**5. Graphene-like models of the structure (8 × 8)**

The increase in the interplanar distance in silicon nitride, detected by the HRTEM method, is a consequence of the weaker (van der Waals) interaction between the atomic planes. The interaction between the silicon nitride layer and the Si substrate also turned out to be weaker than interaction provided by normal covalent bonds. As mentioned above, when the structure (8 × 8) is formed, the ammonia interaction occurs with the mobile silicon adatoms, but not with the dangling bonds of the silicon atoms on the surface, which provides an increased

**41**

**Figure 8.**

*Models of a graphene-like layer of silicon nitride. (a) g-Si3N4; (b) g-Si3N3.*

*Van der Waals and Graphene-Like Layers of Silicon Nitride and Aluminum Nitride*

interlayer distance between the silicon nitride layer and the silicon surface. Taking into account all the experimental data presented, it can be assumed that the structure (8 × 8) has a graphene-like nature. Moreover, the production of graphene-like AlN layers, described in our paper [50], was possible only on such a graphene-like layer of silicon nitride. If only the silicon nitride layer had dangling bonds (silicon or nitrogen), then the AlN layer would have formed in the bulk wurtzite structure (formation of graphene-like AlN is discussed below). Possible graphene-like models of the layer of silicon nitride g-Si3N4 and g-Si3N3 are shown in **Figure 8**. Similar model g-Si3N3 was considered earlier in the theoretical work of Guo [51]. The basis of both structures is the Si3N3 aromatic conjugated rings connected to each other by

structure) or via Si-Si bonds (**Figure 8b**). Atoms of silicon in these structures have

The proposed structures satisfy the available experimental diffraction data, STM/STS, and HRTEM. They reproduce the periodicity (8 × 8) and the characteristic features of the honeycomb structure observed in the STM, taking into account the weakening of interaction with the silicon surface (there are no dangling bonds in the layer) and explaining the metastability of the structure (8 × 8). Metastability is a consequence of the formation of weaker π-bonds than σ-bonds. In the stable structure of Si3N4 (amorphous or crystalline β-Si3N4), all bonds of silicon and

 hybridization of atomic orbitals, forming three σ-bonds in planar configuration. The fourth electron of the silicon participates in the π-bond with the nitrogen atom in the ring. Each nitrogen atom uses three valence electrons, and in the aromatic ring, nitrogen has sp. hybridization, and the third valence electron participates in

hybridization (as in the β-Si3N4 crystal

*DOI: http://dx.doi.org/10.5772/intechopen.81775*

either nitrogen atoms (**Figure 8a**) having sp2

sp2

the π-bond.

nitrogen are σ-bonds.

#### *Van der Waals and Graphene-Like Layers of Silicon Nitride and Aluminum Nitride DOI: http://dx.doi.org/10.5772/intechopen.81775*

interlayer distance between the silicon nitride layer and the silicon surface. Taking into account all the experimental data presented, it can be assumed that the structure (8 × 8) has a graphene-like nature. Moreover, the production of graphene-like AlN layers, described in our paper [50], was possible only on such a graphene-like layer of silicon nitride. If only the silicon nitride layer had dangling bonds (silicon or nitrogen), then the AlN layer would have formed in the bulk wurtzite structure (formation of graphene-like AlN is discussed below). Possible graphene-like models of the layer of silicon nitride g-Si3N4 and g-Si3N3 are shown in **Figure 8**. Similar model g-Si3N3 was considered earlier in the theoretical work of Guo [51]. The basis of both structures is the Si3N3 aromatic conjugated rings connected to each other by either nitrogen atoms (**Figure 8a**) having sp2 hybridization (as in the β-Si3N4 crystal structure) or via Si-Si bonds (**Figure 8b**). Atoms of silicon in these structures have sp2 hybridization of atomic orbitals, forming three σ-bonds in planar configuration. The fourth electron of the silicon participates in the π-bond with the nitrogen atom in the ring. Each nitrogen atom uses three valence electrons, and in the aromatic ring, nitrogen has sp. hybridization, and the third valence electron participates in the π-bond.

