Plasma exposure inducing crystallization of indium oxide film with improved electrical and mechanical properties at room temperature Lei Yang • Jiaqi Zhu • Jie Bai • Yuankun Zhu • Bing Dai • Hailing Yu • Zhenyu Jia • Jiecai Han Received: 24 November 2013 / Accepted: 7 May 2014 / Published online: 3 June 2014 � Springer Science+Business Media New York 2014 Abstract A novel plasma exposure technique has been introduced into conventional magnetron sputtering process to enhance the crystallization of indium oxide (In2O3) films at room temperature. The effect of plasma exposure tech- nique with different pulsed DC voltages on the electrical and mechanical properties of In2O3 films was investigated. It is observed that film crystallization can be significantly enhanced when the pulsed DC voltage (|Vp|) is higher than |-500 V| (|Vp| [ |-500 V|). By applying the plasma exposure process, In2O3 films prepared at room tempera- ture with thickness of 135 nm shows low resistivity of 4.11 9 10-4 X cm, mobility of 42.1 cm2/Vs, and trans- mittance over 80 % in the visible range. Compared with the In2O3 films without plasma exposure process, the In2O3 films with plasma exposure show better crystallization and remarkably higher nanohardness. The plasma exposure technique is a useful candidate technique for enhancing film crystallization at low temperature. Introduction Transparent conducting indium oxide (In2O3) thin films have attracted much attention due to their tremendous importance in optoelectronic devices, such as transparent electrode for flat panel displays and photovoltaic cells [1–3]. For these applications, the performances of In2O3 films are usually tailored by several factors, such as mor- phological, opto-electrical, and mechanical properties. It is vital to optimize these properties during deposition of In2O3 films on semiconductive rigid and flexible substrate. However, the properties of these substrates would degrade due to the high-temperature deposition condition which is commonly needed for the crystallization of In2O3 films. Therefore, low-temperature deposition of crystalline In2O3 films is desired in deposition In2O3 films and the related devices [4, 5]. To approach this dearth of low-temperature crystalliza- tion of In2O3 films, some deposition techniques including bias voltage technique [6, 7], ion/plasma-assisted deposi- tion [8–10], and the high power pulsed magnetron sput- tering (HPPMS) [11, 12] have been developed. The bias voltage technique can fairly improve the crystallization of films, but the quality of crystallization is limited by the poor controllability of bias voltage used for lower energetic range of ion bombardment energy. The ion/plasma-assisted deposition is limited by inefficient cost and the complica- tion of the equipment. HPPMS is beneficial to grow crys- talline films in which ionized sputtering dominate the content. However, the requirement of high power supplies in this method strongly limits its application compared to those such as conventional magnetron sputtering. In a word, there are several methods can be utilized to enhance the crystallization of In2O3 films at low temperature, yet they are some inevitable defects in practice. Therefore, it is urgent to develop a simple, cost-efficient, high-quality, and low-temperature crystalline technique. Recently, we established the low-temperature feasibility of high-quality crystalline In2O3 films deposited on glass substrates at room temperature by a conventional magnetron sputtering along with a novel plasma exposure technique L. Yang � J. Zhu (&) � J. Bai � B. Dai � H. Yu � Z. Jia � J. Han Harbin Institute of Technology, Post-Box 3010, Yikuang Street 2, Harbin 150080, People’s Republic of China e-mail:
[email protected] Y. Zhu School of Materials Science and Engineering, University of Shanghai for Science & Technology, Shanghai 200093, People’s Republic of China 123 J Mater Sci (2014) 49:5955–5960 DOI 10.1007/s10853-014-8314-0 [13, 14]. Plasma exposure technique, using high-pulsed DC voltage to strike the as-deposited films on substrates, is regarded to be beneficial for the crystallization of In2O3 films without the complication of equipments compared with the separated ion/plasma sources and without any heating process. In this work, we study the dependence of different pulsed DC substrate voltages on the structural, opto-electrical, and mechanical properties of crystalline In2O3 films by using plasma exposure technique. Experimental Film preparation In2O3 films were grown on borosilicate glass and Si (100) substrates by magnetron sputtering, respectively. In2O3 ceramic target size (99.95 % purity) was 49 mm in diam- eter. The target was mounted in the magnetrons configu- ration with a distance of 70 mm from the substrates. Before submitting into the growth chamber, the sub- strates were carefully cleaned by using acetone and alcohol in an ultrasonic bath for 30 min, respectively. The