Guohao Yu

1/f noise in MgO double-barrier magnetic tunnel junctions

Low frequency noise has been investigated in MgO double-barrier magnetic tunnel junctions (DMTJs) with tunneling magnetoresistance (TMR) ratios up to 250% at room temperature. The noise shows a 1/f frequency spectrum and the minimum of the noise magnitude parameter is 1.2×10−10 μm2 in the parallel state for DMTJs annealed at 375 °C. The bias dependence of noise and TMR suggests that DMTJs with MgO barriers can be useful for magnetic field sensor applications.

Studies on High-Voltage GaN-on-Si MIS-HEMTs Using LPCVD Si 3 N 4 as Gate Dielectric and Passivation Layer

This paper investigates the performance of AlGaN/gallium nitride (GaN) MIS high electron mobility transistors (MIS-HEMTs). The gate dielectric layer and the surface passivation layer are formed by the low-pressure chemical vapor deposition (LPCVD) Si₃N₄. The LPCVD-Si₃N₄ MIS-HEMTs exhibit a high breakdown voltage (BV) of 1162 V at I_DS = 1 μA/mm, a low OFF-state leakage of 7.7 × 10^-12 A/mm, and an excellent ON/OFF-current ratio of ~10¹¹. Compared with the static ON-resistance of 2.88 mΩ · cm², the dynamic ON-resistance after high OFF-state drain bias stress at 600 V only increases to 4.89 mΩ · cm². The power device figure of merit = BV²/R_ON.sp is calculated to be 469 MW · cm^-2. The LPCVD-Si₃N₄/GaN interface state density is in the range of (1.4-5.3) × 10¹³ eV^-1 cm^-2 extracted by the conventional conductance method. Finally, the gate insulator degradation of GaN-based MIS-HEMTs is analyzed by time-dependent dielectric breakdown test. The lifetime is extrapolated to 0.01% of failures after ten years at 300 K by fitting the data with a power law to a gate voltage of 10.1 V.

Dynamic Characterizations of AlGaN/GaN HEMTs With Field Plates Using a Double-Gate Structure

A novel double-gate AlGaN/GaN HEMT, in which an additional top-gate covers the adjacent regions of a normal gate, was proposed and fabricated for the first time to compare the dynamic characteristics of AlGaN/GaN HEMTs with a source field plate (SFP) and a gate field plate (GFP). During the dynamic characterization, the device was configured in two operation modes: One is the SFP mode with the top gate biased at 0 V, and the other is the GFP mode applying the gate pulse signal on the top gate at the same time. Compared with an AlGaN/GaN HEMT without field plates, both GFP and SFP much improve the dynamic performances. Compared with the SFP, the GFP shows better dynamic performances with a ~ 34% reduction of switch-on delay time and ~ 6% reduction of dynamic on-state resistance. Studying the dynamic characteristics and applying negative voltage on the top gate during the off state, the mechanism differences between the GFP and the SFP are discussed in detail.

Normally Off AlGaN/GaN MIS-High-Electron Mobility Transistors Fabricated by Using Low Pressure Chemical Vapor Deposition Si 3 N 4 Gate Dielectric and Standard Fluorine Ion Implantation

This letter presents a fabrication technology of enhancement-mode (E-mode) AlGaN/GaN metal-insulator- semiconductor high-electron mobility transistors (MIS-HEMTs) using 10 keV fluorine ion implantation. An 8 nm low-pressure chemical vapor deposition silicon nitride layer was deposited on the AlGaN as gate dielectric and energy-absorbing layer that slows down the high energy (10 keV) fluorine ions to reduce the implantation damage. The E-mode MIS-HEMTs exhibit a threshold voltage as high as +3.3 V with a maximum drain current over 200 mA/mm (250 mA/mm for depletion-mode MIS-HEMTs) and a high on/off current ratio of 109. Meanwhile, the E-mode MIS-HEMT dynamic RON is only 1.53 times larger than the static RON after off-state VDS stress of 500 V.

Normally-off p-GaN/AlGaN/GaN high electron mobility transistors using hydrogen plasma treatment

In this letter, we report a method by introducing hydrogen plasma treatment to realize normally-off p-GaN/AlGaN/GaN HEMT devices. Instead of using etching technology, hydrogen plasma was adopted to compensate holes in the p-GaN above the two dimensional electron gas (2DEG) channel to release electrons in the 2DEG channel and form high-resistivity area to reduce leakage current and increase gate control capability. The fabricated p-GaN/AlGaN/GaN HEMT exhibits normally-off operation with a threshold voltage of 1.75 V, a subthreshold swing of 90 mV/dec, a maximum transconductance of 73.1 mS/mm, an ON/OFF ratio of 1 × 107, a breakdown voltage of 393 V, and a maximum drain current density of 188 mA/mm at a gate bias of 6 V. The comparison of the two processes of hydrogen plasma treatment and p-GaN etching has also been made in this work.

