Hyun-Ick Cho

Expression of brain-derived neurotrophic factor in rat dorsal root ganglia, spinal cord and gracile nuclei in experimental models of neuropathic pain

Chronic constriction injury of the sciatic nerve and lumbar L5 and L6 spinal nerve ligation provide animal models for pain syndromes accompanying peripheral nerve injury and disease. In the present study, we evaluated changes in brain-derived neurotrophic factor (BDNF) immunoreactivity in the rat L4 and L5 dorsal root ganglia (DRG) and areas where afferents from the DRG terminates (the L4/5 spinal cord and gracile nuclei) in these experimental models of neuropathic pain. Chronic constriction injury induced significant increase in the percentage of small, medium and large BDNF-immunoreactive neurons in the ipsilateral L4 and L5 DRG. Following spinal nerve ligation, the percentage of large BDNF-immunoreactive neurons increased significantly, and that of small BDNF-immunoreactive neurons decreased markedly in the ipsilateral L5 DRG, while that of BDNF-immunoreactive L4 DRG neurons of all sizes showed marked increase. Both chronic constriction injury and spinal nerve ligation induced significant increase in the number of BDNF-immunoreactive axonal fibers in the superficial and deeper laminae of the L4/5 dorsal horn and the gracile nuclei on the ipsilateral side. Considering that BDNF may modulate nociceptive sensory inputs and that injection of antiserum to BDNF significantly reduces the sympathetic sprouting in the DRG and allodynic response following sciatic nerve injury, our results also may suggest that endogenous BDNF plays an important role in the induction of neuropathic pain after chronic constriction injury and spinal nerve ligation. In addition, the increase of BDNF in L4 DRG may contribute to evoked pain which is known to be mediated by input from intact afferent from L4 DRG following L5 and L6 spinal nerve ligation.

Demonstration of Rashba spin splitting in GaN-based heterostructures

The circular photogalvanic effect, induced by infrared radiation, has been observed in (0001)-oriented n‐GaN low dimensional structures. The photocurrent changes sign upon reversing the radiation helicity demonstrating the existence of spin splitting of the conduction band in k space in this type of materials. The observation suggests the presence of a sizeable Rashba type of spin splitting, caused by the built-in asymmetry at the AlGaN∕GaN interface.

A normally off GaN n-MOSFET with Schottky-barrier source and drain on a Si-auto-doped p-GaN/Si

We have fabricated an enhancement-mode n-channel Schottky-barrier-MOSFET (SB-MOSFET) for the first time on a high mobility p-type GaN film grown on silicon substrate. The metal contacts were formed by depositing Al for source/drain contact and Au for gate contact, respectively. Fabricated SB-MOSFET exhibited a threshold voltage of 1.65 V, and a maximum transconductance(g/sub m/) of 1.6 mS/mm at V/sub DS/=5V, which belongs to one of the highest value in GaN MOSFET. The maximum drain current was higher than 3 mA/mm and the off-state drain current was as low as 3 nA/mm.

Enhancement of InGaN-Based Vertical LED With Concavely Patterned Surface Using Patterned Sapphire Substrate

To improve the external quantum efficiency, we have proposed a new method utilizing surface roughening of vertical-type light-emitting diodes (VT-LEDs) fabricated on hemispherical patterned sapphire substrate by using a laser lift-off technique. The advantages of this method are simple and reproducible in transferring the well-defined patterns on sapphire into GaN layer. The VT-LED with concavely patterned surface showed a nearly twofold increase in the output power compared to the normal planar surface. This improvement in the VT-LED performances is attributed to the increase in the escaping probability of photons from the LED surface.

Formation of Low-Resistivity Nickel Silicide with High Temperature Stability from Atomic-Layer-Deposited Nickel Thin Film

Nickel silicide (NiSi) was formed by annealing a uniform low-resistivity nickel (Ni) film deposited by atomic layer deposition (ALD). A Ni film as-deposited at 220 °C exhibited the lowest sheet resistance of 18 Ω/sq. comparable to that of the film obtained by physical vapor deposition, even though it contained a significant amount of carbon from the metalorganic precursor. It is believed that the carbon is uniformly distributed in the film by partly forming a weak Ni3C phase which eliminates other crystalline defects in the film and hence lowers the resistance of the film. However, the carbon was not observed at the Ni/Si interface and in the silicon bulk except at the film surface after the annealing to form silicide. The existence of carbon at the surface of the film causes the film to maintain a low-resistivity NiSi phase up to 800 °C, without the carbon at the surface, the phase of film is changed to the high-resistivity nickel disilicide (NiSi2) at such a high temperature. The deposition of Ni by ALD and the formation of low-resistivity NiSi with an increased temperature stability can be useful in fabricating advanced devices, such as nanometer scale complementally metal–oxide silicons (CMOSs) or three-dimensional (3-D) MOS devices like Fin-type field-effect transistors (Fin-FETs).

