Precise aperture control of oxidized GaAs surface emitting laser using in-situ monitered oxidation process

Abstract

The precise and reproducible control of oxide aperture diameters in oxide-VCSELs (Vertical Cavity Surface Emitting Lasers) is needed for highly reliable and low-cost manufacturing as well as for realizing small aperture single mode devices.') However, the precise control of oxidation process is still a remaining issue in a production stage. In this paper, we develop a novel in-situ monitoring system of GaAlAs wet oxidation for the precise control of oxide apertures in 850 nm GaAs VCSELs. Figure 1 shows the schematic setup of our oxidation system with in-situ optical monitoring system using an infrared microscope system combined with a steam furnace. Oxidation temperatures and corresponding oxidation rates are ranging from 420 to 460°C and 0.5 to 2.4 p d m i n , respectively. Samples we used include single 40 nm Al,,,,Ga,,,,As layer for oxide-current confinement underneath a 24-paired p-DBR of standard 850 nm VCSEL wafers?) We formed 4 p height and 33 pn square mesas by using standard lithography and inductively coupled plasma etching. Figure 2 shows the top view of an oxidized mesa during wet oxidation observed by an in-situ monitoring microscope. The oxide-aperture diameters are plotted as a function of oxidation time at various substrate temperatures (TJ, which were measured by the in-situ monitoring system. The results show that an oxidation rate increases with an aperture diameter of less than 7 pm under all T, conditions, while an oxidation rate seems almost constant at diameters ranging from 7 to 20 p. In addition, we found that there is dead time before noticeable oxidation, which is dependent on the oxidation temperature T,. Also, there is run-to-run variation, which is shown by different experiments #A and #B as shown in Fig. 3." By using the in-situ monitoring of aperture diameters, we are able to control precisely the diameter in spite of nonlinear nature of such oxidation rates. Figure 4 shows an oxidation process sequence using the in-situ monitoring system. We used a two-step oxidation sequence with different oxidation rates of 2 pdmin . and 0.5 @min. for better control in small oxide apertured devices. The oxidation temperature was reduced to slow down an oxidation rate from 2 pn to 0.5 pn when the aperture size is 6 pn ahead a target size. Figure 5 shows measured results of oxide aperture diameters for three different target diameters of a) 3.5 l~n, b) 6.0 pm and c) 12 pn. As shown in the figure, we archived run-to-run variation of I0.6 pn in all cases of a), b) and c) without any calibration prior to oxidation process. The m-to-run variation without in-situ monitoring is about M p, which could be dramatically improved in our system. Figure 6 shows typical L/ I characteristic of 850 nm GaAs VCSELs with a 12 pm oxide aperture fabricated by our in-situ monitored oxidation process. We believe that this is the first demonstration of the precise control of oxide-apertures by in-situ monitoring.') We expect that ow proposed method may enable us mass-production of oxidized VCSELs including single mode devices.