S. C. Hung

Oxygen sensors made by monolayer graphene under room temperature

The electrical resistivity of monolayer graphene exhibit significant changes upon expose to different concentration of oxygen (O2) at room temperature. The monolayer graphene, grown by chemical vapor deposition with perfect uniformity within 1 cm × 1 cm will attach O2 molecules and enhance the hole conductivity, which will lead to a change of resistivity of graphene thin film. We quantified the change of resistivity of graphene versus different O2 concentration and the detection limit of the simple O2 sensor was 1.25% in volume ratio.

Detection of chloride ions using an integrated Ag∕AgCl electrode with AlGaN∕GaN high electron mobility transistors

AlGaN∕GaN high electron mobility transistors (HEMTs) with an Ag∕AgCl gate exhibit significant changes in channel conductance upon exposing the gate region to various concentrations of chloride (Cl−) ion. The Ag∕AgCl gate electrode, prepared by potentiostatic anodization, changes electrical potential when it encounters Cl− ions. This gate potential changes lead to a change of surface charge in the gate region of the HEMT, inducing a higher positive charge on the AlGaN surface, and increasing the piezoinduced charge density in the HEMT channel. These anions create an image positive charge on the Ag gate metal for the required neutrality, thus increasing the drain current of the HEMT. The HEMT source-drain current was highly dependent on Cl− ion concentration. The limit of detection achieved was 1×10−8M using a 20×50μm2 gate sensing area.

High sensitivity carbon monoxide sensors made by zinc oxide modified gated GaN/AlGaN high electron mobility transistors under room temperature

AlGaN/GaN high electron mobility transistors (HEMTs) with zinc oxide (ZnO) nanowires modified gate exhibit significant changes in channel conductance upon expose to different concentration of carbon monoxide (CO) at room temperature. The ZnO nanowires, grown by chemical vapor deposition (CVD) with perfect crystal quality will attach CO molecules and release electrons, which will lead to a change in surface charge in the gate region of the HEMTs, inducing a higher positive charge on the AlGaN surface, and increasing the piezoinduced charge density in the HEMTs channel. These electrons create an image positive charge on the gate region for the required neutrality, thus increasing the drain current of the HEMTs. The HEMTs source-drain current was highly dependent on the CO concentration. The limit of detection achieved was 400 ppm in the open cavity with continuous gas flow using a 50×50 μm2 gate sensing area.

Characteristics of carbon monoxide sensors made by polar and nonpolar zinc oxide nanowires gated AlGaN/GaN high electron mobility transistor

AlGaN/GaN high electron mobility transistors (HEMTs) with polar and nonpolar ZnO nanowires modified gate exhibit significant changes in channel conductance upon exposure to different concentration of carbon monoxide (CO) at room temperature. The ZnO nanowires, grown by chemical vapor deposition, with perfect crystal quality will attach CO molecules and release electrons, which will lead to a change of surface charge in the gate region of the HEMTs, inducing a higher positive charge on the AlGaN surface, and increasing the piezo-induced charge density in the HEMTs channel. These electrons create an image positive charge on the gate region for the required neutrality, thus increasing the drain current of the HEMTs. The HEMTs source-drain current was highly dependent on the CO concentration. The limit of detection achieved was 400 ppm and 3200 ppm in the open cavity with continuous gas flow using a 50 × 50 μm2 gate sensing area for polar and nonpolar ZnO nanowire gated HEMTs sensor, respectively.

Nonpolar light emitting diode made by m-plane n-ZnO/p-GaN heterostructure

Nonpolar (100) m-plane n-ZnO/p-GaN light-emitting-diodes (LEDs) were grown by chemical vapor deposition on p-GaN templates which was grown by metalorganic chemical vapor deposition on LiAlO2(100) substrate. Direct current (DC) electroluminescence (EL) measurements yielded a peak at 458nm. The EL peak position was independent of drive current and a full width of half maximum (FWHM) of 21.8 nm was realized at 20mA. The current-voltage characteristics of these diodes showed a forward voltage (Vf) of 6V with a series resistance of 2.2 × 105 Ω.

