Atomic Layer Deposition Of Several Nano-thin Films/nano-composite Structures And Their Applications InMicroelectronics And Energy Storage Devices

Atomic layer deposition (ALD) can be defined as a film deposition technology which is based on sequential self-terminating gas-solid reactions. The unique reaction mechanism imparts ALD several distinct advantages, such as excellent three-dimensional conformality, large area uniformity, and sub-monolayer thickness control. Nowadays, ALD has shown particularly great prospect in many areas, such as microelectronics, energy, nanotechnology, catalysis, optics, optoelectronics, biomedicine, and so on. It has gained more and more interests from both academia and industry.The strongest driving force of ALD comes from the microelectronics industry due to its extraordinary advantages in fabrication of deep sub-micron integrated circuits. In this thesis, we mainly focused on the issue of ALD high dielectric (high-k) films integrated with novel semiconductor channel materials. The impact of self-cleaning and surface passivation on the interface quality and electrical properties of ALD high-k gate dielectrics on high mobility GaAs and Ge substrtates were investigated systematically. The growth behavior of ALD Al2O3 dielectric layers on the pretreated graphenes has also been characterized. In addition, we developed a facile approach to fabricate large scale CVD graphene top-gate field-effect transistor (FET) devices based on transferable ALD dielectric films. The electrical properties of graphene top-gate FETs have been evaluated.With the development of ALD, the emerging molecular layer deposition (MLD) was proposed and broadened for purely organic and hybrid inorganic-organic materials. MLD-derived hybrid films possess the advantages of inorganic and organic materials for wide applications in optics, energy, catalysis, etc. Herein, the growth processing, mechanism, and stability of the new Ti-based fumaric acid hybrid thin films were explored thoroughly. Moreover, their applications in charge trapping memory and energy storage devices were also examined.Recently ALD is being investigated widely as a new method of modification on electrodes of lithium-ion batteries (LIBs) via surface coating with evidently improved performances. There exist lots of reports related to ALD coatings on a variety of LIBs’ cathodes and anodes, however until now literature on protecting lithium metal anodes by ALD is quite rare. Hence, we presents two kinds of ALD routes for stabilizing lithium metal anodes. ALD solid-state electrolyte thin films-LixAlyS, served as ex-situ solid electrolyte interface (SEI), and floating ALD oxide films as mechanical protection layer, were adopted and compared to suppress the lithium dendrites formation, respectively. In addition, ALD was also exploited to prepare several nano-composite structure substrates and investigated their surface enhanced Raman scattering (SERS) effects.The main achievements are summarized as follows:I. Atomic layer deposition of dielectric thin films and their applications in microelectronics1. The self-cleaning effect of ALD precursor pulses on GaAs substrates was investigated. It is found that the combination of TMA and TDMAH pretreatment has an optimal self-cleaning effect. The native oxides on GaAs surfaces are removed effectively with improved interface quality and electrical properties. The GaAs/l-nm-Al2O3/2.8-nm-HfO2/Pt sample with TMA and TDMAH combination pretreatment exhibits a capacitance equivalent thickness (CET) of 1.5 nm. In addition, the thermal stability of HfO2 dielectrics on GaAs with Al2O3 or AIN interfacial passivation layer were compared carefully. Experimental results confirm that NH3 plasma also shows the self-cleaning role, and surface native oxides of GaAs are eliminated effectively during AlN deposition. HfO2/AlN/GaAs stacks exhibit excellent thermal stability with a lowest interface trap density (Dit) value of 2.11×1011 eV-1 cm-2 after 500 ℃ annealing.2. The native oxides on Ge substrates can be transformed into GeOxNy by in situ NH3 plasma pretreatment in plasma-enhanced ALD (PEALD) chamber. As a result, interfacial and electrical properties are improved evidently. The 3-nm-thick HfO2/Ge sample with 60 s NH3 plasma pretreatment exhibits a CET of 0.96 nm and a leakage current density of 1.12 mA/cm2 at+1 V gate bias. Moreover, The SiO2 interlayer in situ formed by PEALD was introduced into HfO2/Ge. SiO2 interlayer can sigificantly suppress Ge outdiffusion during postdeposition annealing process. Compared to the ex situ formed SiO2 interlayer by metalorganic CVD (MOCVD), the MOS units with in situ PEALD formed SiO2 interlayer show better electrical performance.3. The effect of surface pretreatment on depositing Al2O3 dielectric films on graphene by ALD was investigated. It is found that uniform Al2O3 thin films can easily be deposited on the graphene after water dipping pretreatment without introduced defects on graphene surface. Transferable ALD Al2O3 films were achieved using Cu or Al foils as sacrificial templates. A facile approach to fabricate large scale CVD graphene tog-gate FET devices has been developed. Transferred ALD Al2O3 films have high quality with a very small gate leakage current density of 75 pA/μm2. The drain current in graphene FETs can be modulated with applied gate voltages. The maximum on-current of 0.85 mA/μm can be obatined at Vds=0.5 V. And the calculated mobility of graphene is 77.7 cm2V-1s-1. Large scale amorphous HfS2 film was successfully prepared by ALD using TEMAH and H2S as precursors. And the growth of HfS2 follows the self-terminating reaction mechanism. In addition, In-situ ALD Al2O3 protecting layer can improve the stability of HfS2 in air.