Thesis (M.S.)--West Virginia University, 2009. / Title from document title page. Document formatted into pages; contains ix, 72 p. : ill. (some col.). Includes abstract. Includes bibliographical references (p. 66-72).

As the current requirements of power devices are moving towards high frequency, high efficiency and high-power density, Silicon-based devices are reaching its limits which are instigating the need to move towards new materials. Gallium Nitride (GaN) has the potential to meet the growing demands due to the wide band-gap nature which leads to various enhanced material properties like, higher operational temperature, smaller dimensions, faster operation and efficient performance. The metal contacts on semiconductors are essential as the interface properties affect the semiconductor performance and device operation. The low resistance ohmic contacts for n-GaN have been well established while most p-GaN devices have still high contact resistivity. Significant work has not been found that focuses on software-based modeling of the device to analyze the contact resistance and implement methods to reduce the contact resistivity. Understanding the interface physics in n-GaN devices using simulations can help in understanding the contacts on p-GaN and eventually reduce its metal contact resistivity. In this work, modeling of the metal-semiconductor interface along with the effect of a heavily doped layer under the metal contact is presented. The extent of reduction in contact resistivity due to different doping and thickness of n++ layer is presented with simulations. These results have been verified by the growth of device based on simulation results and reduction in contact resistivity has been observed. The effect of different TLM pattern along with different annealing conditions is presented in the work. / Master of Science

MoS2, among many other transition metal dichalcogenides (TMDCs), holds great promise for future applications in nano-electronics, opto-electronics and mechanical devices due to its ultra-thin nature, flexibility, sizable band-gap, and unique spin-valley coupled physics. However, there are two main challenges that hinder careful study of this material. Firstly, it is hard to achieve Ohmic contacts to mono-layer MoS2, particularly at low temperatures (T) and low carrier densities. Secondly, materials' low quality and impurities introduced during the fabrication significantly limit the electron mobility of mono- and few-layer MoS2 to be substantially below theoretically predicted limits, which has hampered efforts to observe its novel quantum transport behaviours. Traditional low work function metals doesn't necessary provide good electron injection to thin MoS2 due to metal oxidation, Fermi level pinning, etc. To address the first challenge, we tried multiple contact schemes and found that mono-layer hexagonal boron nitride (h-BN) and cobalt (Co) provide robust Ohmic contact. The mono-layer spacer serves two advantageous purposes: it strongly interacts with the transition metal, reducing its work function by over 1 eV; and breaks the metal-TMDCs interaction to eliminate the interfacial states that cause Fermi level pinning. We measure a flat-band Schottky barrier of 16 meV, which makes thin tunnel barriers upon doping the channels, and thus achieve low-T contact resistance of 3 kohm.um at a carrier density of 5.3x10^12/cm^2. Similar to graphene, eliminating all potential sources of disorder and scattering is the key to achieving high performance in MoS2 devices. We developed a van der Waals heterostructure device platform where MoS2 layers are fully encapsulated within h-BN and electrically contacted in a multi-terminal geometry using gate-tunable graphene electrodes. The h-BN-encapsulation provides excellent protection from environmental factors, resulting in highly stable device performance, even at elevated temperatures. Both optical and electrical characterization confirms our high quality devices, including an ultra-clean interface, a record-high Hall mobility reaching 34,000 cm^2/Vs, and first observation of Shubnikov–de Haas oscillations. The development of Ohmic contact and fabrication of high quality devices are critical to MoS2 application and studying its intrinsic properties. Therefore, the progress made in this work will facilitate efforts to study novel physical phenomena of MoS2 that were not accessible before.