The proposed structures satisfy the available experimental diffraction data, STM/STS, and HRTEM. They reproduce the periodicity (8 × 8) and the characteristic features of the honeycomb structure observed in the STM, taking into account the weakening of interaction with the silicon surface (there are no dangling bonds in the layer) and explaining the metastability of the structure (8 × 8). Metastability is a consequence of the formation of weaker π-bonds than σ-bonds. In the stable structure of Si3N4 (amorphous or crystalline β-Si3N4), all bonds of silicon and nitrogen are σ-bonds.

**Figure 8.** *Models of a graphene-like layer of silicon nitride. (a) g-Si3N4; (b) g-Si3N3.*

*2D Materials*

states (8 × 8) with π-band.

**aluminum nitride**

Waals crystal.

in contrast from the layers of Si and AlN.

**5. Graphene-like models of the structure (8 × 8)**

*HRTEM image of layers of SiN and AlN on the Si (111) surface.*

The increase in the interplanar distance in silicon nitride, detected by the HRTEM method, is a consequence of the weaker (van der Waals) interaction between the atomic planes. The interaction between the silicon nitride layer and the Si substrate also turned out to be weaker than interaction provided by normal covalent bonds. As mentioned above, when the structure (8 × 8) is formed, the ammonia interaction occurs with the mobile silicon adatoms, but not with the dangling bonds of the silicon atoms on the surface, which provides an increased

an appreciable difference in the electronic structures (state densities) β-Si3N4 and (8 × 8) is seen. Moreover, in the works [48, 49], devoted to the calculation of electronic states (0001) β-Si3N4, HOMO states associated with dangling bonds did not found. As it will be shown further, it is better to associate this peak in the density of

**4. HRTEM study of van der Waals structure of silicon nitride and** 

The atomic arrangement of (8 × 8) was also investigated here by the HRTEM method. For these studies, samples with the following sequence of layers were grown on the (111) Si substrate: 2–3 monolayers of silicon nitride with a structure (8 × 8) and thin epitaxial layer of AlN. The interplanar spacing in the Si substrate in the silicon nitride and AlN layers was determined, see **Figure 7**. It turned out that the interplanar spacing between the layers of silicon nitride and also between the last silicon layer and the silicon nitride layer is about 3.3 Å, which is noticeably larger than the interplanar distances in silicon (3.13 Å) and the known thickness of the β-Si3N4 monolayer (2.9 Å). In addition, the layers of silicon nitride differ sharply

The interplanar distances in silicon nitride of 3.3 Å are larger than the interplanar distances Si 3.13 Å and are larger than the thickness of the monolayer β-Si3N4 (2.9 Å). The interplanar distances in the еpitaxial AlN layer are also larger than normal interplanar distances in bulk wurtzite AlN (2.49 Å). Therefore, this epitaxial structure (SiN)2(AlN)4 turned out to be a van der

**40**

**Figure 7.**

## **6. Graphene-like AlN layer formation on Si (111) surface by ammonia MBE**

In the present experiments, an AlN flat ultrathin layer was prepared by using the following two-stage procedure. At first, a clean (1 × 1) silicon surface was exposed under the ammonia flux (10 sccm) at the substrate temperatures of 1050°C, and in the second stage, the AlN layer is formed by the Al deposition when the ammonia flux was switched off and the background ammonia pressure of ~10<sup>−</sup><sup>7</sup> –10<sup>−</sup><sup>8</sup> Torr was achieved.

For the AlN formation, the ammonia flux was turned off at the moment when the best (8 × 8) RHEED pattern with sharp and bright eightfold fractional spots was reached. This moment corresponds to the maximum of the curve in **Figure 1b**. Next, the Al deposition onto the highly ordered (8 × 8) structure was performed. The Al flux was established on the value equivalent to the AlN growth rate of ~0.1 Ml/s. The appearance of the AlN diffraction spots and the transformation of the (8 × 8) structure to a new fourfold structure was observed. The RHEED pattern of AlN and (4 × 4) structure is shown in **Figure 9a**. An intensity profile measured along a horizontal line (A-B) crossing the streaks is shown in **Figure 9b**.

