Sung-Ho Hahm

Normally Off GaN MOSFET Based on AlGaN/GaN Heterostructure With Extremely High 2DEG Density Grown on Silicon Substrate

A normally off GaN MOSFET was proposed by utilizing an extremely high 2-D electron-gas density (> 1014 / cm2) at an AlGaN/GaN heterostructure as source and drain, which can be obtained by controlling the tensile stress accompanied with the growth of GaN on silicon substrate. The fabricated MOSFET with an Al2O3 gate insulator exhibited excellent device performance, such as a threshold voltage of 2 V, drain current of 353 mA/mm, extrinsic transconductance of 98 mS/mm, and field-effect mobility of 225 cm2/V·s.

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.

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).

Nanometer-scale gap control for low voltage and high current operation of field emission array

A new technique for nanometer-scale gap formation was proposed and investigated for the possibility in vacuum microelectronic device application. When the temperature is down at the end of oxidation, the compressive stress induced at the initially connected slim polysilicon patterns forces to be separated into two parts, yielding wedge-shaped cathode and anode tips for a field emission array with extremely small interelectrode distance. The oxidation time and the polysilicon thickness are the key parameters to control the interelectrode gap. The fabricated field emission devices show very high emission current with low turn-on voltage.

Fabrication of a lateral field emission triode with a high current density and high transconductance using the local oxidation of the polysilicon layer

We have proposed and fabricated a lateral type polysilicon field emission triode using conventional photolithography and a LOCOS (local oxidation of polysilicon) in a lateral direction. The techniques employed in this study are very simple and allow for good reproducibility both in shaping sharp electrode tips and controlling the short cathode-to-gate inter-electrode distance. The devices exhibit excellent electrical characteristics such as a low turn-on voltage of 14 V at V/sub GC/=0 V, a stable high emission anode current (I/sub A/) of /spl mu/A/triode over 90 hours with a relatively small gate leakage current (I/sub G/) of 0.23 /spl mu/A/triode (I/sub A//I/sub G//spl ges/400), and large transconductance (g/sub m/) of 57 /spl mu/S/5 triode at v/sub GC/=5 V and V/sub AC/=26 V. These superior field emission characteristics are believed to be due to an increased field enhancement effect which is related to the sharp cathode and gate tips shaped by the LOCOS as well as the high aspect ratio (tip height/radius of tip end) of the cathode tip.

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.

Effect of Al doping in GaN films grown by metalorganic chemical vapor deposition

The effect of Al doping in GaN films grown by metalorganic chemical vapor deposition was investigated using photoluminescence (PL), time-resolved PL, Hall measurements, and reciprocal space map. The electron mobility measured at 300 (150) K by a Hall measurement significantly increased from 170 (185) cm2/V s in the undoped sample to 524 (744) cm2/V s in the Al-doped sample grown with a molar flow rate ratio Al/(Al+Ga) of 0.056. When increasing the incorporation of Al in GaN, the band edge photoluminescence emission intensity was enhanced by about one order of magnitude compared to the undoped GaN. In addition, an increase in the decay lifetime of the GaN band edge emission was observed with the Al-doped GaN. In conclusion, the incorporation of only a small amount of Al in GaN was found to significantly reduce the point-defect-related electron scattering center associated with the compensating acceptors (Ga vacancies or their complexes) and nonradiative recombination centers, thereby improving the electric...

RuO 2 /GaN Schottky contact formation with superior forward and reverse characteristics

This is a first time report of a ruthenium oxide (RuO/sub 2/) Schottky contact on GaN. RuO/sub 2/ and Pt Schottky diodes were fabricated and their characteristics compared. When the RuO/sub 2/ Schottky contact was annealed at 500/spl deg/C for 30 min, the current-voltage (I-V) and capacitance-voltage (C-V) characteristics of the RuO/sub 2/ were dramatically improved. The annealed RuO/sub 2//GaN Schottky contact exhibited a reverse leakage current that was at least two or three orders lower in magnitude than that of the Pt/GaN contact along with a very large barrier height of 1.46 eV, which is the highest value ever reported for a GaN Schottky system.

Dual-wavelength sensitive AlGaN/GaN metal-insulator-semiconductor-insulator-metal ultraviolet sensor with balanced ultraviolet/visible rejection ratios

We proposed and fabricated a metal-insulator-semiconductor-insulator-metal type dual-wavelength sensitive UV sensor by using an AlGaN/GaN hetero-structure layer epitaxially grown on a sapphire substrate and a thin Al2O3 layer inserted between AlGaN and Ni Schottky electrodes to reduce dark current and improve the UV/visible rejection ratio. The proposed sensor shows high photo-responsive current to both UV wavelength regimes with a significantly improved UV/visible rejection ratio under the regime of the GaN-related UV response. Cut-off wavelengths can be controlled by changing the bias below and above 10 V.