Electrical contacts to devices which pose low resistance continue to be of interest as the dimensions of devices decrease and nanotechnology demands better means of creating electrical access. Continued improvement in the performance of ohmic contacts requires techniques to better characterise and quantify the performance of such contacts. In order to study and estimate the resistance of such contacts or the resistance posed by the interface(s) in such contacts, accurate test structures and evaluation techniques need to be used. The resistance posed by an interface is quantified using its specific contact resistivity (SCR), which is denoted using ƒâc (units: £[cm2). Cross Kelvin resistor (CKR) test structures have been used for the measurement of low values of SCR. A simplified approach to this problem of SCR evaluation (developed previously at RMIT University) using the CKR test structures with varying contact sizes was used and during this work was shown to be accurate for the estimation of low values (less than10-8 £[cm2) of SCR. The silicides of interest in this study were titanium silicide (TiSi2) and nickel silicide (NiSi). These thin films are known for their low resistivity and low barrier heights to both n-type and p-type silicon. The research involved thin film formation and substantial materials characterisation of these thin films. The silicide thin films were formed by vacuum annealing metal thin films on silicon substrates. Silicide thin films formed from metal films deposited by DC magnetron sputtering and electron beam evaporation were compared. The composition, crystallographic orientation, and morphology of these thin films were studied using spectroscopy (AES, SIMS, RBS, in situ Raman spectroscopy), diffraction (Bragg-Brentano and glancing angle XRD, RHEED), and microscopy techniques (TEM, SEM, and AFM). TiSi2 and NiSi thin films were also found to be suitable for microsystems fabrication due to their ability to withstand wet etching of silicon using potassium hydroxide. The SCR of aluminium-titanium silicide ohmic contacts was evaluated to be as low as 6 x 10-10 ƒÇcm2, which is the lowest reported for any two- layer single-interface contact. Characterisation of ohmic contacts comprising of aluminium, nickel silicide, and doped silicon (with shallow implants) were also carried out using the same technique. SCR values as low as 5.0 x 10-9 ƒÇcm2 for contacts to antimony-doped silicon and 3.5 x 10-9 £[cm2 to boron-doped silicon were evaluated.

Orientadores: Stanislav Alexandrovich Moshkalev, Mario Antonio Bica de Moraes / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin / Made available in DSpace on 2018-08-11T00:23:59Z (GMT). No. of bitstreams: 1 LeonEras_JorgeAugusto_D.pdf: 28434284 bytes, checksum: 042fbff5bb19c08014235156fdc57ee4 (MD5) Previous issue date: 2007 / Resumo: Não informado / Abstract: Not informed. / Doutorado / Física da Matéria Condensada / Doutor em Ciências

It is challenging to achieve low-resistive ohmic contacts to III-nitride semiconductors due to their wide bandgap. A common way to reduce the contact resistance is to recess the ohmic area prior to metallization. In the minimization of the contact resistance, parameters like the recess depth, anneal temperature and design of the metal stack are commonly optimized. In this work, three other approaches have been evaluated. All experiments were performed on AlGaN/GaN heterostructures. The fabricated ohmic contacts were recess etched, metallized with a Ta/Al/Ta stack, and annealed at 550-575◦C.Firstly, it is shown that the laser writer intensity, transmittance and focus offset during optical lithography affect the contact resistance. The reason is believed to be the variation in the resist profile, which has an impact on the metal coverage. At the optimum intensity/transmittance/focus condition, which generates a relatively medium undercut, a contact resistance of 0.23 Ωmm was obtained.In the second approach, the metal layer of annealed contacts was removed by wet etching, followed by the re-deposition of a metal stack and annealing. The purpose was to increase the amount of N vacancies in the AlGaN, which are responsible for the contact formation. A minimum contact resistance of 0.41 Ωmm was achieved with this method, compared to 0.28 Ωmm with the regular method (without remetallization).In the last approach, the bottom Ta layer was sputtered, whereas evaporation was used in all other cases. The minimum contact resistance was found to be 0.6 Ωmm, which was higher than for the evaporated contacts. The reason was assumed that the thickness of sputtered Ta should be thinner than the evaporated Ta due to its higher density. Moreover, the obtained lower sheet resistance is assumed to caused by the atomic scale damage due to the high energy ions during sputtering. / En utmaning med III-nitrid-halvledare är att uppnå låg-resistivitetskontakter, på grund av deras breda bandgap. Ett konventionellt tillvägagångsätt för att reducera kontaktresistansen är att fördjupa ohmska ytan före metallisering. I strävandet av att minska den ohmska resistansen sker vanligtvis en optimering av följande parametrar, recessddjup, anlöpningstemperatur och metallagersdesign. I detta arbete så har samtliga tre parametrar evaluerats. Alla experiment utfördes på AlGaN/GaNheterostrukturer. De tillverkade ohmska kontakterna var recesssetsade, metalliserade med ett Ta/Al/Ta lager och anlöpt vid 550-575◦C.Den primära undersökningen, visar att laserritar-intensitet, -transmission och fokusförskjutning under optisk litografi inverkar på kontaktresistansen. Anledningen antas vara variation i resistprofilen, vilket påverkar metallbeläggningen. Vid optimal intensitet/transmission/fokus-förhållanden, (som genererar en underskärning), blev den resulterande kontaktresistansen 0.23 Ωmm uppmätt.I en sekundär undersökning, avlägsnas ohmska kontaktens metallager genom våtetsning, följt av en återdeponering av ett nytt metallager, samt anlöpning. Syftet var att öka mängden N-vakanser i AlGaN-lagret, som formar ohmska kontakten. Minsta kontaktresistansen uppmätt var 0.41Wmm, att jämföras med 0.28 Ωmm, som uppnåddes genom den konventionella metoden (utan återmetallisering).Den sista undersökningen jämförde sputtrade med evaporerade bottenlager av Ta, (evaporation användes som standardmetod i de tidigare undersökningarna). Med sputtrning blev den minsta kontakresistansen 0.6 Ωmm, (högre än de evaporerade kontakterna). En hypotetisk förklarning kan vara att det sputtrade Ta-lagret är tunnare än det evaporerade Ta-lagret, på grund av en dess högre densitet. Därutöver, den uppmätta lägre skiktresistansen antas bero på den skada i atomskala som sker vid de höga energi-kollisioner som joner skapar vid sputtrning.

