Infrared and photoelectron spectroscopy of semi-insulating silicon layers

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Abstract

Infrared (IR) spectroscopy, X-ray-induced photoelectron spectroscopy (XPS) and electron microprobe analysis (EMA) were used to study semi-insulating polycrystalline silicon layers (SIPOS) obtained by chemical vapor deposition (CVD) from SiH2 and N2O gases. A mean `bulk' oxygen concentration determined by EMA ranged from 16 to 50 at.%. The position of the Si–O–Si asymmetric stretching band indicates an oxygen-rich phase with a greater oxygen concentration than the mean bulk oxygen concentration determined by EMA technique. In the frame of a two-phase model of SIPOS composed of only pure silicon and homogeneous oxide phases, we calculated the mean bulk oxygen concentration using the density of the oxygen–silicon bonds derived from the normalized integrated absorption intensity of the Si–O–Si stretching vibration mode. We obtained a good agreement with EMA results. The two-phase model is strongly supported by the photoelectron spectroscopy results.

Introduction

Semi-insulating polycrystalline silicon (SIPOS) is a generic name for films of nonstoichiometric silicon oxide deposited by chemical vapor deposition (CVD) using the reaction between silane and oxygen-bearing species such as nitrous oxide [1]. The oxygen contents of the films can be varied from 0 at.% (polycrystalline or amorphous silicon) to 66.7 at.% (silicon dioxide). SIPOS is used in a variety of electronic devices [2]as protective, filtering or antireflection coating in optical applications and as dielectric material in certain microelectronic devices [3].

Despite the large number of studies on SIPOS, some basic questions in respect to structural properties still remain to be clarified. Since the electronic and optical properties of the films are often linked inextricably to the film structure, it is necessary to understand the bonding configuration of films with differing oxygen content. It is still an open question how the oxygen is distributed in SIPOS. The random mixture model (RMM) suggests the mixing of Si and SiO2 at the level of several tetrahedral units, while in the random bond model (RBM) the bonding networks of the films can be represented as a statistically controlled distribution of five Si(Si4−nOn) (n=0,1,2,3,4) bonding configurations [4].

Section snippets

Experimental

Polycrystalline silicon films (SIPOS) were deposited by low pressure chemical vapor deposition (CVD) in a conventional horizontal reactor. The reactive gases were SiH4 and N2O, the working pressure during deposition was adjusted to 57 Pa. The following deposition parameters were used: substrate temperature 648–654°C, the SiH4 flow rate was held at 50 standard cubic cm min−1 (sccm), the gas flow of N2O was varied between 8.0 and 17.5 sccm. SIPOS layers were deposited on Czochralski-grown

Bulk properties of SIPOS films studied by IRS

Fig. 1 presents a series of IR transmission spectra of films produced in the reactor at different levels of NO2 flux. Substrates contributions are subtracted. Films nos. 1 and 2 contain some hydrogen bonding group, the weak Si–H stretching band at 2265 cm−1. Films with higher mean bulk oxygen concentration (Nos. 3, 4, 5) do not show evidence of this band. An absorption band at about 3300 cm−1 is observed in the spectra of these samples.

From the spectra shown in Fig. 1, one can see that the

Discussion

The deviation of the values of oxygen concentration calculated using our model from those obtained by the electron microprobe analysis (EMA) can be explained by the number of errors and simplifications done. First, as shown by Maley [14], the error, which leads to an overestimation of the absorption coefficient for films with smaller content of oxygen, results from coherent multiple reflections in the films and may be as high as 30%. Second, in layers with a larger content of oxygen, oxygen

Conclusions

We have shown that SIPOS layers prepared by CVD method represent a heterogenous structure consisting of pure silicon islands embedded in an oxide phase. The binding energy analysis carried out on statistically significant set of spectra indicates that the islands are differentially charged during a photoemission process.

The most intense band observed in IR spectra has a peak frequency that shifts monotonically from 1030 to 1065 cm−1 with increasing oxygen content. It can be assigned to Si–O–Si

Acknowledgements

This work was supported by projects No. 202/95/1445 and 202/95/1393 from the Grant Agency of Czech Republic and by project No. 113-10/737 from the Grant Agency of Charles University.

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