Influence of nitrogen and magnesium doping on the properties of ZnO films^*

The morphology of the ZnO films grown on the MgO (111) substrates with different doping elements of N and Mg can be seen through the RHEED patterns captured in situ after the growth,^[13] as shown in Fig. 1. Generally speaking, streaky RHEED patterns indicate a flat sample surface and the spots indicate poor consistency on the surface, while the spotty patterns indicate a three-dimensional feature. Figure 1(a) shows the RHEED pattern of the ZnO film without doping, along the [1-210] azimuth and the streaky feature indicates a growth mode close to two-dimensional. Figure 1(b) shows the RHEED pattern of the ZnO film with N doping. The RHEED pattern becomes dotty, indicating that N doping damages the two-dimensional growth of ZnO, which is consistent with the previous results.^[14] Figure 1(c) shows the RHEED patterns of the ZnO film with co-doping of N and Mg elements. It also exhibits a spotty feature, sharper than those in Fig. 1(b), indicating that Mg may promote the growth of N-doped ZnO thin film. Both RHEED patterns in Figs. 1(b) and 1(c) show a small extension along the [1-210] azimuth, implying a smaller coherence on the surface.

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Such an effect is further proved by the AFM images captured from these three film samples, as shown in Fig. 1. The ZnO film without doping shows a relatively smoother morphology (Fig. 1(d)), with an RMS value of 0.60 nm, while N doping induces an island structure (Fig. 1(e)) with an RMS value of 3.46 nm. For the ZnO film with N and Mg co-doping, the AFM image (Fig. 1(f)) shows a decrease of the RMS value to 2.70 nm.

The phase structures of the grown undoped and doped ZnO films were measured by XRD, as shown in Fig. 2. The 2θ values in x axis of Fig. 2 are calibrated by the substrate signal, (i.e., the strong peaks at 37.02° observed before growth of ZnO, which is illustrated in Fig. 2). All three films exhibit a feature at around 34.6°, assigned to ZnO (002). The presence of such a strong ZnO (002) feature, compared to other ZnO features, indicate that the deposited ZnO films are highly oriented. Further inspection of the XRD spectra shows that the position of ZnO (002) peaks shifts slightly after doping (i.e., the 2θ values of the peak for undoped, N-doped, and N–Mg co-doped ZnO films are 34.55°, 34.68°, and 34.66°, respectively). It is speculated that N hinders the growth of ZnO^[15] and then prevents the complete relaxation of the stress, thus causing the excursion of the 2θ of ZnO (002) peak. The small diffraction peak at about 30.7° is likely originated from ZnO (100). It is also noted that, for the same growth parameters, the quality of the co-doped ZnO films is worse than that of the undoped ZnO film, as indicated by the lower intensity and larger FWHM of the ZnO (002) peak.

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The average crystallite size (D) was estimated from the Debye–Scherrer's equation^[16]

where λ represents the wavelength of the x-ray radiation, is the full width at half maximum (FWHM) of diffraction peak, and is the scattering angle. The crystallite size of the ZnO film, the N-doped ZnO film, and N–Mg co-doped ZnO film is 4.6 nm, 2.7 nm, 3.9 nm, respectively, indicating that Mg probably improves the quality of N-doped ZnO films and leads to a smoother morphology than the N-doped ZnO film without Mg doping.

Figure 3(a) shows the synchrotron-based x-ray absorption spectra of the ZnO film (oxygen K-edge) with different incident angles, which shows an obvious polarization dependence with the photon energy higher than 535 eV, indicating that the features at 530–535 eV and above 535 eV could be attributed to the O 2p states that hybridize with the isotropic Zn 4s states and anisotropic Zn 4pd states, respectively.

