Jinki Hong

Magnetic-field-controlled reconfigurable semiconductor logic

Logic devices based on magnetism show promise for increasing computational efficiency while decreasing consumed power. They offer zero quiescent power and yet combine novel functions such as programmable logic operation and non-volatile built-in memory. However, practical efforts to adapt a magnetic device to logic suffer from a low signal-to-noise ratio and other performance attributes that are not adequate for logic gates. Rather than exploiting magnetoresistive effects that result from spin-dependent transport of carriers, we have approached the development of a magnetic logic device in a different way: we use the phenomenon of large magnetoresistance found in non-magnetic semiconductors in high electric fields. Here we report a device showing a strong diode characteristic that is highly sensitive to both the sign and the magnitude of an external magnetic field, offering a reversible change between two different characteristic states by the application of a magnetic field. This feature results from magnetic control of carrier generation and recombination in an InSb p–n bilayer channel. Simple circuits combining such elementary devices are fabricated and tested, and Boolean logic functions including AND, OR, NAND and NOR are performed. They are programmed dynamically by external electric or magnetic signals, demonstrating magnetic-field-controlled semiconductor reconfigurable logic at room temperature. This magnetic technology permits a new kind of spintronic device, characterized as a current switch rather than a voltage switch, and provides a simple and compact platform for non-volatile reconfigurable logic devices.

Gamma-Ray Response of Semi-Insulating CdMnTe Crystals

Semi-insulating Cd0.9Mn0.1Te:In crystals are grown by vertical Bridgman method. The segregation coefficient of Mn in CdTe is nearly 1 so that all the CdMnTe samples obtained from one ingot have nearly a same Mn composition. Also sulfur-based passivation effectively prevent the formation of Te-oxide but large amount of Mn exist as a MnO on the CdMnTe surface. The resistivity of CdMnTe samples are low 1010 Omegacm and well resolved 241Am gamma peaks are seen for all detectors. The difference in spectral response can be attributed to the effect of excess Te and conductivity change due to over-compensation induced by indium segregation in CdMnTe. The mobility-lifetime products evaluated from the dependence of peak location on the bias voltage are 1 times 10-3 cm2/V. The higher mobility-lifetime products in our CdMnTe crystals than other previous reports are assumed due to minimization of impurity contents in MnTe by several zone refining process.

Local Hall effect in hybrid ferromagnetic/semiconductor devices

The authors have investigated the magnetoresistance of ferromagnet-semiconductor devices in an InAs two-dimensional electron gas system in which the magnetic field has a sinusoidal profile. The magnetoresistance of their device is large. The longitudinal resistance has an additional contribution which is odd in applied magnetic field. It becomes even negative at low temperature where the transport is ballistic. Based on the numerical analysis, they confirmed that their data can be explained in terms of the local Hall effect due to the profile of negative and positive field regions. This device may be useful for future spintronic applications.

Defect levels of semi-insulating CdMnTe:In crystals

Using photoluminescence (PL) and current deep-level transient spectroscopy (I-DLTS), we investigated the electronic defects of indium-doped detector-grade CdMnTe:In (CMT:In) crystals grown by the vertical Bridgman method. We similarly analyzed CdZnTe:In (CZT:In) and undoped CdMnTe (CMT) crystals grown under the amount of same level of excess Te and/or indium doping level to detail the fundamental properties of the electronic defect structure more readily. Extended defects, existing in all the samples, were revealed by synchrotron white beam x-ray diffraction topography and scanning electron microscopy. The electronic structure of CMT is very similar to that of CZT, with shallow traps, A-centers, Cd vacancies, deep levels, and Te antisites. The 1.1-eV deep level, revealed by PL in earlier studies of CZT and CdTe, were attributed to dislocation-induced defects. In our I-DLTS measurements, the 1.1-eV traps showed different activation energies with applied bias voltage and an exponential dependence on the tra...

