Cecom - Tr - 92 - B 007 - F a Cascadable , Monolithic Laser / Modulator / Amplifier Transmitter

A. 13-tan wavelength, InGaAsP-InP folded-cavity, siirface-emitting laser with CH4-H2 reactive ion-etched vertical and 45° angled facets was demonstrated for the first time. Continnoos-wave threshold currents of 32 mA have been achieved, with >15 mW CW power for the surface-emitted light These surface-emitting lasers with two dry-etched facets are suitable for wafer-level -esüng and for monolithic integration with other InP-based photonic devices. LONG-WAVELENGTH InP-InGaAsP surface-emitting lasers (SEL's) are useful for optical communications and two-dimensional optoelectronic interconnections employing InP-based optoelectronic integrated circuits [1]. Research on 1.3-fim and 1.55-pm-long wavelength SEL's has been centered on four different types of laser structures: verticalcavity SEL's [2H3], grating SEL's [4], integrated-deflector SEL's (IDSEL's) [5]-[6], and folded-cavity SEL's (FCSEL's) [7H9]. To date, some of the most successful SEL's utilize high temperature (740 °C) mass transported integrated deflectors [5], or wafer-fused GaAs-AlAs vertical-cavity mirrors [2]. Unfortunately, the high temperature processes and/or complicated structures of these devices are not readily compatible with monolithic integration of devices such as transistors and detectors. In this respect, FCSEL's and IDSEL's with etched beamdeflectors for surface-emission, and a simple p-i-n material structure with a fabrication process nearly identical to that of simple etched-facet lasers, can be easily incorporated into InP photonic integrated circuits. Considerable work on etchedfacet InP IDSEL's has recently been reported with threshold currents of approximately 30 mA [6]. Unfortunately, most devices of this type suffer from low output efficiency. On the other hand, compared to an external deflector, the 45° total internal reflection mirror of a FCSEL exhibits superior deflecting efficiency, and requires no high-reflectivity coatings, and high performance FCSEL's based on GaAs material have Manuscript received January 30,1995; revised April 7,1995. This work was supported in pa« by Rome Labs/ARPA, AFOSR, and US Army/CECOM. C-P. Chao, D. G. Garbuzov, and S. R. Forrest are with the Advanced Technology Center for Photonics and Optoelectronic Materials, Department of Electrical Engineering, Princeton University, Princeton. NJ 08544 USA G.-J. Shiau was with the Advanced Technology Center for Photonics and Optoelectronic Materials, Department of Electrical Engineering, Princeton University. Princeton, NJ 08544 USA; he is now with Applied Materials, Inc San Jose, CA 95054 USA L. A DiMarco and M. G. Harvey are with the David Samoff Research Center, Princeton, NJ 08540 USA IEEE Log Number 9412716. already been demonstrated [10]. For InP-InGaAsP FCSEL's, many technologies, such as inclined reactive ion beam etching [6] and focused ion beam etching [8] have been adopted for the formation of the 45° facet, and devices reported to date still exploit one cleaved facet primarily due to the inferior quality of the etched 45° mirror. Following the demonstration of a precisely etched InP-InGaAsP 45° facet [9], in this work we report the fabrication of 1.3-/ixa wavelength, strained MQW InP-InGaAsP FCSEL's with 45° angle and vertical facets. With both facets dry-etched, these devices can be tested at the wafer level, and can be readily integrated with other long-wavelength optoelectronic devices. A low, continuouswave (CW) threshold current of 32 mA and 18% efficiency for the surface-emitted light was achieved for 4-^m-wide by 600-^m-long devices. Threshold current as low as 25 mA was measured for 470-^m-long devices with one facet cleaved. To our knowledge, this is the first demonstration of truly integratable InP-InGaAsP FCSEL's, and the threshold current represents a 30% improvement over the lowest value reported to date for 1.3 ßm wavelength FCSEL's [9]. Furthermore, a CW power > 15 mW for vertical light emission is observed, which is comparable to the best value reported to date for SEL's at this wavelength [6]. The laser wafer was grown by gas-source molecular beam epitaxy (GSMBE) on a S-doped n InP substrate. A 1-Atm thick, lightly Si-doped (2 x 10 cm) InP lower cladding layer was first grown, followed by a 1200-Ä thick, undoped InGaAsP lower waveguide layer which had a bandgap cutoff wavelength of Xg = 1.15 fj.ni. After growth of the undoped active region, a 1200-A-thick, InGaAsP Xg = 1.15 ßm upper waveguide layer was grown. The active region consists of five compressively strained InGaAsP quantum wells with 105-Ä wide barriers (A5 = 1.15 ßm) and 67-A wide wells (Xg = 1.4 ßm). Finally, we grew a 1.2-^m thick, Be-doped (2 x 10 cm to 5 x 10 cm) InP upper cladding layer, and an 800-A-thick, p InGaAs top contact layer Be-doped to ~2 x 10 cm. The device, shown schematically in Fig. 1, exploits a 45° facet, a vertical facet, and a ridge waveguide, all formed by dry etching using a CH4-H2 plasma. The device processing started by patterning 4-/jm-wide by 600-/xm-long SiNx (2000-Ä thick) strips that served as a dry-etching mask for the ridge waveguide. The ridge etching was stopped ~2000 A above the undoped waveguide region. A flat surface 1041-U35/95S04.00 © 1995 JEEE