ISSN 0036-0244, Russian Journal of Physical Chemistry A, 2017, Vol. 91, No. 4, pp. 739–743. © Pleiades Publishing, Ltd., 2017. Original Russian Text © Yu.K. Ezhovskii, 2017, published in Zhurnal Fizicheskoi Khimii, 2017, Vol. 91, No. 4, pp. 691–695.

PHYSICAL CHEMISTRY OF NANOCLUSTERS AND NANOMATERIALS

Molecular Layering of Silicon and Aluminum Oxides on Binary Semiconductors Yu. K. Ezhovskii* St. Petersburg State Technological Institute (Technical University), St. Petersburg, 190013 Russia *e-mail: [email protected] Received March 22, 2016

Abstract—The formation of nanolayers of silicon and aluminum oxides, obtained by means of molecular layering (or atomic layer deposition (ALD technology)) on surfaces of GaAs, InAs, and InSb, is investigated. Conditions for the layer-wise growth of surface nanostructures are established, and some of their dielectric characteristics are estimated. Keywords: molecular layering, nanolayers of silicon and aluminum oxides DOI: 10.1134/S0036024417040070

INTRODUCTION Creating different types of nanodevices is a problem whose solution requires us to develop the methods for synthesis of respective materials and studying the basic patterns of nano-object behavior [1]. A special position is held by electronics as a field of the intense development of new technologies that will enable us to create solid-state nanostructures on the atomic– molecular level. Although molecular beam epitaxy (MBE) [2] allows us to solve a number of critical problems in microelectronics, the high cost of MBE equipment and the problems of batch processing of items have led to the development of alternative methods. ALD (Atomic Layer Deposition) technology has recently been studied intensively with the aim of creating high-quality ultrathin layers [3–5]. The advantages and prospects of this field in creating submicron elements of integral devices have been demonstrated in a number of works [6–8]. ALD technology is based on processes known earlier as molecular layering (ML), the physicochemical basis of which was developed as early as the 1970s by the school of Russian scientists led by Aleskovskii [9]. Using a gas-phase reagent feed and having self-organizing features enables the batch processing of items and thus ensures their economic viability. This work describes the results from studying the synthesis and main dielectric characteristics of SiO2 and Al2O3 nanolayers produced by means of molecular layering on the surfaces of semiconductors used extensively in optoelectronics (GaAs, InAs, and InSb).

EXPERIMENTAL The use of surface chemical reactions enables us to synthesize low-dimensional structures while controlling the composition and thickness of a monolayer. The production of oxide layers is based on self-limiting processes of the chemisorption of a metal halogenide (MCln) and water vapor under conditions of complete surface coverage. On a hydroxylated surface (symbol │), the process occurs according to the reactions:

|−OH) m + MCl n → |−O−) m MCl n −m + m HCl, (1) |−O−) m MCl n −m + (n − m)Н 2О → |−O−) m M (ОН) n− m + (n − m)HCl.

(2)

The m value depends on the distribution and concentration of hydroxyl groups on the surface: with a silicon matrix where m ≈ 2 [10], multiple repeated reactions (1) and (2), with the intermediate removal of excess reagent and reaction products, result in the formation of oxide layers of specified thickness. The temperature conditions for the growth of films of binary compounds AB (Тf) must comply with the relation • T A•,TB• ≤ Tf ≤ T AB ,

where T A•,

TB•,

• T AB

(I)

are the critical temperatures of the condensation of components A, B, and AB compound, respectively. This excludes the direct condensation of any component (A or B), and the process is limited by the formation of a chemisorbed monolayer. Surface reactions (1) and (2) must proceed under nonequilibrium conditions to be complete, which is ensured by the fairly high activity of surface hydroxyl groups. Estimates of this for an gallium arsenide

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d, nm 24

1

2

3

16 3a 8 2a 1a 0

10

20

30

40

50 N

Fig. 1. Dependences of layer thicknesses on the number of ML cycles for (1–3) Al2O3 and (1a–3a) SiO2, obtained on (100) GaAs at Т = (1) 403, (2) 453, and (3) 523 K. Reagent vapor pressure P = 13 Pa; duration of vapor contact τ = 60 s.

