Structure and Optical Properties of Tio2 Thin Films Deposited by ALD

Open Phys. 2017; 15:1067–1071

Research Article

Marek Szindler*, Magdalena M. Szindler, Paulina Boryło, and Tymoteusz Jung

Structure and optical properties of TiO2 thin films deposited by ALD method https://doi.org/10.1515/phys-2017-0137 is an inorganic compound, occurs naturally in three poly- Received Oct 19, 2017; accepted Nov 21, 2017 morphic forms: as the rutile and anatase minerals with a tetragonal structure and brookite with an orthorhom- Abstract: This paper presents the results of study on tita- bic structure. The last two forms pass in the rutile (most nium dioxide thin films prepared by atomic layer depo- ∘ durable) above the 800-900 C temperature. Photoelectro- sition method on a silicon substrate. The changes of sur- chemical properties of TiO are associated with the ab- face morphology have been observed in topographic im- 2 sorption of solar radiation. Titanium dioxide is charac- ages performed with the atomic force microscope (AFM) terized by high absorption in the UV range and up to a and scanning electron microscope (SEM). Obtained rough- few percent absorption of visible VIS radiation. In order ness parameters have been calculated with XEI Park Sys- to increase the range of visible light absorption, which tems software. Qualitative studies of chemical composi- is essential in solar cells and in photodegradation of wa- tion were also performed using the energy dispersive spec- ter, research on the modification of its properties isin trometer (EDS). The structure of titanium dioxide was in- progress [2–4, 13]. Methods of deposition have a signifi- vestigated by X-ray crystallography. A variety of crystalline cant impact on the development of antireflection coatings. TiO2 was also confirmed by using the Raman spectrome- Among many techniques that increase the usefulness of ter. The optical reflection spectra have been measured with the surface of photovoltaic materials, chemical and phys- UV-Vis spectrophotometry. ical deposition methods play an important role in indus- Keywords: photovoltaic; silicon solar cell; atomic layer de- trial practice. One of the depositing methods character- position; antireflection coating ized by dynamic development is the atomic layer deposition (ALD) technique. By using highly reactive precur- PACS: 42.25.Bs, 68.35.Ct, 68.37.-d sors, which react immediately with the substrate to form a monolayer and prevent further reactions, each cycle results in an increase in the thickness of the layer with a 1 Introduction strictly defined value. The most commonly used precur- sors for the preparation of TiO2 are TiCl4, Ti(OCH2CH3)4 The possibility of the development of photovoltaics is and Ti(OCH(CH3)2)4, which react with H2O. In the pro- strongly dependent on the efficiency of silicon solar cells, cess of growth of metal oxides, in which H2O is used as which determines the price of electricity produced from an oxygen precursor, hydroxyl groups can be formed on them. One of the factors affecting the efficiency of the sil- the surface of substrate during the oxygen precursor pulse. icon solar cell is the reduction of light reflection from its Thus, the adsorbed metal precursor may react with sur- front surface. The effect of minimizing the reflectance of face hydroxyl groups and release some ligands in this re- light is achieved, among others, by depositing an antire- action [14, 15]. flection coating. The most popular antireflection coating used in photovoltaics are: SiO2, a-SiNx:H, TiO2, a-Si:C:H, ZnS, Ta2O5, SnxOx, SiNx, MgF2 [1–12]. Titanium dioxide 2 Materials and methodology

*Corresponding Author: Marek Szindler: Institute of Engineer- The TiO2 thin films have been deposited by an atomic ing Materials and Biomaterials, Silesian University of Technology, layer deposition using an R-200 system from Picosun com- Konarskiego 18a, 44-100 Gliwice, Poland; pany. As a precursor of TiO2, titanium tetrachloride (TICl4) Email: [email protected] has been used, which reacted with water enabling the Magdalena M. Szindler, Paulina Boryło, Tymoteusz Jung: Insti- deposition of the thin films. The 2D and 3D topographic tute of Engineering Materials and Biomaterials, Silesian University 2×2µm images were performed with XE-100 Park Systems of Technology, Konarskiego 18a, 44-100 Gliwice, Poland

