Solution-derived Ge-Sb-Se-Te phase-change chalcogenide films

Abstract

Ge-Sb-Se-Te chalcogenides, namely Se-substituted Ge-Sb-Te, have been developed as an alternative optical phase change material (PCM) with a high figure-of-merit. A need for the integration of such new PCMs onto a variety of photonic platforms has necessitated the development of fabrication processes compatible with diverse material compositions as well as substrates of varying material types, shapes, and sizes. This study explores the application of chemical solution deposition as a method capable of creating conformally coated layers and delves into the resulting modifications in the structural and optical properties of Ge-Sb-Se-Te PCMs. Specifically, we detail the solution-based deposition of Ge-Sb-Se-Te layers and present a comparative analysis with those deposited via thermal evaporation. We also discuss our ongoing endeavor to improve available choice of processing-material combinations and how to realize solution-derived high figure-of-merit optical PCM layers, which will enable a new era for the development of reconfigurable photonic devices.

Figure 1

A schematic of chemical solution deposition process. Chalcogenide bulk materials with a target composition prepared using a melt-quench process are ground into powder. The powder is then loaded into a custom-designed solvent to form a solution. Subsequently, substrates are dipped into the solution to create films. Finally, residual solvents are removed using thermal bake.

Figure 2

(a) Solvent–solvent solubility of binary EtSH-en. (b) Solvent–solute solubility of binary EtSH-en + Ge-Sb–Se-Te powder. (c) A summary map indicating optimal process parameters of solvent composition and solute loading level.

Figure 3

(a) A strategy employing a combination of substrate cleaning, solution filtration, and aging processes toward the deposition of high-quality films. (b) In-situ GIXRD data of solution-derived Ge₂Sb₂Se₄Te₁ films with a DFT-simulated diffraction pattern as a reference. (c) Atomic percentages of constituent elements in Ge₂Sb₂Se_5-xTe_x (x = 0.5, 0.6, and 0.7).

Figure 4

(a) In-situ GIXRD data of solution-derived Ge₂Sb₂Se_4.5Te_0.5 films with TE-deposited Ge₂Sb₂Se₄Te₁ films and a DFT-simulated diffraction pattern as references. (b) Transmission of solution-derived Ge₂Sb₂Se_4.5Te_0.5 films prior to and post thermal switching. (c) A series of surface patterns were created using sub-ablation near-bandgap direct laser writing with varying power densities. (d) Raman spectra collected from as-deposited and laser-written areas with a comparison made to those from TE-deposited films.

Figure 5

(a) Transmission spectra of EtSH, en, and a binary EtSH-en solvent as well as a solution consisting of the binary solvent and Ge-Sb–Se-Te powder. (b) The time-dependent transmission spectra of the binary solvent. (c) The time-dependent transmission spectra of the solution. (d) The time-dependent ratio of the characteristic spectral signature at a wavelength of 7.9 µm (EtSH) to that at a wavelength of 7.4 µm (en).

Figure 6

(a) Ge₂Sb₂Se₄Te₁ dissolved in binary en:Edtsh solvent with a volume to volume ratio of 10:1. (b) Complete dissolution of Ge₂Sb₂Se₄Te₁ powder in solvent with a loading level of 2.5 wt%.

Figure 7

(a) Transmission spectra of the solvents, (b) Transmission spectra of the en:Edtsh solution with a volume-to-volume ratio of 10:1 when left standing at room temperature, (c) Transmission measurements of film deposited on ZnSe substrate as a function of bake protocol, (d) Comparison of the atomic % measured between a Ge₂Sb₂Se₄Te₁ bulk alloy and a Ge₂Sb₂Se₄Te₁ film deposited on ZnSe substrate after extended baking of the ZnSe substrate.

Figure 8

(a) XRD measurements of Ge₂Sb₂Se₄Te₁ film and comparison of the peaks with bulk Ge₂Sb₂Se₄Te₁ alloy, (b) SEM image of the en:Edtsh solution-deposited film baked at 350 °C, showing the needle shaped morphology of the Ge₂Sb₂Se₄Te₁.