Temperature effects on sputtered ITO

Temperature effects on sputtered ITO F. Menchini1, L. Serenelli1,2, G. Stracci1, M. Izzi1, E. Salza1, D. Caputo2, G. de Cesare2 and M. Tucci1 1 ENEA, Casaccia Research Centre, Via Anguillarese 301, 00123 Rome (Italy) 2 DIET, University of Rome “Sapienza”, Via Eudossiana 18, 00184 Rome (Italy) Abstract — Indium Tin Oxide (ITO) is widely used in solar cell devices for its excellent electrical and optical characteristics, such as high transparency in the Ultraviolet-Visible range and good conductivity (around 104 :-1 cm-1). In this work we have compared thin (70-150 nm) ITO layers deposited by Direct Current or Radio Frequency sputtering. We have used different substrate temperatures during film growth and have afterwards thermally annealed the samples at different temperatures up to 300°C to investigate the effects on the electrical and optical properties of the material. We have found out that the different growth/annealing conditions induce changes in the optical properties of the samples as well as in the conductivity and carrier concentration. I. INTRODUCTION In heterojunction solar cells, a key point for achieving high efficiency is the deposition of a Transparent Conductive Oxide (TCO) layer as full-area contact on both base and emitter side of the solar cell. Various TCOs have been studied through the years as alternatives to Indium Tin Oxide (ITO), to circumvent its relative high cost and Indium shortage in nature. Aluminum Oxide, Tungsten Oxide and Cerium Oxide [1-3] have attracted interest in the recent past, but ITO still remains the most widely used as TCO in heterojunction (HJ) solar cells [4,5] manufacturing because of its excellent electrical and optical characteristics, such as high transparency in the ultravioletvisible range and good conductivity, typically 80% and 104 S cm-1 respectively for a thin film also useful as antireflection coating [6]. Among the available deposition techniques for ITO growth, the most efficient and widely used is Physical Vapor Deposition (PVD). Both Radio Frequency (RF) and Direct Current (DC) magnetron sputtering are suitable to obtain highquality films for photovoltaic applications. The DC process is very simple and easy to be scaled up on large area, with relatively high growth rates. In turn, the RF process requires the use of a matching network between the power generator and the target, and results in lower deposition rates. If this latter could be a drawback for scalability, it could represent an advantage because of the lower ion bombardment, which, in principle, should reduce stress on the underlying films. In this work we have studied the intrinsic properties of ITO for Photovoltaic applications in silicon heterojunction solar cells [C], where thin films are used to work also as antireflection coating. In particular, we have grown thin (<150 nm) ITO films by in-line, large area DC and RF magnetron sputtering, at different power densities and substrate temperatures. The samples have been subsequently thermally annealed up to 300°C to explore the effects of temperature 978-1-5386-8529-7/18/$31.00 ©2018 IEEE during and after deposition. The results obtained on DC- and RF-grown samples have been compared and discussed. II. EXPERIMENTAL ITO samples have been deposited on Corning glass in an inline, large area deposition system by DC Magnetron Sputtering at a constant pressure of 1.1u10-3 mbar in Argon (Ar) without Oxygen addition. The target was an Indium–Tin alloy with a 90%:10% In:Sn weight ratio and 170 cm2 area. The power has been varied between 200 and 400 W and the substrate temperature during deposition has been varied between Room Temperature (RT) and 180°C. Similar samples have been grown by 13.56 MHz RF sputtering from a 6 inches diameter round target of same ITO alloy at 100W power and 3.3u10-3 mbar pressure, at various substrate temperatures between RT and 180°C. Table I shows a summary of different growth conditions for which the most significant effects have been observed. Each sample name actually refers to several glasses covered with ITO produced with the same sets of deposition parameters. The samples have been characterized in terms of Reflectance (R) and Transmittance (T) by a Perkin