Projects
Novel one-dimensional high-performance FET structures
The last several decades have witnessed great progress in semiconductor device fabrication and integration. However, as the devices reach deep sub-100 nm scale, traditional methods, based on scaling of the device size while maintaining its basic structure, face increasing technological and fundamental challenges. For example, at the nanoscale, device size fluctuations will result in a large spread in device characteristics, including the threshold voltage and on/off current. Increasing demand on the resolution of the equipment and expenses of building and operating the facilities also pushes the traditional top-down approach towards its practical limit and hinders device scaling from reaching true atomic level.
In this project, we focus on high-performance transistor devices fabricated with an alternative, bottom-up approach, in which the critical device size is defined through a chemical synthesis method. This approach is in principle capable of parallel production of massive number of devices with crystal and structures well controlled at the atomic scale. In particular, semiconductor nanowires and nanowire heterostructures, with diameters of a few to tens of nanometers and lengths up to tens of micrometers, will be explored as building blocks of nanoelectronics.
These clean nanoscale devices offer not only smaller size, but also novel properties which are inaccessible or difficult to achieve in larger scale devices. For example, discreteness of electrons comes into play when the Coulomb energy associated with the addition of an individual electron becomes larger than the thermal energy. One-dimensional quantum wires and zero-dimensional quantum dots form when the relevant device size is comparable to the de Broglie wavelength of the carriers. As a result, the electrical and optical properties of these nanoscale devices depend not only on the materials, but can also be custom tailored by the specific device geometry. We plan to probe a number of interesting issues related with these novel 1D systems such as whether confinement effects improve the carrier mobility, new device physics in the ballistic limit, and techniques to optimize the device on-state performance while maintaining acceptable subthreshold behavior.
Non-transistor based memory and logic devices beyond the end of the roadmap
It is generally believed that continued scaling of CMOS technology will finally reach a “brick wall” as the device size approaches a few nanometer scale, due to, for example, escalating fluctuations in the gate threshold voltage resulting from inevitable fabrication spreads, and skyrocketing fabrication costs needed to satisfy increasing demands in fabrication accuracy. Novel, non-FET based devices and architectures will be needed to facilitate the escalating demands of defect tolerance and massive interconnects. Reconfigurable architectures, in particular, crossbar structures in which the active components are hysterestic resistors formed at the points where two nanowire arrays crossing each other, have emerged as a leading candidate. Like field-programmable gate arrays (FPGA), the crossbar scheme offers inherent defect tolerant capability. Furthermore, the simple two-terminal layout of the crossbar structure makes it suitable for aggressive scaling, with the potential that both logic and memory functions can be integrated in the same entity.
In a typical approach, arrays of metallic nanowires are used that act as both interconnects and contact leads, and self-assembled molecules are proposed as the active component at the crosspoints. However, switches based on molecules suffer from serious issues such as low yield, slow switching speed and low on/off ratio, which greatly affect their application potential.
In our approach, an alternative, solid-state based system is explored which in principle precisely addresses the abovementioned issues. Instead of unreliable molecules, a Si based heterostructure material acts as the active component in a metallic nanowire crossbar array. These heterostructure devices can offer scalability down to nanometer scale, comparable to molecule based devices, while in the meantime exhibiting superior properties for hysterestic switching, such as high yield, fast switching speed, well defined thresholds and high on/off ratio. Furthermore, intrinsic rectifying behavior, a property which eliminates crosstalk and is highly desirable in the crossbar scheme, can also be achieved through proper material engineering. The proposed Si heterostructure crossbar arrays will offer up to terabit-scale density (1012 devices/cm2) potential in a defect tolerant, reconfigurable architecture, while being fabricated on a reliable platform that is also compatible with available Si technology. In particular, high density, non-volatile memories with integrated decoder devices that interface the nanoscale components with microscale electronics will be demonstrated. General logic applications based on the crossbar structures will also be explored, with emphasis on a hybrid crossbar/CMOS approach.
