485 research outputs found

    Label-free detection of biomolecules with Ta2O5-based field effect devices

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    Dissertação para obtenção do Grau de Doutor em Nanotecnologias e NanociênciasInternational Iberian Nanotechnology Laboratory (INL

    FLASH MEMORY DEVICES WITH METAL FLOATING GATE/METAL NANOCRYSTALS AS THE CHARGE STORAGE LAYER: A STATUS REVIEW

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    Traditional flash memory devices consist of Polysilicon Control Gate (CG) – Oxide-Nitride-Oxide (ONO - Interpoly Dielectric) – Polysilicon Floating Gate (FG) – Silicon Oxide (Tunnel dielectric) – Substrate. The dielectrics have to be scaled down considerably in order to meet the escalating demand for lower write/erase voltages and higher density of cells. But as the floating gate dimensions are scaled down the charge stored in the floating gate leak out more easily via thin tunneling oxide below the floating gate which causes serious reliability issues and the whole amount of stored charge carrying information can be lost. The possible route to eliminate this problem is to use high-k based interpoly dielectric and to replace the polysilicon floating gate with a metal floating gate. At larger physical thickness, these materials have similar capacitance value hence avoiding tunneling effect.  Discrete nanocrystal memory has also been proposed to solve this problem. Due to its high operation speed, excellent scalability and higher reliability it has been shown as a promising candidate for future non-volatile memory applications. This review paper focuses on the recent efforts and research activities related to the fabrication and characterization of non-volatile memory device with metal floating gate/metal nanocrystals as the charge storage layer