Effectiveness of Self-Carbon and Titanium Capping Layers in NiSi formation with Ni Film Deposited by Atomic Layer Deposition

We firstly deposit a Ni film, directly after removing the native oxide, by atomic layer deposition (ALD) using a N2-hydroxyhexafluoroisopropyl-N1 (Bis-Ni) precursor, H2 as the reactant gas and Ar purging gas at 220 °C at a deposition rate of 1.25 A/cycle. The as-deposited Ni and Ni3C films exhibited sheet resistances of 5 Ω/ (sample B) and 18 Ω/ (sample A), respectively. The formation of a Ni3C phase was easily controlled by varying the flow rate of the H2 reactant as above gas. A rapid thermal process (RTP) was then performed in a nitrogen ambient to form NiSi at different temperatures from 400 to 900 °C. We estimated the process window temperature for the formation of low-resistance NiSi to be between 600 and 800 °C for self-carbon and Ti capping layers, as below while in the case of only Ni deposition the process window temperature changes to 700 to 800 °C. The respective sheet resistances of the films were changed to 3 Ω/ (sample B) and 4 Ω/ (sample A) after silicidation. The reaction between Ni and Si could be increased by the self-carbon and Ti capping layers due to a decrease in the oxidation contamination and impurity incorporation in the Ni film during the silicidation process. This self-carbon capping layer is formed by the carbon-containing Ni3C phase, which segregates to the surface during the annealing process and forms a relatively thick surface layer. Additionally, this layer also protects the surface from oxygen contamination. The deposition of Ni by ALD and the improved formation of the low-resistance NiSi with increased temperature stability will be useful in the fabrication of advanced devices, such as nano meter-scale complementary metal oxide semiconductor (CMOS) or three-dimensional (3-D) devices.

Enhanced Electrical Characteristics of AlGaN/GaN Heterostructure Field-Effect Transistor with p-GaN Back Barriers and Si Delta-Doped Layer

We present the electrical characteristics of an AlGaN/GaN/p-GaN heterostructure field-effect transistor (HFET) with a Si delta-doped layer. The p-GaN layer greatly improves buffer isolation (between neighboring mesas) in the AlGaN/GaN HFET and leads to effective carrier confinement. The Si delta-doped layer compensates not only the carrier depletion caused by the formation of a pn junction, but also even causes an increase in two-dimensional electron gas (2DEG) density. The proposed AlGaN/GaN HFET shows greatly improved electrical characteristics such as high drain current density and transconductance and low buffer and gate leakage currents compared with those of conventional AlGaN/GaN HFETs.

Thermal and electrical properties of 5-nm-thick TaN film prepared by atomic layer deposition using a pentakis(ethylmethylamino)tantalum precursor for copper metallization

We first report on the thermal stability and electrical properties of 5 nm-thick TaN films prepared by atomic layer deposition (ALD) using pentakis(ethylmethylamino)tantalum (PEMAT) and ammonia. The deposition rate of the ALD-TaN process was about ~0.067 nm per cycle in a temperature range between 200 and 250 °C, which is a typical feature of ALD process. In cross sectional transmission electron microscopy (TEM) images, the deposited TaN films exhibited a very smooth and uniform interface. The thermal stabilities of these films were tested by depositing a Cu film of 200 nm thickness on a TaN layer and subsequently performing annealing for 30 min by varying the temperature from 300 to 800 °C in N2 ambient. The high and low-frequency capacitance–voltage (C–V) and breakdown characteristics of a Cu/TaN/SiO2/Si capacitor showed that the barrier properties of thin TaN films against Cu diffusion are inhibited above 500 °C, which is considerably lower than the inhibition temperature estimated by four-point probe or X-ray diffraction (XRD) measurement.

Expression of monocyte chemoattractant protein-1 and its induction by tumor necrosis factor receptor 1 in sensory neurons in the ventral rhizotomy model of neuropathic pain

The expression and role of monocyte chemoattractant protein-1 (MCP-1) in the rat dorsal root ganglion (DRG) and spinal cord was evaluated in the lumbar 5 ventral rhizotomy (L5 VR) model of neuropathic pain. MCP-1 protein expression in the L4/L5 DRG neurons following L5 VR peaked after 3 days, and then declined. Immunohistochemistry showed that no MCP-1 immunoreactivity was observed in the spinal cord after L5 VR, while enzyme-linked immunosorbent assay (ELISA) revealed a small but significant increase in MCP-1 protein content. L5 VR resulted in robust and prolonged mechanical allodynia and thermal hyperalgesia. Administration of anti-MCP-1 neutralizing antibody before and at early time points after L5 VR resulted in a significant attenuation of mechanical allodynia and thermal hyperalgesia, while post-treatment had a weaker effect on established neuropathic pain. Extensive colocalization of tumor necrosis factor receptor 1 (TNFR1) and MCP-1 was observed in the L5 DRG following L5 VR, and treatment with TNFR1 antisense oligonucleotide reduced L5 VR-induced MCP-1 expression in L5 DRG neurons and neuropathic pain behaviors. MCP-1/chemokine (C-C motif) receptor 2 signaling has been proposed as a major regulator of macrophage trafficking. In contrast to the effect on pain behaviors, however, intrathecal administration of anti-MCP-1 neutralizing antibody had no effect on the L5 VR-induced increase in ED-1-immunoreactive macrophages in the L5 DRG and the distal stump of the transected L5 ventral root. These data indicate that increased MCP-1 in DRG neurons might participate in the initiation, rather than the maintenance, of neuropathic pain induced by L5 VR. Furthermore, increased MCP-1 in the DRG is induced by TNF-α/TNFR1 and has no effect on the infiltration of macrophages into the DRG following L5 VR.

Device Characteristics of AlGaN/GaN MIS-HFET Using Al2O3?HfO2 Laminated High-k Dielectric

This is the first report on an AlGaN/GaN metal-insulator-semiconductor-heterostructure field-effect transistors (MIS-HFET) with an Al2O3–HfO2 laminated high-k dielectric, deposited by plasma-enhanced atomic layer deposition (PEALD). Based on capacitance-voltage measurements, the dielectric constant of the deposited Al2O3–HfO2 laminated layer was estimated to be 15. The fabricated MIS-HFET with a gate length of 1.2 µm exhibited a maximum drain current of 500 mA/mm and a maximum transconductance of 125 mS/mm. The gate leakage current was at least 4 orders of magnitude lower than that of the reference HFET. The pulsed current-voltage curve revealed that the Al2O3–HfO2 laminated dielectric effectively passivated the surface of the device.