@phdthesis{oai:sucra.repo.nii.ac.jp:00019692,
 author = {ARIFUZZAMAN, RAJIB},
 month = {},
 note = {XXII, 129p, Mist chemical vapor deposition (Mist-CVD) deposited metal oxide thin films as dielectric materials and wide-bandgap semiconductors have been extensively studied. This study was carried out toward reducing the driving voltage and high transconductance in metal-oxide-semiconductor field-effect transistor (MOS-FET) devices and for their relevance in power electronics, solar cells, and photodetectors. Because mist-CVD takes the benefits of solution process and CVD system, facilitating the production of a well-controlled thin film using an environmental-friendly mist-CVD technique. In this thesis, AlOx was chosen as a model metal oxide as dielectric layer for such applications because of its wide bandgap, high breakdown field, high passivation ability of c-Si, and good thermal stability. The bandgap and dielectric constant also can be controlled by adding guest metal oxide. In this study, TiO₂ was added as a guest metal oxide with AlOx formed Al₁-xTixOy to improve the dielectric constant but let off some barrier height for the application of metal insulator semiconductor field effect transistors (MIS-FET).
 Most studies of thin films using the mist-CVD focus on the synthesis of crystalline thin films on sapphire by adjusting the deposition condition and device performance. On the other side, the amorphous metal oxide thin film as an insulating layer is more preferable to fabricate MIS-FET. However, few studies on the growth mechanism and role of solvent in the synthesis of metal oxide thin films using mist CVD have been performed. To this aim, the following items have been studied in this thesis.
 a) Role of H₂O in the synthesis of amorphous AlOx using mist-CVD
 In this work, the effect of H₂O in the synthesis of amorphous AlOx using atmospheric pressure mist-CVD from aluminum acetylacetonate Al(acac)₃ as source material and CH₃OH/H₂O as the solvent system has been studied. The incorporation of -OH groups into the AlOx network that is fabricated from Al(acac)₃ and using solely methanol as a solvent, generates both malformed Al(OH) network and nonuniformity. But the addition of a small amount of water in the solvent during film growth, though decreases the deposition rate due to the hydrogen bond in water molecules but markedly removes -OH groups from the growth surface of the AlOx network and improves the surface uniformity and refractive index. The effect of exposing these films to H₂O or CH₃OH mists after fabrication was also assessed to understand the surface chemistry on the growth surface. The feature is that the Al(OH) absorption peaks, for AlOx films with CH₃OH solvent alone, decreased markedly together with increase AlO₆ octahedron related peaks by increasing the H₂O mist exposure period, suggesting that H₂O mist removed OH bonds near the surface with etched several mono layers. On the other side, no change appeared in the FT-IR and AFM for the AlOx with CH₃OH:H₂O ratio of 7:3 even after 40 minutes H₂O exposure. For the case of CH₃OH mist exposure, the OH absorption band appeared together with larger surface roughness for AlOx with CH₃OH:H₂O ratio of 7:3, suggesting that the CH₃OH mist is the source of OH group and the H₂O mist acts as a scavenger to the OH near the growing surface of AlOx network. The AlOx thin films grown with CH₃OH:H₂O ratio of 7:3 at 400 ℃ exhibits a recombination velocity of 16 cm/s, a breakdown field of 6.9 MV/cm, and the interface trap density of 4.2×10¹⁰ cm-²eV-¹ which is compatible with the AlOx grown by the other vacuum-based method.
