Investigation of phase segregation in sol–gel derived ZnMgO thin films

Abstract

Highly c-axis-oriented Zn 1-x Mg x O multilayered thin films have been deposited on p-type Si substrates with different concentration of Mg (x = 0.00−0.40) using a sol-gel spin-coating technique. The x-ray diffraction (XRD) shows that single-phase wurtzite thin films start showing phase segregation for a Mg content of x = 0.25 for the sol-gel-derived ZnMgO thin films. The element specific near edge x-ray fine structure (NEXAFS) collected at O K-edge also clearly evidence the phase segregation at x = 0.25. These results also show that films are deposited with wurtzite structure as dominant phase even after phase segregation. The NEXAFS spectra collected at Zn L 3 -edge rule out the presence of any Zn-related defect due to Mg doping. The atomic force microscopy (AFM) depicts the spherical shape of nanosized grains, and grain size varies slightly with Mg content. The single-phase ZnMgO thin films show a band gap tuning from 3.38 to 3.84 eV, which is also consistent with blue shifting of near-band edge PL emission. The electrical resistivity of thin films increases with Mg content before phase segregation. However, the optical band gap, photoluminescence and electrical resistivity show anomalous behavior at phase segregation limit which has been discussed and correlated with each other.

Polarization discontinuity induced two-dimensional electron gas at ZnO/Zn(Mg)O interfaces: A first-principles study

“…33 As a result, the ternary alloy Zn 1−x Mg x O exhibits a sensitive structurecomposition dependence, adopting the wurtzite (B4) form only for low Mg concentrations (x < 0.33). [43][44][45] Nevertheless, metastable structures of wurtzite Zn(Mg)O with very high Mg concentration can be achieved by epitaxial growth on high-quality ZnO crystals. This way, samples with Mg concentrations as high as 60% or more have been demonstrated.…”

“…28 Electronically, the Mg concentration serves to tune the Zn(Mg)O band gap gradually between that of wurtzite ZnO and MgO. 33,[43][44][45] The band gap of Zn 1−x Mg x O for a given x can be estimated from the Vegard's law E…”

“…33 Experimentally, at low concentrations, the band gap increases fairly linearly with Mg concentration. [43][44][45] For higher concentrations, the bowing for the zincblende Zn(Mg)O mixture has been estimated from first-principles calculations to be moderately large, i.e. the band gap increases faster than linear with the Mg concentration.…”

Influence of Mg content on defect-related luminescence of undoped and doped wurtzite MgZnO ceramics

“…With increasing the Mg content, both the enhancement [3][4][5] and weakening [6,7] of the visible emission intensity were observed. In [3][4][5][6]8], the shift of defect-related emission band toward the short wavelength side as a result of alloying ZnO with MgO was found, whereas in [9,10] any change of emission band shape or peak position was not observed. In [11], it was shown that self-activated defect-related emission in sol-gel MgZnO phosphors can be shifted from the orange to green-blue spectral region by changing the annealing temperature and gas ambient.…”

Structural, microstructural, and optical properties of Zn1−x Mg x O thin films grown onto glass substrate by ultrasonic spray pyrolysis

“…The thermodynamic limit solid solubility of MgO in ZnO is reported around 4 % in the bulk [13]. However, the limit solid solubility of MgO in ZnO for thin films has been reported to be around 33 mol % as deposited by pulsed laser deposition (PLD) technique [6], 25 % as deposited by sol-gel spin-coating technique [14] and 43 % as deposited by molecular beam epitaxy (MBE) [15].…”