matrix, performed using the Taft induction constants scale, yielded the value σ*i = 5.1 [11]. We may assume that for InAs and InSb, σ*i > σ*Cl = 2.88 [12], which is enough to produce surface reactions with vapors of silicon and aluminum chlorides. Plates of gallium arsenide (AGChT-23-17 with orientation (100) and AGE-4-16 with orientation (110)), indium arsenide (epitaxial structures IMN10/PAI380-25.5 (111) and (100)), and indium antimonide (ISE-0 with orientation (111)), subjected to preliminary etching in bromine–methanol etchants (1–5% Br), were used as substrates. As can be seen from the ellipsometry data, the thickness of the residual oxide layers on all matrices did not exceed 2 nm. Oxide nanolayers were synthesized via reactions (1) and (2) at pressure Р ≈ 10–100 Pa of the vapors of the respective metal chloride and water, with intermediate removal of excess reagent and reaction products. The process was conducted in a vacuum flow-type unit under a residual gas pressure no higher than 10–1 Pa, as when synthesizing layers on silicon [13]. The layers’ thicknesses were determined from ellipsometry measurements of parameters Δ and Ψ, and calculated in an approximation of the single-layer Drude–Tronston model [14]. The Δ and Ψ parameters were measured using an ellipsometer arranged according to the PQSA scheme [15] with a fixed compensator. An LG-75 laser with a radiation wavelength of 632.8 nm was used as the source of polarized light. Magnetic modulation of the light beam was used to improve the accuracy of measurements. The error in determining these parameters did not exceed ±0.1′. Ellipsometric determination of the refraction index of the synthesized structures allowed identification of

their compositions, also controlled using data from Xray photoelectron spectroscopy (XPS) with HP5950A (AlKα radiation with EKα = 1486 eV) and SER1 (MgКα radiation with ЕKα = 1253 eV) spectrometers. The layers’ dielectric characteristics were determined both on the above matrices and in a metal– dielectric–semiconductor (MDS) system using vacuum-sputtered films of aluminum on silicon that were ~0.1 μm thick.

RESULTS AND DISCUSSION A linear d = f(N) dependence was observed in studying the growth of layers of the above oxides at different substrate temperatures (Fig. 1) and reagent vapor pressures for the synthesized SiO2 and Al2O3 layers. This indicated continuing hydroxyl activity and a uniform quantity of the synthesized surface compound in each cycle. In all cases, film thickness d was proportional to number N of the cycles of treatment of the matrix surface according to reactions (1) and (2): d = d0N, where d0 is the parameter of layer growth indicating the averaged layer thickness within one cycle of treatment for both components and characterizing the structure of the synthesized layer. Based on the ratio between the experimental d0 values and the calculated linear parameters of the layer structure in the direction of growth, we can estimate the degree of surface filling by the growing layer and determine the mechanism of its formation. To determine the conditions of ultimate surface filling by the synthesized groups according to reactions (1) and (2) at a specified synthesis temperature, we performed an ellipsometric study of the kinetics of the chemisorption of metal halogenides and water, based on the thickness of the formed oxide, normalized for one cycle of treatment (d0). At different temperatures, the dependences of the d0 parameter on the duration of reagent contact with the substrate (τ) were similar and differed only in the ultimate d0 values (Fig. 2). Analysis of these dependences showed that the surface filling was isotypical for all of the studied compounds, but with Al2O3 layers its rate was twice that of SiO2 layers. This was most likely due to characteristics of the chemisorption of silicon and aluminum chlorides. A series of d = f(N) dependences obtained at different temperatures was analyzed to identify the mechanism behind the formation of the synthesized nanostructures, and to determine the conditions for layerwise growth. This allowed us to establish the effect the temperature of synthesis (Тf) had on the parameter of layer growth (Fig. 3). As can be seen from our results, the d0 = f(Тf) dependences are notably different for SiO2 and Al2O3. The greater number of surface reactions for SiO2 layers as the temperature rose testifies to

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which maintains its stability up to T ≈ 500 K, is in this case most likely. The layer-wise growth of SiO2 was possible only on GaAs and InAs at T > 525 K, when the d0 parameter approached the monolayer thickness for this oxide (Fig. 3). With InSb, surface filling by silicon-oxide groups did not exceed 0.5. The lower d0 values for InAs and InSb when Тf > 525 and 550 K, respectively, was due to both the drop in the density of hydroxyl groups and the thermochemical instability of these matrices in a silicon halogenide atmosphere. The latter was confirmed by the experimental data on GaAs, which show that ОН-groups are thermally stable under the above conditions (Fig. 3а).

d0, nm 0.6 2 0.4

1

0.2

0

20

40

60 τ, s

Fig. 2. Effect of the duration of contact with reagent vapor on the increment of (1) silicon and (2) aluminum oxide layer within one cycle of treatment at T = 523 K and Р = 13 Pa.

