First Principles Modeling of Non-Equilibrium Gas Phase Environments for AlN and Diamond Growth from the Vapor
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2004-11-15
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Abstract
The purpose of this research has been to understand, at a fundamental level, non-equilibrium gas-phase environments for the vapor deposition of crystals and thin films. This study is based on three fundamental insights:
• the existence of a chemical potential minimum in the gas phase as a function of temperature for crystal and thin film growth from the vapor;
• the approximation of non-equilibrium gas phase environments by a combination of statistical mechanical states in local equilibrium (i.e. a state in chemical equilibrium at each temperature);
• the construction of appropriate combinations of statistical states based on their chemical potential for some given growth conditions.
Using these insights we established two ab initio non-equilibrium models describing the gas phase environments for AlN and diamond growth.
Our gas phase model for AlN sublimation growth contains 62 species and 3 statistical states. The following new information regarding AlN crystal growth was obtained from the model.
• AlmN (m=1,2,3,4) and Alm (m=1,2,3) species are supersaturated in the crystal layer and therefore are proposed as growth precursors; N2 is undersaturated and therefore it is not likely a precursor;
• A transition temperature, Tt, was identified that characterizes growth conditions. It is the temperature at which the AlN source vaporization pressure is equal to the inlet N2 pressure.
• Optimal source temperature and the total temperature difference ΔT between the source and crystal are interpreted in terms of Tt.
The model can quantitatively interpret all available experiments.
Our gas phase model for diamond deposition focuses on the pure hydrogen system, which we assert contains the dominant features of typical growth conditions. In the model there are H atoms and rovibrationally excited H2molecules, which together form 143 metastable statistical states. The establishment of the model consisted of three steps.
? First, a sub-model of H equal probability population was developed to calculate supersaturated hydrogen atom concentrations as the function of temperature. It assumes only those statistical states without spontaneous evolution tendency contributing to the concentration, with each of these states having an equal weight.
? Second, another sub-model of H2 maximum participation was developed to calculate the rovibrational population of H2 as the function of temperature. It assumes that only those statistical states without spontaneous evolution tendency contributing to the concentration. Each of these statistical states contributes the same as its corresponding part within the other statistical state immediately below it along chemical potential axis.
? Finally, the two sub-models are unified into one model based on the highest metastable statistical states populated. In the unified model, H2 maximum participation model is responsible for the measured gas temperatures.
This unified model provides the following new insights:
? Supersaturated H atoms produced at the filament surface are stabilized in the entire gas phase due to the existence of metastable statistical states that are caused by rovibrationally excited H2.
? The overall non-equilibrium concentration of rovibrationally excited H2 is maximized due to the existence of supersaturated H atoms.
? The formation of temperature drops is related to the spontaneously chemical process. All observed ?irregular? temperature drops are further classified into four cases.
? The discrepancy among experimentally measured effective activation energies for H atom production is interpreted based on the embedded relation among filament temperature, gas-phase temperature, total pressure and concentration distributions in the unified model.
? The mechanism of acetylene conversion to methane is attributed to rovibrationally excited H2 addition chemistry due to states |v=2,J=1> ~ |v=3,J=1> of H2.
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gas phase deposition, sublimation growth, supersaturated H atom, molecular excitation, first principles, growth precursor, aluminum nitride, pi-bond formation and breaking, diamond, wide band-gap materials, non-equilibrium, growth mechanism, source temperature, CVD, MOCVD, MBE, concentration profile, temperature profile, concentration distribution, temperature gradient, temperature drops
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Degree
PhD
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Materials Science and Engineering
