Infrared ellipsometry and near-infrared-to-vacuum-ultraviolet ellipsometry study of free-charge carrier properties in In
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Infrared ellipsometry and near-infrared-to-vacuum-ultraviolet ellipsometry study of freecharge carrier properties in In-polar p-type InN Stefan Schöche1, Tino Hofmann1, Nebiha Ben Sedrine2, Vanya Darakchieva2,3, Xinqiang Wang4, Akihiko Yoshikawa5, and Mathias Schubert1 1
Department of Electrical Engineering and Center for Nanohybrid Functional Materials, University of Nebraska-Lincoln, U.S.A. 2 Instituto Tecnológico e Nuclear, Sacavém, Portugal 3 Department of Physics, Chemistry and Biology, Linköping University, Sweden 4 State Key Lab of Artificial Microstructure and Mesoscopic Physics, Peking University, Beijing, China 5 Graduate School of Electrical and Electronics Engineering, Venture Business Laboratory, Chiba University, Chiba, Japan
ABSTRACT We apply infrared spectroscopic ellipsometry (IRSE) in combination with near-infrared to vacuum-ultraviolet ellipsometry to study the concentration and mobility of holes in a set of Mgdoped In-polar InN samples of different Mg-concentrations. P-type behavior is found in the IRSE spectra for Mg-concentrations between 1x1018 cm-3 and 3x1019 cm-3. The free-charge carrier parameters are determined using a parameterized model that accounts for phonon-plasmon coupling. From the NIR-VUV data information about layer thicknesses, surface roughness, and structural InN layer properties are extracted and related to the IRSE results.
INTRODUCTION Recently, it was demonstrated by different groups that p-type conduction in InN can be achieved by Mg-doping [1-4]. Indications for successful p-type conductivity were only found for dopant concentrations of 1x1018 cm-3 + i < ε 2 >= sin 2 Φ ⎢1 + ⎜⎜ ⎟⎟ tan 2 Φ ⎥. ⎢⎣ ⎝ 1 + ρ ⎠ ⎥⎦
In the case of a perfectly smooth, uncovered substrate in air, the pseudo-DF and intrinsic material DF are identical. For all other cases appropriate layer models are necessary for data analysis. The ellipsometry data was analyzed by using a layer-stack model including the DF for the sapphire substrate and the GaN template layer [11]. The NIR-VUV DF of sapphire and GaN are well established and were taken from databases [12]. For c-plane InN, the influence of the extraordinary refractive index on the measurement data is small. Therefore the InN layer was assumed to be isotropic. In order to determine the thickness of the InN layer and surface roughness values, mathematical inversion (point-by-point fit) of the NIR-VUV data was applied to match the DF of InN as close as possible [10]. A single InN layer was assumed and surface roughness was taken into account by adding a surface layer which was modeled using the Bruggeman effective medium theory by weighing equally the DF of InN and air [10]. Multiple angles of incidence were used to minimize parameter correlation. The thickness values of each layer as obtained from the NIR-VUV measurements were used as input parameters for the IRSE analysis. For the IR spectral range, an InN buffer layer was included in the model to allow for different carrier properties in buffer layer and bulk InN. Uniaxial DFs with optical axis p
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