Advances in Oxide-Confined Vertical Cavity Lasers. Photonics Research Department. Albuquerque, NM (505) phone (505) FAX

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1 I Advances in Oxide-Confined Vertical Cavity Lasers Kent D. Choquette, R. P. Schneider, Jr., K. L, Lear, K. M. 6gsC;. H. Q. Hou, H. C. Chui, M. Hagerott Crawford, and W. W. C 7 -. h I4 k#&j Photonics Research Department Sandia National Laboratories Albuquerque, NM (55) phone (55) FAX kdchoqu@sandia.gov Abstract We review the advances made in device fabrication, structure, and performance of vertical-cavity surface emitting lasers (VCSELs) which incorporate the selective oxidation of AlGaAs. DISCLAIMER This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recornmendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the

2 Advances in Oxide-Confined Vertical Cavity Lasers Kent D. Choquette, R. P. Schneider, Jr., K. L. Lear, K. M. Geib, H. Q. Hou, H. C. Chui, M. Hagerott Crawford, and W. W. Chow Photonics Research Department S andia National Laboratories Albuquerque, NM t During the past two years significant performance advancesl-9 have been achieved in selectively oxidized vertical-cavity surface emitting lasers (VCSELs), many of which have established overall benchmark records for semiconductor lasers. For example, Fig. 1 shows a wall plug efficiency (at 1 mw output) of greater than 5%3 and threshold currents less than 1 pa have been obtained from VCSELs containing a stable and low refractive index Al-oxide formed from AlGaAs. These oxidized VCSEL structures leverage the high oxidation selectivity of Al(Ga)As and the capability of forming buried oxide layers within the epilayers of the laser? Here we review the advances made in device fabrication, structure and performance of selectively oxidized VCSELs. Figure 2 shows sketches of several oxidized VCSEL structures which have been demonstrated. The laterally oxidized layers are formed from high Alcontaining AlGaAs or AlAs layers which are typically exposed at a mesa sidewall and oxidized in a steam environment at 4 to 45 "C. Oxide aperture VCSELs, as shown in Fig. 2(a), contain one or more oxide current apertures to funnel the current into the active region. The oxide apertures have been employed under a top dielectric distributed Bragg reflector (DBR) mirror,l or else doped2 or undoped6 monolithic semiconductor DBR mirrors. Oxide aperture VCSELs possess strong electrical and index-guided optical confinement in a planar geometry enabling efficient heat sinking. Instead of oxidizing only a few layers,

3 ~b K. Choquette, et az. "Advances in Oxide-Confined Vertical Cavity Lasers" * all of the low index AlAs layers can be partially laterally oxidized as shown in Fig. 2(b) to form a high index core and low index oxide cladding for greater optical confinement.5 Finally, Fig 2(c) depicts an "all-oxide" VCSEL which incorporates an oxide aperture between high index contrast DBR mirrors composed of GaAs/Al-oxide layers? The oxide DBRs have a very broad mirror stopband due to the large index contrast which relaxes the fabrication tolerances and the overall epilayer thickness, perhaps enabling easier VCSEL manufacture. Enhanced optical and electrical properties have been attained using the various oxidized VCSEL structures. Low threshold currents have been achieved using oxide current apertures under dielectric1 or undoped semiconductor DBRs.6 To achieve low threshold currents, enhanced coupling of spontaneous emission7 and/or reduced optical loss in the undoped mirrofi are important. Unfortunately, these VCSELs also exhibit relatively high threshold voltages due to current restriction or contact resistance effects. Monolithic oxide aperture VCSELs with doped mirrors (see Fig. 2@)) have exhibited record low threshold voltage2 and high wall plug efficiency? Furthermore, threshold currents as low as 58 na have recently been attained in these lasers, as depicted in Fig. 3. Moreover, the threshold currentlvoltage are found to be relatively insensitive to lasing wavelength, due to suppression of the thermally induced mismatch between the laser gaidcavity resonance.8 Finally, oxide aperture VCSELs have been fabricated at emission wavelengths varying from 1.Opm to 64nm,9 indicating the versatility of this structure. The record performance and potential manufacturability of oxide aperture VCSELs make these lasers both technologically and scientifically intriguing. The work at Sandia is supported by the DOE under contract No. DE-AC4-94AL85.

4 References ID. L. Huffaker, D. G. Deppe, K. Kumar, and T. J. Kumar, Appl. Phys. Lett. 65, 97 (1994). 2K. D. Choquette, R. P. Schneider, Jr., K. L. Lear, and K. M. Geib, Electron. Lett. 3, 243 (1994). 3K. L. bar, K. D. Choquette, R. P. Schneider, Jr., S. P. Kilcoyne, and K. M. Geib, Electron. Lett. 31, 28 (1995). 4M. MacDougal and P. D. Dapkus, IEEE Photon. Tech. Lett. 7,229 (1995). 5Y. Hayashi, T. Mukaihara, N. Hatori, N. Ohnoki, A. Matsutani, F. Koyama, and K. Iga, Electron. Lett. 31, 56 (1995). 6G. M. Yang, M. H. MacDougal and P. D. Dapkus, Electron. Lett. 31, 886 (1995). 7D. G. Deppe, D. L. Huffaker, J. Shin, and Q. Deng, IEEE Photon. Tech. Lett. 7, 965 (1995). 8K. D. Choquette, K. L. Lear, R. P. Schneider, Jr., K. M. Geib, J. J. Figiel, and R. Hull, IEEE Photon. Tech. Lett. 7, 1237 (1995). 9K. D. Choquette, R. P. Schneider, Jr., M. H. Crawford, K. M. Geib, and J. J. Figiel, Electron. Lett. 31, 1145 (1995).

5 s 5 K. Choquette, et al. "Advances in Oxide-Confined Vertical Cavity Lasers" Figure Captions Figure 1. Time lines of highest power conversion efficiency and lowest threshold current achieved for vertical-cavity surface emitting lasers. Figure 2. Sketches of oxidized VCSEL structures (heavy lines are oxide layers), which incorporate: (a) buried oxide current apertures; (b) oxidized low index cladding; or (c) semiconductor/al-oxide high index contrast distributed Bragg reflector mirrors. Figure 3. Light output versus current for an ultralow threshold current oxide aperture 84 nm VCSEL exhibiting a threshold current of 58 nm.

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