Colorimetric Porous Photonic Bandgap Sensors With Integrated ...

IEEE SENSORS JOURNAL, VOL. 6, NO. 3, JUNE 2006

661

Colorimetric Porous Photonic Bandgap Sensors With Integrated CMOS Color Detectors Xiaoyue Fang, Vincent K. S. Hsiao, Vamsy P. Chodavarapu, Student Member, Albert H. Titus, Member, IEEE, and Alexander N. Cartwright, Member, IEEE

Abstract—In this paper, the development of a novel colorimetric sensor system based on the integration of complementary metal–oxide–semiconductor (CMOS) color detectors with a modified porous polymeric photonic bandgap sensor is reported. The color detector integrated circuit (IC) is implemented with AMI (AMI Semiconductor) 1.5 m technology, a standard CMOS fabrication process available at MOSIS (http://www.mosis.org). The color detectors are based on the spectral responses of buried double junctions (BDJs) and stacked triple junctions (STJs); the ratio of the photocurrents at the junctions provides spectral information. Both types of color detectors are characterized with a monochromator, and the results are compared. The BDJ color detector is used with a porous photonic bandgap reflection grating whose reflection spectra shifts as a function of the concentration of vapor analyte present. The experimental results verify that the color change of the photonic crystal can be detected and correlated to the change in analyte concentration. The entire system is compact and low power. Index Terms—Analog circuits, color photodetectors, integrated optoelectronics, porous plastics, sensors.

I. INTRODUCTION OLOR is the combination of hue, brightness (lightness), and saturation of an optical source or reflection from a surface. The detection of color provides a wealth of information about a scene, as illustrated by the number of animals with color vision. Color vision arises from the existence of different types of receptors each of which is sensitive to different peak wavelengths of light. The combination of the responses from these produces color perception. Color detection in a sensor system is also important since this can provide additional information about an environment. However, the extent to which the sensor system requires color perception as opposed to optical wavelength selectivity depends on the application. In this paper, we

C

Manuscript received February 7, 2005; revised April 25, 2005. We would like to acknowledge the generous financial support of the National Science Foundation (NSF) IGERT: Biophotonics-Materials and Applications Award #DGE0345408, NSF SENSORS Award #BES-0330240, and the Johnson and Johnson Focused Giving Grant. The associate editor coordinating the review of this paper and approving it for publication was Prof. Eugenii Katz. X. Fang was with the Department of Electrical Engineering, University at Buffalo, The State University of New York, Buffalo, NY 14260 USA. She is now with Aerotek Automotive, Oshawa, ON L1H 8P7, Canada (e-mail: [email protected]). V. K. S. Hsiao, V. P. Chodavarapu, A. H. Titus, and A. N. Cartwright are with the Department of Electrical Engineering, University at Buffalo, The State University of New York, Buffalo, NY 14260 USA (e-mail: [email protected]. edu; [email protected]; [email protected]; [email protected]). Digital Object Identifier 10.1109/JSEN.2006.874021

use monochromatic color detectors for measuring the shift in the wavelength of the incident light. Previously, a novel technique for monochromatic color detection using a buried double p–n junction (BDJ) structure has been presented [1], which is implemented with a standard complementary metal–oxide–semiconductor (CMOS) process [2], [3]. As example applications, the CMOS BDJ detector has been used for fluorescence detection in microarrays [4], and for the detection and measurement of ambient light sources [5], [6]. Applications in (bio)chemical/biological discrimination and seawater pH measurement have also been reported [7]–[9]. The methods for detecting organic chemical vapors can be classified into two broad types: electrical and optical. Optical measurement techniques are considered to be easier, faster, and safer as compared with the electrical methods [10], [11]. One optical method is based on monitoring the changes in the spectrum of light reflected from a periodic porous structure, such as a porous silicon photonic bandgap structure [12]–[14]. The sensor, made of the porous material, is placed in the environment to be monitored. When the analyte vapor diffuses into the pores of the sensor material, the effective refractive index of the material changes, and there is a corresponding change in the reflection peak (as a function of wavelength) of the sensor. Thus, the reflected light is of a different color, and the shift in detected color can indicate the presence and, ultimately, concentration of the analyte [15], [16]. In this paper, we report the first gas-phase analyte sensor using a CMOS-based monochromatic color detector integrated circuit (IC) and a porous photonic bandgap structure. The CMOS IC is fabricated through MOSIS and has a number of color detectors and readout circuitry; this is described in detail in Section II. The integration of the color detectors on standard CMOS chips eliminates any additional optical analysis instrumentation and permits signal conditioning circuitry on-chip. The photonic crystals are fabricated at the University at Buffalo, The State University of New York, using a recently developed method [17]. We discuss the fabrication of the photonic crystal in Section III. Results comparing these two different structures for different-sized active areas, as well as results of vapor detection using the photonic crystals, are presented in Sections IV and V, respectively. II. CMOS COLOR DETECTOR IC A. Detector Structure and Operation The color detectors presented in this paper are based on designs that are described in [1]–[4]. In that design, two standard

