InP-based VCSELs with Buried Tunnel Junction for Optical Communication and Sensing in the 1.3-2.3 μm Wavelength Range

Abstract

During the past few years, vertical-cavity surface-emitting lasers (VCSELs) have emerged as one of the most important semiconductor light sources for numerous applications. While this is true without exception for GaAs-based versions with emission wavelengths below 1 µm, the development of VCSELs for longer wavelengths, such as 1.3 or 1.55 µm suffers from fundamental intrinsic drawbacks of the material systems which are suited for these wavelengths. In particular, the enhanced temperature sensitivity of the material gain in InP-based active regions as well as the poor thermal conductivity of ternary or quaternary compounds hamper the realization of high performance lasers. With novel technological concepts, however, significant progress for long-wavelength VCSELs has been achieved only recently [1][2][3][4][5]. In this paper we present recent progress on high-performance InP-based longwavelength VCSELs incorporating a buried tunnel junction (BTJ) for applications in optical communication and sensing [4]. The BTJ-approach as shown in Fig. 1 addresses the thermal difficulty by the use of a tunnel junction which substitutes electrically high-resistance and high-absorbing p-doped layers by low-resistance n-doped layers. Accordingly, ultralow threshold voltages and series resistances can be achieved along with sharply decreased excess heat generation. Lateral structuring of the tunnel junction results in a self adjusted strong current and index confinement.To reduce the thermal resistance on the heat sink side, a short period and highly reflective hybrid dielectric mirror with gold coating is used. While the InP-substrate on top of the structure is completely removed, an electroplated gold layer on the bottom side serves as an excellent heatsink and provides mechanical stability. An optimized modulation and packaging capability is accomplished with a low parasitic design comprising a thick insulation layer and one side contacting scheme. The entire manufacturing is done on a full wafer level. Device characteristics for singlemode 1.55 µm wavelength VCSELs in cw operation are given in Fig. 2 for room temperature and 80°C, respectively. Owing to the excellent thermal heat management, the lasers exhibit a singlemode optical output power at room temperature well beyond 3 mW even for BTJ-diameters as small as 4.5 µm which corresponds to the highest power density reported for longwavelength VCSELs [5]. For comparison, typical multimode VCSELs with 20 µm BTJ-diameter show threshold currents around 14 mA and peak output powers around 12 mW, respectively. At 80°C, the singlemode output power still reaches 1 mW while the maximum cw operating temperature is around 110°C. The threshold current and voltage are as small as 0.88 mA and 0.93 V, respectively and the differential series resistance is only around 30 Ω at rollover current. The small series resistance facilitates high speed operation as shown in Fig. 2 for wide a wide open 10 Gbit/s back-to-back eye diagram. Fig.1 Cross section of BTJ-VCSEL 113 0-7803-9560-3/06/$20.00

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