Pyroelectric Response in Crystalline Hafnium Zirconium Oxide (Hf 1-x Zr x O 2 ) Thin Films

Transcription

1 Pyroelectric Response in Crystalline Hafnium Zirconium Oxide (Hf 1-x Zr x O 2 ) Thin Films S. W. Smith, 1 A. R. Kitahara, 1, a) M. A. Rodriguez, 1 M. D. Henry, 1 M. T. Brumbach, 1 J. F. 1, b) Ihlefeld 1 Sandia National Laboratories, Albuquerque, New Mexico 87185, USA Pyroelectric coefficients were measured for 20 nm thick crystalline hafnium zirconium oxide (Hf 1-x Zr x O 2 ) thin films across a composition range of 0 x 1. Pyroelectric currents were collected near room temperature under zero applied bias and a sinusoidal oscillating temperature profile to separate the influence of non-pyroelectric currents. The pyroelectric coefficient was observed to correlate with zirconium content, increased orthorhombic/tetragonal phase content, and maximum polarization response. The largest measured absolute value was 48 µcm -2 K -1 for a composition with x = 0.64, while no pyroelectric response was measured for compositions which displayed no remanent polarization (x = 0, 0.91, 1). a) Present address: Carnegie Mellon University b) Corresponding author: jihlefe@sandia.gov

2 Ferroelectric behavior in crystalline HfO 2 -based films has been reported over the last several years with the ferroelectric phase stabilized by a variety of dopants. 1,2 Owing to processing simplicity and silicon compatibility, this has attracted attention for potential nonvolatile memory applications, 3 and more recently to leverage negative differential capacitance for transistor scaling. 4, 5,6 While ferroelectricity was initially unexpected given the simplicity of the compositions and the high polarization values at thicknesses where conventional ferroelectrics suffer from scaling effects, 7 evidence is mounting that the ferroelectric effect is intrinsic to a metastable orthorhombic phase that is stabilized through doping and strain in thin layers. 8,9 As ferroelectrics constitute a class of materials displaying spontaneous polarization, a requisite property of this material class is pyroelectricity: a variation in polarization magnitude with temperature. While there is compelling evidence that the switchable polarization response observed in doped HfO 2 thin films is truly ferroelectric, there have been few reports of pyroelectricity. Those reports that do exist rely on indirect measurements using polarization-field 10, 11 response at differing temperatures or direct measurements with a linear temperature ramp. These measurement approaches are subject to errors related to separating pyroelectric currents from temperature-dependent leakage or thermally generated currents, respectively, and thus do not definitively provide evidence that spontaneous polarization response is intrinsic to the material phase, or that the material is truly pyroelectric. In this work, the pyroelectric coefficients of a composition series of (Hf 1-x Zr x )O 2 thin films (0 x 1) were measured using an oscillating temperature method to unambiguously separate pyroelectric current from other thermally induced currents. 12 Since pyroelectric current is dependent on the rate of temperature change with time when a sinusoidal temperature profile is imposed on a ferroelectric, the pyroelectric component of the current will be 90 out-of-phase with the temperature oscillations. Other thermally stimulated currents will change this ideal phase shift and are mathematically isolated and eliminated by the sine term in equation (1): i p =i 1 sin( ) = pa T 1, (1) where i p is the pyroelectric current amplitude, i 1 is the measured current amplitude, is the phase difference between the temperature and current oscillations, p the pyroelectric coefficient, A the 2

