**3. Techniques for growth nanowire by in situ TEM**

The very first demonstration of nanowire growth by in situ TEM was from Prof. Yang's group which validated the VLS mechanism experimentally using the Au-catalyzed Ge nanowire growth [68]. This seminal experiment was conducted by heating Au nanoclusters along with micrometer-sized Ge particles. They neither used a continuous supply of Ge-precursor nor a closed system. Over the course of time technological advances in the field of TEM paved the way to environmental TEM (ETEM), where the pressure near the sample can be orders of magnitude higher than a conventional TEM. Studies in which a continuous supply of precursors was used were reported [41, 46–49, 69–90].

However, an ETEM is not a necessity for studying CVD nanowire growth in situ by TEM, it is possible by using a closed or isolated cell instead. In principle, it is possible that a cell isolated from the microscope vacuum is used; gaseous precursors can be supplied continuously to this cell by external inlet gas-tubes and removed by outlet tubes, without releasing the gases to the microscope environment. Another strategy is to use completely closed cells, in which powders of precursor material are deposited in the cell and then sealed [91]. These powders are heated intentionally to evaporate it so as to form a vapor-phase supply of precursors to the catalyst for growth [91]. An intermediate method, which is feasible with commercially available instruments for gas handling, is to pre-deposit powdered material on the isolated cell but externally supply carrier gases such as H2 or N2 (no gases are released here to the microscope environment) [92]. The cells have an electron transparent amorphous film both at the top and at the bottom of the cell. An advantage of this strategy is that any ordinary TEM can be used for it. However, the thickness of the top and bottom casing combined could be substantial, reducing the attainable spatial resolution.

**99**

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

In the more conventional open heating cell geometry, there is either one layer of amorphous layer or none, providing better spatial resolution. Commercial chips are available with a few holes made in a thin amorphous film. When a piece of commercially available substrate wafer is loaded vertically in the TEM [69, 70] or a lithographically patterned cantilever chips is used [76, 93] for growing epitaxial aligned nanowires there is no film on top or bottom of the nanowire sample, enabling epitaxial growth and better spatial resolution; in such cases the resolution of the microscope and thickness of the sample would be the bottleneck. TEM resolution is currently restricted by technical limitations, not by the physically attainable limit; over the years TEM resolution has been constantly improving and this evolution is

visible if we look at reports of nanowire growth with in situ TEM as well.

**4. In situ TEM of elemental semiconductor nanowire growth**

tion time, which in turn determines the average nanowire growth rate.

this assumption of instantaneous layer-growth breaks down.

Most theoretical models for nanowire growth kinetics assume instantaneous layer completion and the growth rate is calculated in a nucleation-limited regime [95–98]. This assumption seems to be valid for the VLS growth of elemental nanowires we discussed above. However, we will now discuss in this section about elemental nanowires and the next section about compound nanowires cases where

As mentioned before, the growth can proceed by the VSS route where the catalyst is a solid particle. In the VSS growth of elemental nanowires the layer completion is slow [73, 74, 78, 82]. The incubation time in the VSS case is shorter than in VLS [74, 78]. The solubility of the growth material in the solid catalyst is much lower than in a liquid catalyst, thus a small amount of excess species can increase the chemical potential sufficiently to nucleate a new layer — making the incubation time short [74, 78]. But the limited amount of material present could be insufficient

Si and Ge nanowire growth has been extensively studied by in situ TEM [46, 68–78, 81, 82, 85–88]. Several aspects such as diameter dependance of growth kinetics [70], nucleation kinetics [87], surface faceting [69], surface migration of catalyst (Au) on nanowire (Si) surface [71], tapering [94], and kinking [75] have been investigated. Depending on the growth conditions such as temperature, catalyst particle and precursor pressures the growth proceeds either by the VLS mode [46, 68–72, 74–76, 78, 81, 82, 85–88] or the VSS mode [46, 72–74, 77, 78, 82, 88]. It is interesting to note that VLS growth has been observed to occur even below the

The nanowire catalyst interface is atomically flat, except when a ledge is growing. The layer-by-layer growth of nanowire atomic layers has been studied in situ during the VLS growth of elemental nanowires [74, 78]. A new (bi)layer starts only after the previous one is completely grown (at least for the nanowire diameters studied) [74, 78]. The time each layer takes to complete once it has nucleated can be called ledge-flow time (or layer completion time, also called step-flow time in some references). We will use the term incubation time for the difference between the ending of one layer and the start of the next layer. (This is not to be confused with the incubation time before the birth/nucleation of the nanowire itself). In VLS growth of elemental nanowires each layer grows instantaneously (ledge-flow time ~ 0) while there is a significant incubation/waiting time between successive layer-growth events [74, 78]. This observation can be explained by a very simple argument — the amount of material required to raise the chemical potential high enough to nucleate a layer is sufficient for forming one full layer as soon as it nucleates. So the layer grows rapidly once nucleated [74]. There is a considerable incuba-

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

eutectic temperature [72].

