Institute for Space Systems Operations * 2001 Annual Report * 16-21

Miniature Optical Sensors for Detection of Water and Air Contaminants

Optical devices for measurement of water and air contaminants are being transformed from costly and bulky laboratory equipment into compact and portable sensors fabricated using LEDs and photodiodes.^1-4 These devices, usually the size of table tops, assembled from conventional components are sensitive to elevated temperatures and use statistical sampling to perform measurements. There is an increasing need for miniature, reliable, real-time, integrated on-a chip sensors for characterization of water and air for environmental, industrial, biomedical, petroleum, military, and space applications.⁵ The ultimate goal of the current research is development of such advanced optoelectronic sensors composed of LEDs, photodetectors, optical filters and wave guides integrated on a single chip.

The group III Nitride AlInGaN semiconductor material system is superior for development of advanced integrated multifunctional optoelectronic sensors. The direct wide band gap of these materials can be tuned in a wide range of energies allowing high efficiency optical emission and detection from near UV to near IR.^6,7 The high thermal, mechanical, chemical, and radiation strengths of such materials would allow sensor performance in harsh and super-ambient environments. Our recent results on III Nitride thin film growth using Radio Frequency Molecular Beam Epitaxy indicate that optically active InGaN and AlGaN layers with variable In and Al composition can be deposited and LEDs and photodiodes with tunable characteristics can be fabricated.^7-9

The purpose of our ISSO-funded project was to investigate the development of optoelectronic chemical sensors based on group III-nitride materials. The compounds GaN, AlN, InN, and their alloys are optically active from 650 nm (InN) to 200 nm (AlN) and thus are ideally suited for use in UV-VIS chemical sensors. Emission and detection devices can be separately tailored to specific wavelengths and grown on the same chip. Such integrated devices of AlInGaN materials could offer many advantages over current optical chemical sensors such as high chemical and thermal stability, smaller size, and higher sensitivity.

Technical Plan and Equipment

Experimental Activity
The materials fabrication aspect of this project has focused on two main areas in the past year. The first area was the growth of InGaN which is the material used in the light-emitting region in our device structure. The purpose was to vary the InGaN growth parameters to produce spectral emission in different wavelength regions. Room temperature photoluminescence of the InGaN layers was used to determine the emission wavelengths.

The second area of interest was the improvement of the optical and electrical properties of our base GaN material. In order to produce samples with improved properties, film growth parameters, including initialization and deposition conditions, were varied and their effects measured. The resulting films were analyzed by room temperature photoluminescence, low temperature PL, and Hall effect measurements.

Optoelectronic sensor structures were fabricated by employment of photolithography, reactive ion etching (RIE), and contact deposition by e-beam evaporation resulted in a pattern shown in the SEM (Fig. 1).

For fabrication of the sensor prototypes we mounted blue LEDs from Agilent Technologies with a maximum wavelength of 475 nm and Hamamatsu Si photodiodes on a double side-polished sapphire substrate together with a long-pass optical filter from Oriel with a cut-off wavelength of 475 and 530 nm for prototypes 1 and 2, respectively (Fig. 2). The assembly was sealed with a chemically-resistant water-proof silicone compound to provide safe immersing of the device into various analytes.

Figure 2. Multifunctional sensor prototypes for optical measurements.

TRIAX 320 monochromator from Jobin-Yvon, a 150W xenon lamp from Oriel, and a Stanford SR510 lock-in amplifier were used for spectral characterization of the LEDs and photodetectors in the range from 250 to 1200 nm. The voltage to the LEDs according to their specifications was supplied from a Topward 360D stabilized DC power supply. The signal from the photodetectors was measured as a voltage by a Keithley 2410 Source Meter, and as a current by a Keithley 485 Picoammeter.

Discussion
Each of the two aspects of the project met specific challenges. The growth of InGaN is typically more difficult than growth of GaN due to the greater vapor pressure of the indium species relative to gallium and aluminum at the film growth temperature. This necessitates lowering the deposition temperature so that the indium can incorporate into the InGaN film. The drawback is that the gallium species have less surface mobility at the lower temperature and thus requires a lower growth rate. In addition, when growing InGaN layers that are several hundred angstroms thick, the indium tends to segregate on the surface of the film, thus leading to uneven indium composition within the layer. To overcome these problems, we employed a quasi-superlattice structure where the InGaN was grown by alternately opening and closing the indium shutter with the gallium and nitrogen shutters always open. This would allow any excess indium left over from the direct growth of InGaN to be consumed during the stage when the indium shutter was closed. Control of the indium composition could be obtained by varying the ratio of indium to gallium and the ratio of time the indium shutter was open and closed.

