Direct optical transitions at K- and H-point of Brillouin zone in bulk MoS2, MoSe2, WS2, and WSe2

Kopaczek, Jan; Polak, Maciej P.; Scharoch, P.; Wu, Kedi; Chen, Bin; Tongay, Sefaattin; Kudrawiec, R.

doi:10.1063/1.4954157

Cited by 52 publications

(43 citation statements)

References 37 publications

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“…So far most of them (MoS 2 , MoSe 2 , WS 2 , WSe 2 , ReS 2 , and ReSe 2 ) were studied by MR (photoreflectance, electroreflectance or piezoreflectance) 12,39–41,43–46 , but none of them was studied by PA spectroscopy. In order to discuss chemical trends for the indirect and direct gap in these materials, they are grouped in a few sets of samples going from smaller to larger atoms.…”

Section: Resultsmentioning

confidence: 99%

“…In contrast to PA spectroscopy, the MR technique (photoreflectance (PR), contactless electroreflectance (CER), and piezoreflectance (PzR)) probes exclusively direct optical transitions which are present at singularities of joint optical density of states 39 . For direct gap semiconductors the optical transition with the lowest energy is the band gap transition, but for indirect gap semiconductors the direct optical transitions are observed in MR spectra at energies larger than the fundamental band gap.…”

Section: Introductionmentioning

confidence: 99%

“…For direct gap semiconductors the optical transition with the lowest energy is the band gap transition, but for indirect gap semiconductors the direct optical transitions are observed in MR spectra at energies larger than the fundamental band gap. So far MR spectroscopy was applied few times to study van der Waals crystals 12,39–46 but many of them were not studied yet by this technique. In this work we use MR spectroscopy as a complementary method to study the band gap in van der Waals crystals, which allows to determine the character of band gap observed in PA spectra.…”

Section: Introductionmentioning

confidence: 99%

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Photoacoustic and modulated reflectance studies of indirect and direct band gap in van der Waals crystals

Zelewski

Kudrawiec

2017

Sci Rep

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Photoacoustic (PA) and modulated reflectance (MR) spectroscopy have been applied to study the indirect and direct band gap for van der Waals (vdW) crystals: dichalcogenides (MoS 2 , MoSe 2 , MoTe 2 , HfS 2 , HfSe 2 , WS 2 , WSe 2 , ReS 2 , ReSe 2 , SnS 2 and SnSe 2 ) and monochalcogenides (GaS, GaSe, InSe, GeS, and GeSe). It is shown that the indirect band gap can be determined by PA technique while the direct band gap can be probed by MR spectroscopy which is not sensitive to indirect optical transitions. By measuring PA and MR spectra for a given compound and comparing them with each other it is easy to conclude about the band gap character in the investigated compound and the energy difference between indirect and direct band gap. In this work such measurements, comparisons, and analyses have been performed and chemical trends in variation of indirect and direct band gap with the change in atom sizes have been discussed for proper sets of vdW crystals. It is shown that both indirect and direct band gap in vdW crystals follow the well-known chemical trends in semiconductor compounds.Van der Waals (vdW) crystals are known for a long time 1-14 but in recent years some of them have gained significant interest because of unique mechanical and optical properties of samples obtained by exfoliation of bulk material to atomically thin layers [15][16][17][18][19][20][21][22][23][24][25] . Since pioneering reports on optical properties of MoS 2 layers 15,16 , it is well established that the electronic band structure of MoS 2 strongly varies with the number of layers and exhibits indirect-to-direct band gap transition with the size reduction to a single layer. Similar studies of optical properties and the electronic band structure have been performed on other van der Waals crystals such as MoSe 2 , MoTe 2 , WS 2 , and WSe 2 22-25 . Some of them were investigated quite recently as bulk materials 13,14 but interest in these materials was low due to no potential applications in semiconductor devices. At this moment the situation is different. VdW crystals are exciting because of interesting physics of two-dimensional (2D) layers and their potential applications in novel semiconductor devices [26][27][28] . Because of this even bulk van der Waals crystals are interesting to study since many of them are not explored experimentally. One method which has never been applied to study most of van der Waals crystals is photoacoustic (PA) spectroscopy. We show that this technique is an excellent tool to study the band gap in van der Waals crystals since it is sensitive to indirect gap transitions which cannot be observed in photoluminescence. In combination with modulated reflectance (MR), which is known as a state of the art absorption-like technique to study direct optical transitions 29 , it is possible to conclude about the band gap character in van der Waals crystals and the spectral separation between the indirect and direct gap. In this work we applied PA and MR to study indirect and direct gap in many van der Waals crystals ...

