Monolithic Integrated Piezoelectric MEMS-Tunable ... - Semantic Scholar
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Monolithic Integrated Piezoelectric MEMS-Tunable VCSEL Michael C. Y. Huang, Student Member, IEEE, Kan Bun Cheng, Ye Zhou, Student Member, IEEE, Albert P. Pisano, and Connie J. Chang-Hasnain, Fellow, IEEE
Abstract—A novel actuation mechanism that utilizes the inherent piezoelectric properties of Alx Ga1−x As compounds is presented for short-wavelength microelectromechanical systems tunable vertical-cavity surface-emitting laser centered at 850 nm. Piezoelectric actuation can provide precise bidirectional displacements and linear tuning characteristics with respect to the applied voltage, does not suffer from travel limitations and catastrophic electrical damage, consumes low power (∼1 µW), and has a relatively fast response compared to thermal actuation. In addition, piezoelectric actuation based on Alx Ga1−x As films offers a large piezoelectric coupling coefficient comparable to those of zinc oxide and quartz, and it can be integrated with high-speed electronic and optoelectronic devices. Single-mode emission with a linear continuous tuning range of 3 nm and low power consumption were experimentally obtained. Index Terms—Laser tuning, microelectromechanical systems (MEMS), tunable optoelectronic devices, vertical-cavity surfaceemitting laser (VCSEL).
I. INTRODUCTION ERTICAL-CAVITY surface-emitting lasers (VCSELs) are a new class of semiconductor lasers that emerged during the 1990s [1]–[8]. They operate with a single wavelength emission by virtue of an extremely small cavity length. The main advantages are surface-normal emission for easy output coupling, compatibility for 2-D arrays, device fabrication, and testing at the wafer level, which essentially allow low-cost manufacturing. Also, VCSELs can be directly modulated by varying their bias current up to 10 GHz, which is very desirable in the application of digital and analog fiber-optic communication systems. The integration of VCSEL and microelectromechanical systems (MEMS) gives rise to an ideal wavelength-tunable light source that leverages the advantages from both sides [9]–[12]. Since VCSEL fabrication utilizes a monolithic process, it can be easily integrated with MEMS structures to leverage the mechanical movements. It has been demonstrated that the combination of the two can yield a wide and continuous wavelength-tuning range, utilizing a simple wavelength-control mechanism.
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Manuscript received October 4, 2006; revised February 15, 2007. This work was supported in part by the Defense Advanced Research Projects Agency (DARPA) Chip-Scale Atomic Clock Grant NBCH1020005 and in part by the Berkeley Microfabrication Laboratory. M. C. Y. Huang, Y. Zhou, and C. J. Chang-Hasnain are with the Department of Electrical Engineering and Computer Sciences, University of California, Berkeley, CA 94720 USA (e-mail: [email protected]; yezhou@ eecs.berkely.edu; [email protected]). K. B. Cheng and A. P. Pisano are with the Department of Mechanical Engineering, University of California, Berkeley, CA 94720 USA (e-mail: kbcheng@ me.berkeley.edu; [email protected]). Digital Object Identifier 10.1109/JSTQE.2007.894056
Since the first demonstration by Wu et al. [13], MEMStunable VCSELs have been extensively studied for various applications, including signal routing and switching in optical networks, gas, chemical, and biomolecular sensing [14]–[16], spectroscopy, and optical excitation of rubidium species in an atomic clock system [17]. The mechanical actuation mechanisms previously reported have been either electrostatic or thermal actuation. Electrostatic actuation is often limited by the 1/3 rule, where the maximum deflection can be at the most one third of the original airgap spacing. The most devastating drawback is the possibility of catastrophic electrical discharge that damages VCSELs when pull-in occurs. In addition, relatively large voltage and power are required to actuate the beam, due to the leakage current across an intrinsic GaAs layer that often serves as the sacrificial material. Thermal actuation, on the other hand, creates a large heating source near the laser-active region, which may adversely affect laser reliability and performance. In addition, the actuation speed is fairly low (∼100 Hz) due to long thermal transient time and high actuation power generally required by thermal actuation [12]. Recently, we proposed the design of a monolithic integrated piezoelectric wavelength-tunable VCSEL that exploits the inherent piezoelectric properties of Alx Ga1−x As compounds [18], [19]. Piezoelectric actuation offers several potential advantages [20], [21]. First, it can provide large mechanical deflections in bidirections, both toward the substrate and opposite to it. The movement is not limited by pull-in effect, as opposed to the 1/3-gap limit associated with the electrostatic actuation. More importantly, there will be no catastrophic damage due to capacitor discharge. It is expected to consume low power (∼ 1 µW), and has a relatively fast response speed (limited only by the mechanical structure rather than the piezoelectric effect). The piezoelectric effect has the advantages of high electromechanical coupling strength and linear response, compared to electrostatic actuation. Finally, piezoelectric actuation can be readily extended to other MEMS-tunable optoelectronic devices operating at different wavelengths. In this paper, we present a novel piezoelectric-actuated MEMS-tunable VCSEL structure. The piezoelectric cantilever beam is monolithically integrated within the VCSEL distributed Bragg reflector (DBR) layers by properly designing the doping profile. Single-mode emission (40 dB) and linear, continuous wavelength tuning (∼ 3 nm) were achieved for an 850-nm VCSEL at room temperature under continuous-wave (CW) operation. In addition, we also present the mechanical characterization of the piezoelectric cantilever beams, which
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Fig. 1. Illustration of the cantilever beam under the piezoelectric effect. When applying a reverse-biased voltage, the vertical electrical field across the intrinsic piezoelectric layer causes a net bending moment that deflects the cantilever beam downward. The beam deflection changes the airgap of the tunable VCSEL and, hence, changes the emission wavelength.
can be used in assisting the mechanical actuator design of the MEMS-tunable VCSEL. II. PIEZOELECTRIC BEAM DESIGN The piezoelectric property of AlGaAs is an inherent characteristic that originates from the zinc-blend crystalline structure that lacks center inversion symmetry [22]. Under the presence of a vertical electric field, an intrinsic AlGaAs beam is subject to a longitudinal strain caused by internal polarization that results in a change in its length (∆L). The magnitude of the piezoelectric effect is determined by the magnitude of the applied field and d14 piezoelectric coefficient, which is a function of the aluminum composition. For instance, d14 = 3.59 pC/N for Al0.8 Ga0.2 As and d14 = 2.80 pC/N for Al0.1 Ga0.9 As. The governing piezoelectric coefficient is dependent on the orientation; it shows a maximum in the 110 direction and zero along the 100 direction. The inherent piezoelectric effect of GaAs can be characterized by the piezoelectric matrix as shown in the equation at the bottom of the page. By utilizing this property, a piezoelectric-actuated cantilever structure is proposed, which comprises of a thin n-doped, a thin intrinsic, and a thick p-doped Alx Ga1−x As layer, as shown in Fig. 1. For a reverse voltage applied across the top n-type and the bottom p-type layers, the applied voltage results in an electric field across the middle intrinsic piezoelectric layer of the cantilever beam. Such a vertical electric field produces a longitudinal strain (∆L) in the intrinsic layers via the converse piezoelectric effect. Since the thin piezoelectric layer is rigidly connected to the cantilever beam, which constrains the longitudinal motion of the piezoelectric layer, an equivalent bending moment (Mp) is applied to the end of the beam. The bending
0 [d] = 0 −d14 cos(2Φ)
0 0 0 0 d14 cos(2Φ) 0
Fig. 2. Schematic of the piezoelectric-actuated MEMS-tunable VCSEL. (a) Side view showing the mechanical configuration. (b) Cross-sectional view showing the optical design.
