**7. Independent control of layer nucleation and layer-growth**

The concentration difference between the two different nanowire species in the catalyst [89] implies that both these species could affect the growth in different ways. This was studied in detail, again using Au-catalyzed GaAs (and TMGa and AsH3 as precursors) [90]. Two sets of experiments were investigated – one was a TMGa series (where AsH3 and temperature were kept constant) and a second was AsH3 series (where TMGa and temperature were kept constant). The ledge-flow time was found to decrease drastically with increasing AsH3 flow (**Figure 3b)**; thus the ledge-flow process was understood to be limited by the As availability. This agrees well with the low As % present in the catalyst [89]. (The idea that ledge-flow is limited by As availability was proposed in an earlier study [84] but not elaborately investigated there.) On increasing TMGa flow in a separate experiment the incubation time decreased drastically while the ledge-flow time remained rather unchanged (**Figure 3c**). This indicated that nucleation of a new layer is triggered by excess Ga.

The experimental observations for the TMGa and AsH3 series matched stochastic Monte Carlo simulations done based on mass transport and nucleation theory [89]. An example of how As % and Ga % in the catalyst varies in an almost cyclic way during the simulated layer-growth cycle is shown in **Figure 3d**.

#### **Figure 3.**

*(a) TEM image showing ledge-flow growth of an atomic bilayer. (b,c) Ledge-flow time as a function of As-precursor flow (b) and Ga-precursor flow (c). (d) A representative example of simulation of Ga and As concentrations in the catalyst. (Plots (b), (c) and (d) are adapted from Maliakkal* et al. *[90] with permission. Further permissions should be directed to ACS.)*

**103**

properties.

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

During the incubation process the As concentration in the catalyst is in equilibrium with the ambient vapor. Once the layer nucleation happens the 'excess' As is consumed to form the GaAs nucleus and so As concentration quickly drops to a low level (where the As is in equilibrium with the solid GaAs nanowire). As soon as the layer is grown completely, the As % quickly rises and equilibrates again with the ambient vapor. Once this happens, the As contribution to the liquid supersaturation remains the same over the rest of the incubation period. However, the Ga building up in the catalyst keeps increasing the liquid chemical potential. Eventually, at some point after the liquid chemical potential is higher than the nucleation barrier, stochastically a nucleation event happens. Since the As % remains steady during the latter part of the incubation period, it is the Ga which is

The study demonstrated independent control of layer nucleation (by Ga) and layer completion (by As) in GaAs nanowires growth [90]. The underlying reason for the nucleation of layer and ledge-flow to be controllable independently is the very low solubility of As and the high solubility of Ga in the Au catalyst. Several other III-V and II-VI compound semiconductors also consist of a nonmetallic species (group V or VI, e.g., N, O, P, S), and a metallic species (group II or III, e.g., Ga, In, Zn, Mg) [101]. These nonmetallic species typically dissolve very little in catalyst (gold or other typical transition metal catalysts) while the metallic species readily forms alloys. In cases were the amount of nonmetallic species collected in the catalyst and available for growth is low, the layer growth process will be restricted by availability of this nonmetallic species. Thus, independent control of layer nucleation and growth would be possible in several other nanowire systems too [90]. The occasions where controlling the layer nucleation and growth are extremely relevant could include doping and growth of ternary compounds. In VLS growth, the nucleation stage determines the crystal stacking of the entire atomic layer [1]. However, dopant/impurity incorporation happening would strongly depend on ledge-flow. Since impurity incorporation could be happening due to step trapping, a slow ledge-flow would help limit the impurity incorporation. On the contrary, for

higher dopant incorporation, a fast ledge-flow could be advantageous [90].

Layer nucleation and growth for the different polytypes have been studied by in situ TEM. Before we discuss the key in situ results, let us discuss the concept of polytypism in nanowires and how the metastable structure could form. Nanowires enable the formation of metastable crystalline phases which do not form during its bulk growth. For example, most III-arsenides and III-phosphides form in the zincblende polytype when grown in bulk, because the bulk energy is lower for zincblende than wurtzite phase. But these materials can form in the wurtzite polytype in nanowires due to surface effects [2–4, 44, 110]. (Details of these crystal structures can be found elsewhere [4]). Controlled polytypism has great technological relevance because the electronic band structure depends on crystal structure. For example, GaP in the usual zincblende phase is an indirect bandgap material; while the wurtzite polytype has a pseudo-direct bandgap [111–113]. The valence and conduction bands of the two polytypes are often misaligned, so sections of one polytype in a matrix of the other polytype nanowire can confine electrons and/or holes. This enables crystal phase quantum dots with abrupt interfaces [114, 115]. Compositional quantum dots, on the other hand, often has a gradual variation of the composition (depending on the material combination chosen) deteriorating its

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

triggering nucleation of a new layer [90].

