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 1.3µm and 1.55µm 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.
Figure 1.
Figure 2.
Electrically-pumped, vertical-cavity lasers operating at 1.55µm 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 25µm 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.
Figure 3.
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.[1] 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 1.3µm has, in fact, been demonstrated.[5] 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. [1] H. Takenouchi, T. Kagawa, Y. Ohiso, T. Tadokoro, and T. Kurokawa, Electronics Letters 32 (18), 1671-3 (1996).
Figure 3.
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