Effects of Substrate and Post‐Deposition Annealing on Structural and Optical Properties of (ZnO)_1−x(GaN)_x Films

Abstract

The structural and optical properties of magnetron sputtered thin films of (ZnO) 1Àx (GaN) x deposited on zinc oxide, sapphire, and silicon oxide are studied as a function of strain accumulation and post-deposition anneals at 600-800 C. For the experimental conditions studied, we found that different amounts of tensile strain accumulated in the samples practically does not affect the strong bandbowing effect, that is, optical bandgap, observed in the as-deposited alloys. In its turn, post-deposition annealing results in a reduction of the tensile strain and dislocation density in the films, as measured by both X-ray diffraction and transmission electron microscopy, corroborating an increase in the crystal quality. In addition, the grain size is found to increase with annealing temperature, for example, mean values of 20 nm up to 50 nm were measured for the alloys with x ¼ 0.15. Meanwhile, the full-width at half maximum of the (0002) X-ray diffraction reflection increases with annealing temperature, but with only a small increase in bandgap energies for the x ¼ 0.15 sample. However, this observation was explained combining the experimental data and first-principles calculations based on density functional theory, showing that the increase in the amount of Ga-N bonds lowers the total energy of the system. As such, we conclude that the thermal treatments increase the Ga-N ordering, resulting in several contributions or a widening of the diffraction peaks.

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

“…This is also in agreement with our previous DFT calculations that suggest an increase in the number of Ga-N bonds after thermal treatment. 17 However, the present study reveals that N-N bonding is an alternative path for nitrogen after annealing. It is also important that our results show that the increase in the Ga-N bonds did not lead to phase separation phenomena or clustering (e.g.…”

“…20 EELS spectra acquired from the ZOGN matrix revealed a different shape of the N-K peak compared to that of N 2 , indicating that N is bonded to the metal atoms. A comparison with spectra from a wz-GaN reference sample indicates that the Ga-N bonds proposed by DFT 17 are also present. It should be clarified that the shape of spectrum 1 is the result of overlapped signals along the beam path (void + the surrounding ZOGN matrix).…”

“…[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.…”

“…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. However, the behaviour of N in these alloys is not yet understood, and the proposed Ga-N bonds are not confirmed, warranting a detailed analysis on the nanoscopic scale.…”

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].…”