1530-437X/$20.00 © 2006 IEEE

662


Fig. 1. Buried double-junction structure (not to scale). Fig. 2. Triple-junction structure (not to scale).

p–n junctions are stacked vertically in the CMOS chip. The distribution of the absorbed light in the silicon layer is given by , where is the reflection coefficient of the air–silicon interface, is the incident light intensity, is the absorption coefficient of silicon, and is the depth into the sample. The absorption coefficient of silicon is a function of the wavelength of light and is larger for shorter wavelengths of light. More important, the distribution of photogenerated electron–hole pairs will follow a corresponding exponential decay throughout the depth of the sample. In the design presented above, each p–n junction is at a different depth in the silicon substrate, so there is a different photocurrent generated at each junction. For example, when a flux of longer wavelength light is incident on the detector, the deeper junction produces more current than if it were illuminated with a shorter wavelength light. In other words, the same flux of shorter wavelength light will produce more current in the shallower (near surface) junction and less in the deeper one. Therefore, the ratio of these photocurrents indicates the wavelength of the light incident on the chip. In this paper, we examine two structures: the buried double junction (BDJ) and stacked triple junction (STJ). Our detectors are fabricated using a standard CMOS process, so all layers used to form the detectors are standard layers. The BDJ structure, depicted in Fig. 1, is a pair of stacked p–n junctions formed using the p-substrate/n-well/p-base regions. The STJ structure (Fig. 2) is a triplet of p–n junctions formed using p-substrate/nwell/p-base/n-select regions. The color detector based on the BDJ structure has two p–n junctions. Since the p-substrate is common to the entire chip, the current cannot be directly measured, so the currents that are (see Fig. 1). The STJ strucmeasured are and the sum ture stacks one additional p–n junction on top of the BDJ structure, which produces one additional photocurrent. Since this top junction is between the n-select and p-base layers, it is closer to the surface on which the light is incident than the top junction

Fig. 3. Micrograph of the fabricated chip. Labels (1–9) are the detectors; see Table I for details.

in the BDJ. As a result, the difference between the photocurrent generated at the upper junction and lower junction is larger. Again, the currents that are measured are labeled in Fig 2. B. Integrated Circuit A micrograph of the integrated circuit (chip) is shown in Fig. 3. The advantage of fabrication through a standard CMOS process means that the ultimate cost of such a device will be low, and additional signal processing can be put on-chip. However, we have no control of the depth of each junction. The prototype chip has a number of different devices on it to allow us to characterize the structures, which are detailed in Table I. The

FANG et al.: COLORIMETRIC POROUS PHOTONIC BANDGAP SENSORS WITH INTEGRATED CMOS COLOR DETECTORS

663

TABLE I RESPONSE OF DETECTORS AT 632 nm (HeNe LASER)

Fig. 5. Experimental setup.

vapor. This changes the average refractive index of the material, and, subsequently, the grating spacing according to the formula Fig. 4. Acetone sensor response for difference concentrations.

peripheral area of each detector is covered by metal to prevent the generation of unwanted photocurrents. The chip is fabricated in 1.5 m AMI process available through MOSIS. This process has two metal layers and two polysilicon layers. Our prototype is a 2 mm 2 mm silicon die that is packaged in a 40-pin DIP ceramic package. Since the chip is fabricated through a commercial process, the junction depths and doping concentration of each layer are not publicly available. III. MODIFIED PHOTONIC CRYSTAL The photonic crystal used for our sensor is a made from a polymer material; the periodic structure is formed simply through a holographic process [18], [19]. This has many benefits over the more complex process of etching the periodic structure in materials such as porous silicon [20]. The prepolymer syrup composed of a monomer, photoinitiator, co-initiator, and liquid crystal (LC) are sandwiched between two glasses, and the periodic structure is formed by the optical interference pattern all in one step. The porous grating structure is created by adding a nonreactive solvent, such as acetone or toluene, into the prepolymer syrup. The grating forms when the nonreactive solvent evaporates after opening the cover glass slide [20]. More important, the Bragg reflection notch shifts when the voids inside the grating structure fill with solvent