The observed fundamental (0-1) AlN streak position exactly coincides with the position of the fractional spot (0-5/4); see **Figure 9b**). Then, an AlN in-plane lattice constant was calculated from the relationship 4 × a111Si = 5 × aAlN, where a111Si = 3.85 Å. Hence, the calculated lattice constant is aAlN = 3.08 Å. This value

#### **Figure 9.**

*(a) RHEED pattern of the Si surface (4 × 4); (b) the intensity profile of (4 × 4) structure measured along the line (A-B) crossing the diffraction streaks, from [50].*

**43**

**Figure 10.**

*Van der Waals and Graphene-Like Layers of Silicon Nitride and Aluminum Nitride*

is substantially lower than lateral lattice constant of the bulk wurtzite AlN value being 3.125 Å, which has been measured by the X-ray diffraction method at a high

There is only one more experimental work [57] that dedicated to epitaxial growth of graphite-like hexagonal AlN nanosheets on single crystal Ag (111). It is interesting to note that a simple our calculation of the g-AlN lattice constant using the lateral constant of (111) Ag (2.89 Å) from the diffraction pattern presented by authors of the work gives the value in the range of 3.06–3.09 Å in contrast to the

We have carried out experimental precise measurements of the in-plane AlN lattice constant under the Al and NH3 fluxes supplied onto the surface either separately or simultaneously. The evolution of in-plane lattice constant during the g-AlN formation process is depicted in **Figure 10**. The increasing of the lattice constant from 3.08 to 3.09 Å under Al flux without ammonia flux (i.e., there is no growth of

Then, the Al flux was turned off (at the moment of 340 s) and the NH3 flux (10 sccm) was switched on. The lateral lattice constant of g-AlN was keeping the same value of 3.09 Ǻ, and so, under the ammonia flux, the formed g-AlN is quite stable. The lateral size of the g-AlN islands has been estimated from the RHEED data as

The (4 × 4) structure is not the consequence of the actual reconstruction of the Si (111) surface. The fractional spots (0 1/4), (0 2/4), and (0 3/4) (and others) are detectable in the RHEED pattern (**Figure 9**) as the result of electrons scattering by both the Si and g-AlN crystal lattices, that is, mixing of reciprocal vectors of these lattices. For example, the fractional reciprocal vectors q(0 1/4) is a result of relation q(0 1/4) = q(0 1)AlN − q(0 1)Si, where q(0 1)AlN and q(0 1)Si are integer order reciprocal vectors of g-AlN and Si, respectively. The approximate equality of the fourfold silicon lateral constant and the fivefold wurtzite AlN lateral constant was previously pointed out in some studies (e.g., see paper [53, 54]) with mismatch of ~1.3%. In our case, there is an exact coincidence, and as the result, the fractional beams of (4 × 4) structure are experimentally observed in the RHEED pattern. Thus, an extremely thin and flat of AlN on (111) Si substrate with the lattice constant of a = 3.08 Å is prepared. This value is close to the *ab initio* calculated value of 3.09 Å


*DOI: http://dx.doi.org/10.5772/intechopen.81775*

for the graphene-like aluminum nitride lattice with sp2

*Evolution of an in-plane lattice constant during the g-AlN formation process.*

value presented by the authors of 3.13 Å.

AlN) is clearly visible.

~70–100 Å.

temperature [52].

*Van der Waals and Graphene-Like Layers of Silicon Nitride and Aluminum Nitride DOI: http://dx.doi.org/10.5772/intechopen.81775*

is substantially lower than lateral lattice constant of the bulk wurtzite AlN value being 3.125 Å, which has been measured by the X-ray diffraction method at a high temperature [52].

