Pulsed chemical vapor deposition of conformal GeSe for application as an OTS selector

Abstract

The ovonic threshold switch (OTS) selector based on the voltage snapback of amorphous chalcogenides has received tremendous attention as it provides several desirable characteristics such as bidirectional switching, a controllable threshold voltage, high drive currents, and low leakage currents. GeSe is a well-known OTS selector that fulfills all the requirements imposed by future high-density storage class memories. Here, we report on pulsed chemical vapor deposition (CVD) of amorphous GeSe by using GeCl₂·C₄H₈O₂ as a Ge source and two different Se sources namely bis-trimethylsilylselenide ((CH₃)₃Si)₂Se (TMS)₂Se and bis-triethylsilylselenide ((C₂H₅)₃Si)₂Se (TES)₂Se. We utilized total reflection X-ray fluorescence (TXRF) to study the kinetics of precursor adsorption on the Si substrate. GeCl₂·C₄H₈O₂ precursor adsorption on a 300 mm Si substrate showed under-dosing due to limited precursor supply. On the other hand, the Se precursor adsorption is limited by low reaction efficiency as we learned from a better within-wafer uniformity. Se precursors need Cl sites (from Ge precursor) for precursor ligand exchange reactions. Adsorption of (TMS)₂Se is found to be much faster than (TES)₂Se on a precoated GeCl_x layer. Atomic layer deposition (ALD) tests with GeCl₂·C₄H₈O₂ and (TMS)₂Se revealed that the growth per cycle (GPC) decreases with the introduction of purge steps in the ALD cycle, whereas a higher GPC is obtained in pulsed-CVD mode without purges. Based on this basic understanding of the process, we developed a pulsed CVD growth recipe (GPC = 0.3 Å per cycle) of GeSe using GeCl₂·C₄H₈O₂ and (TMS)₂Se at a reactor temperature of 70 °C. The 20 nm GeSe layer is amorphous and stoichiometric with traces of chlorine and carbon impurities. The film has a roughness of ∼0.3 nm and it starts to crystallize at a temperature of ∼370 °C. GeSe grown on 3D test structures showed excellent film conformality.