moment produces a deflection (δ) at the tip of the cantilever beam that changes the airgap size, which then varies the emission wavelength of the VCSEL. Due to the p-i-n doping choice, the cantilever structure in this particular design could only move toward the substrate. It is expected that a better implementation of the doping scheme, such as n-i-n or p-i-p structure, can enable movement in both directions and, hence, double the potential tuning range. For simplicity and quick proof of concept, the clamped cantilever beam design is adopted as the basic design for our first prototype in which the longitudinal mode (d31 ) is used for actuation. A more sophisticated structure (such as a V-shape structure) can be designed based on the cantilever beam design performance. III. VCSEL DESIGN The schematic of the optical laser design of the MEMStunable VCSEL is shown in Fig. 2. The device consists of a 34-pair n-doped DBR bottom mirror, a λ-cavity layer with the GaAs quantum-well active region, and a movable top mirror. The top mirror is comprised of three parts (starting from the substrate side): fixed four pairs of p-doped DBRs, a variable airgap, and a freely suspended 15 pairs of DBRs mirror that is supported via a cantilever structure. The material of DBR consists of alternating layers of Al0.12 Ga0.88 As and Al0.9 Ga0.1 As. The airgap has a designed free-standing thickness of 1.8 µm (corresponds to 9λ/4 in optical thickness), and d14 sin(2Φ) d14 cos(2Φ) 0
−d14 cos(2Φ) −d14 sin(2Φ) 0
0 0 d14 sin(2Φ)
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is formed by selectively removing the sacrificial layer material (p-doped GaAs) underneath the cantilever. The suspended DBR cantilever is designed with an unevenly distributed p-i-n doping profile (from the substrate side): a thick nine-pair p-doped DBR, a thin four-pair intrinsic DBR, and a thin two-pair n-doped DBR. In such a configuration, the neutral axis of the beam is shifted upward from the center; hence, the longitudinal strain resulting from the piezoelectric actuation can be effectively transferred to a bending moment. However, the intrinsic DBR layers are subject to unintentional doping from the background during the material epitaxy growth, which may limit the maximum electric field allowed for piezoelectric actuation. Electric current injection is conducted through the middle laser contact (via the four pairs of p-doped DBR) and backside ground contact. An aluminum-oxide aperture is formed on an Al0.98 Ga0.02 As layer in the p-DBR section, immediately above the cavity layer, to provide efficient current and optical confinement. The piezoelectric tuning is conducted through the top tuning contact (via the n-DBR) and the middle laser contact. Wavelength tuning is accomplished by applying a reverse voltage bias across the top n-doped layer and the middle p-doped layers. The applied voltage results in a vertical electric field across the intrinsic DBR layers that induce mechanical deflection via the piezoelectric effect. The beam deflection reduces the airgap size and then, consequently, changes the Fabry–Perot resonance of the cavity. Hence, it blue-shifts the emission wavelength of the VCSEL. IV. FABRICATION The fabrication process of the piezoelectric MEMS-tunable VCSEL has a few extra processing steps as compared to that of the conventional VCSEL. First, a top tuning contact consisting of Ni–Ge–Au alloy is deposited by electron beam evaporation on the top n-doped layer. Then, a wet-chemical vertical etch is required to etch down to the bottom DBR layers and form the VCSEL mesa structure. The device then goes through a thermal wet-oxidation process at 450 ◦ C to form the aluminum-oxide aperture for electrical and optical confinement. Typically, an oxide aperture less than 3 µm is required for a single transverse mode emission from VCSEL. After oxidation, we performed another wet-chemical vertical etch to form the cantilever structure, in addition to exposing the embedded layer necessary to form the laser contact. Next, a Ti–Au alloy metal was deposited to form the laser current contact on the p-DBR layers above the cavity. Finally, a wet-chemical selective etch process was carried out to remove the GaAs sacrificial material underneath the cantilever to form the freely-suspending beam. The wetchemical selective etch was followed by a critical point drying process to avoid the surface stiction problem. Fig. 3(a) shows the scanning electron microscope (SEM) image of a fabricated tunable VCSEL device. The bottom surface roughness is mainly due to nonuniformity from the wet-chemical etch process, but it is expected not to have any effect on the device performance. A highly selective citric-acid-based chemical etch is required to remove the GaAs sacrificial layer underneath the cantilever, and form the freely suspending cantilever beam [19], [23].
Fig. 3. SEM image of the fabricated piezoelectric-actuated MEMS-tunable VCSEL. (a) Top view. (b) Side-view zooming in the suspended cantilever head. The suspended piezoelectric cantilever was formed by selectively removing the GaAs sacrificial material underneath the beam.
By using this wet-etch chemistry, highly selective removal of the sacrificial GaAs against Al0.12 Ga0.88 As (the low aluminum component of the DBR) was achieved, with an etch selectivity > 100. In addition, the beam bending induced from the residual stress is negligible, as observed from the SEM image. Compared to the conventional reactive-ion etch (RIE)-based selective etch method, the chemical-based selective etching enables a very low-cost and rapid fabrication cycle, in addition to improving the device yield and reproducibility. Fig. 3(b) shows the zoom-in image of the released cantilever suspending over the VCSEL cavity. The smoothness from the cantilever and bottom mesa surface after the selective etch implies the high selectivity in the wet-chemical GaAs sacrificial layer etches. V. MECHANICAL CHARACTERIZATION The mechanical deflection of the piezoelectric-actuated cantilever beams were experimentally obtained by monitoring their center height change as a function of the applied voltage, using both white light interferometer and atomic force microscopy (AFM) measurement. To optimize the piezoelectric tuning performance, a preliminary experimental study of the piezoelectric effect with different cantilever orientations and beam dimensions has been performed. As the VCSEL epitaxy wafers were grown from (100) GaAs substrate, the orientation of the fabricated cantilevers is rotated from (110) to (100) by lithography. Fig. 4(a) shows the measured vertical deflection for different cantilever beam orientations with respect to (110), but with same dimensions (length of 240 µm and width of 10 µm). Because of
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Fig. 5. Resonant frequency measurement for two piezoelectric-actuated cantilevers with different lengths using an AFM in tapping mode. It shows the fundamental and higher order vibration modes for the two cantilevers. The calculated values of resonant frequency using the ANSYS simulation are 123 and 30 kHz for 120- and 240-µm beams, respectively.
