Branch point energies in ZnO and MgO measured using ZnMgO:Al/Si heterostructures

Zn(1-x)MgxO and Zn(1-x)MgxO:Al films with the Mg content (x) varying in the 0.04-0.25, while keeping the Al concentration ~2.5 at.% in the doped ones, were grown by Atomic Layer Deposition (ALD) on Si(100) substrates for the realization of heterostructures to study the impact of Al doping and Mg alloying on the bands alignment. The analysis of the layers performed by X-ray diffraction measurements, energy-dispersive X-ray spectroscopy and high resolution scanning transmission electron microscopy indicated that the films are almost relaxed and polycrystalline with a columnar structure. Furthermore, all over the investigated Mg content they exhibited a wurtzite crystal structure with the expected nanolaminate Mg and Al distribution within each column as shown in (a). On the other hand, no evidence of precipitates or inclusions was found, thus implying that the extracted parameters from the electrical analysis are of general nature. From capacitance vs voltage measurements performed at RT the conduction band misalignment (ΔEC) was determined. However, as shown in (b), where the results concerning RT Hall measurements performed on the films deposited on glass during the same processes are presented, the Zn(1-x)MgxO and ZnMgO:Al films are degenerately n-type. Hence, the measured ΔEC were corrected for many body effects and the interaction of the electrons with the donors. The so obtained electron affinity (χZn(1−x)MgxO) vs Mg content evaluated using the Si one (4.05 eV) as a reference are reported in (c). These results were used to extract the branch point energies (Fibp) under the assumption of a linear variation between the respective values of the corresponding two binary compound semiconductors, ZnO and MgO. The Fibps were found equal to (2.8 ± 0.2) eV and (3.6 ± 0.4) eV, if referred to the valence band maximum, for ZnO and MgO, respectively. These values are ∼0.5−0.8 eV and ~1.8 eV lower than the theoretically predicted ones for ZnO and MgO, respectively [1-3], even though in agreement with the ones obtained by Mönch in Ref.[3] on the basis of compiled experimental data.