ENVIRONMENTAL INSTITUTE OF HOUSTON


Proposal Title: A low cost, Portable Fluorescence Sensor for Measurements of Water Flows

Place of performance: Space Vacuum Epitaxy Center, University of Houston, 4800 Calhoun, S&R1 R. 724, Houston, TX 77204

Dates of performance: 01/01/99-01/09/99

Investigators: Prof. Wanda Zogozdzon-Wozik, Prof. A. Bensaoula and David Starikov

Amount granted: $15000

PROJECT SUMMARY

Flow meaurement by dye dilution is chosen for its superior accuracy (+/-2%). This flow measurement technique can accurately measure flows as low as 1 gallon per minute and as high as 1 billion gallons per day. It is the method of choice in cases of turbulent, large volume, and rapidly changing flows. It is also commonly used to measure difficult-to-access flows and flows in large diameter pipes. In these situations, other methods are significantly more expensive, highly inaccurate, or impractical to implement. To measure water flow, a dye is injected at a constant rate upstream of a fluorometer which is used to determine the dilution rate. Multiplying the dye injection rate by the dilution ratio gives the flow rate. These highly accurate flow measurements are used to calibrate flow meters, verify flow capacity, settle billing disputes, localize infiltration, determine discharge rates, measure cap performance.

High Performance Integrated Solar-Blind Optoelectronic Chemical Sensor Based on Gallium Nitride

Report of Activitivities

The objective of this project was to develop, fabricate, and evaluate nitride-based solar-blind solid-state integrated chemical sensor structures for chemical reagent identification and concentration measurements. During completion of Task 1, epitaxial growth of GaN layers on sapphire has been optimized. Since fabricated Schottky barrier diode structures exhibited better performance on p-type, than on n-type GaN layers, closer attention was paid to the growth of Mg-doped GaN. Hall effect measurements performed on these layers indicated p-type doping concentrations up to 7 x1017 cm-3 and a carrier mobility of 1.2 cm2/V-sec.

In the frame of Task 2 GaN-based optoelectronic Schottky barrier diode structures have been fabricated and characterized. The photosensitivity of these structures was in the near-UV range of 365-400 nm. A UV/blue optical emission with a maximum peak at 450 nm was observed at forward bias and a broad band emission ranging from near UV to near IR with a maximum peak at 382 nm was observed at reverse bias. The total Lambertian UV power measured from a packaged Light Emitting Diode (LED) in the spectral range 200-300 nm from a forward biased 0.5 mm diameter contact was 0.46 mW. Both the optical emission and the photosensitivity spectra indicated a limit in the near-UV at the wavelength of 365 nm corresponding to the band gap energy of GaN (~3.4 eV). This is a result of the optical emission absorption by the GaN base material.

To avoid such absorption and extend the spectral characteristics of our structures further into the UV part of the spectrum we have developed a process for deposition of electrically conductive UV transparent electrodes based on fluorine-doped semiconductor tin oxide (SnO2) layers. Such layers deposited on sapphire demonstrated up to 80% of the optical transmittance in the UV range from visible down to 260 nm. UV photosensitive Schottky barrier structures fabricated using SnO2 electrodes indicated dark currents in the order of ~ 1 pA and a radiant sensitivity around ~0.01 A/W in the spectral range from at least 250 nm to 475 nm.

The results described above indicate that both light emitting and photosensitive Schottky barrier UV structures based on p-GaN can be fabricated on the same substrate in a single technological process. The next effort in the development of the advanced optoelectronic sensors will be made towards improving of the processing in order to integrate the sensor components onto a single substrate and developing appropriate optical wave guides.