GaP Nanowire VLS Growth Observed in a Closed Gas Cell In Situ TEM Setup

Nanowires, especially those consisting of semiconductor materials, are used for many technical applications, like LEDs, photo detectors, lasers and solar cells [1]. A widely used fabrication process for semiconductor materials is metal organic vapor phase epitaxy (MOVPE), which is capable of layer growth as well as nanowire formation. However, structural investigations of these materials are usually carried out post growth. <i>In-situ</i> studies promise an improved understanding of the growth process, resulting in a performance improvement of fabricated materials. <i>In-situ</i> (scanning) transmission electron microscopy ((S)TEM) is a powerful tool to investigate dynamic processes at the atomic level. Gold catalyzed vapor liquid solid (VLS) growth [2] of GaP nanowires observed in a closed gas cell and heating TEM holder is presented in this study.<br/>A commercially available Protochips Atmosphere closed gas cell in-situ system has been modified to allow the usage of toxic and pyrophoric gases, like precursors used in MOVPE. To this end a gas mixing system, an appropriate gas monitoring system as well as a gas scrubbing system have been added to the setup [3]. The in-situ TEM holder is capable of sustaining pressures and temperatures up to 1 bar and 1000 °C respectively. This enables the TEM holder to act as a MOVPE reactor at a micrometer scale in any TEM. A double C_S-corrected JEOL JEM 2200FS operating at 200 kV was used for the TEM observations. Samples were prepared by depositing Au nanoparticles (NP) from a suspension droplet onto a micro electro mechanical system (MEMS) chip, containing a silicon carbide heating membrane and a silicon nitride membrane acting as viewing window. Tertiarybutylphosphine (TBP) and trimethylgallium (TMGa) were used as precursor gases at partial pressures between 10^-1 hPa and 10^-4 hPa with a V/III ratio of 10. Nitrogen was used as carrier gas at a pressure of several hundret hPa. Nanowire growth is observed in a temperature range of 400 °C to 500 °C.<br/>The observed nanowires show various diameters from 20 nm up to 200 nm, created by gold nanoparticles of different sizes due to clustering and oswald ripening. Furthermore, the contact angle of the liquid catalyst at the triple phase line changes from wire to wire. Thereby multiple nanowire-droplet geometries can be observed under the same growth conditions. It is found that not only growth conditions, like temperature and precursor ratio and partial pressure determine the growth dynamics, but the mentioned geometry has a big influence. This can be seen for example in the growth velocity, which rises with an increasing contact angle. Larger contact angles lead to higher surface ratios of droplet surface to growth plane area. By this the diffusion rate of growth material into the droplet is increased, resulting in a higher crystallization rate at the growth plane, causing a higher growth rate. At a droplet to growth plane surface ratio of 4 growth velocities of 20 nm/s were observed, whereas at a ratio of 8 an increase to 40 nm/s can be seen. Thus, we assume the growth mechanism to be limited by the diffusion of growth material from the precursor vapor into the catalyst droplet under the growth conditions mentioned above.<br/><b>References</b><br/>[1] R. Yan, et al., Nature Photonics 3 (2009), pp. 569-576<br/>[2] R. S. Wagner, W. C. Ellis, Applied Physics Letters 4 (1964), pp. 89<br/>[3] R. Straubinger, et al., Microscopy and Microanalysis 23.4 (2017), pp. 751-757