Huili G. Xing

Exfoliated multilayer MoTe2 field-effect transistors

The properties of multilayer exfoliated MoTe2 field-effect transistors (FETs) on SiO2 were investigated for channel thicknesses from 6 to 44 monolayers (MLs). All transistors showed p-type conductivity at zero back-gate bias. For channel thicknesses of 8u2009ML or less, the transistors exhibited ambipolar characteristics. ON/OFF current ratio was greatest, 1u2009 ×u2009105, for the transistor with the thinnest channel, 6u2009ML. Devices showed a clear photoresponse to wavelengths between 510 and 1080u2009nm at room temperature. Temperature-dependent current-voltage measurements were performed on a FET with 30 layers of MoTe2. When the channel is turned-on and p-type, the temperature dependence is barrier-limited by the Au/Ti/MoTe2 contact with a hole activation energy of 0.13u2009eV. A long channel transistor model with Schottky barrier contacts is shown to be consistent with the common-source characteristics.

Coded-Aperture Imaging Using Photo-Induced Reconfigurable Aperture Arrays for Mapping Terahertz Beams

We report terahertz coded-aperture imaging using photo-induced reconfigurable aperture arrays on a silicon wafer. The coded aperture was implemented using programmable illumination from a commercially available digital light processing projector. At 590 GHz, each of the array element apertures can be optically turned on and off with a modulation depth of 20 dB and a modulation rate of ~ 1.3 kHz. Prototype demonstrations of 4 ×4 coded-aperture imaging using Hadamard coding have been performed. Continuous THz imaging with 8 ×8 pixels has also been demonstrated, using a slowly moving metal strip as the target. In addition, this technique has been successfully applied to mapping THz beams by using a 6 ×6 aperture array at 590 GHz. The imaging results agree closely with theoretical calculations based on Gaussian beam propagation, demonstrating that this technique is promising for realizing real-time and low-cost terahertz cameras for many applications. The reported approach provides a simple but powerful means to visualize THz beams, which is highly desired in quasi-optical system alignment, quantum-cascade laser design and characterization, and THz antenna characterization.

Graphene as transparent electrode for direct observation of hole photoemission from silicon to oxide

We demonstrate the application of graphene as collector material in internal photoemission (IPE) spectroscopy, which enables direct observation of both electron and hole injections at a Si/Al2O3 interface and overcomes the long-standing difficulty of detecting holes in IPE measurements. The observed electron and hole barrier heights are 3.5u2009±u20090.1 eV and 4.1u2009±u20090.1 eV, respectively. Thus, the bandgap of Al2O3 can be deduced to be 6.5u2009±u20090.2 eV, in good agreement with the value obtained by ellipsometry analysis. Our modeling effort reveals that, by using graphene, the carrier injection from the emitter is significantly enhanced and the contribution from the collector electrode is minimal.

Ultrathin MBE-Grown AlN/GaN HEMTs with record high current densities

III-V nitride-based HEMT technology has made rapid progress over the last decade. Benefiting from the extremely high polarization charge, in this work, we demonstrate record high DC current density (2.9 A/mm) and very high extrinsic transconductance (~430 mS/mm) AIN/GaN HEMTs.

Tunnel field-effect transistor heterojunction band alignment by internal photoemission spectroscopy

The electron energy band alignment of a metal-oxide-semiconductor tunnel field-effect transistor heterojunction, W/Al2O3/InGaAs/InAs/InP, is determined by internal photoemission spectroscopy. At the oxide flat-band condition, the barrier height from the top of the InGaAs/InAs valence band and the top of the InP valence band to the bottom of the Al2O3 conduction band is determined to be 3.5 and 2.8u2009eV, respectively. The simulated energy band diagram of the heterostructure is shown to be consistent with the measured band alignments if an equivalent positive charge of 6.0u2009×u20091012u2009cm−2 is present at the Al2O3/InGaAs. This interface charge is in agreement with previously reported capacitance-voltage measurements.

Direct electrical observation of plasma wave-related effects in GaN-based two-dimensional electron gases

In this work, signatures of plasma waves in GaN-based high electron mobility transistors were observed by direct electrical measurement at room temperature. Periodic grating-gate device structures were fabricated and characterized by on-wafer G-band (140–220u2009GHz) s-parameter measurements as a function of gate bias voltage and device geometry. A physics-based equivalent circuit model was used to assist in interpreting the measured s-parameters. The kinetic inductance extracted from the measurement data matches well with theoretical predictions, consistent with direct observation of plasma wave-related effects in GaN-channel devices at room temperature. This observation of electrically significant room-temperature plasma-wave effects in GaN-channel devices may have implications for future millimeter-wave and THz device concepts and designs.

MBE-Grown Ultra-shallow AlN/GaN HFET Technology

Due to large polarization effects, two-dimensional electron gas (2DEG) concentrations higher than 1x10 cm can be produced at the AlN/GaN heterojunction with AlN barriers as thin as 2 nm. This ultra-shallow channel together with the wide bandgap of AlN (6.2 eV) makes AlN/GaN heterojunction field effect transistors (HFET) extremely attractive for high frequency (>100 GHz) high power applications. At Notre Dame, these structures have been grown using molecular beam epitaxy and the record transport properties among III-V nitrides are achieved: a sheet resistance of ~ 170 ohm/square for a single heterostructure at room temperature. HFETs have been fabricated with optical lithographically defined gates. At present the device dc characteristics show a maximum drain current of 800 mA/mm and transconductance of 180 mS/mm for 3 μm long gate. This clearly demonstrates its value toward high speed devices. The development as well as challenges of this technology will be discussed here.

GaN/NbN epitaxial semiconductor/superconductor heterostructures

Epitaxy is a process by which a thin layer of one crystal is deposited in an ordered fashion onto a substrate crystal. The direct epitaxial growth of semiconductor heterostructures on top of crystalline superconductors has proved challenging. Here, however, we report the successful use of molecular beam epitaxy to grow and integrate niobium nitride (NbN)-based superconductors with the wide-bandgap family of semiconductors—silicon carbide, gallium nitride (GaN) and aluminium gallium nitride (AlGaN). We apply molecular beam epitaxy to grow an AlGaN/GaN quantum-well heterostructure directly on top of an ultrathin crystalline NbN superconductor. The resulting high-mobility, two-dimensional electron gas in the semiconductor exhibits quantum oscillations, and thus enables a semiconductor transistor—an electronic gain element—to be grown and fabricated directly on a crystalline superconductor. Using the epitaxial superconductor as the source load of the transistor, we observe in the transistor output characteristics a negative differential resistance—a feature often used in amplifiers and oscillators. Our demonstration of the direct epitaxial growth of high-quality semiconductor heterostructures and devices on crystalline nitride superconductors opens up the possibility of combining the macroscopic quantum effects of superconductors with the electronic, photonic and piezoelectric properties of the group III/nitride semiconductor family.

Band alignment of TFET heterojunctions and post deposition annealing effects by internal photoemission spectroscopy

The tunneling field-effect transistor (TFET) has been attracting increasing attention for low-voltage logic application [1]. Recently, III-V semiconductor TFETs are being extensively explored, showing higher on-state current than Si TFETs [1]–[3]. However, low subthreshold swings (SS) have not yet been demonstrated on any III-V TFETs, possibly due to the high density of traps at the high-k oxide and semiconductor interface [2]–[3]. It has been shown that post deposition anneal (PDA) can improve SS by reducing the interface trap density in III V TFETs [2].