Non-Silicon MOSFETs and Circuits with Atomic Layer Deposited Higher-k Dielectrics

The quest for technologies beyond 14nm node complementary metal-oxide-semiconductor (CMOS) devices has now called for research on higher-k gate dielectrics integration with high mobility channel materials such as III-V semiconductors and germanium. Ternary oxides, such as La2-xYxO3 and LaAlO3, have been considered as strong candidates due to their high dielectric constants and good thermal stability. Meanwhile, the unique abilities of delivering large area uniform thin film, excellent controlling of composition and thickness to an atomic level, which are keys to ultra-scaled devices, have made atomic layer deposition (ALD) technique an excellent choice. In this thesis, we systematically study the atomic layer epitaxy (ALE) process realized by ALD, ALE higher-k dielectric integration, GaAs nMOSFETs and pMOSFETs on (111)A substrates, and their related CMOS digital logic gate circuits as well as ring oscillators. A record high drain current of 376 mA/mm and a small SS of 74 mV/dec are obtained from planar GaAs nMOSFETs with 1μm gate length. La2-xYxO3/GaAs(111)A interfaces are systematically investigated in both material and electrical aspects. The work has expanded the near 50 years GaAs MOSFETs research to an unprecedented level. Following the GaAs work, Ge n- and p-MOSFETs with epitaxial interfaces are also fabricated and studied. Beyond the conventional semiconductors, the complex oxide channel material SrTiO3 is also explored. Well-behaved LaAlO3/SrTiO3 nMOSFETs with a conducting channel at insulating ALD amorphous LaAlO3 - insulating crystalline SrTiO3 interface are also demonstrated

Development of electrical characterization techniques for lifetime prediction and understanding defects in InGaAs-based III-V transistors

Combining the advanced complementary metal-oxide-semiconductor (CMOS) technique, faster and highly packaged circuits have continuous developing for more than fifty years. During this period time, engineers focus on the higher electrical field and accelerate the degradation to make the highly performance of advanced CMOS devices with better and smaller size than before. At this moment, there are plenty of issues are found which can affect the performances ofthe device. For example, for the new material technique, people prefe tor use III-V materials to instead of the traditional silicon, and for the traditional CMOS device, the main reliability issue is bias temperature instability (BTI), NBTI for pMOS and PBTI for nMOS. Therefore, this thesis will talk about investigating the NBTI/PBTI and III-V device performance and lifetime prediction issues. For the whole ofthe thesis, it can be divided by 4 chapters to describe the details, the first chapter would be the introduction of background knowledge and current understanding for how the traditional CMOS device works, and why the new materials could be instead, like III-V device, it shows the advantages and disadvantages for each of them. Moreover, there are some other information would be explained, such as Discharging-based multiple pulse (DMP) method, and hot carrier stress. Those are also topics and directions relate tod this thesis for the further research. Chapter 2 shows a new measurement method, which called Fast reliability screening technique method (VSS). It compared the conventional measurement method, which is constant voltage stress (CVS), is faster and only use a single device. And it also combined the measurement details and results to show that method is more efficiently, which includes the benefits and parameters discussion in the different conditions. Chapter 3 shows a separation traps method of III-V devices with in In0.53Ga0.47As channel. In this chapter, firstly, it shows the introduction of III-V devices for the basic theory and working function, then there are some experiments, which include both AC and DC measurements to show the III-V devices have very fast recovery abilities. Then because of the recovery ability, the devices would work well after charging and discharging even after heavily stress and long stress time. Moreover, combine the different discharge voltage levels to get the whole border trap energy distribution. After that, the chapter 3 shows how to separate the traps into two different types, type A and type B. there are experiments results and details to show how it is divided. Then it also mentioned the method can work with different stress time, channel thickness and temperature dependence. In Chapter 4, the discussion and a summary about previous work has been given, which also points out the future research plans and directions. It includes the lifetime prediction under slow and fast measurements for III-V devices

NOVEL III-V DEVICE ARCHITECTURES FOR APPLICATION IN ADVANCE CMOS LOGIC AND BEYOND

Ph.DDOCTOR OF PHILOSOPH

Modeling & Simulation of High Performance Nanoscale MOSFETs

Silicon-on-insulator (SOI) has been the forerunner of the CMOS technology in the last few decades offering superior CMOS devices with higher speed, higher density and reduced second order effects for submicron VLSI applications.A new type of transistor without junctions and no doping concentration gradients is analysed and demonstrated. These device structures address the challenge of short channel effects (SCEs) resulting with scaling of transistor dimensions and higher performance for deep submicron VLSI integration. Recent experimental studies have invigorated interest in partially depleted (PD) SOI devices because of their potentially superior scalability relative to bulk silicon CMOS devices. SELBOX structure offer an alternative way of suppressing kink effect and self heating effects in PD-SOI devices with a proper selection of oxide gap length. Also in order to mitigate the difficulties in fabrication of ultra thin devices for the semiconductor industry, resulting from scaling of gate length in MOSFET, a new device structure called junctionless (JL) transistors have recently been reported as an alternative device. In conclusion, extensive numerical simulation studies were used to explore and compare the electrical characteristics of SELBOX SOI MOSFET with a conventional single-material gate (SMG) bulk MOSFET. The proposed work investigates the DC and AC characteristics of the junctionless transistors. Also the performance analysis of JL transistors is compared and presented with the conventional DG MOSFET structure. The results presented in this work are expected to provide incentive for further experimental exploration