Unlike SiO2, no signs of activation were observed upon the formation of Al2O3 layers. The high d0 values for aluminum oxide layers that greatly exceed the average sizes of aluminum–oxygen tetrahedra were due to their formation occurring via chemisorption of the (Al2Cl6) dimers that predominate in the vapors’ composition. Reactions (1) and (2) can proceed (with a preferential metal coordination number equal to 4) in two main directions (in a schematic representation):

the activation character of SiCl4 chemisorption. The formation of an activated complex with a hydroxyl group of the type O H Cl Si Cl Cl Cl

Cl

O

OH + Al2Cl6

Al

−HCl

O

OH

Cl

Cl

Al

−HCl

Cl

−HCl

(3a) OH

O H OH

O Al

O

Cl + H2O

O

−HCl

Al OH

OH Al

O

Cl

Al OH + Al2Cl6

H O

O + H2O

Al

O

OH

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OH

(3b)

Al

O

Cl

The predominance of any direction is determined by the topography of the hydroxyl cover. At reduced tem-

d0, nm 0.3 (a)

O

OH

peratures (Тf < 400 K), when the conditions of (I) are not realized and the density of hydroxyl groups is high

d0, nm 1.2 (b) 1

0.2

0.8 2 1

0.1 0

0.4

3 400

500

2 3

600

0

400

500

600 Tf, K

Fig. 3. Dependences of d0 for SiO2 (a) and Al2O3 layers (b) on GaAs (1), InAs (2), and InSb (3) on the temperature of synthesis (P = 13 Pa, τ = 60 s). RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A

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(even molecules of adsorbed water can be present), the formation of both Al2O3 and SiO2 layers proceeds mainly due to interaction between the reagents in the adsorbed layer. High d0 values when Тf < 400 K testified to the polymolecular sorption of reagents. Only when Тf > 400 K for InAs and InSb and Тf > 475 K for GaAs did the parameter of layer growth approximate the doubled sizes of aluminum-oxygen tetrahedra, indicating that the mechanism behind the formation of Al2O3 layers with predominant m = 2 (as was shown in reaction scheme (3a)) was in effect under these conditions. The drop in the d0 values when Т > 473 K for InAs and Т > 453 K for InSb must be the result of the thermochemical instability of these matrices in a halogenide atmosphere, rather than of a reduction in the density of hydroxyl groups. The latter is indicated by the symbate narrowing of the range of the layer-wise formation mechanism for both Al2O3 and SiO2 (Fig. 3) along with the heat of formation in the series GaAs, InAs, InSb. It is noteworthy that the growth of Al2O3 and SiO2 nanolayers upon molecular layering on GaAs, InAs, and InSb followed the pattern for a silicon matrix and other oxides [13], testifying to the general character of this process. Our electron diffraction study of the structure of the synthesized films showed that SiO2 layers were amorphous at all thicknesses and throughout the range of temperatures. Aluminum oxide layers up to 100 nm thick were amorphous when Тf < 450 K, while at high temperatures they contained α-Al2O3. As the Al2O3 layers grew thicker and the temperature of synthesis rose, crystalline fragments started to emerge in its structure as supramolecular formations. The broadening of the diffraction rings observed on their patterns enabled us to estimate the average size of crystallites in the range of 2 to 20 nm, depending on the layer thickness. These crystallites were most likely based on oxide groups that formed on ensembles of regularly spaced hydroxyl groups of uniform surface. Thermal treatment of the aluminum oxide layers on GaAs (Т = 623 K) substantially raised the intensity of reflections on the diffraction images and led to an abrupt increase in crystallite size, resulting in the virtually complete crystallization of aluminum oxide. As can be seen from the XPS spectra of oxide nanolayers, they contained none of the peaks in the energy ranges of 190–200 and 260–270 eV that are characteristic of 2s- and 2p-levels of chlorine. Some spectrum broadening with respect to oxygen states (530–531 eV) indicated the presence of hydroxyl groups. With oxide layers more than 8 nm thick (the depth of electron emission), there was no signal from the substrate in the XPS-spectra, testifying to the layers’ integrity at thicknesses of more than 8 nm. The oxygen/metal ratio ([O]/[Al]) in oxide layers synthesized on GaAs at different temperatures was determined from the juxtaposition of the areas of the

