[20a-E303-1~5] 13.2 Exploratory Materials, Physical Properties, Devices

▲ [20a-E303-5] Optical and Electrical Characterizations of Ge Type II Clathrate Films Grown on Sapphire Substrate

〇(D)Rahul Kumar¹, T. Maeda², R. Tanahashi², Y. Hazama², F. Ohashi², H. S. Jha², T. Kume^1,2 (1.Div. of Environmental and Renewable Energy Systems, Graduate School of Engineering, Gifu University, 2.Department of Electrical, Electronic & Computer Engineering, Gifu University)

Keywords:Clathrate, Optical Properties, I-V Characteristics

Type II Ge clathrates are networks of slightly distorted Ge tetrahedral which traps alkali or alkaline earth metals (guest atoms) inside the cages. These guest atoms are considered as the template for the synthesis of such low-density structures [1]. The type II Ge clathrate structures are generally represented by the formula M_xGe₁₃₆ (x ≤ 24, M = alkali/alkaline earth metals). Metallic (guest) atoms present in the cages lead to the metallic nature to the clathrate. However, the nearly guest free clathrate structure (x~0) shows semiconducting nature with a similar or wider band gap. Recently, nearly guest free, type II Ge clathrate (Ge₁₃₆) has attracted significant research interest in solar photovoltaic application due to its unique properties such as tunable and wide direct band gap [2]. The main objective of this work is to clarify the optical and electrical properties of Ge₁₃₆ films fabricated on sapphire substrates. To prepare Na_xGe₁₃₆ sample, we used Ge films deposited by RF sputtering (at 400 ℃ of substrate temperature for 2 - 3 hrs) on sapphire as starting material. The Ge film reacted with Na vapor in Ar environment in a sealed stainless steel vessel at 500 ℃ for 3 hrs for the synthesis of precursor films. Subsequent annealing under high vacuum (£ 10^-3 Pa) at 300 ℃ for 12 hrs resulted in the formation of the Ge₁₃₆ clathrate films. The Ge clathrate films were characterized by X-Ray diffraction, UV-Vis-NIR & FTIR transmission, and I-V measurements (van der Pauw Method). Fig. 1 shows the I-V characteristics of the Na_xGe₁₃₆ film on sapphire substrate by using Ag electrodes suggesting the Ohmic characteristics of the sample. The estimated resistivity of the film is about 4.0×10^-2 W×cm. Free-carrier absorption and fundamental absorption has been observed below and above 0.3 eV, respectively in optical absorption spectra.