**8. Polytypism in III-V nanowires**

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

*Nanowires - Recent Progress*

solubility in the Au-Ga alloy.

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

The concentration difference between the two different nanowire species in the catalyst [89] implies that both these species could affect the growth in different ways. This was studied in detail, again using Au-catalyzed GaAs (and TMGa and AsH3 as precursors) [90]. Two sets of experiments were investigated – one was a TMGa series (where AsH3 and temperature were kept constant) and a second was AsH3 series (where TMGa and temperature were kept constant). The ledge-flow time was found to decrease drastically with increasing AsH3 flow (**Figure 3b)**; thus the ledge-flow process was understood to be limited by the As availability. This agrees well with the low As % present in the catalyst [89]. (The idea that ledge-flow is limited by As availability was proposed in an earlier study [84] but not elaborately investigated there.) On increasing TMGa flow in a separate experiment the incubation time decreased drastically while the ledge-flow time remained rather unchanged (**Figure 3c**). This

**7. Independent control of layer nucleation and layer-growth**

indicated that nucleation of a new layer is triggered by excess Ga.

The experimental observations for the TMGa and AsH3 series matched stochastic Monte Carlo simulations done based on mass transport and nucleation theory [89]. An example of how As % and Ga % in the catalyst varies in an almost cyclic way during the simulated layer-growth cycle is shown in **Figure 3d**.

*(a) TEM image showing ledge-flow growth of an atomic bilayer. (b,c) Ledge-flow time as a function of As-precursor flow (b) and Ga-precursor flow (c). (d) A representative example of simulation of Ga and As concentrations in the catalyst. (Plots (b), (c) and (d) are adapted from Maliakkal* et al. *[90] with permission.* 

**102**

**Figure 3.**

*Further permissions should be directed to ACS.)*

During the incubation process the As concentration in the catalyst is in equilibrium with the ambient vapor. Once the layer nucleation happens the 'excess' As is consumed to form the GaAs nucleus and so As concentration quickly drops to a low level (where the As is in equilibrium with the solid GaAs nanowire). As soon as the layer is grown completely, the As % quickly rises and equilibrates again with the ambient vapor. Once this happens, the As contribution to the liquid supersaturation remains the same over the rest of the incubation period. However, the Ga building up in the catalyst keeps increasing the liquid chemical potential. Eventually, at some point after the liquid chemical potential is higher than the nucleation barrier, stochastically a nucleation event happens. Since the As % remains steady during the latter part of the incubation period, it is the Ga which is triggering nucleation of a new layer [90].

The study demonstrated independent control of layer nucleation (by Ga) and layer completion (by As) in GaAs nanowires growth [90]. The underlying reason for the nucleation of layer and ledge-flow to be controllable independently is the very low solubility of As and the high solubility of Ga in the Au catalyst. Several other III-V and II-VI compound semiconductors also consist of a nonmetallic species (group V or VI, e.g., N, O, P, S), and a metallic species (group II or III, e.g., Ga, In, Zn, Mg) [101]. These nonmetallic species typically dissolve very little in catalyst (gold or other typical transition metal catalysts) while the metallic species readily forms alloys. In cases were the amount of nonmetallic species collected in the catalyst and available for growth is low, the layer growth process will be restricted by availability of this nonmetallic species. Thus, independent control of layer nucleation and growth would be possible in several other nanowire systems too [90].

The occasions where controlling the layer nucleation and growth are extremely relevant could include doping and growth of ternary compounds. In VLS growth, the nucleation stage determines the crystal stacking of the entire atomic layer [1]. However, dopant/impurity incorporation happening would strongly depend on ledge-flow. Since impurity incorporation could be happening due to step trapping, a slow ledge-flow would help limit the impurity incorporation. On the contrary, for higher dopant incorporation, a fast ledge-flow could be advantageous [90].