*Nanowires - Recent Progress*

In situ imaging techniques on the other hand allows monitoring individual nanowires. Optical microscopes due to the limited spatial resolution are not ideal for observing growth evolution of nanowire (though some studies have been attempted using confocal optical microscopy using photoluminescence measurements [62]). Scanning electron microscopes (SEMs) have better spatial resolution than optical microscopes and could be used to monitor nanowire growth [63–67]. In situ SEM combined with Auger electron spectroscopy has been used to correlate nanowire growth and morphology to surface chemistry [63]. In situ electron backscattered electron diffraction (EBSD) performed during growth in an SEM has been used to study crystal phases and crystallographic orientation [64]. An SEM uses electron scattering from a sample while a transmission electron microscope (TEM) uses the electrons transmitted through a thin sample (preferably less than ~50 nm) to form images. TEMs have better spatial resolution than SEMs. Be it in an in situ SEM or TEM study, a video or a series of images are captured to study the dynamics of the process in relation with the specimen environment. A key advantage of using in situ microscopic techniques, particularly in situ TEM, is that localized or dynamic behavior happening at individual wires could be investigated. One limitation to studying nanowire growth inside a microscope is that electron microscopes require vacuum environment to minimize electron scattering in the air outside the specimen. So, often the growth conditions, e.g. pressure, used for the in situ growth study are slightly modified compared to a conventional growth method. Typical total pressures used in conventional ex situ CVD are much beyond the maximum attainable pressure for in situ TEM experiments. Majority of the pressure in the ex situ CVD case is from the carrier gas. By careful design of the TEM and the growth chamber, it is in principle possible to obtain comparable precursor partial pressures.

**3. Techniques for growth nanowire by in situ TEM**

sors was used were reported [41, 46–49, 69–90].

The very first demonstration of nanowire growth by in situ TEM was from Prof. Yang's group which validated the VLS mechanism experimentally using the Au-catalyzed Ge nanowire growth [68]. This seminal experiment was conducted by heating Au nanoclusters along with micrometer-sized Ge particles. They neither used a continuous supply of Ge-precursor nor a closed system. Over the course of time technological advances in the field of TEM paved the way to environmental TEM (ETEM), where the pressure near the sample can be orders of magnitude higher than a conventional TEM. Studies in which a continuous supply of precur-

However, an ETEM is not a necessity for studying CVD nanowire growth in situ by TEM, it is possible by using a closed or isolated cell instead. In principle, it is possible that a cell isolated from the microscope vacuum is used; gaseous precursors can be supplied continuously to this cell by external inlet gas-tubes and removed by outlet tubes, without releasing the gases to the microscope environment. Another strategy is to use completely closed cells, in which powders of precursor material are deposited in the cell and then sealed [91]. These powders are heated intentionally to evaporate it so as to form a vapor-phase supply of precursors to the catalyst for growth [91]. An intermediate method, which is feasible with commercially available instruments for gas handling, is to pre-deposit powdered material on the isolated cell but externally supply carrier gases such as H2 or N2 (no gases are released here to the microscope environment) [92]. The cells have an electron transparent amorphous film both at the top and at the bottom of the cell. An advantage of this strategy is that any ordinary TEM can be used for it. However, the thickness of the top and bottom casing combined could be substantial, reducing the attainable spatial resolution.

**98**

In the more conventional open heating cell geometry, there is either one layer of amorphous layer or none, providing better spatial resolution. Commercial chips are available with a few holes made in a thin amorphous film. When a piece of commercially available substrate wafer is loaded vertically in the TEM [69, 70] or a lithographically patterned cantilever chips is used [76, 93] for growing epitaxial aligned nanowires there is no film on top or bottom of the nanowire sample, enabling epitaxial growth and better spatial resolution; in such cases the resolution of the microscope and thickness of the sample would be the bottleneck. TEM resolution is currently restricted by technical limitations, not by the physically attainable limit; over the years TEM resolution has been constantly improving and this evolution is visible if we look at reports of nanowire growth with in situ TEM as well.