Elmer Lambda 950 UltraViolet-Visible-NearInfrared (UV-Vis-NIR) spectrophotometer. Bulk film resistivity has been evaluated by measuring the sheet Resistance (Rsh) with a Four Point Probe tool and thickness by a Tencor Alphastep contact profilometer. After deposition, the samples have been thermally annealed in a static IR furnace under Nitrogen flux for 10 minutes at different temperatures from 100°C to 300°C, and again characterized. Two most representative samples (DC6 and DC7) have been also characterized in terms of mobility (P), free carrier density (N) and resistivity (U) in a Biorad Hall profiler according to the Van der Paw configuration. III. RESULTS AND DISCUSSION We have started by characterizing the DC-grown samples. The absorption coefficient D has been calculated by the approximated formula ͳ ሺͳ െ ܴሻ ߙ ൌ ݈݊ ‫ݐ‬ ܶ where t is the film thickness in cm, R is the Reflectance and T the Transmittance measured as a function of wavelength. 3128 TABLE I SUMMARY OF ITO GROWING CONDITIONS RF RF1 RF2 Dep. T (°C) RT RT RT 130 180 100 100 RT 180 1.5 In Fig. 1 the absorption coefficients of samples grown at RT and different powers are reported, both for the as deposited (as-dep) samples and after a thermal treatment at 300°C. It can be seen that the as-dep samples show only small differences. Thermal annealing induces a lowering of absorption in the Vis range, and the sample grown at lower power becomes more transparent than the others. 5 D (cm-1) 4 10 300 400 500 600 700 1.0 0.5 0.0 0 50 100 150 200 300 Annealing temperature (°C) Fig. 2. Resistivity as a function of thermal annealing temperature in samples grown at RT and different powers. Lines are only a guide for the eye. DC1-RT DC4-130°C DC5-180°C 800 5 10 Fig. 1. Absorption coefficient before (solid line) and after (dashed line) thermal annealing at 300°C in samples grown at different powers and RT. In Fig. 2 we report the effects on bulk resistivity (U) of thermal annealing performed in several temperature steps on the same samples reported in Fig. 1. All samples show a steplike decreasing trend of U with increasing annealing temperature. The transition temperatures for the three samples, defined as the temperature at which the slope of the curves in Fig. 2 is maximum and the ITO films change drastically their characteristics, are very similar and decrease from about 195°C to about 180°C with increasing deposition power. The U values for the as-dep samples decrease with increasing power, while the values reached after the 250°C treatment are comparable for all samples. D (cm-1) Wavelength (nm) 978-1-5386-8529-7/18/$31.00 ©2018 IEEE 250 The differences between samples grown at different powers are not pronounced, therefore we have focused on the 200 W power, because it has been observed that the lower the power, the lower the damages induced on underlying amorphous layers as commonly used in heterojunction solar cells [8]. Another set of samples has been grown at 200 W and different substrate temperatures. Fig. 3 shows the absorption coefficient before and after thermal treatment. In this case, the as-dep samples show noticeable differences, and the higher the deposition temperature, the lower the absorption. After a thermal treatment, the absorption decreases for all samples in the UV-Vis range. DC1-200W DC2-300W DC3-400W 10 DC1-200W DC2-300W DC3-400W -3 Power (W) 200 300 400 200 200 U(10 : cm) DC Sample DC1/DC6 DC2 DC3 DC4 DC5/DC7 4 10 300 400 500 600 700 800 Wavelength (nm) Fig. 3. Absorption coefficient before (solid line) and after (dashed line) thermal annealing at 300°C in samples grown at different substrate temperatures and 200 W. 3129 DC1-RT DC4-130°C DC5-180°C 1.5 Concerning the RF samples, their absorption coefficients are much lower than those of the DC samples, and the higher the deposition temperature, the better the transparency in the 400-1200 nm range. Samples grown at RT show a slight increase in transparency after a thermal annealing at 180°C. DC6-RT DC6-RT-annealed DC7-180°C RF1-RT RF1-RT-annealed RF2-180°C 5 10 D (cm-1) Fig. 4 shows the variations of resistivity with thermal treatment temperature on the same samples reported in Fig. 3. Again, as for the samples grown at different powers (Fig. 2), a decreasing trend with increasing annealing temperature is observed, and the lower the deposition temperature, the higher the resistivity. In this case the transition temperatures are very different for the three samples, and decrease from about 195°C for the samples grown at RT to 160°C and 155°C for the samples grown at higher temperatures. As for the samples grown at different powers (Fig. 2), the U values after thermal annealing at 250°C become comparable for all the ITO films grown at different temperatures. 