Fig. 4. Mechanical characterization on the piezoelectric-actuated cantilever beams. (a) Cantilever deflection as a function of the applied voltage for different beam orientations with respect to (110), all with the same dimensions (length of 240 µm and width of 10 µm). (b) Cantilever deflection as a function of the applied voltage for three different beam lengths, all aligned along the110 direction.
crystal asymmetry in Alx Ga1−x As compound, the piezoelectric coefficient governing the piezoelectric actuation varies with crystal orientation, which results in decreasing magnitude of piezoelectric actuation toward the 100 direction (45◦ ). As expected from the GaAs piezoelectric matrix, the piezoelectric actuation exhibits a strong dependence on the orientation, where the beam deflection is largest for beams aligned to 110 direction (140 nm), and the deflection magnitude decreases as the angle of the beam orientation increases toward 100 direction. The angular dependence is a clear demonstration of the piezoelectric effect, which cannot be induced by thermal or electrostatic actuation. When increasing the length of the cantilever beam, the deflection magnitude increases due to the decrease in beam stiffness. Fig. 4(b) shows the vertical deflection measured by AFM for three different cantilever beam lengths aligned along 110 direction. Maximum deflections of 10, 130, and 270 nm were measured for beam lengths of 120, 240, and 360 µm, respectively. Similar to Fig. 4(a), the mechanical deflection exhibits a
highly smooth and linear characteristic, as expected for a piezoelectric actuation. It is important to note that the piezoelectricactuation-induced beam deflection has very negligible effect on the cantilever beam curvature, since the deflection magnitude is often less than 0.1% of the beam length. Fig. 5 shows the resonance frequency measurement for two of the piezoelectric cantilever beams of different lengths (120 and 240 µm), showing the fundamental and higher order vibration modes. The measurement was performed by using AFM in the tapping mode, where the AFM cantilever is driving the MEMS piezoelectric cantilever at different frequencies and monitoring the deflection magnitude. The beams were driven to resonance in simple flexural mode in air, and the resonance frequency was measured to be ∼ 93 kHz for a beam length of 120 µm and ∼23 kHz for 240 µm. The measured resonance frequencies, in general, agree well with those from our 2-D plane strain model and ANSYS simulations, where the calculated values are 123 and 30 kHz, respectively. The discrepancy is mainly due to the air damping and considerable mechanical coupling between the AFM tip and cantilever beams at lower resonance frequencies. VI. EXPERIMENT RESULTS By integrating the piezoelectric cantilever (200 µm × 15 µm) with the VCSEL cavity, CW wavelength tuning using the piezoelectric actuation was experimentally demonstrated in room temperature operation. Fig. 6 shows the VCSEL optical characteristics (without applying the tuning voltage) for output power versus bias current (LI), and voltage versus bias current (VI). The device exhibits a lasing threshold current of 1.2 mA and maximum output power of ∼ 1.2 mW with the slope efficiency of 0.3 mW/mA. When applying a bias voltage across the tuning contacts, the piezoelectric effect deflects the cantilever beam, which translates into a change in the airgap and, consequently, varies the laser emission wavelength. Fig. 7(a) shows the measured emission spectra of the piezoelectric-actuated device under various
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Fig. 6. Measured optical characterization showing the LI and IV characteristic of a fabricated VCSEL (without external tuning voltage). The VCSEL exhibits a maximum output power of 1.2 mW and the threshold current is 1.2 mA.
tuning voltages. With the oxide aperture of 3 µm, a singlemode emission (40 dB) is obtained throughout the entire range of ∼ 3 nm. In addition, the peak spectral intensity remains fairly constant throughout the entire tuning range, indicating that tilting loss from cantilever deflection is negligible. Fig. 7(b) shows the emission wavelength as a function of the applied voltage across the top tuning electrodes and the corresponding IV characteristic across the p-i-n structure within the piezoelectric cantilever beam. Within the voltage range from −1 to 8 V, ∼ 3 nm of continuous and linear wavelength tuning is obtained with very small leakage current (< 10 nA) across the voltage range. Hence, the device requires very low power for piezoelectric actuation (< 1µW), which is much smaller than typical electrostatic-actuated MEMS-tunable VCSELs (∼ 100 µW). In addition, the linear wavelength-tuning characteristic with respect to the applied voltage, shown in piezoelectric actuation, can simplify and improve the wavelength control as compared to the electrostatic actuation, which has a quadratic wavelengthtuning behavior. With further optimization of the optical and mechanical designs, it is anticipated that the wavelengthtuning range can be easily increased for a wide range of applications. VII. CONCLUSION We demonstrated a monolithic integrated piezoelectric MEMS-tunable VCSEL that utilizes the intrinsic piezoelectricity in Alx Ga1−x As compound. The actuation is governed by the applied electric field and the piezoelectric coefficient of the material, which is dependent on the aluminum mole fraction and crystal orientation. We experimentally demonstrated the mechanical deflection for piezoelectric beams of different orientations and lengths, where a maximum physical deflection of 270 nm was achieved with a 360-µm piezoelectric cantilever design. This design led to an integrated piezoelectricactuated MEMS wavelength-tunable VCSEL with room temperature CW operation. Single transverse-mode emission (40 dB) with linear continuous wavelength tuning of ∼ 3 nm was obtained for an 850-nm VCSEL with output power of 1.2 mW. Piezoelectric actuation has low power dissipation (< 1µW), and can eliminate the possibility of catastrophic damage created by pull-in effects for electrostatic-actuated MEMS devices. In addition, it offers a large potential tuning range and is highly promising for many tunable MEMS optoelectronic device implementations. ACKNOWLEDGMENT The epitaxy wafer was grown by Land Mark Optoelectronic Corporation.
Fig. 7. Wavelength-tuning characteristics of the piezoelectric-actuated MEMS-tunable VCSEL. (a) Continuous wavelength spectra under various applied voltages. A continuous wavelength-tuning range of 3 nm is obtained with a single-mode emission of 40-dB SMSR. (b) Lasing wavelength as a function of the tuning voltage and the IV characteristic across the cantilever p-i-n junction plotted with the same x-axis. The tuning exhibits a very linear characteristic as a function of the applied voltage, and the leakage current across the p-i-n junction is < 10 nA throughout the entire tuning range.