    Fabrication and Pseudo-Analog Characteristics of Ta2O5 -Based ReRAM Cell

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    Memristori on yksi elektroniikan peruskomponenteista vastuksen, kondensaattorin ja kelan lisäksi. Se on passiivinen komponentti, jonka teorian kehitti Leon Chua vuonna 1971. Kesti kuitenkin yli kolmekymmentä vuotta ennen kuin teoria pystyttiin yhdistämään kokeellisiin tuloksiin. Vuonna 2008 Hewlett Packard julkaisi artikkelin, jossa he väittivät valmistaneensa ensimmäisen toimivan memristorin. Memristori eli muistivastus on resistiivinen komponentti, jonka vastusarvoa pystytään muuttamaan. Nimens mukaisesti memristori kykenee myös säilyttämään vastusarvonsa ilman jatkuvaa virtaa ja jännitettä. Tyypillisesti memristorilla on vähintään kaksi vastusarvoa, joista kumpikin pystytään valitsemaan syöttämällä komponentille jännitettä tai virtaa. Tämän vuoksi memristoreita kutsutaankin usein resistiivisiksi kytkimiksi. Resistiivisiä kytkimiä tutkitaan nykyään paljon erityisesti niiden mahdollistaman muistiteknologian takia. Resistiivisistä kytkimistä rakennettua muistia kutsutaan ReRAM-muistiksi (lyhenne sanoista resistive random access memory). ReRAM-muisti on Flash-muistin tapaan haihtumaton muisti, jota voidaan sähköisesti ohjelmoida tai tyhjentää. Flash-muistia käytetään tällä hetkellä esimerkiksi muistitikuissa. ReRAM-muisti mahdollistaa kuitenkin nopeamman ja vähävirtaiseman toiminnan Flashiin verrattuna, joten se on tulevaisuudessa varteenotettava kilpailija markkinoilla. ReRAM-muisti mahdollistaa myös useammin bitin tallentamisen yhteen muistisoluun binäärisen (”0” tai ”1”) toiminnan sijaan. Tyypillisesti ReRAM-muistisolulla on kaksi rajoittavaa vastusarvoa, mutta näiden kahden tilan välille pystytään mahdollisesti ohjelmoimaan useampia tiloja. Muistisoluja voidaan kutsua analogisiksi, jos tilojen määrää ei ole rajoitettu. Analogisilla muistisoluilla olisi mahdollista rakentaa tehokkaasti esimerkiksi neuroverkkoja. Neuroverkoilla pyritään mallintamaan aivojen toimintaa ja suorittamaan tehtäviä, jotka ovat tyypillisesti vaikeita perinteisille tietokoneohjelmille. Neuroverkkoja käytetään esimerkiksi puheentunnistuksessa tai tekoälytoteutuksissa. Tässä diplomityössä tarkastellaan Ta2O5 -perustuvan ReRAM-muistisolun analogista toimintaa pitäen mielessä soveltuvuus neuroverkkoihin. ReRAM-muistisolun valmistus ja mittaustulokset käydään läpi. Muistisolun toiminta on harvoin täysin analogista, koska kahden rajoittavan vastusarvon välillä on usein rajattu määrä tiloja. Tämän vuoksi toimintaa kutsutaan pseudoanalogiseksi. Mittaustulokset osoittavat, että yksittäinen ReRAM-muistisolu kykenee binääriseen toimintaan hyvin. Joiltain osin yksittäinen solu kykenee tallentamaan useampia tiloja, mutta vastusarvoissa on peräkkäisten ohjelmointisyklien välillä suurta vaihtelevuutta, joka hankaloittaa tulkintaa. Valmistettu ReRAM-muistisolu ei sellaisenaan kykene toimimaan pseudoanalogisena muistina, vaan se vaati rinnalleen virtaa rajoittavan komponentin. Myös valmistusprosessin kehittäminen vähentäisi yksittäisen solun toiminnassa esiintyvää varianssia, jolloin sen toiminta muistuttaisi enemmän pseudoanalogista muistia.The memristor is one of the fundamental circuit elements in addition to a resistor, capacitor and an inductor. It is a passive component whose theory was postulated by Leon Chua in 1971. It took over 30 years before any known physical examples were discovered. In 2008 Hewlett Packard published an article where they manufactured a device which they claimed to be the first memristor found. The memristor, which is a concatenation of memory resistor, is a resistive component that has an ability to change its resistance. It can also remember its resistance value without continuous current or voltage. Typically, a memristor has at least two resistance states that can be altered. This is the reason why memristors are also called resistive switches. Resistive switches can be used in memory technologies. A memory array that has been built using resistive switches is called ReRAM (resistive random access memory). ReRAM, like Flash memory, is a non-volatile memory that can be programmed or erased electrically. Flash memories are currently used e.g. in memory sticks. However, compared to Flash, ReRAM has faster operating speed and lower power consumption, for instance. It could potentially replace current memory standards in future. A ReRAM memory cell can also store multiple bits instead of binary operation (”0” or ”1”). Typically there exists multiple intermediate resistance states between ReRAM’s limiting resistances that could be utilized. Such memory could be called analog, if the amount of intermediate states is not limited to discrete levels. Analog memories make it possible to build artificial neural networks (ANN) efficiently, for instance. ANNs try to model the behaviour of brain and to perform tasks that are difficult for traditional computer programs such as speech recognition or artificial intelligence. This thesis studies the analog behaviour of Ta 2 O 5 -based ReRAM cell. Manufacturing process and measurement results are presented. The operation of ReRAM cell is rarely fully analog as there exists limited amount of intermediate resistance states. This is the reason why operation is called pseudo-analog. Measurement results show that a single ReRAM cell is suitable for binary operation. In some cases, a single cell can store multiple resistance values but there exists significant variance in resistance states between subsequent programming cycles. The proposed ReRAM cell cannot operate as pseudo-analog ReRAM cell in itself as it needs an external current limiting component. Improving the manufacturing process should reduce the variability such that the operation would be more like a pseudo-analog memory.Siirretty Doriast

    Study of High-k Dielectrics and their Interfaces on Semiconductors for Device Applications

