Atanu Das

GaN Thin Film Based Light Addressable Potentiometric Sensor for pH Sensing Application

Gallium nitride (GaN) is a material with remarkable properties, including wide band gap, direct light emission and excellent chemical stability. In this study, a GaN-based light addressable potentiometric sensor (LAPS) with Si3N4 ~50 nm as a sensing membrane is fabricated. By modulated optical excitation from an ultraviolet 365 nm light-emitting diode, the photoresponse characteristic and related pH sensitivity of the fabricated GaN-based LAPS is investigated. A Nernstian-like pH response with pH sensitivity of 52.29 mV/pH and linearity of 99.13% is obtained. These results of the GaN-based LAPS show great promise and it could be used as a single chemical sensor or integrated optoelectronic chemical sensor array for biomedical research with high spatial resolution.

Fabrication and thermal analysis of flip-chip light-emitting diodes with different numbers of Au stub bumps

The thermal performance of flip-chip (FC) light-emitting diodes (LEDs) with different numbers of Au stub bumps has been investigated by using thermosonic bonder. The LEDs were mounted on the aluminium nitride (AlN) sub-mounts which have superior thermal conductivity (230 W/mK), and the high power Chip-on-Plate (COP) package was proposed to be used for our measurement. In order to understand the thermal performance of the high power FC-LEDs, the experimental measurement and finiteelement model (FRM) numerical simulation have been used. It is found that the thermal performance of our 1 1m m 2 FC-LEDs can only be improved when using at least 6 Au stub bumps as interconnected metals. Moreover, the surface temperature of FC-LEDs is significantly reduced while using 20 Au stub bumps.

Enhanced nanoscale resistive switching memory characteristics and switching mechanism using high-Ge-content Ge0.5Se0.5 solid electrolyte

We demonstrate enhanced repeatable nanoscale bipolar resistive switching memory characteristics in Al/Cu/Ge0.5Se0.5/W, as compared with Al/Cu/Ge0.2Se0.8/W structures, including stable AC endurance (>105 cycles), larger average SET voltage (approximately 0.6 V), excellent data retention (>105 s) at 85°C, and a high resistance ratio (>104) with a current compliance of 8 μA and a small operation voltage of ±1.5 V. A small device size of 150 × 150 nm2 and a Cu nanofilament with a small diameter of 30 nm are both observed by high-resolution transmission electron microscope in the SET state. The GexSe1 − x solid electrolyte compositions are confirmed by both energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy. The switching mechanism relies on the smaller barrier heights for holes rather than for electrons; the positively charged Cuz+ ions (i.e., holes) migrate through the defects in the GexSe1 − x solid electrolytes during SET/RESET operations. Hence, the Cu nanofilament starts to grow at the Ge0.5Se0.5/W interface, and starts to dissolve at the Cu/Ge0.5Se0.5 interface, as illustrated in the energy band diagrams. Owing to both the higher barrier for hole injection at the Cu/Ge0.5Se0.5 interface than at the Cu/Ge0.2Se0.8 interface and greater thermal stability, the resistive switching memory characteristics of the Al/Cu/Ge0.5Se0.5/W are improved relative to the Al/Cu/Ge0.2Se0.8/W devices. The Al/Cu/Ge0.5Se0.5/W memory device can also be operated with a low current compliance of 1 nA, and hence, a low SET/RESET power of 0.61 nW/6.4 pW is achieved. In addition, a large memory size of 1,300 Pbit/in2 is achieved with a small nanofilament diameter of 0.25 Å for a small current compliance of 1 nA.

An observation of charge trapping phenomena in GaN/AlGaN/Gd2O3/Ni–Au structure

Charge trapping, especially electron trapping phenomena in GaN/AlGaN/Gd2O3/Ni–Au metal-oxide-semiconductor structure have been investigated. Owing to crystallization of Gd2O3 film after annealing at 900 °C in ambient air for 30 s, a significant memory window of 1.6 V is observed under 5 V@100 ms programming pulse compared with that of as-deposited sample. The fabricated structure exhibits no erase phenomena under large negative bias of −20 V. Only time dependent natural charge loss is occurred. Even so, 0.9 V of memory window is still remained after 21 h of retention. Good endurance of 103 cycles with 2.0 V memory window is also obtained.

Investigation of temperature dependent threshold voltage variation of Gd2O3/AlGaN/GaN metal-oxide-semiconductor heterostructure

Temperature dependent threshold voltage (Vth) variation of GaN/AlGaN/Gd2O3/Ni-Au structure is investigated by capacitance-voltage measurement with temperature varying from 25°C to 150°C. The Vth of the Schottky device without oxide layer is slightly changed with respect to temperature. However, variation of Vth is observed for both as-deposited and annealed device owing to electron capture by the interface traps or bulk traps. The Vth shifts of 0.4V and 3.2V are obtained for as-deposited and annealed device respectively. For annealed device, electron capture process is not only restricted in the interface region but also extended into the crystalline Gd2O3 layer through Frenkel-Poole emission and hooping conduction, resulting in a larger Vth shift. The calculated trap density for as-deposited and annealed device is 3.28×1011∼1.12×1011 eV−1cm−2 and 1.74×1012∼7.33×1011 eV−1cm−2 respectively in measured temperature range. These results indicate that elevated temperature measurement is necessary to characteri...

