Elsevier

Optical Materials

Volume 107, September 2020, 109967
Optical Materials

Enhanced selective solar absorption of surface nanotextured semi-insulating 6H–SiC

https://doi.org/10.1016/j.optmat.2020.109967Get rights and content

Highlights

  • Single-crystal 6H-SiC films have been surface nanotextured with fs- laser pulses.

  • The modification threshold laser fluence has been found to be around 0.7 J/cm2.

  • Solar absorptance has shown a maximum value >75%.

  • Solar selectivity has reported the significant values of 1.7 at 1000 K.

Abstract

Surface treatments were performed on single-crystal semi-insulating 6H–SiC by femtosecond pulsed laser irradiation, aimed at analyzing the effect of the laser-induced periodic surface structures (LIPSSs) on the films’ optical properties. The surface morphology study of the laser-induced nanostructures allows determining the modification threshold fluence of about 0.7 J cm−2 as well as detecting fine (160 nm) and coarse (450–550 nm) ripples according to different values of the laser pulse fluence (ΦP) released to the material. Micro-Raman spectroscopy allows determining the presence of undesired amorphous structural phases when ΦP exceeds 2.15 J cm−2, whereas no compositional variations occur for lower values of ΦP. Samples treated on the entire surface with the pulse fluence conditions to obtain fine ripples were optically tested. Although the long-range order is progressively lost as the accumulated laser fluence increases, the heaviest treated samples show solar absorptance values > 75% and spectral selectivity up to 1.7 projected at the operating temperature of 1000 K, thus pointing out the suitability of fs-laser surface textured 6H–SiC to act as a selective solar absorber for energy conversion devices operating at high temperature.

Introduction

Silicon carbide (SiC) is a wide bandgap semiconductor (WBS) with outstanding mechanical, electronic and thermal properties [1]. These remarkable characteristics make SiC one of the most promising alternatives to silicon for applications such as high power and high frequency electronics and micro-electro-mechanical systems (MEMS) [[2], [3], [4]]. Conversely, although sporadically proposed [5], the application of SiC in photovoltaic conversion was hampered by the wide bandgap energy of SiC polytypes (2.39, 3.28, 3.05 eV of 3C, 4H, 6H polytypes, respectively) with respect to the useful photon energy band of solar spectrum. However, if a controlled optical and electronic defect engineering is performed, WBSs can act as even more efficient active materials for photovoltaics than the optimal single-junction semiconductors predicted from the Shockley-Queisser theory, consisting of a semiconductor with a bandgap of 1.1 eV (successively corrected to 1.34 eV with a more accurate analysis of the solar spectrum) [6,7]. Indeed, a controlled introduction of intermediate band defects can increase the optimal bandgap value of the active semiconductor up to about 2 eV, where a 63% efficiency limit is expected, whereas 55% efficiency is evaluated for the wide bandgap energy of 3 eV [8]; therefore, such a defect engineering strategy can be the key-enabling tool for the exploitation of SiC-based photovoltaic cells, characterized by the additional capability compared to standard semiconductors to operate in harsh environments (high temperature, high radiation fluxes, aggressive chemical agents). In this context, the use of a WBS such as diamond (5.47 eV bandgap) has already been successfully introduced for solar concentrated power converters based on thermionic and photon-enhanced thermionic emission (PETE) [9,10], where defected black diamond plates were used as active materials. The controlled defect-engineering of black diamond plates was performed by the formation of ultrashort laser-induced periodic surface structures (LIPSSs) on the diamond surface, resulting in the introduction of optical and electronic defect levels [[11], [12], [13]]. LIPSS, which can be successfully and easily obtained on semiconductors [14], metals [15], and dielectrics without the use of lithographic tools [16], were able to dramatically enhance diamond interaction with solar radiation up to 99% solar absorptance values [17]. In addition, surface nano-engineering can be exploited to introduce electronically active defect states within the electronic bandgap of WBSs in order to increase their sub-bandgap quantum efficiency, as in the case successfully implemented on diamond. Such a strategy can be applied also in SiC for enhancing its photovoltaic conversion capability [6]. Finally, WBSs can also enable the operational concept of isothermal high-temperature solar cells based on internal photo and thermal electron emission, thus strengthening the future use of WBSs for solar applications [18].