 b) Synthesis of AlOx thin films monitored by a fast-scanning mobility particle analyzer
 In this topic, the effect of deposition parameters on the film properties of mist-CVD a-AlOx films from aluminum acetylacetonate (Al(acac)₃) and CH₃OH/H₂O mixture (=7:3 volume ratio) is demonstrated. Different deposition parameters, such as the flow rate of dilution gas (N2) (Fd), furnace temperature (Tf), solution concentration, and mesh bias (Vm) were optimized via the analysis of the size distribution of mist precursors using a fast-scanning mobility particle analyzer. The film morphology, the rigidity of the AlOx network, and junction property at the AlOx/n-type crystalline Si (n-Si) were dominated by the size distribution of the mist precursors determined by the deposition parameters. Further, the mesh bias supply during film growth promoted the miniaturization of the size distribution of the charged mist particles. Consequently, a marked increase in the number density of the mist particles resulted in an increased refractive index (n) of the AlOx thin films with small surface roughness values. Furthermore, such property of the AlOx films improved the junction property at the AlOx/n-Si interface. In this study, the correlation between the size distribution of mist particles, which is dependent on the deposition parameters, and the film and interface properties, is presented together with MIS-FET performance for the AlOx thin films obtained by mist-CVD. By adjusting the mesh bias of 9 kV, the interface trap density for AlOx/n-Si metal-oxide-semiconductor capacitor can be reduced to 9.5×10⁹ cm-²eV-¹, and the fixed charge density can be increased by 5.1×10¹² cm-²eV-¹. Field-effect mobility of 41.4 cm²/(V s) was obtained, with a threshold voltage of 2.75 V and an on/off ratio of 10⁷, using a mechanically exfoliated MoSe₂ flake as a channel layer, which is compatible with the AlOx grown by the other vacuum-based method.
 c) Synthesis of mist-CVD Al₁-xTixOy thin films improved the dielectric properties
The large bandgap dielectric material is essential to suppress the charge injection from electrodes into dielectrics that cause leakage current. On the other side, the dielectric material with a high dielectric constant is important to scall down the device size. The trade-off between these two properties is clearly noticed. One effective proposal for balancing between κ and Eg is employing aluminum titanium oxide [((Al₂O₃)₁-xTiO₂)x] (Al₁-xTixOy) with intermediate properties of Al₂O₃(κ ~9, Eg~ 7 eV) and TiO₂(κ ~50, Eg ~3 eV). As a third topics, we investigated the synthesis of amorphous aluminum titanium oxide Al₁-xTixOy thin films from Al(acac)₃ and Ti(acac)₄ mixture using CH₃OH/H₂O as a solvent by mist chemical vapor deposition (mist CVD) for application as a high dielectric material. The Ti composition ratio x in the Al₁-xTixOy thin films depends on the Al(acac)₃ and Ti(acac)₄ mixing ratios as well as CH₃OH/H₂O volume ratio. A bandgap energy of Al₁-xTixOy films was decreased from 6.38 to 4.25 eV and the surface roughness also decreased when the Ti composition ratio was increased from 0 to 0.54. The capacitance-voltage plot revealed that the dielectric constant of Al₁-xTixOy thin films increased from 6.23 to 25.12. Consequently, Al₁-xTixOy thin films with a bandgap energy of 5.12 eV and a dielectric constant of 13.8 were obtained by adjusting the ratio x of 0.26. This Al₀.₇₄Ti₀.₂₆Oy layer was applied as a gate dielectric layer for the metal-insulator-semiconductor field-effect transistors (MIS-FETs) using a mechanically exfoliated two-dimensional (2D) transition metal dichalcogenide (TMDC), MoSe₂, and As-doped WSe₂ flakes as a channel layer. The MoSe₂ based MIS-FETs with source/drain gold electrodes exhibit n-channel behavior with a field-effect mobility of 85 cm²/ (V∙s), a threshold voltage of 0.92 V, and an on/off ratio of ～10⁶. As-doped WSe₂ based MIS-FETs with source/drain platinum electrodes also showed an ambipolar behavior, which was applied for use in logic applications. These findings suggest that Al₀.₇₄Ti₀.₂₆Oy by mist CVD is promising as a high-k material for TMDC based MIS-FETs., Abstract ....................................................................................... I
Acknowledgments ................................................................... IV
List of Publications and Presentations .................................. VI
List of Tables ......................................................................... XII
List of Figures ....................................................................... XIII
List of Abbreviation ............................................................. XIX
Chapter 1: Introduction and Motivation ................................ 2
 1.1 Thin film ........................................................................................ 2
 1.2 Metal oxide and transition metal oxide thin films ....................... 5
 1.3 Benefit of Al₂O₃ as insulating/dielectric layer.............................. 6
 1.4 Thin-film fabrication method and benefits of mist-CVD ............ 8
 1.5 Motivation of this study .............................................................. 11
 1.6 Structure of this dissertation ...................................................... 13