Improved tunneling magnetoresistance in (Ga,Mn)As/AlOx/CoFeB magnetic tunnel junctions

We fabricated (Ga,Mn)As/AlOx/Co40Fe40B20 magnetic tunnel junctions with ferromagnetic semiconductor/insulator/ferromagnetic metal (S/I/F) structure. The treatments of pre-annealing and post-plasma cleaning on the (Ga,Mn)As film were introduced before the growth of the subsequent layers. A high tunneling magnetoresistance (TMR) ratio of 101% is achieved at 2 K, and the spin polarization of (Ga,Mn)As, P = 56.8%, is deduced from Julliere’s formula. The improved TMR ratio is primarily due to the improved magnetism of (Ga,Mn)As layer by low-temperature annealing and cleaned interface between (Ga,Mn)As and AlOx attained by subsequent plasma cleaning process.

Electric-field control of CoFeB/IrMn exchange bias system

The electrically controlled spintronic devices based on magnetism at the interface is a key challenge today. We have studied the top sub./CoFeB/IrMn and bottom sub./IrMn/CoFeB pinned exchange bias systems as a function of electric field at room temperature deposited on the (011)-Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT) piezoelectric substrate. It was found that the electric-field tuning of exchange bias was very small although both the structures show good regular magnetoelectric coupling. We propose an alternative way to control the exchange bias via electric field in the multilayered structure by inserting a metallic spacer layer between exchange bias bilayer and bottom free magnetic layer, i.e., PMN-PT/CoFeB/Cu/CoFeB/IrMn. We successfully tuned the exchange bias of such multilayer structure as function of electric field at room temperature. Our results show a step forward in utilizing electrically controlled multiferroic systems for practical applications.

AlGaN/GaN metal-insulator-semiconductor high electron mobility transistors with reduced leakage current and enhanced breakdown voltage using aluminum ion implantation

This letter has studied the performance of AlGaN/GaN metal-insulator-semiconductor high electron mobility transistors on silicon substrate with GaN buffer treated by aluminum ion implantation for insulating followed by a channel regrown by metal–organic chemical vapor deposition. For samples with Al ion implantation of multiple energies of 140 keV (dose: 1.4 × 1014 cm−2) and 90 keV (dose: 1 × 1014 cm−2), the OFF-state leakage current is decreased by more than 3 orders and the breakdown voltage is enhanced by nearly 6 times compared to the samples without Al ion implantation. Besides, little degradation of electrical properties of the 2D electron gas channel is observed where the maximum drain current IDSmax at a gate voltage of 3 V was 701 mA/mm and the maximum transconductance gmmax was 83 mS/mm.

Fabrication of normally-off AlGaN/GaN metal–insulator–semiconductor high-electron-mobility transistors by photo-electrochemical gate recess etching in ionic liquid

We characterized an ionic liquid (1-butyl-3-methylimidazolium nitrate, C8H15N3O3) as a photo-electrochemical etchant for fabricating normally-off AlGaN/GaN metal–insulator–semiconductor high-electron-mobility transistors (MIS-HEMTs). Using the ionic liquid, we achieved an etching rate of ~2.9 nm/min, which is sufficiently low to facilitate good etching control. The normally-off AlGaN/GaN MIS-HEMT was fabricated with an etching time of 6 min, with the 20 nm low-pressure chemical vapor deposition (LPCVD) silicon nitride (Si3N4) gate dielectric exhibiting a threshold voltage shift from −10 to 1.2 V, a maximum drain current of more than 426 mA/mm, and a breakdown voltage of 582 V.

Off-state electrical breakdown of AlGaN/GaN/Ga(Al)N HEMT heterostructure grown on Si(111)

Electrical breakdown characteristics of AlxGa1−xN buffer layers grown on Si(111) are investigated by varying the carbon concentration ([C]: from ∼1016 to 1019 cm−3), Al-composition (x = 0 and 7%), and buffer thickness (from 1.6 to 3.1 μm). A quantitative relationship between the growth conditions and carbon concentration ([C]) is established, which can guide to grow the Ga(Al)N layer with a given [C]. It is found that the carbon incorporation is sensitive to the growth temperature (T) (exponential relationship between [C] and 1/T) and the improvement of breakdown voltage by increasing [C] is observed to be limited when [C] exceeding 1019 cm−3, which is likely due to carbon self-compensation. By increasing the highly resistive (HR) Al0.07Ga0.93N buffer thickness from 1.6 to 3.1 μm, the leakage current is greatly reduced down to 1 μA/mm at a bias voltage of 1000 V.