where is the reflected wavelength, is the average refractive index of grating film, and is the grating spacing. The simplicity of the fabrication, combined with the CMOS color detectors, creates a straightforward, low-cost sensor system. Fig. 4 is the reflection spectra of grating cell filled with different concentrations of acetone vapor. The Bragg wavelength shifts from 540 to 580 nm with only 9% of acetone vapor. Increasing the concentration of the acetone vapor not only changed the average index of the grating but also the effective grating spacing. The change in the reflected light can be observed even with the naked eye. IV. EXPERIMENTAL RESULTS AND MEASUREMENTS A. Monochromator Characterization The color detectors are first characterized using a monochromator to provide a narrowband optical input [full-width at halfmaximum (FWHM) of approximately 5 nm) and using a tungsten lamp light source. The incident light intensity on the detectors is wavelength dependent, and the average intensity is approximately 1 W/mm . There are four BDJ structures on the chip and two STJ structures (see Fig. 3 and Table I). For the BDJ structures, we plot the responses for the two larger detectors. The two junctions are biased as shown in Fig. 5. is the is the current through current through the top junction, and the bottom junction. To reiterate, since the p-substrate is always connected to ground, current readout can only be taken from

664


the range of 425 to 700 nm. We used a low-pass filter for measurements below 525 nm to prevent frequency doubling in the monochromator, and this was removed at 525 nm. Thus, what appear to be prominent sidelobes at 570 nm are a result of the change in the light intensity hitting the detectors; this produces the change in current. The peak at 470 nm for the top junction currents ( ) occur because the responsivity for silicon drops for shorter wavelengths [21]. We also analyze the STJ structures with the monochromatic light input. There are two sizes of STJ-based detectors: 200 m 200 m and 500 m 500 m. In this device, is the current through the top p–n junction (n-select/p-base), is the current through the middle p–n junction (p-base/n-well), and is the current through the bottom p–n junction (n-well/p-substrate). In this triple-junction structure, the P-substrate and P-base are internally connected and grounded (connected to , and are ground on-chip). The output currents , read directly from the chip, and the current ratios are calculated from these. Fig. 5(a) shows the current versus wavelength for each size. For both sizes of the triple-junction detector, the top diode has maximum response at approximately 450 nm while the maximum for the bottom diodes occurs between 500 and 700 nm. as the indicator of wavelength; We use the ratio this ratio is good in a spectral range of 425 to 600 nm [see Fig. 5(b)]. The double-junction detector working range begins at wavelengths around 425 nm. At wavelengths longer than 600 nm, the currents are still wavelength dependant, but since little is absorbed at the top junction, the ratio becomes nearly constant. B. Comparison of Two Structures

2

Fig. 6. (a) Experimental results for the BDJ structure with the 500 m 500 m- and the 300 m 300 m-sized detectors; (b) comparison of normalized values; currents are normalized to the peak value for each size.

2

P-base and N-well, which are currents and , respectively. Fig. 6(a) shows the current versus wavelength for both the sizes. We can see that the bottom junction produces a larger response for longer wavelengths, while the top junction produces a larger current at shorter wavelengths, as expected. In addition, the larger detector produces a larger current. In order to better compare the two sizes, we normalize the currents from each detector, i.e., set the largest current value for each detector to unity and calculate the other values proportionally. This is shown in Fig. 6(b). Both diodes have nearly the same response at the top junction while the smaller diode has a larger normalized deeper junction response. This results in the larger size detector having a larger ratio range, which is shown in Fig. 6(b). The difference due to size is because the contacts for the detectors are located around the edge of each; therefore, fewer of the carriers generated in the larger detector at the lower junction may be collected at the contacts. Based on these experimental results, the 300- m detector is acceptable, but the 500- m detector is better since the ratio is higher. Thus, the current ratio is a good measure of spectral information in

