Eunsung Shin

Vanadium Oxide Thin-Film Variable Resistor-Based RF Switches

Vanadium dioxide (VO2) is a unique phase change material (PCM) that possesses a metal-to-insulator transition property. Pristine VO2 has a negative temperature coefficient of resistance, and it undergoes an insulator-to-metal phase change at a transition temperature of 68 °C. Such a property makes the VO2 thin-film-based variable resistor (varistor) a good candidate in reconfigurable electronics to be integrated with different RF devices such as inductors, varactors, and antennas. Series single-pole single-throw (SPST) switches with integrated VO2 thin films were designed, fabricated, and tested. The overall size of the device is 380 μm × 600 μm. The SPST switches were fabricated on a sapphire substrate with integrated heating coil to control VO2 phase change. During the test, when VO2 thin film changed from insulator at room temperature to metallic state (low-resistive phase) at 80 °C, the insertion loss of the SPST switch was <;3 dB at 10 GHz. In addition, the isolation of the SPST improved to better than 30 dB when the temperature dropped to 20 °C. These tunable characteristics of the RF switch provide evidence for VO2 as a useful PCM for the broad range of applications in reconfigurable electronics.

Memristor-based pattern recognition for image processing: an adaptive coded aperture imaging and sensing opportunity

Adaptive coded aperture (diffraction) sensing, an emerging technology enabling real-time, wide-area IR/visible sensing and imaging, could benefit from new high performance biologically inspired image processing architectures. The memristor, a novel two terminal passive device can enable significantly powerful biologically inspired processing architectures. This device was first theorized by Dr. Leon Chua in 1971. In 2008, HP Labs successfully fabricated the first memristor devices. Due to its unique properties, the memristor can be used to implement neuromorphic functions as its dynamics closely model those of a synapse, and can thus be utilized in biologically inspired processing architectures. This paper uses existing device models to determine how device parameters can be tuned for the memristor to be used in neuromorphic circuit design. Specifically, the relation between the different models and the number of states the device can hold are examined.

Thin film, nanoparticle, and nanocomposite fabrication by through thin film ablation

We have developed a laser-based technique for fabricating thin films, nanoparticles, and nanocomposite thin films. The process is denoted Through Thin Film Ablation (TTFA) and entails using a thin film target that is ablated from the backside. The deposits produced by this process show neither evidence of the larger (micrometer-sized) particles nor the extent of agglomeration that are produced by conventional laser ablation. The TTFA technique offers the potential for fabricating a wide variety of materials in thin film, nanoparticle, and nanocomposite form. In this paper we present the results of Fe nanoparticle synthesis by TTFA including nanoparticle size distributions, time of flight distributions, and time resolved spectral data. These data provide a more complete understanding of the TTFA process.

Lithium based memristive device

The effort of investigating memristor material continues. This paper demonstrates the latest results of such effort by our group. These include memristor device design and measurement based on stacked lithium niobate and aluminum oxide. I-V sweeping results show good switch memristive characteristic. Other preliminary results in this paper include temperature dependency study, pulse voltage write/read and retention characterization. From these results, lithium niobate based device show the temperature stability and multiple stages of write/read and long retention period. All these demonstrate its potential for the application of neuromorphic computing in the future.

Fabrication, characterization, and modeling of memristor devices

This paper describes the fabrication of memristor devices based on titanium and hafnium oxides. The device cross sectional area is varied to observe the impact this has on the current-voltage characteristic. A modeling technique is then utilized that is capable of matching the current-voltage characteristics of memristor devices. The model was able to match the titanium oxide device described in this paper with 13.58% error. The device model was then used in a neuromorphic simulation showing that a circuit based on this device is capable of learning logic functions.

Fabrication and testing of memristive devices

As semiconductor devices have shrunk further into the nanoscale regime, a new device, the memristor, has been discovered that has the potential to transform neuromorphic computing systems. This device is considered as the fourth fundamental circuit element. It was first theorized by Dr. Leon Chua in 1971 and has been discovered by HP labs in 2008. This paper describes initial efforts at fabricating the memristor devices and examining their properties. Two versions of memristor devices have been fabricated at the University of Dayton and the Air Force Research Laboratory utilizing varying thicknesses of the TiO2 dielectric layers. Our results show that the devices do exhibit the characteristic hysteresis loop in their I-V plots. Further refinement in the devices to achieve stronger hysteresis will be carried out as future work.

