Zhikai Tang

Excitonic ultraviolet laser emission at room temperature from naturally made cavity in ZnO nanocrytal thin films

Abstract Hexagonally shaped ZnO nanocrystal thin films were fabricated on sapphire(0001) substrates by laser molecular beam epitaxy. Nanocrystal structure was investigated by atomic force microscopy and transmission electron microscopy. Epitaxial growth of ZnO nanocrystal thin films on sapphire substrates was found to occur in a spiral and grain growth mode. The grain growth mode was interpreted by taking higher order epitaxial relationship of oxygen sublattice units between ZnO and sapphire into account. Nanocrystal size could be tuned from 50 to 200 nm controlling film thickness, growth conditions and stoichiometry of the target. The films having small nanocrystal size of about 50 nm showed excitonic stimulated emission having peak energy of 3.2 eV at room temperature with a very low threshold (24 kW cm−2). Mode transition from excitonic stimulated emission to electron hole plasma appeared above another threshold (50 kW cm−2). Well defined Fabry–Perot cavity mode was observed in the emission spectra measured from side edge of the film. It was concluded that the grain boundaries between nanocrystals serve not only as potential barriers confining excitons but also as cavity mirrors.

600-V Normally Off

In this letter, 600-V normally-OFF SiNx/AlGaN/GaN metal-insulator-semiconductor high-electron-mobility transistor (MIS-HEMT) is reported. Normally-OFF operation and low OFF-state gate leakage are obtained by using fluorine plasma ion implantation in conjunction with the adoption of a 17-nm SiNx thin film grown by plasma-enhanced chemical vapor deposition as the gate insulator. The normally-OFF MIS-HEMT exhibits a threshold voltage of +3.6 V, a drive current of 430 mA/mm at a gate bias of 14 V, a specific ON-resistance of 2.1 mΩ·cm2 and an OFF-state breakdown voltage of 604 V at a drain leakage current of 1 μA/mm with VGS=0 V, and the substrate grounded. Effective current collapse suppression is obtained by AlN/SiNx passivation as proved by high-speed pulsed I-V and low-speed high-voltage switching measurement results.

{\rm SiN}_{x}

We report an in situ low-damage pre-gate treatment technology in an atomic layer deposition (ALD) system prior to the ALD- Al₂O₃ deposition, to realize high-quality Al₂O₃/III-nitride (III-N) interface. The technology effectively removes the poor quality native oxide on the III-N surface while forming an ultrathin monocrystal-like nitridation interlayer (NIL) between Al₂O₃ and III-N surface. With the pre-gate treatment technology, high-performance Al₂O₃(NIL)/GaN/AlGaN/GaN metal-insulator-semiconductor high-electron-mobility transistors are demonstrated, exhibiting well-behaved electrical characteristics including suppressed gate leakage current, a small subthreshold slope of ~64 mV/dec, and a small hysteresis of ~0.09 V.

/AlGaN/GaN MIS-HEMT With Large Gate Swing and Low Current Collapse

The physical mechanism of passivation of AlGaN/GaN HEMTs by AlN thin film prepared with plasma-enhanced atomic layer deposition (PEALD) is investigated by characterizing Ni- Al2O3/AlN-GaN/AlGaN/GaN metal-insulator-semiconductor (MIS) diodes. The dielectric stack Al2O3/AlN (13/2 nm) exhibits similar capability in suppressing the current collapse in AlGaN/GaN HEMTs as the 4-nm PEALD-AlN thin film used in our previous work but delivers much lower vertical leakage to facilitate the capacitance-voltage characterizations. Exceptionally large negative bias (<; -8 V) is required to deplete the 2-D electron gas in the MIS diodes C-V measurement. By virtue of quasi-static C-V characterization, it is revealed that positive fixed charges of ~ 3.2 × 1013 e/cm2 are introduced by the PEALD-AlN. The positive fixed charges are suggested to be polarization charges in the monocrystal-like PEALD-AlN. They can effectively compensate the high-density slow-response acceptor-like interface traps, resulting in effective suppression of current collapse.

High-Quality Interface in

We report a high-performance normally-off Al2O3/AlN/GaN MOS-channel-high electron mobility transistor (MOSC-HEMT) featuring a monocrystalline AlN interfacial layer inserted between the amorphous Al2O3 gate dielectric and the GaN channel. The AlN interfacial layer effectively blocks oxygen from the GaN surface and prevents the formation of detrimental Ga-O bonds. Frequency-dispersion in C-V characteristics and threshold voltage hysteresis are effectively suppressed, owing to improved interface quality. The new MOSC-HEMTs exhibit a maximum drain current of 660 mA/mm, a field-effect mobility of 165 cm2/V·s, a high on/off drain current ratio of ~1010, and low dynamic on-resistance degradation.

