725. Long-Wavelength Resonant Vertical-Cavity. LED/Photodetector with a 75-nm Tuning Range. G. L. Christenson, A. T. T. D. Tran, Z. H. Zhu, Y. H. Lo, M. Hong ...
IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 9, NO. 6, JUNE 1997
725
Long-Wavelength Resonant Vertical-Cavity LED/Photodetector with a 75-nm Tuning Range G. L. Christenson, A. T. T. D. Tran, Z. H. Zhu, Y. H. Lo, M. Hong, J. P. Mannaerts, and R. Bhat
Abstract— A design for a highly tunable long-wavelength LED/photodetector has been investigated. The device consists of a GaAs-based distributed Bragg reflector (DBR) that is waferbonded to InP-based active layers, with a surface-micromachined tunable top DBR mirror to produce the wavelength shift. A 1.5-m device has been fabricated with a continuous tuning range of 75 nm. An extinction ratio of greater than 20 dB existed across the entire tuning range. Index Terms— Frequency control, indium materials/devices, light-emitting diodes, microelectromechanical devices, semiconductor device bonding, tunable circuits/devices.
L
ONG-WAVELENGTH transmitters and receivers are needed for optical communications and interconnects. Vertical-cavity devices have been extensively studied for their compact size, circular beam profile, single longitudinal mode, and wafer-scale fabrication and testing. In addition to these properties, a large continuous tuning range is desirable for applications such as wavelength-division multiplexing (WDM), in which several different wavelengths are required. Several methods have been used in the past to tune the wavelength of vertical cavity devices, including mechanically changing the thickness of an added spacer layer [1] and using current injection to produce temperature-induced changes in the refractive index of the active layers [2]–[4], but the tuning ranges have been limited to 10 nm with these methods. Another method that has been explored recently for GaAs-based vertical-cavity surface-emitting lasers (VCSEL’s) is the use of a micromachined movable top mirror that has accomplished an 18-nm tuning range [5], [6]. Longwavelength devices are more problematic due to the difficulty of high-reflectivity back-mirror fabrication. To date, there has been negligible tuning of these devices. The 1.5- m device structure presented here has attained a continuous wavelength tuning of 75 nm during operation as both a light-emitting diode (LED) and photodetector by using a tunable micromachined Manuscript received October 29, 1996; revised February 10, 1997. This work was supported in part by the National Science Foundation, DARPA, and JSEP/Air Force Office of Scientific Research. The work of G. L. Christenson was supported by a Department of Defense NDSEG fellowship grant. G. L. Christenson and Y. H. Lo are with the School of Electrical Engineering, Cornell University, Ithaca, NY 14853 USA. A. T. T. D. Tran is with the School of Applied and Engineering Physics, Cornell University, Ithaca, NY 14853 USA. Z. H. Zhu is with the School of Electrical Engineering, Cornell University, Ithaca, NY 14853 USA on leave from Zhejiang University, China. M. Hong and J. P. Mannaerts are with Lucent Technologies, Murray Hill, NJ 07974 USA. R. Bhat is with Bellcore, Red Bank, NJ 07701 USA. Publisher Item Identifier S 1041-1135(97)04056-1.