Multigate MOSFETs for digital performance and high linearity, and their fabrication techniques

The aggressive downscaling of complementary metal–oxide–semiconductor (CMOS) technology is facing great challenges to overcome severe short-channel effects. Multigate MOSFETs are one of the most promising candidates for scaling beyond Si CMOS, due to better electrostatic control as compared to conventional planar MOSFETs. Conventional dry etching-induced surface damage is one of the main sources of performance degradation for multigate transistors, especially for III-V high mobility materials. It is also challenging to increase the fin aspect ratio by dry etching because of the non-ideal anisotropic etching profile. Here, we report a novel method, inverse metal-assisted chemical etching (i-MacEtch), in lieu of conventional RIE etching, for 3D fin channel formation. InP junctionless FinFETs with record high-aspect-ratio (~ 50:1) fins are demonstrated by this method for the first time. The i-MacEtch process flow eliminates dry-etching-induced plasma damage, high energy ion implantation damage, and high temperature annealing, allowing for the fabrication of InP fin channels with atomically smooth sidewalls. The sidewall features resulting from this unique and simplified process ensure high interface quality between high-k dielectric layer and InP fin channel. Experimental and theoretical analyses show that high-aspect-ratio FinFETs, which could deliver more current per area under much relaxed horizontal geometry requirements, are promising in pushing the technology node ahead where conventional scaling has met its physical limits. The performance of the FinFET was further investigated through numerical simulation. A new kind of FinFET with asymmetric gate and source/drain contacts has been proposed and simulated. By benchmarking with conventional symmetric FinFET, better short-channel behavior with much higher current density is confirmed. The design guidelines are provided. The overall circuit delay can be minimized by optimizing gate lengths according to different local parasites among circuits in interconnection-delay-dominated SoC applications. Continued transistor scaling requires even stronger gate electrostatic control over the channel. The ultimate scaling structure would be gate-all-around nanowire MOSFETs. We demonstrate III-V junctionless gate-all-around (GAA) nanowire (NW) MOSFETs for the first time. For the first time, source/drain (S/D) resistance and thermal budget are minimized by regrowth using metalorganic chemical vapor deposition (MOCVD) in III-V MOSFETs. The fabricated short-channel (Lg=80 nm) GaAs GAA NWFETs with extremely narrow nanowire width (WNW= 9 nm) show excellent transconductance (gm) linearity at biases (300 mV), characterized by the high third intercept point (2.6 dBm). The high linearity is especially important for low power applications because it is insensitive to bias conditions

Terahertz Technology and Its Applications

The Terahertz frequency range (0.1 – 10)THz has demonstrated to provide many opportunities in prominent research fields such as high-speed communications, biomedicine, sensing, and imaging. This spectral range, lying between electronics and photonics, has been historically known as “terahertz gap” because of the lack of experimental as well as fabrication technologies. However, many efforts are now being carried out worldwide in order improve technology working at this frequency range. This book represents a mechanism to highlight some of the work being done within this range of the electromagnetic spectrum. The topics covered include non-destructive testing, teraherz imaging and sensing, among others

InAs Nanowire Devices and Circuits

Since the introduction of the transistor and the integrated circuit, the semiconductor industry has developed at a remarkable pace. By continuously fabricating smaller and faster transistors, it has been possible to maintain an exponential increase in performance, a phenomenon famously described by Moore’s Law. Today, billions of transistors are integrated on a single chip and the size of a transistor is on the scale of tens of nanometres. Until recently, the improvements in performance and integration density have been mostly driven by scaling down the transistor size. However, as the length scale is rapidly approaching that of only a few atoms, this scaling paradigm may not continue forever. Instead, the research community, as well as the industry, is investigating alternative structures and materials in order to further increase the performance. One emerging technology for use in future electronic circuits is transistors based on nanowires. The nanowire transistor structure investigated in this work combines a number of key technologies to achieve a higher performance than traditional Si-based transistors. Epitaxially grown nanowires are naturally oriented in the vertical direction, which means that the devices may be fabricated from the bottom and up. This three-dimensional structure allows a higher integration density and enables the gate to completely surround the channel in a gate-all-around configuration. Combined with a high-k dielectric, this results in an excellent electrostatic gate control. Furthermore, nanowires have the unique ability to combine semiconductor materials with significantly different lattice constants. By introducing InAs as a channel material, a much higher electron mobility than for Si is achieved. In this work, simulations of nanowire-based devices are performed and the ultimate performance is predicted. A nanowire transistor architecture with a realistic footprint is proposed and a roadmap is established for the scaling of the device structure, based on a set of technology nodes. Benchmarking is performed against competing technologies, both from a device and circuit perspective. The physical properties of nanowire transistors, and the corresponding capacitor structure, are investigated by band-structure simulations. Based on these simulations, a ballistic transport model is used to derive the intrinsic transistor characteristics. This is combined with an extensive evaluation and optimization of the parasitic elements in the transistor structure for each technology node. It is demonstrated that an optimized nanowire transistor has the potential to operate at terahertz frequencies, while maintaining a low power consumption. A high quality factor and extremely high integration density is predicted for the nanowire capacitor structure. It is concluded that InAs nanowire devices show great potential for use in future electronic circuits, both in digital and analogue applications