## **8. Polytypism in III-V nanowires**

Layer nucleation and growth for the different polytypes have been studied by in situ TEM. Before we discuss the key in situ results, let us discuss the concept of polytypism in nanowires and how the metastable structure could form. Nanowires enable the formation of metastable crystalline phases which do not form during its bulk growth. For example, most III-arsenides and III-phosphides form in the zincblende polytype when grown in bulk, because the bulk energy is lower for zincblende than wurtzite phase. But these materials can form in the wurtzite polytype in nanowires due to surface effects [2–4, 44, 110]. (Details of these crystal structures can be found elsewhere [4]). Controlled polytypism has great technological relevance because the electronic band structure depends on crystal structure. For example, GaP in the usual zincblende phase is an indirect bandgap material; while the wurtzite polytype has a pseudo-direct bandgap [111–113]. The valence and conduction bands of the two polytypes are often misaligned, so sections of one polytype in a matrix of the other polytype nanowire can confine electrons and/or holes. This enables crystal phase quantum dots with abrupt interfaces [114, 115]. Compositional quantum dots, on the other hand, often has a gradual variation of the composition (depending on the material combination chosen) deteriorating its properties.

Now let us first briefly discuss a simplified explanation for the occurrence of the metastable wurtzite structure in nanowires. During VLS growth, at appropriate catalyst contact angles, the nucleus of each layer is preferably formed at the triplephase-line because it eliminates the energy cost of a preexisting liquid segment [1]. For nucleation happening at the triple-phase-line the nanowire surface energy is a key factor [1]. The surface energy of possible wurtzite side facets could be lower than the zincblende counterparts [1]. In such cases, the wurtzite structure can be more favorable than zincblende for energy minimization, depending on the catalyst supersaturation and relevant interface energies [1]. Extensive models proposed by several groups to correlate the observed crystal structure at different conditions can be found elsewhere [1, 96, 103, 104, 116–118].

In GaAs nanowire growth studied ex situ for a very wide range of V/III ratios it was found that very low V/III ratios gives zincblende, a higher V/III results in wurtzite, and an even higher V/III give zincblende again [110]. A high V/III ratio (i.e. higher AsH3, which may also be interpreted as effectively lesser Ga) is associated with a smaller catalyst; while a low V/III results in bulged up catalyst with high contact angle [110]. There are also two intermediate transition regimes with mixed structures [110]. Theoretical models of Au-catalyzed GaAs growths could also simulate three different growth regimes [103, 104]. Often in typical experiment series a narrower range of V/III is studied, and thus it may happen that only a zincblende to wurtzite transition or only a wurtzite to zincblende transition is observed on increasing V/III ratio.

Jacobsson *et al.* observed two growth regimes in situ — at moderate V/III ratio wurtzite segments grew, and at low V/III the catalyst bulges and zincblende segments grew [84]. The wurtzite growth occurred while the nanowire-catalyst interface was one flat plane and the ledge-flow growth was gradual [84]. During zincblende growth the interface showed an oscillating truncated corner; the ledge-flow was "too rapid to observe" but was correlated to the cyclic dynamics of the truncation [84]. According to the study zincblende phase grows if any edge is truncated, whereas wurtzite grows only when the interface is flat [84]. They speculated that the truncation happening in the low V/III regime would make nucleation occur away from the triple-phase-line which in turn makes zincblende the preferred structure. (Note the authors had not claimed an if-and-only-if condition between interface geometry and crystal structure. However, in my personal experience, some readers misinterpreted that truncation was a necessity for low V/III zincblende growth.) The droplet contact angle was the key parameter in deciding the crystal structure [84]. Au:Ga ratio in the droplet was found to be not critical, hence a similar crystal structure–geometry correlation was speculated to be applicable to self-catalyzed wires too [84].

Polytypism in self-catalyzed GaAs nanowires was recently studied using in situ TEM-MBE by Panciera *et al* [99]. In this work the three regimes were observed as shown in **Figure 4e**, **g** and **h** (zincblende at low V/III ratio, wurtzite at higher V/III ratio, zincblende at even higher V/III ratio) [99]. In the high V/III zincblende regime, (which was not experimentally achievable in Jacobsson *et al.*), the nucleation was found to occur at the triple-phase-line, the ledge-flow was slow, and no truncation was observed. Their observations consistent with Jacobsson *et al.* include (i) control of crystal structure by contact angle, (ii) gradual ledge-flow and flat interface during wurtzite growth, and (iii) truncation and rapid ledge-flow observed in the low V/III zincblende growth [99].