3 area of the device, the angular frequency of the temperature oscillations, and T 1 the amplitude of the temperature oscillations. The pyroelectric response is then assessed with respect to film composition, polarization response, and phase assemblage. Continuous 100 nm thick, (111)-oriented TaN bottom electrodes were deposited via 30 off-axis rf-magnetron sputtering at 3.3 W/cm 2 under 5 mtorr argon, from a TaN target onto 001- oriented n-type silicon substrates within a Kurt J. Lesker Lab 18 instrument. 20 nm thick (Hf 1- xzr x )O 2 films were then prepared on the TaN-coated silicon substrates by thermal atomic layer deposition (ALD) at 150 C in an Ultratech Savannah flow-through style reactor. Tetrakis(dimethylamino)hafnium (TDMA Hf, at 75 C) and tetrakis(dimethylamino)zirconium (TDMA Zr, at 75 C) were used as precursors for HfO 2 and ZrO 2 ALD cycles, respectively, water was used as an oxidant for both, and nitrogen as a carrier gas. 13 The relative number of cycles of TDMA Hf and TDMA Zr were adjusted to control film composition with a 10 cycle super-cycle. Blanket 20 nm thick TaN top electrodes were deposited on the ALD (Hf,Zr)O 2 films using the same conditions as used for the bottom electrode. Films were then annealed for 30 s at 600 C in a flowing nitrogen atmosphere within a rapid thermal annealer (Surface Science Integration Solaris 150). Platinum top contacts, ~200 µm in diameter and 90 nm thick, were deposited via rf-magnetron sputtering at 6.6 W/cm 2 under 3 mtorr argon through a shadow mask. Individual electrodes were isolated using reactive ion etching with the platinum contacts serving as a hard mask. Samples were mounted to an anodized aluminum carrier and etched within a PlasmaTherm VersaLine inductively coupled plasma reactive ion etcher (ICP RIE). A plasma etch chemistry similar to that of a mixed-mode Bosch silicon etch, 14 (ICP power of 900 W, bias power of 15 W, and SF 6 and C 4 F 8 gas flows of 50 and 90 sccm, respectively, and a chamber pressure of 15 mtorr) was used. This etch chemistry creates a fluorine-rich plasma environment, which anisotropically etches the TaN film with minimal contribution to the etch rate from milling while placing a thin CF x Teflon-like coating along the metal sidewalls. 14 Total etch time was 30 seconds. Polarization-electric field (P(E)) measurements were conducted with a Radiant Precision Multiferroic Workstation with a 100 Hz sweep rate. Plotted data is from the 4 th set of nested loops collected up to 6 V to minimize initial wake up effects. 15 Pyroelectric currents were 3

4 measured by affixing the sample (with silver epoxy), and a thermocouple (Omega CO1 K-type, with Kapton tape and thermally conductive paste) to a 1.1 mm thick alumina coupon. The coupon was affixed to a Peltier cooler (TE Technology, model VT ) driven by a 10 V peak-peak, 15 mhz sinusoidal wave generated by a Keysight 33500B function generator. A diagram of the measurement geometry is provided as Figure S1 in the Supplemental Information. A Hewlett-Packard 4192 LF impedance analyzer was used to polarize the samples with a 4 V bias (2 MV/cm) and to measure the capacitance and loss tangent at 1 khz with a 30 mv rms oscillator level prior to measuring the pyroelectric current. A Keithley 6512 electrometer was connected to the top and bottom contacts on the (Hf 1-x Zr x )O 2 films to measure the pyroelectric current generated by the temperature oscillations under zero applied bias. A Siemens model D500-2 powder diffractometer using Cu K radiation and a diffracted beam LiF monochromator was used for Grazing-Incidence X-Ray Diffraction (GIXRD) measurements to determine film phase assemblage. Three phases are commonly observed in (Hf, Zr)O 2 -based ferroelectrics: a monoclinic linear dielectric phase associated with HfO 2 -rich compositions, an orthorhombic ferroelectric phase associated with intermediate compositions, and a tetragonal antiferroelectric-consistent phase, associated with ZrO 2 rich compositions. 8, The orthorhombic and tetragonal phases have similar diffraction patterns and cannot be separated by GIXRD and will be referred to as the orthorhombic/tetragonal phase in this report. A series of diffraction patterns were collected using grazing angles of 0.5, 1.0, 1.5, and 2.0. The patterns were collected using the following parameters: range, step-size of and a count time of 30 seconds. No instrumental artifacts were detected in the patterns (e.g. Laue peaks), which allowed all patterns to be merged to improve data quality. The relevant peak locations to distinguish the monoclinic and orthorhombic/tetragonal phases occurred in the range. Because the patterns showed a significantly broad peak above associated with the TaN electrode, the XRD patterns had the background modeled and subtracted in this range within the program Jade 9.6 (Materials Data, Inc. Livermore, CA) so as to better compare the peaks for the monoclinic and orthorhombic/tetragonal phases. To improve clarity, the XRD patterns were smoothed using a Savitzky-Golay (17 point) filter within the Jade program. Refined weight fractions of monoclinic and orthorhombic/tetragonal phases were derived based on profile fitting of peak areas and 4

