Materials Chemistry and Physics 66 (2000) 172–176 Growth process analysis of a-Si 1-x N x :H films probed by X-ray reflectivity E. Bontempi a , L.E. Depero a , L. Sangaletti b,* , F. Giorgis c , C.F. Pirri c a Structural Chemistry Laboratory, Dipartimento di Ingegneria Meccanica, Istituto Nazionale per la Fisica della Materia, Università di Brescia, Via Branze 38, 25123 Brescia, Italy b Istituto Nazionale per la Fisica della Materia and Dipartimento di Matematica e Fisica, Università Cattolica del Sacro Cuore, Via Musei 41, 25121 Brescia, Italy c Istituto Nazionale per la Fisica della Materia and Dipartimento di Fisica, Politecnico di Torino, C.so Duca degli Abruzzi 24, 10129 Torino, Italy Abstract Amorphous silicon–nitrogen (a-Si 1-x N x :H) alloys deposited by ultra high vacuum plasma-enhanced chemical vapor deposition (PECVD) have been characterized by X-ray reflectivity (XRR) measurements in order to investigate their structural properties. From the analysis of XRR data, the thickness, density, interface and surface roughness of the films have been evaluated. © 2000 Elsevier Science S.A. All rights reserved. Keywords: X-ray reflectivity (XRR); Plasma-enhanced chemical vapor deposition (PECVD); Silicon nitride; Alloys; Light-emitting devices (LED) 1. Introduction Hydrogenated amorphous silicon nitride alloys (a-Si 1-x - N x :H) have been extensively used for a wide variety of mi- croelectronic and optoelectronic applications such as solar cells, thin film transistors, optical sensors and light-emitting devices (LEDs) [1]. In fact, these alloys have a tunable opti- cal gap from 1.9 up to 5 eV depending on nitrogen content. In addition, the alloying enhances the room temperature ra- diative efficiency by several orders of magnitude while shifts the emission band towards higher energies. These properties render silicon–nitrogen alloys very appealing for LEDs. In the last decade, many efforts have been devoted to the study of nanometric multilayers based on amorphous silicon and its alloys, like the a-SiN and a-SiC based systems, where the thickness control is a crucial step. Amorphous nanomet- ric multilayers are very promising for optoelectronic appli- cations since the radiative properties are strongly enhanced with respect to the corresponding bulk film having the com- position of deeper layers [2]. Due to the strong influence of interface quality, layer thickness and density on the efficiency of single- and multi-layer based devices, a careful analysis of these pa- rameters is mandatory. Among the available techniques for structural and morphological characterization, X-ray reflec- tivity (XRR) is finding large applications in the analysis of thin layers. In particular, XRR can provide non-destructive * Corresponding author. E-mail address: sangalet@dmf.bs.unicatt.it (L. Sangaletti). information about density, thickness, and surface and inter- face roughness of single- and multi-layer samples. Semi- conductor materials and optoelectronic devices constitute an area of application for XRR. Studies in this field in- clude SiO 2 layers for MOSFET devices [3,4], SiGe/Si [5–8], (AlGa)As/GaAs [9] or a-Si:H/a-Si 3 N 4 :H [10] super- lattices. In a-Si:H/a-Si 3 N 4 :H devices, the control over the single-layer thickness on atomic scale and the interface quality are recognized as crucial for efficient light emission. Recently, the XRR technique has been applied to the study of the structure of silicon nitride thin films grown by r.f. magnetron sputtering [11]. Very much like the electromagnetic radiation in the visi- ble, X-rays can also be specularly reflected from plane sur- faces. This is related to the fact that at short wavelengths characteristic of X-rays the diffraction index of the material is slightly less than one, which makes it possible to have total reflection of X-rays at the air/material interface. Thorough descriptions of XRR can be found in review papers (see, e.g. [12] and references therein) and in a recent book by Holy et al. [13]. At typical X-ray frequencies the diffraction index can be expressed as n = 1 - δ - iβ (1) where δ=ρ e e 2 λ 2 /2π mc 2 , β =μλ/4π , ρ e is the electronic density (Z electrons per atom), μ the linear absorption coef- ficient for energies far from the X-ray absorption threshold, and λ the X-ray wavelength. 0254-0584/00/$ – see front matter © 2000 Elsevier Science S.A. All rights reserved. PII:S0254-0584(00)00338-2