However, it is not necessary that the low V/III zincblende growth can occur only with truncation. The growth of zincblende with a bulged particle (high contact angle) at low V/III ratio, but without truncation, has also been reported (**Figure 4f**) [89]. In this study the V/III ratio was decreased to observe bulging of the particle,

**105**

**Figure 4.**

*permission as per Creative Commons license.*

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

but a lower limit was set on the V/III ratio intentionally to avoid the truncation regime [89]. The V/III ratio was varied in small steps and maintained for some time to reach steady state [89] – this could be the reason why this intermediate zincblende regime with high contact angle and without truncation was observed in this study unlike the other two in situ studies discussed above [84, 99]. Thus, we can infer that the zincblende growth at low V/III does not necessarily require truncation. More detailed investigation is necessary to say if the nucleation happened at the triple-phase-line or the center in this case, and if the ledge-flow was gradual or instantaneous. Whether the presence of truncation makes the nucleation preferable

*(a)-(d) Schematic representation of crystal phases observed as a function of V/III ratio or catalyst contact angle. The catalyst-nanowire interface is either a single plane (b-d) or truncated (a). (e)-(h) shows TEM images with zincblende (ZB) or wurtzite (WZ) structures. Scalebars correspond to 5 nm. Inset of (e) shows a dynamic truncated corner during the zincblende growth at low V/III ratio. (e), (g) and (h) are adapted with permission from Panciera* et al. *[99] Copyright (2020) American Chemical Society. Image (f) showing low V/ III zincblende growth even without truncation is from another study – Maliakkal et al. [89]. Adapted with* 

A heuristic explanation for the choice of crystal structure based on the currently available data [84, 89, 99] and theoretical calculations [1, 49] on GaAs VLS growth is as follows. The metastable wurtzite phase can grow only if the layer nucleates at the triple-phase-line [49]. If the nucleation is at triple-phase-line, either wurtzite or zincblende can form depending on the supersaturation and surface energies [1]. When nucleation occurs away from the triple-phase-line, it can form only zincblende structure [1, 49]. Now let us look at crystal structure as a function of the contact angle. Glas *et al.* predicted that nucleation is preferred at the triplephase-line for a range of contact angles [π − βc; βc], where the critical angle βc is a function of the relevant interface energies [1, 49]. Thus, at intermediate contact angles (i.e. intermediate V/III ratios), nucleation occurs at the triple-phase-line and wurtzite structure is formed as observed( **Figure 4c**,**g**) [99]. For the lower contact angle (**Figure 4d**,**h**), nucleation was reported to be zincblende and at the triplephase-line, [99] which demands that zincblende structure would have been the lower energy nucleus phase at those growth conditions [1]. For the higher contact angle regime, zincblende was found to grow, even without truncation [89]. It would have happened either (i) with nucleation away from the triple-phase-line giving

away from the triple phase line is also an open question.

*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*

#### **Figure 4.**

*Nanowires - Recent Progress*

on increasing V/III ratio.

self-catalyzed wires too [84].

observed in the low V/III zincblende growth [99].

be found elsewhere [1, 96, 103, 104, 116–118].

Now let us first briefly discuss a simplified explanation for the occurrence of the metastable wurtzite structure in nanowires. During VLS growth, at appropriate catalyst contact angles, the nucleus of each layer is preferably formed at the triplephase-line because it eliminates the energy cost of a preexisting liquid segment [1]. For nucleation happening at the triple-phase-line the nanowire surface energy is a key factor [1]. The surface energy of possible wurtzite side facets could be lower than the zincblende counterparts [1]. In such cases, the wurtzite structure can be more favorable than zincblende for energy minimization, depending on the catalyst supersaturation and relevant interface energies [1]. Extensive models proposed by several groups to correlate the observed crystal structure at different conditions can

In GaAs nanowire growth studied ex situ for a very wide range of V/III ratios it was found that very low V/III ratios gives zincblende, a higher V/III results in wurtzite, and an even higher V/III give zincblende again [110]. A high V/III ratio (i.e. higher AsH3, which may also be interpreted as effectively lesser Ga) is associated with a smaller catalyst; while a low V/III results in bulged up catalyst with high contact angle [110]. There are also two intermediate transition regimes with mixed structures [110]. Theoretical models of Au-catalyzed GaAs growths could also simulate three different growth regimes [103, 104]. Often in typical experiment series a narrower range of V/III is studied, and thus it may happen that only a zincblende to wurtzite transition or only a wurtzite to zincblende transition is observed

