Visualizing metal-mediated nucleation and growth of GaN

TL;DR

This study combines real-time in situ environmental TEM with Gibbs free-energy analyses to dissect metal-mediated GaN nucleation and growth, contrasting GaN formation from Ga droplet arrays with and without GaN pre-nuclei. It reveals a decisive role for the Ga/GaN interface in facilitating NH3 decomposition and nitrogen incorporation, lowering the effective temperature for growth, and shows a strong temperature dependence that governs whether nucleation occurs inside Ga droplets or at interfaces. At low temperatures, growth is confined to Ga/GaN interfaces; at intermediate temperatures, NH3 decomposition proceeds in both environments enabling nucleation and growth in Ga droplets and at interfaces; at high temperatures, improved crystallinity and epitaxial templating by pre-nuclei drive well-defined GaN morphologies, including polytype mixtures. Overall, the work provides fundamental insights into metal-mediated GaN growth and offers guidance for tailoring GaN-based nanostructures by controlling the interface and temperature.

Abstract

Understanding the atomic-scale mechanisms governing metal-mediated nucleation and growth of gallium nitride (GaN) and related alloys is critical for tailoring their structural and functional properties in advanced electronic, optoelectronic, and quantum devices. Using real-time environmental transmission electron microscopy (E-TEM) in conjunction with Gibbs free energy calculations, we elucidate the distinct processes of GaN nucleation and growth from Ga droplet arrays with and without GaN pre-nuclei. For the lowest temperatures, although GaN nucleation at Ga droplet arrays is not observed, GaN growth occurs preferentially at pre-existing GaN nuclei, presumably due to the reduced Gibbs free energy for NH3 decomposition at Ga/GaN interfaces. For intermediate to high temperatures, E-TEM reveals nucleation and growth of GaN from Ga droplets with and without GaN nuclei, with enhanced crystallinity for the GaN nuclei, due to epitaxial templating. These results highlight the critical role of the Ga/GaN interface in facilitating NH3 decomposition and GaN growth, offering fundamental insights into metal-mediated nucleation and growth of GaN and related materials.

Figures (5)

