Thin Solid Films 570 (2014) 16–19 Contents lists available at ScienceDirect Thin Solid Films journal homepage: www.elsevier.com/locate/tsf Control of preferred (222) crystalline orientation of sputtered indium tin oxide thin films Duy Phong Pham a, Bach Thang Phan a,b, Van Dung Hoang a, Huu Truong Nguyen a, Thi Kieu Hanh Ta b, Shinya Maenosono c, Cao Vinh Tran a,⁎ a b c Laboratory of Advanced Materials, University of Science, Vietnam National University, Ho Chi Minh, Viet Nam Faculty of Materials Science, University of Science, Vietnam National University, Ho Chi Minh, Viet Nam Japan Advanced Institute of Science and Technology, Nomi, Ishikawa, Japan a r t i c l e i n f o Article history: Received 18 February 2014 Received in revised form 30 July 2014 Accepted 29 August 2014 Available online September 2014 Keywords: Indium tin oxide Thin films Texture Conductivity Surface roughness Sputtering a b s t r a c t We report a two-step growth process for the fabrication of (222)-plane textured indium tin oxide (ITO) films A thin ITO seed layer was grown in mixed Argon + Oxygen gases, followed by a thick ITO deposited in Argon gas X-Ray diffraction shows that the sputtered ITO films exhibit strongly preferred (222) crystalline orientation The (222)-plane textured ITO films have high transmittance above 80% in the visible range and carrier concentration, mobility and resistivity in the range of 1021 cm−3, 40 cm2/Vs and 10−4 Ω·cm, respectively The surface roughness of our (222) textured ITO films is 1.4 nm, which is one of the smallest value obtained from sputtered ITO thin films © 2014 Elsevier B.V All rights reserved Introduction Tin-doped indium oxide (ITO) has been known as a transparent electrode in several optoelectronic devices such as liquid crystal displays, solar cells, organic light-emitting diodes (OLEDs), smart window, touch screen, and other flat panel displays due to its high optical transmittance and low electrical resistivity [1–6] Recently, the organic lightemitting diodes (OLEDs), which are one of the most promising candidates for flat panel displays, demand a very flat surface of ITO film [7] for improving electroluminescence efficiency and display lifetime [8] In general, homogeneity and surface roughness are very important for the reliability of devices since the organic layers in the OLEDs have thicknesses of only about 100 nm [9] In particular, the peak-to-valley roughness of ITO film has a linear relationship with the reverse leakage current of devices [8] Also, the surface morphology of ITO films considerably affects the patterning properties during the fabrication process of flat panel displays [10] Many published literatures show that electrical and optical properties of ITO thin films strongly depend on its preferential crystallographic orientation The ITO films with (400) crystallographic orientation have smaller optical band-gap, less effective “Sn” doping and larger grain size than the (222) textured films [11] Nakaya et al proposed that the ITO films with the (222) preferred orientation ⁎ Corresponding author E-mail address: tcvinh@hcmus.edu.vn (C.V Tran) http://dx.doi.org/10.1016/j.tsf.2014.08.041 0040-6090/© 2014 Elsevier B.V All rights reserved experience little deterioration at its interface with an over-lying film, thereby improving the light emission characteristics and lifetime of devices [12] In addition, since there is a small lattice mismatch between the neighboring oxygen-oxygen (O-O) distance on the close-packed ITO (222) and ZnO (002) planes, it benefits the initial nucleation and subsequent growth of high quality ZnO materials on (222) ITO substrates [13,14], probably leading to a good contact for carrier transport in solar cells based on ZnO substance materials Kim et al have found that the preferential orientations of the ITO thin films depend on the oxygen partial pressure An ITO film grown with pure Ar gas shows a preferential (400)-plane orientation parallel to the substrate surface while the preferential orientation of films changed from (400) to (222) plane when even a small amount of O2 was added to the Ar sputtering environment It was also observed that the diffraction intensity of the (222) peak decreased as the oxygen partial pressure increased [15] Moreover, most publications indicate that the (222) textured ITO films grown in mixed Ar + O2 gases have poor conductivity compared with the (400) textured ITO films grown in Ar gas environment The reason for this is the reduction density of oxygen vacancies, which is the main contributor of electrical carriers in the ITO film In this paper, we report the procedure to prepare (222) textured ITO films with high conductivity grown in Ar gas environment instead of mixed Ar + O2 gases The proposed procedure is the two-step sputtering process, in which a thin oxygen seed layer of indium tin D.P Pham et al / Thin Solid Films 570 (2014) 16–19 oxide (O-ITO) was sputtered in the mixed Ar + O2 gases prior to the deposition of the thick overhead ITO films in Ar gas Experiments The ITO thin films were prepared on soda-lime glass substrate by dc magnetron sputtering The target was commercial ceramic target with 10 wt.