Demonstration of Piezoelectric Actuated GaAs-Based MEMS Tunable ...
IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 18, NO. 10, MAY 15, 2006
1197
Demonstration of Piezoelectric Actuated GaAs-Based MEMS Tunable VCSEL Michael C. Y. Huang, Student Member, IEEE, Kan Bun Cheng, Ye Zhou, Bala Pesala, Connie J. Chang-Hasnain, Fellow, IEEE, and Albert P. Pisano
Abstract—We report the first experimental demonstration of a novel piezoelectric actuated microelectromechanical systems (MEMS) tunable vertical-cavity surface-emitting laser (VCSEL). A large physical deflection was obtained with the piezoelectric actuated MEMS cantilever beam monolithically integrated with the VCSEL distributed Bragg reflector. Single-mode emission and continuous tuning were achieved at room temperature under continuous-wave operation. Index Terms—Laser tuning, microelectromechanical systems (MEMS), vertical-cavity surface-emitting laser (VCSEL).
I. INTRODUCTION
M
ICROELECTROMECHANICAL systems (MEMS) tunable vertical-cavity surface-emitting laser (VCSEL) has been extensively studied [1] for various applications including signal routing and switching in optical networks, biomolecular sensing and spectroscopy, and optical excitation of rubidium species in an atomic clock system. MEMS tunable structures are desirable because they provide for a large and continuous tuning range with high precision, fast response, and the compatibility with current optoelectronic fabrication processes. The actuation mechanisms previously reported have been either electrostatic or thermal actuation. The electrostatic actuation is often limited by the 1/3 rule, where the maximum deflection can be at most 1/3 of the original airgap spacing. The most devastating drawback is the possibility of electrical catastrophic discharge that damages VCSEL when pull-in occurs [2]. 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 it generally requires high actuation power [3]. Recently, we proposed the design of a monolithic piezoelectric tunable VCSEL that exploits the inherent piezoelectric properAs compounds [4]. Piezoelectric actuation ties of the Al Ga offers several potential advantages. First, it can provide large displacements in bidirections, both toward the substrate and opposite to it. The movement is not limited by the pull-in effect, as opposed to the 1/3 gap limit with electrostatic actuation. Most Manuscript received September 6, 2005; revised February 4, 2006. This work was supported in part by the Defense Advanced Research Projects Agency (DARPA) Chip-Scale Atomic Clock (CSAC) under Grant NBCH1020005. M. C. Y. Huang, Y. Zhou, B. Pesala, 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]). K. B. Cheng and A. P. Pisano are with the Department of Mechanical Engineering, University of California, Berkeley, CA 94720 USA. Digital Object Identifier 10.1109/LPT.2006.873923
Fig. 1. Schematic 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. (Color version available online at http://ieeexplore.ieee.org.)
importantly, there will be no catastrophic damages due to capacitor discharge. It is expected to consume very low power (1 W), and has a fast response speed (limited only by the mechanical structure rather than the piezoelectric effect). Finally, piezoelectric actuation can be readily extended to other MEMS tunable optoelectronic devices operating at different wavelengths. In this work, we report the first demonstration of the novel piezoelectric actuated MEMS tunable VCSEL. The piezoelectric cantilever beam is monolithically integrated within the VCSEL distributed Bragg reflector (DBR). Single-mode emission and continuous tuning were achieved at room temperature under continuous-wave (CW) operation. II. PIEZOELECTRIC VCSEL DESIGN As is an inherent The piezoelectric property of Al Ga characteristic that originates from its lack of center symmetry in the zinc-blend crystalline structure [5]. Under the presence of a vertical electric filed, a longitudinal strain is produced. The magnitude of the piezoelectric effect is determined by the piezoelectric coefficient, which is a function of the aluminum composition and the orientation with respect to (110). By utilizing this property, a piezoelectric actuated cantilever structure is proposed, which comprises thin n-doped, thin inAs layers, as shown in Fig. 1. trinsic, and thick p-doped Al Ga 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
1041-1135/$20.00 © 2006 IEEE
1198
IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 18, NO. 10, MAY 15, 2006
Fig. 2. Schematic of the piezoelectric actuated MEMS tunable VCSEL from the (a) side view and (b) cross-sectional view. (Color version available online at http://ieeexplore.ieee.org.)