5 reference intensity ratios (RIR) to derive phase fractions. The orthorhombic/tetragonal (111 orthorhombic, 011 tetragonal) reflection at ~ was fit along with the two reflections from the monoclinic phase at 28.4 and , 17, 20 which represent the 1 01 and 111 peaks, respectively. The total range in the data that was fit was 28-33º 2. X-ray photoelectron spectroscopy was performed within a Kratos Axis Ultra DLD instrument. To accurately measure film compositions, a thin layer of the film was sputtered away using a Kratos Minibeam III ion gun. Monochromatic Al Kα ( ev) X-rays operating at 120 W were utilized for excitation. The analysis area was an elliptical area of 300 x 700 microns. Survey spectra were recorded with 160 ev pass energy, 500 mev step sizes, and 100 ms dwell times averaged over 12 scans. Data processing was performed with CasaXPS Version The binding energy axes of all spectra were aligned to the main component of the C 1s peak to ev. Kratos relative sensitivity factors built-in to CasaXPS were used for quantification, C 1s at 1.0, F 1s at 3.6, O 1s at 2.81, and Zr 3d at ZrO 2 samples were analyzed to check the ratio of Zr:O ratio of 1:2. The Hf 4f relative sensitivity factor was adjusted to 7.00 to obtain a 1:2 ratio for Hf:O in HfO 2. Peak areas were obtained using Shirley backgrounds for all components. Polarization-field response for compositions of x = 0, 0.32, 0.64, and 1 are displayed in Figure 1 with responses for all compositions provided in Figure S2 of the Supplementary Materials. The XPS-determined zirconium fraction is provided as the actual composition along with the ALD cycle fraction ratio in each plot. Each film contains slightly less zirconium than may be expected from the cycle ratio owing to the smaller growth per cycle of ZrO 2 relative to HfO 2. From the data, it is seen that pure HfO 2 (Fig. 1(a)) is a linear dielectric with no notable nonlinear response or opening of the polarization curves. As the ZrO 2 content increases, the P(E) response opens up (Figs. 1(b) and (c)), consistent with that expected for a ferroelectric material and prior observed responses. 16 The ZrO 2 end-member composition displays an antiferroelectriclike behavior at high fields (2.5 MV/cm), where the polarization response becomes non-linear with hysteresis, but no apparent remanence of the polarization is observed. For the films displaying a ferroelectric-consistent response (x = 0.23 to 0.64), the polarization continues to increase with increased driving field. While evidence of polarization saturation is observed, some of this effect can also be attributed to a finite level of leakage current, which is present in the 5

6 highest driving field condition, 3 MV/cm. 21 This notwithstanding, the polarization responses for these compositions display ferroelectric-consistent behavior. Unlike prior research where a peak polarization magnitude was observed for an ~x = 0.50 composition, 16 the maximum polarization in this work was observed for an x = 0.64 composition. The origin of the composition difference of peak polarization is unclear, but may be related to different processing conditions, film thicknesses, 22 and electrodes between the two studies resulting in differing phase assemblages. To complement the polarization responses, the GIXRD patterns for each composition over the 2 range of 26 to 33 are shown in Figure 2. HfO 2, shown on the bottom, possesses peaks consistent with the 1 11 and 111 reflections of the monoclinic phase. 20 As the zirconium content increases, the monoclinic phase peak intensities decrease as the orthorhombic/tetragonal phase peak intensities (~30.2 in 2 ) increase. The calculated weight fractions of each phase are provided in Fig. 2(b). It is observed that increased zirconium content results in films possessing larger fractions of orthorhombic/tetragonal phase with respect to the monoclinic phase and the films with a higher orthorhombic/tetragonal phase content display a larger polarization response with applied field for compositions containing up to x = The full diffraction patterns collected for each composition are provided in Figure S3 in the Supplementary Materials. Two examples of the pyroelectric measurement output, one for a composition that showed polarization-field hysteresis with a finite level of remanent polarization (Hf 0.36 Zr 0.64 O 2 ) and one that had no apparent remanent polarization (ZrO 2 ), are shown in Figures 3(a) and 3(b), respectively. For the Hf 0.36 Zr 0.64 O 2 composition, sinusoidal oscillations of the current are observed for a sinusoidal temperature profile with an amplitude of ±2.8 K at room temperature. Fitting both the current and temperature data to sinusoidal curves reveals that the current response is 94 degrees out-of-phase with the temperature signal and that the current density amplitude was ±1.25 x10-9 A cm -2, which equates to a calculated absolute pyroelectric coefficient of 48 µcm -2 K -1. The 4 non-ideal phase shift was considered in the coefficient calculation. The origin of the phase shift is not explicitly known, but may be due to thermally stimulated currents. For the ZrO 2 composition, the measured current response to the sinusoidal temperature oscillations resulted in no measured pyroelectric current, as no clear sinusoidal current could be identified within the limits of our measurement system (5 fa). Since ZrO 2 displayed no remanent 6

