InAlGaAs/InP MQW Structures Grown by MOVPE in Nitrogen Atmosphere for 1.55 µm VCSELs

Abstract

m emitting InGaAlAs/InP MQW vertical cavity surface emitting lasers (VCSEL) structures grown by MOVPE under nitrogen atmosphere are presented. Structure design and growth conditions were optimized in order to obtain MQWs with high structural and optical properties. Electrically and optically VCSELs based on these active layers and fused AlGaAs/GaAs mirrors were opbtained. On optically pumped VCSEL a single mode CW emission power of 3 mW at 24 o C and 1.2 mW at 73 o C were measured. An output power of more than 1.5 mW at 20 o C and 0.2 mW at 70 o C were achieved for single mode electrically pumped VCSEL. Long-wavelength VCSELs are among most promising low-cost light sources for local-area and long-haul fiber-based optical communication systems and optical interconnection. The main limitation to realizing commercial devices has been high-temperature performance. The most commonly used active material system for 1.55-µm emission is InP-InGaAsP, which is limited in VCSEL applications by poor temperature characteristics. In contrast, InAlGaAs/InP material system is very attractive for long wavelength emitting lasers due to superior high temperature performance compared with standard InGaAsP quantum well lasers. This advantage is due to the conduction band offset in InAlGaAs system, dEc =0.72dEg, is considerably larger than in InGaAsP system (dEc =0.40dEg ) (1). This larger conduction band offset has been predicted to result in better electron confinement in the conduction band and, therefore, higher temperature stability. We describe here the MOVPE growth of strain-compensated InGaAlAs/InP MQW vertical-cavity 1.55 µm emitting structures in nitrogen atmosphere. The use of nitrogen as carrier gas in the MOVPE of III/V-based materials was proved to be effective in obtaining highly homogeneous and high purity layers and a reduction of carbon and oxygen incorporation in Ga(Al)As layers has been demonstrated (2). To our knowledge, there are no publications on the growth of InAlGaAs/InP using nitrogen as a carrier gas. Results on high power and high-temperature operation VCSEL devices based on these structures, with both optical and electrical pumping, are presented. 2. Experimental procedure InAlGaAs structures were grown by low pressure MOVPE under nitrogen atmosphere in an AIXTRON 200/4 reactor. The source materials were trimethylindium, trimethylgallium, trimethylaluminium, arsine and phosphine. Disilane and carbon tetrabromide were used as dopant sources. The V/III ratio was close to 100 and the growth rate was 1-2 µm/h. The MQW structure consists of 6 In 0.7Ga 0.2Al0.1As quantum wells (~1.2% compressively strained) surrounded by 7 In0.41Ga0.4Al0.19As barriers (~0.8% tensile strained) and InP cap and buffer layers. The QWs composition was tuned for a photoluminescence emission at ~1.53-1.55 µm with barriers with band gap at ~1.25 µm. Epitaxy was performed on (001) exactly oriented and 2 degree misoriented InP substrates at different growth temperatures in order to optimize the uniformity, as well as the optical and structural quality of the material. Grown structures were studied by standard room temperature photoluminescence (PL) measurement, Nomarski microscopy, Atomic Force Microscopy (AFM), Transmission Electron Microscopy (TEM) and high resolution X-Ray Diffraction (XRD).