The (4 × 4) structure is not the consequence of the actual reconstruction of the Si (111) surface. The fractional spots (0 1/4), (0 2/4), and (0 3/4) (and others) are detectable in the RHEED pattern (**Figure 9**) as the result of electrons scattering by both the Si and g-AlN crystal lattices, that is, mixing of reciprocal vectors of these lattices. For example, the fractional reciprocal vectors q(0 1/4) is a result of relation q(0 1/4) = q(0 1)AlN − q(0 1)Si, where q(0 1)AlN and q(0 1)Si are integer order reciprocal vectors of g-AlN and Si, respectively. The approximate equality of the fourfold silicon lateral constant and the fivefold wurtzite AlN lateral constant was previously pointed out in some studies (e.g., see paper [53, 54]) with mismatch of ~1.3%. In our case, there is an exact coincidence, and as the result, the fractional beams of (4 × 4) structure are experimentally observed in the RHEED pattern. Thus, an extremely thin and flat of AlN on (111) Si substrate with the lattice constant of a = 3.08 Å is prepared. This value is close to the *ab initio* calculated value of 3.09 Å for the graphene-like aluminum nitride lattice with sp2 -like bonding [55, 56].

There is only one more experimental work [57] that dedicated to epitaxial growth of graphite-like hexagonal AlN nanosheets on single crystal Ag (111). It is interesting to note that a simple our calculation of the g-AlN lattice constant using the lateral constant of (111) Ag (2.89 Å) from the diffraction pattern presented by authors of the work gives the value in the range of 3.06–3.09 Å in contrast to the value presented by the authors of 3.13 Å.

We have carried out experimental precise measurements of the in-plane AlN lattice constant under the Al and NH3 fluxes supplied onto the surface either separately or simultaneously. The evolution of in-plane lattice constant during the g-AlN formation process is depicted in **Figure 10**. The increasing of the lattice constant from 3.08 to 3.09 Å under Al flux without ammonia flux (i.e., there is no growth of AlN) is clearly visible.

Then, the Al flux was turned off (at the moment of 340 s) and the NH3 flux (10 sccm) was switched on. The lateral lattice constant of g-AlN was keeping the same value of 3.09 Ǻ, and so, under the ammonia flux, the formed g-AlN is quite stable. The lateral size of the g-AlN islands has been estimated from the RHEED data as ~70–100 Å.

**Figure 10.** *Evolution of an in-plane lattice constant during the g-AlN formation process.*

*2D Materials*

was achieved.

**by ammonia MBE**

**6. Graphene-like AlN layer formation on Si (111) surface** 

flux was switched off and the background ammonia pressure of ~10<sup>−</sup><sup>7</sup>

along a horizontal line (A-B) crossing the streaks is shown in **Figure 9b**.

In the present experiments, an AlN flat ultrathin layer was prepared by using the following two-stage procedure. At first, a clean (1 × 1) silicon surface was exposed under the ammonia flux (10 sccm) at the substrate temperatures of 1050°C, and in the second stage, the AlN layer is formed by the Al deposition when the ammonia

For the AlN formation, the ammonia flux was turned off at the moment when the best (8 × 8) RHEED pattern with sharp and bright eightfold fractional spots was reached. This moment corresponds to the maximum of the curve in **Figure 1b**. Next, the Al deposition onto the highly ordered (8 × 8) structure was performed. The Al flux was established on the value equivalent to the AlN growth rate of ~0.1 Ml/s. The appearance of the AlN diffraction spots and the transformation of the (8 × 8) structure to a new fourfold structure was observed. The RHEED pattern of AlN and (4 × 4) structure is shown in **Figure 9a**. An intensity profile measured

The observed fundamental (0-1) AlN streak position exactly coincides with the position of the fractional spot (0-5/4); see **Figure 9b**). Then, an AlN in-plane lattice constant was calculated from the relationship 4 × a111Si = 5 × aAlN, where a111Si = 3.85 Å. Hence, the calculated lattice constant is aAlN = 3.08 Å. This value

*(a) RHEED pattern of the Si surface (4 × 4); (b) the intensity profile of (4 × 4) structure measured along the* 

–10<sup>−</sup><sup>8</sup>

Torr

**42**

**Figure 9.**

*line (A-B) crossing the diffraction streaks, from [50].*

**Figure 11.** *Atomic models of (a) graphene-like AlN; (b) wurtzite AlN.*

The epitaxial growth of AlN was initiated by turning on the Al flux, keeping the same ammonia flux (at the moment of 390 s). The fractional streaks of the (4 × 4) structure are gradually dimmed together with the increasing of the fundamental (0 1) g-AlN streak intensity. At the moment of about 420 s, a structure (1 × 1) of g-AlN appears. Further growth of AlN leads to the lattice constant conversion from 3.09 to 3.125 Å. The value 3.125 Å corresponds to bulk value of the wurtzite AlN lattice constant at high temperature [52].