Cette thèse s’inscrit dans le cadre du développement d’une filière de transistors de puissance à base de nitrure de Gallium au CEA-LETI. Ces transistors, en particulier les HEMT utilisant l’hétérostructure AlGaN/GaN, présentent des propriétés très utiles pour les applications de puissance. L’essor de cette technologie passe notamment par le développement de contacts ohmiques peu résistifs. Cette thèse a pour objectif d’approfondir la compréhension des mécanismes de formation du contact ohmique sur une structure AlGaN/GaN. Dans un premier temps, une étude thermodynamique sur une dizaine de métaux de transition utilisables comme base de l’empilement métallique du contact a été menée, ce qui a permis de retenir une métallisation Ti/Al. Puis, les différentes réactions physico-chimiques de cet empilement avec des substrats nitrurés ont été étudiées en faisant varier la composition et les températures de recuit de formation du contact ohmique. Enfin, plusieurs études sur structure AlGaN/GaN couplant caractérisations électriques et physico-chimiques ont permis d’identifier des paramètres décisifs pour la réalisation d’un contact ohmique, peu résistif et nécessitant une faible température de recuit. / This PhD is part of the development of Gallium nitride based power transistors at the CEA-LETI. These transistors, especially those based on AlGaN/GaN heterostructure, are very promising for power electronics applications. The goal of this PhD is to increase the knowledge of the mechanisms responsible for the ohmic contact formation on a AlGaN/GaN structure. First, a thermodynamic study of several transition metals has been performed, leading us to select Ti/Al metallization. Then, the multiple physico-chemical reactions of this stack with nitride substrates have been studied depending on the stack composition and the annealing temperature. Finally, several studies on AlGaN/GaN structure coupling both physico-chemical and electrical characterizations reveal different decisive parameters for the formation of an ohmic contact with a low-resistance and a low annealing temperature.

