A detailed investigation of the growth mechanism of ultra-thin silicon nitride (Si3N4) films on Si(111) substrates, their structure, morphology and surface chemistry down to atomic scale have been investigated using various surface analytical techniques such as low energy electron diffraction (LEED), scanning tunneling microscopy (STM) and ESCA microscopy. A radio frequency N2 plasma source from Epi Uni-bulb has been used for the nitridation of atomically clean Si(111) surfaces. The substrate temperatures during the nitridation process were ranging from 600–1050°C and the plasma exposure times were varied from 5 s for initial nucleation up to 45 min for saturation thickness. The initial stage of N nucleation on Si(111), how the structure and morphology of the nitride films depend on thickness and temperature, surface atomic reconstructions and the nitride film chemical composition are discussed here. All findings are explained in terms of thermally activated inter-diffusion of Si and N atoms as well as the surface adatom diffusion/mobility.
- silicon nitride thin film
- growth mechanism
- surface reconstruction
In semiconductor technology, one of the most important parts is the formation of homogeneous insulating layers and are of high practical importance as it is an integral part all kinds of integrated circuits. Due to the recent miniaturization in nanotechnology, insulating layer thickness needs to be precisely controlled down to a few nanometer ranges. This nanometer scale insulating films not only reduce the devices size, but also provide a platform for novel device fabrication such as resonant tunneling diodes  or memory devices for magnetic tunnel junction .
Since last few decades, SiO2 on silicon (Si) is found to be the most useful insulating material in VLSI device technology because of its high quality and superior homogeneity. However, continuous miniaturization of device size now demands a thickness of this insulating layer down to a few atomic layers. Here, the homogeneity of the insulating film in terms of morphology and chemical purity becomes more pronounced. Very small fluctuations in thickness of the oxide layer may lead to the break-down due to the enhanced electron tunneling as this insulating barriers is extremely thin. Hence, demand for new materials with a higher dielectric constant is of high priority which can replace the SiO2 layer to overcome this issue. In this regard, crystalline silicon nitride (Si3N4) films received considerable attention to replace the existing SiO2 gate dielectric materials, as it is compatible with existing Si processing technologies as well as larger dielectric constant and diffusion resistive materials properties [3, 4]. Plasma assisted amorphous silicon nitride layer has recently been used as high performance gate dielectric . High thermal stability and refractive index of Si3N4 make it capable for high temperature structural ceramics and anti-reflective coating materials, respectively. Finally, crystalline Si3N4 on Si can also serve as a substrate for highly demanding GaN growth to integrate the opto-electronic devices to the well-established silicon based device technology [6, 7]. Therefore, better understanding of the initial Si3N4 films growth on Si, their structural and morphological evolution as well as chemical properties in a very local (atomic) scale are of high technological importance.
1.1 Structure and properties
Silicon nitride is a structural ceramics, which exhibits high mechanical strength at room as well as elevated temperature. It can also be useful because of its high fracture toughness. It shows a very high thermal stability, up to 1600°C in air and also has a much larger dielectric constant (ε = 7.5) as compared to the conventional SiO2 (ε = 3.8). It is a semiconducting material with an energy band gap of about 4.7 eV, which is almost half of SiO2 (9 eV). Usually the bulk Si3N4 has been produced by sintering method and structurally appeared as poly-crystalline ceramic. Crystalline silicon nitride appears with two different phases, such as α-phase and β-phase of Si3N4. Both the phases have space groups of P31c and P63 for α- and β-Si3N4, respectively and the structures appeared with a hexagonal symmetry. Although there were some contradiction about these two phases, finally it has been well accepted that the only stable phase is the β-Si3N4 phase whereas the α-Si3N4 phase is a meta-stable one and has a lattice constant along c-axis (0001) just double of the β-phase.
