L. B. Chang

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.

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.

Barrier Height Enhancement of Ni/n-Type InP Schottky Contact Using a Thin Praseodymium Interlayer

A praseodymium interlayer with a thickness of 100 A at Ni/InP Schottky contacts is shown to enhance the barrier height and reduce the reverse-bias current density. It is demonstrated that even though the contact was baked for 8 h at a temperature of 300°C, its barrier height and reverse-bias current density could still be maintained at about 1.05 eV and 1.86×10-10 A/cm2 at -3 V, respectively. It is believed that praseodymium has high reactivity with oxygen to form praseodymium oxide with a wide band gap and a strong chemical bond. Similar to metal-insulator-semiconductor diodes, praseodymium can effectively increase the Schottky barrier height and reduce the reverse leakage current.

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...

The enhancement of InGaAs Schottky barrier height by the addition of Pr2O3 and In2O3 in the liquid phase epitaxy

In this work, a barrier height of ∼0.7 eV is constantly observed from the In0.53Ga0.47As Schottky diodes, regardless of the utilization of different metals as Schottky contacts. By the addition of Pr2O3 and In2O3 in the liquid phase epitaxy, a very low background impurity concentration is also obtained. The low background concentration is credited to the gettering effect from the addition of Pr and oxygen in the growth melt. The high Schottky barrier is attributed to the formation of a stable oxide layer on the surface of the epilayer, which in turn forms a metal-insulator-semiconductor structure in our Schottky diodes. The high Schottky barrier is very stable even at a high measuring temperature and was repeatedly obtained after four months of exposure to the environment.

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.

Study of AlGaInP multiquantum-well/double heterostructure light-emitting diodes with In-added GaP window layer regrown by antimony-based liquid phase epitaxy

The characteristics of surface-emitting AlGaInP multiquantum-well (MQW) and double heterostructure (DH) light-emitting diodes (LEDs) with a GaP window layer regrown by a GaP source melt with the addition of indium in the liquid phase epitaxy (LPE) system are studied. Using antimony in the source melt instead of gallium eliminates the phenomenon of melt back at the top layer of the LED during the LPE regrowth process. Meanwhile, adding In and Zn to the source melt significantly increases the GaP growth rate and effectively reduces residual melt during the regrowth process. Thus, the AlGaInP LED with a Zn-doped GaP window layer regrown by In being added, Sb-based LPE exhibits a lower turn-on voltage and higher light output power than that without the regrown GaP window layer. However, the emission peak wavelength of the MQW LED shifts a little after the LPE regrowth process. The DH LED does not shift. Thus, the DH structure is more suitable than the MQW structure for the LPE regrowth process of GaP.

Characterization of thermal and optical behaviors of flip-chip LED packages with various underfills

The goal of this study is to experimentally and numerically study the thermal and optical behaviors of flip-chip (FC) LED packages with and without underfill. For thermal characterization, the junction temperature (Tj) and thermal resistance (Rth) are evaluated experimentally by a junction temperature tester and an infrared thermal imager, and numerically by ANSYS simulation. After the simulation model validation, the effect of bu mp number and underfill thermal conductivity on Tj and Rth was further investigated. For optical characterization, the light output of the packages are measured by an integrating sphere system and calculated by TracePro commercial software. Both thermal and optical results for these packages will be presented and discussed in terms of parameters such as number of bumps and underfill in this paper.

Performances of planar GaAs and InGaAs Schottky diodes fabricated by Pr 2 O 3 -added liquid phase epitaxy

This work reports on the improved performance of planar GaAs and InGaAs Schottky diodes fabricated by rare element Pr related oxides Pr2O3-added liquid phase epitaxy. We verify that impurity concentrations are suppressed by two orders, and that electron mobility is significantly enhanced. With Pr2O3 added, the cutoff frequency of Schottky diodes increases in planar InGaAs but decreases in planar GaAs, because of higher contact resistance.

An observation of charge trapping phenomena in GaN/AlGaN/Gd 2 O 3 MOS schottky structure

Charge trapping specially electron trapping phenomenon is observed in GaN/AlGaN/Gd2O3 MOS schottky structure for first time. Under positive pulse programming, a significant hysteresis window is observed. A 4.4V hysteresis window is observed under +10V@100ms pulse programming. It is worthy to mention that no erase phenomenon is observed, even very large negative bias such as −40V. Only time dependent natural charge loss is observed. Even so, a 1.76V hysteresis window is still remained after 14 hours of retention. It is inferred that our fabricated MOS structure is behaved as a storage node under positive pulse programming. Good charge retention is observed and it may be used as the Write Once Read Many (WORM) memory in future.