In order to fabricate good optoelectronic devices, optical and electrical film quality are important parameters. One of the issues of GaN growth is the lack of lattice-matched substrates on which to grow epilayers. This results in GaN layers with large numbers of dislocations which can reduce the optical quality of the material as well as the mobility of the free carriers. In order to address these problems, we worked to optimize the initiation of film growth on the substrate, the deposition of the AlN buffer layers, and the growth parameters of the GaN overlayers. Mainly, the substrate preparation, buffer layer growth temperature, and buffer layer thickness were varied. The effects on the optical quality were gauged by the full width at half maximum (FWHM) of the room temperature photoluminescence and the electrical quality judged by measuring the free carrier mobility using the Hall effect.

The sensor design concept was based on employment of optically transparent sapphire substrates that can be used as windows to separate the laterally configured sensor components from the analyte. One of the main advantages of such a configuration is its use as a flow-through, in-line sensor. In addition, lateral setup of the components allows employment of a totally planar design, which significantly simplifies the fabrication process. One of the possible drawbacks is a possible high background signal resulting from the direct illumination of the photodetector by the LED light transferred by the sapphire substrate. Experiments in this task were set up to determine the background noise and investigate the sensor prototype ability to measure fluorescence signals above this background. Two sensor prototypes were fabricated in order to verify this configuration concept by measurements of fluorescence from a single-component analyte with excitation and emission in a blue and a green band, respectively.

Results
For the growth of InGaN, we were able to demonstrate films that were optically active from 371 nm to 456 nm as shown in the room temperature photoluminescence, in Fig. 3. Emission from the films was typically single-peaked which indicates that segregation of the indium during the growth was mostly avoided. The layers were grown with indium/gallium flux ratios from 0.72 to 1.11 and In + Ga / Ga shutter ratios from 1 to 0.72.

Figure 3. Photoluminescence of various InGaN layers emitting at different wavelengths.

For the improvement of our base GaN layers, we were able to achieve layers with improved electrical and optical properties. As the left axis of Fig. 4 illustrates, the electron mobility of similarly doped GaN films was increased from 88.5 cm²/Vs to over 300 cm²/Vs. At the same time, the optical quality was improved as shown on the right axis of Fig. 4 by the reduction of room temperature PL FWHM values from 59 meV to 53 meV. In addition to room temperature measurements, our better layers exhibited excellent photoluminescence at 10K. Fig. 5 is a LT PL example of a layer with very bright near-band-edge emission at 354.6 nm with a narrow FWHM of 16 meV. The low temperature result is further indication of the optical quality of the recent films.

	Figure 4. Progression of Si-doped GaN films showing increased carrier mobility and reduced photoluminescence FWHM values. and n represent photoluminescence and mobility respectively.
	Figure 5. Photoluminescence at 10 K of typical improved Si-doped GaN film showing single peak and narrow line width.

Using the second sensor prototype we have measured fluorescence of a Fluorescein™ dye in methanol solutions at different pH values (Fig. 6). Also, fluorescence was measured with the second sensor prototype from chlorophyll extracted from three groups of green leaves (Fig. 7).

	Figure 6. Fluorescence of a Fluorescein™ dye in methanol solutions at different pH measured with the Prototype 2.
	Figure 7. Fluorescence measured with Prototype 2 from chlorophyll extracted from three groups of green leaves.

The detection limit in these measurements was poor because of a background signal resulting from the direct illumination of the photodiode from the LED through the sapphire substrate and by light scattered by other components. Enhancement of the signal was achieved by placing a metallic mirror in front of the sapphire substrate. With this set up we performed measurements based on: (a.) optical absorption in potassium and cobalt salts (Fig. 8); (b.) variation of the refractive index of commonly used organic solvents (Fig. 9);and (c.) scattering by alumina powders of various sizes (Fig. 10). These results indicate that several sensor functions can be achieved using the lateral component setup.

	Figure 8. Measurements of optical absorption by potassium and cobalt salts performed using Prototype 1.
	Figure 9. Measurements of the refractive index variation of commonly used organic solvents performed by using Prototype 1.
	Figure 10. Optical scattering by alumina powder of various sizes performed using Prototype 1.

In conclusion, the growth of III Nitrides by RF MBE needs to be further optimized in order to decrease the density of the vertical defects present in the layers. Fabrication and testing of III-N-based sensor simulators proved the concept of lateral component configuration in characterizing compounds used in various application fields. Applicability and multi-functionality of these sensors were confirmed by measurements of various fluorescing, absorbing, and scattering analytes. Miniaturized sensors based on this technology will be invaluable in a variety of space flight applications. Other applications may be found in environmental, biomedical, and petroleum related problems.