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[3] C. Wilmsem, H. Temkin, and L. A. Coldren, Vertical-Cavity SurfaceEmitting Lasers. New York: Cambridge Univ. Press, 1999. [4] K. Iga, “Surface-emitting laser—Its birth and generation of new optoelectronics field,” IEEE J. Sel. Topics Quantum Electron., vol. 6, no. 6, pp. 1201–1215, Nov.–Dec. 2000. [5] A. J. Danner, J. J. Raftery, Jr., T. Kim, P. O. Liesher, A. V. Giannopoulos, and K. D. Choquette, “Progress in photonic crystal vertical cavity lasers,” IEICE Trans. Electron., vol. E88-C, pp. 944–950, 2005. [6] F. Koyama, “Recent advances of VCSEL photonics,” J. Lightw. Technol., vol. 24, no. 2, pp. 4502–4513, Dec. 2006. [7] N. Nishiyama, C. Caneau, B. Hall, G. Guryanov, M. H. Hu, X. S. Liu, M. J. Li, R. Bhat, and C. E. Zah, “Long-wavelength vertical-cavity surfaceemitting lasers on InP with lattice matched AlGaInAs-InP DBR grown by MOCVD,” IEEE J. Sel. Topics Quantum Electron., vol. 11, no. 5, pp. 990– 998, Sep.–Oct. 2005. [8] M. Ortsiefer, R. Shau, G. Bohm, F. Kohler, and M. C. Amann, “Lowthreshold index-guided 1.5 µm long-wavelength vertical-cavity surfaceemitting laser with high efficiency,” Appl. Phys. Lett., vol. 76, pp. 2179– 2181, 2000. [9] C. J. Chang-Hasnain, “Tunable VCSEL,” IEEE J. Sel. Topics Quantum Electron., vol. 6, no. 6, pp. 978–987, Nov.–Dec. 2000. [10] J. Boucart, R. Pathak, Z. Dongxu, M. Beaudoin, P. Kner, S. Decai, R. J. Stone, R. F. Nabiev, and W. Yuen, “Long wavelength MEMS tunable VCSEL with InP-InAlGaAs bottom DBR,” IEEE Photon. Technol. Lett., vol. 15, no. 9, pp. 1186–1188, Sep. 2003. [11] M. Li, W. Yuen, G. S. Li, and C. J. Chang-Hasnain, “Top-emitting micromechanical VCSEL with a 31.6-nm tuning range,” IEEE Photon. Technol. Lett., vol. 10, no. 1, pp. 18–20, Jan. 1998. [12] F. Riemenschneider, M. Maute, H. Halbritter, G. Boehm, M. C. Amann, and P. Meissner, “Continuously tunable long-wavelength MEMS-VCSEL with over 40-nm tuning range,” IEEE Photon. Technol. Lett., vol. 16, no. 10, pp. 2212–2214, Oct. 2004. [13] M. S. Wu, E. C. Vail, G. S. Li, W. Yuen, and C. J. Chang-Hasnain, “Tunable micromachined vertical cavity surface emitting laser,” Electron. Lett., vol. 31, pp. 1671–1672, 1995. [14] G. Totschnig, M. Lackner, R. Shau, M. Ortsiefer, J. Rosskopf, M. C. Amann, and F. Winter, “1.8 µm vertical-cavity surface-emitting laser absorption measurements of HCl, H2 O and CH4 ,” Meas. Sci. Technol., vol. 14, pp. 472–478, 2003. [15] C. F. R. Mateus, M. C. Y. Huang, P. Li, B. T. Cunningham, and C. J. ChangHasnain, “Compact label-free biosensor using VCSEL-based measurement system,” IEEE Photon. Technol. Lett., vol. 16, no. 7, pp. 1712–1714, Jul. 2004. [16] C. F. R. Mateus, M. C. Y. Huang, C. J. Chang-Hasnain, J. E. Foley, R. Beatty, P. Li, and B. T. Cunningham, “Ultra-sensitive immunoassay using VCSEL detection system,” Electron. Lett., vol. 40, no. 11, pp. 649– 651, May 27, 2004. [17] J. Kitching, S. Knappe, M. Vukicevic, L. Hollberg, R. Wynands, and W. Weidmann, “A microwave frequency reference based on VCSELdriven dark line resonances in Cs vapor,” IEEE Trans. Instrum. Meas., vol. 49, no. 6, pp. 1313–1317, Dec. 2000. [18] G. Piazza, K. Castelino, A. P. Pisano, and C. J. Chang-Hasnain, “Design of a monolithic piezoelectrically actuated microelectromechanical tunable vertical-cavity surface-emitting laser,” Opt. Lett., vol. 30, pp. 896–898, 2005. [19] M. C. Y. Huang, K. B. Cheng, Z. Ye, B. Pesala, C. J. Chang-Hasnain, and A. P. Pisano, “Demonstration of piezoelectric actuated GaAs-based MEMS tunable VCSEL,” IEEE Photon. Technol. Lett., vol. 18, no. 10, pp. 1197–1199, May 2006. [20] L. Li, P. Kumar, S. Kanakraju, and D. L. DeVoe, “Piezoelectric AlGaAs bimorph microactuators,” J. Micromech. Microeng., vol. 16, pp. 1062– 1066, 2006. [21] P. Kumar, L. Lihua, L. Calhoun, P. Boudreaux, and D. DeVoe, “Fabrication of piezoelectric Al0.3Ga0.7As microstructures,” Sens. Actuators A, Phys., vol. A115, pp. 96–103, 2004. [22] S. Adachi, “GaAs, AlAs, and Alx Ga1−x As: Material parameters for use in research and device applications,” J. Appl. Phys., vol. 58, pp. R1–R29, 1985. [23] T. Kitano, S. Izumi, H. Minami, T. Ishikawa, K. Sato, T. Sonoda, and M. Otsubo, “Selective wet etching for highly uniform GaAs/Al0.15 Ga0.85 As heterostructure field effect transistors,” J. Vac. Sci. Technol. B, Microelectron., vol. 15, pp. 167–170, 1997.
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Michael C. Y. Huang (S’02) received the B.S. degree in electrical engineering and computer sciences from the University of California, Berkeley, in 2002, where he is currently working toward the Ph.D. degree in electrical engineering. His research interests include nanostructured materials, nanosemiconductor and microsemiconductor optoelectronic devices, and their applications.
Kan Bun Cheng received the B.Eng. degree in mechanical engineering from Hong Kong University of Science and Technology, Kowloon, Hong Kong, in 2002. He is currently working toward the Ph.D. degree in applied science and technology at the University of California, Berkeley. His research interests include microsensors/nanosensors and actuators, and semiconductor materials and devices for harsh environment.
