Open Access

Depth-Sensitive Raman Investigation of Metal-Oxide-Semiconductor Structures: Absorption as a Tool for Variation of Exciting Light Penetration Depth

Paweł Borowicz
Published: 12 January 2016
Journal of Spectroscopy , Volume 2016, pp 1-14; doi:10.1155/2016/1617063

Abstract: Presented work focuses the attention on two regions of MOS structure placed in the vicinity of the semiconductor/dielectric interface, in particular: on part of dielectric layer and thin layer of the substrate. In the presented work the application of absorption as a tool that can vary the absorption depth of excitation light into the semiconductor substrate is discussed. The changes of the absorption depth of visible light allows to obtain Raman signal from places in the substrate placed at different distances from the dielectric/semiconductor interface. The series of Raman spectra obtained from visible excitation in the case of varying absorption depth allowed to analyze the structure of the substrate as a function of distance from the interface. Deep ultraviolet Raman study regarding part of silicon dioxide layer placed directly at the interface is not discussed so far which makes the analysis of the structure of this part of dielectric layer possible. Comparison of reported in this work Raman data with structure of silicon/silicon dioxide interface obtained from other experimental techniques proves the applicability of proposed methodology.1. IntroductionThe progress in miniaturization of Metal-oxide-semiconductor- (MOS-) type electronic devices results in limitation of active area of semiconductor substrate to the thin layer placed in the vicinity of interface between semiconductor substrate and dielectric layer. An example of such device is High Electron Mobility Transistors (HEMTs). The thickness of active area in this device is limited to several dozen nanometers [1].Raman spectroscopy detects small shifts in frequencies of normal modes caused by small differences between parameters of crystal or molecular structure like bond lengths or bond angles. This accuracy makes from this experimental technique a very efficient tool for structural study. An example of this type of application of Raman spectroscopy is delivered by study of changes in semiconductor structure caused by implantation [2].Optical microscopy is the experimental technique which offers high spatial resolution. The transverse resolution is determined by diffraction limit of microscopic objective. The diameter of Airy spot can be calculated according to Rayleigh or Sparrow criteria [3]. In the case of the microscopic lenses with high numerical aperture (), = 0.55, the dimension of Airy spot is placed in the range between 500 nm and 700 nm [3]. Spatial resolution of Raman microscopy was applied in the investigation of spatial distribution of such parameters like mechanical stress in semiconductor substrate [4, 5] or channel temperate in HEMTs [6, 7]. The thermal effect observed in HEMTs is caused by self-heating present in the case of current flow between source and drain of the transistor.The axial dimension of the focus depends also on . The distribution of the intensity across the laser beam can be described by Gaussian function. The axial dimension of the laser beam focus in the case of confocal microscopes is not smaller than 1 μm [8]. The thickness of active area in today’s electronic devices is at the least order of magnitude smaller than axial dimension of laser beam focus determined from geometrical optics. Because of this it is necessary to introduce the procedure that can avoid the limitation coming from geometrical optics.The most important property that can help to overcome the limitation of axial diameter of laser beam focus is absorption. Absorption coefficient of each material is a function of wavelength of incident light. It means that one can change the penetration depth of the light into the material by choice of the irradiation wavelength. The dependence between wavelength and absorption of the material was applied for investigation of ohmic contacts with additional carbon layer formed at different temperatures [9]. Silicide film mixed with carbon atoms is transparent for visible light, because this visible irradiation of the ohmic contact through silicide layer causes the Raman scattering in carbon layer placed between silicide film and silicon carbide substrate. The deep-ultraviolet excitation applied in the same configuration cannot reach the above-mentioned carbon layer due to strong absorption of the silicide layer mixed with carbon structures. Therefore Raman scattering excited in deep-ultraviolet spectral range delivers information about two types of carbon species:(i)carbon layer which is built on the free surface of silicides due to carbon atom diffusion;(ii)carbon clusters placed inside of silicide layer [9].The other problem which was investigated by means of extraction of signal generated in thin layer from large background is related to properties of the interface between silicon carbide (SiC) and dielectric layer. The Near Interface Traps (NITs) in the MOS-type structures can decrease the mobility of charge carriers even by two orders of magnitude. Extensive