Browsing by Subject "Chemical vapor deposition."

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Open Access
Atomic layer deposition of metal oxide thin films and nanostructures
(2013) Dönmez, İnci
With the continuing scaling down of microelectronic integrated circuits and increasing need for three-dimensional stacking of functional layers, novel or improved growth techniques are required to deposit thin films with high conformality and atomic level thickness control. As being different from other thin film deposition techniques, atomic layer deposition (ALD) is based on selflimiting surface reactions. The self-limiting film growth mechanism of ALD ensures excellent conformality and large area uniformity of deposited films. Additionally, film thickness can be accurately controlled by the number of sequential surface reactions. Gallium oxide (Ga2O3) thin films were deposited by plasma-enhanced ALD (PEALD) using trimethylgallium as the gallium precursor and oxygen plasma as the oxidant. A wide ALD temperature window was observed from 100 to 400 °C, where the deposition rate was constant at ~0.53 Å/cycle. The deposition parameters, composition, crystallinity, surface morphology, optical and electrical properties were studied for as-deposited and annealed Ga2O3 films. In order to investigate the electrical properties of the deposited films, metal-oxide-semiconductor capacitor structures were fabricated for a variety of film thicknesses and annealing temperatures. Ga2O3 films exhibited decent dielectric properties after crystallization upon annealing. Dielectric constant was increased with film thickness and decreased slightly with increasing annealing temperature. As an additional PEALD experiment, deposition parameters of In2O3 thin films were studied as well, using the precursors of cyclopentadienyl indium and O2 plasma. Initial results of this experiment effort are also presented. Accurate thickness control, along with high uniformity and conformality offered by ALD makes this technique quite promising for the deposition of conformal coatings on nanostructures. This thesis also deals with the synthesis of metal oxide nanotubes using organic nanofiber templates. Combination of electrospinning and ALD processes provided an opportunity to precisely control both diameter and wall thickness of the synthesized nanotubes. As a proof-ofconcept, hafnia (HfO2) nanotubes were synthesized using three-step approach: (i) preparation of the nylon 6,6 nanofiber template by electrospinning, (ii) conformal deposition of HfO2 on the electrospun polymer template via ALD using the precursors of tetrakis(dimethylamido)hafnium and water at 200 °C, and (iii) removal of the organic template by calculation to obtain freestanding HfO2 nanotubes (hollow nanofibers). When the same deposition procedure was applied on nanofibers with different average fiber diameters, thinner HfO2 wall thicknesses were obtained for the templates having smaller diameters due to insufficient exposure of precursor molecules to saturate their extremely large surface area. Thus, “exposure mode” was applied to obtain the desired wall thickness while coating high-surface area nanofibers. We present the experimental efforts including film deposition parameters, structural, elemental, and morphological properties of HfO2 nanotubes.
Open Access
Chemical vapor deposition of boron nitride nanotubes
(2013) Çiftçi, Niyazi Okan
Since ancient times materials that are available and used by people constitute indispensable constituent parts of world history. The historical ages are named after the materials which paved an irrevocable way to this stream and revolutionize the progress of history by changing the rhythm of anthropological breakthrough irreversibly. People have used these materials either directly borrowing from nature or by transforming those that are given. As we approach to modern society two methods namely a top-down method using transmission of experiences of before generations conventionally and bottom-up method by application of modern science and technology to understand subtleties of materials and engineering them for the specific goals. Boron nitride nanotubes are one class of these materials having superior properties to the conventional ones. High strength, electrically insulator property, controllable wide band gap and high oxidation resistance as compared to carbon nanotubes put them to the first ranks for the design of the future devices for our modern society. Hence a simple and inexpensive way of production of this kind of materials is utmost importance. For this in this thesis an inexpensive and a very practical way of BNNTs production is elucidated. The reactants boron (B), iron (III) oxide (Fe2O3) and magnesium oxide (MgO) are used as solid precursors. A CVD furnace reaching up till 1200 oC with a special design of added concentric quartz tubes equipped with NH3 and Argon gas served as reaction chamber.