Ye Zhou (S’06) received the B.Eng. degree in electronic engineering from Tsinghua University, Beijing, China, in 2003. He is currently working toward the Ph.D. degree in electrical engineering at the University of California, Berkeley. His research interests include novel-structure vertical-cavity surface-emitting lasers, microelectromechanical systems, and nanoelectromechanical systems optoelectronic devices, nanophotonics, and optical interconnects.
Albert P. Pisano received the B.S., M.S., and Ph.D. degrees in 1976, 1977, and 1981, respectively, mechanical engineering from Columbia University, New York. Since 2004 he has been a Professor and the FANUC Chair of Mechanical Systems in the Department of Mechanical Engineering, University of California, Berkeley, with a joint appointment in the Department of Electrical Engineering and Computer Science. He was the Director of the Electronics Research Laboratory as well as Berkeley Sensor and Actuator Center (BSAC), University of California. Prior to joining the faculty at the University of California, he held research positions with the Xerox Palo Alto Research Center, Palo Alto, CA, Singer Sewing Machines Corporate R&D Center, San Francisco, CA, and General Motors Research Laboratories, Warren, MI. From 1997 to 1999, he was a Program Manager for the MEMS Program at the Defense Advanced Research Projects Agency (DARPA), Arlington, VA. He was a founder in four start-up companies in the area of transdermal drug delivery, transvascular drug delivery, sensorized catheters, and manufacturing equipment. His current research interests include microelectromechanical systems for a wide variety of applications, including RF components, power generation, drug delivery, strain sensors, biosensors, and disk-drive actuators. Prof. Pisano is a member of the National Academy of Engineering since 2001.
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Connie J. Chang-Hasnain (M’88–SM’92–F’98) received the B.S. degree from the University of California, Davis, in 1982 and the M.S. and Ph.D. degrees from the University of California, Berkeley, in 1984 and 1987, respectively, in electrical engineering. From 1987 to 1992 she was a member of Technical Staff at Bellcore. From 1992 to 1995, she was an Associate Professor of electrical engineering with Stanford University, Stanford, CA. Since 1996, she has been a Professor of electrical engineering with the University of California, Berkeley, where she is the John R. Whinnery Chair Professor and the Chair of the Nanoscale Science
and Engineering Graduate Group. She is also the Director of the Center for Optoelectronic Nanostructured Semiconductor Technologies. Her research interests include nanomaterials and optoelectronic devices, and their applications. Dr. Chang-Hasnain is a Fellow of the Optical Society of America and the Institution of Electrical Engineers. She is an honorary member of A. F. Ioffe Institute since 2005. She was named a Presidential Faculty Fellow, a National Young Investigator, a Packard Fellow, a Sloan Research Fellow, and the Outstanding Young Electrical Engineer of the Year by Eta Kappa Nu. She was the recipient the 1994 IEEE LEOS Distinguished Lecturer Award, the 2000 Curtis W. McGraw Research Award from the American Society of Engineering Education, the 2003 IEEE William Streifer Scientific Achievement Award, and the 2005 Gilbreth Lecturer Award from the National Academy of Engineering.
IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 13, NO. 2, MARCH/APRIL 2007
Monolithic Integrated Piezoelectric MEMS-Tunable VCSEL Michael C. Y. Huang, Student Member, IEEE, Kan Bun Cheng, Ye Zhou, Student Member, IEEE, Albert P. Pisano, and Connie J. Chang-Hasnain, Fellow, IEEE
Abstract—A novel actuation mechanism that utilizes the inherent piezoelectric properties of Alx Ga1−x As compounds is presented for short-wavelength microelectromechanical systems tunable vertical-cavity surface-emitting laser centered at 850 nm. Piezoelectric actuation can provide precise bidirectional displacements and linear tuning characteristics with respect to the applied voltage, does not suffer from travel limitations and catastrophic electrical damage, consumes low power (∼1 µW), and has a relatively fast response compared to thermal actuation. In addition, piezoelectric actuation based on Alx Ga1−x As films offers a large piezoelectric coupling coefficient comparable to those of zinc oxide and quartz, and it can be integrated with high-speed electronic and optoelectronic devices. Single-mode emission with a linear continuous tuning range of 3 nm and low power consumption were experimentally obtained. Index Terms—Laser tuning, microelectromechanical systems (MEMS), tunable optoelectronic devices, vertical-cavity surfaceemitting laser (VCSEL).
I. INTRODUCTION ERTICAL-CAVITY surface-emitting lasers (VCSELs) are a new class of semiconductor lasers that emerged during the 1990s [1]–[8]. They operate with a single wavelength emission by virtue of an extremely small cavity length. The main advantages are surface-normal emission for easy output coupling, compatibility for 2-D arrays, device fabrication, and testing at the wafer level, which essentially allow low-cost manufacturing. Also, VCSELs can be directly modulated by varying their bias current up to 10 GHz, which is very desirable in the application of digital and analog fiber-optic communication systems. The integration of VCSEL and microelectromechanical systems (MEMS) gives rise to an ideal wavelength-tunable light source that leverages the advantages from both sides [9]–[12]. Since VCSEL fabrication utilizes a monolithic process, it can be easily integrated with MEMS structures to leverage the mechanical movements. It has been demonstrated that the combination of the two can yield a wide and continuous wavelength-tuning range, utilizing a simple wavelength-control mechanism.