in the intrinsic layers via the converse piezoelectric effect. The thin piezoelectric layer is rigidly connected to the cantilever beam, which constrains the longitudinal motion of the piezoelectric layer and results in an equivalent bending moment applied to the beam. The bending moment produces a deflection 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. The schematic of the optical design of the MEMS tunable VCSEL is shown in Fig. 2. The device consists of a bottom n-DBR mirror, a -cavity layer with the active region, and a top mirror. The top mirror is comprised of three parts (starting from the substrate side): a fixed p-DBR, an airgap, and a freely suspended DBR that is supported via a cantilever structure. The suspended DBR cantilever consists of an unevenly distributed p-i-n structure: a thick p-DBR, a thin i-DBR, and a thin n-DBR, wherein such configuration enables the piezoelectric actuation mentioned above. Electric current injection is conducted through the middle contact (via the p-DBR) and backside contact. An aluminum oxide aperture is formed on an Al Ga As layer in the p-DBR section above the cavity layer to provide efficient current and optical confinement. The piezoelectric tuning is conducted through the top contact (via the n-DBR) and the middle contact. III. FABRICATION The fabrication process of the piezoelectric MEMS tunable VCSEL is the same as the existing electrostatic MEMS tunable VCSEL. The epitaxial wafer was obtained from Land Mark Optoelectronics. The process includes three metal depositions, cantilever structure formation, VCSEL mesa formation, oxidation, and cantilever beam release followed by critical point drying. Fig. 3 shows the scanning electron microscope (SEM) image of a fabricated MEMS tunable VCSEL. The final device contains m cantilever beam with a m head. a A highly selective etch is required to remove the GaAs sacrificial layer underneath the cantilever and form the freely sus-
Fig. 3. SEM image of a fabricated device from (a) top view and (b) side view. The suspended VCSEL cantilever was formed by selectively removing the GaAs sacrificial material underneath the beam.
Fig. 4. Measured cantilever deflection as a function of applied voltage for beams aligned in different angles in respect to (110) direction.
pending cantilever beam. The release process utilized a highly selective citric-acid-based etchant [6]. The etchant was prepared from 1 : 1 anhydrous citric acid: DI water (w/w). The citric acid was mixed with NH OH solution to adjust its pH to 6.5. Finally, the mixture of the pH-adjusted citric acid and H O solution in a volume ratio of 5 : 1 is heated to 60 C. The heating drastically increases the diffusion of the etchant, especially for the release of the large cantilever head. In addition, photoresist was used to cover all metal surfaces during the selective etch to minimize any electrochemical charging effect. By using this wet-etch chemistry, highly selective removal of the sacrificial GaAs against Al Ga As (the low aluminum component of the DBR) was achieved. Compared to the conventional reactiveion-etching-based selective etch method, the chemical-based selective etching enables very rapid fabrication cycle and improve the device yield and reproducibility. IV. EXPERIMENT RESULTS To verify the piezoelectric actuation of the cantilever strucm aligned in difture, simple cantilever beams ferent orientations were fabricated. Fig. 4 shows the measured
HUANG et al.: DEMONSTRATION OF PIEZOELECTRIC ACTUATED GaAs-BASED MEMS TUNABLE VCSEL
1199
Fig. 5. L–I and I –V characteristic at 0-V tuning bias. The maximum power output is around 900 W and threshold current is 1.2 mA.
beam deflection as a function of the applied voltage for beams with different orientations in respect to (110). The vertical profile of the beam was measured using Wyko white light interferometer and confirmed by atomic force microscope. As expected from the literature [5], the piezoelectric actuation exhibits a strong dependence on the orientation, where the beam deflection is largest for beams aligned to (110) and almost zero deflection for beams aligned to (100). A maximal physical deflection of 150 nm was obtained with this cantilever design. By integrating the piezoelectric cantilever arm m with the VCSEL cavity, CW wavelength tuning using the piezoelectric actuation was experimentally demonstrated in room temperature operation. Fig. 5 shows the light output– current–voltage ( – – ) characteristic of one fabricated MEMS tunable VCSEL, where the maximum output power is 1 mW and threshold current is 1.2 mA. When applying a bias voltage across the tuning contacts, the piezoelectric effect deflects the cantilever beam, which translates into the change in the laser emission wavelength. Fig. 6(a) shows the CW emission spectra of the device when . With the the active region is electrically pumped at AlO optical confinement, single-mode emission with 30-dB sidemode suppression ratio was obtained throughout the tuning range. Fig. 6(b) shows the emission wavelength as a function of the applied voltage and the – characteristic of the p-i-n structure in the cantilever beam. Within the voltage range from 10 to 5 V, 1 nm of continuous and precise wavelength tuning is obtained with very small leakage current ( nA) across the intrinsic piezoelectric layer. Further increasing the bias voltage would cause a dramatic reduction of the electric field and an increase of the forward-biased current, through which the thermal effect would counteract the beam movement. Also, the variation of airgap induces a phase change for the electric field inside the VCSEL optical cavity, where the phase change does not have linear correspondence to the emission wavelength. In addition, the limited tuning range was due to the physical deflection of 10 nm with this particular mechanical design. Future designs will incorporate the one used in simple MEMS experiment with 150-nm physical displacement to lead to 30-nm wavelength tuning.