Advanced semi-classical Monte Carlo modeling of Si, Ge, InGaAs, and MoS₂ n-channel FETs for novel CMOS

Scaling-down of silicon (Si) based complementary-metal-oxide-semiconductor (CMOS) technologies are approaching material limits. For high-performance applications, high thermal velocity channel materials, such as indium-gallium-arsenide (InGaAs) and germanium (Ge), are viable alternatives to Si to extend the limits of CMOS downscaling. The unique mechanical and electrical properties of two-dimensional atomic crystals, such as single-layer molybdenum disulfide (MoS₂), combined with soft, flexible, and curvilinear substrates, enable new device functionalities and concepts in the field of low-power flexible electronics not achievable with Si channels. While the intrinsic electron mobility of MoS₂ is rather low, strain engineering may provide a pathway for improving electron transport. Silicon, InGaAs, Ge, and MoS₂ n-channel MOSFETs were explored via first-principles computational tools including density functional theory and particle-based ensemble semi-classical Monte Carlo methods to better understand and enable the rational design of end-of-the-roadmap CMOS and potential beyond-CMOS technologies. The impact of contact geometry and transmissivity and gate length scaling on quasi-ballistic nanoscale Si, Ge, and InGaAs n-channel FinFETs was studied. FinFETs with end, saddle/slot, and raised source and drain contacts and the same saddle/slot contact geometry with different gate lengths, according to the projections of industry roadmaps, were simulated. Simulated Si FinFETs exhibited relatively limited degradation in performance due to non-ideal contact transmissivities, more limited sensitivity to contact geometry with non-ideal contact transmissivities, some contact-related advantage for Si 〈110〉 channel devices, and limited sensitivity to gate length scaling. Simulated InGaAs FinFETs were highly sensitive to modeled contact geometry, specific contact resistivity, the band structure model, and gate length scaling. Simulated Ge FinFETs showed substantial degradation due to non-ideal contact transmissivities, sensitivity to gate length scaling, and a large orientation-related advantage for Ge 〈110〉 channel devices. The impact of tensile strain on the intrinsic performance limits of monolayer MoS₂ n-channel MOSFETs was studied. 200 and 15 nm gate length MoS₂ MOSFETs with end contacts subject to different types and amounts of strain were simulated. Simulated MoS₂ MOSFETs displayed improved performance with strain due to lower effective mass and larger inter-valley separation, which is largely reduced due to non-ideal contact transmissivities.Materials Science and Engineerin

Vertical InAs Nanowire Devices and RF Circuits

Recent decades have seen an exponential increase in the functionality of electronic circuits, allowing for continuous innovation, which benefits society. This increase in functionality has been facilitated by scaling down the dimensions of the most important electronic component in modern electronics: the Si-based MOSFET. By reducing the size of the device, more transistors per chip area is possible. Smaller MOSFETs are also faster and more energy-efficient. In state of the art MOSFETs, the key dimensions are only few nanometers, rapidly approaching a point where the current scaling scheme may not be maintained. Research is ongoing to improve the device performance, mainly focusing on material and structural improvements to the existing MOSFET architecture. In this thesis, MOSFETs based on nanowires, are investigated. Taking advantage of the nanowire geometry, the gate can be wrapped all-around the nanowires for excellent control of the channel. The nanowires are made in a high-mobility III-V semiconductor, InAs, allowing for faster electrons and higher currents than Si. This device type is a potential candidate to either replace or complement Si-based MOSFETs in digital and analogue applications. Single balanced down-conversion mixer circuits were fabricated, consisting of three vertically aligned InAs nanowire MOSFETs and two nanowire resistors. These circuits are shown to operate with voltage gain in the GHz-regime. Individual transistors demonstrated operation with gain at several tens of GHz. A method to characterise the resistivity and metal-semiconductor contact quality has been developed, using the transmission line method adapted for vertical nanowires. This method has successfully been applied to InAs nanowires and shown that low-resistance contacts to these nanowires are possible. To optimise the performance of the device and reach as close to intrinsic operation as possible, parasitic capacitances and resistances in the device structure need to be minimised. A novel self-aligned gate-last fabrication method for vertical InAs nanowire transistors has been developed, that allows for an optimum design of the channel and the contact regions. Transistors fabricated using this method exhibit the best DC performance, in terms of a compromise between the normalised transconductance and sub-threshold swing, of any previously reported vertical nanowire MOSFET