Low Temperature Transparent ITO-based Contacts for Mid-IR Applications
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Low Temperature Transparent ITO-based Contacts for Mid-IR Applications F. Genty1,2, S. Margueron2, S. Ould Saad Hamady2, J.C. Petit2, H. Srour2, A. Karsaklian dal Bosco1,2, J. Sadok1, J. Huguenin2, M. Bouirig1 and J. Jacquet1,2 1 Supélec, 2 rue Edouard Belin, 57070 Metz, France 2 LMOPS, Laboratoire Matériaux Optique Photonique et Systèmes, EA 4423, Université Paul Verlaine Metz / Supélec, 2 rue Edouard Belin, 57070 Metz, France ABSTRACT In this work, thin films of Indium Tin Oxide (ITO)-based materials were tested as potential candidates for mid-IR transparent contacts on Te-doped GaSb and Si-doped InAs semiconductor wafers. Since these contacts are devoted to be inserted in Sb-based devices which are generally MBE-grown at ~450°C, low-temperature fabrication processes were particularly tested with a maximum temperature of annealing of 400°C. 50 nm-thick ITO films were deposited on glass, Te-doped GaSb and Si-doped InAs wafers and resistivity of 8.10-4 Ω.cm combined with ~80% of transmittance at 2 µm and ohmic contacts with a specific resistance of 3.10-4 Ω.cm2 were obtained. Then, in order to improve these properties in the mid-IR, other ITObased materials were tested: In doped ZnO (IZO) and Zn doped ITO (ITZO). The first results obtained on these materials show that the insertion of 10% of Zn in classical ITO structure results in a degradation of the electrical properties of the layer without a real impact on its optical transmittance near 2 µm. Concerning IZO, a large improvement of the transmittance in the whole visible-mid-IR wavelength range was observed for annealed samples at a temperature as low as 350°C. However, the electrical resistivity appears very sensible to the temperature of annealing. INTRODUCTION Materials from InGaAlAsSb system grown on InAs or GaSb substrates offer a great potential for applications in the mid-Infrared (mid-IR) wavelength range. A number of highly efficient photonic devices were developed in this spectral range [1,2]. Photovoltaics and thermophotovoltaics devices can also be developed since a non neglectible part of the solar emission covers the 2 µm to 2.5 µm range. In the case of these latter devices, the use of transparent conducting oxides (TCO) contacts would allow to cover the whole surface of the front side of the device and to limit the conversion losses obtained when using metallic grids [3]. Moreover, the development of efficient transparent contacts should be also interesting for GaSbbased mid-IR emitting Vertical Cavity Surface Emitting Lasers (VCSELs). Indeed, the performances of these devices are currently limited by several processing problems intrinsically linked to the nature of GaSb. In particular, it is for example impossible to realize oxide diaphragms or proton implantation, process usually carried out to improve the performances of VCSEL devices by limiting the volume of excited active layer and by ensuring a better carrier repartition in the structure. The use of a transparent contact could lead to a better homogenization of the carriers in the structure,
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