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Manuscript received October 4, 2006; revised February 15, 2007. This work was supported in part by the Defense Advanced Research Projects Agency (DARPA) Chip-Scale Atomic Clock Grant NBCH1020005 and in part by the Berkeley Microfabrication Laboratory. M. C. Y. Huang, Y. Zhou, and C. J. Chang-Hasnain are with the Department of Electrical Engineering and Computer Sciences, University of California, Berkeley, CA 94720 USA (e-mail: [email protected]; yezhou@ eecs.berkely.edu; [email protected]). K. B. Cheng and A. P. Pisano are with the Department of Mechanical Engineering, University of California, Berkeley, CA 94720 USA (e-mail: kbcheng@ me.berkeley.edu; [email protected]). Digital Object Identifier 10.1109/JSTQE.2007.894056
Since the first demonstration by Wu et al. [13], MEMStunable VCSELs have been extensively studied for various applications, including signal routing and switching in optical networks, gas, chemical, and biomolecular sensing [14]–[16], spectroscopy, and optical excitation of rubidium species in an atomic clock system [17]. The mechanical actuation mechanisms previously reported have been either electrostatic or thermal actuation. Electrostatic actuation is often limited by the 1/3 rule, where the maximum deflection can be at the most one third of the original airgap spacing. The most devastating drawback is the possibility of catastrophic electrical discharge that damages VCSELs when pull-in occurs. In addition, relatively large voltage and power are required to actuate the beam, due to the leakage current across an intrinsic GaAs layer that often serves as the sacrificial material. Thermal actuation, on the other hand, creates a large heating source near the laser-active region, which may adversely affect laser reliability and performance. In addition, the actuation speed is fairly low (∼100 Hz) due to long thermal transient time and high actuation power generally required by thermal actuation [12]. Recently, we proposed the design of a monolithic integrated piezoelectric wavelength-tunable VCSEL that exploits the inherent piezoelectric properties of Alx Ga1−x As compounds [18], [19]. Piezoelectric actuation offers several potential advantages [20], [21]. First, it can provide large mechanical deflections in bidirections, both toward the substrate and opposite to it. The movement is not limited by pull-in effect, as opposed to the 1/3-gap limit associated with the electrostatic actuation. More importantly, there will be no catastrophic damage due to capacitor discharge. It is expected to consume low power (∼ 1 µW), and has a relatively fast response speed (limited only by the mechanical structure rather than the piezoelectric effect). The piezoelectric effect has the advantages of high electromechanical coupling strength and linear response, compared to electrostatic actuation. Finally, piezoelectric actuation can be readily extended to other MEMS-tunable optoelectronic devices operating at different wavelengths. In this paper, we present a novel piezoelectric-actuated MEMS-tunable VCSEL structure. The piezoelectric cantilever beam is monolithically integrated within the VCSEL distributed Bragg reflector (DBR) layers by properly designing the doping profile. Single-mode emission (40 dB) and linear, continuous wavelength tuning (∼ 3 nm) were achieved for an 850-nm VCSEL at room temperature under continuous-wave (CW) operation. In addition, we also present the mechanical characterization of the piezoelectric cantilever beams, which
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Fig. 1. Illustration of the cantilever beam under the piezoelectric effect. When applying a reverse-biased voltage, the vertical electrical field across the intrinsic piezoelectric layer causes a net bending moment that deflects the cantilever beam downward. The beam deflection changes the airgap of the tunable VCSEL and, hence, changes the emission wavelength.
can be used in assisting the mechanical actuator design of the MEMS-tunable VCSEL. II. PIEZOELECTRIC BEAM DESIGN The piezoelectric property of AlGaAs is an inherent characteristic that originates from the zinc-blend crystalline structure that lacks center inversion symmetry [22]. Under the presence of a vertical electric field, an intrinsic AlGaAs beam is subject to a longitudinal strain caused by internal polarization that results in a change in its length (∆L). The magnitude of the piezoelectric effect is determined by the magnitude of the applied field and d14 piezoelectric coefficient, which is a function of the aluminum composition. For instance, d14 = 3.59 pC/N for Al0.8 Ga0.2 As and d14 = 2.80 pC/N for Al0.1 Ga0.9 As. The governing piezoelectric coefficient is dependent on the orientation; it shows a maximum in the 110 direction and zero along the 100 direction. The inherent piezoelectric effect of GaAs can be characterized by the piezoelectric matrix as shown in the equation at the bottom of the page. By utilizing this property, a piezoelectric-actuated cantilever structure is proposed, which comprises of a thin n-doped, a thin intrinsic, and a thick p-doped Alx Ga1−x As layer, as shown in Fig. 1. For a reverse voltage applied across the top n-type and the bottom p-type layers, the applied voltage results in an electric field across the middle intrinsic piezoelectric layer of the cantilever beam. Such a vertical electric field produces a longitudinal strain (∆L) in the intrinsic layers via the converse piezoelectric effect. Since the thin piezoelectric layer is rigidly connected to the cantilever beam, which constrains the longitudinal motion of the piezoelectric layer, an equivalent bending moment (Mp) is applied to the end of the beam. The bending
0 [d] = 0 −d14 cos(2Φ)
0 0 0 0 d14 cos(2Φ) 0
Fig. 2. Schematic of the piezoelectric-actuated MEMS-tunable VCSEL. (a) Side view showing the mechanical configuration. (b) Cross-sectional view showing the optical design.
moment produces a deflection (δ) at the tip of the cantilever beam that changes the airgap size, which then varies the emission wavelength of the VCSEL. Due to the p-i-n doping choice, the cantilever structure in this particular design could only move toward the substrate. It is expected that a better implementation of the doping scheme, such as n-i-n or p-i-p structure, can enable movement in both directions and, hence, double the potential tuning range. For simplicity and quick proof of concept, the clamped cantilever beam design is adopted as the basic design for our first prototype in which the longitudinal mode (d31 ) is used for actuation. A more sophisticated structure (such as a V-shape structure) can be designed based on the cantilever beam design performance. III. VCSEL DESIGN The schematic of the optical laser design of the MEMStunable VCSEL is shown in Fig. 2. The device consists of a 34-pair n-doped DBR bottom mirror, a λ-cavity layer with the GaAs quantum-well active region, and a movable top mirror. The top mirror is comprised of three parts (starting from the substrate side): fixed four pairs of p-doped DBRs, a variable airgap, and a freely suspended 15 pairs of DBRs mirror that is supported via a cantilever structure. The material of DBR consists of alternating layers of Al0.12 Ga0.88 As and Al0.9 Ga0.1 As. The airgap has a designed free-standing thickness of 1.8 µm (corresponds to 9λ/4 in optical thickness), and d14 sin(2Φ) d14 cos(2Φ) 0
−d14 cos(2Φ) −d14 sin(2Φ) 0
0 0 d14 sin(2Φ)
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is formed by selectively removing the sacrificial layer material (p-doped GaAs) underneath the cantilever. The suspended DBR cantilever is designed with an unevenly distributed p-i-n doping profile (from the substrate side): a thick nine-pair p-doped DBR, a thin four-pair intrinsic DBR, and a thin two-pair n-doped DBR. In such a configuration, the neutral axis of the beam is shifted upward from the center; hence, the longitudinal strain resulting from the piezoelectric actuation can be effectively transferred to a bending moment. However, the intrinsic DBR layers are subject to unintentional doping from the background during the material epitaxy growth, which may limit the maximum electric field allowed for piezoelectric actuation. Electric current injection is conducted through the middle laser contact (via the four pairs of p-doped DBR) and backside ground contact. An aluminum-oxide aperture is formed on an Al0.98 Ga0.02 As layer in the p-DBR section, immediately above the cavity layer, to provide efficient current and optical confinement. The piezoelectric tuning is conducted through the top tuning contact (via the n-DBR) and the middle laser contact. Wavelength tuning is accomplished by applying a reverse voltage bias across the top n-doped layer and the middle p-doped layers. The applied voltage results in a vertical electric field across the intrinsic DBR layers that induce mechanical deflection via the piezoelectric effect. The beam deflection reduces the airgap size and then, consequently, changes the Fabry–Perot resonance of the cavity. Hence, it blue-shifts the emission wavelength of the VCSEL. IV. FABRICATION The fabrication process of the piezoelectric MEMS-tunable VCSEL has a few extra processing steps as compared to that of the conventional VCSEL. First, a top tuning contact consisting of Ni–Ge–Au alloy is deposited by electron beam evaporation on the top n-doped layer. Then, a wet-chemical vertical etch is required to etch down to the bottom DBR layers and form the VCSEL mesa structure. The device then goes through a thermal wet-oxidation process at 450 ◦ C to form the aluminum-oxide aperture for electrical and optical confinement. Typically, an oxide aperture less than 3 µm is required for a single transverse mode emission from VCSEL. After oxidation, we performed another wet-chemical vertical etch to form the cantilever structure, in addition to exposing the embedded layer necessary to form the laser contact. Next, a Ti–Au alloy metal was deposited to form the laser current contact on the p-DBR layers above the cavity. Finally, a wet-chemical selective etch process was carried out to remove the GaAs sacrificial material underneath the cantilever to form the freely-suspending beam. The wetchemical selective etch was followed by a critical point drying process to avoid the surface stiction problem. Fig. 3(a) shows the scanning electron microscope (SEM) image of a fabricated tunable VCSEL device. The bottom surface roughness is mainly due to nonuniformity from the wet-chemical etch process, but it is expected not to have any effect on the device performance. A highly selective citric-acid-based chemical etch is required to remove the GaAs sacrificial layer underneath the cantilever, and form the freely suspending cantilever beam [19], [23].