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    This thesis has focused on two emerging applications of high-k dielectrics in Metal-Oxide-Semiconductor Field Effect Transistors (MOSFETs) and in Metal-InsulatorSemiconductor High Electron Mobility Transistors (MIS-HEMTs). The key aim has been to propose the best routes for passivation of semiconductor/high-k oxide interfaces by investigating the band alignments and interface properties of several oxides, such as Tm2O3, Ta2O5, ZrO2, Al2O3 and MgO, deposited on different semiconductors: Si, Ge, GaN, InGaAs and InGaSb. The electrical characterisation of fabricated MIS capacitor and (MIS)-HEMT devices have also been performed. Thulium silicate (TmSiO) has been identified as a promising candidate for integration as interfacial layer (IL) in HfO2/TiN MOSFETs. The physical properties of Tm2O3/IL/Si interface have been elucidated, where IL (TmSiO) has been formed using different post-deposition annealing (PDA) temperatures, from 550 to 750 °C. It has been found that the best-scaled stack (sub-nm IL) is formed at 550 °C PDA with a graded interface layer and a strong SiOx (Si 3+) component. A large valence band offset (VBO) of 2.8 eV and a large conduction band offset (CBO) of 1.9 eV have been derived for Tm2O3/Si by X-ray photoelectron spectroscopy (XPS) and variable angle spectroscopic ellipsometry. Further increase of device performance can be achieved by replacing Si with GaN for high frequency, high power and high-temperature operation. In this thesis, several GaN cleaning procedures have been considered: 30% NH4OH, 20% (NH4)2S, and 37% HCl. It has been found that the HCl treatment shows the lowest oxygen contamination and Garich surface, and hence has been used prior sputtering of Ta2O5, Al2O3, ZrO2 and MgO on GaN. The large VBOs of 1.1 eV and 1.2 eV have been derived for Al2O3 and MgO on GaN respectively, using XPS and Kraut’s method; the corresponding CBOs are 2.0 eV and 2.8 eV respectively, taking into account the band gaps of Al2O3 (6.5 eV) and MgO (7.4 eV) determined from XPS O 1s electron energy spectra. The lowest leakage currents were obtained for devices with Al2O3 and MgO, i.e. 5.3 ×10-6 A/cm2 and 3.2 ×10-6 A/cm2 at 1 V, respectively in agreement with high band offsets (> 1 eV). Furthermore, the effect of different surface treatments (HCl, O2 plasma and 1-Octadecanethiol (ODT)) prior to atomic layer deposition of Al2O3 on the GaN/AlGaN/GaN heterostructure has been investigated. The MIS-HEMTs fabricated using the low-cost ODT GaN surface treatment have been found to exhibit superior performance for power switching applications such as a low threshold voltage, VT of -12.3 V, hysteresis of 0.12 V, a small subthreshold voltage slope (SS) of 73 mV/dec, and a low density of interface states, Dit of 3.0 x10^12 cm-2eV-1. A comprehensive novel study of HfO2/InGaAs and Al2O3/InGaSb interfaces have also been conducted for use in III-V based MOSFETs. The addition of the plasma H2/TMA/H2 pre-cleaning has been found to be very effective in recovering etch damage on InGaAs, especially for (110) orientation, and led to the improvement of electrical characteristics. Furthermore, the combination of H2 plasma exposure and forming gas anneal yielded significantly improved metrics for Al2O3/InGaSb over the control HCltreated sample, with the 150 W plasma treatment giving both the highest capacitance and the lowest stretch out

    Atomic layer deposition of high-k dielectrics from novel cyclopentadienyl-type precursors

