1,721,253 research outputs found
Efficient Dislocation Reduction Methods for Integrating Gallium Nitride HEMTs on Si
Full text linkGallium Nitride (GaN) and its alloys with InN and AlN, the III-nitrides, are of interest for a variety of high power-high frequency electronics and optoelectronics applications. However, unlike Si and GaAs technology that have been developed on native substrates, III-nitride devices have been developed on non-native substrates such as Si, sapphire and SiC. This is because bulk cheap native III-nitride substrates are unavailable. Among the known substrates, III-nitride technology development on Si is desirable because of its large substrate size and low cost. However, the large lattice and thermal expansion mismatch between the III-nitrides films and Si substrate leads to a high level of dislocations, 1010 cm-2, and tensile stress which results in cracking. For successful integration of crack free and low dislocation density GaN on Si various kinds of transition layer schemes are used that help to incorporate a compressive growth stress to neutralize the tensile thermal mismatch stresses and also to reduce dislocation densities to levels required by devices. These transition schemes, ranging from 400 nm to 7 m, involve the use of graded AlGaN layers, high/low temperature interlayers and superlattices.
The aim of the research described in this thesis was a systematic comparison of the different transition layer schemes currently used with the objective of increasing the efficiency of integrating device quality, crack free, low dislocation density, <109 cm-2, GaN with Si. A metal organic chemical vapor deposition equipped with an in-situ stress monitor was used for growth. Transmission electron microscopy was used for quantitative measurement of dislocation density.
The research shows, for the first time, that all transition layer optimization depends critically on the Si surface made available for growth of the first AlN layer. It needs to be optimally cleaned such that it is oxide free and smooth. A quantitative TEM comparison of various currently used transition layer schemes shows that while they have interesting mechanistic differences, they are not very different in their dislocation reduction efficiency. All of them yield a final dislocation density in a probe GaN layer of 1-3×109 cm-2. In contrast, a combination of Si doping and compressive growth stress has a synergistic effect on dislocation reduction. A simple 210 nm transition layer based on this understanding, the lowest reported yet, yields GaN layers that are crack free and have lower <1x109 cm-2 dislocation density, than those obtained by the aforementioned more complicated schemes. High electron mobility transistor characteristics performance on the probe GaN layers obtained on these transition layers supports the structural observations above
Anodized Zirconia Nanostructures
Full text linkElectrochemical anodization is a facile technique to synthesize ordered oxide nanostructures. Though the number of materials exhibiting anodized nanostructures has increased considerably in the recent years, only nanoporous alumina and nanotubular titania have been investigated extensively for various applications. Anodized nanostructures, nanotubes and nanopores, of zirconia are also of considerable interest for applications such as templates, sensors and solid-oxide fuel cells. In spite of the potential applications of zirconia, these nanostructures have been barely studied. As most of these applications require elevated temperatures in excess of 400C, thermal stability becomes an important attribute. Even though zirconia (Tm=2715C) has as higher melting point than alumina(Tm = 2072C), literature reports and initial research showed that the thermal stability of anodized zirconia was limited to 500C-1 h compared to 1000C-4 h for alumina. The work carried out as a part of this research showed that halide ions used in the synthesis are the possible cause for the lower thermal stability. Chemical treatment of the zirconia membranes to neutralize the halide ions helped enhance the stability to 1000C-1 h, thus, improving their usability for most of the applications mentioned above. Most of the current reported work on aluminum, zirconium, and titanium is predominantly limited to anodization of foils which can only yield free-standing nanostructures. As synthesis of these nanostructures on a substrate would further facilitate their usage, supported anodized zirconia nanostructures were synthesized by anodizing sputtered zirconium films. This study showed that the anodized morphology depends strongly on the sputtered film microstructure, which changes in accordance with the Thornton’s zone diagrams. A general approach thus developed is expected to be applicable to anodization of all metallic films. Most applications involving zirconia also require stabilization against a tetragonal-monoclinic phase transformation by suitable alloying such as with yttria. Towards this end, routes to develop anodized yttria-stabilized zirconia nanostructures, which are nonexistent, were explored. The synthesis of yttria stabilized zirconia nanostructures with no detectable monoclinic phase was achieved. Yttrium alloying using a solution treatment was found to enhance stability of the supported nanostructures to 900C-16 h, which makes it possible to now evaluate these nanostructures, especially for micro-SOFC applications
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Stress and Microstructural Evolution During the Growth of Transition Metal Oxide Thin Films by PVD
Full text linkSystem on Chip (SoC) and System in Package (SiP) are two electronic technologies that involve integrating multiple functionalities onto a single platform. When the platform is a single wafer, as in SOC, it requires the ability to deposit various materials that enable the different functions on to an underlying substrate that can host the electronic circuitry. Transition metal oxides which have a wide range of properties are ideal candidates for the functional material. Si wafer on which micro-electronics technology is widely commercialized is the ideal host platform.
