CC BY
34
YflK 537.311.33:621.382
BecTHHK Cn6ry. Cep. 4. 2012. Bun. 3
A. P. Baraban, V. E. Drozd, I. O. Nikiforova, V. A. Dmitriev, V. A. Prokof'ev, A. A. Gadzhala, O. P. Matveeva
ELECTRONIC STRUCTURE OF THIN Ta2O5 FILMS ON SILICON
In recent years, solid-state electronics and optoelectronics pay special attention to working out technologies for production and investigation of dielectric layers with high compared with SiO2 dielectric permittivity [1, 2]. These layers are layers of Ta2O5, and who, above all, can be used as an insulator in the storage capacitor of static and dynamic RAM, and the new generation "dry" capacitors, characterized by the maximum ratio of the capacity/dimensions, as well as in the outer layers of the dielectric block of memory elements in the silicon-based [1, 2], and in the memory static cell made in a single ALD cycle using dielectrics with different widths of the forbidden gaps [3].
The aim of this work was to obtain Ta2O5 layers on silicon (silicon dioxide) and studying the properties of the formed silicon—insulator structures.
In this paper for the synthesis of Ta2 O5 layers used well-tested previously, for example, the synthesis of Si—Al2O3 structures [4], atomic layer deposition method. The growth of Ta2O5 films was carried out either on a standard single-crystal silicon wafers KDB-40 (NA = 3 • 1014 cm-3), 100 mm in diameter, or on plates with a layer of thermal SiO2 (oxidized silicon wafers KDB-40 in the "dry" oxygen at 1'000 °C, the thickness of oxide 50 nm) using two reagents — tantalum chloride (TaCl5) and water. Since, tantalum chloride is a crystalline substance at room temperature, the main technological challenge for the successful implementation of the method of molecular layering was to design and build equipment that provides a well-controlled evaporation process TaCl5. As a result of the test experiment has been set temperature range (180-250 C temperature of the silicon substrate, the evaporator temperature is 45-90 C) at which a further synthesis of Si—Ta2O5 structures and Si—SiO2—Ta2O5 structures were carried out. To ensure a high growth rate the synthesis of the films was carried out on a rotating substrate.
To study the electrical properties of Ta2O5 films obtained on silicon substrates using the method of high-frequency capacitance-voltage characteristics (C-V characteristics). Join C-V characteristics was carried out in metal—insulator—semiconductor (MIS) and the electrolyte—insulator—semiconductor (EIS) systems. As the field electrodes we used aluminum and electrolyte — 1 N sodium sulfate solution. In addition, photoluminescence (PL) and excitation photoluminescence (EPL) spectra were recorded at the facility Fluorolog®-3, manufactured by HORIBA Jobin Yvon. The 450 W xenon lamp for photoluminescence excitation was used. Spectra were recorded in photon counting mode with a photomultiplier R928P. Scan speed was 150 nm/s. Accuracy — 0.5 nm. When measuring the spectra was set: excitation wavelength (the emission wavelength in the case of photoluminescence excitation spectra), spectral width of excitation monochromator (2 or 10 nm), spectral width of the monochromator luminescence recording (5 or 10 nm) and the scanning area. The spectra were corrected for spectral sensitivity of the apparatus and the intensity of the excitation lamp.
Si—Ta2O5 structures with insulator layer thickness 20-100 nm were synthesized. Energy diagram of the formed Si—Ta2O5 structures, as shown by fotoinjection measurements, characterized by low potential barrier for electrons 0.4 eV) at the silicon—insulator interface,
<s> A. P. Baraban, V. E. Drozd, I. O. Nikiforova, V. A. Dmitriev, V. A. Prokof'ev, A. A. Gadzhala, O. P. Matveeva, 2012
which is consistent with published data [1]. This made it difficult to correct measurements of C-V characteristics especially with the field metal electrode. Therefore, the electrical properties of Si—Ta2O5 structures were investigated with a field electrolytic electrode, providing a large potential barrier height for electrons at the outer edge of the insulator and substantially reduce the magnitude of through current. However, these measurements because of the high conductivity of the system were only estimated. For more accurate measurements of high-frequency C-V characteristics the Si—SiO2—Ta2O5 structure were used, in which SiO2 layer blocked the possibility of electron exchange between the silicon substrate and the synthesized layer of Ta2O5 (Fig. 1). As a result, based on measurements of high-frequency C-V characteristics in EIS system was obtained value of relative permittivity for Ta2O5 layers in the thickness range 20-100 nm, which was 13.5 ± 0.5. This value was obtained by measuring the thickness of the insulator layer with a scanning helium ion microscope Zeiss ORION on the image cross-sectional sample.