AlGaN/GaN metal–insulator–semiconductor high electron mobility transistors (MIS-HEMTs) are promising candidates for next generation high-efficiency and high-voltage power applications. The excellent physical properties of GaN-based materials, featuring high critical electric field and large carrier saturation velocity, combined to the high carrier density and large mobility of the two-dimensional electron gas confined at the AlGaN/GaN interface, enable higher power density minimizing power losses and self-heating of the device. However, the advent of the GaN-based MIS-HEMT to the industrial production is still hindered by technological challenges that are being faced in parallel. Among them, one of the biggest challenge is represented by the insertion of a gate dielectric in MIS-HEMTs compared to Schottky-gate HEMTs, which causes operational instability due to the presence of high-density trap states located at the dielectric/III-nitride interface or within the dielectric. The development of a gold-free ohmic contact technology is another important concern since the high-volume and cost-effective production of GaN-based transistors also depends on the cooperative manufacturing of GaN-based devices in Si production facilities, where gold represents an undesidered source of contamination. In fact, even though over the past years there have been multiple attemps to develop gold-free ohmic contacts, there is still no full understanding of the contact formation and current transport mechanism. The first objective of this work was the investigation of a gold-free and low-resistive ohmic contact technology to AlGaN/GaN based on sputtered Ta/Al-based metal stacks annealed at low temperatures. A low contact resistance below 1 Ω mm was obtained using Ta/Al-based metal stacks annealed at temperatures below 600 °C. The ohmic behavior and the contact properties of contact resistance, optimum annealing temperature and thermal stability of Ta/Al-based contacts were studied. The nature of the current transport was also investigated indicating a contact mechanism governed by thermionic field emission tunneling through the AlGaN barrier. Finally, gold-free Ta/Al-based ohmic contacts were integrated in MIS-HEMTs fabricated on a 150 mm GaN-on- Si substrate, demonstrating to be a promising contact technology for AlGaN/GaN devices and revealing to be beneficial for devices operating at high temperatures. The optimization of the MIS-gate structure in terms of trap states at the dielectric/III-nitride interface and inside the dielectric in MIS-HEMTs using atomic layer deposited (ALD) Al2O3 as gate insulator was the second focus of this work. First, the MIS-gate structure was improved by an O2 plasma surface preconditioning applied before the Al2O3 deposition and by an N2 postmetallization anneal applied after gate metallization, which significantly reduced trap states at the Al2O3/GaN interface and within the dielectric. Afterwards, the effectiveness of these treatments was demonstrated in Al2O3-AlGaN/GaN MIS-HEMTs by pulsed current–voltage measurements revealing improved threshold voltage stability. Lastly, it was shown that also the lower annealing temperatures used for the formation of Ta/Al-based ohmic contacts, processed before gate dielectric deposition, are beneficial in terms of trap states at the ALD-Al2O3/GaN interface, representing a new aspect to be considered when using an ohmic first fabrication approach.

Thesis (Ph. D.)--Ohio State University, 2006. / Title from first page of PDF file. Includes bibliographical references (p. 209-217).

Germanium (Ge) is the most compatible semiconductor material with silicon-based complementary metal-oxide semiconductor technology, which has higher electron and hole mobility than Si, leading to enhanced device performance. In addition, semiconductor nanowires (NWs) have attracted significant attention as promising candidates for next-generation nanoscale devices. Due to their unique geometry and physical properties, NWs show excellent optical and electrical properties such as quantum size effects, enhanced light absorption, and high biological and chemical sensitivity. Furthermore, high response to light irradiation is one of the most significant properties of semiconductor NWs, which makes them excellent candidates for photodetectors. Hence, Ge NWs are promising high-mobility nanostructures for optoelectronic devices. Despite constant improvement in the performance of single NW-based devices, determining their electrical properties remains challenging. Here, a symmetric six-contact Hall bar configuration is developed for top-down fabricated highly doped Ge NWs with different widths down to 30 nm, which simultaneously facilitates Hall effect and four-probe resistance measurements. Furthermore, accurate control of doping and fabrication of metal contacts on n-type doped Ge NWs with low resistance and linear characteristics remain significant challenges in Ge-based devices. Therefore, a combined approach is reported to fabricate Ohmic contacts on n-type doped Ge NWs using ion implantation and rear-side flash lamp annealing. This approach allows the fabrication of axial p–n junctions along the single NWs with different widths. The fabricated devices demonstrated rectifying characteristics in dark conditions. The photoresponse of the axial p–n junction photodetectors was investigated under three different illumination wavelengths of 637 nm, 785 nm, and 1550 nm. Moreover, the fabricated axial p–n junction photodetector demonstrated a high-frequency response up to 1 MHz at zero bias.