Surface passivation of semiconducting oxides by self-assembled nanoparticles

Physiochemical interactions which occur at the surfaces of oxide materials can significantly impair their performance in many device applications. As a result, surface passivation of oxide materials has been attempted via several deposition methods and with a number of different inert materials. Here, we demonstrate a novel approach to passivate the surface of a versatile semiconducting oxide, zinc oxide (ZnO), evoking a self-assembly methodology. This is achieved via thermodynamic phase transformation, to passivate the surface of ZnO thin films with BeO nanoparticles. Our unique approach involves the use of BexZn1-xO (BZO) alloy as a starting material that ultimately yields the required coverage of secondary phase BeO nanoparticles, and prevents thermally-induced lattice dissociation and defect-mediated chemisorption, which are undesirable features observed at the surface of undoped ZnO. This approach to surface passivation will allow the use of semiconducting oxides in a variety of different electronic applications, while maintaining the inherent properties of the materials.

2 reaches at 2θ = 34.42° of the bulk ZnO(0002) value. The multiple diffraction peaks of the asgrown BZO(0.06) film tend to be a single peak at T A ≥ 700 °C. These results clearly indicate that annealing induces strain relaxation and Be redistribution in these metastable alloy films on highly mismatched substrates. Extra diffraction peaks in both annealed films appear around 2θ = 41.0° at T A ≥ 800 °C. These peaks correspond to the wurtzite BeO(0002) indicative of the thermally-driven nucleation-and-growth of BeO nanoparticles (NPs) in the transformed alloy films. 3

Supplementary Information 2
The effects of thermal annealing on the optical absorption properties of the BZO(0.02 and 0.06) films was examined and compared to those of undoped ZnO. All of the as-grown and annealed films are highly transparent above 80 % in UV and visible range as shown in Fig. 2a.
The absorption coefficient, , of the films was calculated using optical transmittance spectra and Fig. 2b shows 2 as a function of photon energy for different Be concentrations and annealing temperature (T A ). The absorption edge is determined by linear extrapolation of the sharp onset to the horizontal portion of the spectra for the as-grown films and shifts to higher energies with Be composition. In addition, an increase in the tailing of the spectra is observed due to band gap widening and structural deterioration induced by the incorporation of Be in the host ZnO lattice (see Supplementary Fig. 1a). Significant changes in the position and tailing of the optical absorption edge in both the ZnO and BZO films are also observed with increasing T A . The absorption edge (optical band gap energy) of the as-grown and annealed ZnO films, increases up to T A = 800 °C, while the energy decreases with further increase in T A ≥ 900 °C ( Supplementary Fig. 2c). The former is due to an increase in thermally-induced donor-like point defects, a reduction in the carrier-trap centers (e.g. charged structural defects or grain boundaries), and subsequent conduction band-filling effects in the annealed undoped films 6 .
This corresponds to an increase in the carrier concentration in the films, which were determined by Hall effect measurements (see Fig. 7a in the main text). The latter for T A ≥ 900 °C primarily arises from thermal decomposition and the resulting structural deterioration of the ZnO lattice.
This compensates the conduction electrons through the introduction of deep acceptor levels in the band gap, together with tailing effects. The red-shift of the optical band gap leads to insulating behavior in the high-temperature-annealed ZnO films (T A ≥ 900 °C) and in turn no optical response was found at T A = 950 °C. On the other hand, a continuous decrease in the optical band gap energy of both BZO films was found with T A mainly due to loss of Be from the film bulk as a result of thermal redistribution and lattice strain relaxation (presented in our previous work 7 .). However, it should be noted that the optical values obtained from the annealed 4 alloy films are affected by many-body interactions, namely, electron-electron and electronphonon interactions, which result in band gap renormalization 8 . The XPS spectra were collected by varying the emission angle, !"# , of the photoelectrons within the XPS probing geometry (see insert of Supplementary Fig. 3a) to increase the XPS surface sensitivity. Figure 3a shows the normalized XPS Zn 3s and Be 1s core level spectra for the annealed BZO films. The Be 1s peak intensity increases with decreasing !"# , indicating