Y.K. Park

High efficiency organic light-emitting devices with Al/NaF cathode

An organic light-emitting device (OLED) with an Al/NaF (1.5 nm) cathode exhibited a highly enhanced performance comparable to the one with the best Al/LiF cathode. This suggests that NaF can be an alternative to commonly used LiF interlayer for improved OLED performance. Photoelectron spectroscopy studies revealed that the intensity of the gap states and the amount of the valence band shift at Al/NaF/Alq3 interface surpassed those of Al/LiF/Alq3 interface, suggesting that the observed performance improvement is directly related with these features.

Localization accuracy of single current dipoles from tangential components of auditory evoked fields

We investigated the localization accuracy of single current dipoles from the tangential components of auditory evoked fields. The tangential fields were measured using planar gradiometers arranged in a way so as to detect two orthogonal field components parallel to a flat plane. Field responses to 1 kHz pure tones were recorded and equivalent current dipoles (ECDs) of N1m peak were estimated based on a locally fitted spherical conductor model. As a measure of localization accuracy, the standard deviation of the coordinates of the ECDs of N1m was obtained from repeated measurements for one subject. The estimated ECDs had a standard deviation of 5.5 mm and their mean location was at the supratemporal plane in the sylvian fissure on the MR image of the subject. In order to investigate the contribution of various errors to the localization accuracy, simulations using a sphere model and experiments using a realistically shaped skull phantom were performed. It was found that the background noise, which consisted of instrumental noise and spontaneous brain fields, was the main source of the errors that could explain the observed standard deviation. Further, the amount of systematic error was much less than the standard deviation due to the background noise. These results suggest that the volume currents in a non-spherical conductor shape such as the temporal region do not produce substantial errors in the localization of current dipoles from tangential components of auditory evoked fields.

Noise characteristics of double relaxation oscillation superconducting quantum interference devices with reference junction

We fabricated double relaxation oscillation SQUIDs (DROSs) with a reference junction and investigated their noise characteristics. Because of the large flux-to-voltage transfer of 1-3 mV/0, the room-temperature dc preamplifier could readout SQUID output voltage directly and the contribution of the preamplifier input noise to the total system noise was negligible. The flux noise of the DROS planar gradiometer is about 4 µ0 Hz-1/2 at 100 Hz, corresponding to a field gradient noise of 1 fT cm-1 Hz-1/2, when operated inside a moderately shielded room. Because of the large modulation voltage of about 100 µV, stable flux-locked-loop operation was possible against drift in the input offset voltage.

An integrated planar gradiometer based on a double relaxation oscillation SQUID

The design and performance of an integrated planar gradiometer based on a double relaxation oscillation SQUID (DROS) are presented. The DROS was made from hysteretic junctions and the devices were fabricated by a simple four-level process. The signal SQUID loop is a gradiometric type with two square holes connected in parallel and a reference junction is used instead of the reference SQUID. The high flux-to-voltage transfer coefficient of typically enabled direct readout by a simple electronics with a modest voltage noise. The pickup coil integrated on the same wafer as the SQUID consists of two planar coils connected in series and has a baseline of 30 mm. The overall size of the device is . The field gradient noise is in the white region and at 1 Hz.

Magnetic tunnel junctions with a tunnel barrier formed by N2O plasma

We investigated a magnetic tunnel junction (MTJ) with a tunnel barrier formed by N2O plasma. Compared with a MTJ with a tunnel barrier formed by conventional O2 plasma, the MTJ fabricated with N2O plasma shows much lower specific junction resistance and a comparably high tunneling magnetoresistance ratio. In particular, it was found that N2O plasma oxidation is quite important in the junction with a thin tunnel barrier, for which O2 plasma cannot be used. From x-ray photoelectron spectroscopy, we observed that N2O plasma oxidation leads to the slight nitridation of the Al2O3 layer and significantly reduces the oxidation of the bottom electrode, especially for a thin tunnel barrier. Thus, we conclude that the use of N2O plasma in forming the tunnel barrier is effective for achieving a low junction resistance and for minimizing the oxidation of the bottom electrode during plasma oxidation in MTJs.

