E. Hall*, G. Almuneau**, J.K. Kim**, A. Huntington*, R. Naone*, H. Kroemer**, L.A. Coldren**
* Materials Dept., University of California, Santa Barbara, CA 93106
** Electrical and Computer Engineering Dept., University of California, Santa Barbara, CA 93106
The extension of vertical cavity surface-emitting lasers (VCSELs) to the telecommunications-significant wavelengths of 1.3Ám and 1.55Ám continues to be an intense area of research. Although the most successful approach to date has involved the wafer-fusion of AlGaAs-based distributed Bragg reflectors (DBRs) to InP-based active regions, there has also been considerable research into a single-epitaxial structure on InP. This approach would significantly reduce processing requirements but has been slowed by the lack of a mature materials combination that has enough refractive index contrast to make highly-reflective, low-loss DBRs.
In this article, we examine two materials combinations that would provide the needed index contrast for monolithic VCSELs on InP, focusing first on arsenide-antimonide mixed group-V materials and then on laterally-oxidized AlInAs.
The arsenide-antimonides are an analog to the successful GaAs/AlGaAs combination on GaAs (used commonly to produced 850-980nm VCSELS), with the introduction of antimony allowing lattice-matching to InP. Because of the low index of AlAsSb (n~3.1), AlAsSb/AlxGa1-xAsSb (x ~ 0.2) DBRs can achieve very high reflectivity with relatively few periods as shown by the calculation in Fig. 1. This high index contrast leads to a lower penetration depth than traditional InGaAsP-based DBRs and, therefore, implies lower optical loss in the structure. The reflectivity spectrum of a n-doped mirror employing this materials combination is shown in Fig. 2, demonstrating the large stop-band and high reflectivity achieved with 20.5 periods.
Electrically-pumped, vertical-cavity lasers operating at 1.55Ám produced in a single epitaxial growth using these antimonide-based DBRs have recently been demonstrated. Two n-type mirrors were used to decrease both the voltage and optical loss, and a heavily-doped tunnel junction was placed at a standing-wave null of the mode to provide electron-hole conversion from the top n-mirror. The active region consists of five strain-compensated AlInGaAs quantum wells.
The L-I results from a 25Ám diameter pillar operated in pulsed- mode at room temperature are shown in Fig. 3. The threshold current is ~7mA, corresponding to a current density, Jth, of only 1.4kA/cm2. The external differential quantum efficiency of this device was ~18% and the maximum power was about 2mW.
The lateral oxidation of AlInAs for even higher index contrast is also a potentially exciting approach. Although this ternary has a very poor index contrast with the available high index materials (Dn ~ 0.3 for AlInAs/InAlGaAs), oxidation of AlInAs lowers its refractive index from 3.2 to ~2.5. As shown by the calculation in Fig. 1, very high reflectivity can be achieved for ~10 periods. A DBR based on this scheme and centered at 1.3Ám has, in fact, been demonstrated. However, because of
the low aluminum content of this alloy (~48%), both high temperatures and long times are needed to oxidize this material, degrading the surface quality of these InP-based structures. A significant increase in oxidation rate has recently been achieved by growing the AlInAs layer as a strain-compensated digital alloy of AlAs and InAs. As shown in Fig. 4, this scheme allows a decrease in either oxidation time or temperature and should enable oxide-based mirrors without harming surface quality This work was supported by the Heterogeneous Optoelectronics Technology Center.  H. Takenouchi, T. Kagawa, Y. Ohiso, T. Tadokoro, and T. Kurokawa, Electronics Letters 32 (18), 1671-3 (1996).
High Temperature 1300 nm VCSELs for single-mode fiber-optic communication
Manufacturing of Oxide VCSEL at Hewlett Packard
Tapered-apertures for high-efficiency miniature VCSELs
Return to Table of Contents..