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    The atomic layer deposition (ALD) method was applied for fabricating high permittivity (high-k) dielectrics, viz. HfO2, ZrO2 and rare earth oxides, which can be used to replace SiO2 as gate and capacitor dielectric. The dielectrics were processed by ALD using novel cyclopentadienyl (Cp, -C5H5) precursors together with water or ozone as the oxygen source. ALD, which has been identified as an important thin film growth technique for microelectronics manufacturing, relies on sequential and saturating surface reactions of alternately applied precursors, separated by inert gas purging. The surface-controlled nature of ALD enables the growth of thin films of high conformality and uniformity with an accurate thickness control. The ALD technique is introduced and ALD processes for HfO2, ZrO2 and rare earth oxide films, as well as the applications of the high-k dielectrics in microelectronics are reviewed. The need for developing new ALD processes for the high-k materials is emphasized. ALD processes for HfO2 and ZrO2 were developed using Cp-type precursors. The effect of different oxygen sources, namely water or ozone, on the film growth characteristics and properties of the ALD-processed films was examined in detail. The oxide films were stoichiometric, with impurity levels below even 0.1 at-% for C or H. Electrical measurements showed promising dielectric properties such as high permittivity values and low leakage current densities. Other properties, such as structure, interfacial layer thickness and morphology, were also characterized. Compared to films processed by water, the ozone-processed films on H-terminated Si showed improved dielectric properties, as well as higher density, lower roughness and better initial growth rate. In addition, in situ gas-phase measurements by quadrupole mass spectrometry (QMS) were performed in order to study the ZrO2 growth mechanism. A number of Cp-precursors were tested for the ALD of several rare earth oxide films. The thermal stability of many of the precursors was limited, but nevertheless, ALD-type processes were developed for Y2O3 and Er2O3 films. High reactivity of the Cp-precursors towards water resulting in high growth rates (1.2-1.7 Å/cycle) and purity of the Y2O3 and Er2O3 films were realized. Despite the detected partial decomposition of the (CpMe)3Gd precursor, Gd2O3 films with high growth rate and purity as well as effective permittivity of about 14 were deposited. Finally, promising processes for ternary scandates, namely YScO3, GdScO3, and ErScO3, were developed using either Cp- or β-diketonate-based processes. These as-deposited ternary films were amorphous exhibiting high effective permittivity (14-15), low leakage current density, and resistance towards crystallization upon annealing even up to 800°C.reviewe

    Low-temperature amorphous oxide semiconductors for thin-film transistors and memristors: physical insights and applications

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    While amorphous oxides semiconductors (AOS), namely InGaZnO (IGZO), have found market application in the display industry, their disruptive properties permit to envisage for more advanced concepts such as System-on-Panel (SoP) in which AOS devices could be used for addressing (and readout) of sensors and displays, for communication, and even for memory as oxide memristors are candidates for the next-generation memories. This work concerns the application of AOS for these applications considering the low thermal budgets (< 180 °C) required for flexible, low cost and alternative substrates. For maintaining low driving voltages, a sputtered multicomponent/multi-layered high-κ dielectric (Ta2O5+SiO2) was developed for low temperature IGZO TFTs which permitted high performance without sacrificing reliability and stability. Devices’ performance under temperature was investigated and the bias and temperature dependent mobility was modelled and included in TCAD simulation. Even for IGZO compositions yielding very high thermal activation, circuit topologies for counteracting both this and the bias stress effect were suggested. Channel length scaling of the devices was investigated, showing that operation for radio frequency identification (RFID) can be achieved without significant performance deterioration from short channel effects, which are attenuated by the high-κ dielectric, as is shown in TCAD simulation. The applicability of these devices in SoP is then exemplified by suggesting a large area flexible radiation sensing system with on-chip clock-generation, sensor matrix addressing and signal read-out, performed by the IGZO TFTs. Application for paper electronics was also shown, in which TCAD simulation was used to investigate on the unconventional floating gate structure. AOS memristors are also presented, with two distinct operation modes that could be envisaged for data storage or for synaptic applications. Employing typical TFT methodologies and materials, these are ease to integrate in oxide SoP architectures