{\rm Al}_{2}{\rm O}_{3}/{\rm GaN}/{\rm GaN}/{\rm AlGaN}/{\rm GaN}

An effective passivation technique that yields low off-state leakage and low current collapse simultaneously in high-voltage (600-V) AlGaN/GaN high-electron-mobility transistors (HEMTs) is reported in this letter. The passivation structure consists of an AlN/SiN_x stack with 4-nm AlN deposited by plasma-enhanced atomic layer deposition and 50-nm SiN_x deposited by PECVD. The AlN/ SiN_x-passivated HEMTs with a gate-drain distance of 15 μm exhibit a high maximum drain current of 900 mA/mm, a low off-state current of 0.7 μA/mm at V_DS = 600 V, and a steep subthreshold slope of 63 mV/dec. Compared with the static on-resistance of 1.3 mΩ·cm², the dynamic on-resistance after high off-state drain bias stress at 650 V only increases to 2.1 mΩ·cm². A high breakdown voltage of 632 V is achieved at a drain leakage current of 1 μA/mm .

MIS Structures With In Situ Pre-Gate Plasma Nitridation

The relationship between band gap and biaxial stress in wurtzite ZnO thin films has been investigated by side-inclination x-ray diffraction technique and optical absorbance spectrum as well as ab initio calculation. The experimental result shows that differing from other semiconductor thin films with hexagonal structure, such as GaN, the band gap of ZnO thin films increases with the increase in biaxial tensile stress. For explaining the difference, ab initio calculation is performed to simulate the relationship between band gap and biaxial stress of wurtzite ZnO and GaN. The calculated result indicates that the band gap of ZnO increases under biaxial tensile stress but GaN is opposite, supporting our experimental result. The band offset calculation shows that the conduction-band minimum (CBM) and the valence-band maximum (VBM) of ZnO and GaN offset to low energy under biaxial tensile stress. The VBM offset of ZnO is larger than the CBM, responsible for the increase in band gap. The VBM offset of GaN is sm...

Mechanism of PEALD-Grown AlN Passivation for AlGaN/GaN HEMTs: Compensation of Interface Traps by Polarization Charges

In this letter, the radio-frequency (RF) transmission properties of single-walled carbon nanotubes (CNTs) have been characterized up to the frequency of 12 GHz in a carbon nanotube field-effect transistor (CNFET) configuration using a two-port S-parameter method for the first time. The RF characteristics of the CNTs were measured from the drain to the source of the CNFET. A resistance, inductance, and capacitance model has been proposed, and the element values have been extracted. Without the effect of the parasitics, the RF signal transmission in the CNTs presents no degeneration even at 12 GHz. The capacitive contact between CNTs and metal electrodes is reported

Al 2 O 3 /AlN/GaN MOS-Channel-HEMTs With an AlN Interfacial Layer

Carbon nanotube (CNT) has a potential to be used as nanoscale transmission lines and as high-performance passive components in nanoelectronics. In this letter, we discuss single-walled carbon nanotubes (SWNTs) that were grown using a chemical-vapor deposition technique. Both the dc and ac two-terminal resistances were measured and analyzed. The transmission properties of the SWNTs were shown by comparing the S parameters of a device with CNTs with those of a device without a CNT. The radio-frequency characterization of the CNTs was carried out using the network analyzer to frequency of 20GHz.

High-Voltage (600-V) Low-Leakage Low-Current-Collapse AlGaN/GaN HEMTs with AlN/SiN x Passivation

We report the interface characterization of Al2O3/AlN/GaN MOS (metal-oxide-semiconductor) structures with an AlN interfacial layer. A thin monocrystal-like interfacial layer (AlN) is formed at the Al2O3/GaN to effectively block oxygen from the GaN surface and prevent the formation of detrimental Ga-O bonds. The suppression of Ga-O bonds is validated by X-ray photoelectron spectroscopy of the critical interface. Frequency-dispersion in C-V characteristics has been significantly reduced, owing to improved interface quality. Furthermore, using the conventional conductance method suitable for extracting the interface trap density Dit in MOS structures, Dit in the device with AlN was determined to be in the range of 1011–1012 eV−1 cm−2, showing one order of magnitude lower than that without AlN. Border traps near the gate-dielectric/GaN interface were identified and shown to be suppressed by the AlN interfacial layer as well.