Nano- and Quantum-electromechanical systems (Wayne Yip Fung)
Nanoscale, high-Q, UHF mechanical resonators have attracted broad interest from both the engineering and scientific communities due to their potential in applications such as highly integrated communications equipments, ultrasensitive position and mass sensors, as well as fundamental research on quantum electromechanical systems. For example, nanomechanical resonators have been proposed to work as on-chip bandpass filters and oscillators, replacing bulky off-chip passive components. Unique opportunities to access the quantum-limited operation in nanomechanics can also be reached when a GHz resonator is cooled down to cryogenic temperature, such that the vibration energy quantum becomes comparable or even higher than the thermal energy. Simply scaling the microscale VHF resonators down to nanoscale to obtain a higher resonant frequency, however, will not be sufficient. For example, surface effects, such as internal loss due to surface roughness, will be pronounced in nanoscale resonators due to the increased surface-to-volume ratio. Furthermore, the length of the beam is normally scaled down faster than the width due to lithography limitations, resulting in decreased aspect ratio and hence increased clamping loss. As a result, even though high quality factors can be readily achieved on VHF micromechanical resonators, Q of UHF fundamental mode resonators has been limited to < 1000, well below the requirement of 103-105 needed to realize the proposed novel applications.
In this project, we address these abovementioned issues by utilizing chemically synthesized single-crystalline nanowires. Unlike resonators fabricated via lithography and etching processes, the chemically synthesized nanowire devices offer minimal internal dissipation due to the smooth surface and the single-crystalline material structure. The small diameter of the nanowires also ensures that GHz frequency resonators can be achieved while maintaining the aspect ratio of the beam, hence minimizing the clamping loss as well. As the first milestone of the proposed work, UHF, high-Q mechanical resonators will be demonstrated using chemically synthesized, single-crystalline Si nanowires. The nanowire mechanical resonators will be characterized using transduction methods such as magnetomotive and electromotive techniques. Furthermore, incorporation of piezoelectric actuation will be explored by substituting Si nanowries with AlN nanowires, which offers a much simplified actuation scheme.
Following the successful demonstration of nanowire mechanical resonators, integrated mechanical-electrical devices and quantum electromechanical operations will be carried out. Particularly, ultra-sensitive position displacement measurement will be performed by coupling a GHz resonator with a single-electron transistor, and quantum-limited zero-point motion detection will be attempted when the device is cooled to its ground state. Entanglement of the mechanical and electrical degrees of freedom will be also studied by coupling the nanomechanical resonator with a charge based two-level system. These quantum electromechanical studies will not only provide insight into quantum coherence in macroscale systems, but may also offer novel applications such as ultra-sensitive force and displacement measurement at the quantum level, and quantum information processing using mechanical devices.
Sensors and photovoltaic devices based on nanomaterials
This project focuses on the synthesis of semiconducting oxide nanowires, such as ZnO and Sn-doped In2O3 nanowires arrays and their applications in nanoscale devices and sensors.
Nanowires growth
Growth of important oxide semiconductor nanowires such as ZnO, SnO2, and In2O3, will be explored by a simple vapor transport method. Following nanowire growth, SEM, TEM, XRD, Raman, and PL will be used to characterize the material properties of the synthesized nanowires samples.
Functional Doping of these nanowires
It is necessary to dope the nanowires in a controlled manner in order to obtain advanced nanowires devices, For example, p-type ZnO nanowires can be obtained and ZnO nanowire p-n junction can be fabricated if Sb can be doped into ZnO nanowires. These doped nanowire structuress will be useful in a range of studies, from electronic devices to electrically injected nanowire lasers.
Device applications
- Gas sensors — nanowires’s merit: high surface to volume ratio;
- Large-area thin film transistors —- nanowires’s merit: high mobility;
- Solar cells —- nanowires’s merit: high surface to volume ratio and mobility;
- Vacuum electron field emission devices —- very high field enhancement factor
Nanowire-based quantum devices