Positrons as interface-sensitive probes of polar semiconductor heterostructures

Group-III nitrides in their wurtzite crystal structure are characterized by large spontaneous polarization and significant piezoelectric contributions in heterostructures formed of these materials. Polarization discontinuities in polar heterostructures grown along the (0001) direction result in huge built-in electric fields on the order of megavolt per centimeter. We choose the III-nitride heterostructures as archetypal representatives of polar heterostructures formed of semiconducting or insulating materials and study the behavior of positrons in these structures using first-principles electronic-structure theory supported by positron annihilation experiments for bulk systems. The strong electric fields drive positrons close to interfaces, which is clearly seen in the predicted momentum distributions of annihilating electron-positron pairs as changes relative to the constituent bulk materials. Implications of the effect to positron defect studies of polar heterostructures are addressed.

Figure 1

Comparison of experimental and computational Doppler spectra of AlN and InN normalized to that of GaN. The markers and curves denote the measured and calculated results, respectively.

Figure 2

(Color online) Macroscopic averages of the positron density (red solid curves) in (a) GaN/AlN(0001), (b) AlN/GaN(0001), (c) $\text{GaN}/\text{AlN}\left(11\overline{2}0\right)$, and (d) $\text{AlN}/\text{GaN}\left(11\overline{2}0\right)$ heterostructures. The positron potentials averaged over the planes perpendicular to the interface normal are also shown (dotted blue curves). For the polar structures in (a) and (b) the straight envelope lines through the potential minima are shown whereas for the nonpolar structures in [(c) and (d)] the energy difference between the straight horizontal lines reflects the positron energy offset between the adjacent layers.

Figure 3

(Color online) Same as Fig. 2 for polar (a) GaN/InN(0001) and (b) InN/GaN(0001), as well as for nonpolar (c) $\text{GaN}/\text{InN}\left(11\overline{2}0\right)$ and (d) $\text{InN}/\text{GaN}\left(11\overline{2}0\right)$ heterostructures.

Figure 4

(Color online) Doppler spectra calculated for the polar heterostructures (a) GaN/AlN(0001) and AlN/GaN(0001); (b) InN/GaN(0001) and GaN/InN(0001), shown as ratio to the bulk GaN spectrum. Also the corresponding ratios for bulk AlN and InN and the Ga vacancy in GaN are shown.