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Section: Resultsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Photoacoustic and modulated reflectance studies of indirect and direct band gap in van der Waals crystals

Zelewski

Kudrawiec

2017

Sci Rep

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“…Our approach then is to favor least vulnerable characterization methods, such as modulation spectroscopy, whilst endeavoring to assess such complications where possible. As a differential method PR is unaffected by scattering losses so may study textured samples in cases where reflection/transmission spectroscopy would fail due to diffuse scattering; PR probes critical points in the joint density of states so is ideal for studying direct transitions whilst being insensitive to indirect transitions 45 and avoiding complications from energyand orientation-dependent absorption magnitudes. Having said this, a consensus does emerge on the raised ∼1.35 eV optical gap seen in amorphous material; 7,8,10,25,26,46 the 1.18 eV PR band gap and numerous XRD reflections suggest negligible amorphous content in this CSS material.…”

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confidence: 99%

Band gap temperature-dependence of close-space sublimation grown Sb2Se3 by photo-reflectance

et al. 2018

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The candidate photovoltaic absorber antimony selenide Sb2Se3 has been prepared by the commercially attractive close-space sublimation method. Structure, composition, and morphology are studied by x-ray diffraction, scanning electron microscopy, and energy dispersive spectroscopy. Large rhubarb-like grains favorable for photovoltaics naturally develop. The temperature-dependence of the direct band gap is determined by photoreflectance between 20 and 320 K and is well described by the Varshni and Bose–Einstein relations, blue-shifting with decreasing temperature from 1.18 to 1.32 eV. The 300 K band gap matches that seen in high quality single-crystal material, while the 0 K gap is consistent with that found in first-principles calculations, further supporting the array of beneficial photovoltaic properties indicated for this material.

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“…Like photoreflectance (PR), CER modulates the built-in electric field in the sample, which changes the reflectance at energies corresponding to direct optical transitions such as the direct bandgap [57,58]. An important advantage of CER over PR spectroscopy is the elimination of problems with the modulation of the refractive index inside the sample which leads to an unwanted Fabry-Perot oscillation [59].…”

Section: Resultsmentioning

confidence: 99%

Gas Source Techniques for Molecular Beam Epitaxy of Highly Mismatched Ge Alloys

et al. 2016

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Ge and its alloys are attractive candidates for a laser compatible with silicon integrated circuits. Dilute germanium carbide (Ge 1−x C x ) offers a particularly interesting prospect. By using a precursor gas with a Ge 4 C core, C can be preferentially incorporated in substitutional sites, suppressing interstitial and C cluster defects. We present a method of reproducible and upscalable gas synthesis of tetrakis(germyl)methane, or (H 3 Ge) 4 C, followed by the design of a hybrid gas/ solid-source molecular beam epitaxy system and subsequent growth of defect-free Ge 1−x C x by molecular beam epitaxy (MBE). Secondary ion mass spectroscopy, transmission electron microscopy and contactless electroreflectance confirm the presence of carbon with very high crystal quality resulting in a decrease in the direct bandgap energy. This technique has broad applicability to growth of highly mismatched alloys by MBE.

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Direct optical transitions at K- and H-point of Brillouin zone in bulk MoS2, MoSe2, WS2, and WSe2

Cited by 52 publications

References 37 publications

Photoacoustic and modulated reflectance studies of indirect and direct band gap in van der Waals crystals

Photoacoustic and modulated reflectance studies of indirect and direct band gap in van der Waals crystals

Band gap temperature-dependence of close-space sublimation grown Sb2Se3 by photo-reflectance

Gas Source Techniques for Molecular Beam Epitaxy of Highly Mismatched Ge Alloys

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