Ruby N. Ghosh

Fiber-optic oxygen sensor using molybdenum chloride cluster luminescence

We report on a reflection-mode fiber-optic oxygen sensor based on the 3O2 quenching of the red emission from hexanuclear molybdenum chloride clusters. Measurements of the probe operating in a 0%–21% gaseous oxygen environment have been obtained, a range suitable for biological and automotive applications. The luminescence signal increases with decreasing oxygen concentration in accordance with theory. We observe clearly resolvable steps in the sensor response for changes of 0.1% absolute oxygen concentration in the 0%–1.0% range. The response time of the fiber probe is theoretically predicted to be 1 s.

Measuring the Electron's Charge and the Fine-Structure Constant by Counting Electrons on a Capacitor

The charge of the electron can be determined by simply placing a known number of electrons on one electrode of a capacitor and measuring the voltage, Vs, across the capacitor. If Vs is measured in terms of the Josephson volt and the capacitor is measured in SI units then the fine-structure constant is the quantity determined. Recent developments involving single electron tunneling, SET, have shown bow to count the electrons as well as how to make an electrometer with sufficient sensitivity to measure the charge.

Interface states in high-temperature gas sensors based on silicon carbide

Silicon carbide (SiC)-based metal-insulator-semiconductor devices are attractive for gas sensing in automotive exhausts and flue gases. The response of the devices to reducing gases has been assumed to be due to a reduced metal work function at the metal-oxide interface that shifts the flat band capacitance to lower voltages. We have discovered that high temperature (700 K) exposure to hydrogen results not only in the flat-band voltage occurring at a more negative bias than in oxygen, but also in the transition from accumulation (high capacitance) to inversion (low capacitance) occurring over a relatively narrow voltage range. In oxygen, this transition is broadened, indicating the creation of a high density of interface states. We present a model of the hydrogen/oxygen response based on two independent phenomena: a chemically induced shift in the metal-semiconductor work function difference and the passivation/creation of charged states at the SiO/sub 2/-SiC interface that is much slower than the work function shift. We discuss the effect of these results on sensor design and the choice of operating point.

Monolithic integration of GaAs light‐emitting diodes and Si metal‐oxide‐semiconductor field‐effect transistors

A monolithic optoelectronic circuit, consisting of a GaAs light‐emitting diode (LED) driven by a Si metal‐oxide‐semiconductor (MOS) transistor, has been fabricated. Light output as a function of applied gate voltage was measured. The LED’s were fabricated in GaAs layers on Ge‐coated Si substrates containing MOS transistors. Normal transistor performance was observed after the GaAs LED fabrication, indicating that GaAs and Si processing technologies appear to be compatible.

Optical dissolved oxygen sensor utilizing molybdenum chloride cluster phosphorescence

We report on an optical oxygen sensor for aqueous media. The phosphorescent signal from the indicator, K2Mo6Cl14, immobilized in a polymer matrix, is quenched by ground state O32. Continuous measurements (Δt=10 s) over 36 h in oxygen atmospheres (0%–21%) were obtained with a signal to noise ratio better than 150. Photobleaching was not observed over ∼13 000 measurements. The senor response at 10, 22, and 37 °C water is governed by bimolecular collisional quenching, as evidenced by a linear fit to the Stern–Volmer equation for dissolved oxygen in the range 0<[O2]<3×10−4.

A portable optical human sweat sensor

We describe the use of HNQ (2-hydroxy-1,4-naphthoquinone or Lawsone) as a potential sweat sensor material to detect the hydration levels of human beings. We have conducted optical measurements using both artificial and human sweat to validate our approach. We have determined that the dominant compound that affects HNQ absorbance in artificial sweat is sodium. The presence of lactate decreases the reactivity of HNQ while urea promotes more interactions of sodium and potassium ions with HNQ. The interactions between the hydroxyl group of HNQ and the artificial sweat components (salts, lactic acid, and urea) were investigated comprehensively. We have also proposed and developed a portable diode laser absorption sensor system that converts the absorbance at a particular wavelength range (at 455 ± 5 nm, where HNQ has an absorbance peak) into light intensity measurements via a photocell. The absorbance intensity values obtained from our portable sensor system agrees within 10.4% with measurements from a laboratory based ultraviolet-visible spectrometer. Findings of this research will provide significant information for researchers who are focusing on real-time, in-situ hydration level detection.