7 polarization, this lack of pyroelectric current suggests a lack of spontaneous polarization. The pure HfO 2 composition, which displayed a linear dielectric response and no remanent polarization, also revealed no pyroelectric response. The pyroelectric measurement output for all compositions can be seen in Figure S4 in the Supplementary Materials. The absolute pyroelectric coefficients for the full composition array are shown in Figure 4 and summarized in Table I. Pyroelectric currents were measured and coefficients calculated for compositions of 0.23 x 0.91 and increased as the ZrO 2 content increased, peaking for the x = 0.64 composition with a value of 48 µcm -2 K -1, before decreasing at higher ZrO 2 contents. This trend is consistent with the composition-dependent polarization response. The increase in pyroelectric coefficient appears to correlate with the apparent increase in orthorhombic/tetragonal phase fraction. However, it is unclear at this time if the relative phase fraction differences drive changes in magnitude of the polarization and pyroelectric coefficient, or if an intrinsic increase in polarization and pyroelectric response is occurring along with compositional change. Additional studies on phase-pure materials would be warranted to isolate this effect. Regardless, the polarization magnitude, pyroelectric coefficient, and fraction of orthorhombic/tetragonal phase appear to be correlated noting that the pure HfO 2 and ZrO 2 films did not demonstrate a measurable pyroelectric response. The pyroelectric coefficient, relative permittivity and loss tangent measured at 1 khz, and the infrared detector figures of merit (F I : current responsivity, F V : voltage responsivity, and F D : signal to noise figure of merit) 23 for the (Hf 1-x Zr x )O 2 thin films are given in Table I. The pyroelectric coefficient for the x = 0.64 film is similar to that reported for silicon-doped HfO 2 measured with the direct pyroelectric current measurement with a linear temperature ramp (52 µcm -2 K -1 ), 11 and reported values for PVDF (25 µcm -2 K -1 ), 24 although they are lower than what has been measured for commonly used ferroelectric pyroelectrics, such as LiTaO 24 3 or PZT. 25 However, (Hf 1-x Zr x )O 2 films have a much lower dielectric constant than PZT or LiTaO 3 and result in infrared figures of merit that may be compelling for device applications. Additionally, HfO 2 -based ferroelectrics have potential device integration advantages, including: being easily formed into conformal thin films that can be prepared on high aspect ratio or tortuous substrates, a CMOS compatible material set, stable in multiple annealing 7