Integrating oxides with Si, generally in the form of thin films as required by microelectronics technology, is however a challenge. It starts with the fact that the properties of crystalline oxides to be exploited in performing various functions are direction dependent. Thus, thin films of these oxides need to be deposited on Si in certain crystallographic orientations. Even if a suitably oriented Si wafer surface were available, it does not always provide for epitaxial growth a critical requirement for controlling the crystalline orientation of thin films. This is because Si surface is covered by an amorphous oxide of Si (SiOx). Thus, during growth of the functional oxide, an ambience in which the Si itself will not oxidize needs to be provided. In addition, during thin film growth on either Si or SiOx surface stresses are generated from various sources. Stress and its relaxation are also associated with the formation and evolution of defects. Both, stress and defects need to be managed in order to harness their beneficial effects and prevent detrimental ones.
Given the requirement of SoC technology and the problem associated, the research work reported in this thesis was hence concerned with the precise controlling the stress and microstructure in oxide thin films deposited on Si substrates. In order to do so a versatile, ultra high vacuum (UHV) thin film with a base pressure of 10-9 Torr was designed and built as part of this study. The chamber is capable of depositing films by both sputtering (RF & DC) and pulsed laser ablation (PLD). The system has been designed to include an optical curvature measurement tool that enabled real-time stress measurement during growth.
Doped zirconia, ZrO2, was chosen as the first oxide to be deposited, as it is among the few oxides that is more stable than SiOx. It is hence used as a buffer layer. It is shown in this thesis that a change in the growth rate at nucleation can lead to (100) or (111) textured films. These two are among the most commonly preferred orientation. Following nucleation a change in growth rate does not affect orientation but affects stress. Thus, independent selection of texture and stress is demonstrated in YSZ thin films on Si. A quantitative model based on the adatom motion on the growth surface and the anisotropic growth rates of the two orientations is used to explain these observations. This study was then subsequent extended to the growth on platinized Si another commonly used Si platform..
A knowledge of the stress and microstructure tailoring in cubic zirconia on Si was then extended to look at the effect of stress on electrical properties of zirconia on germanium for high-k dielectric applications. Ge channels are expected to play a key role in next generation n-MOS technology. Development of high-k dielectrics for channel control is hence essential.
Interesting stress and property relations were analyzed in ZrO2/Ge. Stress and texture in pulsed laser deposited (PLD) oxides on silicon and SrTiO3 were studied. It is shown in this thesis that stress tuning is critical to achieve the highest possible dielectric constant. The effect of stress on dielectric constant is due to two reasons. The first one is an indirect effect involving the effect of stress on phase stability. The second one is the direct effect involving interatomic distance. By stress control an equivalent oxide thickness (EOT) of 0.8 nm was achieved in sputter deposited ZrO2/Ge films at 5 nm thickness. This is among the best reported till date.
Finally, the effect of growth parameters and deposition geometry on the microstructural and stress evolution during deposition of SrTiO3 on Si and BaTiO3 on SrTiO3 by pulsed laser deposition is the same chamber is described
Epitaxial Integration Of Functional Oxides On Silicon (100)
No full textEpitaxial integration of BaTiO3 (BTO) and other functional oxides with (100) Si is essential to exploit their functional capabilities using the well-established Si-CMOS technological platform. However, such oxide integration is impeded by the presence of native amorphous SiOx on Si surfaces and its formation during oxide deposition. Buffer layers of oxides that are thermodynamically more stable than SiOx are often introduced in order to mitigate the same. Heterogenous integration either employs an expensive molecular beam epitaxy process (STO layer) or a complex transition scheme (BTO/LNO/CeO2/YSZ/Si). While BaTiO3 – a ferro electric of importance for various electronic and photonic applications- is the main oxide considered herein, similar approaches have also helped integrate Ga2O3 – an emerging oxide of importance to power electronic and UV applications-as described towards the end of the thesis.
Epitaxial growth of a CMOS compatible buffer layer titanium nitride (TiN) via reactive PLD (RPLD) is demonstrated in chapter 2. By implementing geometric modifications (eclipsed off-axis), TiN films with record low particulate density ~ 6x103 cm-2 and at high growth rates of ~1μm/hr are obtained. In chapter 3, the feasibility of BaTiO3 integration on Si (100) using this single TiN transition layer is explored. The polarization (c-axis) axis in BaTiO3 and its relative orientation w.r.t substrate normal dictates its application as a memory device (out of plane BTO) or an electro-optic (in-plane BTO) modulator. In the literature lattice-matched substrates are employed to achieve the same. However, this is not possible on Si, as the substrate Si is fixed. Epitaxial BaTiO3 films with both c-axis in-plane and out-of-plane polarization are demonstrated on TiN/Si(100). This change in polarization direction is brought about very simply by changing the growth temperature. Though good quality c-OP BaTiO3 films grown via TiN/Si are suited for memory applications, integrated electro-optic (E-O) applications based on c-IP BaTiO3 require an insulating buffer to limit the light propagation losses. In chapter 4, the feasibility of epitaxial growth of in-plane BaTiO3 via insulating the MgO layer on Si (100) using PLD is explored. The epitaxial in-plane BaTiO3 films on MgO/Si are electro-optically active and have the potential to be used in on-chip E-O modulators. In the last chapter of this thesis, heterogeneous integration of a binary wide bandgap semiconductor β-Ga2O3 (4.8 eV) on Si (100) platform is investigated to enable compact focal plane sensor arrays in deep ultraviolet regime on a CMOS chip. For the first-time epitaxial integration of β-Ga2O3 on (400) oriented silicon on insulator (SOI) (100) substrate has been reported
Soil and Water Assessment Tool
No full textPresentation on Soil and Water Assessment Tool (SWAT) delivered by Dr. Raghavan Srinivasan, one of the developers of Soil and Water Assessment Tool (SWAT) at Texas A&M University and experienced in linking climate change impact on water and land related issues
Integration of AlGaN with (111) Si Substrate by MOCVD
No full textAlGaN is an important semiconductor material for electronic and optoelectronic applications.
