Hyoung-soo Ko

Origin of surface potential change during ferroelectric switching in epitaxial PbTiO3 thin films studied by scanning force microscopy

We investigated the surface potential of the ferroelectric domains of the epitaxial PbTiO3 (PTO) films using both Kelvin probe and piezoresponse force microscopy. The surface potential changes as a function of applied biases suggested that the amount and sign of surface potentials depend on the correlation between polarization and screen charges. It also suggested that the trapped negative charges exist on the as-deposited PTO surfaces. Injected charges and their resultant surface potentials are investigated by grounded tip scans. The results unveiled the origin of surface potential changes during ferroelectric switching in the epitaxial PTO films.

Effect of local surface potential distribution on its relaxation in polycrystalline ferroelectric films.

We have studied the effect of local surface potential distribution on its relaxation in the polycrystalline ferroelectric thin films. A lower surface potential region, i.e., potential pit, is generated near a grain boundary. The deep potential pit has a faster relaxation than the area far away from the potential pit due to the acceleration of the screen charge draining near the grain boundary and the domains formed by applying higher voltage have a faster relaxation due to the larger gradient of screen charge distribution. In addition, the surface potential and its relaxation depend on the sign of applying voltage. The result shows that the surface potential distribution may influence significantly to the reliability of bit signal on the memory devices.

High-resolution field effect sensing of ferroelectric charges.

Nanoscale manipulation of surface charges and their imaging are essential for understanding local electronic behaviors of polar materials and advanced electronic devices. Electrostatic force microscopy and Kelvin probe force microscopy have been extensively used to probe and image local surface charges responsible for electrodynamics and transport phenomena. However, they rely on the weak electric force modulation of cantilever that limits both spatial and temporal resolutions. Here we present a field effect transistor embedded probe that can directly image surface charges on a length scale of 25 nm and a time scale of less than 125 μs. On the basis of the calculation of net surface charges in a 25 nm diameter ferroelectric domain, we could estimate the charge density resolution to be as low as 0.08 μC/cm(2), which is equivalent to 1/20 electron per nanometer square at room temperature.

Investigation of 1T DRAM cell with non-overlap structure and recessed channel

In this paper, a capacitor-less 1T DRAM cell transistor with non-overlap structure and recessed channel is presented. Because of the non-overlap structure between gate and source/drain, GIDL (Gate Induced Drain Leakage) current is efficiently suppressed at hold condition. This results in more than 1 s retention time at 25 °C and 100 ms at 85 °C

An endurance-free ferroelectric random access memory as a non-volatile RAM

We demonstrate endurance characteristics of a 1T1C, 64 Mb FRAM in a real-time operational situation. To explore endurance properties in address access time tAA of 100 ns, we establish a measurement set-up that covers asymmetric pulse-chains corresponding to D1- and D0-READ/RESTORE/WRITE over a frequency range from 1.0 to 7.7 MHz. What has been achieved is that endurance cycles approximate 5.9 times 1024 of cycle times in an operational condition of VDD = 2.0 V and 85degC in the developed 64 Mb FRAM. Donor concentration due to build-up of oxygen vacancy in a ferroelectric film has also been evaluated to 2.3 times 1020 cm-3 from I-V-t measurements.

Characterization of Sensitivity and Resolution of Silicon Resistive Probe

The resistive probe, which can detect the variation in probe resistance while scanning over positive/negative biased bits, has been considered as the one of the most suitable candidate for used as a high-speed, ultralarge-storage systems. Ultralarge-storage density has been demonstrated by the electrical recording technique in which surface charge is read in ferroelectric materials, which is named as scanning resistive probe microscopy (SRPM). Because of the difficulty of fabricating, the SRPM device little has been previously done on relating probe properties with process parameters. As it is important in probe design to analyze the properties of sensitivity and resolution for different process and device parameters, the parameters of the fabricated microscope are embedded in the simulation performed in this study. To obtain the optimum resistive probe design, we analyze the effect of different process parameters on the properties. In the first part of the paper, the device structure fabricated to identify the electrostatic charge effects between the tip and the poly silicon domain (surface) is presented. In the second part, we explain the process conditions of the resistive probe. In the third part, we present the sensitivity and resolution for different process conditions. The probe is placed in contact with a poly silicon domain (surface) to measure the field sensitivity, and a 0.3% resistance change per voltage applied to the surface is detected with a spatial resolution of less than 1000 nm. The sensitivity and resolution for different implantation doses, wet oxidation time, and resistances in the high-doped n-type region are also presented. We found that there is a correlation between sensitivity and resolution for different implantation doses and annealing times. Although the sensitivity decreases as resistance increases, the change is less than 10% even at a resistance of 200 Ω. Therefore, the resistance of the cantilever can be neglected in our study of probe sensitivity.

High-Resolution Field Effect Sensing of Ferroelectric Charges

Nanoscale manipulation of surface charges and their imaging are essential for understanding local electronic behaviors of polar materials and advanced electronic devices. Electrostatic force microscopy and Kelvin probe force microscopy have been extensively used to probe and image local surface charges responsible for electrodynamics and transport phenomena. However, they rely on the weak electric force modulation of cantilever that limits both spatial and temporal resolutions. Here we present a field effect transistor embedded probe that can directly image surface charges on a length scale of 25 nm and a time scale of less than 125 μs. On the basis of the calculation of net surface charges in a 25 nm diameter ferroelectric domain, we could estimate the charge density resolution to be as low as 0.08 μC/cm(2), which is equivalent to 1/20 electron per nanometer square at room temperature.

A methodology to characterize device-level endurance in 1T1C (1-transistor and 1-capacitor) FRAM

We present a mimicking methodology to describe device-level endurance in a 1T1C, 64 Mb FRAM (ferroelectric random access memory). Device-level endurance of FRAM must clarify all the issues raised from destructive read-out READ/WRITE. To explore endurance properties in a real-time operational situation, we have established a measurement set-up that covers asymmetric pulse chains corresponding to Data 1 (D1) and Data 0 (D0) READ/RESTORE over a frequency range from 1.0 to 7.7 MHz. The cycle-to-failure of 5.9 × 1024 cycles in an operational condition of 7.7 MHz and 85 °C, has been obtained from extrapolation to VDD = 2.0 V in a voltage acceleration. We compare testing results with those of D1¿D0 populations of bit-line potential.

Analysis and modeling of resistive probes

A simple model of the resistive probe was proposed for the first time to enhance its sensitivity. The model classifies output current into three components: sensing current, unmodulated current, and punch-through current. Based on it, the electrical performance of the resistive probe was analyzed. The proposed model is expected to contribute to improvement of the sensing performance of the resistive probe.

Design Optimization of Scanning Resistive Microscopy (SRM) Probe for Spatial Resolution Improvement

We suggest a new design of resistive probe (RP) for spatial resolution improvement by Si tip shape control. The major change of the new design is wedge-like probe formation rather than pyramidal shape, which results in the reduced removal of high doped volume near the slanting surface of probe. Therefore, we could reduce spatial resolution deterioration mainly due to low doped volume of the sloped surface. The newly designed resistive probe (RP) showed about 40 nm transition (which is assumed to be a distance taken signal rising from 10% to 90% of saturation level) in bit writing and reading experiment on the real ferroelectric media, which is a few times improvement compared to the original one.