In Situ TEM Studies of III-V Nanowire Growth Mechanism

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

Vapor-solid-solid growth dynamics in GaAs nanowires

Semiconductor nanowires are promising material systems for coming of age nanotechnology. The usage of the vapor solid solid (VSS) route, where the catalyst used for promoting axial growth of nanowire is a solid, offers certain advantages compared to the common vapor liquid solid (VLS) route (using liquid catalyst). The VSS growth of group-IV elemental nanowires have been investigated by other groups in situ during growth in a transmission electron microscope (TEM). Though it is known that compound nanowire growth has different dynamics compared to monoatomic semiconductors, the dynamics of VSS growth of compound nanowires has not been understood. Here we investigate VSS growth of compound nanowires by in situ microscopy, using Au-seeded GaAs as a model system. The growth kinetics and dynamics at the wire-catalyst interface by ledge-flow is studied and compared for liquid and solid catalysts at similar growth conditions. Here the temperature and thermal history of the system is manipulated to control the catalyst phase. In the first experiment discussed here we reduce the growth temperature in steps to solidify the initially liquid catalyst, and compare the dynamics between VLS and VSS growth observed at slightly different temperatures. In the second experiment we exploit thermal hysteresis of the system to obtain both VLS and VSS at the same temperature. The VSS growth rate is comparable or slightly slower than VLS growth. Unlike in the VLS case, during VSS growth we see several occasions where a new layer starts before the previous layer is completely grown, i.e. multilayer growth. Understanding the VSS growth mode enables better control of nanowire properties by widening the range of usable nanowire growth parameters

In situ analysis of catalyst composition during gold catalyzed GaAs nanowire growth

Semiconductor nanowires offer the opportunity to incorporate novel structures and functionality into electronic and optoelectronic devices. A clear understanding of the nanowire growth mechanism is essential for well-controlled growth of structures with desired properties, but the understanding is currently limited by a lack of empirical measurements of important parameters during growth, such as catalyst particle composition. However, this is difficult to accurately determine by investigating post-growth. We report direct measurement of the catalyst composition of individual gold seeded GaAs nanowires inside an electron microscope as they grow. The Ga content in the catalyst during growth increased with both temperature and Ga precursor flux. A direct comparison of the calculated phase diagrams of the Au-Ga-As ternary system to the measured catalyst composition not only lets us estimate the As content in the catalyst but also indicates the relevance of phase diagrams to understanding nanowire growth

The emergence of macroscopic currents in photoconductive sampling of optical fields

Photoconductive field sampling enables petahertz-domain optoelectronic applications that advance our understanding of light-matter interaction. Despite the growing importance of ultrafast photoconductive measurements, a rigorous model for connecting the microscopic electron dynamics to the macroscopic external signal is lacking. This has caused conflicting interpretations about the origin of macroscopic currents. Here, we present systematic experimental studies on the signal formation in gas-phase photoconductive sampling. Our theoretical model, based on the Ramo–Shockley-theorem, overcomes the previously introduced artificial separation into dipole and current contributions. Extensive numerical particle-in-cell-type simulations permit a quantitative comparison with experimental results and help to identify the roles of electron-neutral scattering and mean-field charge interactions. The results show that the heuristic models utilized so far are valid only in a limited range and are affected by macroscopic effects. Our approach can aid in the design of more sensitive and more efficient photoconductive devices