X-ray reflectivity spectra of ultrathin films and nanometric multilayers: Experiment and simulation
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L. Sangaletti Istituto Nazionale per la Fisica della Materia and Dipartimento di Matematica e Fisica, Universita` Cattolica del Sacro Cuore, Via dei Musei 41, 25123 Brescia, Italy
F. Giorgis and C.F. Pirri Istituto Nazionale per la Fisica della Materia and Dipartimento di Fisica, Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129 Torino, Italy (Received 8 March 2001; accepted 15 June 2001)
Amorphous silicon–nitrogen (a–Si1–xNx:H) alloys, thin films, and multilayers deposited by ultrahigh-vacuum plasma-enhanced chemical vapor deposition were studied and modeled by x-ray reflectivity (XRR) measurements. The analysis of XRR data obtained from the single-layer samples allowed us to calculate the density, thickness, and interface roughness of each layer. To check the deposition parameters, the deviation (tnom − texp)/(tnom) of the measured thickness texp from the nominal thickness tnom was evaluated. Based on these results, a simulation of a multilayer film, obtained by deposition alternating stoichiometric and substoichimetric layers was carried out. It is shown that the best fitting is obtained by introducing into the XRR calculation a thickness distribution with a standard deviation related to the deviation (tnom − texp)/(tnom) estimated for the single layers.
I. INTRODUCTION
Advances in material growth techniques such as molecular-beam epitaxy (MBE), chemical vapor deposition (CVD), and chemical beam epitaxy (CBE) allow the precise control and the reproducibility of each layer of the sample, a fundamental requirement for the production of the device. Due to the strong influence of interfaces, layer thicknesses, and densities on the efficiency of single and multilayer-based devices, a careful analysis of these parameters is mandatory. Among the available techniques for structural and morphological characterization, x-ray reflectivity (XRR) has found large applications in the study of thin layers. In particular, XRR provides nondestructive information about density, thickness, and surface and interface roughness of singlelayer and multilayer samples. Examples of application of XRR in the study of semiconductor materials and optoelectronic devices include SiO2 layers for metal-oxide semiconductor field-effect transistor devices,1,2 SiGe/ Si,3,4 (AlGa)As/GaAs,5 and a–Si:H/a-Si3N4:H6 superlattices. In a–Si:H/a-Si3N4:H devices, the control of the single-
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J. Mater. Res., Vol. 16, No. 9, Sep 2001 Downloaded: 15 Mar 2015
layer thickness on atomic scale and the interface quality are recognized to be crucial for the efficiency of the light emission. Recently, the XRR technique has been applied to the study of the structure of silicon nitride thin films grown by radio frequency (rf) magnetron sputtering.7 Many experimental and theoretical works have been done for the evaluation of electronic and structural properties of a–Si1–xNx:H material with a composition close to stoichiometry8–10 but only a few a
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