Fig. 6. (a) CW tuning spectra for the MEMS tunable VCSEL. One nanometer of continuous wavelength tuning is obtained with single-mode emission 30-dB sidemode suppression ratio; (b) lasing wavelength as a function of tuning voltage and the I –V characteristic of the cantilever p-i-n junction plotted with the same x axis.
V. CONCLUSION We demonstrated piezoelectric tunable MEMS in As compound, where a large physical deflection of Al Ga 150 nm was achieved with a piezoelectric cantilever design. This design also led to the first monolithic integrated piezoelectrically actuated MEMS tunable VCSEL with room- temperature CW operation. Piezoelectric actuation can eliminate catastrophic damages created by the pull-in effects in electrostatic MEMS. In addition, it offers a large potential tuning range and is highly promising for many tunable MEMS device implementations. REFERENCES [1] C. J. Chang-Hasnain, “Tunable VCSEL,” IEEE J. Sel. Topics Quantum Electron., vol. 6, no. 6, pp. 978–987, Nov./Dec. 2000. [2] C. F. R. Mateus, C. H. Chang, L. Chrostowski, S. Yang, S. Decai, R. Pathak, and C. Chang-Hasnain, “Widely tunable torsional optical filter,” IEEE Photon. Technol. Lett., vol. 40, no. 6, pp. 819–821, Jun. 2002. [3] 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. [4] 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, no. 8, pp. 896–898, 2005. As: material parameters for use [5] S. Adachi, “GaAs, AlAs and Al Ga in research and device applications,” J. Appl. Phys., vol. 58, no. 3, pp. R1–R29, 1985. [6] T. Kitano, S. Izumi, H. Minami, T. Ishikawa, K. Sato, T. Sonoda, and M. Otsubo, “Selective wet etching for highly uniform Ga As heterostructure field effect transistors,” J. Vac. GaAs=Al Sci. Technol. B, vol. 15, pp. 167–170, 1997.
1197
Demonstration of Piezoelectric Actuated GaAs-Based MEMS Tunable VCSEL Michael C. Y. Huang, Student Member, IEEE, Kan Bun Cheng, Ye Zhou, Bala Pesala, Connie J. Chang-Hasnain, Fellow, IEEE, and Albert P. Pisano
Abstract—We report the first experimental demonstration of a novel piezoelectric actuated microelectromechanical systems (MEMS) tunable vertical-cavity surface-emitting laser (VCSEL). A large physical deflection was obtained with the piezoelectric actuated MEMS cantilever beam monolithically integrated with the VCSEL distributed Bragg reflector. Single-mode emission and continuous tuning were achieved at room temperature under continuous-wave operation. Index Terms—Laser tuning, microelectromechanical systems (MEMS), vertical-cavity surface-emitting laser (VCSEL).
I. INTRODUCTION
M
ICROELECTROMECHANICAL systems (MEMS) tunable vertical-cavity surface-emitting laser (VCSEL) has been extensively studied [1] for various applications including signal routing and switching in optical networks, biomolecular sensing and spectroscopy, and optical excitation of rubidium species in an atomic clock system. MEMS tunable structures are desirable because they provide for a large and continuous tuning range with high precision, fast response, and the compatibility with current optoelectronic fabrication processes. The actuation mechanisms previously reported have been either electrostatic or thermal actuation. The electrostatic actuation is often limited by the 1/3 rule, where the maximum deflection can be at most 1/3 of the original airgap spacing. The most devastating drawback is the possibility of electrical catastrophic discharge that damages VCSEL when pull-in occurs [2]. 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 it generally requires high actuation power [3]. Recently, we proposed the design of a monolithic piezoelectric tunable VCSEL that exploits the inherent piezoelectric properAs compounds [4]. Piezoelectric actuation ties of the Al Ga offers several potential advantages. First, it can provide large displacements in bidirections, both toward the substrate and opposite to it. The movement is not limited by the pull-in effect, as opposed to the 1/3 gap limit with electrostatic actuation. Most Manuscript received September 6, 2005; revised February 4, 2006. This work was supported in part by the Defense Advanced Research Projects Agency (DARPA) Chip-Scale Atomic Clock (CSAC) under Grant NBCH1020005. M. C. Y. Huang, Y. Zhou, B. Pesala, 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]). K. B. Cheng and A. P. Pisano are with the Department of Mechanical Engineering, University of California, Berkeley, CA 94720 USA. Digital Object Identifier 10.1109/LPT.2006.873923
Fig. 1. Schematic 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. (Color version available online at http://ieeexplore.ieee.org.)
