Dielectric function spectra and critical-point energies of Cu2ZnSnSe4 from 0.5 to 9.0 eV

Abstract

We present dielectric function e ¼ e 1 þ ie 2 spectra and critical-point energies of Cu 2 ZnSnSe 4 determined by spectroscopic ellipsometry from 0.5 to 9.0 eV. We reduce artifacts from surface overlayers to the maximum extent possible by performing chemical-mechanical polishing and wet-chemical etching of the surface of a Cu 2 ZnSnSe 4 thin film. Ellipsometric data are analyzed by the multilayer model and the e spectra are extracted. The data exhibit numerous spectral features associated with critical points, whose energies are obtained by fitting standard lineshapes to second energy derivatives of the data. The experimental results are in good agreement with the e spectra calculated within the GW quasi-particle approximation, and possible origins of the pronounced critical-point structures are identified. V

Spectroscopic ellipsometry study of Cu2ZnSnSe4 bulk crystals

“…These observed transitions might occur at the Z(0, 0, 0.5) point of the first Brillouin zone of the CZTSe. 8 Our results are in fair agreement with the identified transitions at 2.42 and 4.01 eV from ellipsometry in thin films 8 and with 2.45 and 3.94 eV transitions observed by thermoreflectance in bulk. 12 Note that in these previous studies, the authors used an excitonic line shape for the data analysis, while we applied interband electronic transitions from CPs of 2D type to describe the e(E) function, in accordance to the typical approach for the e(E) function analysis of close related ternary chalcopyrite compounds.…”

“…Second, the derived band gap value depends on the selected linear part in Tauc plots. We cannot compare our E 0 values with earlier ellipsometry studies on CZTSe as the band gap feature was not analyzed 8 or even was not measured. 12 In the higher energy region, we determined values of 2.29-2.32 and 3.73-3.92 eV for E 1A and E 1B transitions, respectively.…”

“…The investigation of the kesterites optical properties and, in particular, their optical constants, is of great interest for both fundamental and application reasons. Some optical constants data from theoretical ab-initio calculations [7][8][9][10] and from a few experimental reports on CZTSe polycrystalline thin films 8,9,11 and bulk crystals 12 have already been reported. With the aim of get a better understanding of the optical constants, we have studied the room temperature spectroscopic ellipsometry (SE) of CZTSe bulk crystals, grown by the Bridgman method.…”

“…Recent band structure calculations suggest that E 0 type transition is due to a direct electronic transition at the U:(000) point. 7,8 From the absorption measurements, band gap values of 1.0-1.02 eV (Refs. 9, 11, and 12) were found.…”

“…The fundamental absorption edge E 0 ¼ E g is observed near 1 eV in agreement with Chen et al 10 calculations, and high energy features E 1A (E 1B ) are found at about 2.3 (4.0) eV, respectively, in agreement with the previous reports. 8,11,12 According to the Adachi's model for the dielectric function (MDF), the electronic transitions in the neighborhood of CP contribute to the features of the complex dielectric function e(E). 17 We employed this approach and modelled the pseudo-dielectric function of the CZTSe crystals.…”