base pressure was evacuated to \2 9 10-4 Pa by turbo molec- ular pump. High-purity argon (99.99 %) with the gas flow rate of 100 standard cubic centimeter per minute (SCCM) was used as the sputtering gas. And the total growth pressure was set to be 0.5 Pa. Before film deposition, a 20 min pre-sputtering was proposed to remove the contaminants on the target surface. The In2O3 thin films were deposited at room temperature with and without the plasma exposure technique, respec- tively. The process of the deposition with plasma exposure technique composed of normal sputtering and the plasma exposure, which comprised a period. The sputtering step was made at a constant power density of 1.1 W/cm2 and no deliberate ion bombardment of the substrates. Subse- quently, the target power supply was turned off, and a metal shield was placed between the In2O3 target and the substrate to avoid mutual influence in the subsequent plasma exposure process. During the plasma exposure, the total pressure of argon was regulated to be 5.0 Pa. After that, a pulsed DC voltage with various peak voltages (|Vp|) from |-400 V| to |-900 V| was applied, and a duty cycle of 50 % was also used to the substrate by unipolar-pulsed DC power supply at a frequency of 50 kHz. Plasma was created near the film surface by this pulsed DC voltage during the plasma exposure. In this case, the ratio of the duration time between plasma exposure and sputtering was controlled to be 2 and the total period was set at 75 min. The whole deposition included five periods in this work. The process of the deposition without plasma exposure technique, only the sputtering was set to be 25 min at room temperature. Table 1 describes the details of the growing condition and thickness of the deposited films. Film characterization The film thickness was measured using Talysurf PGI 1240 profilometer with a resolution of 0.8 nm, and film com- position was characterized by energy-dispersive X-ray spectroscopy (EDS) attached to a scanning electron microscope (SEM) system (Quanta 200FEG, USA). Film structure was investigated by glancing incident X-ray dif- fraction (GIXRD, PANalytical X’ Pert Pro) with Cu Ka radiation with a wavelength of 1.54 Å. In order to reveal the spatial orientation distribution of crystalline grains, pole figures were characterized using a four-circle diffractometer (D/max-IIIA), operating at 40 kV and 40 mA. The pole figure measurement is conducted at fixed 2h according to plane of (222) and (400) by rotating the sample around the normal (azimuth angle u: 0�–360�) and parallel (polar angle v: 0�–70�) to the substrate. Film surface morphology was studied by atomic force microscope (AFM) with Veeco MultiMode. Optical trans- mittance in range of 280–850 nm was measured using a perkin elmer lambda 950 dual beam photo-spectrometer. The electrical properties were characterized by Hall mea- surements in the Van der Pauw geometry using an Ecopia HMS-3000 system with a 0.6 T magnet. The hardness of indium oxide films was obtained by MTS Nano Indenter XP system. Table 1 Thickness, crystallite size, chemical composition, and electrical properties of the thin films as the function of the pulsed DC voltage The pulsed DC voltage (V) Thickness (nm) Crystallite size (nm) O/In ratio Carrier concentration (cm-3) Mobility (cm2/Vs) Resistivity (9104 X cm) – 150 – 1.30 4.22 9 1020 26.4 5.61 -400 140 – 1.32 3.98 9 1020 29.0 5.41 -500 140 – 1.36 3.90 9 1020 38.7 4.13 -600 135 14.3 1.39 3.53 9 1020 39.9 4.43 -700 135 15.1 1.40 3.61 9 1020 42.1 4.11 -800 128 18.9 1.42 2.40 9 1020 38.0 6.84 -900 121 19.3 1.45 1.99 9 1020 33.8 9.28 5956 J Mater Sci (2014) 49:5955–5960 123 Results and discussion Structural properties The XRD patterns of indium oxide films deposited on glass substrates with and without plasma exposure technique under different pulsed DC voltages (Vp) are shown in Fig. 1a. Under the lower pulsed DC voltages (|Vp| \ |-500 V|), the In2O3 films are amorphous with XRD fea- tures as the same as the as-deposited one. The pronounced crystalline structure is observed at |Vp| [ |-500 V|, indi- cating that the plasma exposure technique with the higher pulsed DC voltages can promote the crystallinity in In2O3 films. The variation of crystalline orientation is related to the effect of the ion bombardment by energetic Ar? ions during plasma exposure. At relatively lower the pulsed DC voltages (|-500 V| \ |Vp| \ |-700 V|), it is expected that Ar? ion bombardment supplies energy to the film surface, enhancing the adatom mobility, and knocking atoms nearby the surface into the regular atomic sites of crystal- line In2O3. This is more in favor for more oxygen incor- porated into structure leading to form more the preferential orientation of (222) than (400), which is good agreement with the previous research [15]. However, at the relatively higher pulsed DC voltages (|-700 V| \ |Vp| \ |-900 V|), the change in the preferential orientations