The temperature-dependency of the optical band gap of ZnO measured by electron energy-loss spectroscopy in a scanning transmission electron microscope

Granerød, Cecilie Skjold; Galeckas, Augustinas; Johansen, K. M.; Vines, Lasse; Prytz, Øystein

doi:10.1063/1.5023316

Cited by 11 publications

(8 citation statements)

References 52 publications

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“…The standard deviation is below 0.01 eV based on 19 measurements in the ZnO layer, and we therefore consider this variation to be significant. This is also consistent with the optical band gap of ZnO and its temperature dependence as previously found by EELS [36], as well as by other conventional band gap methods [37]. In samples with high enough electron concentrations, such as ours, the Fermi level moves into the conduction band and thus states below this level are filled and not available for optical transitions from the valence band.…”

Section: B Band Gap Analysissupporting

confidence: 90%

Direct observation of conduction band plasmons and the related Burstein-Moss shift in highly doped semiconductors: A STEM-EELS study of Ga-doped ZnO

et al. 2018

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The combination of high optical transparency and low electrical resistivity has made transparent conductive oxides (TCOs) a key technology in many optoelectronic applications. Furthermore, the study of TCOs yields insight into many fundamental parameters of semiconductors. For example, the high charge carrier concentration results in an apparent shift in the band gap, the so-called Burstein-Moss shift, in addition to plasmonic resonances in the near infrared regime. While both effects are related to the carrier concentration and band structure, their lateral distribution and interaction with boundary conditions such as interfaces and surfaces are difficult to assess, and a direct observation of the local distribution has remained elusive. Here we employ electron energy-loss spectroscopy in scanning transmission electron microscopy (STEM-EELS) for direct observation of spatially resolved plasmonic resonances and Burstein-Moss shift in gallium doped zinc oxide (GZO), one of the most widely used TCOs. A 25 nm thick GZO film with a carrier concentration of 7 × 10 20 cm −3 has been grown epitaxially on a nominally undoped ZnO film. The GZO film shows a renormalized Burstein-Moss shift of ∼0.5 eV, in accordance with that expected from Hall effect measurements. The plasma resonance of the conduction band electrons is located at 0.82 eV in the bulk GZO, and with a substantial increase in intensity towards the sample surface, where a surface plasmon energy of 0.54 eV is observed. Hence we have directly measured differences in optical properties between the two films, the local variation across the GZO film has been studied, and we are also directly observing the difference between bulk and surface properties of GZO.

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Section: B Band Gap Analysissupporting

confidence: 90%

Direct observation of conduction band plasmons and the related Burstein-Moss shift in highly doped semiconductors: A STEM-EELS study of Ga-doped ZnO

et al. 2018

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“…The apparent band gap, E g , for ZOGN thin films was estimated using transmission measurements, as well as STEM-EELS, both yielding a significant reduction down to ∼2.4 -2.6 eV for x ≤ 0.15. Using EELS represented as spatially resolved maps discriminating ∼5 nm pixels [96] together with EDX maps, it was furthermore found that the reduction in apparent E g was close to homogeneous over the probed area, with no indications of segregation into ZnO-or GaN phases with band gap energies of ∼3.4 -3.5 eV. Our experimentally obtained band bowing is also in remarkable agreement with our theoretically obtained results, utilizing the GW approximation.…”

Section: Band Gap Evolution In (Zno) 1−x (Gan) X Alloyssupporting

confidence: 86%

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

Olsen¹,

Bazioti²,

Azarov³

et al. 2018

Semicond. Sci. Technol.

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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.

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“…2 probe regions of ∼150 μm in diameter and 200 × 400 nm 2 , respectively. The EELS data can also be presented in the form of spatially resolved maps discriminating ∼5 nm pixels [39]. Figure 4 shows (a) an example of such map for x = 0.15, (b) onset for a number of pixels as indicated by arrows in the map, and (c) corresponding dark-field image of the mapped area of the sample.…”

Section: A Experimental Resultsmentioning

confidence: 99%

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

et al. 2019

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It is known that (ZnO) 1−x (GaN) x alloys demonstrate remarkable energy band bowing, making the material absorb in the visible range, in spite of the binary components being classical wide band gap semiconductors. However, the origin of this bowing is not settled; two major mechanisms are under debate: Influence of the orbital repulsion and/or formation of a defect band. In the present work, we applied a combination of the absorption and emission measurements on the samples exhibiting an outstanding nanoscale level of (ZnO) 1−x (GaN) x homogeneity as monitored by the high resolution electron microscopy equipped with the energy dispersive x-ray analysis and the electron energy loss spectroscopy; moreover the experimental data were set in the context of the computational analysis of the alloys employing density functional theory and quasiparticle GW approximation. A prominent discrepancy in the band gap values as deduced from the absorption and emission experiments was observed systematically for the alloys with different compositions and interpreted as evidence for the absorption gap shrinking due to the defect band formation. Computational data support the argument, revealing only minor variations in the bulk of the conduction and valence band structures of the alloys, except for a characteristic "tail" in the vicinity of the valence band maximum. As such, we conclude that the energy gap bowing in (ZnO) 1−x (GaN) x alloys is due to the defect band formation, presumably at the top of the valence band maximum.

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The temperature-dependency of the optical band gap of ZnO measured by electron energy-loss spectroscopy in a scanning transmission electron microscope

Cited by 11 publications

References 52 publications

Direct observation of conduction band plasmons and the related Burstein-Moss shift in highly doped semiconductors: A STEM-EELS study of Ga-doped ZnO

Direct observation of conduction band plasmons and the related Burstein-Moss shift in highly doped semiconductors: A STEM-EELS study of Ga-doped ZnO

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

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

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The temperature-dependency of the optical band gap of ZnO measured by electron energy-loss spectroscopy in a scanning transmission electron microscope

Cited by 11 publications

References 52 publications

Direct observation of conduction band plasmons and the related Burstein-Moss shift in highly doped semiconductors: A STEM-EELS study of Ga-doped ZnO

Direct observation of conduction band plasmons and the related Burstein-Moss shift in highly doped semiconductors: A STEM-EELS study of Ga-doped ZnO

Bandgap bowing in crystalline (ZnO)1−x (GaN) x thin films; influence of composition and structural properties

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

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