Effect Of Plasma-Cracked Selenium on the Molecular Beam Epitaxy Synthesis of In2Se3 Thin Films
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Ray, Ethan D.
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Abstract
Indium sesquiselenide (In2Se3) is a chalcogenide semiconductor material with broad novel applications in optoelectronics, non-volatile memory, and advanced neuromorphic computing due to its multiple stable phases, most notably, β-In2Se3, γ-In2Se3, and κ-In2Se3. Key issues with synthesizing In2Se3 involve a lack of scalability of pure-phase material and the incorporation of defects due to high-temperature synthesis requirements. Traditional molecular beam epitaxy (MBE) depositions of In2Se2 use thermal evaporation to sublimate Indium and Selenium precursors before deposition, resulting in low phase selectivity, unreacted Selenium clusters, and poor film morphology due to limited surface reactivity.
To address these limitations, this work introduced and assessed the use of a radiofrequency plasma-cracker to break up Selenium into small diatomic or monatomic constituents to generate highly reactive species before deposition. This shift aimed to lower the temperature barriers required to form single-phase β-In2Se3, γ-In2Se3, and κ-In2Se3 while improving their surface morphologies. In this work, a comprehensive comparison using Raman spectroscopy and atomic-force microscopy (AFM) was made between films grown using thermally evaporated Selenium and plasma-cracked Selenium, with Indium consistently produced using thermal evaporation. Further, nucleation and growth between 2-minute and 60-minute depositions were assessed to provide early-stage analysis.
The results of this work show that plasma-cracked selenium showed no improvement or impact on the nucleation characteristics of low-temperature (400°C) films and led to the formation of significant Selenium clustering at longer deposition times. Further, high-temperature films (700°C) deposited using plasma-cracked Selenium led to irregular Selenium cluster formations and Indium-rich mixed phases, likely due to selenium source depletion and plasma instability. Intermediate deposition temperatures (500°C) using plasma-cracked Selenium led to single-phase β-In₂Se₃ growth, pointing towards the merit of this method to modestly lower temperature requirements for In2Se3 formation.
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Undergraduate Research Option Thesis