Optical simulation and fabrication of HfMoN/HfON/Al 2 O 3 spectrally selective coating N. Selvakumar a,n , K. Prajith a , A. Biswas a,b , Harish C. Barshilia a a Nanomaterials Research Laboratory, Surface Engineering Division, CSIR-National Aerospace Laboratories, Bangalore 560017, India b Atomic & Molecular Physics Division, Bhabha Atomic Research Centre, Mumbai 400085, India article info Article history: Received 9 February 2015 Received in revised form 8 April 2015 Accepted 21 April 2015 Available online 14 May 2015 Keywords: Spectroscopic ellipsometry Optical constants HfMoN Spectrally selective coating Reflectance abstract Detailed ellipsometric studies were carried out to measure the refractive indices (n) and extinction coefficients (k) of the HfMoN(H)/HfMoN(L)/HfON/Al 2 O 3 tandem absorber in the wavelength range of 300–1000 nm. The experimentally measured ellipsometric parameters have been fitted with simulated spectra using the Tauc–Lorentz model for generating dispersion of optical constants of the individual layers. Using these n and k values a three layer HfMoN/HfON/Al 2 O 3 spectrally solar selective coating was designed and simulated using Essential Macleod software. The optical constants and thicknesses obtained from the simulation were used to develop the three layer tandem absorber with high absorptance and low emittance. The three layer tandem absorber has been deposited on stainless steel (SS) substrates using a magnetron sputtering system. The spectrophotometric data showed that the three layer tandem absorber exhibits high absorptance of 0.95 with low emittance of 0.14 on SS substrate and these values are similar to that of four layer HfMoN(H)/HfMoN(L)/HfON/Al 2 O 3 tandem absorber. & 2015 Elsevier B.V. All rights reserved. 1. Introduction One of the fruitful utilization of solar energy for meeting the energy requirement of mankind is its conversion into thermal energy using solar collectors, which collect energy from the Sun by absorbing the solar radiation and transfer it to some other med- ium [1]. The main quality of a solar collector should be its high absorptance (α) and low emittance (ε) at higher operating tem- perature (T 4400 °C) in air and vacuum. The photothermal con- version efficiency of a solar collector is often enhanced by depositing a spectrally selective coating on the surface of the collector. One of the approaches to achieve such spectrally selec- tive coating is to deposit a graded index tandem absorber on metallic surface where the refractive index (n) and extinction coefficient (k) decrease continuously from the substrate to the surface [2,3]. In order to reduce the front surface reflection, anti- reflection coating of one or two layers is generally deposited on top of the absorber coating. The optical properties of the tandem absorber mainly depend on the optical constants and thicknesses of the individual absorber layers, antireflection coatings and also on the infrared reflectance of the substrate material. In our previous work, the deposition of four layer tandem absorber (i.e., HfMoN(H), high metal content)/HfMoN(L), low metal content)/HfON/Al 2 O 3 ) with high absorptance of 0.95 and low emittance of 0.14 was reported on stainless steel (SS) substrate [4]. The four layer tandem absorber exhibited high thermal sta- bility in air (475° C for 34 h) and vacuum (600 °C for 450 h) for longer durations. The reason for choosing HfMoN material for high temperature solar thermal applications has been reported in our previous work. In the present paper, we have reported the detailed ellipsometric studies of HfMoN(H)/HfMoN(L)/HfON/Al 2 O 3 tandem absorber deposited on SS substrate and also the design and simulation of the three layer tandem absorber (i.e., HfMoN/HfON/Al 2 O 3 ) using Essen- tial Macleod software. The optical constants of single layer HfMoN (H), HfMoN(L) and HfON coatings were measured in the wavelength range of 300–1000 nm and these values were used for the design of three layer tandem absorber using Essential Macleod software. To the best of our knowledge, the reflectance and optical constants data of HfMoN have not been reported. The thicknesses of all the indi- vidual layers and the deposition process conditions for the HfMoN layer were varied in order to achieve the high absorptance of 0.95 and low emittance of 0.14. The total reflectance of the three layer tandem absorber was measured using an integrating sphere acces- sory in the UV–vis–NIR spectrophotometer in the wavelength range of 300–2500 nm. The specular reflectance of the coating at various angles of incidence was measured using a Universal Reflectance Accessory in the UV–vis–NIR spectrophotometer in the wavelength range of 400–800 nm. The spectrophotometric results showed that HfMoN is the main absorber layer and the top HfON/Al 2 O 3 is the Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/solmat Solar Energy Materials & Solar Cells http://dx.doi.org/10.1016/j.solmat.2015.04.031 0927-0248/& 2015 Elsevier B.V. All rights reserved. n Corresponding author. Fax: þ91 80 2521 0113. E-mail addresses: selvakumar@nal.res.in (N. Selvakumar), harish@nal.res.in (H.C. Barshilia). Solar Energy Materials & Solar Cells 140 (2015) 328–334