top distributed Bragg reflector (DBR) and wafer bonding to combine InP-based active layers with a high-reflectivity GaAs-based back-side DBR mirror. The device structure with electrical connections is shown in Fig. 1(a). An SEM of the membrane is shown in Fig. 1(b). The vertical cavity consists of a GaAs-based DBR mirror grown on a GaAs substrate, InP-based strain-compensated multiple quantum well (MQW) active layers, and a micromachined tunable top DBR mirror. Fig. 1(c) shows a SEM of the bonded region, indicating no discontinuity in the layer structure due to the wafer bonding process. For room temperature, continuous-wave (CW) operation, a laser driver injects current into the MQW layers, resulting in light emission from the optical aperture of the structure. This light is collected by a multimode optical fiber leading to an optical spectrum analyzer that monitors the output. When a voltage is applied to the contact on the top mirror, electrostatic forces move the mirror toward the substrate. This decreases the Fabry–Perot cavity length and the output shifts continuously to shorter wavelengths. The InP-based active layers are grown by organometallic vapor phase epitaxy and consist of 12 compressively strained InGaAsP quantum wells (QW’s) with alternating tensile-strained barrier layers. The bottom DBR mirror is composed of 27.5 pairs of alternating /4 GaAs and AlAs layers grown by molecular beam epitaxy. The GaAs system is chosen for the bottom mirror instead of InP since there is a much larger refractive index difference between GaAs and AlAs than lattice-matched InP-based materials, resulting in a sufficient reflectivity with fewer layer pairs. In order to incorporate the InP-based active material with the GaAsbased mirror, wafer bonding is used [7], [8]. In this process, the epitaxial layers on the two wafers are fused together in a hydrogen furnace after extensive surface preparations. The InP substrate is removed with HCl, leaving the InP-based MQW layers on the GaAs–AlAs DBR mirror and the GaAs substrate. The bonded epitaxial layer is consistent over the entire area of the wafer, with no visible defects or peeling. The integrity of the bonded surface remains intact throughout the rest of the processing, indicating strong chemical bonds between the two materials. Surface micromachining is then used to fabricate the suspended top mirror of the resonant cavity [9]. A sacrificial polyimide layer is used to form an air gap between the suspended structure and the top of the InP active layers on the wafer surface. The suspended membrane consists of a second DBR mirror and a TiW layer that provides support and acts
1041–1135/97$10.00 1997 IEEE
726
IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 9, NO. 6, JUNE 1997
(a)
Fig. 2. Output power of the LED as the applied tuning voltage increases from 0 to 47.5 V. The inset shows the output power of the LED with no applied bias, showing the high extinction ratio over a large frequency range.
(b)
Fig. 3. Measured (hollow) and theoretical (solid) shifts in the emission wavelength with an applied membrane voltage from 0–50 V for three devices with different mechanical structures.
(c) Fig. 1. (a) Schematic cross section of a vertical-cavity LED. A laser driver supplies the bias current to the MQW region, resulting in light emission from the top of the device. A suspended mirror is adjusted electrostatically by the application of a bias voltage to change the output wavelength. (b) SEM 140 photograph of the suspended membrane with dimensions of 140 m m. (c) SEM of the defect-free InP–GaAs bonded interface.
2
as the membrane contact. The DBR mirror is composed of 4.5 pairs of alternating /4 layers of Si and SiO , resulting in 99% reflectivity from 1.4 to 1.8 m. Etch holes are fabricated in the top membrane for faster release. Corrugation is added to increase the strength of the TiW membrane. The dimensions of the membrane are 140 m 140 m. The cantilever support beams have widths of 10 m and lengths of 160 m. The optical aperture is 14 m in diameter and the air gap between the membrane and the substrate is approximately 3 m, which is slightly greater than twice the active layer wavelength. During wavelength tuning, a voltage is applied between the TiW contact and the top contact of the active
layer structure, producing an electrostatic force that pulls the membrane toward the substrate and blue-shifts the emission wavelength. Since the thickness of the spin-on polyimide that determines the cavity length is not uniform over a large area, the devices have different central wavelengths across the wafer. This can be overcome by biasing each device to obtain the required wavelengths, since all have similar tuning characteristics. The emission profile for the device is shown in Fig. 2 for applied tuning voltages from 0 to 47.5 V. The central wavelength shifts continuously from 1510.5 nm to shorter wavelengths with increasing applied voltage. A plot of the emission wavelength shift with applied membrane voltage for three devices with different mechanical structures is shown in Fig. 3. Theoretical curves are also given for comparison. The increased deflection at higher voltage is due to a decrease in the gap size, since the wavelength shift is proportional , where is the applied to and are the thicknesses of the membrane voltage,
