Spectroscopic ellipsometry investigation of electronic states and optical properties of thin films from Ge₃₀As_xSe_70-x system

Highlights

• The thin Ge₃₀As_xSe_70-x films were analysed by spectroscopic ellipsometry (SE).

• The correlation between SE and X-ray photoelectron spectroscopy data were found.

• Thermo- optical coefficients of the refractive index and band gap were calculated.

• The temperature coefficient of the linear expansion was determined.

Abstract

This paper deals with the investigation of the optical properties of thin films from the Ge₃₀As_xSe_70-x system. The complex permittivity, $\widehat{\epsilon }={\epsilon }_1+i{\epsilon }_2$, and the optical band gap, E_g^opt, were determined by spectroscopic ellipsometry measurements. The spectra of ε₂ in the ultraviolet spectral range were analysed on the base of the existing literature data for the valence band spectra obtained by X-ray photoelectron spectroscopy. It was found that the absorption in the spectral range 2.0–6.5 eV is related to the bonding and anti-bonding p-orbitals of Ge, Se and As atoms. The temperature coefficients of linear expansion, refractive index and the band gaps were determined. The evaluated values for the non-linear refractive index, γ, and the two-photon absorption coefficient, β, showed that the thin films exhibit a highly non-linear refractive index at the telecommunication wavelength.

Introduction

The wide region of glass formation in the As-Se-Ge system enables variation of the properties of the glasses from this system over a wide range [1]. Because of this reason, the As-Se-Ge glasses and thin films find application in different devices for acousto-optics [2], preparation of infrared transmitting glass-ceramics [3] or manufacturing of different optical elements such as fibers for all-optical switching [4] and waveguides for supercontinuum generation [3].

Because of the wide glass-forming region within the Ge-As-Se system, various structural units depending on composition can be observed in both glasses and thin films [5]. The structural investigations reported show that their glassy network is mainly consisted of layered [AsSe_3/2] pyramids and three dimensionally linked [GeSe_4/2] tetrahedral units, as well selenium chains Se_n and As₄Se₄ structural units [6, 7]. The Ge−As−Se glasses show a completely disordered structure. Numerous investigations of As-Ge-Se glasses [8], [9], [10], [11] have shown that the mean coordination number (MCN) plays a key role in determining their structural and physical properties. The investigations of the elastic phase transitions and structural properties show that into the glass-forming region As-Ge-Se system can be observed three distinct phases - the Flexible phase (0 < MCN < 2.285), Intermediate Phase (2.285 < MCN < 2.51) and stressed-rigid phase (MCN > 2.51) [12, 13]. A structural investigation by the X-ray absorption fine structure (EXAFS) [7] demonstrated that there are not Ge-Se and Se- Se bonds but also Ge-Ge, Ge-As, and As-As bonds in highly selenium-poor compositions and Se−Se bonds in selenium-rich samples. In case of Se-deficient glasses, a nanophase separation to Ge-rich phase and As-rich phase occurs [12, 14, 15]. In [10] it is shown that the glasses with different composition and same MCN = 2.5 have similar glass network and that the variation of the chemical composition slightly modifies the physical properties. Due to the lower optical losses, the investigations of the optical properties were focused on bulk glasses and as-deposited thin films from As-Ge-Se system with values mean coordination number, MCN ≤ 2.5 [16], [17], [18], [19], [20]. For the glasses with a mean coordination number around 2.45 it was observed that they possess high photo- and thermal stability i.e. negligible changes of the refractive index occur after illumination or thermal annealing [4].

The As-Se-Ge glasses with a germanium content of 30–35 at. % occupy an intermediate state [21]. For example, the commercially available infrared optical glass AMTIR-1 (Ge₃₃As₁₂Se₅₅) has low optical losses, while the Ge₃₅As₁₅Se₅₀ glass demonstrated ~ 3 times higher optical losses [22]. The role of the homopolar bonds to the increase of the nonlinear refractive index, n₂, in chalcogenide glasses is still discussed in some papers [23], [24], [25]. According to [11] the increase of the optical losses in As-Ge-Se glasses with MCN > 2.64 are due to appearance of ethane-like structural units, Ge₂Se_6/2, in the glassy network. For some arsenic-containing glasses, a significant increase of the values of n₂ is observed when the numbers of the homopolar As-As bonds in the glassy network increases [23, 24]. According to the authors of [25] the higher non-linear refractive index of the Ge₃₅As₁₅Se₅₀ and Ge₃₃As₁₂Se₅₅ glasses is due to presence of the Ge-Ge bonds in the glass’ network. They have found that these glasses possess high non-linear refractive index, n₂, and two times higher values of the figure of merit, FOM = n₂ / βλ (where β is non-linear absorption coefficient and λ is wavelength) in comparison with the glasses with lower mean coordination number (Ge₂₂As₂₀Se₅₈ and Ge₂₅As₁₀Se₆₅). These non-linear optical properties make them appropriate materials for highly nonlinear and high-index contrast fibers for telecommunication applications.

Therefore, the purpose of our work, presented here, is to investigate the optical properties by spectral ellipsometry of thin As-Ge-Se layers along the Ge₃₀As_xSe_70-x section. Various optical components based on chalcogenide materials are used in different laser systems, but due to high power of the laser's irradiation they could change their optical properties due to the temperature variation during the exploitation of devices. In order to eliminate the structural stress, the thin films are usually annealed at a temperature below their glass-transition temperature. Therefore, we also trace the changes of the thickness and optical band gap of annealed Ge₃₀As_xSe_70-x films in the temperature range of 25 to 250°C.