IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 13, NO. 2, FEBRUARY 2001 97 Selectively Etched Undercut Apertures in AIAsSb-Based VCSELs E. Hall, S. Nakagawa, Student Member, IEEE, G. Almuneau, J. K. Kim, and L. A. Coldren, Fellow, IEEE Abstract—Apertures were formed in single-growth, AsSb-based, long-wavelength (1.55 m) vertical-cavity surface-emitting lasers by laterally etching the active region. The materials contrast be- tween the AlAsSb-based mirrors and the AlInGaAs-based active region leads to a high selectivity for the etch, allowing long aper- tures to be formed with minimal etching of the mirrors. Lasers showing reduced threshold currents and increased efficiencies were demonstrated using these apertures. Index Terms—Quantum-well lasers, semiconductor lasers, sur- face-emitting lasers. I. INTRODUCTION S IGNIFICANT recent progress in the development of ver- tical-cavity surface-emitting lasers (VCSELs) emitting at 1.3–1.55 m is quickly making these light sources a viable option as high-performance components for optical fiber net- works. In addition to offering cost advantages through such fea- tures as on-wafer testing, VCSELs also have inherent advan- tages over edge-emitters such as scalability to two-dimensional arrays. Although many of the best results for these devices have re- sulted from the wafer-fusion [1], [2] or metamorphic growth [3] of AlGaAs-based distributed Bragg reflector (DBR) mirrors with InP-based active regions, there is still considerable interest in the monolithic growth of long-wavelength VCSELs com- pletely lattice-matched to InP. This approach would reduce the cost and scaling issues associated with the wafer fusion process and avoid the possible reliability problems associated with the heavily dislocated metamorphic mirrors. The AlGaAsSb–AlAsSb materials combination lat- tice-matched to InP has shown to be an attractive option for the DBRs in an InP-based monolithic VCSEL because of the high refractive index contrast associated with this materials pair. Room-temperature pulsed lasing has been demonstrated in a VCSEL employing two AlGaAsSb-based DBRs and an AlGaInAs-based active region [4]. Unlike shorter wavelength AlGaAs-based VCSELs, there is no natural oxidizable material in an InP-based mono- lithic VCSEL from which an oxide aperture can be formed. In wafer-fused devices, an AlAs aperture placed in the GaAs-based mirror is typically used, but even for these devices, the resistive fused interface—which lies between the aperture Manuscript received August 6, 2000; revised October 30, 2000. This work was supported by the Heterogeneous Optoelectronics Technology Center. The authors are with the Department of Electrical and Computer Engi- neering, University of California, Santa Barbara, CA 93106 USA (e-mail: ehall@xanadu.ece.ucsb.edu). Publisher Item Identifier S 1041-1135(01)01049-7. Fig. 1. Schematic of VCSEL structure showing the undercut aperture thanks to the different material systems used for the mirrors and active region. and the active region—defeats much of the current confining effect of the aperture. These devices would also benefit, therefore, from an InP-based aperture. In the InP-lattice-matched materials, there has been work at- tempting to increase the slow lateral oxidation rate of AlInAs [5] or produce an oxide from AlAsSb without the thin Sb-metal layer that typically forms above the oxide [6]. In this letter, we present a method to form an aperture in VCSELs with AlAsSb DBRs by taking advantage of the radi- cally different materials used for the active region and the DBRs in these devices. A selective etch consisting of citric acid and hydrogen peroxide removes the AlInGaAs-based active region while barely etching the AlGaAsSb-based DBRs, confining the current and the mode, therefore, to the center of the structure. We have applied this etch to devices similar to those presented previously [4], and the reduction in threshold current achieved while maintaining a constant etched-pillar width is presented here. II. EXPERIMENTAL SETUP The VCSELs, which were grown by molecular beam epitaxy on an n-doped (Sn) InP substrate, are very similar to the devices previously reported. A schematic of this structure is shown in Fig. 1. The bottom and top DBRs consist, respectively, of 23 and 32 pairs of AlAs Sb and Al Ga As Sb -layers lattice-matched to InP. Lattice-matching was achieved by using previously calibrated group-V induced reflection high-energy electron diffraction oscillations and then growing at conditions with near-unity antimony incorporation rates [7]. This method ensures both reproducibility of results without daily repetition of the calibrations and also reliability of lattice-matching throughout a single growth. Both mirrors are also doped uniformly n-type with PbTe and have linearly 1041–1135/01$10.00 © 2001 IEEE