4 10 400 1.0 -3 U(10 : cm) 3 10 600 800 1000 1200 Wavelength (nm) 0.5 0.0 Fig. 5. Absorption coefficient in the UV-Vis-NIR spectral range for samples grown by DC and RF sputtering at different temperatures. 0 50 100 150 200 250 300 Annealing temperature (°C) Electrical characterizations have been performed on the samples in terms of mobility (P), free carrier density (N) and resistivity (U), and the results are reported in Figs. 6, 7 and 8. 1.4 Fig. 4. Resistivity as a function of thermal annealing temperature in samples grown at 200W and different substrate temperatures. Lines are only a guide for the eye. 978-1-5386-8529-7/18/$31.00 ©2018 IEEE 1.0 -3 U (10 : cm) On the basis of the previous observations, we have selected the two deposition conditions corresponding to the most different measured properties, i.e. 200 W power and RT and 180°C substrate temperature, respectively. New sets of samples (DC6 and DC7), twins of DC1 and DC5, have been deposited. The samples have been thermally annealed at different temperatures and optically characterized in terms of Reflectance and Transmittance in a range extended to 1200 nm, so as to cover the one of interest for crystalline silicon based heterojunction solar cells. Electrical characterizations have been carried out by Hall measurements. Analogous sets of samples (RF1 and RF2) have been deposited by RF sputtering and compared to DC6 and DC7. Figure 5 shows the absorption coefficient of the samples grown at RT and 180°C by both DC and RF process. It is evident that the DC samples are less transparent than the RF ones in the whole spectral range. For DC samples, a thermal annealing at 180°C induces only small changes on D, because the transition temperature has not been reached yet. On the other hand, growing the sample directly at 180°C leads to a higher improvement of transparency in the 400-650 nm range. DC6-RT DC7-180°C RF1-RT RF2-180°C 1.2 0.8 0.6 0.4 0.2 0.0 0 50 100 150 200 250 300 Annealing temperature (°C) Fig. 6. Resistivity as a function of annealing temperature for samples grown at different substrate temperatures, by both DC and RF sputtering. Lines are only a guide for the eye. From Fig. 6 it is seen that the resistivity of the as-dep RF samples (around 4u10-4 :cm) is less influenced by the deposition temperature with respect to that of DC samples, which varies from 1.2u10-4 to 0.4u10-4 :cm for RT and 180°C deposition, respectively. In both RF samples U is lower 3130 21 3x10 DC6-RT DC7-180°C RF1-RT RF2-180°C 21 -3 N (cm ) 2x10 21 1x10 20 1x10 0 50 100 150 200 250 300 Tann (°C) Fig. 7. Variation of free carrier concentration of ITO samples deposited by DC and RF sputtering at RT and 180°C after different thermal annealing treatments. Lines are only a guide for the eye. From Fig. 8 it is immediate to note that just after deposition the RF samples are characterized by a higher mobility (26.6 cm2/V s) with respect to the one of the DC samples (10 cm2/V s). The mobilities of RF and DC samples show a decreasing and increasing trend, respectively, with increasing annealing temperature, with a convergence of the values for all samples around 16 cm2/V s. The coupled analysis of N and μ can explain the resistivity variations reported in Fig. 6. For the DC samples, the global increase of both N and P with increasing annealing temperature results in a corresponding decrease of U. For the RF1 sample, the annealing causes a decrease in both μ and N, with an enhancement of resistivity, especially at 300°C. In 978-1-5386-8529-7/18/$31.00 ©2018 IEEE sample RF2 a compensation between the increase in N and the decrease in μ with increasing annealing temperature does not produce large variations in the film resistivity. DC6-RT DC7-180°C RF1-RT RF2-180°C 