Bandgap bowing in crystalline (ZnO)_1−x (GaN) _x thin films; influence of composition and structural properties

Abstract

Highly crystalline thin films of (ZnO) 1−x (GaN) x were synthesised using RF magnetron sputtering, with x ranging from 0 to 0.20. The band gap of the alloys showed, as estimated, a significant reduction down to ∼2.5 eV for x > 0.07, by employing UV-VIS transmission measurements and electron energy loss spectroscopy, compared to the band gap energies of the two host materials, i.e. E ZnO g = 3.37 eV and E GaN g = 3.51 eV. The reduced band gap results in an extension of the absorption for the alloys well into the visible part of the spectrum. Structural analysis, utilizing x-ray diffraction, Rutherford backscattering spectrometry and transmission electron microscopy, yielded highly crystalline films, with columnar grains and a good heteroepitaxial relation to the Al 2 O 3 substrate. The unit cell of the alloys was found to be rotated 30 • with respect to the that of the substrate, in order to minimize the lattice mismatch to the substrate. An increase in c-lattice constant as a function of GaN content (x) was found, opposite to that predicted by Vegard's law, and explained in terms of strain, as well a high density of threading dislocations. The effects of thermal annealing in N 2 atmosphere after growth were analysed, both experimentally and using computational calculations employing density functional theory. Optically no large effects were found, especially in the estimated band gap energies. In terms of crystal structure, an increase in grain size was detected, reduced strain and c-lattice parameters approaching the expected values from Vegard's law, reduced dislocation density and an overall increase in crystalline quality. On the other hand, a systematic peak-broadening of the (0002) x-ray diffraction reflection was detected, attributed to an increase in Ga-N bonds. Moreover, for the films with higher x, an interfacial layer with a higher Ga-content compared to the remaining film was detected, attributed to the formation of zinc blende phases resulting from the accumulation of stacking faults. Nano-sized voids consisting of molecular N 2 were also found after post-deposition annealing, where the formation of voids was attributed to the agglomeration of Zn-and Ga-vacancies. The filling of voids with molecular N 2 was found to be a stabilization mechanism for the vacancy clusters, indicating that N is not stable in the O substitutional site. Finally, a deeper investigation of the mechanisms governing the band bowing effect in the (ZnO) 1−x (GaN) x alloys was undertaken, combining experimental and computational results. The results revealed the formation of a GaN-like defect band above the valence band maximum of ZnO to be the cause of the reduced band gap, oppositely to the explanation used in the literature, with orbital repulsions within the valence band, pushing the valence band maximum upward in energy.

Evidence of defect band mechanism responsible for band gap evolution in (ZnO)1−x(GaN)x alloys

“…Post-deposition isochronal anneals at 600, 700, and 800 • C in nitrogen atmosphere were employed to improve structural and optical properties. More detailed descriptions of the sample preparation, annealing, and structural characterization can be found elsewhere [6,18].…”

“…The band bowing effect occurs when the apparent band gap of the alloy is reduced as a function of composition, in a nonlinear fashion, with respect to the two unmodified materials. Various synthesis routes have been used to fabricate the ZOGN alloy, targeting the absorption tunability to meet the specific needs of different applications; for example nanowires were deposited using a sol-gel method [3] and high-temperature vapor-phase diffusion reactions [4], whereas pulsed-laser deposition [5] and magnetron sputtering [6] resulted in highly crystalline thin films. Solid solution synthesis was utilized to fabricate bulk samples for water-splitting applications [1,[7][8][9].…”

Effects of Substrate and Post‐Deposition Annealing on Structural and Optical Properties of (ZnO)1−x(GaN)x Films

“…The origin of this bandbowing effect is still under debate, where orbital‐repulsion within the valence band and type‐II band alignment have both been suggested as plausible candidates. This bandgap narrowing has been exploited for water‐splitting applications, but is also a potential candidate for various optoelectronic applications as the alloy has been successfully fabricated as highly crystalline thin films as well …”

Formation of N2 bubbles along grain boundaries in (ZnO)1−x(GaN)x: nanoscale STEM-EELS studies

“…(ZnO) 1Àx (GaN) x (ZOGN) alloys are intriguing candidates for bandgap engineering, since the bandgap energy (E g ) of the system is reduced in comparison to its binary compounds (E ZnO g B 3.3 eV, E GaN g B 3.4 eV) due to a strong band bowing effect. 1 By tuning the alloy composition (x), E g can be tailored from the UV-range into the visible part of the spectrum, making ZOGN a strong candidate for photovoltaic 2,3 and solar watersplitting [4][5][6] applications. However, the overall micro and nanostructure can strongly affect the semiconductor properties and device performance.…”

“…[9][10][11][12] Moreover, chemically induced phenomena such as phase separation and elemental clustering can play a detrimental role in the functional properties of an alloy system. Post-deposition annealing usually improves the crystalline quality [13][14][15][16] and recently Olsen et al 2,17,18 conducted a comprehensive study on magnetron sputtered ZnO-rich ZOGN thin films, elucidating the influence of composition and post-deposition annealing on the film properties, as well as investigating the mechanisms governing the band bowing effect. In the same study, 17 DFT calculations revealed that the total energy of the system decreased by increasing the amount of Ga-N bonds, predicting a re-arrangement of the nitrogen bonds during postdeposition annealing.…”