Effect of Strain and Surface Proximity on the Acceptor Grouping in ZnO

Abstract

According to the present knowledge, the level of zinc oxide conductivity is determined by donor and acceptor complexes involving native defects and hydrogen. In turn, recently published low-temperature cathodoluminescence images and scanning photoelectron microscopy results on ZnO and ZnO/N films indicate grouping of acceptor and donor complexes in different crystallites, but the origin of this phenomenon remains unclear. The density functional theory calculations on undoped ZnO presented here show that strain and surface proximity noticeably influence the formation energy of acceptor complexes, and therefore, these complexes can be more easily formed in crystallites providing appropriate strain. This effect may be responsible for the clustering of acceptor centers only in certain crystallites or near the surface. Low-temperature photoluminescence spectra confirm the strong dependence of acceptor luminescence on the structure of the ZnO film.

Figure 1

(a) Relaxed structure of V_ZnH in the unstrained ZnO. (b) Relaxed structure of the QD. Purple and red balls represent Zn²⁺ and O^2– ions, respectively. Smaller cyan balls denote pseudohydrogen atoms (H* and H**) employed for surface passivation. The numbers 1–5 denote, respectively, the Zn (or vacancy) sites: 1: c, 2: m₁, 3: m₂, 4: m₃, and 5: m₄. (c,d) Variations of the energy of the band gap (c) and average Zn–O bond length (d) under different strains for ZnO bulk.

Figure 2

(a) Defect formation energies of unstrained ZnO as functions of ε_F. (b) Variation of the E_form of V_Zn, H_i⁰ and on the biaxial strain.

Figure 3

(a–c) Total DOSs of ZnO/H_i for unstrained and 4% tensile and compressive strained crystal, respectively. (d–f) Contribution (×10 times) of s(H) orbital for unstrained and 4% tensile and compressive strained crystal, respectively. Green dashed horizontal lines correspond to the VBM and CBM unstrained crystal. (g–i) Atomic configuration and the density of charge states of unstrained and 4% tensile and compressive strained crystal, respectively. Contours go from 0.0009 to 0.5 electron/Bohr³.

Figure 4

(a) Total DOS and (b–d) contributions of p(O), d(Zn), and s(H) orbitals of QD ZnO with H_i at m₁ site. Horizontal dashed lines correspond to the VBM and CBM states. (e–g) Relaxed crystal structure of QD with H interstitial in the m₁, m₂, and m₄ configurations, respectively.

Figure 5

(a–e) Total (orange color) and the contribution of p(O) orbital (dark cyan color) spin-resolved DOS of ZnO containing a V_Zn: (a,b) bulk under 0 and 4% compressive strain, respectively. (c–e) QD with vacancy at the m₁, c, and m₃ sites, respectively. Left and right panels denote the spin-down and -up channels, respectively.

Figure 6

Calculated atomic configurations and isosurfaces of spin density corresponding to 0.05 electron/Bohr³ for the neutral zinc vacancy: (a) in the bulk ZnO under 4% compressive strain, (b–d) in QD at c, m₁, and m₃ positions, respectively.

Figure 7

(a–d) Total spin-resolved DOS of ZnO containing a V_ZnH: (a) unstrained, (b) ε_xy = 4%, (c) ε_xy = −4%, and (d) ε_zz = −4%. Left and right panels denote the spin-down and -up channels, respectively. Green dashed horizontal lines correspond to the VBM and CBM unstrained crystal. a_t↓, e_t↓ levels, and Δ_a–e are depicted for the unstrained crystal. (e–h) Yellow isosurface represents the spin-density distribution of the ZnO containing a V_ZnH: (e) unstrained, (f) ε_xy = 4%, (g) ε_xy = −4%, and (h) ε_zz = −4%.

Figure 8

(a) X-ray diffractograms of ZnO films deposited on a-Al₂O₃ (blue) and c-Al₂O₃ (black) substrate; (b) photoluminescence spectra of ZnO/a-Al₂O₃ (up) and ZnO/c-Al₂O₃ (down) measured at 5K using excitation energy of 3.483 eV.