In Situ Environmental TEM Observation of Early-Stage Nucleation Behavior of GaN Growth on SiN Substrate

Semiconductor polytype heterostructures, which consist of chemically homogeneous structures formed via an abrupt change in crystal structure, offer opportunities for performance exceeding those of composition-based semiconductor heterostructures. Of particular interest are heterostructures formed via an abrupt change in atomic plane stacking sequence, such as the transition from the wurtzite (WZ) polytype to the zincblende (ZB) polytype. It has been suggested that the formation of ZB segments within WZ nanowires (NWs) can act as quantum dots (QDs) in NWs, which are promising candidates for single-photon emitters.<br/>We recently discovered a Ga-mediated molecular-beam epitaxy (MBE) process to nucleate ZB and WZ GaN NWs on Si(001) [1]. Key to this process is a Ga pre-deposition step, in which Ga droplet arrays are formed prior to NW growth. We have also examined the origins of polytype selection during metal-mediated epitaxy of GaN NWs. Quantitative EDS reveals a notably higher average Si atomic fraction in ZB NWs than in WZ NWs. Correspondingly, DFT calculations predict that incorporation of Si atomic fractions &gt; 0.08 onto the Ga sublattice stabilizes ZB GaN. We hypothesize that the high Ga BEP during the Ga pre-deposition enables dissolution of excess Si into the liquid Ga, thereby stabilizing ZB GaN. To further study the initial stages of the GaN growth process under real time, environmental transmission electron microscopy (E-TEM) was utilized to observe nucleation and growth of GaN from Ga droplets under ammonia exposure.<br/>In this work, in preparation for E-TEM studies, Ga droplet samples were prepared using MBE on micro-electromechanical systems (MEMS)-based chips with SiN_x thin film windows as substrates. A specialized holder was designed for MBE growth to securely mount the chips, using a mask to expose only the SiN_x thin film windows and prevent deposition on the electrodes, avoiding electrical shorts. Two different growth conditions were utilized: (1) Ga droplets and (2) pre-nucleated GaN within Ga droplets using N plasma in MBE. In the absence of pre-nitridation in MBE, circular Ga droplets with the diameter of 24.3 ± 0.2 nm were formed on SiN_x film of MEMS-based chips. On the other hand, exposure of Ga droplets to N plasma leads to faceted GaN formation at the edges of the Ga droplets, resulting in partially-nucleated GaN within Ga droplets. Since both samples were exposed to the atmosphere after MBE growth, thin oxide layers (1-2 nm) were observed on their surfaces.<br/>With Ga droplet samples under ammonia exposure in E-TEM, GaN formation was not observed after heating up to 800°C. Instead, Ga desorption and oxidation occurred. Interestingly, with pre-nucleated GaN within Ga droplets, after heating up to 800°C during ammonia exposure, Ga desorption occurred first at lower temperature (~ 500°C), followed by facet formation at higher temperatures (&gt; 600°C). High-resolution TEM (HRTEM) after ammonia exposure shows uniform crystal orientation within particles, indicating epitaxial growth of GaN in E-TEM along pre-nucleated GaN. The competition between Ga desorption rate, ammonia decomposition rate, and GaN growth rate on Ga/SiNx and Ga/GaN interface will be discussed.<br/><br/>[1] Lu, H., S. Moniri, C. Reese, S. Jeon, A. Katcher, T. Hill, H. Deng, and R.S. Goldman. 2021. “Influence of gallium surface saturation on GaN nanowire polytype selection during molecular- beam epitaxy.” Appl. Phys. Lett. 119:031601.<br/><br/>This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award # DE-SC0023222. This research used the Electron Microscopy facility of the Center for Functional Nanomaterials, which is a U.S. Department of Energy Office of Science User Facility, at Brookhaven National Laboratory under Contract No. DE-SC0012704.