In order to function properly, the detectors must produce a current ratio that is spectrally dependent. For the BDJ detector, we obtain and from the chip directly, calculate from this, and use as the color/wavelength measure. For the STJ detector, since it is not possible to get or directly, the ratio is used as the wavelength indicator. Comparing Fig. 6 with Fig. 7, it is clear that the STJ structure produces larger currents than the BDJ structure for the same size device. However, the current ratio, not absolute value, is used to indicate color. The graph in Fig. 8 depicts the current ratios for both structures. From this, we see that the BDJ detectors have a larger change in ratio across the wavelengths 425–670 nm; this indicates that the BDJ detectors have much better resolution than the STJ detectors. This is true for the smaller 300 m 300 m BDJ detector as well. In addition, at longer wavelengths, the STJ detectors change in current ratio is extremely small. However, the additional p–n junction enables the STJ structure to respond better to lower intensity light. C. Photonic Crystal Acetone Sensor The photonic crystal exhibits a shift in the reflection peak as a function of acetone concentration in air. Fig. 3 shows the response of the photonic-crystal sensor when illuminated with white light. Clearly, the shift occurs from 540 to 635 nm while the acetone concentration increases from 0% to 22%. A drop of acetone (0.01 g, with a relative evaporation rate of


665

Fig. 8. Comparison of the current ratios for both types of detectors.

2

Fig. 7 (a) Experimental results for the STJ structure with the 200 m 200 m- and the 500 m 500 m)-sized devices; (b) comparison of the normalized currents.

2

7.7 surface [22].

) is dropped onto 25-mm square crystal

V. PHOTONIC CRYSTAL WITH INTEGRATED CMOS COLOR DETECTORS m BDJ detector to detect the We use the 500 m reflected signal from the photonic-crystal sensor. In our experiment, the CMOS color detector is positioned facing the photonic crystal. A tiny drop of acetone is dropped onto the crystal surface. At this time, the acetone concentration in air surrounding the crystal reaches a maximum, and it reflects in the red/orange wavelengths; the detector IC produces a response that indicates a longer wavelength spectrum. As the acetone evaporates, i.e., the acetone concentration decreases, the reflected color of the light changes to yellow, and then to green (to shorter and shorter wavelengths). The test setup is shown in Fig. 5. Thus, we measure a change over time of the acetone concentration. Fig. 9 is a plot of the measured and calculated currents as a and that are function of time. The three curves are

Fig. 9. Measurements with the photonic-crystal sensor. The plot shows typical changes in the currents and current ratio with time. The “zero” time on the x axis is actually 55–65 s after the acetone drop is placed into the chamber. Thus, the total time of measurement until the response stops changing can be up to 100 s.

directly read from the chip output and that is calculated from the measured currents. The axis is time, with the time starting when the reflected light changes to red. During the time period shown, the reflected light changes from red (wavelength of approximately 635 nm) to green (wavelength of approximately 540 nm). We see that the top-junction current increases as the color changes from longer wavelengths to shorter wavelengths, while the bottom-junction photocurrent decreases. Therefore, as indicated in Fig. 9 with respect to the right-side scale, the curincreases. Since the spectrum of the reflected rent ratio light is not as narrow as the monochromator output, the ratios cannot be directly compared. However, the change in the current and current ratio is correct and correctly indicates the color shift, and in turn, the acetone concentration in the air. VI. CONCLUSION In this paper, a color detector has been designed and fabricated with AMI 1.5 technology. This detector utilized buried double-junction (BDJ) or triple-junction structure. Since junctions at different depths have different spectral responses, the