Various groups have already described the crystal structure of β-Si3N4 and for the bulk crystals. A lattice constant of ‘a’ = 7.595 Å in the hexagonal (X-Y) plane whereas ‘c’ = 2.902 Å in the vertical (Z) direction are obtained . A schematic model of the β-Si3N4 structure (top view), containing 14 atoms in the lateral plane (normal to c-axis) can be seen in Figure 1 where half of the atoms are placed below (at Z = −c/4 plane) and the other half above (at Z = c/4 plane) the X-Y plane. Local geometrical configuration indicates a combination of sp3 and sp2 hybridized orbital for Si and N atoms, respectively. This structure can also be considered as tetrahedrons of Si3N4 complex network, connected through their corners. Further structural information of β-phase of silicon nitride and how it is related to the α-phase has also be reported elsewhere [8, 9].
1.2 Different approaches
Formation of crystalline silicon nitride films has been performed using various growth methods. Chemical vapor deposition (CVD) growth process has been used to grow silicon nitride where a mixture of silicon-hydrogen (SiH4, Si2H6, Si3H8) and nitro-hydro compounds (NH3, N2H4, HN3) are used. In general, resulting films are usually appeared in an amorphous phase, non-stoichiometric and significantly contaminated . However, without supplying any Si -H2 source compound, it also possible to obtain nitride layers by direct nitridation of a Si substrate at elevated temperature. Si(111) surface exhibits a threefold rotational symmetry. Moreover, its 2 × 2 unit cell is having a lattice mismatch of only about 1.1% with the ‘a’ axis of hexagonal Si3N4. These two properties make it a very compatible substrate for the growth of Si3N4 films in the (0001) lattice direction. It can be done by exposing the atomically clean Si substrate to various N2 compound reactive gases such as NH3 [11, 12, 13, 14, 15, 16, 17], NO [18, 19, 20, 21] or other gaseous at high nitridation temperatures or by post exposure thermal annealing. In addition, ion bombardment methods [22, 23, 24, 25] or sputter deposition technique have also been employed. However, use the pure nitrogen gas would be the simplest and easiest way for the nitridation of the Si surface. But a very high growth pressure is required due to the inertness of molecular nitrogen. This may lead to a huge out gassing and finally lead to a severe contamination within the nitride film. If we can provide an atomic nitrogen source, this problem can be solved. Therefore, nitrogen plasma sources can be used for the exposure of active nitrogen flux on Si surface. The atomic nitrogen exposure on Si(111) and Si(001) surfaces at relatively lower nitridation temperatures leads to the formation of amorphous nitride layer and appears with a highly disorder interface of nitride/Si. However, a crystalline interface and well-ordered films of hexagonal β-Si3N4 films have only been observed for nitridation temperature only above 700°C [13, 14, 15, 16, 17, 22, 23, 24, 25, 26, 27]. Epitaxial β-Si3N4 formation on Si(111) by thermal annealing of N-irradiated Si(111) surface has been reported by Yamabe et al. . Silicon nitride growth kinetics and surface thermodynamics at elevated temperature under ammonia exposure is recently been reported . N2-plasma assisted surface nitridation of Si(111) followed by vacuum annealing at high temperature (900°C) results in a better quality Si3N4 film .
Various analytical characterization methods have been employed to investigate the growth mechanism and structural properties of thermally grown nitride layers. In general, surface probing instruments collect the information on lateral averaging over the surface. As diffraction method, low energy electron diffraction (LEED), reflected high energy electron diffraction (RHEED) [28, 29] are used for surface whereas low angle X-ray diffraction (XRD) is used for interface studies. Similarly, to investigate the chemical properties as well as bonding configuration, Auger electron spectroscopy (AES), photoelectron spectroscopy with X-ray (XPS) and ultraviolet light (UPS), electron energy loss spectroscopy (EELS), thermal desorption spectroscopy(TDS) etc. have been used. However, for atomic scale local information such as growth kinetics and surface reconstructions, direct microscopic imaging using low energy electron microscopy (LEEM), atomic force microscopy and scanning tunneling microscopy have been utilized. X-ray reflectivity (XRR) has also been carried out in some cases, to find the information about nitride layer thickness and subsurface interface.