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2
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Fig. 1. High-frequency C-V characteristics for structures with Ta2O5 layers in EIS system: 1 — Si—Ta2O5 (100 nm); 2 — Si—SiO2 (55 nm); 3 — Si—SiO2 (55 nm)—Ta2O5 (100 nm)
Based on the analysis of the C-V characteristics for each type of field electrode values of the flat-band potential (Vfb) and effective charges embedded in a dielectric (Qfb) were determined. For electrolytic field electrode: Vfb = 0.39 V, Qfb = —4.1 • 1011 cm~2. For the aluminum field electrode: Vfb = —0.54 V, Qfb = —4.3 • 1011 cm~2. It is seen that the effective charge in both cases is practically the same as the difference between Vfb (0.93 V) for both field electrodes corresponds to the difference in work function of the field electrodes (0.95 V).
To investigate the nature of electronic processes in silicon—insulator structures, we used the method of field cycles [5]. The method was to obtain the dependence of the flat-band potential for investigated structure (determined by measuring the high-frequency C-V characteristics) of the average electric field generated in the insulator layer of the pre-polarization the structure in the electrolyte ("+" on silicon). Fig. 2 shows the dependence of the flat-
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Fig. 2. Dependence of the flat-band potential on the electric field in the SiO2 layer for different structures (except Si—Ta2O5 structures):
1 — Si—Ta2O5; 2 — Si—SiO2 (55 nm)—Ta2O5 (100 nm); 3 — Si—SiO2 (55 nm)—Ta2O5 (60 nm); 4 — Si—SiO2 (55 nm)
band potential on the electric field in the SiO2 layer for different structures. Found that for all structures prior field exposure resulted in the formation of an effective positive charge embedded in an insulator layers. For the structure of Si—SiO2 (55 nm), this behavior is typical, and for the used technology of forming the oxide layer due to the formation of positively charged defects in SiO2 by dissipation of hot electrons energy [5].
In the case of Si—Ta2O5 structures formation of the built positive charge most likely due to the capture of holes by the traps in the insulator layer. Depending on the flat-band potential of the electric field for the multilayer structures are similar to the curve for the Si—SiO2 structures, but a marked increase in the effective charge began at lower fields. Ability to capture of holes in a layer of tantalum pentoxide in this case was excluded because the hole current component blocked by the presence of silicon oxide. The observed change in the charge state of these structures is almost entirely determined by changes in the charge state of SiO2 layer in the flow-through the electron current and is due to the processes of energy dissipation of hot electrons.
The presence of tantalum pentoxide layer on silicon oxide reduces the "effective" potential barrier for electron injection from the electrolyte: instead of one barrier, the value of ~ 4.2 eV (characteristic boundary SiO2—electrolyte), the electrons cross the first potential barrier at the electrolyte—tantalum pentoxide boundary (1.33 eV) and then the barrier at the dielectrics boundary (2.87 eV). In this case, as was established by measuring, the current through the sample with two-layer dielectric takes more current than in the structures of Si—SiO2, with the same value of the average electric field in the SiO2. As a result, in the case of tantalum pentoxide structures in SiO2 is injected and, accordingly, is heated more electrons and the dissipation of excess energy which leads to more intense defect formation and appears to change the flat-band potential at lower values of the field in the SiO2 layer (see Fig. 2). At the same time with greater thickness of tantalum pentoxide observed large
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magnitude of through current and processes of change in the charge state of the silicon dioxide layer occurred in weak electric fields. This is due, apparently, with an additional heating of electrons in the layers of Ta2O5 (even as far away from the ideal dielectric processes of electron heating can occur [6]), which facilitates the process of electrons injection into the layer of silicon dioxide. A more extensive defect formation in the Si—SiO2—Ta2O5 structures at fixed electric field strength in a silicon dioxide layer is manifested in the development of an irreversible breakdown in a more weak electric fields (the position of the rightmost point on the curves of Fig. 2).