8 environments 26 for process integration, and as oxides they are likely more robust at elevated temperatures than polymer films like PVDF. Further, from the perspective of applications where large changes in polarization with temperature may be problematic, the low pyroelectric coefficient could be advantageous in possessing a limited response. In summary, the polarization and pyroelectric response in 20 nm thick (Hf 1-x Zr x )O 2 films for a range of compositions was assessed. Increased polarization magnitude with increasing ZrO 2 content was observed for compositions up to x = Films that exhibited remanent polarization also displayed a pyroelectric response with the maximum absolute pyroelectric coefficient measured at 48 µcm -2 K -1 for the x = 0.64 composition. The composition dependence of the pyroelectric response magnitude trended with the polarization magnitude and with the content of the orthorhombic/tetragonal phase fraction. The observation of a pyroelectric response for (Hf 1- xzr x )O 2 films that display a switchable polarization provides additional evidence that the polarization is intrinsic to the crystal structure for these compositions and that the polarization hysteresis observed is truly a ferroelectric response. See supplementary material for Figure S1 a pyroelectric measurement Peltier coolerthermocouple-sample schematic, Figure S2 polarization-electric field curves for the full composition array, Figure S3 the full 10º to 55º 2 GIXRD patterns for the full composition array, and Figure S4 pyroelectric current and temperature oscillations for the full composition array. The authors gratefully acknowledge Dr. Paul G. Clem for his critical review of this manuscript. This work was supported by the Laboratory Directed Research and Development Program at Sandia National Laboratories, a multi-mission laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy s National Nuclear Security Administration under Contract DE-AC04-94AL

9 References: 1. M. H. Park, Y. H. Lee, H. J. Kim, Y. J. Kim, T. Moon, K. D. Kim, J. Müller, A. Kersch, U. Schröder, T. Mikolajick and C. S. Hwang, Adv. Mater. 27 (11), (2015). 2. T. S. Böscke, J. Müller, D. Bräuhaus, U. Schröder and U. Böttger, Appl. Phys. Lett. 99 (10), (2011). 3. J. Müller, T. S. Böscke, S. Müller, E. Yurchuk, P. Polakowski, J. Paul, D. Martin, T. Schenk, K. Khullar, A. Kersch, W. Weinreich, S. Riedel, K. Seidel, A. Kumar, T. M. Arruda, S. V. Kalinin, T. Schlösser, R. Boschke, R. v. Bentum, U. Schröder and T. Mikolajick, Electron Devices Meeting (IEDM), 2013 IEEE International, (2013). 4. K.-S. Li, P.-G. Chen, T.-Y. Lai, C.-H. Lin, C.-C. Cheng, C.-C. Chen, Y.-J. Wei, Y.-F. Hou, M.-H. Liao, M.-H. Lee, M.-C. Chen, J.-M. Sheih, W.-K. Yeh, F.-L. Yang, S. Salahuddin and C. Hu, Electron Devices Meeting (IEDM), 2015 IEEE International 15, (2015). 5. C. H. Cheng and A. Chin, IEEE Electron Device Lett. 35 (2), (2014). 6. M. Hoffmann, M. Pešić, K. Chatterjee, A. I. Khan, S. Salahuddin, S. Slesazeck, U. Schroeder and T. Mikolajick, Adv. Funct. Mater. 26 (47), (2016). 7. J. F. Ihlefeld, D. T. Harris, R. Keech, J. L. Jones, J.-P. Maria and S. Trolier-McKinstry, J. Am. Ceram. Soc. 99 (8), (2016). 8. X. Sang, E. D. Grimley, T. Schenk, U. Schröder and J. M. LeBeau, Appl. Phys. Lett. 106 (16), (2015). 9. T. Shiraishi, K. Katayama, T. Yokouchi, T. Shimizu, T. Oikawa, O. Sakata, H. Uchida, Y. Imai, T. Kiguchi, T. J. Konno and H. Funakubo, Appl. Phys. Lett. 108 (26), (2016). 10. M. H. Park, H. J. Kim, Y. J. Kim, T. Moon, K. D. Kim and C. S. Hwang, Nano Energy 12, (2015). 11. M. Hoffmann, U. Schröder, C. Künneth, A. Kersch, S. Starschich, U. Böttger and T. Mikolajick, Nano Energy 18, (2015). 12. E. J. Sharp and L. E. Garn, J. Appl. Phys. 53 (12), 8980 (1982). 13. D. M. Hausmann, E. Kim, J. Becker and R. G. Gordon, Chem. Mater. 14, (2002). 9

