In Situ TEM Studies of III-V Nanowire Growth Mechanism

*Carina B. Maliakkal*

### **Abstract**

Growing nanowires inside a transmission electron microscope (TEM) and observing the process in situ has contributed immensely to understanding nanowire growth mechanisms. Majority of such studies were on elemental semiconductors – either Si or Ge – both of which are indirect bandgap semiconductors. Several compound semiconductors on the other hand have a direct bandgap making them more efficient in several applications involving light absorption or emission. During compound nanowire growth using a metal catalyst, the difference in miscibility of the nanowire species inside the metal catalyst are different, making its growth dynamics different from elemental nanowires. Thus, studies specifically focusing on compound nanowires are necessary for understanding its growth dynamics. This chapter reviews the recent progresses in the understanding of compound semiconductor nanowire growth obtained using in situ TEM. The concentrations of the nanowire species in the catalyst was studied in situ. This concentration difference has been shown to enable independent control of layer nucleation and layer growth in nanowires. In situ TEM has also enabled better understanding of the formation of metastable crystal structures in nanowires.

**Keywords:** compound nanowire, transmission electron microscopy, ledge-flow, semiconductor, GaAs, in situ techniques, wurtzite, zincblende, polytypism

#### **1. Introduction**

The high surface-to-volume ratio and the high aspect ratio of the nanowire geometry paves the way to a plethora of interesting advantages. Growing materials as nanowires has enabled the formation of metastable crystal phases, in turn enabling crystal structure tuning [1–4]. Integration of different lattice-mismatched materials into the same structure was also achieved; compared to growth of heteroepitaxial films, defect-free growth is easier in nanowires because of the small diameter (a few 10 or 100 nanometers) and small interfacial area [5–7]. Yet another advantage of nanowire growth is to form alloy compositions which are unstable in the bulk phase [8]. Materials generally grown in the nanowire morphology can be broadly classified as elemental and compound [9]. Metallic nanowires (Ti, Fe, Co, Ni, Sn etc.) and elemental semiconductors (e.g. Si, Ge) fall under the category of elemental materials. Stochiometric compound nanowires are either compound semiconductors (e.g. GaAs, ZnO) or insulators (e.g. Al2O3, Si3N4). Alloy nanowires are also possible, e.g. SixGe1-x, AlxGa1-xAs. Controlling the electronic, bandgap-engineering related, structural, compositional, morphological, mechanical and optical properties of semiconductor nanowires enables its application in devices such

solar cells, [10–13] electronics, [14–16] LEDs, [16, 17] LASERS, [18, 19] photodetectors, [20, 21] thermoelectrics, [22, 23] biosensors [24, 25] and qubits [26–29].

Nanowires can be fabricated by a top-down approach (where regions of a film are selectively etched) or by a bottom-up approach (where the nanowires are grown on a substrate) [30]. Top-down approach often result in rough defected surfaces [9]. One way to grow nanowires bottom-up is by electrochemical deposition [31–34]. The bottom-up growth of nanowires from a gas phase precursor supply is what we will discuss in more detail here. The bottom-up nanowires growth from gas phase are done either with or without a foreign metal catalyst. The nanowire growth using a metal catalyst was proposed to proceed by the vapor–liquid–solid or 'VLS' mechanism [35]. According to the VLS mechanism, the nanowire elements or their precursor supplied in the vapor (V) phase gets dissolved in the liquid (L) 'catalyst' and after supersaturation precipitates out as the solid (S) nanowire (**Figure 1**). The metallic liquid, in addition to providing a nucleation point for the solid nanowire, fosters the gathering and in some cases the decomposition of precursors – hence often called 'catalyst' [9]. Later a similar growth mode called the vapor-solid–solid (VSS) was also proposed for when the catalyst is a solid, instead of the liquid catalyst in VLS [36, 37]. When there is no foreign catalyst used the nanowire growth can proceed in either of the two ways: (i) a self-catalyzed mode where the metallic element of the nanowire forms the liquid catalyst droplet [38–40] or (ii) a non-catalyzed vapor-solid route where the material from the vapor phase directly attaches to the solid nanowire without any liquid or solid catalyst [41]. The yield of nanowire growth without an external catalyst can be increased by the use of selective-area dielectric mask to keep some areas unfavorable for nucleation; small openings in the mask acting as preferential nucleation site for nanowires [42, 43].