    Semiconductor Nanowire MOSFETs and Applications

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    Semiconductor nanowires have aroused a lot of scientific interest and have been regarded as one of the most promising candidates that would make possible building blocks in future nanoscale devices and integrated circuits. Employing nanowire as metal‐oxide‐semiconductor field‐effect transistor (MOSFET) channel can enable a gate‐surrounding structure allowing an excellent electrostatic gate control over the channel for reducing the short‐channel effects. This chapter introduces the basic physics of semiconductor nanowires and addresses the problem of how to synthesize semiconductor nanowires with low‐cost, high‐efficiency and bottom‐up approaches. Effective integration of nanowires in modern complementary metal‐oxide‐semiconductor (CMOS) technology, specifically in MOSFET devices, and non‐volatile memory applications is also reviewed. By extending the nanowire MOSFET structure into a universal device architecture, various novel semiconductor materials can be investigated. Semiconductor nanowire MOSFETs have been proved to be a strong and useful platform to study the physical and electrical properties of the novel material. In this chapter, we will also review the investigations on topological insulator materials by employing the nanowire field‐effect transistor (FET) device structure

    Development of pyroelectric sensor integrated with two-dimensional transistor

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    Progress in the complementary metal-oxide semiconductors (CMOS) integrated circuit technology and the acceptance of silicon based microelectronics have revolutionised the electronics world. Advancement in human endeavour relies on the improvement of our processes and the discovery of new materials with beneficial properties. The advent of two dimensional (2D) materials has created exciting new possibilities. Possibilities that not only include the replacement of silicon in microelectronics but also for application in other areas such as photonics. Also, the quest to move beyond silicon and its oxide has seen increased interest in potential high dielectric and/or pyroelectric materials. The primary motivation for this work is to improve the output of Pyreos’s lead zirconium titanate (PZT) based pyroelectric sensor by integrating the device with a 2D tungsten diselenide (WSe2) field effect transistor (FET). A systematic approach has been taken to realise this goal by first exploring the material properties individually. From varied literature, two dimensional (2D) materials have shown promise in both electrical and optical properties. In this work, studies have been conducted on the 2D material WSe2, which has been deposited via exfoliation and pulse laser deposition (PLD). Though PLD WSe2 offers good control during deposition, our findings show that the exfoliated WSe2 has better electrical and optical properties. The field effect mobility calculated for the exfoliated WSe2 and PLD-WSe2 FET is 12.06 cm2/Vs and 5.66 × 10-2 cm2/Vs respectively. A maskless lithography method has been employed for the fabrication of the devices and tests were carried out using a Keithley parameter analyser, Raman and photoluminescence (PL) spectroscopy. The PL spectra showed that the exfoliated WSe2 has a bandgap of 1.6 eV while the PLD WSe2 possessed no bandgap, thereby limiting its applications. We studied the breakdown characteristics of a 20 nm anodic tantalum, a potential high dielectric gate oxide. At an electric field of 1.5 MV/cm, the leakage current extracted for the as-deposited, 200 °C and 400 °C annealed anodic tantalum are 10-5, 10-2 and 1 A/cm2 respectively while the breakdown field is 5.4, 5.1 and 3.3 MV/cm respectively. Upon integration of the anodic tantalum with a WSe2 FET, a field effect mobility of 0.9 cm2V-1s-1 has been realised. Analysis has also been conducted on the PZT pyroelectric device to understand its surface profile using energy backscatter diffraction (EBSD) and a factorial design of experiment. From the EBSD analysis, the sputtered PZT possesses a grain size of about 100 nm and exists in tetragonal crystals having good correlation for an orientation along the [111] direction normal to the surface. Results and understanding from the factorial design of the experiment enabled the integration of PZT with a WSe2 FET. While the standalone PZT based sensor possesses a 1 nA/mm2 current density output, the integrated PZT based WSe2 FET has an output of 16 nA/mm2. This improvement in the overall current output is over ten orders of magnitude. Thus, we have succeeded in amplifying the output of the PZT pyroelectric based sensor. The experimental outcomes of this thesis would help the research community in developing higher performing integrated sensors and possibly exposing other areas of interest on inexpensive and simple methods for producing materials with high dielectric constants
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