Influence of patterned sapphire substrates with different symmetry on the light output power of InGaN-based LEDs

This paper aims to investigate the light output power (LOP) of InGaN-based light-emitting diodes (LEDs) grown on patterned sapphire substrates (PSSs) with different symmetry. The GaN epitaxial layers grown on the hexagonal lattice arrangement PSS (HLAPSS) have a lower compressive strain than the ones grown on the square lattice arrangement PSS (SLAPSS). The quantum-confined Stark effect (QCSE) is also affected by the residual compressive strain. Based on the experimentally measured data and the ray tracing simulation results, the InGaN-based LED with the HLAPSS has a higher LOP than the one with the SLAPSS due to the weaker QCSE within multiple-quantum wells (MQWs).

Physical and Memory Characteristics of Atomic-Layer-Deposited High-κ Hafnium–Aluminum-Oxide Nanocrystal Capacitors with Iridium-Oxide Metal Gate

The physical and memory characteristics of high-κ hafnium–aluminum-oxide (HfAlO) nanocrystals in an n-Si/SiO2/HfO2/Al2O3/iridium-oxide (IrOx) structure have been investigated. The high-κ HfAlO nanocrystal in the SiO2/HfO2/Al2O3 layers is formed owing to the diffusion of Al2O3 and HfO2 films after high-temperature annealing at 900 °C. High-κ HfAlO nanocrystals with a small diameter of 5 nm and a high density of 1.7 ×1012 cm-2 have been confirmed by high-resolution transmission electron microscopy and X-ray photoelectron spectroscopy. Owing to the formation of high-κ HfAlO nanocrystals, a large hysteresis memory window of ΔV≈4.4 V at a sweeping gate voltage of ±10 V is observed compared with that of as-deposited memory capacitor. A hysteresis memory window of ΔV≈1.4 V with a small sweeping gate voltage of ±7 V is also observed. Good endurance of 104 cycles with a large memory window of ΔV≈3.6 V is obtained. A significant memory window of ΔV≈1.3 V is observed after a retention time of 5×104 s, owing to the charge confinement in the high-κ HfAlO nanocrystals.

Effective Treatment on AlGaN/GaN MSM-2DEG Varactor with ( NH4 ) 2S / P2S5 Solution

The effect of surface passivation using (NH 4 ) 2 S and (NH 4 ) 2 S/P 2 S 5 on a AlGaN/GaN-based metal-semiconductor-metal diode above a two-dimensional electron gas (MSM-2DEG) varactor was investigated. The surface property, capacitance ratio (C max /C min ), and leakage current of the prepared samples were studied before and after treatments using X-ray photoelectron spectroscopy and capacitance―voltage and current―voltage analyses. It showed that the (NH 4 ) 2 S/P 2 S 5 -treated sample had the most excellent surface state and C max /C min and the least leakage current because of either reduced native oxide or deposited phosphorus compounds only provided by (NH 4 ) 2 S/P 2 S 5 and sulfide upon the surface, also validated by having the highest sheet carrier density. Hence, these promising results promote further potential for varactor applications.

Improvement of Surge Protection by Using an AlGaN/GaN-Based Metal-Semiconductor-Metal Two-Dimensional Electron Gas Varactor

In this paper, a varactor with metal–semiconductor–metal diodes on top of the (NH4)2S/P2S5-treated AlGaN/GaN two-dimensional electron gas epitaxial structure (MSM-2DEG) is proposed to the surge protection for the first time. The sulfur-treated MSM-2DEG varactor properties, including current–voltage (I–V), capacitance–voltage (C–V), and frequency response of the proposed surge protection circuit, are presented. To verify its capability of surge protection, we replace the metal oxide varistor (MOV) and resistor (R) in a state-of-the-art surge protection circuit with the sulfur-treated MSM-2DEG varactor under the application conditions of system-level surge tests. The measured results show that the proposed surge protection circuit, consisted of a gas discharge arrester (GDA) and a sulfur-treated MSM-2DEG varactor, can suppress an electromagnetic pulse (EMP) voltage of 4000 to 360 V, a reduction of 91%, whereas suppression is to 1780 V, a reduction of 55%, when using only a GDA.

Carrier Concentrations in Degenerate Semiconductors Having Band Gap Narrowing

The density-of-states effective mass approximation and the conduction-band effective mass approximation are employed to formulate carrier concentrations and the diffusivity-mobility relationship (DMR) for heavily doped n-semiconductors exhibiting band gap narrowing. These are very suitable for the investigation of electrical transport also in heavily doped p-semiconductors. Numerical calculations indicate that the DMR depends on a host of parameters including the temperature, carrier degeneracy, and the non-parabolicity of the band structure.