Low Resistivity Boron Doped Amorphous Silicon-Germanium Alloy Films Obtained with a Low Frequency Plasma
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Low Resistivity Boron Doped Amorphous Silicon-Germanium Alloy Films Obtained with a Low Frequency Plasma A. Heredia-J1, A. Torres-J2, F.J. De la Hidalga-W2, A. Jaramillo-N1, J. Sanchez-M3 C. Zúñiga-I2. M. Basurto P3. and A. Pérez1. 1
Optics and 2Electronics Departments, Instituto Nacional de Astrofísica, Óptica y Electrónica, INAOE Luis E. Erro # 1, CP 72840, Tonantzintla, Puebla, México 3 Uversidad Autónoma del Estado de Morelos, UAEM Cuernavaca, Morelos, México [email protected] Abstract- The structural and electrical properties of boron doped amorphous silicon-germanium alloy films, obtained using a low frequency plasma enhanced chemical vapor deposition (LF PECVD), are presented in this contribution. These thin films were deposited on a substrate heated at 270°C, and by decomposing a mixture of silane, germane, and diborane gases. The chemical bond structure was studied by Infrared Spectroscopy. Our results show that, for a constant diborane flow, the increase of germane flow enhances the incorporation of boron into the film; the peak at 2540 cm-1 becomes larger as the Ge content increases. Transport of carriers was studied by measuring current-voltage curves as a function of temperature. The conductivity increased from 10-6 to 10 (Ω-cm)-1, while the refraction index increased from 3.312 to 4.4458, for an increasing Ge content; this makes the films suitable for optical waveguide applications. On the other hand, the activation energy varied from 0.668 to 0.220 eV when the sample was doped with boron. The AFM images showed that the surface roughness was improved for an alloy with 50% of Ge. Introduction Hydrogenated amorphous silicon (a-Si:H) and silicon-germanium alloys (a-Si1-xGex) are important semiconductor films for several microelectronics devices, such as photodetectors, bolometers, and thin film transistors (TFTs). In particular, there is considerable interest in hydrogenated amorphous silicon-germanium (a-Si1-xGx:H) thin films to fabricate in bolometers and integrated optical devices [1]. The preparation of amorphous films by the decomposition of silane using plasma attracted great interest because the first amorphous semiconductor, with a low density of intrinsic defects, was produced using such systems; plasma decomposition also allowed for obtaining alloys and doped films. Plasma deposition is currently used for growing a-Si, a-Ge, SiC, SiGe, and some other amorphous films. Radio frequency (RF) discharge is a conventional method for depositing Si-Ge films, the advantages of using low frequency are such as: simple technical realization, a possibility to achieve high uniformity and deposition rate (Vd). In LF discharge, the surface of the growing film can be bombarded by ions with higher energies than in RF discharge because of thicker near electrode sheath [2]. In several works the mixture SiH4+GeH4 has been used in depositions of Si-Ge films by low frequency plasma enhanced chemical vapor deposition (PECVD) [3]. The different investigations have demonstrated that the optoelectronic prope
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