Diamond Growth on Ge Substrates for IR Windows Applications

Germanium (Ge) is a crucial material for LWIR (Long-Wave InfraRed) applications. Its unique combination of optical, electronic, and thermal properties makes it an ideal material for a wide range of LWIR applications, from telecommunications and imaging to advance photonic and optoelectronic devices [1]-[2]. To further enhance the optical properties, nano-structuration processes were developed to form a graded-index medium between air and the Ge substrate [3]. However, optimal optical properties alone are insufficient for most applications; mechanical resilience is also critical. Unprotected Ge substrates suffer from low mechanical resistance, brittleness, and challenges in chemical/mechanical cleaning for long-term measurements. Initial attempts to protect Ge substrates with diamond-like carbon layers were unsuccessful, as the required thickness for effective protection compromised the transmittance properties of Ge IR windows.<br/>This study addresses these challenges by exploring diamond film deposition using microwave plasma-assisted chemical vapor deposition (MPCVD). Diamond offers a unique combination of high mechanical strength, mechanical-chemical durability, and transparency across ultraviolet, visible, and infrared ranges. However, several limitations hinder straightforward diamond deposition on Ge substrate: (i) Ge's low melting point, leading to thermal damage and cracking or delamination of the film, (ii) high thermal expansion mismatch between the two materials causing spontaneously diamond layer delamination, and (iii) poor adhesion of the diamond film due to Ge being a non-carbide forming material.<br/>To overcome these limitations, intermediate silicon nitride thin film (SiNx) is used in this present work. Our investigation will focus on studying the efficiency of SiNx to protect Ge substrate and enhance the adhesion of the diamond film. We focus also on determining the optimal SiNx thickness requisite for achieving the diamond adhesion while preserving optical transparency. Extensive surface and interface characterization, including scanning electron microscope (SEM) and x-ray photoelectron spectroscopy (XPS), were employed for this purpose. Moreover, optimizing the diamond film quality and thickness is essential. This includes controlling deposition parameters to ensure uniform coverage and mitigating thermal stress to avoid cracking or delamination. These steps are crucial for maintaining the integrity of the Ge substrate and enhancing the performance of the optical devices. Finally, nanostructured Ge surfaces were developed in this work using nano-imprint lithography, resulting in periodic conical diamond structures. This study delves into the efficiency of the diamond deposition on a nanostructured Ge substrate for a better optical performance.<br/><br/>[1] V. Reboud <i>et al.</i>, « Germanium based photonic components toward a full silicon/germanium photonic platform », <i>Prog. Cryst. Growth Charact. Mater.</i>, vol. 63, n 2, p. 1 24, juin 2017, doi: 10.1016/j.pcrysgrow.2017.04.004.<br/>[2] L. Vivien <i>et al.</i>, « Germanium on silicon photodetectors for telecom wavelengths », présenté à Integrated Optoelectronic Devices 2007, J. A. Kubby et G. T. Reed, Éd., San Jose, CA, févr. 2007, p. 647707. doi: 10.1117/12.700621.<br/>[3] T. P. Pasanen, J. Isometsä, M. Garin, K. Chen, V. Vähänissi, et H. Savin, « Nanostructured Germanium with &gt;99% Absorption at 300–1600 nm Wavelengths », <i>Adv. Opt. </i><i>Mater.</i>, vol. 8, n^o 11, p. 2000047, juin 2020, doi: 10.1002/adom.202000047.