Proton NMR and Magnetic Susceptibility in a-Si:H
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Proton NMR and Magnetic Susceptibility in a-Si:H Jonathan Baugh, Daxing Han, Alfred Kleinhammes, and Yue Wu Department of Physics and Astronomy, University of North Carolina, Chapel Hill, NC 27599-3255, [email protected] ABSTRACT Proton nuclear magnetic resonance (NMR) is an important tool for characterizing the structure of a-Si:H over a wide range of length scales including short- and medium-range order as well as nanostructures. Some of the structural information obtained by NMR is based directly on proton NMR characteristics such as the lineshape and dipolar interactions that probe distances between hydrogen atoms and hydrogen environment. Based on such information it is shown that hydrogenated multi-vacancy model for Si-H clusters is fully consistent with both experimental and theoretical observations. It is demonstrated that changes of short- and medium-range order can be detected by measuring the magnetic susceptibility χ. Here χ is not directly related to NMR characteristics and NMR is used merely as a sensitive and accurate magnetometer. The result indicates that a-Si:H prepared by hot-wire CVD (HWCVD) and plasma-enhanced CVD (PECVD) with high H-dilution have higher structural order compared to conventional a-Si:H. By carrying out NMR measurements on single a-Si:H thin film new NMR features were observed such as the orientation dependence of the proton NMR spectrum with respect to the magnetic field. Based on such orientation dependence, strong evidence of aligned nano-channels was obtained in some device quality a-Si:H films. INTRODUCTION Recently, HWCVD and high H-dilution PECVD a-Si:H have received growing interest because of the reduced metastability effect observed in such films [1, 2]. There are a number of experimental evidences indicating that the structural order is enhanced in such films compared to conventional a-Si:H [3, 4, 5, 6, 7]. In the past, NMR has played an important role in the understanding of hydrogen microstructures in a-Si:H [6, 8, 9, 10, 11]. A systematic NMR study of HWCVD and high H-dilution PECVD a-Si:H films could help reveal the unique structural properties of such films. Numerous experiments over the past 25 years have demonstrated the essential role of hydrogen in determining the properties of a-Si:H [12]. It is known that the incorporated hydrogen atoms are mainly in the form of Si-H bonds in device-quality a-Si:H [12]. The general understanding is that Si-H bonds terminate dangling bonds and release strains in an otherwise over-coordinated amorphous silicon network. However, the structure of a-Si:H is more complicated than a uniform random network. One of the first indications of structural inhomogeneity comes from 1H NMR. Instead of a single featureless proton peak, the 1H NMR spectrum of a-Si:H was found to have two distinct components, a narrow peak with a typical full width at half maximum (FWHM) of a few kHz and a broad peak with a typical FWHM of about 25 kHz [9, 10, 11]. The narrow peak was attributed to isolated Si-H whereas the broad peak was attributed to Si-H
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