Experimental Determination of Valence Band Offsets of ZnGeN2 and (ZnGe)0.94Ga0.12N2 with GaN

Abstract

Abstract Body: Zinc germanium nitride (ZnGeN2) is the II-IV-N2 analogue of GaN which has a band gap (Eg) very close to that of GaN (~3.4 eV) and a lattice mismatch of <0.1% with GaN [1]. Based on first principles calculations, the predicted valence band offset (VBO) of ZnGeN2 with GaN can be as high as 1.4 eV, [2] which provides great opportunities to design high efficiency InGaN/ZnGeN2 heterostructure quantum wells for visible light emitters [1]. However, there has been no report on the experimentally determined VBO at the ZnGeN2/GaN heterointerface, due to the lack of availability of high quality materials. In this work, we have experimentally determined the VBOs of ZnGeN2 and a 94% ZnGeN2 – 6% GaN alloy ((ZnGe)0.94Ga0.12N2) with GaN using Kraut’s method. The VBO between two materials A and B are determined using the core energy levels (ECL) in bulk A, bulk B and at the A/B heterointerface and the position of the valence band maxima (VBM) in bulk A and bulk B. X-ray photoemission spectroscopy (XPS) was used to measure the ECL and VBM values. To determine the VBO between ZnGeN2 and GaN, a GaN/c-sapphire template, a 16-nm-thick ZnGeN2 film and a 2-nm-thick ZnGeN2 layer grown on GaN template were used as the bulk GaN, bulk ZnGeN2 and ZnGeN2/GaN heterostructure, respectively. ZnGeN2 films were grown on GaN/c-sapphire templates via metalorganic chemical vapor deposition (MOCVD) [3,4]. By using the measured Zn 3d (Ge 3d) core levels from ZnGeN2 and the Ga 3d core levels from GaN, the VBO of ZnGeN2 with GaN was extracted to be 1.45±0.15 eV (1.65±0.15 eV). These values are comparable to the values predicted from first-principles calculations using explicit interface calculations [2]. Similarly, the VBO of (ZnGe)0.94Ga0.12N2 with GaN was determined. The samples used include a GaN/c-sapphire template as bulk GaN, a 20-nm-thick (ZnGe)0.94Ga0.12N2 films as bulk (ZnGe)0.94Ga0.12N2 and a 3-nm-thick (ZnGe)0.94Ga0.12N2 film as the (ZnGe)0.94Ga0.12N2/GaN heterostructure. These samples were grown on GaN/c-sapphire templates via MOCVD. The experimentally extracted VBO of (ZnGe)0.94Ga0.12N2 with GaN was 1.29±0.15 eV. Assuming a linear dependence of the VBO with composition x of (ZnGe)1-xGa2xN2, the predicted VBO of (ZnGe)0.94Ga0.12N2 from the theoretically calculated VBO of ZnGeN2 would be 1.32 eV. Thus, the experimentally measured VBO results agree well with the theoretically predicted values. The conduction band offsets were derived using the determined VBO values and the energy band gap Eg of the materials. The band gaps of ZnGeN2 and (ZnGe)0.94Ga0.12N2 were estimated from the inelastic energy loss features of N 1s peaks in the XPS spectra of 16-nm-thick ZnGeN2 and 20-nm-thick (ZnGe)0.94Ga0.12N2 samples. The Eg values determined by this method were 3.0±0.2 eV for ZnGeN2 and 3.1±0.2 eV for (ZnGe)0.94Ga0.12N2. The lower values of Eg as compared to the predicted band gap values (~3.4 eV) can be due to the presence of disorder in the cation sublattice. In conclusion, the valence band offsets of MOCVD-grown (ZnGe)1-xGa2xN2 with GaN, with x = 0 and 0.06 were determined experimentally using XPS. The measured VBO values are comparable to the predicted values from first-principles calculations. The results from this study will expand device designs based on pure III-nitrides to III-nitrides/II-IV-N2, which can potentially address key challenges in III-nitride based electronic and optoelectronic device technologies. This work is supported by the U.S. DOE (DOE SSL: DE-EE0008718) and by the NSF (DMREF: SusCHeM: 1533957).