2. Experimental details
Highly oriented p-type Si(111) wafers (boron doped, 0.02° miscut angle) were used as substrates and a clean Si(111) surface with well reconstructed 7 × 7 structure was prepared prior to the nitridation process. This preparation was routinely checked by both LEED and STM, and a clear 7 × 7 reconstruction was reproducibly observed. The sample temperature was measured using an infrared pyrometer with an uncertainty of 20°C. Si substrate nitridation process was performed by exposing the 7 × 7 reconstructed clean Si(111) surface to atomic nitrogen flux at different substrate temperatures. A commercial radio frequency (RF) plasma source from Epi Uni-bulb was used as atomic nitrogen sources. Plasma chamber pressure was maintained at about 10−5 mbar during plasma discharge. The pressure was controlled by a leak valve connected to the inlet line of N2 gas. The plasma source was operated at with a RF power of 450 W. The exposure time of the active nitrogen flux on Si substrate was varied such a way that the coverage of the nitride films starts from sub-monolayer and ends up to the saturation thickness. Crystalline quality and surface reconstructions of the nitride layer were characterized by LEED. The surface morphology, growth kinetics, as well as nucleation process and the real space atomic structure were studied using an in-situ STM. Chemical composition and stoichiometry of the film are studied using ESCA microscopy and X-ray photoelectron spectroscopy, which finally provides the information about film thickness and its homogeneity.
3.1 LEED results
An average information about the Si3N4 surface reconstruction grown on Si(111) and its crystalline quality after the nitridation process at different substrate temperatures ranging from 600–1100°C, have been investigated using low energy electron diffraction (LEED) method. Prior to any nitride formation, clean Si(111) surface appears with a sharp 7 × 7 LEED patterns. LEED patterns appear with a very faint “8 × 8” reconstruction for nitridation below 600°C. This finding indicates a pour crystallinity of the nitride films and mostly amorphous silicon nitrides later is formed. With increasing nitridation temperature, crystalline quality of the nitride films drastically improves and appears with a sharp “8 × 8” LEED patterns. After nitridation for about 15 min under the RF-plasma source at different temperatures, the evolution of the “8 × 8” structures has been shown in Figure 2. The LEED patterns of clean Si(111)-7 × 7 have been shown in Figure 2(a). The yellow and cyan circles represent the (0,0) and (1,0) diffraction spots of the reconstructed Si(111) surface. After nitridation at 700°C, faint diffraction spots of Si3N4-“8 × 8” structure (1, 0) starts to appear. Increase the nitridation temperature results in a much brighter LEED spots along with an additional faint superstructures of “8/3 × 8/3” . In case of a further increase in nitridation temperature to 920°C (Figure 2(b)), the LEED patterns get significantly sharpen. The “8/3 × 8/3” superstructures (red circles) appear in a much brighter contrast and dominate in lower beam energy. From the positions of the diffraction spots of the Si3N4-“8 × 8” structure and comparing those with the initial Si(111) diffraction spots, a periodicity of about 2.79 Å has been observed for the “8 × 8” structures. However for the “8/3 × 8/3” superstructures, the periodicity appears to be 10.2 Å.
For surface nitridation above 950°C, another type of surface reconstruction of Si3N4/Si(111) appears in LEED pattern, which is known as the ‘quadruplet structure’ or 3/4 × 3/4 reconstruction. This pattern appears in coexistence with the existing “8 × 8” structure. With further increasing in nitridation temperature, the quadruplet structure starts to dominate over the “8 × 8” patterns. Clear LEED patterns of quadruplet structure have been observed only above 1000°C (Figure 2(c)). This is appeared in bright spots (blue ellipse) with the crystalline domains angular rotations of ±5° and ± 10°, with respect to the underlying Si lattice. The (1, 0) spots of the Si3N4 “8 × 8” structure has also been observed within the purple circles. By comparing the quadruplet structure LEED spots with the (1, 0) diffraction spot of Si(111), a surface lattice periodicity of about 2.88 Å, has been observed for the quadruplet structures. It has been observed that the quadruplet structures usually appear with a contaminated surface, sometimes even at lower nitridation temperatures [13, 31].
