Quantification of Hydrogen and Determination of the Binding State in a-Si:H:D by Thermal Desorption Spectroscopy
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QUANTIFICATION OF HYDROGEN AND DETERMINATION OF THE BINDING STATE IN a-Si:H:D BY THERMAL DESORPTION SPECTROSCOPY NORIKUNI YABUMOTO, YASUJI MURAMATSU AND MASAHARU OSHIMA NTT Electrical Communications Laboratories, 3-9-11 Midori-cho, Musashino-shi, Tokyo 180 Japan ABSTRACT Thermal desorption spectroscopy (TDS) is used to quantify the amount of hydrogen in hydrogenated amorphous silicon(a-Si:H) as well as to determine the binding state. It is possible to measure the quantity and binding state of hydrogen in 100-8000A thick a-Si:H films. Hydrogen is incorporated in the 100A thick filn at a rate of about three times greater than that in the over 1000A thick film. Hydrogen in all films is bonded to silicon with three kinds of binding states at 2.8, 3.1 and 3.3eV. a-Si:H:D is the origin of prepared from SiH /D 2 plasma in order to clarify hydrogen in the films which are usually deposited from SiH4 and diluted H2 gas. TDS spectra of H 2, HD and D2 show that there is more hydrogen from SiH4 in the film in each of the binding states, than from the diluent H2 . INTRODUCTION Physical properties, such as photoconductivity and the optical gap, of hydrogenated amorphous silicon (a-Si:H) deposited from SiH4 /H 2 plasma are influenced by the number of dangling bonds. The quantity and binding state of hydrogen which terminates dangling bonds are strongly related to these properties. Secondary ion mass spectroscopy(SIMS), Rutherford back Scattering, and infrared absorption are usually used to measure hydrogen atoms in a-Si:H. The first two methods determine the method can quantity but not the binding state, while the last quantify the hydrogen atoms which combine with silicon(Si-H, SiH2 and Si-H 3 ) but not the quantity of isolated hydrogen. Thermal desorption spectroscopy(TDS) can determine the binding state from the peak temperature of the desorption spectrum[l]. If the desorption spectrum can be quantified, the amount of hydrogen in different binding states can be determined. The quantification method of gas effusion was used, in which hydrogen gas was effused from the a-Si:H into a chamber with a fixed volume, and then the pressure increase was measured[2]. In this method, hydrogen was quantified but the binding state was not determined, since the TDS spectrum was not obtained. However, a calibration curve was determined between the rate of H2 leaking through a mass flowmeter into the TDS apparatus being evacuated and the mass spectrum intensity. Using this calibration curve and the TDS spectrum, it was possible to measure the quantity and
Mat. Res. Soc. Symp. Proc. Vol. 75. 1987 Materials Research Society
714
binding state of hydrogen in a-Si:H films. The present paper describes the quantification method of the TDS spectrum and the reliability of quantified value. The dependence of hydrogen concentration on film thickness is investigated using this method. In addition, the origin of hydrogen in a-Si:H is clarified from TDS spectrum using a-Si:H:D prepared from a SiH4 /D 2 plasma. EXPERIMENTS Samples The samples were prepared
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