from (222) to (400) is attributed to the effect of the higher energetic Ar? ion bombardment. The radiation damage to the growing surface is a dominant process at higher energy ion bom- bardment. In contrast to the (222) plane, the (400) plane is more resistant to the high energetic ion bombardment [16, 17]. Therefore, the (400) plane becomes the preferential orientation of In2O3 films. Similar phenomena are also observed in ITO films grown at the higher radio frequency power [17]. The variation in degree of crystal orientation under two the pulsed DC voltages (|Vp| = |-500 V| and |-800 V|) is shown in Fig. 1b. Both the films show a mixed (400) and (222) texture. Under |Vp| = |-500 V|, the (400) pole figure shows that a majority of reflections are at the center of the pole, indicating that the (400) plane is moderately parallel to the substrate surface. In the pole figure (222), the most of the (222) diffracted intensity maxima are scattered around the center of the pole, showing that (222) plane nearly parallel to the substrate surface. Another group of reflection is in the form of a board ring pattern at a diffraction angle of *60�, which respond to {110} in-plane texture. In contrast to |Vp| = |-500 V|, the (400) pole figure under |Vp| = |-800 V| shows that h400i axis orientation excel- lently normal to the substrate surface, due to most of the diffracted intensity maxima concentrated at the center of the pole with few scattered reflection surrounding it. The (222) pole figure shows that the titled four spots correspond to the diffraction intensities from {110} in-plane texture. Fig. 1 a The XRD patterns of indium oxide films as function of the pulsed DC voltages. b Pole figure for (400) and (222) diffraction of In2O3 films grown on the glass substrate at different pulsed DC voltage (|Vp| = |-500 V| and |-800 V|). c Relative intensity ratio of (222)/(400) and crystallite size calculated from (400) peak with the various pulsed DC voltages J Mater Sci (2014) 49:5955–5960 5957 123 A ring pattern tending toward four spots pattern has proved the increasing degree of orientation, which is in line with the XRD study. Figure 1c shows that the relative intensity of (222)/(400) firstly increases with increasing the pulsed DC voltages (|Vp|) and then decreases. Meanwhile, to investigate the trend of crystallite size with the pulsed DC voltages, the Scherrer equation calculates the crystallite size from (400) and peak error bars are presented on the graph [18], the mean crystallite size gradually increases with increasing |Vp|, and is shown in Table 1. Surface morphology Micrographs obtained by atomic force micrographs (AFM) of In2O3 films prepared by RF magnetron sputtering with and without plasma exposure technique at various pulsed DC voltages are shown in Fig. 2a. Under the lower pulsed DC voltage (|Vp| \ |-600 V|), uniformly distributed features with no pinholes or island structures are observed over a 2 9 2 lm area. With increasing |Vp| (|Vp| [ |-600 V|), the surface of samples becomes conical shape and spaced islands emerged all over the whole surface. The different surface features of samples result from the energetic varia- tion of the Ar? ion bombardment during plasma exposure steps with a series of |Vp| (|-400 V| \ |Vp| \ |-900 V|). Under the lower pulsed DC voltage, the primary effect of the Ar? ion bombardment may enhance the surface diffusion of atoms and anneal the defects leading to uniformly distrib- uted features, whereas the main influence of the high ener- getic Ar? ion bombardment results in the damage of the grown surface at higher pulsed DC voltage [17]. Figure 2b shows root mean square (RMS) surface rough- ness as a function of the pulsed DC voltages. It is observed that the roughness of the In2O3 films increases from 0.314 to 7.23 nm monotonously as the pulsed DC voltage increases from |-400 V| to |-900 V|. When the pulsed DC voltage is below |-600 V|, the surface of samples is smoother than the as-deposited one. However, the RMS surface roughness dra- matically increases above |-700 V|. The considerable dif- ference in roughness values for samples reveals that the plasma exposure technique is a promising technique in regu- lating the surface roughness of the films. On the other hand, the gradually increase of the grain size of the In2O3 films may also contribute to the significant increase of RMS. Electrical properties The dependence of electrical resistivity (q), Hall mobility (l), carrier concentration (n) on the pulsed DC voltage of the In2O3 films are shown in Fig. 3. The electrical prop- erties of the films are highly influenced by the pulsed DC voltage. When |Vp| increases from |-400 V| to |-900 V|, the electrical resistivity initially decreases and then increases. When |Vp| = |-700 V|, the lowest resistivity of 4.11 9 10-4 X cm is achieved. Hall mobility of the films gradually increases with the rise of pulsed DC voltage, the maximum of 42.1 