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Utilizing In Situ Reflection High Energy Electron Diffraction to Investigate the Structural Transformations of Oxide Thin Films /

Record Type:	Electronic resources : Monograph/item
Title/Author:	Utilizing In Situ Reflection High Energy Electron Diffraction to Investigate the Structural Transformations of Oxide Thin Films // Alexandra J Howzen.
Author:	Howzen, Alexandra J.,
Description:	1 electronic resource (259 pages)
Notes:	Source: Dissertations Abstracts International, Volume: 86-07, Section: B.
Contained By:	Dissertations Abstracts International86-07B.
Subject:	Physical chemistry. -
Online resource:	https://pqdd.sinica.edu.tw/twdaoapp/servlet/advanced?query=31764513
ISBN:	9798302180162

Utilizing In Situ Reflection High Energy Electron Diffraction to Investigate the Structural Transformations of Oxide Thin Films /
Howzen, Alexandra J.,

Utilizing In Situ Reflection High Energy Electron Diffraction to Investigate the Structural Transformations of Oxide Thin Films /Alexandra J Howzen. - 1 electronic resource (259 pages)

Source: Dissertations Abstracts International, Volume: 86-07, Section: B.

A custom atomic layer deposition (ALD) chamber equipped with reflection high energy electron diffraction (RHEED) was designed, manufactured, and calibrated to study the structure of surfaces and thin films. The oxides studied include aluminum oxide, hafnium oxide, and gallium oxide with a focus on gallium oxide due to its many polymorphs that can exist depending on the deposition temperature, precursor dosage or chemistry, and substrate structure or orientation. In these studies, amorphous, polycrystalline, and epitaxial films were studied with the RHEED-equipped ALD system and supported by other measurements.The first stage of this project consisted of the design of an ALD chamber with RHEED. The chamber body was chosen to have an inner diameter of 10 in, which ensured that nearby system components did not exceed 250 °C to prevent degradation of the Kalrez o-rings. Flow simulations were also performed to locate any areas that could harbor vortices and set a maximum flow velocity that would ensure laminar flow through the chamber. Considerations for RHEED incorporation were also addressed such as understanding how pressure will affect the quality of RHEED by evaluating the mean free path as a function of temperature. Other considerations such as maximum viable chamber pressure to maintain less than 1x10-6 Torr in the RHEED electron source were also discussed. Calibration was also required to determine the camera length of this custom system using a known sample, and the wafer surface temperature by observing the temperature curve of a thermocouple bonded to a wafer surface. This system was used to deposit and observe the structural transformations of thin, oxide films described in this work.Aluminum oxide on c-plane sapphire, hafnium oxide on silicon, and gallium oxide on silicon were initially investigated to test the capabilities of the newly manufactured chamber. A 3.3 nm thick, amorphous alumina film was deposited ex situ then heated to 640 °C with RHEED irradiation. There were no definitive indicators that the film crystallized, but a change in the observed RHEED pattern indicate that a structural rearrangement may have taken place. Three, approximately 30 nm, amorphous hafnia films were deposited on Si chips ex situ and then heated to 675 °C while being probed using RHEED. RHEED indicated that crystallization occurred between 410 and 415 °C which is similar to crystallization temperatures recorded in literature. The observed ring pattern was indexed. Approximately 5 nm of gallium oxide was deposited on Si ex situ and annealed under RHEED irradiation. A bright halo was observed beginning as early as 190 °C and becoming more prominent as the temperature increased to 575 °C. The RHEED ring pattern observed was unfortunately unable to be indexed. These preliminary measurements proved that this custom-built chamber was capable of observing the polycrystalline crystallization 0f ultrathin ALD films.Thin films of gallium oxide were deposited in an amorphous (low temperature) and, separately, epitaxial form (high temperature) to evaluate how their crystallinity impacted their final structure. The films were then annealed at ultra-high vacuum (UHV) under RHEED observation. The film deposited at 303 °C was crystalline while the one deposited at 217 °C was amorphous and crystallized during the post-deposition anneal. All samples exhibited a streak pattern as observed by RHEED, so the widths between the streaks were converted to d-spacings and the changes in these d-spacings were observed during the deposition process (for films that deposited crystalline) and the annealing procedure. These d-spacings were used to determine the in- and out-of-plane orientation relationship and make observations about the potential strain of each film. Ex situ x-ray diffraction was used to confirm the epitaxial orientation of films and cross-sectional transmission electron microscopy evaluated the film/substrate interface and overall film structure.A series of amorphous gallium oxide films were deposited on sapphire to investigate how varying film thickness influences crystallization behavior and final film structure. Films between 2 and 9 nm were deposited and annealed to 675 °C for this investigation. The streak pattern widths were again converted into d-spacings to evaluate the in- and out-of-plane epitaxial relationship between the film and the substrate. To evaluate film structural stability, a series of films were annealed one or more times in vacuum. Complete desorption of films less than 3 nm was observed upon a single annealing procedure, so it was investigated that desorption could be suppressed by lowering the maximum temperature, annealing at a higher pressure, or introducing a small partial pressure of oxygen into the chamber during annealing. Structural differences were observed between 6 and 9 nm thick films where thicker films more closely resembled films that deposited epitaxially. Ex situ x-ray photoelectron spectroscopy was used to determine the thickness of all films.It was observed in the previous experiments that the surface of gallium oxide thin films can change with increasing film thickness, annealing temperature, or annealing ambient gas environment. RHEED pattern simulations were performed in an effort to understand how surface symmetry or the presence of twins can influence the observed RHEED pattern. Overall, this work has shown that RHEED is a useful tool to understand the structural nuances that occur within thin films during epitaxy, crystallization, and annealing under varying atmospheres.