For information about the electronic structure formed Ta2O5 layers, we used the method of photoluminescence (PL). The emission spectra were obtained of Si—SiO2 (55 nm)—Ta2O5 (100 nm), Si—SiO2 (55 nm), Si—Ta2O5 (100 nm), Si—Ta2O5 (40 nm) samples at excitation energies of 3.35 eV (370 nm) and 4.13 eV (300 nm). There have also been obtained excitation functions of PL in 450 nm (2.75 eV) region. It was found that the shape of the spectral distribution of PL in the case of all structures with Ta2O5 layers practically coincide. In this case, we can see an increase in luminescence intensity with increasing thickness of the layer of tantalum pentoxide. Fig. 3 shows the photoluminescence and photoluminescence excitation spectra of the Si—SiO2 (55 nm)—Ta2O5 (100 nm) sample.
Shown in Fig. 3 PL spectra almost entirely due to radiative processes in the Ta2O5 layer, as the PL for SiO2 layers is characterized by another type of spectral distribution under the given conditions of excitation and, most importantly, completely different excitation functions of PL in 450 nm region. The obtained PL spectra and PL excitation spectra were approximate by Gaussian distributions. An example of such an approximation is shown in
E, eV
Fig. 3. PL spectra (1 ), (2) and excitation PL spectra (3) for Si—SiO2 (55 nm)—Ta2O5
(100 nm) structures
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Fig. 4- An example of an approximation the obtained PL spectra (1) by Gaussian distributions: 2.6 (2); 2.8 (3); 3-0 (4); 3-2 eV (5); 6 — difference between the obtained PL spectra
and approximation
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Fig. 4. The result was a set of bands common to all samples containing tantalum pentoxide layers: 2.6 ± 0.1; 2.8 ± 0.1; 3.0 ± 0.1; 3.3 ± 0.1; 4.4 ± 0.1 eV. The dispersion of all bands was 0.20 ± 0.05 eV.
The above information on the decomposition of the photoluminescence spectra and excitation photoluminescence spectra show a rather complicated electronic structure of luminescence centers in the layers of tantalum pentoxide. Judging from the spectral manifestation of the luminescence as luminescence centers are the most likely, the so-called oxygen-deficient defects, which can be attributed, and tantalum ions are located in the oxide matrix. Obtained only by the PL results do not permit an unambiguous conclusion about the nature of the observed luminescence, i. e. preferred intracenter PL or PL arising from the participation of the band structure of the oxide layer. However, under the totality of the results, we consider the second possibility more likely: the presence of defects (luminescence centers) in the oxide layer is accompanied by the formation of a number of energy levels in the band gap of tantalum pentoxide. As a result, we can offer the following diagram of the electronic structure of tantalum pentoxide layer formed on the surface of silica (silicon dioxide) by atomic layer deposition (Fig. 5). This scheme allows us to explain the spectral composition of the
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Fig. 5. The electronic structure of tantalum pentoxide layer formed on the surface of silica
PL with excitation in the band 300 nm and in the band 370 nm, as well as the type of excitation function of the luminescence of 450 nm. The presence of a maximum in the photoluminescence excitation function at ~ 4.4 eV in the case of tantalum pentoxide layer thickness of 100 nm, due either to the characteristics of the distribution of the density of states in the conduction band, or the impossibility of PL excitation over the entire thickness of the oxide layer due to intense absorption in the high energy part of spectra (self-absorption edge). To establish the nature of the defects, which can lead to the appearance of the PL, more research is needed.
Observation of photoluminescence indicated the presence in tantalum pentoxide band gap unfilled energy levels localized at 2.8-3.4 eV above the valence band, which due to their energy position led to the formation of hole conduction channel in the Si—Ta2O5-electrolyte system. It is the presence of a high hole conductivity in such a system and explains the problems with the measurement of high-frequency capacitance. In the presence of a multilayer system SiO2 layer blocks the flow of hole current and allows the use of high-frequency capacitance measurements to study the electrical characteristics of formed structures.
Thus, by the low-temperature atomic layer deposition method Ta2O5 layers of various thicknesses (20-100 nm), which have increased the value of relative permittivity, were synthesized. On the basis of high-frequency capacitance and photoluminescence measurements have identified the main electrical characteristics of the formed structures, and proposed a model of their electronic structure.
References
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Статья поступила в редакцию 3 апреля 2012 г.