cm2/Vs is obtained at |Vp| = |-700 V|. On one hand, it is due to the reduction of defects in the grain because of enhancement of crystallization. Therefore, the migration velocity of the carriers is accelerated by the reduction the scattering centers caused by defects. On the other hand, the increase of Hall mobility is attributed to the decrease of grain-boundary scattering resulting from the improvement in film quality as confirmed from X-ray dif- fraction measurement [19]. At the lower of the pulsed DC voltages, carrier con- centration of the In2O3 films slowly decreases due to a reduction of amount of oxygen vacancies. While Ar? ion bombardments become stronger at |Vp| [ |-700 V|, indium and oxygen atoms will be in more favor of binding by more energetic ions bombardment. Then, the film has a compo- sition relatively close to stoichiometry leading to rapidly Fig. 2 a AFM images (2 9 2 lm) of the In2O3 film surface morphology with plasma exposure technique at different pulsed DC voltages (a as-grown, b |-500 V|, c |-600 V|, d |-700 V|, e |-800 V|, f |-900 V|). b Surface roughness of the In2O3 films as a function of the pulsed DC voltages 5958 J Mater Sci (2014) 49:5955–5960 123 decrease the O vacancy density (see in Table 1) [20]. Therefore, carrier concentration sharply reduces during relative the high-pulsed DC voltages. Optical properties The transmittance of In2O3/glass stacks as the function of the pulsed DC voltage is presented in Fig. 4. Average transmittance is[80 % from 280 to 850 nm in all of In2O3 films. Sharp absorption edges were observed around \500 nm in all the films. The absorption coefficient (a) is calculated using the relation T = exp(-ad), where T is the transmittance and d is the film thickness. The relation between the absorption coefficient and the incident photon energy (hm) is given by the following equation [21, 22]: ðahmÞ2 ¼ Aðhm � EgÞ; ð1Þ where A and Eg are optical constant and optical bandgap, respectively. The Eg can be determined by extrapolations of the linear regions of the plots to zero absorption. The inset of Fig. 4 shows the relationship (ahm)2 and hm for In2O3 films with plasma exposure technique under the different pulsed DC voltage. According to Burstein–Moss shift, bandgap decreases with the decrease in carrier con- centration. It is observed that the concentration of the films decreases with the decrease of Vp. The optical bandgap is found to be varying between 3.45 and 3.58 eV for the various pulsed DC voltages. Mechanical properties Figure 5 shows the hardness and elastic modulus of the In2O3 films on glass substrate as the function of the pulsed DC voltage. The hardness and elastic modulus of the films are determined for films over an indentation depth of *30 nm. As shown in Fig. 5, sample A deposited without plasma exposure technique has only hardness of *6.06 GPa. The values of hardness and elastic modulus increased with the decrease of the pulsed DC voltage. The improvement in hardness for these films is due to the plasma exposure tech- nique used during the preparation of samples. The plasma exposure technique stimulates a plasma near the surface, leading to ion bombardment in favor of the formation of crystalline indium oxides in order to enhance the mechanical properties. The maximum of hardness and elastic modulus existing in |Vp| = |-700 V| is *12.96, *160.03 GPa, respectively. This is because that higher energetic ions with plasma exposure technique induce durability of {400} plane oriented growth of the films [17, 18]. However, the hardness value decreases when the |Vp| is higher than |-800 V|, which is attributed to the radiation damage of surface induced by the high energetic ion bombardment. This is supported by AFM analysis which exhibits the higher surface roughness. Conclusions The plasma exposure technique was used to grow the crystallization of indium oxide films at room temperature. Fig. 3 Electrical properties of In2O3 thin films with plasma exposure technique as function of the pulsed DC voltage Fig. 4 Transmission spectra for In2O3 films with plasma exposure technique under different pulsed DC voltages (a as-grown, b |-500 V|, c |-700 V|, d |-900 V|); Inset (ahm)2 of the same series of films plotted against photon energy Fig. 5 Hardness and elastic modulus of In2O3 films with plasma exposure technique as function of the pulsed DC voltage J Mater Sci (2014) 49:5955–5960 5959 123 It is observed that plasma exposure technique is beneficial to enhance crystallization. The surface roughness of the films can also be exactly tuned by plasma exposure tech- nique. Crystalline In2O3 films exhibits excellent optical properties with transmittance over 80 % (including the glass substrates) in range of 280–850 nm. In addition, the plasma exposure technique by applying the appropriate pulsed DC voltage also improves the electrical properties of the In2O3 films. 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