Optical constants and band gap of wurtzite Al1−xScxN/Al2O3 prepared by magnetron sputter epitaxy for scandium concentrations up to x = 0.41

Martina Baeumler,Oliver Ambacher,Joachim Wagner,Mario Prescher,Agnė Žukauskaitė,Yuan Lu,Nicolas Kurz,Lutz Kirste,Tim Christoph

doi:10.1063/1.5101043

Martina Baeumler, Oliver Ambacher + Show 7 more

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https://doi.org/10.1063/1.5101043

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Abstract

Wurtzite Al1−xScxN thin films with scandium Sc concentrations up to x = 0.41 were prepared by reactive pulsed DC magnetron co-sputtering at heater temperatures between 300 °C and 400 °C on Al2O3 substrates. Spectroscopic ellipsometry was used to determine the dielectric functions of wurtzite Al1−xScxN by modeling the spectra with a three-layer model involving parametric oscillator functions for the Al1−xScxN layers. By combining ellipsometry with transmission spectroscopy, we determined the composition dependence of the optical band gap Eg. For x &lt; 0.25, the experimentally determined band gap follows the theoretical prediction with an offset, which arises from residual sub-band gap absorption. For higher Sc concentrations, for which the band gap is expected to become indirect, the experimental data deviate from the theoretical values for the direct band gap. No absorption that can be attributed to cubic ScN crystallites was observed up to x = 0.41, indicating a high phase purity in line with X-ray diffraction data.

Highlights

Aluminum scandium nitride (Al1−xScxN) is of increasing interest for its pronounced piezoelectric properties.[1]
The full width at half maximum (FWHM) of the Al1−xScxN 0002 reflection rocking curves (ω-scans) increased toward higher Sc concentration from 0.9° to 1.7° for the samples in Series 1 and from 1.0° to 2.1° for Series 2, respectively. This increase indicates a slight degradation of the film quality for higher Sc concentrations, all films exhibit a high degree of c-axis orientation.[10]
The dielectric functions of wurtzite Al1−xScxN/Al2O3 prepared by magnetron sputter epitaxy were determined for Sc concentrations up to x = 0.41

Summary

Introduction

Aluminum scandium nitride (Al1−xScxN) is of increasing interest for its pronounced piezoelectric properties.[1] In addition, due to the possibility to tune the band gap in the UV range, its optical properties are getting attention as well.[2,3] Depending on the Sc content, Al1−xScxN can be grown as wurtzite[1,4] as well as rock salt (cubic) material.[4,5,6] Wurtzite c-axis oriented Al1−xScxN grown by reactive magnetron co-sputtering on silicon Si(001) and sapphire Al2O3(0001) substrates has been reported up to about x = 0.43.1,7–11 At higher Sc content, the material is prone to phase separation, cubic inclusions, and elemental segregation due to metastability.[4,12,13] Similar observations were made for Al1−xScxN sputtered on TiN(111)/Al2O3(0001) with up to x = 0.3 at higher process temperatures.[14] High quality wurtzite material was realized by plasma assisted molecular beam epitaxy for x = 0.14 – 0.24.15 Based on ab initio mixing enthalpy calculations, the transition region between wurtzite and cubic phases was predicted to be at Sc content x ≈ 0.54 or even higher.[2,13]

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