(a) (b)

(e) (f)

∘ Figure 1: AFM 2D and 3D images of the surface topography of TiO2 thin films deposited on glass substrate at 300 C after 630 (a-b), 830 (c-d) and 1030 cycles (e-f) Structure and optical properties of TiO2 thin films Ë 1069

(a) (b)

∘ Figure 2: SEM images of the surface topography of TiO2 thin films deposited on glass substrate at 300 C after 630 (a), 830 (b), 1030 cycles (c) and EDS spectrum (d)

ning electron microscope (SEM) images were taken with a Zeiss Supra 35. Qualitative studies of chemical composi- tion were also performed using the energy dispersive spectrometer (EDS). The structure of titanium dioxide was in- vestigated by X-ray crystallography. A variety of crystalline TiO2 was also confirmed by using the Raman spectrometer. The TiO2 thin films have been examined by using an evolu- tion 220 UV-Vis spectrometer Thermo Scientific company.

3 Results and discussion

Figure 3: The diffraction pattern of ALD titanium dioxide thin film ∘ The films deposited at 300 C by ALD method on a silicon substrate have been analyzed, based on the number of de- atomic force microscope (AFM) and the RMS and Ra co- position cycles (Figures 1a-e). It has been found out in the efficient values were determined with XEI software. Scan- images, that repetitive aggregations of atoms have a simi- 1070 Ë M. Szindler et al.

Figure 4: Raman spectrum of ALD titanium dioxide thin film

Table 1: Summary of roughness parameters for deposited TiO2 thin films

No. Temperature Number Surface area RMS Ra ∘ [ C] of cycle [µm2] [nm] [nm] 1 300 630 4.0 2.52 2.01 2 300 830 4.0 1.78 1.43 3 300 1030 4.0 1.18 0.95

the TiO2 thin films have a granular structure and with increasing number of cycles of deposition the aggregations of atoms take milder forms. The microanalysis of the as- prepared thin films has been carried out by the energy dispersive X-ray spectroscopy. In Figure 2d is shown the EDS spectra of thin film of ALD TiO2. There are observed peaks at about 0.5 and 4.5 keV in these EDS spectra’s, which are assigned to oxygen and titanium respectively. Such an analysis can be confirmed by the presence of titanium dioxide. The structural studies were implemented by using X- ray method (Figure 3). Registered diffraction pattern of ti- Figure 5: The spectrum of reflection for bare silicon and with differ- tanium dioxide thin film shows the reflection characteris- ent number of TiO2 ALD cycles tic for the titanium dioxide - a variety of anatase (coming from the sample). Further structural testing of deposited lar geometrical features. With increasing the number of de- thin films is made by using a Via Reflex Raman spectrom- position cycles, aggregations of atoms take milder forms, eter equipped with an Ar ion laser with a 514.5 nm length. decreasing surface roughness. After 630 cycles of deposi- Spectral range is 150 – 3200 cm−1 (Figure 4). Using confo- tion, the RMS (root mean square) value is equal to 2.52 nm, cal microscope images, the data point on the sample was while after 1030 cycles, the RMS value decreases to 3.13 nm presented. The results, processed by the WiRE 3.1 program, (Table 1). determining the structure of the deposited thin films as an The morphology of TiO2 layers deposited by ALD is anatase, confirmed earlier research done by the X-ray crys- homogeneous and uniforms (Figures 2a-c). The surface tallography. doesn’t show any discontinuities, cracks, pores and de- The reflection of bare silicon compared with different fects. The SEM research confirmed the AFM results that cycles of TiO2 is shown in Figure 5. The best results were Structure and optical properties of TiO2 thin films Ë 1071