3.2 STM results
Exposure of atomic nitrogen on the Si(111) surface at elevated substrate temperatures results in silicon nitride formation. A clear understanding of the silicon nitride growth mechanism and its impact on the structural properties of the as grown film is of high fundamental interest. STM studies of various silicon nitride films after nitridation at various temperatures, as well as Si3N4 film thicknesses starting from sub-monolayer coverage up to the saturation thickness can be seen here. The initial nucleation stage of silicon nitride films, their various growth steps along with the evolution of surface morphology and finally surface atomic structures of are discussed with atomic precision.
The closer view STM images of free standing nitride island and nitridation at the initial domain boundary of Si(111)-7 × 7 surface are shown in Figure 6. The Si3N4 structures are now appeared with an ordered atomic arrangement of a honeycomb-like “8 × 8” (Figure 6). Within islands/pits, Si(111)-7 × 7 structures are not observed any more. A clearer appearance of the “8 × 8” structure also suggests a drastic improvement in nitride films crystalline quality. Apart from the structural improvements, the shape and size of nitride nucleation centers have significantly been altered. The “8 × 8” nucleation pits within the terrace area remain still triangular, however, the free standing islands become in truncated shape and start to become a hexagonal shape as shown in Figure 6(a) . Direct nucleation of nitride layer (without forming pit/patch) has also started after second step of annealing at 900°C, at the domain boundary regions of initial the 7 × 7 terrace, as can be seen as bright dots in Figure 6(b). This way, nitride layer connects between two different nucleation pits/islands and further proceeds to form a continuous layer on Si surface. The structural quality of the “8 × 8”- Si3N4 layer has also been improved with the nitridation/annealing temperature, which is also in agreement with our earlier LEED observations.
Empty state STM images of Si3N4 surface grown at 850°C are shown in Figure 8(a) and (b). Honeycomb-like “8 × 8” surface reconstruction of Si3N4 surface is formed after 30 s of RF-plasma nitridation. The structure appears as a defective and in quasi-periodic network of local disordering (Figure 8(a)). A closer view of this honeycomb-like structures nicely resolved with atomic resolution is depicted in Figure 8(b). However, the short range lattice disorders can also be observed here along with the loop like structures. These disordered state can be attributed to the interfacial states of Si3N4/Si(111). An autocorrelation of this surface shows a periodicity of about 30.6 Å, indicating a “8 × 8”- Si3N4 structure .
3.3 ESCA results
Apart from STM imaging, ESCA microscopy technique is also employed to investigate the of surface chemical properties of the thin silicon nitride films grown on clean Si(111) substrates. Contrast observed in ESCA microscopy image usually occurs from two factors: (a) chemical inhomogeneity, i.e., any kind of compositional fluctuation and (b) film thickness inhomogeneity, i.e., surface roughening leading towards nitride layer thickness fluctuation. To perform a comparative study, two types of nitride samples are used here. In one hand, a flat and smooth nitride film grown at a relatively low nitridation temperature is used. In other hand, nitride film with a rough surface morphology grown by nitridation at higher temperature is tested.
