Wide-bandwidth Distributed Bragg Reflector Using AlAs Oxide/GaAs Multilayers

Abstract

Distributed Bragg reflectors (DBRs) are used in a wide variety of optoelectronic devices, including vertical cavity surface emitting lasers (VCSELs), resonant cavity detectors, and phototransistors. Because of the low refractive index ratio of 3.5/3.0 for typical materials, many pairs of the constituent materials must be grown to achieve a reflectivity of greater than 99%, and the band for which the reflectivity is greater than 90% is only around 100 nm. Furthermore, the spectral bandwidth and reflectivity are very sensitive to the thickness and thickness uniformity of the layers. For this reason, highly sophisticated growth control techniques must be employed to control the thickness. To relax this requirement as well as increase the spectral bandwidth , the use of two materials that have a much larger refractive difference must be used. In this report, we describe the fabrication of a wide bandwidth high reflectivity DBRs using the native oxide of AlAs as the low refractive index layer and GaAs as the high refractive index layer so that the refractive index ratio is increased from 1.2 in GaAs/AlAs to 2.3 in GaAs/AlAs oxide. The use of high index ratio mirrors in the GaAs material system has been shown previously[l,2], where the AlAs is etched away and replaced either with air or acrylic resin; however, this technique requires great care to keep the DBR from collapsing and needs supports on the side to hold up the GaAs layers. In contrast, our oxide/GaAs DBR structure is a robust, self-supporting structure An advantage of this structure due to the wide bandwidth is the insensitivity to the angle of incoming light. This wide bandwidth and low angular sensitivity benefit broadband devices such as light emitting diodes and solar cells by increasing light utilization. . The structure, shown in Figure 1, is grown by MOCVD, patterned with stripes, and etched to expose the AlAs layers for subsequent wet thermal oxidation[3]. The oxidation rate of AlAs is much faster than that of GaAs, which allows for the total consumption of AlAs while the GaAs is left unoxidized. The native oxide of AlAs is formed by flowing N2 bubbled through H20 at 90°C over the sample at 425°C. The sample is taken out when the AlAs is completely oxidized. The index of refraction of the oxide is approximately 1.55. The combination of the native oxide with GaAs, which has an index of refraction of 3.5 at 1 pm, creates a pair with a high refractive ratio of 2.26. The reflectivity spectrum of a 3 pair oxide/GaAs DBR is shown in Fig. 2. The absolute reflectivity is calibrated using Au as a reference. The peak reflectivity is 99.5f0.3%, and the bandwidth is 434 nm. By comparison, a structure with 16 pairs of AlAs/GaAs gives a reflectivity of 99.5% with a bandwidth of only 110 nm. The AlAs/GaAs structure is also 2 times thicker than the oxide/GaAs structure. We will present oxidation rates as well as dependence of oxide quality on growth conditions. Also, characterization of the structure by SEM and XTEM will be shown.