importantly, there will be no catastrophic damages due to capacitor discharge. It is expected to consume very low power (1 W), and has a fast response speed (limited only by the mechanical structure rather than the piezoelectric effect). Finally, piezoelectric actuation can be readily extended to other MEMS tunable optoelectronic devices operating at different wavelengths. In this work, we report the first demonstration of the novel piezoelectric actuated MEMS tunable VCSEL. The piezoelectric cantilever beam is monolithically integrated within the VCSEL distributed Bragg reflector (DBR). Single-mode emission and continuous tuning were achieved at room temperature under continuous-wave (CW) operation. II. PIEZOELECTRIC VCSEL DESIGN As is an inherent The piezoelectric property of Al Ga characteristic that originates from its lack of center symmetry in the zinc-blend crystalline structure [5]. Under the presence of a vertical electric filed, a longitudinal strain is produced. The magnitude of the piezoelectric effect is determined by the piezoelectric coefficient, which is a function of the aluminum composition and the orientation with respect to (110). By utilizing this property, a piezoelectric actuated cantilever structure is proposed, which comprises thin n-doped, thin inAs layers, as shown in Fig. 1. trinsic, and thick p-doped Al Ga 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
1041-1135/$20.00 © 2006 IEEE
1198
IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 18, NO. 10, MAY 15, 2006
Fig. 2. Schematic of the piezoelectric actuated MEMS tunable VCSEL from the (a) side view and (b) cross-sectional view. (Color version available online at http://ieeexplore.ieee.org.)
in the intrinsic layers via the converse piezoelectric effect. The thin piezoelectric layer is rigidly connected to the cantilever beam, which constrains the longitudinal motion of the piezoelectric layer and results in an equivalent bending moment applied to the beam. The bending moment produces a deflection 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. The schematic of the optical design of the MEMS tunable VCSEL is shown in Fig. 2. The device consists of a bottom n-DBR mirror, a -cavity layer with the active region, and a top mirror. The top mirror is comprised of three parts (starting from the substrate side): a fixed p-DBR, an airgap, and a freely suspended DBR that is supported via a cantilever structure. The suspended DBR cantilever consists of an unevenly distributed p-i-n structure: a thick p-DBR, a thin i-DBR, and a thin n-DBR, wherein such configuration enables the piezoelectric actuation mentioned above. Electric current injection is conducted through the middle contact (via the p-DBR) and backside contact. An aluminum oxide aperture is formed on an Al Ga As layer in the p-DBR section above the cavity layer to provide efficient current and optical confinement. The piezoelectric tuning is conducted through the top contact (via the n-DBR) and the middle contact. III. FABRICATION The fabrication process of the piezoelectric MEMS tunable VCSEL is the same as the existing electrostatic MEMS tunable VCSEL. The epitaxial wafer was obtained from Land Mark Optoelectronics. The process includes three metal depositions, cantilever structure formation, VCSEL mesa formation, oxidation, and cantilever beam release followed by critical point drying. Fig. 3 shows the scanning electron microscope (SEM) image of a fabricated MEMS tunable VCSEL. The final device contains m cantilever beam with a m head. a A highly selective etch is required to remove the GaAs sacrificial layer underneath the cantilever and form the freely sus-
Fig. 3. SEM image of a fabricated device from (a) top view and (b) side view. The suspended VCSEL cantilever was formed by selectively removing the GaAs sacrificial material underneath the beam.
Fig. 4. Measured cantilever deflection as a function of applied voltage for beams aligned in different angles in respect to (110) direction.