In case of smooth nitride films, a very week modulation in ESCA image contrast has been observed for both scans using Si-2p bulk and nitride binding energies (images not shown here). ESCA microscopy image using N-1 s components also appears in a very similar manner. Both findings indicate towards the surface chemical homogeneity of the nitride film. To further confirm the chemical uniformity of this surface, high resolution XPS study has also been tested using of Si-2p photoemission line. A chemical shift of 2.9 eV has been found within the Si-2p spectra. This information can suggest the nitride film stoichiometry and in good agreement with the reported literature for Si3N4 compound formation [33, 34]. The thickness of the nitride film can also be estimated using this known Si3N4 stoichiometry. Total integral intensities of both nitride and bulk Si-2p components are used to calculate the film thickness. A detailed analysis of the nitride film thickness calculation has already been reported elsewhere [35, 36].
However, for nitride films of rough surface morphology, grown at higher nitridation temperatures appear with a strong contrast in ESCA microscopy images as shown in Figure 9. Photoelectron signals from: (a) N-1 s line and (b) Si-2p bulk line are used for the spectro-micrographs here. An inverted contrast in ESCA microscopy images is observed here appear with a strong contrast (Figure 9(a) and (b)). This finding can be explained in terms of nitride film thickness fluctuation. The existence of any bare Si(111) surface can be excluded from our earlier STM observations, where a continuous nitride layer with a thickness modulation was observed for this type of nitridation process.
In case of a smooth and uniform nitride films, the contrast in ESCA microscopy image for N-1 s line can be attributed to silicon nitride film stoichiometry, i.e., chemical inhomogeneity. In that case, Si-2p bulk component should be homogeneous and the ESCA microscopy is expected to be without any significant imaging contrast. In contrast, however, a strong contrast is observed also for the Si-2p bulk line. It is a clear indication of thickness fluctuation rather any chemical inhomogeneity within the nitride film. Figure 9(c) and (d) show a further investigation related to the film thickness variation of nitride films. Figure 9(c) shows a closer view of ESCA micrograph using the N-1 s line. Individual XPS spectra of the Si-2p binding energy measured at two different locations (marked bright and the dark areas within the ESCA micrographs) are recorded and the correlation between these two spectra is compared (Figure 9(c)). Both spectra clearly indicate the contrast within the ESCA micrograph is mostly due to the lateral fluctuation in the nitride film thickness rather not due to any chemical inhomogeneity.
From our STM studies of silicon nitride formation it is quite clearly that the initial Si3N4 nucleation always starts at the surface steps of Si(111) substrate in two ways: (a) in one hand direct formation of nucleation patches at the step edges of initial Si(111) surface or (b) on other hand formation of additional steps by creating triangular nucleation etch pits within the 7 × 7-Si(111) terrace areas. Initial nucleation at the surface steps can be correlated to the defect induced nucleation of N atom. As more dangling bonds are available at the step edges, they act as chemically more active with respect to the terrace area of 7 × 7-Si(111) domains. As a result, it can facilitate the initial nucleation of nitride layer. In addition, surface steps give more degrees of freedom for the nitride layer growth leading towards the relaxation of nitride lattice strain induced from the mismatch between the deposited nitride material and the Si substrate lattice.
Larger size of the nitride nucleation centers with a lower number density for higher nitridation temperature are explained in terms of enhanced thermal diffusion of the N and Si adatoms. Thermal diffusion starts to dominate above 750°C, which results in a heterogeneous nucleation of pits/patches at substrate defects, i.e. at Si(111)-7 × 7 domain boundaries and surface steps. Si3N4 nucleation above 850°C occurs exclusively at the surface steps, which can concluded as follows. There are no such domain boundaries of 7 × 7 structure due to surface phase transition at about 830°C from the (7 × 7) → (1 × 1) . Moreover, at higher nitridation temperature the surface diffusion of Si and N adatoms gets larger. The triangular shape of the etch pits/patches may be influenced by the three-fold crystal symmetry of the underlying Si(111)-7 × 7 substrate.
