Shuiming Li

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.

Normally OFF GaN-on-Si MIS-HEMTs Fabricated With LPCVD-SiN x Passivation and High-Temperature Gate Recess

Low-current-collapse normally OFF GaN-on-Si MIS high-electron-mobility transistors (MIS-HEMTs) are fabricated with low-pressure chemical-vapor-deposited SiN_x (LPCVD-SiN_x) passivation and high-temperature low-damage gate-recess technique. The high-thermal-stability LPCVD-SiN_x enables a passivation-prior-to-ohmic process strategy and effectively suppresses deep states at the passivation/HEMT interface. The fabricated MIS-HEMTs feature a high V_TH of +0.85 V at the drain current of 1 μA/mm and a remarkable ON/OFF current ratio of 10¹⁰ while reduced dynamic ON-resistance as compared to plasma-enhanced chemical-vapor-deposited SiO₂ passivation. High field-effect channel mobility of 180 cm²/V·s is achieved, leading to a high maximum drain current density of 663 mA/mm.

Oxygen vacancy formation, crystal structures, and magnetic properties of three SrMnO3−δ films

The crystal structures and magnetic properties of the 40u2009nm brownmillerite SrMnO2.5 film, perovskite SrMnO3-δ film, and mixed-phase film have been systematically investigated. The features of the oxygen vacancy ordering superstructure in the brownmillerite SrMnO2.5 film are observed from HRSTEM as follows: the dark stripes with a periodicity of four (110) planes of the cubic perovskite appearing at an angle of 45° with the substrate-film interface and extra reflection spots in fast Fourier transformation patterns along the (001) plane. When annealing the brownmillerite SrMnO2.5 film under higher oxygen pressure, the top portion undergoes structure transition into perovskite SrMnO3-δ as seen in the mixed-phase film consisting of the perovskite SrMnO3-δ phase dominating at the top part and the brownmillerite SrMnO2.5 phase dominating at the bottom part. The magnetic properties and Mn valences of the brownmillerite SrMnO2.5 film indicate that this film, similar to the bulk, is antiferromagnetic with TN at 37...

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 140u2009keV (dose: 1.4u2009×u20091014 cm−2) and 90u2009keV (dose: 1u2009×u20091014u2009cm−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 3u2009V was 701u2009mA/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.

Abnormal magnetic ordering and ferromagnetism in perovskite ScMnO3 film

Bulk multiferroic ScMnO3 is the stable hexagonal phase, and it is very difficult to prepare its perovskite orthorhombic phase even under high pressure. We fabricated the orthorhombic ScMnO3 thin film by pulsed laser deposition through suitable substrate LaAlO3 and found that nano-scale twin-like domains are naturally formed in the thin film. Magnetic properties of the orthorhombic ScMnO3 thin films show that, besides normal antiferromagnetic ordering at 47u2009K, an anomalous magnetic transition occurs at 27u2009K for 60u2009nm film and at 36u2009K for 150u2009nm film only along the c-axis, which is absent in the ab-plane. Moreover, the second magnetic transition for both films is suppressed when the applied field increases from 1 kOe to 10 kOe. In addition, the ferromagnetism shows up in both films at 10u2009K, and saturation magnetization increases dramatically in 60u2009nm film compared with 150u2009nm film. We propose that the second magnetic transition might be more of lattice strain effect and also related to magnetism-induced fer...

Direct observation of magnetic domain evolution in the vicinity of Verwey transition in Fe3O4 thin films

We report a direct observation of magnetic domain evolution near the Verwey transition (TV) in Fe3O4 films. We found the stripe domains in the Fe3O4/Mg2TiO4 film while the irregular domains in the Fe3O4/MgO film and the similar characters of magnetic domains in the vicinity of TV for both samples: the bigger domain size and the higher contrast of the phase signal below TV and the more disordered domain images at TV. Remarkably, the magnetic behaviors can be well understood and the domain-wall energy and the demagnetizing energy can be calculated from the magnetic domains near TV in the Fe3O4/Mg2TiO4 film. Our work presents a demonstration of the low-temperature magnetic domains and gives a new perspective to understand the Verwey transition in Fe3O4 thin films.

Stress evolution in AlN and GaN grown on Si(111): experiments and theoretical modeling

We introduce a temperature dependent anisotropic model for the stresses in gallium nitride (GaN) and aluminum nitride (AlN) films grown on Si(111) substrates and their epiwafer bow effects caused by thermal mismatch between the film and substrate. The model is verified by Raman scattering experiments with carefully prepared samples. The stresses analyzed from Raman frequency shifts in experiments show excellent agreement with the stresses from finite element modeling simulations. The interaction force mechanisms and the impact factors are compared. The analysis provides an insight in understanding the defect behaviors in film growth. Our model could be useful in the evaluation of the residual stresses and deformations in film growth control, post thermal process in device manufacture, packaging, and reliability estimation.

Single orthorhombic b axis orientation and antiferromagnetic ordering type in multiferroic CaMnO3 thin film with La0.67Ca0.33MnO3 buffer layer

The detailed crystal structure and antiferromagnetic properties of a 42u2009nm thick CaMnO3 film grown on a LaAlO3 substrate with a 9u2009nm La0.67Ca0.33MnO3 buffer layer have been investigated. Compared with a CaMnO3 film directly grown on a LaAlO3 substrate, only one kind of orthorhombic b axis orientation along the [100] axis of the substrate is observed in the CaMnO3 film with a La0.67Ca0.33MnO3 buffer layer. To determine the antiferromagnetic ordering type of our CaMnO3 film with a buffer layer, the first-principles calculations were carried out with the results, indicating that the CaMnO3 film, even under a tensile strain of 1.9%, is still a compensated G-type antiferromagnetic order, the same as the bulk. Moreover, the exchange bias effect is observed at the interface of the CaMnO3/La0.67Ca0.33MnO3 film, further confirming the antiferromagnetic ordering of the CaMnO3 film with a buffer layer. In addition, it is concluded that the exchange bias effect originates from the spin glass state at the La0.67Ca0.33...