pending cantilever beam. The release process utilized a highly selective citric-acid-based etchant [6]. The etchant was prepared from 1 : 1 anhydrous citric acid: DI water (w/w). The citric acid was mixed with NH OH solution to adjust its pH to 6.5. Finally, the mixture of the pH-adjusted citric acid and H O solution in a volume ratio of 5 : 1 is heated to 60 C. The heating drastically increases the diffusion of the etchant, especially for the release of the large cantilever head. In addition, photoresist was used to cover all metal surfaces during the selective etch to minimize any electrochemical charging effect. By using this wet-etch chemistry, highly selective removal of the sacrificial GaAs against Al Ga As (the low aluminum component of the DBR) was achieved. Compared to the conventional reactiveion-etching-based selective etch method, the chemical-based selective etching enables very rapid fabrication cycle and improve the device yield and reproducibility. IV. EXPERIMENT RESULTS To verify the piezoelectric actuation of the cantilever strucm aligned in difture, simple cantilever beams ferent orientations were fabricated. Fig. 4 shows the measured
HUANG et al.: DEMONSTRATION OF PIEZOELECTRIC ACTUATED GaAs-BASED MEMS TUNABLE VCSEL
1199
Fig. 5. L–I and I –V characteristic at 0-V tuning bias. The maximum power output is around 900 W and threshold current is 1.2 mA.
beam deflection as a function of the applied voltage for beams with different orientations in respect to (110). The vertical profile of the beam was measured using Wyko white light interferometer and confirmed by atomic force microscope. As expected from the literature [5], the piezoelectric actuation exhibits a strong dependence on the orientation, where the beam deflection is largest for beams aligned to (110) and almost zero deflection for beams aligned to (100). A maximal physical deflection of 150 nm was obtained with this cantilever design. By integrating the piezoelectric cantilever arm m with the VCSEL cavity, CW wavelength tuning using the piezoelectric actuation was experimentally demonstrated in room temperature operation. Fig. 5 shows the light output– current–voltage ( – – ) characteristic of one fabricated MEMS tunable VCSEL, where the maximum output power is 1 mW and threshold current is 1.2 mA. When applying a bias voltage across the tuning contacts, the piezoelectric effect deflects the cantilever beam, which translates into the change in the laser emission wavelength. Fig. 6(a) shows the CW emission spectra of the device when . With the the active region is electrically pumped at AlO optical confinement, single-mode emission with 30-dB sidemode suppression ratio was obtained throughout the tuning range. Fig. 6(b) shows the emission wavelength as a function of the applied voltage and the – characteristic of the p-i-n structure in the cantilever beam. Within the voltage range from 10 to 5 V, 1 nm of continuous and precise wavelength tuning is obtained with very small leakage current ( nA) across the intrinsic piezoelectric layer. Further increasing the bias voltage would cause a dramatic reduction of the electric field and an increase of the forward-biased current, through which the thermal effect would counteract the beam movement. Also, the variation of airgap induces a phase change for the electric field inside the VCSEL optical cavity, where the phase change does not have linear correspondence to the emission wavelength. In addition, the limited tuning range was due to the physical deflection of 10 nm with this particular mechanical design. Future designs will incorporate the one used in simple MEMS experiment with 150-nm physical displacement to lead to 30-nm wavelength tuning.
Fig. 6. (a) CW tuning spectra for the MEMS tunable VCSEL. One nanometer of continuous wavelength tuning is obtained with single-mode emission 30-dB sidemode suppression ratio; (b) lasing wavelength as a function of tuning voltage and the I –V characteristic of the cantilever p-i-n junction plotted with the same x axis.
V. CONCLUSION We demonstrated piezoelectric tunable MEMS in As compound, where a large physical deflection of Al Ga 150 nm was achieved with a piezoelectric cantilever design. This design also led to the first monolithic integrated piezoelectrically actuated MEMS tunable VCSEL with room- temperature CW operation. Piezoelectric actuation can eliminate catastrophic damages created by the pull-in effects in electrostatic MEMS. In addition, it offers a large potential tuning range and is highly promising for many tunable MEMS device implementations. REFERENCES [1] C. J. Chang-Hasnain, “Tunable VCSEL,” IEEE J. Sel. Topics Quantum Electron., vol. 6, no. 6, pp. 978–987, Nov./Dec. 2000. [2] C. F. R. Mateus, C. H. Chang, L. Chrostowski, S. Yang, S. Decai, R. Pathak, and C. Chang-Hasnain, “Widely tunable torsional optical filter,” IEEE Photon. Technol. Lett., vol. 40, no. 6, pp. 819–821, Jun. 2002. [3] 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. [4] 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, no. 8, pp. 896–898, 2005. As: material parameters for use [5] S. Adachi, “GaAs, AlAs and Al Ga in research and device applications,” J. Appl. Phys., vol. 58, no. 3, pp. R1–R29, 1985. [6] T. Kitano, S. Izumi, H. Minami, T. Ishikawa, K. Sato, T. Sonoda, and M. Otsubo, “Selective wet etching for highly uniform Ga As heterostructure field effect transistors,” J. Vac. GaAs=Al Sci. Technol. B, vol. 15, pp. 167–170, 1997.
