Quantitative study of gas-phase reactions in ZnO thin-film coating process

A comprehensive investigation of the gas-phase reaction pathways of diethylzinc (DEZn) and tert -butanol ( t -BuOH) during metal-organic chemical vapor deposition (MOCVD) is conducted to improve the deposition quality of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mrow> <mml:mi mathvariant="normal">ZnO</mml:mi> </mml:mrow> </mml:math> thin films from a micro-mechanistic perspective. This study employs quantum chemical calculations based on density functional theory (DFT) to analyze the reaction kinetics and thermodynamics of the DEZn and t -BuOH system, in order to identify the reaction mechanism and the most probable gas-phase reaction products at different temperatures under excess t -BuOH conditions. Results indicate that the gas-phase product distribution is governed by DEZn pyrolysis. At low temperatures ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mi>T</mml:mi> <mml:mo>&lt;</mml:mo> <mml:mn>523.15</mml:mn> <mml:mspace width="0.25em"/> <mml:mrow> <mml:mi mathvariant="normal">K</mml:mi> </mml:mrow> </mml:math> ), the reaction is hindered by a complexation-dominated mechanism, inhibiting Zn(OH) 2 formation and resulting in poor-quality, island-like film growth. A mechanistic shift occurs at 523.15 K due to partial pyrolysis of DEZn, transitioning the system to a bimolecular collision-dominated regime yielding primarily <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mo stretchy="false">(</mml:mo> <mml:mrow> <mml:mi mathvariant="normal">ZnOH</mml:mi> </mml:mrow> <mml:msub> <mml:mo stretchy="false">)</mml:mo> <mml:mn>2</mml:mn> </mml:msub> </mml:math> . Optimal film quality is achieved in the complete pyrolysis zone ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mn>583.15</mml:mn> <mml:mtext>–</mml:mtext> <mml:mn>673.15</mml:mn> <mml:mspace width="0.25em"/> <mml:mspace width="0.25em"/> <mml:mrow> <mml:mi mathvariant="normal">K</mml:mi> </mml:mrow> </mml:math> ), where <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mo stretchy="false">(</mml:mo> <mml:mrow> <mml:mi mathvariant="normal">ZnOH</mml:mi> </mml:mrow> <mml:msub> <mml:mo stretchy="false">)</mml:mo> <mml:mn>2</mml:mn> </mml:msub> </mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mrow> <mml:mi mathvariant="normal">HZnOH</mml:mi> </mml:mrow> </mml:math> synergistically promote ordered layered <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mrow> <mml:mi mathvariant="normal">ZnO</mml:mi> </mml:mrow> </mml:math> growth. These findings suggest that by utilizing temperature to modulate the supply ratio of monomers <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi mathvariant="normal">C</mml:mi> </mml:mrow> <mml:mn>2</mml:mn> </mml:msub> <mml:msub> <mml:mrow> <mml:mi mathvariant="normal">H</mml:mi> </mml:mrow> <mml:mn>5</mml:mn> </mml:msub> <mml:mrow> <mml:mi mathvariant="normal">ZnH</mml:mi> </mml:mrow> </mml:math> and dimers <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mo stretchy="false">(</mml:mo> <mml:msub> <mml:mrow> <mml:mi mathvariant="normal">C</mml:mi> </mml:mrow> <mml:mn>2</mml:mn> </mml:msub> <mml:msub> <mml:mrow> <mml:mi mathvariant="normal">H</mml:mi> </mml:mrow> <mml:mn>5</mml:mn> </mml:msub> <mml:mrow> <mml:mi mathvariant="normal">Zn</mml:mi> </mml:mrow> <mml:msub> <mml:mo stretchy="false">)</mml:mo> <mml:mn>2</mml:mn> </mml:msub> </mml:math> under excess t -BuOH condition, the structural properties of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mrow> <mml:mi mathvariant="normal">ZnO</mml:mi> </mml:mrow> </mml:math> films can be precisely controlled.