Morphological changes during post-annealing as well as improvement in structural quality can also be attributed to the temperature induced enhanced surface diffusion of Si and N adatoms. In one hand, high temperature promotes a better diffusion of surface Si atoms. On other hand, crystalline Si3N4 formation under atomic N exposure demands a proper supply of adatom species. Hence, an improvement in crystalline quality is expected for the higher nitridation temperature. LEED observations can also be explained in a very similar manner. The continuous supply of Si atoms can be linked to groove and hole formation on Si(111) surface by removal of the upper terrace. This will act as a Si source for further nitride formation. The enlargement of nitride nucleation patches or free standing islands can also be explained in terms of coalescence effect or by local nitride re-growth. The transition of free change standing nitride islands from a triangular to truncated /distorted-hexagonal shape can be explained by crystal rotational symmetry
The atomically resolved honey-comb like “8 × 8” surface reconstruction of the β-Si3N4 (0001) surface can only be observed for very thin nitride films (coverage below 2ML), which appears in a loop like way. In addition, no real long range symmetry has been found in STM imaging. This honeycomb-like appearance with many local disordering in STM images can be correlated in following ways. In one hand, the STM images can easily get influenced by the Si3N4-Si(111) interface states (very thin film). As there is a significant lattice mismatch between the substrate and epilayer, it may lead to a highly distorted bonding configuration. On other hand, STM imaging can also be modulated by the local electronic states of underlying Si(111) substrate (only valid for thin overlayer). In contrast, “8/3 × 8/3” super structures show a clear atomic reconstructions with long range symmetry. As it appears for a relatively thicker layer, background electronic influence such as sub-surface information can be ignored. However, a weak three fold surface modulation has been observed for thicker nitride films which is in good agreement with the proposed structural model of Bauer et al. 
STM findings of roughening of the silicon nitride surface at higher nitridation temperature are also complementary with our ESCA microscopy results. Nitride films appear as chemically homogeneous, however, contrast in the ESCA images are mostly due to the thickness fluctuation of the nitride film. The stoichiometry, i.e., chemical composition of the nitride films is found to be Si3N4 for both, smooth and rough surface morphology nitride films grown at lower and higher nitridation temperatures, respectively. The contrast in ESCA microscopy is only observed for higher nitridation temperatures as it results in formation of nitride films of inhomogeneous thickness.
In summary, high quality crystalline silicon nitride thin films have successfully been grown on clean Si(111) substrate by elevated temperature exposure of active nitrogen from a RF-plasma source. Initial nucleation process, nitridation temperature and atomic N exposure duration dependent films structure and morphology, surface atomic reconstructions and chemical properties of the β-Si3N4 /Si(111) have been investigated in details using different surface characterization techniques such as STM, LEED and ESCA microscopy. Initial Si3N4 nucleation strongly determine by the Si(111) surface defects. It always occurs at the step edges (upper terrace) and terraces by nucleation pit formation. Lower nitridation temperature generally results in nitride films of poor crystalline quality but appears with a smooth surface morphology. Whereas, a highly crystalline Si3N4 film can be achieved for nitridation at higher temperatures. Moreover, the surface morphology gets severely roughen by forming holes and grooves on Si(111) terrace. An atomically resolved honeycomb-like reconstruction of “8 × 8” surface periodicity is observed for very thin Si3N4 films. However, for thicker films grown at higher nitridation temperature shows an atomically resolved “8/3 × 8/3” superstructure in STM. Both the findings are complementary and in good agreement with LEED results. ESCA microscopy measurements confirm a Si3N4 stoichiometry of the nitride films. It also suggests a thickness fluctuation for a nitride growth at higher temperature. Finally, this type of crystalline Si3N4 films have a huge potential to successfully replace the existing SiO2 dielectric layer on Si(111) for device technology. Furthermore, it can also provide a platform for crystalline growth of group III nitrides on Si(111), which can further integrate the optoelectronic devices to the existing well established Si based technology .
The author is very much grateful to Prof Jens Falta and his coworkers of University of Bremen, Germany, for all kind of experimental supports as well as all short of valuable scientific knowledge and discussions.
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