Mingda Zhu

Extraordinary Control of Terahertz Beam Reflectance in Graphene Electro-absorption Modulators

We demonstrate a graphene-based electro-absorption modulator achieving extraordinary control of terahertz reflectance. By concentrating the electric field intensity in an active layer of graphene, an extraordinary modulation depth of 64% is achieved while simultaneously exhibiting low insertion loss (∼2 dB), which is remarkable since the active region of the device is atomically thin. This modulator performance, among the best reported to date, indicates the enormous potential of graphene for terahertz reconfigurable optoelectronic devices.

Efficient terahertz electro-absorption modulation employing graphene plasmonic structures

We propose and discuss terahertz (THz) electro-absorption modulators based on graphene plasmonic structures. The active device consists of a self-gated pair of graphene layers, which are patterned to structures supporting THz plasmonic resonances. These structures allow for efficient control of the effective THz optical conductivity, thus absorption, even at frequencies much higher than the Drude roll-off in graphene where most previously proposed graphene-based devices become inefficient. Our analysis shows that reflectance-based device configurations, engineered so that the electric field is enhanced in the active graphene pair, could achieve very high modulation-depth, even ∼100%, over a wide frequency range up to tens of THz.We propose and discuss terahertz (THz) electro-absorption modulators based on graphene plasmonic structures. The active device consists of a self-gated pair of graphene layers, which are patterned to structures supporting THz plasmonic resonances. These structures allow for efficient control of the effective THz optical conductivity, thus absorption, even at frequencies much higher than the Drude roll-off in graphene where most previously proposed graphene-based devices become inefficient. Our analysis shows that reflectance-based device configurations, engineered so that the electric field is enhanced in the active graphene pair, could achieve very high modulation-depth, even ∼100%, over a wide frequency range up to tens of THz.

Terahertz imaging employing graphene modulator arrays

In this paper we propose and experimentally demonstrate arrays of graphene electro-absorption modulators as electrically reconfigurable patterns for terahertz cameras. The active element of these modulators consists of only single-atom-thick graphene, achieving a modulation of the THz wave reflectance > 50% with a potential modulation depth approaching 100%. Although the prototype presented here only contains 4x4 pixels, it reveals the possibility of developing reliable low-cost video-rate THz imaging systems employing single detector.

1.9-kV AlGaN/GaN Lateral Schottky Barrier Diodes on Silicon

In this letter, we present AlGaN/GaN lateral Schottky barrier diodes on silicon with recessed anodes and dual field plates. A low specific ON-resistance R_ON,SP (5.12 mQ · cm²), a low turn-ON voltage (<; 0.7 V), and a high reverse breakdown voltage (BV) (>1.9 kV) were simultaneously achieved in devices with a 25-μm anode/cathode distance, resulting in a power figure-of-merit BV²/R_ON,SP of 727 MW · cm^-2. The record high BV of 1.9 kV is attributed to the dual field-plate structure.

Near unity ideality factor and Shockley-Read-Hall lifetime in GaN-on-GaN p-n diodes with avalanche breakdown

Textbook-like device characteristics are demonstrated in vertical GaN p-n diodes grown on bulk GaN substrates. These devices show simultaneously an avalanche breakdown voltage (BV) of >1.4 kV under reverse bias, an ideality factor plateau of ∼2.0 in a forward bias window followed by a near unity ideality factor of 1.1, which are consistently achieved over a temperature range of 300–400 K. At room temperature (RT), the diode with a mesa diameter of 107 μm showed a differential on-resistance Ron of 0.12 mΩcm2, thus resulting in a record figure-of-merit BV2/Ron of ∼16.5 GW/cm2, which is the highest ever demonstrated in any semiconductors. Analytical models are used to fit experimental I-Vs; based on the recombination current with an ideality factor of ∼2.0, a Shockley-Read-Hall lifetime of 12 ns is extracted at RT with an estimated recombination center concentration of 3 × 1015 cm−3.