English

ISBN: 9798302180162Subjects--Topical Terms:

1981412
Physical chemistry.
Subjects--Index Terms:

Epitaxy

Utilizing In Situ Reflection High Energy Electron Diffraction to Investigate the Structural Transformations of Oxide Thin Films /
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Flow simulations were also performed to locate any areas that could harbor vortices and set a maximum flow velocity that would ensure laminar flow through the chamber. Considerations for RHEED incorporation were also addressed such as understanding how pressure will affect the quality of RHEED by evaluating the mean free path as a function of temperature. Other considerations such as maximum viable chamber pressure to maintain less than 1x10-6 Torr in the RHEED electron source were also discussed. Calibration was also required to determine the camera length of this custom system using a known sample, and the wafer surface temperature by observing the temperature curve of a thermocouple bonded to a wafer surface. This system was used to deposit and observe the structural transformations of thin, oxide films described in this work.Aluminum oxide on c-plane sapphire, hafnium oxide on silicon, and gallium oxide on silicon were initially investigated to test the capabilities of the newly manufactured chamber. A 3.3 nm thick, amorphous alumina film was deposited ex situ then heated to 640 °C with RHEED irradiation. There were no definitive indicators that the film crystallized, but a change in the observed RHEED pattern indicate that a structural rearrangement may have taken place. Three, approximately 30 nm, amorphous hafnia films were deposited on Si chips ex situ and then heated to 675 °C while being probed using RHEED. RHEED indicated that crystallization occurred between 410 and 415 °C which is similar to crystallization temperatures recorded in literature. The observed ring pattern was indexed. Approximately 5 nm of gallium oxide was deposited on Si ex situ and annealed under RHEED irradiation. A bright halo was observed beginning as early as 190 °C and becoming more prominent as the temperature increased to 575 °C. The RHEED ring pattern observed was unfortunately unable to be indexed. These preliminary measurements proved that this custom-built chamber was capable of observing the polycrystalline crystallization 0f ultrathin ALD films.Thin films of gallium oxide were deposited in an amorphous (low temperature) and, separately, epitaxial form (high temperature) to evaluate how their crystallinity impacted their final structure. The films were then annealed at ultra-high vacuum (UHV) under RHEED observation. The film deposited at 303 °C was crystalline while the one deposited at 217 °C was amorphous and crystallized during the post-deposition anneal. All samples exhibited a streak pattern as observed by RHEED, so the widths between the streaks were converted to d-spacings and the changes in these d-spacings were observed during the deposition process (for films that deposited crystalline) and the annealing procedure. These d-spacings were used to determine the in- and out-of-plane orientation relationship and make observations about the potential strain of each film. Ex situ x-ray diffraction was used to confirm the epitaxial orientation of films and cross-sectional transmission electron microscopy evaluated the film/substrate interface and overall film structure.A series of amorphous gallium oxide films were deposited on sapphire to investigate how varying film thickness influences crystallization behavior and final film structure. Films between 2 and 9 nm were deposited and annealed to 675 °C for this investigation. The streak pattern widths were again converted into d-spacings to evaluate the in- and out-of-plane epitaxial relationship between the film and the substrate. To evaluate film structural stability, a series of films were annealed one or more times in vacuum. Complete desorption of films less than 3 nm was observed upon a single annealing procedure, so it was investigated that desorption could be suppressed by lowering the maximum temperature, annealing at a higher pressure, or introducing a small partial pressure of oxygen into the chamber during annealing. Structural differences were observed between 6 and 9 nm thick films where thicker films more closely resembled films that deposited epitaxially. Ex situ x-ray photoelectron spectroscopy was used to determine the thickness of all films.It was observed in the previous experiments that the surface of gallium oxide thin films can change with increasing film thickness, annealing temperature, or annealing ambient gas environment. RHEED pattern simulations were performed in an effort to understand how surface symmetry or the presence of twins can influence the observed RHEED pattern. Overall, this work has shown that RHEED is a useful tool to understand the structural nuances that occur within thin films during epitaxy, crystallization, and annealing under varying atmospheres.
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