Some of the common techniques for growing nanowires with gas phase precursors by the aforementioned mechanisms include chemical vapor deposition (CVD), metalorganic CVD (MOCVD) and molecular beam epitaxy (MBE). These systems were initially designed for growing thin films and later adapted for growing nanowires. Usually for growing nanowires in any of these systems the catalyst-coated substrate is loaded into the system, the system is closed and precursors are supplied at

#### **Figure 1.**

*Nanowire growth with a liquid catalyst is explained by the VLS mechanism. Accordingly, the supplied vapor phase precursor species dissolves in the liquid catalyst and at appropriately high supersaturation crystallizes atomic layers of the solid nanowire. The TEM image shown here was captured in situ while an atomic layer was growing.*

**97**

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

dispersive X-ray spectroscopy (XEDS), photoluminescence, etc.

rapid growth of the nucleus to beyond the critical size) [49].

**2. In situ techniques**

specific technological applications.

appropriate temperature and pressure. In conventional growth systems, either there is no in situ monitoring during growth or there is some large-area indirect monitoring. MBE systems sometimes monitors the crystal structure of the surface layer by RHEED (reflection high-energy electron diffraction). Some MOCVD systems are equipped with in situ light reflectance monitors which can be used to estimate the increase in sample height and surface roughening. These methods are used conventionally for tracking growth of thin films from large areas of the sample. Using these techniques for monitoring nanowires demand some modifications. After growing nanowires, the samples can be elaborately analyzed ex situ by methods relevant to the study. Typical characterization techniques are scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), energy

Such ex situ characterizations could suffice for studying nanowire morphology, crystallinity and composition. Analysis of nanowires from multiple growths with different parameters can help indirectly understand the growth mechanism to some extent. Still the dynamics of the growth understood based on such ex situ characterizations are largely speculative. Moreover, some attributes of the nanowire could be different while growing (at high temperature with precursor supply) and after cooling down the system for post mortem analysis. Monitoring nanowire growth in situ certainly has advantages in elucidating the growth mechanism and dynamics. An example of a phenomenon which was discovered only due to in situ observation of individual nanowires is truncation — where the nanowire-catalyst interface has a dynamic non-flat surface near the triple-phase-line [44–48]. (Triple-phase-line refers to the periphery of the interface between the nanowire and the catalyst droplet where the vapor, liquid and solid phases meet.) Another interesting in situ observation was that the nucleation of wurtzite layer happens at, or at least very close to, a corner of the triple-phase-line (observation of the precise location being elusive due to the 'limited' temporal resolution compared to the expected extremely

Observing and characterizing the nanowires while they are growing is called in situ growth monitoring. Strictly speaking, 'in operando' is the exact word, but we stick to 'in situ' to conform to popular usage. In situ techniques can provide directly interpretable and time-resolved observations enabling better understanding of the growth mechanism, which in turn empowers better control of nanowire growth for

In situ characterization of nanowire crystal structure or nanowire morphology has been reported using various techniques. In situ RHEED attached to MBE systems can be used to follow crystal structure changes and nucleation/birth of ensemble of nanowires [50, 51]. By modifying the optical reflectometry techniques that have been used conventionally in MOCVD systems, the nanowire diameter and length evolution has been monitored in situ in real time for an ordered array of nanowires [52]. Combining finite difference frequency domain simulations with in situ reflectometry enabled monitoring growth of randomly positioned nanowires (i.e. periodic array was not a necessity) [53]. In situ X-ray diffraction (XRD) has been used to study crystal phase of the nanowire [54, 55] and the catalyst phase [56]. In situ infrared spectroscopy has been used to correlate surface chemistry during nanowire growth to its morphology [57–59] or the choice of growth direction [60]. Line-of-sight quadrupole mass spectrometry in situ was used to study different stages of nanowire growth including nanowire nucleation [61]. All these techniques give ensemble averaged results.

*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*

solar cells, [10–13] electronics, [14–16] LEDs, [16, 17] LASERS, [18, 19] photodetectors, [20, 21] thermoelectrics, [22, 23] biosensors [24, 25] and qubits [26–29].