oxygen (Еbond = 530–531 eV) and aluminum (Еbond = 74.6 eV–Al2р, Еbond = 118.8 – Al2s) peaks. Our results showed it was ~4 when Тf = 423 K, and fell to 1.5–1.6 as the temperature rose when Тf = 573 K. The higher oxygen content at lower temperatures was most likely due to oxide hydration when Т < 423 K. IR spectroscopy studies performed using the multiple frustrated total internal reflection (MFTIR) of films of aluminum oxide synthesized on GaAs elements at Тf = 553 K corroborated the results obtained via XPS. Absorption maxima that were characteristic of Al–O bonds with metal coordinations of 4 and 6 were clearly visible at ν = 744 and 835 cm−1 [16]. The dielectric characteristics of nanolayers were estimated using samples of synthesized oxide layers more than 10 nm thick, where the conductivity did not depend on the layer thickness and was determined by the temperature of synthesis. Along with an increase in Тf, there was a drop in conductivity down to minimal values for the conditions of layer-wise nanostructure growth (Тf = 500–550 K), and the specific conductivity was σ = (1–3) × 10−14 Ω−1 cm−1 for both oxides. The dielectric permittivity was ε = 8–10 for Al2O3 and ε = 3.9–4.3 for SiO2. When Тf < 400 K, the conductivity grew by 3–4 orders of magnitude as a result of oxide hydration (in accordance with the XPS data on the oxygen/metal ratio in the synthesized layers). CONCLUSIONS Nanolayers of silicon and aluminum oxides were synthesized on surfaces of GaAs, InAs, and InSb by means of molecular layering (Atomic Layer Deposition). Analysis of the temperature factor shows that oxide nanostructures can be formed by alternate chemisorption vapors of metal halogenide and water in accordance with three mechanisms: (1) reactions between components in a polymolecular adsorbed layer with the formation of hydrated oxides; (2) the sequential growth of monomolecular layers (a layerwise growth mechanism); and (3) the formation and subsequent development of two-dimensional islandlike structures. It was established that under conditions of layer-wise growth, SiCl4 chemisorption has the character of activation. Layer-wise SiO2 growth is observed only when Тf > 500 K; with aluminum chloride, it proceeds with the formation of dimers according to a non-activation mechanism. The results from estimating the dielectric characteristics of SiО2 and Al2O3 films obtained under conditions of layer-wise growth show that nanolayers were characterized by fairly high parameters. This allows the application of molecular layering in creating quality dielectric structures in the elemental bases of micro- and nanoelectronics systems (e.g., gate dielectrics in MDS-structures, capacity units, and barrier layers).

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ACKNOWLEDGMENTS This work was supported by the RF Ministry of Science and Education; and by the Russian Science Foundation, project no. 14-13-00597. REFERENCES 1. Nanotechnology Research Directions: Vision for Nanotechnology in the Next Decade, Ed. by M. C. Roko, R. S. Williams, and P. Alivisatos (Kluwer, Boston, 2000). 2. Molecular-Beam Epitaxy and Heterostructures, NATO ASI Series, Ed. by L. Chang and K. Ploog (Springer, Netherlands, 1985), Vol. 87. 3. T. Suntola, Mater. Sci. Rep. 4, 261 (1989). 4. T. Seidel, A. Londergan, and L. Winkler, Solid State Technol., No. 5, 67 (2003). 5. J. Nishizava and T. Kurabayash, Chem. Sustainable Dev., No. 8, 5 (2000). 6. X. Ming, Solid State Technol. 44, 70 (2001).

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7. J. Gelatos, L. Chen, H. Chung, and R. Thakur, Solid State Technol. 2, 44 (2003). 8. R. L. Puurunen, J. Appl. Phys. 97, 121301 (2005). 9. V. B. Aleskovskii, Vestn. Akad. Nauk SSSR, No. 6, 48 (1975). 10. Yu. K. Ezhovskii and P. M. Vainshtein, Russ. J. Appl. Chem. 71, 235 (1998). 11. Yu. K. Ezhovskii, Russ. Chem. Rev. 73, 195 (2004). 12. The Chemist’s Reference Book (Khimiya, Moscow, Leningrad, 1971), Vol. 3 [in Russian]. 13. Yu. K. Ezhovskii, Russ. J. Phys. Chem. A 85, 447 (2011). 14. R. Azzam and N. Bashara, Ellipsometry and Polarized Light (North-Holland, Amsterdam, 1977). 15. V. K. Gromov, Introduction to Ellipsometry (Leningr. Gos. Univ., Leningrad, 1986) [in Russian]. 16. A. A. Tsyganenko, P. P. Mardilovich, G. N. Lysenko, and A. I. Trokhimets, Advances in Photophysics (Leningr. Gos. Univ., Leningrad, 1987), No. 9, p. 28 [in Russian].

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