Chemical phase transitions of the HfO2∕SiON∕Si nanolaminate by high-temperature thermal treatments in NO and O2 ambient

Temperature-dependent chemical phase transitions of the HfO2∕SiON∕Si(100) nanolaminate in O2 and NO ambient have been investigated using high-resolution photoemission spectroscopy. Hf4f, Si2p, O1s, and N1s photoemission spectra were measured after annealing the nanolaminate at a temperature between 750 and 1150°C. These spectra show that the chemical phase transitions of the nanolaminate strongly dependend on the ambient gases. The nanolaminate in an O2 ambient is stable below 750°C but the HfO2 and SiON layers dissociate by producing Hf silicides above 950°C. In contrast, the nanolaminate in NO ambient does not transit into Hf silicides up to 1050°C. Interestingly, the HfO2 and SiON layers transforms into HfOxNy and SiNx layers, respectively, with a high thermal stability, where the HfOxNy layers are a high-k dielectric material and the SiNx layers work as a barrier against both Si and O diffusion

Infiltration-reaction process for fabricating YBa2Cu3O7−δ superconductor

Abstract A new process for fabricating YBa2Cu3O7−δ superconductor was developed using infiltration-reaction process. A stack of Y2BaCuO5 and YBa2Cu3O7−δ compacts was heated at 1000–1200 °C and then cooled. By the infiltration-reaction process YBa2Cu3O7−δ superconductor where Y2BaCuO5 dispersions under 5 ωm were densely and evenly distributed has been fabricated without residual liquid phase. The resulting YBa2Cu3O7−δ superconductor showed large hysteresis and Jc from magnetization measurement.

Characteristics of the planar second-order high-Tc SQUID gradiometer

We have studied the planar-type single-layer second-order high-Tc SQUID gradiometer that detects the transverse field gradient, d2Bz/dx2. The gradiometer consists of three parallel-connected rectangular pickup loops that are directly coupled to a SQUID. The pickup loops are designed in such a way that the SQUID measures the difference in the differential screening currents of neighbouring pickup loops. The SQUID has either step-edge or bicrystal Josephson junctions. The gradiometer was made from a single layer of YBa2Cu3O7 film on a 1 cm × 1 cm chip. The response of the gradiometer was tested in various flux distributions generated by three identical multiturn film coils patterned on the same chip in parallel with each pickup loop. The balanced gradiometer responded sensitively only to the second-order field gradient. Reduction of the inductive coupling between loops improved the intrinsic balance.

Determination of transit time distribution and Rabi frequency by applying regularized inverse on Ramsey spectra

The authors report on a method to determine the Rabi frequency and transit time distribution of atoms that are essential for proper operation of atomic beam frequency standards. Their method, which employs alternative regularized inverse on two Ramsey spectra measured at different microwave powers, can be used for the frequency standards with short Ramsey cavity as well as long ones. The authors demonstrate that uncertainty in Rabi frequency obtained from their method is 0.02%.

Nitrogen effects on lowering specific junction resistance and suppressing Mn diffusion in a magnetic tunnel junction

We investigated the annealing effects of a magnetic tunnel junction on the Fe bottom pinned layer, of which the top surface was exposed to nitrogen plasma for a few tens of seconds prior to the deposition of the Al layer. The nitrogen-treated junction shows a lower magnetoresistance (MR) ratio and specific junction resistance (RA) than the untreated normal junction. However, after annealing at 230 °C, the MR ratio of the nitrogen-treated junction recovers to optimal values of the normal junction, while the RA remains lower than that of the normal junction. Furthermore, the nitrogen-treated junction shows less reduction of MR ratio and RA value with annealing at 270 °C for 3 h than the normal junction. From x-ray photoelectron spectroscopy and auger electron spectroscopy depth profiles, it is found that the nitrogen, which was initially at an interface between Al and Fe, diffuses into both the Al and FeMn layers after annealing at 230 °C. It seems that the nitrogen plays an important role in reducing Mn di...