In this work, we are firstly interested in studying the optical absorption and the solar selectivity of surface nanotextured SiC plates and in correlating the optical properties to structural and morphological characteristics. Among the different polytypes of silicon carbide, 6H–SiC (α-SiC) is the most common one, characterized by 3.05 eV bandgap, high electron mobility (650 cm2 V−1 s−1), outstanding breakdown electric field (3 × 107 V cm−1), high thermal conductivity (3.6 W cm−1 K−1), chemical inertness and stability at high temperature (operating temperature of 6H–SiC p-n junctions is up to 1000 K) [19]. The first attempts of laser nanotexturing on optical properties of 6H–SiC have been performed by Zhao et al. [20], which reported a 39% enhancement of the total absorption integrated from 1.25 to 3.0 eV, whereas Song et al. [21] reported interesting birefringence properties of the treated surfaces. However, samples therein used were not intrinsic, but n-doped and therefore characterized by a considerable starting optical absorption and a starting intrinsic semi-transparent material is fundamental for fully demonstrating the effectiveness of WBS defect-engineering.

Here we show, for the first time to the best of our knowledge, the significant enhancement of the solar absorption and selectivity of semi-insulating (i.e. resistivity > 105 Ω cm at room temperature) 6H–SiC samples, achieved by nano-texturing the material surface with femtosecond laser pulses. Moreover, we discuss the influence of laser parameters on the formation of surface nanostructuring, and consequently on the overall optical properties of large nanostructured areas, with an analysis particularly oriented towards solar applications.

Section snippets

Materials and methods

Samples used in this work are freestanding single-crystal (orientation <0001>) 10 × 10 × 0.25 mm3 semi-insulating 6H–SiC plates purchased from Xiamen Powerway Advanced Material Co., Ltd., with nominal bulk resistivity > 105 Ω cm and surface roughness < 1 nm.

A Ti:Sapphire femtosecond laser (100 fs pulse duration, linear polarization, wavelength λfs = 800 nm, adjustable repetition rate up to 1 kHz) was used for LIPSSs fabrication. The laser beam was produced by a regeneratively amplified

SEM characterization of single treated spots

A preliminary investigation was carried out by producing single treated spots on the 6H–SiC surfaces, aimed at analyzing the effect of laser treatments at different laser pulse fluences on the resulting morphological and structural properties. Table 1 summarizes the relevant experimental parameters for the treatments, all obtained by applying a fixed number of pulses N = 1000 at a repetition rate of 100 Hz and by employing the laser pulse energy as the key parameter to obtain different spots

Conclusions

Semi-insulating single-crystal 6H–SiC was successfully nanotextured by using femtosecond pulsed laser irradiation, and the related modification fluence threshold was found to be 0.7 J cm−2. Coherently with previous literature [26], single-pulse fluence values < 1.1 J cm−2 induce LIPSSs with periodicity of about 160 nm at any N, whereas ΦP > 1.41 J cm−2 produces coarser LIPSSs of 450–550 nm period at high N values; for ΦP > 2.15 J cm−2, nanostructures become irregular and Si–C bonding starts

CRediT authorship contribution statement

M. Mastellone: Investigation, Formal analysis, Data curation, Writing - original draft, Visualization. A. Bellucci: Investigation, Methodology, Writing - review & editing, Conceptualization. M. Girolami: Investigation, Validation, Writing - review & editing. R.M. Montereali: Investigation. S. Orlando: Investigation, Methodology. R. Polini: Investigation, Writing - review & editing. V. Serpente: Investigation, Validation, Writing - review & editing. E. Sani: Investigation. V. Valentini:

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors are grateful to Mr. Antonello Ranieri of CNR-IC for his technical support.

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