Synthesis of Ag Nanoparticles by Through Thin Film Ablation

There are several applications that require a thin layer of nanoparticles that are spread uniformly and without agglomeration over a surface. Among these applications are carbon nanotube synthesis and the development of fuel cells, advanced sensors, and optoelectronic devices. There is a need for improved processes of forming these nanoparticles. Among the numerous methods that have been developed, pulsed laser ablation has proved (ElShall et al., 1995; Burr et al., 1997, Seraphin et al., 1997, Geohegan et al., 1998; Lowndes et al, 1998, Makimura et al., 1998; Becker et al., 1998; Lowndes et al., 1999; Makino et al., 1999; Link and El-Sayed, 2000; Mafune et al., 2000; Mafune et al., 2001; Ogawa et al., 2000; Tang et al., 2001; Ozawa et al., 2001; Hata et al., 2001; Barnes et al., 2002) to be especially effective because of the potential for congruent ablation, the ability to produce nanoparticles of high purity, the ability to deposit nanoparticles on room temperature substrates, and the relative simplicity of the process. One problem with conventional pulsed laser ablation is the observation that the deposited material, in addition to containing nanoparticles, also contains large, m-sized particles. These are formed through a process denoted splashing (Ready, 1963), which occurs when a transient liquid layer is formed in the irradiated volume of the target; liquid droplets can be ejected from the target by the recoil pressure of the expanding gas on the transient liquid layer. Traditionally, splashing has been minimized (but not eliminated) by ablating very smooth targets or by using low laser energy densities. Target splashing is an issue in nanoparticle synthesis since the deposition of large particles within a field of nanoparticles makes it considerably more difficult to exploit the unique properties of isolated, non-agglomerated nanoparticles. Target splashing is also an issue because it represents material waste. There is, therefore, a need for a laser ablation process that further reduces or, ideally, eliminates splashing in nanoparticle synthesis. In addition, there is the further need for a process that minimizes nanoparticle agglomeration

Thermally controlled vanadium dioxide thin film microwave devices

Vanadium dioxide (VO2) thin films have unique insulator to metal transition above the critical temperature of 68 °C. In this study, VO2 thin films were deposited on a sapphire substrate for thermally controllable RF/microwave devices such as switches, limiters and resonators. The selectively deposited VO2 thin film based devices showed ~2 kΩ at room temperature and less than 2 Ω at 70 °C. These thermally controlled varistors were utilized in shunt as well as series configurations for microwave switching and in resonant structures. Switching devices designed using a VO2 shunt varistor showed good isolation (>20 dB) and low insertion loss (<;1 dB) up to 20 GHz. This paper presents two types of shunt varistor devices designed and fabricated on sapphire substrates.

Nanomaterials produced by laser ablation techniques. Part II: High spatially resolved nondestructive characterization of nanostructures

We studied nanoparticles by several high resolution microscopic methods as scanning electron microscopy (SEM), transmission electron microscopy (TEM) and scanning probe techniques especially atomic force microscopy (AFM) in contact and non-contact mode. While AFM in non-contact mode gives reliable information for 100 nm range nanoparticles it fails for smaller particles, showing lack of reproducibility. TEM and SEM prove to be reliable. By SEM imaging the agglomeration behavior and the structure of agglomerates are discussed in detail.

Filament formation in lithium niobate memristors supports neuromorphic programming capability

Memristor crossbars are capable of implementing learning algorithms in a much more energy and area efficient manner compared to traditional systems. However, the programmable nature of memristor crossbars must first be explored on a smaller scale to see which memristor device structures are most suitable for applications in reconfigurable computing. In this paper, we demonstrate the programmability of memristor devices with filamentary switching based on LiNbO3, a new resistive switching oxide. We show that a range of resistance values can be set within these memristor devices using a pulse train for programming. We also show that a neuromorphic crossbar containing eight memristors was capable of correctly implementing an OR function. This work demonstrates that lithium niobate memristors are strong candidates for use in neuromorphic computing.