GaN-on-GaN p-n power diodes with 3.48 kV and 0.95 mΩ-cm2: A record high figure-of-merit of 12.8 GW/cm2

We report GaN p-n diodes on free-standing GaN substrates: a record high Baligas figure-of-merit (V<;sub>B<;/sub><;sup>2<;/sup>/ Ron) of 12.8 GW/cm<;sup>2<;/sup> is achieved with a 32 μm drift layer and a diode diameter of 107 μm exhibiting a BV > 3.4 kV and a R<;sub>on<;/sub> <; 1 mΩ-cm<;sup>2<;/sup>. The leakage current density is low: 10<;sup>-3<;/sup> - 10<;sup>-4<;/sup> A/cm<;sup>2<;/sup> at 3 kV. A record low ideality factor of 1.1-1.3 is signature of high GaN quality. These are among the best-reported GaN p-n diodes.

High breakdown single-crystal GaN p-n diodes by molecular beam epitaxy

Molecular beam epitaxy grown GaN p-n vertical diodes are demonstrated on single-crystal GaN substrates. A low leakage current <3 nA/cm2 is obtained with reverse bias voltage up to −20 V. With a 400 nm thick n-drift region, an on-resistance of 0.23 mΩ cm2 is achieved, with a breakdown voltage corresponding to a peak electric field of ∼3.1 MV/cm in GaN. Single-crystal GaN substrates with very low dislocation densities enable the low leakage current and the high breakdown field in the diodes, showing significant potential for MBE growth to attain near-intrinsic performance when the density of dislocations is low.

Impact of CF4 plasma treatment on threshold voltage and mobility in Al2O3/InAlN/GaN MOSHEMTs

The shift of the threshold voltage Vth in Al2O3/InAlN/GaN metal–oxide–semiconductor high-electron-mobility transistors (MOSHEMTs) is demonstrated by CF4 plasma treatments. The accompanying channel mobility degradation is monitored to understand the tradeoff design space. The effective negative charge introduced by the F plasma treatments at the oxide interface is found to be as high as −0.73 × 1013 cm−2 (mobility > 500 cm2 V−1 s−1), sufficient to fully compensate for the net polarization charge in Al0.15GaN/GaN HEMTs. Although it is difficult to obtain Vth 0 V owing to the high polarization charges in InAlN, these MOSHEMTs with 1 µm gates show very low leakage (~1 × 10−11 A/mm), low hysteresis, and low dispersion.

Ultralow-Leakage AlGaN/GaN High Electron Mobility Transistors on Si With Non-Alloyed Regrown Ohmic Contacts

Without employing gate dielectrics, AlGaN/GaN high-electron mobility transistors (HEMTs) on Si with non-alloyed regrown ohmic contacts exhibit record-low leakage currents ~10-12 A/mm, high ON/OFF current ratios 1011. Compared with HEMTs with conventional alloyed ohmic contacts, HEMTs with non-alloyed contacts show a reduction of 106 in leakage current, a steeper subthreshold slope, and 50% improvement in breakdown voltage. These observations indicate that avoiding high-temperature alloyed ohmic processes can lead to improved device performance.

Two-dimensional electron gases in strained quantum wells for AlN/GaN/AlN double heterostructure field-effect transistors on AlN

Double heterostructures of strained GaN quantum wells (QWs) sandwiched between relaxed AlN layers provide a platform to investigate the quantum-confined electronic and optical properties of the wells. The growth of AlN/GaN/AlN heterostructures with varying GaN quantum well thicknesses on AlN by plasma molecular beam epitaxy (MBE) is reported. Photoluminescence spectra provide the optical signature of the thin GaN QWs. Reciprocal space mapping in X-ray diffraction shows that a GaN layer as thick as ∼28 nm is compressively strained to the AlN layer underneath. The density of the polarization-induced two-dimensional electron gas (2DEG) in the undoped heterostructures increases with the GaN QW thickness, reaching ∼2.5 × 1013/cm2. This provides a way to tune the 2DEG channel density without changing the thickness of the top barrier layer. Electron mobilities less than ∼400 cm2/Vs are observed, leaving ample room for improvement. Nevertheless, owing to the high 2DEG density, strained GaN QW field-effect transis...