Experimental elucidation of vacancy complexes associated with hydrogen ion-induced splitting of bulk GaN

We present a detailed study of the thermal evolution of H ion-induced vacancy related complexes and voids in bulk GaN implanted under ion-cut conditions. By using transmission electron microscopy, we found that the damage band in as-implanted GaN is decorated with a high density of nanobubbles of $\sim 1–2\text{nm}$ in diameter. Variable energy Doppler broadening spectroscopy showed that this band contains vacancy clusters and voids. In addition to vacancy clusters, the presence of $V_{\text{Ga}}$, $V_{\text{Ga}}{\text{-H}}_2$, and $V_{\text{Ga}}{V}_{\text{N}}$ complexes was evidenced by pulsed low-energy positron lifetime spectroscopy. Subtle changes upon annealing in these vacancy complexes were also investigated. As a general trend, a growth in open-volume defects is detected in parallel to an increase in both size and density of nanobubbles. The observed vacancy complexes appear to be stable during annealing. However, for temperatures above $450°\text{C}$, unusually large lifetimes were measured. These lifetimes are attributed to the formation of positronium in GaN. Since the formation of positronium is not possible in a dense semiconductor, our finding demonstrates the presence of sufficiently large open-volume defects in this temperature range. Based on the Tao-Eldrup model, the average lattice opening during thermal annealing was quantified. We found that a void diameter of 0.4 nm is induced by annealing at $600°\text{C}$. The role of these complexes in the subsurface microcracking is discussed.

Figure 1

XTEM image of H-implanted free-standing GaN for an implantation energy of 50 keV and a fluence of $2.6\times {10}^{17}{\text{cm}}^{-2}$. H ion-induced nanobubbles are visible as 1–2 nm bright spots.

Figure 2

XTEM image of H-implanted GaN substrate implanted under the conditions described in Fig. 1 and annealed at $600°\text{C}$ for 5 min.

Figure 3

(Color online) Thermoevolution of the normalized $S$ parameter of H-implanted GaN samples as a function of incident positron energy. For comparison, the spectrum recorded from bulk GaN is also shown.

Figure 4

(Color online) The evolution of the average positron lifetime as a function of the positron energy for the virgin, as-implanted, and annealed GaN samples. The symbols are the same as in Fig. 3.

Figure 5

(Color online) The decomposition of the lifetime spectra for the virgin, as-implanted, and annealed GaN samples extracted at a positron energy of 10 keV.

Figure 6

XTEM image of H-implanted GaN substrate under the conditions described in Fig. 1 and annealed at $400°\text{C}$ for 2 min.

Figure 7

(Top) XTEM image of H-implanted GaN substrate under the conditions described in Fig. 1 and annealed at $600°\text{C}$ for 2 min. (Bottom) High magnification XTEM image of the damage band.