Thus, the transformation from graphite-like (sp2 -hybridization, see **Figure 11a**) to wurtzite structure of AlN (sp3 -hybridization, **Figure 11b**) is observed. This transition is similar to the transition metastable graphene-like silicon nitride of the structure (8 × 8) to stable amorphous phase a-Si3N4. The maximal thickness of the g-AlN layer of ~5–6 AlN monolayers was estimated using the AlN growth rate that is less than the theoretically predicted value of 22–24 monolayer [58]. The difference between calculations and experimental data might be attributed to the competition with the bulk stabilization mechanism involving structural defects and roughening, which were not taken into account in work [58]. The similar discrepancy between theoretically predicted and experimentally measured thickness of the sp2 -sp3 transition was noticed for ZnO [59].

### **7. Conclusion**

Systematic studies of the structure (8 × 8) by the methods RHEED, STM/ STS, and HRTEM were carried out. It is found that the structure (8 × 8) is formed within 5–7 s during nitridation of the Si (111) surface at the temperature range 950–1150°C. The formation rate of the structure (8 × 8) is independent on the temperature. The kinetics of the thermal decomposition of this two-dimensional layer of silicon nitride has been studied. It is established that the structure (8 × 8) is a metastable phase, and with further nitridation, a transition to the stable amorphous phase of Si3N4 occurs. In the structure (8 × 8), the honeycomb structure with the side length of hexagon of 6 Å was found for the first time, on which the adsorption phase of silicon is located with a periodicity of 10.2 A. The interplanar spacing in the epitaxial structure (SiN)2(AlN)4 on the (111) Si surface are measured: 3.3 Å in silicon nitride layer and 2.86 Å in AlN. These interlayer distances correspond to the weak van der Waals interaction between the layers. Scanning tunneling spectroscopy in the filled states revealed a peak of 1.1 eV below the Fermi level. Comparison of the measurements in the STS of the metastable phase of silicon nitride with measurements made on a clean surface of Si (111)–(7 × 7) and on an amorphous Si3N4 layer helped us in identifying the peak −1.1 eV as the π-bonding band of the structure (8 × 8). The band gap between the bonding and antibonding orbitals is

**45**

**Author details**

wurtzite structure.

**Table 1.**

**Acknowledgements**

silicon nitride and aluminum nitride.

Nos. 17-02-00947, 18-52-00008).

Hungary

Vladimir G. Mansurov1

Konstantin S. Zhuravlev1

of Sciences, Novosibirsk, Russia

provided the original work is properly cited.

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

, Timur V. Malin1

and Bela Pecz2

1 Rzhanov Institute of Semiconductor Physics Siberian Branch of Russian Academy

2 Thin Film Physics Department, Institute of Technical Physics and Materials Science Centre for Energy Research, Hungarian Academy of Sciences, Budapest,

, Sergey A. Teys1

,

\*, Yurij G. Galitsyn1

, Ildiko Cora2

\*Address all correspondence to: mansurov@isp.nsc.ru

*Van der Waals and Graphene-Like Layers of Silicon Nitride and Aluminum Nitride*

**Lateral lattice constant Interplanar** 

found of 2.2 eV. Consequently, the 2D silicon nitride layer is a semiconductor. The data obtained allowed us to propose new graphene-like model structures (8 × 8). The models are planar graphene-like structures of g-Si3N3 and/or g-Si3N4. Owing to the formation of graphene-like Si3N3 layer, it is possible to synthesize graphenelike g-AlN, the lateral lattice constant of 3.08 Å, and the interplanar distance of 2.86 Å. When the AlN thickness of 5–6 monolayers is reached, g-AlN passes into the

g-Si3N3 (g-Si3N4) 10.2 Å (5.9 Å hex side) 3.3 Å < 4 eV 2.2 eV g-AlN 3.08 Å 2.86 Å ~ 3.0 eV [55]

