Probing Wave Functions of Electrically Active Shallow Level Defects by Means of High-Frequency Pulsed ENDOR in Wide Bandgap Materials: SiC, AlN, ZnO, and AgCl

Abstract

In the high-frequency ENDOR experiments, the hyperfine (HF) interaction between the unpaired electron of the shallow donor or shallow acceptor and the nuclear spins of the Coulombic center and the surrounding atoms is determined, which is then translated into the spin density of the electronic wave function at the various atomic positions. The results of studying the spatial distribution of wave functions for shallow donors in ZnO, AgCl, AlN, and SiC crystals, ZnO-based nanostructures, and shallow boron acceptors in SiC will be presented. The change of the electronic wave function of a shallow donor in ZnO quantum dots (QDs) when entering the regime of quantum confinement by using the nuclear as probes has been observed. The model, based on the effective mass approximation (EMA), that describes a 1s-like wave function with the Bohr radius of ~ 1.5 nm for distant shells was tested. The EMA does not yield an appropriate description of the electronic wave function when the radius of the QD is reduced below the Bohr radius. The direct reconstruction of the wave function of the intrinsic shallow electronic center (SEC) and self-trapped excitons in AgCl was presented. The SEC was suggested to be an electron that is shallowly trapped by two adjacent silver ions on a single cationic site (split-interstitial position), so-called “latent image” in silver halides. The shallowly trapped electron of the STE is shown to behave like a hydrogen 1s electron, centered on the Ag+ lattice position, with a Bohr radius r0 = 1.51 nm that is in agreement with Bohr radius of SEC (r0 = 1.66 nm). For SEC in AgBr, r0 = 2.48 nm. It was demonstrated that dynamic nuclear polarization of nuclear spins due to hyperfine interactions with ligand nuclei can be achieved in ZnO (and based QDs) and AgCl by saturating the high-frequency EPR transition of a shallow donor at low temperatures corresponding to a high Boltzmann factor. Several types of shallow donors were indicated in AlN crystals: (i) affected by the DX-relaxation and (ii) with normal behavior. The strong HF interaction for light-induced SD in AlN support the assignment to the impurity in anionic sublattice (e.g. oxygen in N position). At the same time, a shallow donor with normal behavior can belong to Si or C in the Al position. The electronic structure of shallow donors and shallow acceptors in silicon carbide was investigated by the ENDOR method. The spin density of the N donor corresponding to the observed ENDOR lines was established to be p like in character and located mainly on the Si atoms for the k site in 4H-SiC, whereas for the three sites in 6H-SiC the spin density is s-like in character and located mainly on the C atoms. An explanation for the difference in the electronic wave function of the N donor in 4H-SiC and 6H-SiC can be found in the large difference in the band structure of the two polytypes and in the position of the minima in the Brillouin Zone. The electronic density for shallow B acceptor substituting Si in the k position is distributed in an ellipsoidal shape with the main symmetry axis making an angle of 70° with the c axis, i.e., along the direction of the B–C with main spin density.