Fig. 3. SEM image of the fabricated piezoelectric-actuated MEMS-tunable VCSEL. (a) Top view. (b) Side-view zooming in the suspended cantilever head. The suspended piezoelectric cantilever was formed by selectively removing the GaAs sacrificial material underneath the beam.
By using this wet-etch chemistry, highly selective removal of the sacrificial GaAs against Al0.12 Ga0.88 As (the low aluminum component of the DBR) was achieved, with an etch selectivity > 100. In addition, the beam bending induced from the residual stress is negligible, as observed from the SEM image. Compared to the conventional reactive-ion etch (RIE)-based selective etch method, the chemical-based selective etching enables a very low-cost and rapid fabrication cycle, in addition to improving the device yield and reproducibility. Fig. 3(b) shows the zoom-in image of the released cantilever suspending over the VCSEL cavity. The smoothness from the cantilever and bottom mesa surface after the selective etch implies the high selectivity in the wet-chemical GaAs sacrificial layer etches. V. MECHANICAL CHARACTERIZATION The mechanical deflection of the piezoelectric-actuated cantilever beams were experimentally obtained by monitoring their center height change as a function of the applied voltage, using both white light interferometer and atomic force microscopy (AFM) measurement. To optimize the piezoelectric tuning performance, a preliminary experimental study of the piezoelectric effect with different cantilever orientations and beam dimensions has been performed. As the VCSEL epitaxy wafers were grown from (100) GaAs substrate, the orientation of the fabricated cantilevers is rotated from (110) to (100) by lithography. Fig. 4(a) shows the measured vertical deflection for different cantilever beam orientations with respect to (110), but with same dimensions (length of 240 µm and width of 10 µm). Because of
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Fig. 5. Resonant frequency measurement for two piezoelectric-actuated cantilevers with different lengths using an AFM in tapping mode. It shows the fundamental and higher order vibration modes for the two cantilevers. The calculated values of resonant frequency using the ANSYS simulation are 123 and 30 kHz for 120- and 240-µm beams, respectively.
Fig. 4. Mechanical characterization on the piezoelectric-actuated cantilever beams. (a) Cantilever deflection as a function of the applied voltage for different beam orientations with respect to (110), all with the same dimensions (length of 240 µm and width of 10 µm). (b) Cantilever deflection as a function of the applied voltage for three different beam lengths, all aligned along the110 direction.
crystal asymmetry in Alx Ga1−x As compound, the piezoelectric coefficient governing the piezoelectric actuation varies with crystal orientation, which results in decreasing magnitude of piezoelectric actuation toward the 100 direction (45◦ ). As expected from the GaAs piezoelectric matrix, the piezoelectric actuation exhibits a strong dependence on the orientation, where the beam deflection is largest for beams aligned to 110 direction (140 nm), and the deflection magnitude decreases as the angle of the beam orientation increases toward 100 direction. The angular dependence is a clear demonstration of the piezoelectric effect, which cannot be induced by thermal or electrostatic actuation. When increasing the length of the cantilever beam, the deflection magnitude increases due to the decrease in beam stiffness. Fig. 4(b) shows the vertical deflection measured by AFM for three different cantilever beam lengths aligned along 110 direction. Maximum deflections of 10, 130, and 270 nm were measured for beam lengths of 120, 240, and 360 µm, respectively. Similar to Fig. 4(a), the mechanical deflection exhibits a
highly smooth and linear characteristic, as expected for a piezoelectric actuation. It is important to note that the piezoelectricactuation-induced beam deflection has very negligible effect on the cantilever beam curvature, since the deflection magnitude is often less than 0.1% of the beam length. Fig. 5 shows the resonance frequency measurement for two of the piezoelectric cantilever beams of different lengths (120 and 240 µm), showing the fundamental and higher order vibration modes. The measurement was performed by using AFM in the tapping mode, where the AFM cantilever is driving the MEMS piezoelectric cantilever at different frequencies and monitoring the deflection magnitude. The beams were driven to resonance in simple flexural mode in air, and the resonance frequency was measured to be ∼ 93 kHz for a beam length of 120 µm and ∼23 kHz for 240 µm. The measured resonance frequencies, in general, agree well with those from our 2-D plane strain model and ANSYS simulations, where the calculated values are 123 and 30 kHz, respectively. The discrepancy is mainly due to the air damping and considerable mechanical coupling between the AFM tip and cantilever beams at lower resonance frequencies. VI. EXPERIMENT RESULTS By integrating the piezoelectric cantilever (200 µm × 15 µm) with the VCSEL cavity, CW wavelength tuning using the piezoelectric actuation was experimentally demonstrated in room temperature operation. Fig. 6 shows the VCSEL optical characteristics (without applying the tuning voltage) for output power versus bias current (LI), and voltage versus bias current (VI). The device exhibits a lasing threshold current of 1.2 mA and maximum output power of ∼ 1.2 mW with the slope efficiency of 0.3 mW/mA. When applying a bias voltage across the tuning contacts, the piezoelectric effect deflects the cantilever beam, which translates into a change in the airgap and, consequently, varies the laser emission wavelength. Fig. 7(a) shows the measured emission spectra of the piezoelectric-actuated device under various
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Fig. 6. Measured optical characterization showing the LI and IV characteristic of a fabricated VCSEL (without external tuning voltage). The VCSEL exhibits a maximum output power of 1.2 mW and the threshold current is 1.2 mA.