obtained for silicon with 830 and 1030 cycles of TiO2. The [3] Alzamani M., Shokuhfar A., Eghdam E., Mastali S., Sol-gel fab- light reflection was minimized below 10% in the range of rication and enhanced optical properties, photocatalysis, and surface wettability of nanostructured titanium dioxide films, 200 nm to 600 nm. Mat. Sci. Semicon. Proc., 2013, 16(4), 1063-1069. [4] Li X.G., Wang X.G., Shen J., Broadband Antireflective Coatings with Different Film Designs by Sol-Gel Method, Rare Metal Mat. Eng., 2012, 43(3), 296-299. 4 Conclusions [5] Wan Y.M., Bullock J., Cuevas A., Passivation of c-Si surfaces by ALD tantalum oxide capped with PECVD silicon nitride, Sol. En- The results and their analysis show that the atomic layer erg. Mat. Sol. C, 2015, 142, 42-46. deposition method allows the deposition of homogenous [6] Rubio F., Denis J., Albella J.M., Martinez-Duart J.M., Sputtered Ta2O5 antireflection coatings for silicon solar-cells, Thin Solid thin films of TiO2 with the desired geometric characteris- Films, 1982, 90(4), 405-408. tics and good optical properties. The irregularities on the [7] Moghadam R.Z., Ahmadvand H., Jannesari M., Design and fabri- surface of the deposited layers do not exceed 4 nm and the cation of multi-layers infrared antireflection coating consisting light reflection in a wide range does not exceed 10% with- of ZnS and Ge on ZnS substrate, Infrared Phys. Techn., 2016, 75, out texturing the silicon surface. 18-21. Considering the optimization of the antireflection [8] Benyahia K., Benhaya A., Aida M.S., ZnS thin films deposition coating production, it is preferable to deposit 830 of ALD by thermal evaporation for photovoltaic applications, J. Semi- cond., 2015, 36(10), 1-4. cycles. Further deposition does not significantly affect the [9] Wang L.S., Chen F.X., Optimization of wide angle reduction of light reflection. Increasing the thickness of MgF2/ZnS/Al2O3 passivation and antireflection film for the coating may lead to increased absorption of light in silicon solar cells, J. Optoelectron. Adv. M., 2012, 14(11-12), the layer and this can lead to a decrease of solar cells effi- 929-934. ciency. However, the technology of atomic layer deposition [10] Kessels W.M.M., Hong J., Van Assche F.J.H, Moschner J.D., Lauinger T., High-rate deposition of a-SiN: H for photovoltaic ap- makes titanium dioxide thin films a good potential mate- plications by the expanding thermal plasma, J. Vac. Sci. Technol. rial for optics, optoelectronics and photovoltaics. A, 2002, 20(5), 1704-1715. [11] Marques F.C., Sprayed SnO2 antireflection coating on textured Acknowledgement: The publication was co-financed by silicon surface for solar cell applications, IEEE T Electron. Dev., the statutory grant of the Faculty of Mechanical Engineer- 1998, 45(7), 1619-1622. ing of the Silesian University of Technology in 2017. [12] Richter A., Benick J., Hermle M., Boron Emitter Passivation With Al2O3 and Al2O3/SiNx Stacks Using ALD Al2O3, IEEE J. Photo- volt., 2013, 3(1), 236-245. [13] El-Amin A.A., Zaki A.A., Improving the Eflciency of Multicrys- References talline Silicon by Adding an ARC Layer in the Front Device, Silicon-NETH, 2017, 9(1), 53-58. [14] Aarik J., Aidla A., Mändar H., Uustare T., Atomic layer deposition [1] Parashar P.K., Sharma R.P., Komarala V.K., Double-layer antire- of titanium dioxide from TiCl4 and H2O: investigation of growth flection from silver nanoparticle integrated SiO layer on silicon 2 mechanism, Appl. Surf. Sci., 2001, 172(1-2), 148-158. wafer: effect of nanoparticle morphology and SiO film thick- 2 [15] Aarik L., Arroval T., Rammula R., Mändar H., Sammelselg V., ness, J. Phys. D. Appl. Phys., 2016, 50(3), 1-11. Aarik J., Atomic layer deposition of TiO2 from TiCl4 and O3, Thin [2] Huang J.J., Lee Y.T., Self-cleaning and antireflection properties Solid Films, 2013, 542, 100-107. of titanium oxide film by liquid phase deposition, Surf. Coat. Tech., 2013, 231, 257-260.