40 30 2 P (cm /Vs) than that of the RT grown DC sample, but comparable to that of the DC sample deposited at 180°C. The effect of thermal annealing on resistivity is quite different for the 4 sets of samples. While for both DC samples a step-function decrease is observed, confirming what shown in Fig. 4, the RF samples show different behaviors depending on the deposition temperature. Indeed, for ITO deposited at 180°C (RF2), a small increment with thermal annealing temperature is recorded. On the other hand, for the films deposited by RF power at RT (RF1) a considerable increase is observed. Figures 7 and 8 show respectively the free carrier density and mobility and their variation after thermal treatment. The N trends in Fig. 7 can be directly correlated to those of U values in Fig. 6. Indeed the relative differences in N values of the DC samples before thermal annealing reflect those of U (Fig. 6), with values of 1.6×1021 cm-3 for the DC sample grown at RT, and lower but similar values (around 6.5×1020 cm-3) for the DC sample grown at 180°C and for both the RF samples. After annealing, the N values for the DC samples have raised up to over 2.2×1021 cm-3, while the RF samples show a much lower variability. 20 10 0 0 50 100 150 200 250 300 Tann (°C) Fig. 8. Mobility of ITO samples deposited by DC sputtering at RT and 180°C, measured after different thermal annealing treatments. As a comparison N of a RF sputtered ITO is shown as blue star. Lines are only a guide for the eye. Summing up all these considerations, it can be established that the RF technique is able to produce ITO films having lower resistivity, higher mobility and lower carrier concentration with respect to the DC samples grown at RT. DC samples grown at 180°C are comparable to RF samples in terms of electrical parameters, even if their transparency remains lower. However the resistivity of films deposited by RF can be negatively affected by a thermal treatment, especially when carried out at temperatures above 180°C. Layers deposited via DC sputtering can suffer from very high free carrier absorption, affecting the transparency in the IR part of the spectrum, especially when deposited at 180°C, due to N concentrations in the order of 1021 cm-3. Thermal treatments further increase this concentration as well as the mobility, which corresponds to an improvement in transport properties. The illustrated different electro-optical properties of ITO films grown by DC or RF techniques can be exploited when designing the cells manufacturing process sequence, by choosing between the different deposition conditions depending on the properties to be privileged, i.e. transparency or conductivity, and on the thermal process the device has to undergo. IV. CONCLUSIONS In this work we have shown the effect of temperature on DC and RF sputtered ITO thin films. We have considered both substrate heating during the deposition and subsequent 3131 thermal treatments, finding out that in all cases higher temperatures lead to better results in terms of transparency. For DC sputtered ITO, mobility has been found to be slightly dependent on substrate deposition temperature and subsequent thermal treatments. The free carrier concentration has resulted higher for samples deposited at 180°C, with an increase after thermal annealing for both deposition temperatures. These effects are reflected on the absorption coefficient, which is lower in the IR part of the spectrum for samples deposited at higher temperatures. The RF sputtering technique produces ITO film with better characteristics just after deposition with respect to samples grown by DC sputtering at RT. In particular, the absorption coefficient is lowered by almost one order of magnitude in the 450-1200 nm range, while the resistivity is less than one half. Nevertheless the conductivity of these films is worsened by post-deposition thermal treatments, especially when performed at temperatures above 180°C. The knowledge of the different temperature responses of DC and RF sputtered ITO layers could be beneficial when designing the device manufacturing process sequence. [2] [3] [4] [5] [6] [7] [8] REFERENCES [1] M. Wimmer, F. Ruske, S. Scherf, B. 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