Nanoscale mapping of optical band gaps using monochromated electron energy loss spectroscopy

Zhan, Wei; Granerød, Cecilie Skjold; Johansen, K. M.; Jensen, Ingvild Julie Thue; Kuznetsov, A. Yu.; Prytz, Øystein

doi:10.1088/1361-6528/aa5962

Cited by 15 publications

(28 citation statements)

References 39 publications

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“…All STEM imaging and EDX investigations were performed at an acceleration voltage of 300 kV with a probe convergence angle of 21 mrad and a resulting spatial resolution of approximately 0.08 nm. For band gap measurements in EELS an acceleration voltage of 60 kV was used to reduce relativistic effects that might otherwise mask the band gap signal [20]. The spectra were acquired using a Gatan GIF 965 spectrometer, with a collection angle of approximately 20 mrad.…”

Section: Methodsmentioning

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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Section: Methodsmentioning

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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“…However, we have previously 8 found that this model gives a high goodness of fit (R 2 > 0.97) for ZnO, and furthermore that the extracted band gap is close to that found by optical methods. 1 With c and E g as fitting parameters, the curve fit of Eq. (1) to the spectra in Fig.…”

Section: Iðeþmentioning

confidence: 99%

“…The band gap is a fundamental property of semiconductors, and control and manipulation of the optical band gap is of increasing interest both from a scientific view and for development of new applications. [1][2][3] This has led to the field of band gap engineering, where the optical band gap is manipulated through alloying or nanoscale modification of the structure.…”

Section: Introductionmentioning

confidence: 99%

“…5 Previously, these energy loss onsets have been mapped with excellent spatial resolution using STEM-EELS, thereby allowing direct access to the optical band gap at the length scales relevant to modern semiconductor device nanostructuring and band gap engineering. 1,[6][7][8] The band gap energy is predetermined by the structure and the chemical composition of the material and can be affected by factors such as pressure (strain/stress) and temperature. Increasing the temperature typically leads to a decrease in the band gap energy, caused by a combination of thermal lattice expansion 9,10 and the temperature-dependency of the electronphonon interaction.…”

Section: Introductionmentioning

confidence: 99%

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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

Granerød

Galeckas

Johansen

et al. 2018

Journal of Applied Physics

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The optical band gap of ZnO has been measured as a function of temperature using Electron Energy-Loss Spectroscopy (EELS) in a (Scanning) Transmission Electron Microscope ((S)TEM) from approximately 100 K up towards 1000 K. The band gap narrowing shows a close to linear dependency for temperatures above 250 K and is accurately described by Varshni, Bose-Einstein, P€ assler and Manoogian-Woolley models. Additionally, the measured band gap is compared with both optical absorption measurements and photoluminescence data. STEM-EELS is here shown to be a viable technique to measure optical band gaps at elevated temperatures, with an available temperature range up to 1500 K and the benefit of superior spatial resolution.

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“…Advances in aberration correction 15 and electron beam monochromation 16 over the past decade have facilitated the application of valence EELS toward a few semiconductor bandgap mapping efforts [17][18][19] although these studies have primarily targeted wide-gap materials (>3.0 eV). 18,20 While an energy resolution of 10 meV, or better, has been reported in a few cases, 21 and has allowed the opportunity to extract such information in narrower-gap materials (<3.0 eV), 17,22 FWHM values in the range 100-200 meV are more typical and accessible in most monochromated instruments.…”

Section: Introductionmentioning

confidence: 99%

Bandgap profiling in CIGS solar cells via valence electron energy-loss spectroscopy

Deitz

Karki

Marsillac

et al. 2018

Journal of Applied Physics

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A robust, reproducible method for the extraction of relative bandgap trends from scanning transmission electron microscopy (STEM) based electron energy-loss spectroscopy (EELS) is described. The effectiveness of the approach is demonstrated by profiling the bandgap through a CuIn 1-x Ga x Se 2 solar cell that possesses intentional Ga/(In þ Ga) composition variation. The EELSdetermined bandgap profile is compared to the nominal profile calculated from compositional data collected via STEM-based energy dispersive X-ray spectroscopy. The EELS based profile is found to closely track the calculated bandgap trends, with only a small, fixed offset difference. This method, which is particularly advantageous for relatively narrow bandgap materials and/or STEM systems with modest resolution capabilities (i.e., >100 meV), compromises absolute accuracy to provide a straightforward route for the correlation of local electronic structure trends with nanoscale chemical and physical structure/microstructure within semiconductor materials and devices.

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Nanoscale mapping of optical band gaps using monochromated electron energy loss spectroscopy

Abstract: Abstract.Using monochromated Electron Energy Loss Spectroscopy (EELS) in a probecorrected Scanning Transmission Electron Microscope (STEM) we demonstrate band gap mapping in ZnO/ZnCdO thin films with a spatial resolution below 10 nm and spectral precision of 20 meV.

Cited by 15 publications

References 39 publications

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

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

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

Bandgap profiling in CIGS solar cells via valence electron energy-loss spectroscopy

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