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Figure 3(b) compares the experimental oxygen K-edge spectra of ZnO with and without N doping. The projected density of states (PDOS) from the DFT calculations are also shown (in the lower layer of Fig. 3(b)) with the atomic models illustrated in the inset. Significant differences between the spectra of ZnO with and without N doping can be observed, especially at the lower energy region. Features in the photon energy regions of 530–540 eV, 540–550 eV and above 550 eV are attributed to the hybridized states of O 2p–Zn 4s, O 2p–Zn 4p, and O 2p–Zn 4d, respectively.^[17] The peak positions of the PDOS contributed by P_x, P_y, P_z orbitals are easily distinguished at about 7.6 eV above the Fermi level, which corresponds to the binding energy at about 534.7 eV. The peak at about 534 eV for the N-doped ZnO is higher than that for the ZnO without doping. This could be explained by the smaller number of valence electrons of N (5 electrons from 2s²2p³) than those of oxygen (6 electrons from 2s²2p⁴). When some oxygen sites in ZnO have been occupied by nitrogen atoms, less charges are transferred from Zn to nearby oxygen sites due to the interference of doped nitrogen. Thus, the oxygen in N-doped ZnO has more unoccupied states than that in ZnO without doping. In the nitrogen K-edge spectrum as shown in Fig. 3(c), there is a strong resonant peak P₁ at 401 eV, which is the characteristic triple bonded N₂ bound state resonance, as well as a shape resonance in the continuum around 400.9 eV.^[18] As metal nitrides have two sharp resonances at the photo energies ranging from 397.5 eV to 401.5 eV, assigned to the transitions of N 1s electrons to the p–d (t_2g) and p–d (e_g) hybridized orbitals, respectively,^[19] the first sharp resonance (P₂: 399.4 eV) in Fig. 3(c) could be attributed to the Zn–N peak. The second peak of Zn–N overlaps with the N₂ peak (P₁: 401 eV). Peak 3 (P₃: 402.8 eV) could be attributed to the transitions of orbitals.^[20] It has also been reported that, NO₂ (N(1s) → 2b₁),^[21] N₂O (N(1s) → π*(N_c)),^[22] and NO (N(1s) → π*)^[21] correspond to the band features at 403.28 eV, 404.7 eV, and 399.5 eV, respectively, and might have contributed partially to the above three peaks and the background.

As reported by Limpijumnong et al.,^[23] N could exist as substitutional diatomic molecules, such as NO, NC, and N₂ on the oxygen sites in ZnO wurtzite structure. The N 1s XPS spectra for ZnO films without and with Mg doping are shown in Figs. 3(d) and 3(e), respectively (after smoothing from the raw data). Without Mg doping, there is a Zn–N peak at 395.8 eV,^[24] as well as (N₂)_O peak^[25] at around 405.30 eV. In between, there are four additional peaks visible, with the peak at 402.5 eV assigned to N–O surface contamination or N₂ trapped within the film during deposition,^[26] the peak at around 400.93 eV assigned to sp² nitrogen bonded to three carbon neighbors,^[27] the peak at around 398.05 eV assigned to C–N,^[28,29] and the peak at around 399.18 eV assigned to C–H.^[30]

For the N–Mg co-doped ZnO film (Fig. 3(e)), there is also a peak at around 395.54 eV assigned to Zn–N,^[24] and the peaks at around 399.79 eV and 398.66 eV were assigned to −NH–,^[31] C–N,^[32] and N–O in the NO molecule.^[33] Compared with the N-doped ZnO film without Mg, the film with Mg shows no obvious feature at the energy range higher than 402 eV, suggesting that Mg could inhibit (N₂)_O production, while at the energy range from 397 eV to 402 eV, a hybridization peak arises in the film with Mg, which is split into several small peaks in the film without Mg. If atomic sensitivity factor is taken into account,^[34] combining the XPS data from the N-doped ZnO and Mg–N co-doped ZnO's zinc (Fig. 3(f)), the concentration ratio of N:Zn in the Mg–N co-doped ZnO film is nearly twice that in the N-doped ZnO film, using the range from 397 eV to 402 eV. We therefore speculate that Mg promotes the inducing of N, which could have resulted in Mg–N bonding at 399.79 eV.