Energy-gap Dependence on the MN mole fraction and temperature in CdMnTe crystal

We measured the dependence of the energy gap in Bridgman-grown Cd1−xMnxTe crystals, 0≤x≤0.25, on the Mn mole fraction and temperatures from 40 to 300 K. We determined the Mn mole fraction and energy gap, respectively, from electron probe microanalysis and near-infrared Fourier-transform infrared transmission spectra. The energy gap increased linearly with an increase in the Mn content in the crystal and with a decrease in temperature. We formulated new equations from these experimental results, wherein we expressed the energy gap as a function of Mn mole fraction and temperature. Also, we compare our findings with published results.We measured the dependence of the energy gap in Bridgman-grown Cd1−xMnxTe crystals, 0≤x≤0.25, on the Mn mole fraction and temperatures from 40 to 300 K. We determined the Mn mole fraction and energy gap, respectively, from electron probe microanalysis and near-infrared Fourier-transform infrared transmission spectra. The energy gap increased linearly with an increase in the Mn content in the crystal and with a decrease in temperature. We formulated new equations from these experimental results, wherein we expressed the energy gap as a function of Mn mole fraction and temperature. Also, we compare our findings with published results.

Improved Carrier-Transport Properties of Passivated Cdmnte Crystals

By analyzing photoconductive decay curves, we compared the surface recombination velocities of semi-insulating CdMnTe:In crystals grown by the vertical Bridgman method with or without surface passivation. Sulfur passivation effectively prevents the formation of a conductive Te oxide layer on the CdMnTe surface and reduces the surface recombination velocities by about one third. We demonstrated, from IR observations of the distribution maps of Te precipitates, that their configuration affects the anomalous photoconductive decay curves and the gamma-ray spectrum in some areas of the CdMnTe crystal. Notably, not only the size but also the spatial configuration of the Te precipitates modulates the carrier-transport properties.

Magnetic bead counter using a micro-Hall sensor for biological applications

Micro-Hall sensors have been fabricated, and various numbers of micron-size magnetic beads have been placed within the sensor area. The Hall resistances measured at room temperature are found to be proportional to the number of the beads, and are in good agreement with the numerically simulated results presented in this study. Our sensors are designed to measure the number of beads between zero and full-scale signals for a given number range of interest. The effects of miniaturizing the beads and sensors to nanoscale are also discussed.

The comparison on the performance of a gamma-ray spectrometer with the variation of Pt(Au)/CdZnTe/Pt(Au) interface

Abstract Cd1−xZnxTe grown by high-pressure Bridgman method (HPB) has been known to be a good candidate for gamma-ray detection material. The metal contacting conditions are very important for the performance of gamma-ray detector. Therefore, it is essential to find proper contact technology. For these purposes, various contact methods (electroless deposition, thermal vacuum evaporation) and materials (Au, Pt) are attempted. From the measurement of current–voltage characteristics, gamma-ray spectroscopy and auger electron spectroscopy (AES), the physical properties of metal contact on the CdZnTe are analyzed and contact methods compared with one another. Electroless Pt deposition is found to have better properties than others in many respects.

An electrical switching device controlled by a magnetic field-dependent impact ionization process

An abrupt change of conductance at a threshold magnetic field was observed in a device consisting of a nonmagnetic narrow-gap semiconductor. The conductance varies more than 25 times as the magnetic field increases. The threshold magnetic field can be tuned using a bias voltage from zero to several hundred Gauss. This large magnetoconductance effect is caused by the magnetic field-dependent impact ionization process. A theoretical model is proposed, and calculations based on this model simulate the experimental results closely. This device may be a good candidate for an electrical switching device controlled by a magnetic field.

Effect of oxidizing the ferromagnetic electrode in magnetic tunnel junctions on tunneling magnetoresistance

The ferromagnetic layer in magnetic tunnel junctions (MTJs) was oxidized with varying O2 concentrations, and the corresponding effect on spin-dependent transport was studied. As expected from our previous results for MTJs with an over-oxidized AlOx tunnel barrier, a partially oxidized ferromagnetic layer plays an important role in spin-dependent transport. As the temperature is lowered, the junction resistance increases dramatically, and the tunneling magnetoresistance (TMR) is strongly suppressed. Increasing the O2 concentration enhances the increase of resistance and suppression of TMR. This work supports our previous conclusion that oxidizing the ferromagnetic layer generates localized magnetic moments, which act as a scattering center for spin-polarized tunneling electrons.