Ni, Ir, Pd, Pt, and silver oxide Schottky contacts were fabricated on the Zn-polar face of hydrothermally grown, bulk ZnO. A relationship was found between the barrier height of the contact and the free energy of formation of its “metal” oxide. This is consistent with the dominating influence of oxygen vacancies (VO) which tend to pin the ZnO Fermi level close to the VO (+2,0) defect level at approximately 0.7eV below the conduction band minimum. Therefore, a key goal in the fabrication of high quality Schottky contacts should be the minimization of oxygen vacancies near the metal-ZnO interface.

We examine ZnSnN2, a member of the class of materials contemporarily termed “earth-abundant element semiconductors,” with an emphasis on evaluating its suitability for photovoltaic applications. It is predicted to crystallize in an orthorhombic lattice with an energy gap of 2 eV. Instead, using molecular beam epitaxy to deposit high-purity, single crystal as well as highly textured polycrystalline thin films, only a monoclinic structure is observed experimentally. Far from being detrimental, we demonstrate that the cation sublattice disorder which inhibits the orthorhombic lattice has a profound effect on the energy gap, obviating the need for alloying to match the solar spectrum.

calculations revealed that altering the degree of cation disorder from none to maximal disorder using the special quasirandom structure (SQS) approach can change the fundamental band gap from 2.09 to 1.12 eV. The possibility of realizing such a variation experimentally was supported by the observation of the cation-disorder-induced wurtzite phase in molecular-beam epitaxy-grown thin fi lms as opposed to the theoretically more stable orthorhombic phase. [ 3 ] The cation-ordered orthorhombic structure is a superstructure of wurtzite structure, with similar tetrahedral bonding, as described in ref. [ 7 ] .Here, the observed absorption edges as a function of free electron density for the differently grown fi lms are consistent with the room temperature fundamental band gap varying from 1.0 to 2.0 eV as the cation ordering increases. Moreover, the conduction and valence band density of states (DOS) have been calculated using hybrid DFT for the cation-ordered orthorhombic and cation-disordered SQS pseudowurtzite phases. Comparison of these DOS with photoemission data from single crystal ZnSnN 2 fi lms grown under different conditions indicates that samples with different degrees of cation ordering have been synthesised, opening up a new method for tuning the band gap of ZnSnN 2 fi lms.The growth conditions for molecular-beam epitaxy (MBE) of the single crystal ZnSnN 2 thin fi lms are summarized in Table 1 . The substrate temperature, the Zn:Sn fl ux ratio and the N 2 pressure were adjusted to vary the degree of cation disorder in the ZnSnN 2 in order to experimentally test the theoretical predictions. X-ray diffraction indicates that all the samples have the wurtzite structure rather than the fully ordered orthorhombic structure (and also that Zn-nitride and Sn-nitride phases are below the detection limit); specifi cally, the peak at 22° which is characteristic of the orthorhombic structure is absent for all fi lms. [ 3 ] Thus, XRD characterization alone is insuffi cient to exclude the possibility of a signifi cant variation of the degree of cation disorder within the samples grown under the range of conditions employed. Consequently, evidence of cation disorder induced variations in the band gap and band structure has been sought and found in the optical, transport and photoemission properties of the ZnSnN 2 fi lms. It is important to note here that in the nonequilibrium growth technique used in this study (plasma-assisted MBE), substrate temperature and fl ux ratio have a profound infl uence on adatom migration length, and hence how atoms incorporate into an epitaxially driven structure. Therefore, higher growth temperature is likely to produce a more ordered fi lm than a lower growth temperature.

Planar Pd, Pt, Au, and Ag Schottky diodes with low ideality factors were fabricated on the Zn-polar (0001) and O-polar (0001¯) faces of bulk, single crystal ZnO wafers. The diodes were characterized by current-voltage and capacitance-voltage measurements. A polarity effect was observed for Pt and Pd diodes with higher quality barriers achieved on the O-polar face. No significant polarity effect was observed for Au or Ag diodes. The highest barriers were achieved with Ag as the Schottky metal with barrier heights varying between 0.77 and 1.02eV. This is possibly due to varying degrees of oxidation of the Ag contacts.