The change in composition of AlGaN (AlN to GaN) provides a range of bandgaps extending
from 6.01 eV, far ultraviolet, to 3.4 eV. This higher bandgap results in a higher breakdown
voltage, than GaN one of the current materials of choice, in the devices made out of it.
Carrier transport is also less sensitive to temperature variation. Hence, AlGaN with high Al
fraction is a suitable candidate for power transistor technology. For optoelectronic
applications like UV-photodetectors and UV-emitters, the full range of AlGaN provides the
tunability in wavelength ranging from 206 nm (AlN) to 360 nm (GaN). As the solar spectrum
ranges from about 250 nm to 2500 nm, AlGaN with high Al fraction is useful for solar-blind
UV applications. AlGaN UV emitters on the other hand can be used in water purification.
Till date all these developments have been carried out by growing AlGaNs on expensive
substrates like SiC, sapphire or freestanding AlN. But the growth of AlGaN on Si (111)
substrates are desirable as opposed to commonly used substrates such as sapphire, SiC or AlN
owing to its higher thermal conductivity (except SiC), low cost and availability in large area.
Integration with Si opens up the possibility to integrate the multifarious applications of
AlGaN with the economic viability of Si (111) substrates. The present work focuses on the
integration of AlGaN on Si (111) substrates by MOCVD. The bounds placed on the
competing requirements composition, thickness, stress, defect density and surface roughness
due to the physico-chemical aspects of AlGaN growth have been identified. Using such
understanding an AlGaN/AlGaN high electron mobility transistor and a UV detector have
been demonstrated
Titania Nanostructures for Photocatalytic and Photovoltaic Applications
Full text linkTitania has been the focus of attention for several decades owing to its chemical
stability, non-toxicity, inexpensiveness and robust surface chemistry. Its technological
applications include use in diverse areas such as photocatalytic reactors, antibacterial coatings, dye sensitive solar cells (DSSC) and more recently the perovskite solar cells to name a few. All of these applications are based on the ability to inject or generate electronhole pairs in titania and transport them to a suitable interface at which they are ejected to
either engender a reaction as in photocatalysis or drive a load as in photovoltaics. From a technological perspective it is also important that such science be achieved and controlled in
supported titania structures.
The research reported in this thesis, thus, started with the development of a process for
obtaining adherent titania films by oxidation of sputtered Ti films on stainless steel, a very commonly used substrate. Challenges that had to be overcome included the need to oxidize titanium to obtain the right phase mixture while preventing film cracking or delamination due to compressive stresses generated during anodic oxidation of Ti.
During this process of obtaining nanostructured TiO2 through anodization, it was serendipitously discovered that planar TiO2 films obtained by oxidation of sputtered Ti films did significantly better than anodized nanoporous titania in bactericidal studies. This was then replicated in organic dye degradation studies. Analysis of the material showed that this improved performance was due to the unintentional contamination during sputtering by Cu,
Zn, Mo possibly due to arcing across brass contacts. This quaternary system was then
systematically explored and it was shown that an optimal metastable composition in the Ti-
Cu-Mo oxide ternary system performs the best. DFT studies showed that this was due to
introduction of shallow and deep states in the band gap that, depending on the level of
dopants, either enhances carrier lifetimes or leads to recombination.
In continuation of this work on supported titania structures by oxidation of Ti, a novel photoanode for use in dye sensitized photovoltaics was developed by oxidation of Ti foam.
This results in an interconnected 3-D network of TiO2 that possess at its core a network of Ti. Such architecture was designed to provide a large surface area for anchoring the sensitizer while simultaneously reducing the distance that charge carriers have to travel before reaching the ohmic contacts to prevent recombination losses. The thesis discusses the preparation of such anodes, the properties of the 3-D oxide and cells, with up to 4% efficiency, developed using such anodes. Reasons for such behaviour and avenues for further exploration to
improve cell efficiency will also be discussed
- …
