Amphoteric Be in GaN: Experimental Evidence for Switching between Substitutional and Interstitial Lattice Sites

We show that Be exhibits amphoteric behavior in GaN, involving switching between substitutional and interstitial positions in the lattice. This behavior is observed through the dominance of ${\mathrm{Be}}_{\mathrm{Ga}}$ in the positron annihilation signals in Be-doped GaN, while the emergence of $V_{\mathrm{Ga}}$ at high temperatures is a consequence of the Be impurities being driven to interstitial positions. The similarity of this behavior to that found for Na and Li in ZnO suggests that this could be a universal property of light dopants substituting for heavy cations in compound semiconductors.

Figure 1

The $S$ parameter measured as a function of positron implantation energy (implantation depth) in selected MBE-grown Be-doped GaN thin films before and after annealing. The dashed line corresponds to the bulk $S$ parameter value of the $p$-type GaN reference.

Figure 2

Relative $(S,W)$ parameters measured in Be-doped GaN. Theoretically calculated parameters for a variety of defects are shown in addition to the experimentally determined $(S,W)$ parameters typical of isolated Ga vacancies in GaN. The calculated effect of $V_{\mathrm{Ga}}$ complexed with one or more nitrogen vacancies (not shown for the sake of clarity) is consistent with Ref. [27]; i.e., the data points have a similar $W$ and clearly higher $S$ than $V_{\mathrm{Ga}}$. We also show the theoretically calculated positron lifetime values for the defects, relative to the GaN lattice lifetime ${\tau }_B$. A single lifetime is shown for defects with similar calculated lifetime values, such as the different configurations of $V_{\mathrm{Ga}}\text{−}{\mathrm{O}}_{\mathrm{N}}$ and $V_{\mathrm{Ga}}$, or the different ${\mathrm{Be}}_{\mathrm{Ga}}$ complexes. The calculations were performed for two different configurations for each of the $V_{\mathrm{Ga}}$ complexes.

Figure 3

Ratio curves obtained from coincidence Doppler broadening experiments in Ga-polar samples and from theoretical calculations. The comparison of experimental and theoretical data clearly shows that defects related to ${\mathrm{Be}}_{\mathrm{Ga}}$ are detected in Be-doped GaN.

Figure 4

Average positron lifetimes (bottom) extracted from the lifetime spectra measured in the low-O HNPS-grown bulk GaN as a function of measurement temperature. The data obtained in Mg- and high-O Be-doped GaN (also the second lifetime component, top) from earlier experiments [31, 32] are shown for comparison.