Open Access
Graphene based high frequency electronics
(2010) Pinçe, Erçağ
Recent advances in chemical vapor deposition of graphene on large area substrates stimulate a significant research effort in order to search for new applications of graphene in the field of unusual electronics such as macroelectronics. The primary aim of this work is to use single layer of graphene for applications of high frequency electronics. This thesis consists of both theoretical and experimental studies of graphene transistors for the use of radio frequency electronics. We have grown graphene layer using chemical vapor deposition technique on large area copper substrates. The grown graphene layers are then transferred onto dielectric substrates for the fabrication of graphene transistors. The theoretical part of the thesis is focused on the understanding the performance limits of the graphene transistor for high frequency operation. We investigate the intrinsic high frequency performance of graphene field effect transistors using a self consistent transport model. The self-consistent transport model is based on a nonuniversal diffusive transport that is governed by the charged impurity scattering. The output and transfer characteristics of graphene field effect transistors are characterized as a function of impurity concentration and dielectric constant of the gate insulator. These experimental and theoretical studies shape the basis of our research on the graphene based radio frequency electronics.
Open Access
Growth and characterization of boron nitride thin films and nanostructures using atomic layer deposition = Bor nitrür ince filmlerin ve nanoyapıların atomik katman biriktirme yöntemi ile büyütülmesi ve karakterizasyonu
(2014) Haider, Ali
Being a member of III-nitride family, boron nitride (BN) and its nanostructures have recently attracted a lot of attention, mainly due to their distinctive and superior material properties, including wide band gap, high-temperature stability, high oxidation and corrosion resistance, as well as high thermal conductivity. This versatile material has found applications in UV emission, lubrication, composite reinforcement, gas adsorption, cosmetics, and thermal management. For modern electronic applications, it is imperative to obtain high quality BN films on large area substrates with a controlled thickness in order to fulfill the entire spectrum of hBN applications. Also, a facile method such as atomic layer deposition (ALD) using non halide precursors is necessary to obtain BN films at low temperatures compliant with the standards in terms of having nontoxic byproducts. ALD is a special case of chemical vapor deposition (CVD), in which two or more precursors are sequentially exposed to substrate surface separated by purging periods. In comparison with other thin film growth methods, hall mark of ALD is self limiting growth mechanism which enables deposition of highly uniform and conformal thin films with sub-angstrom thickness control. The precise and conformal layer by layer growth of ALD can be exploited to achieve growth of BN hollow nanofibers (HNFs) on high aspect ratio electrospun polymer nanofibrous templates. BN HNFs fabricated by combination of ALD and electrospinning can be utilized to address and solve important constraints associated with previous methods of fabrication such as severe preparation conditions, limited control over morphology, and low purity of the resulting BN HNFs. In this thesis, we report on the controlled deposition of BN films and its nanostructures with the use of a hollow-cathode plasma source integrated (HCPA-ALD) reactor and present detailed materials characterization results of deposited thin films and fabricated nanostructures. Depositions are carried out at low substrate temperatures (less than 450 °C) using sequential injection of nonhalide triethylboron (TEB) and N2/H2 plasma as boron and nitrogen precursors, respectively. The deposition process parameters such as pulse length of TEB and substrate temperature, as well as the influence of post-deposition annealing are studied. Moreover, another nonhalide alternative precursor named tris(dimethyl)amidoboron (TDMAB) was studied for deposition of BN films. Initial experiments were performed using TDMAB and N2/H2 plasma as boron and nitrogen precursor. In addition to BN thin film growth studies, we report on electrospun polymeric nanofibrous template-based fabrication and characterization of AlN/BN bishell HNFs. Synthesized AlN/BN bishell HNFs were found to be polycrystalline with a hexagonal structure along with lowimpurity content.