Baoshun Zhang

Mechanical characterization of suspended GaN microstructures fabricated by GaN-on-patterned-silicon technique

The mechanical properties of high-quality suspended GaN microstructures fabricated by GaN-on-patterned-silicon technique are characterized. Micro-Raman scattering is used to study the stress distribution in the GaN microstructures, and the measured results show that the stress in GaN microbeams decreases 47% when the silicon underneath the microbeams is removed. Microbeam bending test is used to measure the Young’s modulus of GaN films grown on silicon (111) substrate, yielding a Young’s modulus of 330 GPa.

Enhancement-Mode Operation of Nanochannel Array (NCA) AlGaN/GaN HEMTs

In this letter, enhancement-mode (E-mode) AlGaN/ GaN high electron mobility transistors (HEMTs) were demon- strated based on lateral scaling of the 2-D electron gas channel using nanochannel array (NCA) structure. The NCA structure consists of multiple parallel channels with nanoscale width defined by electron-beam lithography and dry etching. Because of the improved gate control from the channel sidewalls and partially relaxed piezoelectric polarization, the fabricated 2 μm-gate-length NCA-HEMT with a nanochannel width of 64 nm showed a thresh- old voltage of +0.6 V and a higher extrinsic transconductance of 123 mS/mm, compared to -1.6 V and 106 mS/mm for the conventional HEMT with μm-scale channel width. The scaling of threshold voltages, peak transconductance, and gate leakage as a function of the nanochannel width were investigated. Small-signal RF performance of NCA-HEMTs were characterized for the first time and compared with those of conventional HEMTs.

Analysis of surface roughness in Ti/Al/Ni/Au Ohmic contact to AlGaN/GaN high electron mobility transistors

A mechanism of the formation of the bulges on the surface of Ti/Al/Ni/Au Ohmic contact in AlGaN/GaN high electron mobility transistors is proposed. According to the analysis of TEM images and corresponding electron dispersive x-ray spectra, the bulges were found to consist of Ni–Al alloy in the body and Au–Al alloy surrounding. We deduce that the bulges were formed due to Ni–Al alloy aggregation in some local areas during the rapid thermal annealing process, which accounts for the rough surface morphology.

Te‐Doped Black Phosphorus Field‐Effect Transistors

Element doping allows manipulation of the electronic properties of 2D materials. Enhanced transport performances and ambient stability of black-phosphorus devices by Te doping are presented. This provides a facile route for achieving airstable black-phosphorus devices.

Scaling behavior of hysteresis in multilayer MoS2 field effect transistors

Extrinsic hysteresis effects are often observed in MoS2 field effect devices due to adsorption of gas molecules on the surface of MoS2 channel. Scaling is a common method used in ferroics to quantitatively study the hysteresis. Here, the scaling behavior of hysteresis in multilayer MoS2 field effect transistors with a back-gated configuration was investigated. The power-law scaling relations were obtained for hysteresis area (⟨A⟩) and memory window (ΔV) with varying the region of back-gate voltage (Vbg,max). It is interesting to find that the transition voltage in the forward sweep (VFW) and in the backward sweep (VBW) shifted to the opposite directions of back-gate voltage (Vbg) with increasing Vbg,max. However, when decreasing Vbg,max, VFW shifted to positive and reversibly recovered, but VBW almost kept unchanged. The evolution of ⟨A⟩, ΔV, VFW, and VBW with Vbg,max were discussed by the electrons transferring process between the adsorbate and MoS2 channel.

Giant spin-torque diode sensitivity in the absence of bias magnetic field.

Microwave detectors based on the spin-torque diode effect are among the key emerging spintronic devices. By utilizing the spin of electrons in addition to charge, they have the potential to overcome the theoretical performance limits of their semiconductor (Schottky) counterparts. However, so far, practical implementations of spin-diode microwave detectors have been limited by the necessity to apply a magnetic field. Here, we demonstrate nanoscale magnetic tunnel junction microwave detectors, exhibiting high-detection sensitivity of 75,400 mV mW−1 at room temperature without any external bias fields, and for low-input power (micro-Watts or lower). This sensitivity is significantly larger than both state-of-the-art Schottky diode detectors and existing spintronic diodes. Micromagnetic simulations and measurements reveal the essential role of injection locking to achieve this sensitivity performance. This mechanism may provide a pathway to enable further performance improvement of spin-torque diode microwave detectors.

Comparison of the properties of GaN grown on complex Si-based structures

With the aim of investigating the possible integration of optoelectronic devices, epitaxial GaN layers have been grown on Si(Ill) semiconductor-on-insulator (SOI) and on Si/CoSi2/Si(111) using metalorganic chemical vapor deposition. The samples are found to possess a highly oriented wurtzite structure, a uniform thickness, and abrupt interfaces. The epitaxial orientation is determined as GaN(0001)//Si(111), GaN[1120]//Si[110], and GaN[1010]//Si[112], and the GaN layer is tensilely strained in the direction parallel to the interface. According to Rutherford backscattering/channeling spectrometry and (0002) rocking curves, the crystalline quality of GaN on Si(111) SOI is better than that of GaN on silicide. Room-temperature photoluminescence of GaN/SOI reveals a strong near-band-edge emission at 368 nm (3.37 eV) with a full width at half-maximum of 59 meV. (c) 2005 American Institute of Physics.

Characterization of Leakage and Reliability of SiN x Gate Dielectric by Low-Pressure Chemical Vapor Deposition for GaN-based MIS-HEMTs

In this paper, we systematically investigated the leakage and breakdown mechanisms of the low-pressure chemical vapor deposition (LPCVD) silicon nitride thin film deposited on AlGaN/GaN heterostructures. The LPCVD-SiNx gate dielectric exhibits low leakage and high breakdown electric field. The dominant mechanism of the leakage current through LPCVD-SiNx gate dielectric is identified to be Poole-Frenkel emission at low electric field and Fowler-Nordheim tunneling at high electric field. Both electric-field-accelerated and temperature-accelerated time-dependent dielectric breakdown of the LPCVD-SiNx gate dielectric were also investigated.

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.

Fabrication of Large-Area Suspended MEMS Structures Using GaN-on-Si Platform

In this letter, piezosensitive elements featuring large-size suspended gallium nitride (GaN) microstructures are fabricated with a two-step dry-release technique using the GaN-on-Si platform. The suspended microstructures are integrated with highly piezosensitive AlGaN/GaN heterostructures as sensing units to realize the GaN-based integrated microsensors. To characterize the residual-stress distribution of the fabricated microstructures, micro-Raman spectroscopy is employed. A microaccelerometer structure with a 250 times 250-mum2 proof-mass area is fabricated with the proposed fabrication technique, and the piezoresponse properties of the integrated sensing elements are characterized through bending experiment.