% SnO2 (99.99% purity) impurity The substrate was kept at a distance of cm from the target The substrate temperature and sputtering power were maintained at 350 °C and 50 W during the deposition, respectively In order to deposit an oxygen seed layer of indium tin oxide (O-ITO), the vacuum chamber was evacuated down to pressure 5.3 × 10− Pa prior to deposition Then the oxygen reactive gas was introduced into the chamber and the required pressure, for example 4.2 × 10−1 Pa, was set Argon gas was introduced thereafter till the preset pressure reached 5.3 × 10−1 Pa Both argon inert gas flow and oxygen reactive gas flow were controlled by a mass flow controller The thin O-ITO seed layer was firstly grown on glass in mixed (O2 + Ar) gases at 5.3 × 10−1 Pa The thickness of O-ITO seed layer is about nm Then, the vacuum chamber was evacuated down to pressure 5.3 × 10−4 Pa again for the following deposition of the 300-nm thick ITO layer in pure Ar gas at 5.3 × 10− Pa The thickness of films was monitored by using the Quartz oscillator (XTM/2-INFICON (USA)) The crystalline phases of the films were characterized in the θ–2θ mode by using a D8 Advance (Bruker) X-ray diffractometer (XRD) with Cu Kα radiation (λ = 0.154 nm) Electrical properties of films were carried out using Hall measurements (Ecopia HMS-3000) The optical transmittance spectra were measured using a UV–vis (Jasco V-530) in the wavelength range from 200 nm to 1100 nm The surface morphology was investigated by Atomic force microscopy (5500 AFM SYSTEM-AGILENT, Tapping mode) and scanning electron microscopy (SEM, JEOL JSM-7401F, operating voltage is 30 kV) The work function was measured by Ultraviolet Photoelectron Spectroscopy (UPS) using a Model AC-2 instrument (RIKEN KEIKI) 17 attained by other authors by growing ITO thin films in pure Ar gas and using not only magnetron sputtering but also other methods [16–19] In contrast, the ITO film with O-ITO seed layer, shows a prominently strong (222) peak, which can be understood that grain growth in the (222) direction is obviously favored against growth in other directions [20] This indicates that the thin O-ITO seed layer has significant effect on the crystal grain orientation of an overhead ITO film In addition, SEM images reveal the significant influence of the O-ITO seed layer on surface morphology of the overhead ITO layer The visible difference of surface morphology of the ITO thin films prepared with and without the O-ITO seed layer is shown in Fig It can be seen that the ITO film with O-ITO layer reveals “grain structure” (Fig 2a) while the film without O-ITO layer shows “domain structure” (Fig 2b) This “domain structure” is also called a “grain–subgrain” structure [10] or “domain–grain” structure [7] of conventionally sputtered ITO thin films in an oxygen-deficient environment, which has been obtained by other authors [7,10,21] The SEM images strongly show that there is transformation from “domain structure” into “grain structure” corresponding to the (400) into (222) texture, respectively, due to an introduction of the initial O-ITO seed layer prior to conventionally deposited ITO layer only in pure Ar gas Fig exhibits the estimated surface roughness (RMS) obtained from AFM analysis There are distinct differences in surface roughness between the two samples with and without O-ITO layer of 1.4 nm and 3.7 nm in a scan area of μm × μm, respectively Jung et al [7] reported that the ITO samples prepared by dc magnetron sputtering have an RMS roughness in the range 2–4 nm Raoufi et al showed AFM images of asdeposited and annealed ITO thin films revealing the formation of a porous granular surface with surface roughness values in the range of 0.847–3.846 nm [22] Hotovy et al reported that ITO thin films grown Results and discussion Fig shows X-ray diffraction patterns of the ITO thin films prepared with and without the O-ITO seed layer It is obvious that the ITO thin film without O-ITO seed layer reveals polycrystalline structure with differently orientated crystalline planes such as (400), (222), (211), (440), and (622) Among these planes, it has been found that there is preferential growing competition between (222) and (400) planes, the (400) plane preferential orientation This structural characteristic has been Fig X-ray diffraction patterns of ITO thin films with and without O-ITO layer Fig SEM images of a) the (222) textured ITO film with O-ITO layer and b) the ITO without initial O-ITO layer 18 D.P Pham et al / Thin Solid Films 570 (2014) 16–19 Fig The optical transmittance of ITO samples with and without O-ITO layer Fig AFM images of a) the (222) textured ITO film with initial O-ITO layer and b) the ITO film without O-ITO layer by RF sputtering deposition have a surface roughness value in the range of 2.3–8.2 nm [23] The RMS surface roughness of ITO samples from 1.7 to 3.8 nm grown at different substrate temperatures was reported by Malathy et al [24] The low surface roughness of 3.7 nm obtained from our ITO film without O-ITO layer is consistent with the above published values With the presence of the O-ITO seed layer, the surface of overhead ITO film becomes smoother with lower roughness (RMS = 1.4 nm) In conjunction with the SEM images shown in Fig 2, this can be attributed to the difference in height of “domain–grains”, which seems to be a factor in inducing different surface roughness in ITO films For the ITO film without O-ITO layer, the domains with different morphologies have different protrusions on the surface of the film, and thus create steps and edges leading to high surface roughness