Optical Properties of Amorphous Silicon-Yttrium Films

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Optical Properties of Amorphous Silicon-Yttrium Films Shmyryeva Alexandra N.1 and Semikina Tetyana V.2 1 Faculty of Electronics, National Technical University of Ukraine “KPI”, Prospect Peremogy 37, 03056, Kiev, Ukraine 2 Teikyo University of Science and Technology 2525 Yatsusawa, Uenohara-machi, Kitatsuru-gun Yamanashi, 409-0193, Japan e-mail: [email protected] ABSTRACT This paper presents and discusses the results of measuring IR reflection and ellipsometric parameters, optical microscopy and AFM the mixed phase of amorphous Si:Y films with microcrystalline inclusions. These films were obtained by electron-beam evaporation of siliconyttrium alloys with different Y concentration (5-30 %) at two substrate temperatures (370 and 620 0C). INTRODUCTION One of the main issues in amorphous silicon technology is the problem of reducing the trap states density, which is aimed to increase the charge carrier mobility, photosensitivity and stability of device performances [1, 2]. The known methods of hydrogen and halogens incorporating to amorphous silicon composition provide some progress, but do not resolve the problem of characteristic degradation of these materials. To decrease the degradation of electrical characteristics, it is possible, for example, to deposit α-Si:H in a cyclic process with consecutive annealing in hydrogen plasma [1, 2], or to combine the amorphous films together with nanocrystalline inclusions [3]. However, still unresolved is the issue of α-Si:H film deposits with stable electrical characteristics. Clear dependence of the latter on film properties and technological conditions have not been derived yet [1, 2, 4]. This work is devoted to synthesis and investigation of new amorphous silicon films with yttrium, namely α-Si:Y deposited in a vacuum without hydrogen. It is known that electron bonds break during the process of amorphous silicon preparation because of the high internal mechanical stresses. Hydrogen is used to fill the breaks in the bonds, but its influence on photoelectrical properties of α-Si film is not simple. The maximum photosensitivity can be reached with appropriate hydrogen concentrations. However, hydrogen atoms simply dissociate from amorphous silicon matrix. Therefore, in materials containing hydrogen one can observe considerable degradation of parameters when illuminated. This degradation decreases the photosensitivity and solar cell efficiency. In order to minimize the trap centre concentration, offered was an idea to prepare mixed-phase amorphous-microcrystalline films. Green [4] came to the same conclusion, that mixed phase amorphous microcrystalline materials seem to be resistant to the above degradation effect. EXPERIMENTAL DETAILS Our films were obtained using electron-beam evaporation of the target at high frequency in crossed electric and magnetic fields in a vacuum on silicon substrates. The specially prepared silicon alloys with yttrium were used as a target. The yttrium concentration in these alloys varied

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from 5 to 30 % relative to the s