CHRISTENSON et al. LONG-WAVELENGTH RESONANT VERTICAL-CAVITY LED/PHOTODETECTOR
top DBR mirror and the air gap, and and are the respective permittivities. At 50 V, the central wavelength of the top curve is 1452 nm. The reflectivity of the backside mirror drops off at this wavelength. Above this voltage, instead of shifting to shorter wavelengths, the device resonance discontinuously hops to a central wavelength of 1527 nm due to the higher reflectivity of the back-side mirror at this wavelength. With an increasing membrane bias, this second mode can then be tuned to shorter wavelengths. In fact, the voltage can be increased to produce an output wavelength that tunes from 1527 nm down continuously again to 1452 nm, leading to a continuous tuning range spanning 75 nm. The emission profile has an extinction ratio of greater than 20 dB that is presently limited by the measurement apparatus. The linewidth is 4–6 nm throughout the tuning range, which is slightly higher than desired since current confinement has been omitted in the present design. As a result, absorption and diffraction of light at the metal contact on the top side of the active region broadens the spectrum. When the active layers are reverse-biased and light is sent into the optical aperture, the device functions as a tunable narrow-band resonant cavity photodetector, with an absorption profile that is nearly identical to the emission profile obtained with forward bias. In addition, the tuning characteristics of the detector, such as central wavelength and wavelength shift with tuning voltage, very closely match that of the emitter. In conclusion, we have demonstrated a 1.5- m tunable LED/photodetector with a high extinction ratio and a very large continuous tuning range of 75 nm. The mechanical structure has a high-frequency cutoff of 5 kHz, which can be extended if the compressive stress in the structure is reduced. For this design, wafer bonding and surface micromachining have been integrated to fully exploit various properties not available otherwise. Due to its vertical cavity, tunability, and easy two-dimensional array formation, the device has potential applications in optical communication and interconnect. For
727
WDM applications, an array of transmitters and/or receivers can be made simultaneously on a chip with a single fabrication process. By tuning the devices individually, the complete array of WDM wavelengths can be obtained and adjusted on a single small chip, increasing the throughput and decreasing costs. ACKNOWLEDGMENT The device was fabricated at the Cornell Nanofabrication Facility, which is supported by the National Science Foundation, Cornell University, and its industrial affiliates. REFERENCES [1] N. Yokouchi, T. Miyamoto, T. Uchida, Y. Inaba, F. Koyama, and K. ˚ continuous tuning of a GaInAsP/InP vertical-cavity surfaceIga, “40 A emitting laser using an external mirror,” IEEE Photon. Technol. Lett., vol. 4, pp. 701–703, 1992. [2] T. Wipiejewski, K. Panzlaff, E. Zeeb, and K. J. Ebeling, “Tunable extremely low threshold vertical-cavity laser diodes,” IEEE Photon. Technol. Lett., vol. 5, pp. 889–892, 1993. [3] C. J. Chang-Hasnain, J. P. Harbison, C. E. Zah, L. T. Florez, and N. C. Andreadakis, “Continuous wavelength tuning of two-electrode vertical cavity surface emitting lasers,” Electron. Lett., vol. 27, pp. 1002–1003, 1991. [4] L. Fan, M. C. Wu, H. C. Lee, and P. Grodzinski, “10.1 nm range continuous wavelength-tunable vertical cavity surface-emitting lasers,” Electron. Lett., vol. 30, pp. 1409–1410, 1994. [5] 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. [6] M. C. Larson and J. S. Harris, Jr., “Wide and continuous wavelength tuning in a vertical-cavity surface-emitting laser using a micromachined deformable-membrane mirror,” Appl. Phys. Lett., vol. 68, pp. 891–893, 1996. [7] Z. L. Liau and D. E. Mull, “Wafer fusion: A novel technique for optoelectronic device fabrication and monolithic integration,” Appl. Phys. Lett., vol. 56, pp. 737–739, 1990. [8] Y.-H. Lo, R. Bhat, D. M. Hwang, M. A. Koza, and T. P. Lee, “Bonding by atomic rearrangement of InP/InGaAsP 1.5 m wavelength lasers on GaAs substrates,” Appl. Phys. Lett., vol. 58, pp. 1961–1963, 1991. [9] A. T. T. D. Tran, Y. H. Lo, Z. H. Zhu, D. Haronian, and E. Mozdy, “Surface micromachined Fabry–Perot tunable filter,” IEEE Photon. Technol. Lett., vol. 8, pp. 393–395, 1996.