In contrast, the ITO films grown on the O-ITO seed layer with (222) preferred orientation not show significant difference in the height of the domains or grains, thus showing a smoother surface Hall measurement results of ITO thin films prepared with and without the O-ITO seed layer are listed in Table 1, in which the O-ITO seed layer was grown at various partial oxygen gas pressures in the range from 1.3 × 10−4 Pa to 4.2 × 10-1 Pa The results show that the electrical properties are identical and stable with carrier concentration, mobility and resistivity in the range of 1021 cm−3, 40 cm2/Vs and 10−4 Ω·cm, respectively In addition, not only electrical properties but also the characteristic of transmittance spectra of samples in the wavelength range from 200 nm to 1100 nm, as shown in Fig 4, is also identical The result shows that the average transmittance in visible range of all samples is the same and over 80% Furthermore, both films have the same work function (4.7 eV and 4.77 eV) The high value of work function is required for efficient hole injection in OLEDs From consideration of the data, it is preliminarily concluded that the changing preferential orientation to prominently (222) orientated ITO film due to O-ITO seed layer has a trivial effect on the low resistivity while maintaining high transmittance in the visible range and high work function, which are necessary for using the ITO films as transparent conducting electrodes in devices It has been known that the usual film growth process can be simply expressed in two steps: 1) the nucleation process; and 2) subsequently, the growth process The effect of the oxygen partial pressure on changing the preferential orientation in the corresponding literature was considered in terms of two overall steps in the film growth process In our opinion, the preferential orientation development of crystalline grains mainly depends on the initial orientations during the nucleation process As proved in our study, the changing preferential orientation to (222) prominent plane is caused only by the partial oxygen gas pressure in the initial nucleation process Fig shows XRD patterns of ITO films with the O-ITO seed layer grown at various partial oxygen gas pressures in the range from 1.3 × 10−4 Pa to 4.2 × 10-1 Pa It can be obviously seen Table Electrical properties of ITO films with O-ITO seed layer deposited at various partial oxygen gas pressures from 1.3 × 10−4 Pa to 4.2 × 10-1 Pa Sample Base pressure (Pa) Deposition pressure (Pa) Partial oxygen pressure (Pa) Carrier concentration ×1021 cm−3 Carrier mobility cm2/V·s Resistivity ×10−4 Ω·cm S1 S2 S3 S4 S5 5.3 × 10−4 5.3 × 10−1 1.3 8.0 6.0 4.2 1.0 1.1 1.1 0.8 1.0 35 45 45 42 37 1.7 1.2 1.2 1.8 1.7 × × × × 10−4 10−4 10−3 10−1 D.P Pham et al / Thin Solid Films 570 (2014) 16–19 19 Acknowledgments We would like to thank Professor Derrick Mott (Japan Advanced Institute of Science and Technology — JAIST) for your assistance with our manuscript Your proofreading and editing greatly helped the readability of our work References Fig XRD diffraction pattern of ITO films with O-ITO layer deposited at various partial oxygen gas pressures from 1.3 × 10−4 Pa to 4.2 × 10-1 Pa that the (222) oriented crystalline plane grows strongly and becomes the preferential orientation in the overhead ITO films as the partial oxygen gas pressures in the O-ITO seed layer preparation increased As a result, it can be inferred that high partial oxygen gas pressure in the O-ITO layer deposition process is favorable to create dominant (222) crystal seed grains, from which (222) oriented crystal grains grow extensively The growth mechanism can be explained as follows: The (222) nucleation is a primary nucleation due to the natural structure of indium atom, a face-centered tetragonal structure, in which the (111) plane is the lowest energy plane At the first stage of the deposition process, the indium atoms arrived and aggregated on the surface substrate to make the (111) plane, where oxygen atom was absorbed to generate the (222) nucleation In poor oxygen partial pressure and the high substrate temperature (350 °C), the (400) nucleation can be formed competitively with the (222) nucleation because of the longer diffusion length and higher mobility of metal adatoms In the rich oxygen partial pressure at the same substrate temperature of 350 °C, the (222) nucleation is more favorable Since the thin seed O-ITO layer has a preferred (222) orientation, the following ITO deposition in Ar atmosphere grows directly on the (222) nucleation Consequently, the overhead ITO thin film grows uniquely in the (222) orientation Conclusions With the initial thin oxygen rich seed layer indium tin oxide (O-ITO) grown in mixed Ar + O2 gases, we can grow the overhead ITO films in pure Ar gas with strongly preferred (222) crystalline orientation The (222) textured ITO films have the same optical and electrical properties compared to the published (400) textured ITO films with high transmittance above 80% in the visible range with carrier concentration, mobility and resistivity in the range of 1021 cm−3, 40 cm2/Vs and 10−4 Ω·cm, respectively In 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(222) nucleation Consequently, the overhead ITO thin film grows uniquely in the (222) orientation Conclusions With the initial thin oxygen rich seed layer indium tin oxide (O-ITO)