10 14. H. V. Jansen, M. J. de Boer, S. Unnikrishnan, M. C. Louwerse and M. C. Elwenspoek, J. Micromech. Microeng. 19 (3), (2009). 15. D. Zhou, J. Xu, Q. Li, Y. Guan, F. Cao, X. Dong, J. Müller, T. Schenk and U. Schröder, Appl. Phys. Lett. 103 (19), (2013). 16. J. Müller, T. S. Böscke, U. Schröder, S. Müller, D. Brauhaus, U. Böttger, L. Frey and T. Mikolajick, Nano Lett. 12 (8), (2012). 17. L. Lutterotti and P. Scardi, J. Appl. Crystallogr 23 (4), (1990). 18. Q. Zeng, A. R. Oganov, A. O. Lyakhov, C. Xie, X. Zhang, J. Zhang, Q. Zhu, B. Wei, I. Grigorenko, L. Zhang and L. Cheng, Acta Crystallogr. C Struct. Chem. 70 (Pt 2), (2014). 19. S. Müller, J. Müller, A. Singh, S. Riedel, J. Sundqvist, U. Schröder and T. Mikolajick, Adv. Funct. Mater. 22 (11), (2012). 20. F. M. Spiridonov and L. N. Komissarova, Russ. J. Inorg. Chem. (Engl. Transl.) 15, 445 (1970). 21. J. Franco and G. Shirane, Ferroelectric Crystals. (Dover Publications, New York, 1993). 22. M. Hoffmann, U. Schröder, T. Schenk, T. Shimizu, H. Funakubo, O. Sakata, D. Pohl, M. Drescher, C. Adelmann, R. Materlik, A. Kersch and T. Mikolajick, J. Appl. Phys. 118 (7), (2015). 23. R. W. Whatmore, Rep. Prog. Phys. 49 (12), (1986). 24. B. Ploss and S. Bauer, Sensor. Actuat. A-Phys. 26 (1-3), (1991). 25. R. W. Whatmore, J. Electroceram. 13, (2004). 26. M. H. Park, H. J. Kim, Y. J. Kim, T. Moon, K. D. Kim, Y. H. Lee, S. D. Hyun and C. S. Hwang, J. Mater. Chem. C 3 (24), (2015). 10

11 Table and Figure Captions: TABLE I. Absolute pyroelectric coefficient, relative permittivity, loss tangent, and IR detector figures of merit for (Hf 1-x Zr x )O 2 composition array. FIG.1. Polarization-field response for 20 nm thick (a) HfO 2, (b) Hf 0.68 Zr 0.32 O 2, (c) Hf 0.36 Zr 0.64 O 2, and (d) ZrO 2 with ALD cycle ratio and XPS determined compositions provided for each. FIG. 2. (a) GIXRD patterns for the (Hf 1-x Zr x )O 2 composition array, labeled with mole fraction ZrO 2, with bar markers denoting the locations of the peaks for the monoclinic 20 and orthorhombic 8, 18 and tetragonal 17 phases. (b) Calculated weight percentages for monoclinic and orthorhombic/tetragonal phases as a function of mole fraction ZrO 2. FIG. 3. Measured room temperature pyroelectric current response and temperature oscillations for 20 nm thick (a) Hf 0.36 Zr 0.64 O 2 and (b) ZrO 2 films. The measured current from the Pt/TaN/(Hf,Zr)O 2 /TaN capacitor structure is shown in black and the temperature profile is shown in blue as a function of time. FIG. 4. Room temperature absolute pyroelectric coefficient for 20 nm (Hf 1-x Zr x )O 2 films as a function of mole fraction ZrO 2 determined by XPS. Pure HfO 2 and ZrO 2 films displayed no pyroelectric response and are indicated by red diamonds at 0 µcm -2 K

12 TABLE I. HfO 2 Hf 0.77 Zr 0.23 O 2 Hf 0.68 Zr 0.32 O 2 Hf 0.58 Zr 0.42 O 2 Hf 0.44 Zr 0.56 O 2 Hf 0.36 Zr 0.64 O 2 Hf 0.22 Zr 0.78 O 2 Hf 0.09 Zr 0.91 O 2 ZrO 2 p (µcm -2 K -1 ) r (1kHz) tan (1 khz) F V (m 2 C -1 ) F D 10 6 (Pa -1/2 ) F I (mv -1 )

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