Influence of Interface States on High Temperature SiC Sensors and Electronics

Silicon carbide based metal/oxide/semiconductor (MOS) devices are well suited for operation in chemically reactive high temperature ambients. The response of catalytic gate SiC MOS sensors to hydrogen-containing species has been assumed to be due to the formation of a dipole layer at the metal/oxide interface, which gives rise to a voltage translation of the high frequency capacitance voltage (C-V) curve. From in-situ C-V spectroscopy, performed in a controlled gaseous environment, we have discovered that high temperature (800 K) exposure to hydrogen results in (i) a flat band voltage occurring at a more negative bias than in oxygen and (ii) the transition from accumulation to inversion occurring over a relatively narrow voltage range. In oxygen, this transition is broadened indicating the creation of a large number of interface states. We interpret these results as arising from two independent phenomena ‐ a chemically induced shift in the metal/semiconductor work function difference and the passivation/creation of charged states (D IT) at the SiO2/SiC interface. Our results are important for both chemical sensing and electronic applications. MOS capacitance gas sensors typically operate in constant capacitance mode. Since the slope of the C-V curve changes dramatically with gas exposure, we discuss how sensor-to-sensor reproducibility and device response time are influenced by the choice of operating point. For electronic applications understanding the environmentally induced changes in DIT is crucial to designing drift-free MOS devices. Our results are applicable to n-type SiC MOS devices in general, independent of the specifics of sample fabrication.

Interface states in high temperature SiC gas sensing

Silicon carbide based metal-oxide-semiconductor (MOS) devices are attractive for gas sensing in harsh, high temperature environments. The response of catalytic gate SiC sensors to hydrogen-containing species has been assumed to be due to the formation of a dipole layer at the metal/oxide interface which gives rise to a voltage translation of the high frequency capacitance voltage (C-V) curve. We have discovered that high temperature (800 K) exposure to hydrogen results in (i) a flat band voltage occurring at a more negative bias than in oxygen and (ii) the transition from accumulation (high capacitance) to inversion (low capacitance) occurring over a relatively narrow voltage range. In oxygen, this transition is broadened indicating the creation of a large number of interface states. We interpret these results as arising from two independent phenomena - a chemically induced shift in the metal/semiconductor work function difference and the passivation/creation of charged states at the SiO/sub 2//SiC interface. MIS capacitance sensors typically operate in constant capacitance mode. These results. affect sensor sensitivity since the slope of the C-V curve changes dramatically with gas exposure.

Sensing mechanisms of high temperature silicon carbide field-effect devices

Metal-insulator-silicon carbide devices have been used for gas sensing in automotive exhausts, because the large band gap of SiC allows high temperature operation up to 1200 K in chemically reactive environments. The sensor response to hydrogen containing species is due to two mechanisms whose effects are difficult to distinguish: the chemical modification of the barrier height at the metal-insulator interface and the creation/passivation of charged states at the insulator-silicon carbide interface. We describe an experimental technique combining in-situ photoemission and in-situ capacitance-voltage spectroscopy to separate the contribution of each phenomenon. Our experiment elucidates the sensing mechanism of high temperature SiC based gas sensors.

The role of oxygen in hydrogen sensing by a platinum-gate silicon carbide gas sensor: An ultrahigh vacuum study

We report several experiments under ultrahigh vacuum conditions that elucidate the role of oxygen in the functioning of silicon carbide field-effect gas sensors with nonporous platinum gates. The devices studied are shown to be sensitive both to hydrogen and to propene. All of the results are consistent with oxygen acting through its surface reactions with hydrogen. Three specific aspects are highlighted: the need, under some conditions, for oxygen to reset the device to a fully hydrogen-depleted state; competition between hydrogen oxidation and hydrogen diffusion to metal/oxide interface sites, leading to steplike behavior as a function of the oxygen:hydrogen ratio (λ-sensing); and the removal of sulfur contamination by oxygen.