666


ratio of the photocurrents at shallower and deeper junctions provides spectral information. Both of the structures are tested with monochromator. The experimental results from the chip show that the BDJ detectors function well over a spectral range of 425 to 700 nm. The BDJ detectors excel the triple-junction detectors with better resolution and wider usable range, but the triple-junction detectors have better responsivity, which is better for detecting in lower light intensity. However, it is possible for a different BDJ detector structure to approach the responsivity of the STJ, but for commercial process-based CMOS detectors, the STJs will always be larger. The size of the detector is a function of the diffusion and well regions. The 500 m 500 m BDJ color detector was chosen to use in an acetone sensor. The acetone sensor is a photonic crystal that reflects different wavelengths of light at different acetone concentrations in the air when illuminated with white light. The color detector chip produces an output signal (current ratio) that indicates a change in reflected wavelength from the crystal. REFERENCES [1] G. N. Lu, M. B. Chouikha, G. Sou, and M. Sedjil, “Color detection using a buried double p–n junction structure implemented in the CMOS process,” Electron. Lett., vol. 32, pp. 594–596, 1996. [2] G. N. Lu, G. Sou, F. Devigny, and G. Guilland, “Design and testing of a CMOS BDJ detector for integrated microanalysis systems,” Microelectron., vol. 32, pp. 227–234, 2001. [3] G. N. Lu, J. M. Galvan, C. Jeloyan, G. Goumy, and V. Marcoux, “Sensitivity estimation of CMOS optical BDJ detector,” Mater. Sci. Eng. C, Biomimetic Mater., Sensors Syst., vol. C21, pp. 203–210, 2002. [4] G. N. Lu, G. Guillaud, G. Sou, F. Devigny, M. Pitaval, and P. Morin, “Investigation of CMOS BDJ detector for fluorescence detection in microarray analysis,” in Proc. 1st Annu. Int. IEEE-EMBS Special Topic Conf. Microtechnologies in Medicine Biology, 2000, pp. 381–386. [5] G. de Graaf and R. F. Wolffenbuttel, “Smart optical sensor systems in CMOS for measuring light intensity and color,” Sens. Actuators A, Phys., vol. A67, pp. 115–119, 1998. [6] ——, “Optical CMOS sensor system for detection of light sources,” Sens. Actuators A, Phys., vol. A110, pp. 77–81, 2004. [7] M. Sedjil, G. N. Lu, G. Michard, and F. Prevot, “A colorimetric method with the use of BDJ detector for seawater pH measurement,” Analytica Chimica Acta, vol. 377, pp. 179–184, 1998. [8] D. P. Poenar, T. M. Siu, and T. O. Kiang, “Color sensor for (bio)chemical/biological discrimination and detection,” Mater. Sci. Semiconductor Process., vol. 5, pp. 17–22, 2002. [9] Y.-J. Kook, J.-H. Lee, Y.-J. Park, and H.-S. Min, “A new CMOS pixel for a biochip,” in Proc. 9th Korean Conf. Semiconductors, Chunan, Korea, pp. 209–210. [10] T. Gao, J. Gao, and M. J. Sailor, “Tuning the response and stability of thin film mesoporous silicon vapor sensors by surface modification,” Langmuir, vol. 18, pp. 9953–9957, 2002. [11] Y. Y. Li, F. Cunin, J. R. Link, T. Gao, R. E. Betts, S. H. Reiver, V. Chin, S. N. Bhatia, and M. J. Sailor, “Polymer replicas of photonic porous silicon for sensing and drug delivery applications,” Science, vol. 299, pp. 2045–2047, 2003. [12] F. Cunin, T. A. Schmedakes, J. R. Link, Y. Y. Li, J. Koh, S. N. Bhatia, and M. J. Sailor, “Biomolecular screening with encoded porous-silicon photonic crystals,” Nature Mater., vol. 1, pp. 39–41, 2002. [13] V. S.-Y. Lin, K. Motesharei, K.-P. S. Dancil, M. J. Sailor, and M. R. Ghadiri, “A porous silicon-based optical interferometric biosensor,” Science, vol. 278, pp. 840–843, 1997. [14] J. Diener, N. Künzner, D. Kovalev, E. Gross, V. Y. Timoshenko, G. Polisski, and F. Koch, “Dichroic Bragg reflectors based on birefringent porous silicon,” Appl. Phys. Lett., vol. 78, pp. 3887–3889, 2001. [15] L. Pancheri, C. J. Oton, Z. Gaburro, G. Soncini, and L. Pavesi, “Improved reversibility in aged porous silicon NO2 sensors,” Sens. Actuators B, Chem., vol. 97, pp. 45–48, 2004. [16] Z. Gaburro, L. Pavesi, C. Baratto, G. Faglia, and G. Sberveglieri, “A porous silicon microcavity as an optical and electrical multiparamteric chemical sensor,” NATO Science Series, II: Mathematics, Physics and Chemistry, Frontiers of Multifunctional Nanosystems vol. 57, pp. 399–412, Springer, 2002, E. V. Buzaneza and P. Scharff, Eds..