study of the properties of SiC/SiO2 interface showed that carbon plays very important role in formation of the defects that can be candidates for NITs [10–13]. The most important Raman bands generated by species built from carbon atoms [14] are placed in the same range of Raman shift as two-phonon spectrum of different polytypes of SiC [15, 16]. Application of two different excitation wavelengths, in particular visible and ultraviolet, made the extraction of the scattering coming from the interface from background which is formed by two-phonon Raman scattering in SiC substrate possible. The extraction was possible due to significantly different penetration depths of exciting radiation from both used spectral ranges [17].The other area where the decreasing of two-phonon Raman scattering plays a key role is related to Raman study of properties of dielectric layer. The problem was discussed in the literature for the system composed of Si substrate and SiO2 layer. Standard configuration of Raman apparatus includes excitation in visible spectral range. In this case significant two-phonon signal generated in Si substrate is observed [18]. The intensity of this second-order Raman scattering is strong enough even to mask the signal form SiO2 layer [19]. Application of deep-ultraviolet excitation makes possible the observation of Raman scattering generated in SiO2 layer [20]. The increase of Si absorption due to change of the excitation wavelength from visible spectral range to deep-ultraviolet results in reduction of radiation penetration depth by about 30 times. In turn, the intensity of two-phonon Raman scattering coming from Si substrate becomes negligible and the signal from silicon oxide layer becomes detectable. This was shown for ~50 nm thick SiO2 layer placed on Si substrate by comparison with bulk material which was commercially available quartz glass Suprasil I [20]. The price to pay for this advantage is long irradiation time. The reason for this “price” is small efficiency of Raman effect in the case of SiO2 combined with small thickness of investigated material. The typical thickness of SiO2 layer of today’s electronic structures is about two orders of magnitude smaller than the axial dimension of the focus of laser beam.This work focuses the attention on the properties of thin layer of semiconductor substrate in the vicinity of semiconductor/dielectric interface. The Si/SiO2 system is used as an example. The change of the power density of exciting light on the sample results in change of effective absorption depth. Effective absorption depth is the thickness of the investigated material which is active from the point of view of measured Raman signal under certain power density. Since the definition of the effective absorption depth is a crucial point in the interpretation of experimental data it will be discussed in detail in the next chapter Experimental where also the methodology of data analysis is described. The systematic change of power density makes possible to record Raman signal from material with different thickness. As a result one can get depth profile of structural properties of investigated material.Another point that will be discussed here is the appearance of crystal-like structures of silicon dioxide that should be placed at the Si/SiO2 interface [21]. As was mentioned above application of deep-ultraviolet excitation in order to reduce two-phonon signal from Si substrate was discussed for amorphous part of SiO2 layer [20]. However, Raman signal observed for this type of excitation contains also traces of narrow lines. These traces will be compared here with Raman spectra reported for crystalline forms of silicon dioxide.The paper is organized according to the following outline. Section 2 presents method of sample preparation and their characterization, Raman apparatus, and methodologies of data analysis and measurements. The special attention was paid to two aspects:(i)the description of the mathematical model which links the power density of irradiation with thickness of the layer of material from which the Raman scattering is recorded;(ii)the discussion of two experimental parameters which are changed if the power density of exciting light is varied: half-angle of the maximum cone of light collected by microscope objective and the dimension of the laser-beam spot.Section 3 presents the measured data. The results of the investigation are discussed in Section 4.2. Experimental2.1. Samples Preparation and CharacterizationSilicon dioxide films were manufactured in Division of Silicon Microsystem and Nanostructure Technology (Institute of Electron Technology, Warsaw, Poland). Three-inch diameter p-type silicon wafers were used as substrates. Samples were characterized by means of spectroscopic ellipsometry, transmission electron microscopy, and transmission/reflection spectroscopy. Details of sample preparation and characterization were already presented in the literature [20].2.2. Raman ApparatusRaman spectra were measured with mic
Keywords: lenses / Raman spectroscopy / Si substrate / porous silicon / silicon dioxide / power density / visible light / quartz / Raman spectra

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