Title:
Understanding the scandium catalytic effect for the improvement of MBE-grown low-temperature scandium aluminum nitride films
Understanding the scandium catalytic effect for the improvement of MBE-grown low-temperature scandium aluminum nitride films
Author(s)
Marshall, Emily N.
Advisor(s)
Doolittle, William A.
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Abstract
Scandium aluminum nitride (ScAlN) is a wide bandgap semiconductor that has garnered significant attention due to its excellent ferroelectric properties, high spontaneous polarization greater than that of other III-nitrides, and high piezoelectric polarization boasting a piezoelectric coefficient greater than that of AlN.[1] ScAlN demonstrates potential for use in electronic, optical, ferroelectric, and acoustic devices.[1] However, to develop improved ScAlN-based devices, one must first improve the ScAlN material itself by minimizing defects during growth, thus reducing potential electrical, acoustic, and optical scattering, and improving device performance.[2,3]
ScAlN has been grown via sputtering,[4] metalorganic chemical vapor deposition (MOCVD),[5] and traditional molecular beam epitaxy (MBE),[6,7] but has seen recent improvements when grown metal-rich and at low temperatures using metal modulated epitaxy (MME), a specialized technique of MBE that enables careful control of surface metal accumulation.[2] However, to fully utilize the benefits of MME growth of ScAlN, one must maintain ~1 monolayer (ML) of metal coverage on the growth surface,[2] which requires knowledge of the material’s growth rate. While growth rate is assumed to be constant across temperature and III/V ratio for the metal-rich growth of most binary III-nitrides (AlN, GaN, InN) below the desorption and decomposition temperatures,[8] it has been shown that the growth rate for Sc-containing nitrides varies as a function of substrate temperature[2] and Sc surface coverage[2,6,9]. The variation in growth rate is believed to be due to a Sc catalytic effect on the cracking of molecular nitrogen.[2,6,9] Understanding and quantifying this catalytic effect is key to improved control of ScAlN growth via MME.
At the core of understanding the Sc catalytic effect is understanding metal catalysis itself, especially as it pertains to the cracking of molecular nitrogen. This thesis discusses how the notoriously strong N_2 triple bond is formed, key properties of transition metal catalysts, and how electron transfer facilitates the weakening of the dinitrogen bond. With an understanding of the fundamentals of metal catalysis, this thesis concludes by revisiting the Sc catalytic effect, discussing the observation of other metal catalytic effects, and suggesting future work to help quantify the effects of catalysts on growth rate.
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Date Issued
2024-04-30
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