 References: ........................................................................................ 14
Chapter 2: Experimental and Analytic methodologies ....... 24
 2.1 Mist-CVD system ........................................................................ 24
  2.1.1 Mist generation unit ................................................................................... 25
  2.1.2 Mist transport unit ...................................................................................... 27
  2.1.3 Reaction unit .............................................................................................. 28
 2.2 MoSe₂-based MIS-FET fabrication............................................ 29
  2.2.1 Deposition of AlOx thin films by mist-CVD ............................................... 29
  2.2.2 Preparation of MoSe₂ and As-doped WSe₂ flakes and transferred on to AlOx/p+-Si by mechanically exfoliation technique ............................................ 31
  2.2.3 Electrodes pattering and forming on MoSe₂-based MIS-FET ................. 34
  2.3 Analytic methodology ................................................................. 35
  2.3.1 Chemical Structure by Fourier Transform Infrared (FT-IR) Spectroscopy .............................................................................................................................. 35
 2.3.2 Optical parameters by Spectroscopic Ellipsometry ................................... 37
 2.3.3 Morphological study by Atomic Force Microscope (AFM) ...................... 42
 2.3.4 Minority carrier lifetime utilize by Micro-photoconductive decay (μ-PCD) .............................................................................................................................. 44
 2.3.5 Surface observation by Optical microscope ............................................... 46
 2.3.6 Elemental Composition and chemical bonding analysis by X-ray photoelectron spectroscopy (XPS) ....................................................................... 47
 2.3.7 Crystallographic structure by X-ray diffraction ........................................ 48
 2.3.8 Size distribution of mist precursors by the particle counter ...................... 49
 2.3.9 Van der Pauw method for Hall measurement ........................................... 53
 2.3.10 Junction properties analysis by C-V measurement ................................. 55
 2.3.10 Carrier transport parameters of MIS-FET .............................................. 62
 References .......................................................................................... 64
Chapter 3: Results and Discussions....................................... 68
 3.1 Role of H₂O in the synthesis of amorphous AlOx using mist-CVD from Al(acac)₃ and CH₃OH/H₂O system for surface passivation and electrical insulator layers .................................................................. 68
  3.1.1 Synthesis of AlOx by mist-CVD with Al(acac)₃ and CH₃OH/H₂O ........... 69
  3.1.2 Post-fabrication exposure of Al(OH) and AlOx network to H₂O and CH₃OH ................................................................................................................. 73
  3.1.3 Role of H₂O as an additive during the growth of mist-CVD of AlOx thin films ...................................................................................................................... 75
  3.1.4 Junction properties at the mist-CVD AlOx/p-Si interface ......................... 77
 3.2 Synthesis of AlOx thin films monitored by a fast-scanning mobility particle analyzer and applied as a gate insulating layer in the field-effect transistors ................................................................. 80
  3.2.1 Film properties of AlOx thin films at different deposition parameters ..... 80
  3.2.2 Effect of Vm on the film deposition of AlOx thin films by mist-CVD ........ 86
  3.2.3 Size distribution of mist particles monitored by scanning mobility particle analyzer ................................................................................................................ 88
  3.2.4 Effect of Vm on the junction properties at the AlOx/n+-Si interface and FET performance ................................................................................................ 94
 3.3 Synthesis of mist chemical vapor deposited Al₁-xTixOy thin films and their application to a high dielectric material ........................ 100
  3.3.1 Synthesis of Al₁-xTixOy thin films by mist-CVD ....................................... 100
  3.3.2 Junction property at the Al1-xTixOy/n-Si interface ................................... 105
  3.3.3 Al₁-xTixOy as a gate insulator layer for MIS-FETs ................................. 112
 References ........................................................................................ 120
Chapter 4: Conclusion and Future Work ........................... 126
 4.1 Summary of Results .................................................................. 126
 4.2 Future Work ............................................................................. 128, 指導教員 : 白井肇, text, application/pdf},
 school = {埼玉大学},
 title = {AlOx Based Thin Films Synthesized by Mist-CVD and Applied as a Gate Insulating Layer in the Field-Effect Transistors},
 year = {2021},
 yomi = {アリフザマン, ラジブ}
}