**spacing**

**Heat of formation** **Band gap**

**Table 1** summarizes some important characteristics of graphene-like phases of

This work was supported by the Russian Foundation for Basic Research (Grant

*DOI: http://dx.doi.org/10.5772/intechopen.81775*

*Structural and thermodynamic characteristics of g-AlN and g-Si3N3.*

*Van der Waals and Graphene-Like Layers of Silicon Nitride and Aluminum Nitride DOI: http://dx.doi.org/10.5772/intechopen.81775*


**Table 1.**

*2D Materials*

**Figure 11.**

The epitaxial growth of AlN was initiated by turning on the Al flux, keeping the same ammonia flux (at the moment of 390 s). The fractional streaks of the (4 × 4) structure are gradually dimmed together with the increasing of the fundamental (0 1) g-AlN streak intensity. At the moment of about 420 s, a structure (1 × 1) of g-AlN appears. Further growth of AlN leads to the lattice constant conversion from 3.09 to 3.125 Å. The value 3.125 Å corresponds to bulk value of the

is similar to the transition metastable graphene-like silicon nitride of the structure (8 × 8) to stable amorphous phase a-Si3N4. The maximal thickness of the g-AlN layer of ~5–6 AlN monolayers was estimated using the AlN growth rate that is less than the theoretically predicted value of 22–24 monolayer [58]. The difference between calculations and experimental data might be attributed to the competition with the bulk stabilization mechanism involving structural defects and roughening, which were not taken into account in work [58]. The similar discrepancy between theoreti-

Systematic studies of the structure (8 × 8) by the methods RHEED, STM/ STS, and HRTEM were carried out. It is found that the structure (8 × 8) is formed within 5–7 s during nitridation of the Si (111) surface at the temperature range 950–1150°C. The formation rate of the structure (8 × 8) is independent on the temperature. The kinetics of the thermal decomposition of this two-dimensional layer of silicon nitride has been studied. It is established that the structure (8 × 8) is a metastable phase, and with further nitridation, a transition to the stable amorphous phase of Si3N4 occurs. In the structure (8 × 8), the honeycomb structure with the side length of hexagon of 6 Å was found for the first time, on which the adsorption phase of silicon is located with a periodicity of 10.2 A. The interplanar spacing in the epitaxial structure (SiN)2(AlN)4 on the (111) Si surface are measured: 3.3 Å in silicon nitride layer and 2.86 Å in AlN. These interlayer distances correspond to the weak van der Waals interaction between the layers. Scanning tunneling spectroscopy in the filled states revealed a peak of 1.1 eV below the Fermi level. Comparison of the measurements in the STS of the metastable phase of silicon nitride with measurements made on a clean surface of Si (111)–(7 × 7) and on an amorphous Si3N4 layer helped us in identifying the peak −1.1 eV as the π-bonding band of the structure (8 × 8). The band gap between the bonding and antibonding orbitals is



transition was


wurtzite AlN lattice constant at high temperature [52]. Thus, the transformation from graphite-like (sp2

*Atomic models of (a) graphene-like AlN; (b) wurtzite AlN.*

cally predicted and experimentally measured thickness of the sp2

wurtzite structure of AlN (sp3

noticed for ZnO [59].

**7. Conclusion**

**44**

*Structural and thermodynamic characteristics of g-AlN and g-Si3N3.*

found of 2.2 eV. Consequently, the 2D silicon nitride layer is a semiconductor. The data obtained allowed us to propose new graphene-like model structures (8 × 8). The models are planar graphene-like structures of g-Si3N3 and/or g-Si3N4. Owing to the formation of graphene-like Si3N3 layer, it is possible to synthesize graphenelike g-AlN, the lateral lattice constant of 3.08 Å, and the interplanar distance of 2.86 Å. When the AlN thickness of 5–6 monolayers is reached, g-AlN passes into the wurtzite structure.

**Table 1** summarizes some important characteristics of graphene-like phases of silicon nitride and aluminum nitride.