Nanowires can be fabricated by a top-down approach (where regions of a film are selectively etched) or by a bottom-up approach (where the nanowires are grown on a substrate) [30]. Top-down approach often result in rough defected surfaces [9]. One way to grow nanowires bottom-up is by electrochemical deposition [31–34]. The bottom-up growth of nanowires from a gas phase precursor supply is what we will discuss in more detail here. The bottom-up nanowires growth from gas phase are done either with or without a foreign metal catalyst. The nanowire growth using a metal catalyst was proposed to proceed by the vapor–liquid–solid or 'VLS' mechanism [35]. According to the VLS mechanism, the nanowire elements or their precursor supplied in the vapor (V) phase gets dissolved in the liquid (L) 'catalyst' and after supersaturation precipitates out as the solid (S) nanowire (**Figure 1**). The metallic liquid, in addition to providing a nucleation point for the solid nanowire, fosters the gathering and in some cases the decomposition of precursors – hence often called 'catalyst' [9]. Later a similar growth mode called the vapor-solid–solid (VSS) was also proposed for when the catalyst is a solid, instead of the liquid catalyst in VLS [36, 37]. When there is no foreign catalyst used the nanowire growth can proceed in either of the two ways: (i) a self-catalyzed mode where the metallic element of the nanowire forms the liquid catalyst droplet [38–40] or (ii) a non-catalyzed vapor-solid route where the material from the vapor phase directly attaches to the solid nanowire without any liquid or solid catalyst [41]. The yield of nanowire growth without an external catalyst can be increased by the use of selective-area dielectric mask to keep some areas unfavorable for nucleation; small openings in the mask acting as preferential nucleation site for nanowires [42, 43]. Some of the common techniques for growing nanowires with gas phase precursors by the aforementioned mechanisms include chemical vapor deposition (CVD), metalorganic CVD (MOCVD) and molecular beam epitaxy (MBE). These systems were initially designed for growing thin films and later adapted for growing nanowires. Usually for growing nanowires in any of these systems the catalyst-coated substrate is loaded into the system, the system is closed and precursors are supplied at

*Nanowire growth with a liquid catalyst is explained by the VLS mechanism. Accordingly, the supplied vapor phase precursor species dissolves in the liquid catalyst and at appropriately high supersaturation crystallizes atomic layers of the solid nanowire. The TEM image shown here was captured in situ while an atomic layer* 

**96**

**Figure 1.**

*was growing.*

appropriate temperature and pressure. In conventional growth systems, either there is no in situ monitoring during growth or there is some large-area indirect monitoring. MBE systems sometimes monitors the crystal structure of the surface layer by RHEED (reflection high-energy electron diffraction). Some MOCVD systems are equipped with in situ light reflectance monitors which can be used to estimate the increase in sample height and surface roughening. These methods are used conventionally for tracking growth of thin films from large areas of the sample. Using these techniques for monitoring nanowires demand some modifications. After growing nanowires, the samples can be elaborately analyzed ex situ by methods relevant to the study. Typical characterization techniques are scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (XEDS), photoluminescence, etc.

Such ex situ characterizations could suffice for studying nanowire morphology, crystallinity and composition. Analysis of nanowires from multiple growths with different parameters can help indirectly understand the growth mechanism to some extent. Still the dynamics of the growth understood based on such ex situ characterizations are largely speculative. Moreover, some attributes of the nanowire could be different while growing (at high temperature with precursor supply) and after cooling down the system for post mortem analysis. Monitoring nanowire growth in situ certainly has advantages in elucidating the growth mechanism and dynamics. An example of a phenomenon which was discovered only due to in situ observation of individual nanowires is truncation — where the nanowire-catalyst interface has a dynamic non-flat surface near the triple-phase-line [44–48]. (Triple-phase-line refers to the periphery of the interface between the nanowire and the catalyst droplet where the vapor, liquid and solid phases meet.) Another interesting in situ observation was that the nucleation of wurtzite layer happens at, or at least very close to, a corner of the triple-phase-line (observation of the precise location being elusive due to the 'limited' temporal resolution compared to the expected extremely rapid growth of the nucleus to beyond the critical size) [49].