tuning voltages. With the oxide aperture of 3 µm, a singlemode emission (40 dB) is obtained throughout the entire range of ∼ 3 nm. In addition, the peak spectral intensity remains fairly constant throughout the entire tuning range, indicating that tilting loss from cantilever deflection is negligible. Fig. 7(b) shows the emission wavelength as a function of the applied voltage across the top tuning electrodes and the corresponding IV characteristic across the p-i-n structure within the piezoelectric cantilever beam. Within the voltage range from −1 to 8 V, ∼ 3 nm of continuous and linear wavelength tuning is obtained with very small leakage current (< 10 nA) across the voltage range. Hence, the device requires very low power for piezoelectric actuation (< 1µW), which is much smaller than typical electrostatic-actuated MEMS-tunable VCSELs (∼ 100 µW). In addition, the linear wavelength-tuning characteristic with respect to the applied voltage, shown in piezoelectric actuation, can simplify and improve the wavelength control as compared to the electrostatic actuation, which has a quadratic wavelengthtuning behavior. With further optimization of the optical and mechanical designs, it is anticipated that the wavelengthtuning range can be easily increased for a wide range of applications. VII. CONCLUSION We demonstrated a monolithic integrated piezoelectric MEMS-tunable VCSEL that utilizes the intrinsic piezoelectricity in Alx Ga1−x As compound. The actuation is governed by the applied electric field and the piezoelectric coefficient of the material, which is dependent on the aluminum mole fraction and crystal orientation. We experimentally demonstrated the mechanical deflection for piezoelectric beams of different orientations and lengths, where a maximum physical deflection of 270 nm was achieved with a 360-µm piezoelectric cantilever design. This design led to an integrated piezoelectricactuated MEMS wavelength-tunable VCSEL with room temperature CW operation. Single transverse-mode emission (40 dB) with linear continuous wavelength tuning of ∼ 3 nm was obtained for an 850-nm VCSEL with output power of 1.2 mW. Piezoelectric actuation has low power dissipation (< 1µW), and can eliminate the possibility of catastrophic damage created by pull-in effects for electrostatic-actuated MEMS devices. In addition, it offers a large potential tuning range and is highly promising for many tunable MEMS optoelectronic device implementations. ACKNOWLEDGMENT The epitaxy wafer was grown by Land Mark Optoelectronic Corporation.
Fig. 7. Wavelength-tuning characteristics of the piezoelectric-actuated MEMS-tunable VCSEL. (a) Continuous wavelength spectra under various applied voltages. A continuous wavelength-tuning range of 3 nm is obtained with a single-mode emission of 40-dB SMSR. (b) Lasing wavelength as a function of the tuning voltage and the IV characteristic across the cantilever p-i-n junction plotted with the same x-axis. The tuning exhibits a very linear characteristic as a function of the applied voltage, and the leakage current across the p-i-n junction is < 10 nA throughout the entire tuning range.
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Michael C. Y. Huang (S’02) received the B.S. degree in electrical engineering and computer sciences from the University of California, Berkeley, in 2002, where he is currently working toward the Ph.D. degree in electrical engineering. His research interests include nanostructured materials, nanosemiconductor and microsemiconductor optoelectronic devices, and their applications.
Kan Bun Cheng received the B.Eng. degree in mechanical engineering from Hong Kong University of Science and Technology, Kowloon, Hong Kong, in 2002. He is currently working toward the Ph.D. degree in applied science and technology at the University of California, Berkeley. His research interests include microsensors/nanosensors and actuators, and semiconductor materials and devices for harsh environment.
Ye Zhou (S’06) received the B.Eng. degree in electronic engineering from Tsinghua University, Beijing, China, in 2003. He is currently working toward the Ph.D. degree in electrical engineering at the University of California, Berkeley. His research interests include novel-structure vertical-cavity surface-emitting lasers, microelectromechanical systems, and nanoelectromechanical systems optoelectronic devices, nanophotonics, and optical interconnects.
Albert P. Pisano received the B.S., M.S., and Ph.D. degrees in 1976, 1977, and 1981, respectively, mechanical engineering from Columbia University, New York. Since 2004 he has been a Professor and the FANUC Chair of Mechanical Systems in the Department of Mechanical Engineering, University of California, Berkeley, with a joint appointment in the Department of Electrical Engineering and Computer Science. He was the Director of the Electronics Research Laboratory as well as Berkeley Sensor and Actuator Center (BSAC), University of California. Prior to joining the faculty at the University of California, he held research positions with the Xerox Palo Alto Research Center, Palo Alto, CA, Singer Sewing Machines Corporate R&D Center, San Francisco, CA, and General Motors Research Laboratories, Warren, MI. From 1997 to 1999, he was a Program Manager for the MEMS Program at the Defense Advanced Research Projects Agency (DARPA), Arlington, VA. He was a founder in four start-up companies in the area of transdermal drug delivery, transvascular drug delivery, sensorized catheters, and manufacturing equipment. His current research interests include microelectromechanical systems for a wide variety of applications, including RF components, power generation, drug delivery, strain sensors, biosensors, and disk-drive actuators. Prof. Pisano is a member of the National Academy of Engineering since 2001.
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Connie J. Chang-Hasnain (M’88–SM’92–F’98) received the B.S. degree from the University of California, Davis, in 1982 and the M.S. and Ph.D. degrees from the University of California, Berkeley, in 1984 and 1987, respectively, in electrical engineering. From 1987 to 1992 she was a member of Technical Staff at Bellcore. From 1992 to 1995, she was an Associate Professor of electrical engineering with Stanford University, Stanford, CA. Since 1996, she has been a Professor of electrical engineering with the University of California, Berkeley, where she is the John R. Whinnery Chair Professor and the Chair of the Nanoscale Science
and Engineering Graduate Group. She is also the Director of the Center for Optoelectronic Nanostructured Semiconductor Technologies. Her research interests include nanomaterials and optoelectronic devices, and their applications. Dr. Chang-Hasnain is a Fellow of the Optical Society of America and the Institution of Electrical Engineers. She is an honorary member of A. F. Ioffe Institute since 2005. She was named a Presidential Faculty Fellow, a National Young Investigator, a Packard Fellow, a Sloan Research Fellow, and the Outstanding Young Electrical Engineer of the Year by Eta Kappa Nu. She was the recipient the 1994 IEEE LEOS Distinguished Lecturer Award, the 2000 Curtis W. McGraw Research Award from the American Society of Engineering Education, the 2003 IEEE William Streifer Scientific Achievement Award, and the 2005 Gilbreth Lecturer Award from the National Academy of Engineering.