Ⅱ. ALD/MLD of novel thin films and their application in energy storage devices1. New Ti-based fumaric acid hybrid thin films were prepared by MLD. A strong temperature-dependent growth characteristic has been observed. With increasing the temperature from 180 ℃ to 300 ℃, the C:O:Ti atomic ratio in films varies from 8.35:7.49:1.00 to 4.66:4.80:1.00 with the obviously reduced growth rate. The structure of hybrid films changes from bridging bonding modes at 200 ℃ to bridging/bidentate mixed bonding modes at elevated temperatures of 250 and 300 ℃. The composition of hybrid films grown at 350 ℃ shows a dramatic decrease in C and O elemental composition (C:O:Ti=1.97:2.76:1.00) due to the thermal decomposition of the fumaric acid precursor. The produced by-product H2O changes the structure of the hybrid film, resulting in the formation of more Ti-O bonds at high temperatures. The stability of the hybrid film against chemical and thermal treatment was explored carefully. In addition, Ti-based fumaric acid hybrid films were investigated as charge trapping layer (CTL) in nonvolatile charge trapping memory. They show a great storage ability for hole due to the huge π bonding of fumaric acid molecules. At the sweeping voltage of ±14 V, the samples with 10 nm CTL exhibit very large memory window of 8.01 V, comparable to the performance of nanocrystal memory using ALD Pt and Ir.2. The porous TiO2 derived from Ti-based fuamric acid hybrid films was utilized as electrode materials in suppercapacitors and lithium-ion batteries. Due to the huge specific surface area, the porous TiO2 nanostructures display promising electrochemical performances. In supercapacitors, porous TiO2 (-0.067mg/cm2) shows a high areal capacitance of 270.2 mF/cm2 at 1.25 mA/cm2, which is～236 times higher than ALD TiO2 thin films. Moreover, it has excellent rate capability and 65.5% of the capacitance can be maintained when the current density increases to 10 mA/cm2. In lithium-ion batteries, porous TiO2 also possesses a high areal capacitance of 30.4 μA/cm2 at 130 μA/cm2 with a high coulombic efficiency (CE) of 99%.3. We developed an ALD process for growing novel solid-state electrolytes, lithium aluminum sulfide (LixAlyS).50 nm LixAlyS films with Li/Al ALD cycle ratio of 1:1 show a relatively high ionic conductivity of 2.5×10-7 S/cm at room temperature, higher than most of solid-state electrolytes deposited by ALD. The ultrathin LixAlyS coatings were used as ex-situ SEI to protect the lithium metal anodes. It is found that LixAlyS coatings can stabilize the Li-electrolyte interface and reduce the interfacial impedance of lithium-metal anodes in contact with organic electrolyte up to five times. LixAlyS can effectively suppress dendrite formation, thereby doubling the lifetime of Li-Cu asymmetric cells.4. Floating ALD oxide films were studied as mechanical protecting layer to stabilize the lithium metal anodes. A novel electrode structure with a floating ultrathin oxide film protection has beeb achieved by combining the ALD oxide films and chemical etching. The floating thin films could move up and down to accommodate large volume change during electrochemical cycling, effectively suppressing dendrite formation and increasing the CE by～10% without the decrease of CE in the initial 150 charge-discharge cycles. In contrast, the ALD films directly coated Cu foils without etching degrade the cell performances because the protection layer can’t move up and down to protect the lithium metal due to a very strong chemical bonding between the protection layer and Cu. Although both LixAlyS ex-situ SEI and floating ALD oxide films can effectively suppress the lithium dendrites formation and improve the cyclability, floating oxide films show better performance due to its better mechanical strength.Ⅲ. ALD of nano-composit structures and their applications in SERS1. The fabrication Au nanoparticles (NPs)/nanogap/Au NPs composite-structures related to SERS effect were studied systematically. This plasmonic nanostructures can be fabricated feasibly by the combination of magnetron sputtering, rapid thermal annealing (RTA), atomic layer deposition process and chemical etching. The nanogap size can be easily and precisely tuned to nanometer scale by adjusting the thickness of ALD Al2O3 layer. Experimental results show that Au NPs/Al2O3/Au NPs structures exhibit much stronger Raman signal of methylene blue (MB) molecules after etching. The stronger Raman signal, the smaller nanogap. The finite-difference time-domain (FDTD) method has been used to simulate the electromagnetic field distribution of Au NPs/Al2O3/Au NPs structures before and after etching. The theoritical calculation confirms that the enhanced electromagnetic field locates between the gap of Au NPs.2 nm-nanogap produces about 10 times electromagnetic field, in good aggreement with experimental data of 14 times.2. Ir nanoparticles (NPs)/ZnO nanowires (NWs) composite structures were investigated as SERS substrates by depositing Ir NPs on ZnO NWs using ALD. A small amount of Ir NPs can enhance Raman intensity moderate degree. However, with increasing the amount of Ir NPs, Ir NPs/ZnO NWs structures show very strong absorption for visible light.200 cycles-Ir NPs/ZnO NWs can absorb more than 99% of visible light at 633 nm, resulting in the decomposition of MB molecules due to the heat effect. Although Ir NPs/ZnO NWs composite structures have worse SERS effect, they shows exciting prospect in catalysis and desalination due to their efficient light absorption in visible light region.