[17] V. K. S. Hsiao, T. C. Lin, G. S. He, A. N. Cartwright, P. N. Prasad, L. V. Natarajan, V. P. Tondiglia, and T. J. Bunning, “Optical microfabrication of highly reflective volume Bragg gratings,” Appl. Phys. Lett., vol. 86, pp. 13113–1–13113-3, 2005. [18] L. V. Natarajan, C. K. Shepherd, D. M. Brandelik, R. L. Sutherland, S. Chandra, V. P. Tondiglia, D. Tomlin, and T. J. Bunning, “Switchable holographic polymer-dispersed liquid crystal reflection gratings based on thiol-ene photopolymerization,” Chem. Mater., vol. 15, pp. 2477–2484, 2003. [19] V. P. Tondiglia, L. V. Natarajan, R. L. Sutherland, D. Tomlin, and T. J. Bunning, “Holographic formation of electro-optical polymer-liquid crystal photonic crystals,” Adv. Mater., vol. 14, pp. 187–191, 2002. [20] V. Mulloni and L. Pavesi, “Porous silicon microcavities as optical chemical sensors,” Appl. Phys. Lett., vol. 76, pp. 2523–2525, 2000. [21] S. M. Sze, Physics of Semiconductor Devices, 2nd ed. New York: Wiley, 1981. [22] VWR Material Safety Data Sheet [Online]. Available: http://www. vwrsp.com/msds/10/JT9/JT9002-2.htm

Xiaoyue Fang received the B.Eng. degree from Tongji University, China, in 1998 and the M.S. degree from the University of Buffalo (UB), The State University of New York, Buffalo, in 2004, both in electrical engineering. She was a Research Assistant in the Analog VLSI Laboratory at UB. She is currently with Aerotek Automotive, Oshawa, ON, Canada. Her interests include analog VLSI and integrated sensor systems.

Vincent K. S. Hsiao is currently working toward the Ph.D. degree in the Department of Electrical Engineering of the University of Buffalo (UB), The State University of New York, Buffalo. Currently, he is a Research Assistant in the Institute for Lasers, Photonics and Biophotonics, UB. His research areas of interest are one-dimensional periodic porous polymer structures and nanoparticle patterning in polymer matrix using holographic interference pattern.

Vamsy P. Chodavarapu (S’03) received the B.E. degree in electronics and instrumentation engineering from Osmania University, India, in 2001 and the M.S. degree in electrical engineering from the University at Buffalo (UB), The State University of New York, Buffalo, in 2003. Currently, he is working toward the Ph.D. degree in electrical engineering at the same university. His research interests are CMOS integrated chemical and biological sensors, optical sensor integration, and data acquisition and processing.

Albert H. Titus (S’86–M’97) received the B.S. and M.S. degrees from the University at Buffalo (UB), The State University of New York, Buffalo, in 1989 and 1991, respectively, and the Ph.D. degree from the Georgia Institute of Technology, Atlanta, in 1997. Prior to joining the faculty at Buffalo, he was an Assistant Professor at the Rochester Institute of Technology, Rochester, NY. Currently, he is an Assistant Professor in the Department of Electrical Engineering at the University at Buffalo, The State University of New York, Buffalo. His research interests include analog VLSI implementations of artificial vision, hardware and software artificial neural networks, hardware implementations of decision-making aids, optoelectronics, and integrated sensor systems. Dr. Titus is a member of the INNS, ASEE, and SPIE. He has been a reviewer for many journals and conferences and has numerous research grants from federal and private sources, including an NSF CAREER Award.


Alexander N. Cartwright (S’94–M’95) received the B.S. and Ph.D. degrees in electrical and computer engineering from the University of Iowa, Iowa City. He is an Associate Professor of Electrical Engineering, Director of the Institute for Lasers, Photonics and Biophotonics and Co-Director of the Electronics Packaging Laboratory at the University at Buffalo (UB), The State University of New York, Buffalo. His research is focused on III–Nitride materials, quantum dot materials, optical nondestructive testing of stress and strain for device reliability, optical sensors, biophotonics, nanophotonics, and nanoelectronics.

667

Dr. Cartwright received an NSF CAREER Award in 1998 and a Department of Defense Young Investigator Award for research on optical properties of III–Nitride materials in 2000. More recently, he was the recipient of the 2002 State University of New York’s Chancellor’s Award for Excellence in Teaching.