Structural Characterization of Molecular Beam Epitaxy Grown GaInNAs and GaInNAsSb Quantum Wells by Transmission Electron
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Structural Characterization of Molecular Beam Epitaxy Grown GaInNAs and GaInNAsSb Quantum Wells by Transmission Electron Microscopy Tihomir Gugov, Mark Wistey, Homan Yuen, Seth Bank and James S. Harris Jr. Solid State and Photonics Laboratory, Stanford University, Stanford, CA 94305, U.S.A. ABSTRACT In the past decade, the quaternary GaInNAs alloy has emerged as a very promising material for lasers in the 1.2-1.6 µm range with application in telecommunication fiberoptic networks. While most of the challenges in growing high quality laser material with emission wavelength out to 1.3 µm have been successfully resolved, extending the emission beyond 1.3 µm has proven to be quite difficult. Achieving emission out to 1.5 µm requires higher In (up to 40%) and N (up to 2%) compositions. This makes the growth of this thermodynamically unstable alloy quite difficult with phase segregation occurring even at lower growth temperatures. Recently, adding small amounts of antimony has dramatically improved the quality of the material and high luminescence has been demonstrated at wavelengths beyond 1.5 µm. In this study, high-resolution transmission electron microscopy (HRTEM) was used in a novel way in conjunction with dark-field (DF) TEM to elucidate the role of antimony in improving the material quality. The results show that antimony improves the material uniformity via reduction of the local compositional fluctuations of indium. INTRODUCTION The GaInNAs(Sb) material system is considered a great candidate for optical sources in fast (gigabit) metro and local area networks. By changing the composition of the alloy, the emission can be tuned to cover the 1.3-1.55 µm range which is the preferred window for telecommunication applications. GaInNAs(Sb) based lasers are expected to have lower cost and better performance compared to the presently employed InGaAsP/InP Bragg grating and distributed feedback (DFB) lasers. Growth on GaAs, an inexpensive and well-developed technology, is sure to greatly reduce the fabrication costs. The large conduction band offset intrinsic to this system ensures better thermal performance and provides an opportunity for the lasers to operate uncooled. In order to utilize the full potential of this alloy, the foremost prerequisite is the ability to grow high quality material. This alloy is characterized by a large miscibility gap which requires that it be grown at low temperatures in metastable regimes. A lot of the growth challenges have been addressed for 1.3 µm emission alloy (containing up to 30% In and 2% N) and several groups [1-4] have been successful in fabricating lasers at this wavelength. The effort to push emission to 1.55 µm, which requires compositions of up to 40% In, has proven next to impossible due to the tendency of the material to phase segregate in this high In concentration regime [5]. Very recently, the addition of a small amount of Sb has put the race for 1.55 µm material back on track by dramatically improving the optical properties of the alloy [6-8]. In this study, we p
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