Effect of temperature on CO detection sensitivity of ZnO nanorod-gated AlGaN/GaN high electron mobility transistors

The carbon monoxide (CO) detection sensitivities of ZnO nanorod-gated AlGaN/GaN high electron mobility transistors were measured over a range of temperatures from 25–150 °C. Once the sensor was exposed to the CO-containing ambient, the drain current, I, of the high electron mobility transistors increased due to chemisorbed oxygen on the ZnO surface reacting with CO, forming CO2 and releasing electrons to the oxide surface. Although the sensor could detect CO as low as 100 ppm at room temperature, the detection sensitivity, ΔI/I, was only around 0.23%. By increasing the sensor temperature to 150 °C, the detection sensitivity was improved by a factor of over 30% to 7.5%.

Integration of Selective Area Anodized AgCl Thin Film with AlGaN/GaN HEMTs for Chloride Ion Detection

We have demonstrated a selective area AgCl anodization process, which can be integrated with the fabrication of AlGaN/GaN high electron mobility transistors (HEMTs) for chloride ion detection. A limit of detection of chloride ion concentration achieved was 1 X 10 -8 M using a 20 X 50 μm anodized Ag/AgCl layer on the HEMT gate sensing area. Unlike the conventional open-circuit potential measurement used for the electrochemical measurement, the drain current of the HEMT was monitored as the output signal in our sensor. The effects of anodization bias voltage and time on the AgCl film properties were investigated. A continuous anodized AgCl film was achieved with the bias voltage of 0.5 or 1 V. However, AgCl films anodized at higher biases (5 V) were not continuous and a larger grain size was obtained. Energy-dispersive X-ray analysis was used to analyze the composition of the anodized AgCl and showed that slightly chloride deficient films (around Ag:Cl = 57:43) were obtained.

Effects of semiconductor processing chemicals on conductivity of graphene

Graphene layers on SiO2/Si substrates were exposed to chemicals or gases commonly used in semiconductor fabrication processes, including solvents (isopropanol, acetone), acids, bases (ammonium hydroxide), UV ozone, H2O, and O2 plasmas. The recovery of the initial graphene properties after these exposures was monitored by measuring both the layer resistance and Raman 2D peak position as a function of time in air or vacuum. Solvents and UV ozone were found to have the least affect, while oxygen plasma exposure caused an increase of resistance of more than 3 orders of magnitude. Recovery is accelerated under vacuum but changes can persist for more than 5 h. Careful design of fabrication schemes involving graphene is necessary to minimize these interactions with common processing chemicals.

Microstructure of InN quantum dots grown on AlN buffer layers by metal organic vapor phase epitaxy

InN quantum dots (QDs) were grown over 2in. Si (1 1 1) wafers with a 300nm thick AlN buffer layer by atmospheric-pressure metal organic vapor phase epitaxy. When the growth temperature increased from 450to625°C, the corresponding InN QDs height increased from 16to108nm while the density of the InN QDs decreased from 1.6×109cm−2to3.3×108cm−2. Transmission electron microscopy showed the presence of a 2nm thick wetting layer between the AlN buffer layer and InN QDs. The growth mechanism was determined to be the Stranski–Krastanov mode. The presence of misfit dislocations in the QDs indicated that residual strain was introduced during InN QDs formation. From x-ray diffraction analysis, when the height of the InN QDs increased from 16to62nm, the residual strain in InN QDs reduced from 0.45% to 0.22%. The residual strain remained at 0.22% for larger heights most likely due to plastic relaxation in the QDs. The critical height of the InN QDs for releasing the strain was determined to be 62nm.

Oxygen sensors made by monolayer graphene

The electrical resistivity of monolayer graphene exhibit significant changes upon expose to different concentration of oxygen (O2) at room temperature. The monolayer graphene, grown by chemical vapor deposition (CVD) with perfect uniformity within 1cm×1cm will attach O2 molecules which will act as a p-type dopant and enhance the hole conductivity, make a change of resistivity of graphene thin film. We quantified the change of resistivity of graphene versus different O2 concentration and the detection limit of the simple O2 sensor was 1.25% in volume ratio.