Jacobsson *et al.* observed two growth regimes in situ — at moderate V/III ratio wurtzite segments grew, and at low V/III the catalyst bulges and zincblende segments grew [84]. The wurtzite growth occurred while the nanowire-catalyst interface was one flat plane and the ledge-flow growth was gradual [84]. During zincblende growth the interface showed an oscillating truncated corner; the ledge-flow was "too rapid to observe" but was correlated to the cyclic dynamics of the truncation [84]. According to the study zincblende phase grows if any edge is truncated, whereas wurtzite grows only when the interface is flat [84]. They speculated that the truncation happening in the low V/III regime would make nucleation occur away from the triple-phase-line which in turn makes zincblende the preferred structure. (Note the authors had not claimed an if-and-only-if condition between interface geometry and crystal structure. However, in my personal experience, some readers misinterpreted that truncation was a necessity for low V/III zincblende growth.) The droplet contact angle was the key parameter in deciding the crystal structure [84]. Au:Ga ratio in the droplet was found to be not critical, hence a similar crystal structure–geometry correlation was speculated to be applicable to

Polytypism in self-catalyzed GaAs nanowires was recently studied using in situ TEM-MBE by Panciera *et al* [99]. In this work the three regimes were observed as shown in **Figure 4e**, **g** and **h** (zincblende at low V/III ratio, wurtzite at higher V/III ratio, zincblende at even higher V/III ratio) [99]. In the high V/III zincblende regime, (which was not experimentally achievable in Jacobsson *et al.*), the nucleation was found to occur at the triple-phase-line, the ledge-flow was slow, and no truncation was observed. Their observations consistent with Jacobsson *et al.* include (i) control of crystal structure by contact angle, (ii) gradual ledge-flow and flat interface during wurtzite growth, and (iii) truncation and rapid ledge-flow

However, it is not necessary that the low V/III zincblende growth can occur only with truncation. The growth of zincblende with a bulged particle (high contact angle) at low V/III ratio, but without truncation, has also been reported (**Figure 4f**) [89]. In this study the V/III ratio was decreased to observe bulging of the particle,

**104**

*(a)-(d) Schematic representation of crystal phases observed as a function of V/III ratio or catalyst contact angle. The catalyst-nanowire interface is either a single plane (b-d) or truncated (a). (e)-(h) shows TEM images with zincblende (ZB) or wurtzite (WZ) structures. Scalebars correspond to 5 nm. Inset of (e) shows a dynamic truncated corner during the zincblende growth at low V/III ratio. (e), (g) and (h) are adapted with permission from Panciera* et al. *[99] Copyright (2020) American Chemical Society. Image (f) showing low V/ III zincblende growth even without truncation is from another study – Maliakkal et al. [89]. Adapted with permission as per Creative Commons license.*

but a lower limit was set on the V/III ratio intentionally to avoid the truncation regime [89]. The V/III ratio was varied in small steps and maintained for some time to reach steady state [89] – this could be the reason why this intermediate zincblende regime with high contact angle and without truncation was observed in this study unlike the other two in situ studies discussed above [84, 99]. Thus, we can infer that the zincblende growth at low V/III does not necessarily require truncation. More detailed investigation is necessary to say if the nucleation happened at the triple-phase-line or the center in this case, and if the ledge-flow was gradual or instantaneous. Whether the presence of truncation makes the nucleation preferable away from the triple phase line is also an open question.

A heuristic explanation for the choice of crystal structure based on the currently available data [84, 89, 99] and theoretical calculations [1, 49] on GaAs VLS growth is as follows. The metastable wurtzite phase can grow only if the layer nucleates at the triple-phase-line [49]. If the nucleation is at triple-phase-line, either wurtzite or zincblende can form depending on the supersaturation and surface energies [1]. When nucleation occurs away from the triple-phase-line, it can form only zincblende structure [1, 49]. Now let us look at crystal structure as a function of the contact angle. Glas *et al.* predicted that nucleation is preferred at the triplephase-line for a range of contact angles [π − βc; βc], where the critical angle βc is a function of the relevant interface energies [1, 49]. Thus, at intermediate contact angles (i.e. intermediate V/III ratios), nucleation occurs at the triple-phase-line and wurtzite structure is formed as observed( **Figure 4c**,**g**) [99]. For the lower contact angle (**Figure 4d**,**h**), nucleation was reported to be zincblende and at the triplephase-line, [99] which demands that zincblende structure would have been the lower energy nucleus phase at those growth conditions [1]. For the higher contact angle regime, zincblende was found to grow, even without truncation [89]. It would have happened either (i) with nucleation away from the triple-phase-line giving

zincblende structure or (ii) with nucleation at the triple-phase-line only, but with the zincblende structure having lower energy at those growth conditions. Note that in the above explanation or in references [1, 49] truncation was not explicitly needed to explain crystal phase switching. At extremely high contact angles and at extremely low contact angles, there could be either truncation or large tapering; [84, 99] but truncation is not a necessity for zincblende growth. The truncation might be responsible for the observed quasi-instantaneous ledge-flow though.