• Figure 1: (a) Temperature-dependence of the Gibbs free energy, $\Delta G$, for NH$_3$ decomposition in liquid Ga (purple) and at the liquid Ga/GaN interface (pink). Intersections with the horizontal dashed line ($\Delta G = 0$) indicate the temperatures at which each pathway becomes thermodynamically favorable. For both pathways, the local environments used in the computation are shown in the insets: N adatoms in liquid Ga (top right) and at the GaN/Ga interface (bottom left).(b) Low-magnification high-angle annular dark field-scanning transmission electron micrograph (HAADF-STEM) of Ga droplet arrays, with energy-dispersive spectroscopy (EDS) maps obtained from the white box region shown as insets. The EDS maps reveal droplets consisting of Ga-rich cores (purple) surrounded by thin oxide shells (blue), while the SiN substrate contains uniform concentrations of Si (yellow) and N (green). (c) Low-magnification HAADF-STEM of Ga droplet arrays with GaN pre-nuclei, with atomic-resolution HAADF-STEMs obtained from the white box regions in (d) and (f). The FFT from (f), shown in (g), is well-indexed to ZB GaN $[1\bar{1}0]$.
• Figure 2: Nucleation and growth at the lowest temperatures: in-situ TEM images during NH$_3$ exposure with $\Delta T / \Delta t = 100~^{\circ}\mathrm{C}/\mathrm{min}$ and $T_{\text{target}}=460~\degree\text{C}$. (a) Heating and NH$_3$ exposure profiles for Ga droplet arrays. (b) Starting arrays of Ga droplets with oxide shells, with a tracking particle labeled with white dashed circles in (b)-(d). (c, d) Sequential TEM images at 6 min and 9 min showing the onset of surface restructuring and formation of small particles from Ga leakage following NH$_{3}$-etching of the Ga$_x$O$_y$ shells. (e) Heating and NH$_3$ exposure profiles for arrays of Ga droplet with GaN pre-nuclei. (f) Starting arrays of Ga droplets with GaN pre-nuclei. (g) Low-magnification TEM image of arrays of Ga droplets with GaN pre-nuclei after exposure of NH$_3$ at 10 min. Time-resolved TEM images at (h) 24 min and (i) 24 min 45 sec, showing progressive lateral GaN growth (colored in red) along the Ga/GaN interface.
• Figure 3: Nucleation and growth at intermediate temperature: in-situ TEM images during NH$_3$ exposure with $\Delta T / \Delta t = 100~^{\circ}\mathrm{C}/\mathrm{min}$ and $T_{\text{target}}=550~\degree\text{C}$. (a) Heating and NH$_3$ exposure profiles for Ga droplet arrays. Real-time TEM images (b) at t = 0 and (b) at 12 min. The yellow arrows in (c) mark the darker contrast on the surface. The surfaces of Ga droplet arrays have roughened, likely due to the etching of the oxide shell by NH$_3$. (d) Darker contrast appears at the center of the particle, with atomic resolution images of the blue-boxed region showing in (e) 13 min and (f) 15 min. The FFTs taken from (e) and (f) are both indexed to (g, h) WZ GaN $[0001]$. (i) Heating and NH$_3$ exposure profiles for arrays of Ga droplets with GaN pre-nuclei. Real-time TEM images of the arrays of Ga droplets with GaN pre-nuclei (j) at t = 0, (k) at 6 min and (l) at 29 min. Ga begins to desorb from smaller Ga/GaN particles, leaving oxide shells on the substrate. Reduced contrast between the Ga/GaN particles and the SiN substrate is likely due to Ga wetting the substrate surface. Tracked particles are marked in each frame with white dotted circles, blue arrows, and yellow arrows for reference. (m) HRTEM image of a GaN particle formed in E-TEM. (n) provides a close-up view, with the FFTs of (n) indexed to (o) WZ GaN $[0001]$ orientation.
• Figure 4: Nucleation and growth at high temperature: in-situ TEM images during NH$_3$ exposure with $\Delta T / \Delta t = 400~^{\circ}\mathrm{C}/\mathrm{sec}$ and $T_{\text{target}}=850~\degree\text{C}$. (a) Heating and NH$_3$ exposure profiles for Ga droplet arrays. (b-c) The evolution of Ga droplet arrays from circular to irregular shape during exposure to NH$_3$. (d) HRTEM image of an example particle showing WZ GaN formation. (e) provides a close-up view, with an FFT shown in the inset, further confirming the formation of WZ GaN. (f) Heating and NH$_3$ exposure profiles for arrays of Ga droplet with GaN pre-nuclei. The arrays of Ga droplets with GaN pre-nuclei transform from (g) circular morphology prior to growth into (h) triangular shape following growth. (i) Atomic-resolution HRTEM image of GaN reveals a mixed polytype GaN nanostructure that transitions from the ZB to the WZ polytypes. (j) Atomic-resolution HRTEM image from the orange-boxed regions in (i), with blue, pink, and yellow dots illustrating ABC stacking along the $[111]$ direction. (k) Atomic-resolution image from the green-boxed regions in (i), with blue and pink dots representing ABAB stacking along the [0001] direction. The FFTs taken from orange- and green-boxed regions are indexed to (l) ZB GaN $[1\bar{1}0]$ and (m) WZ GaN $[11\bar{2}0]$, respectively, with ZB $[111]$ parallel to WZ $[0001]$.
• Figure 5: In situ TEM images during NH$_3$ exposure with $\Delta T / \Delta t = 400~^{\circ}\mathrm{C}/\mathrm{sec}$ and $T_{\text{target}}=850~\degree\text{C}$. (a) Heating and NH$_3$ exposure profiles for arrays of Ga droplets with GaN pre-nuclei. (b) Low-magnification TEM image of the arrays of Ga droplets with GaN pre-nuclei. Real-time TEM images from 5 min to 5 min 7.5 sec (c-g) showing faceted particles that grow along GaN pre-nuclei and (h-l) nucleation and growth of GaN. White dotted circles mark the same positions for reference. Slight focus shift occurs due to rapid heating.