Acknowledgements
We would like to acknowledge NASA (collaborative agreement to the Space Vacuum Epitaxy Center (SVEC), the Texas Higher Education Coordinating Board (Texas ATP), and a Texas Space Grant Consortium (TSGC) grant for supporting this work.

References
¹H. Chuang, P. Macuch, and M. B. Tabacco. "Optical Sensors for Detection of Bacterial. 1. General Concepts and Initial Development," Anal. Chem. 73.3 (2001): 462-66.
²G. Liebsch, I. Klimant, B. Frank, G. Holst, and O. S. Wolfbeis. "Luminescence Lifetime Imaging of Oxygen, pH, and Carbon Dioxide Distribution Using Optical Sensors," Appl. Spectrosc. 54.4 (2000): 548-59.
³A.-C. Chang, J. B. Gillespie, and M. B. Tabacco. "Enhanced Detection of Live Bacteria Using a Dendrimer Thin Film in an Optical Biosensor," Anal. Chem. 73.3 (2001): 467-70.
⁴Y. Kostov, C. R. Albano, G. Rao. "All Solid-State GFP Sensor," Biotechnology and Bioengineering 70.4 (2000): 473-77.
⁵W. Lukosz. Sensors and Actuators B: Chemical (Oct. 1995).
⁶S. J. Pearton, C. R. Abernathy, F. Ren, F., Shul, R. J. Kilcoyne, S. P. Hagerott-Crawford, M. Zolper, J. C. Wilson, R. G. Schwartz, and J. M. Zavada. "Process development for III-V Nitrides," Materials Science and Engineering B 38.1/2 (1996): 138.
⁷I. Akasaki. "Progress in Crystal Growth and Future Prospects of Group III Nitrides by Metalorganic Vapor-Phase Epitaxy," J. Crystal Growth 195.1/4 (1998): 248.
⁸M. A. L. Johnson, J. D. Brown, J. W. Cook, Jr., J. F. Schetzina, H. S. Kong, and J. A. Edmond. "Molecular Beam Epitaxy Growth and Properties of GaN, InGaNI, and GaN/InGaN Quantum Well Structures," JVST B 16.3 (1998): 1282-85.
⁹D. Starikov, C. Boney, I. Berishev, I. C. Hernandez, and A. Bensaoula. "Radio-Frequency Molecular Beam Epitaxy Growth of III Nitrides for Microsensor Applications," JVST B 19.4 (2001):1404-08.

Publications
Starikov, D., C. Boney, I. Berishev, I. C. Hernandez, and A. Bensaoula. "Radio-Frequency Molecular Beam Epitaxy Growth of III Nitrides for Microsensor Applications," J. Vac. Sci.Tech. B 19.4 (2001): 1404-08.
Starikov, D., C. Boney, N. Medelci, J.-W. Um, A. Bensaoula, M. Larios-Sanz, and G. E. Fox. "Experimental Simulation of Integrated Optoelectronic Sensors Based on III Nitrides," J. Vac. Sci.Tech. B (Sept./Oct. 2001). (Accepted for publication.)

Theses or student reports
Um, J.-W. "Integrated Optoelectronic Sensor Based on GaN and its Simulation." M.S. degree, Department of Chemistry, 1999-2001.

Presentations
Starikov, D., C. Boney, J.-W. Um, N. Medelci, and A. Bensaoula. "Development of Integrated Optoelectronic Sensors Based on III Nitrides." 48th American Vacuum Society Int'l Symp., San Francisco, CA, Oct. 28-Nov. 2, 2001.
Bensaoula, A., D. Starikov, and C. Boney. "III-N Multifunctional Chemical Sensors on Si Substrates," invited talk, Barcelona Technical Institute-Univ. of Barcelona, Spain, Aug. 9, 2001.
Starikov, D., C. Boney, J.-W. Um, and A. Bensaoula. "Integrated Multifunctional Sensors Based on III-Nitrides," NanoSpace 2001, Galveston, TX, March 13-16, 2001.

Funding and proposals
Starikov, D. "Nitride-Based Structures for Implantable Glucose Sensors." Sept. 1, 2002-Aug. 31, 2004, NIH, $200,000; pending.
-. "Self-Aligned Multi-Color Photodetectors Based on III-Nitrides for Advanced Flame/Fire Detection." Jan. 1, 2002-Dec. 31, 2003, Texas Higher Education Coordinating Board, Texas ARP, $175,000.

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Institute for Space Systems Operations - 2001 Annual Report
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