reported [89]. X-ray energy dispersive spectroscopy (XEDS) spectroscopy was used in situ to study the catalyst composition during Au-catalyzed GaAs growth performed inside an ETEM. Trimethylgallium (TMGa) and arsine (AsH3) were used as the precursors. The XEDS measurement was conducted in the TEM mode by condensing the beam to a small region and positioning it in the front part of the catalyst (like depicted in **Figure 2a**). Since the nanowire was growing, the sample stage was constantly repositioned so that the beam is all the time on the catalyst itself, and not hitting the nanowire part. The XEDS signal from Au, Ga and As was studied. The catalyst had a significant amount of Ga alloyed with the Au. The Ga % in the catalyst was found to increase with both temperature and the Ga precursor flux. **Figure 2b** shows the Ga % as function of the V/III ratio i.e. the ratio of the group V precursor to the group III precursor. These experiments were done in the 420–500°C temperature range. At these temperatures, the catalyst interaction with nanowire depends on the TMGa flow – (a) in the absence of a TMGa flow the catalyst particle can etch the nanowire (similar to what was reported by Tornberg *et al*. [108]); (b) at an intermediate TMGa flow there is neither growth nor etching; (c) at a slightly higher TMGa there is nanowire growth where the Ga % increases with increasing TMGa flow to a quasi-steady state and the catalyst bulges due to the additional Ga; (d) eventually there is a regime with truncated nanowire-catalyst interface and (e) finally at even higher TMGa the catalyst bulges and topples.

The As signal in the EDX spectra was too low to be conclusively attributed to be arising from the catalyst and was suspected to be due to scattered signal from the nanowire [89]. The As content was however estimated by an indirect method calculating phase diagrams or liquidus lines for different As % and comparing the Ga % in these calculations to the measured Ga % value. The estimated minimum As % in the catalyst was ~0.01%. For the nanowire dimensions used, this would be

#### **Figure 2.**

*In situ catalyst composition measurement. (a) TEM image of a nanowire. The catalyst composition was measured in situ by XEDS by condensing the beam in the front of the catalyst. (b) the Ga % in the catalyst is plotted as a function of the V/III ratio. The As % measured in the catalyst was negligible. (a) and (b) are adapted from Maliakkal* et al. *2019 [89] with permission as per creative commons license.*

#### *Nanowires - Recent Progress*

less than the amount of As required to form one complete bilayer. Indirect estimates based on ex situ growth, [95] phase diagrams, [109] and theoretical calculations related to Au-catalyzed GaAs nanowire growth [102, 103] also suggested low As solubility in the Au-Ga alloy.
