APPLIED PHYSICS LETTERS

VOLUME 73, NUMBER 7

17 AUGUST 1998

Ferroelectric characterization of highly „0001…-oriented YMnO3 thin films grown by chemical solution deposition Woo-Chul Yi,a) Joon-Seon Choe, Chang-Rok Moon, and Sook-Il Kwun Department of Physics, Seoul National University, Seoul 151-742, Korea

Jong-Gul Yoon Department of Physics, University of Suwon, Kyung-gi-do 445-743, Korea

~Received 27 April 1998; accepted for publication 11 June 1998! Highly ~0001!-oriented thin films of YMnO3 were grown directly on Si substrates by chemical solution deposition. The crystallinity of the films was investigated by using x-ray diffraction: u –2u scan, rocking curve, and pole figure. Analysis of the x-ray photoelectron spectroscopy data and Rutherford backscattering spectroscopy spectrum showed that the films had a single phase of stoichiometric YMnO3. The ferroelectric properties of YMnO3 were investigated by measuring the temperature dependence of the capacitance–voltage characteristics in the metal/ferroelectric/ semiconductor structure. Screening of the ferroelectricity of YMnO3 thin film at room temperature was discussed in conjunction with the charge effects. © 1998 American Institute of Physics. @S0003-6951~98!01433-8#

Nonvolatile memory devices using ferroelectric ~FE! materials have attracted much attention in recent years. One of these devices is in the form of a metal/FE/semiconductor field effect transistor ~MFSFET! as a nondestructive readout device. So far, several FE/Si structures have been fabricated using such oxide ferroelectrics as Bi4Ti3O12 , PbZrx Ti12x O3, and SrBi2Ta2O3. 1–3 However, it is difficult to fabricate ferroelectric oxide films directly on Si substrates because of the thermodynamic reactions between the constituent elements of the film and the substrate. Recently, YMnO3 thin film has been proposed as another promising candidate.4 YMnO3 has several advantages compared with other ferroelectric materials, especially for the application to MFSFET, because of its low dielectric constant ~about 20 at room temperature! and no volatile constituent elements. YMnO3 crystal has been reported to have a hexagonal structure and a spontaneous polarization P s along the c-axis with a value of 5.5 mC/cm2. Although the ferroelectricity of bulk YMnO3 was discovered in 1963,5 observations of the ferroelectricity in YMnO3 thin film are not yet satisfactory.4 In this work, we report the growth of highly ~0001!oriented YMnO3 films on Si by chemical solution deposition ~CSD!. The structural and electrical properties of YMnO3 films in a metal/FE/semiconductor ~MFS! structure were investigated. The screening of the ferroelectricity of YMnO3 thin film at room temperature ~RT! will be discussed in conjunction with the charge effects. To prepare the YMnO3 precursor, yttrium tri-i-propoxide and manganese ~III! acetyl-acetonate were dissolved in 2methoxyethanol with small amounts of acetic acid. The Si substrates were dipped in 7:1 buffered HF solution for 3 min to remove the native SiO2 on Si. The precursor was deposited on the substrate by spin coating followed by the rapid thermal annealing ~RTA! process in air at 850 °C for 40 s for

each layer. In order to obtain films of desired thickness, the processes were performed repeatedly. Final crystallization of the RTA-processed films were carried out in air at 800 °C for 1 h by conventional furnace annealing ~CFA!. Structural characterization of the films was performed by using x-ray diffraction ~XRD!: u-2u scan, rocking curve, and pole figure. The XRD patterns of the YMnO3 films grown on an n-type Si~100! substrate at different annealing conditions are shown in Fig. 1. For the film grown at the optimum condition, only the (000h) peaks consistent with the hexagonal YMnO3 phase were observed indicating the growth of highly ~0001!-oriented YMnO3 film. The XRD data also show that the high temperature annealing processes ~both RTA and CFA! are necessary to obtain highly ~0001!-

FIG. 1. XRD patterns of YMnO3 thin films on Si~100! at different annealing conditions: ~a! CFA (750 °C, 1 h! with RTA (750 °C, 40 s!, ~b! only RTA ~850 °C, 40 s!, and ~c! CFA ~800 °C, 1 h! with RTA ~850 °C, 40 s!. The inset is the rocking curve scan of ~0004! peak of the YMnO3 film. Symbols ¯ 1) ~*! peaks of YMnO3, and the indicate ~0004! ~s!, ~0002! ~d!, (112 substrate ~s! peak.

a!

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© 1998 American Institute of Physics

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Appl. Phys. Lett., Vol. 73, No. 7, 17 August 1998

FIG. 2. XPS ~Al K a source! result of a YMnO3 thin film in a wide scan. The inset is the XPS of Mn 2p state.

oriented YMnO3 thin films. This may be related to the nucleation and growth characteristics of the film. Unlike the various vapor deposition techniques, the CSD is not atom-byatom growth, and therefore, relatively long distance diffusion is inevitable for the crystallization if the local atomic arrangement does not resemble that of the crystalline phase. For YMnO3 film, the nucleation of ~0001!-oriented grains seems to be favorable because the ~0001! plane of hexagonal YMnO3 is the close-packed one. The close-packed structure is compatible with dense random packing of an amorphous phase, and thus, crystallization without long distance diffusion may be allowed. Moreover, in a high temperature RTA process, heterogeneous nucleation is thermodynamically much favored over homogeneous nucleation, which may result in the growth of ~0001!-oriented YMnO3 film. Minimization of lattice misfit strain between film and substrate, of course, affects the growth orientation of the film. However, threefold symmetry of hexagonal YMnO3 makes it difficult to discuss the ~0001! growth orientation of the film in conjunction with the lattice mismatch with the Si~100! substrate. The rocking curve scan for the ~0004! XRD peak ~inset of Fig. 1! showed a full width half maximum ~FWHM! value of 1.96°. The pole figure of the film revealed 12-fold poles ¯ 2 % planes at a >61° in the Schultz geometry6 on the of $ 112 Si~100! plane. These results indicate that YMnO3 thin film has a mosaic structure with highly c-axis ~ferroelectric axis! oriented grains. This orientation of the film is ideal for the MFSFET devices because the reversible ferroelectric polarization is important for gate programming in these devices. Also, preferred or epitaxial thin films show good ferroelectric properties and lower coercive field than polycrystalline films.7 The surface morphology and film thickness observed by scanning electron microscope revealed distribution of grains of an average size about 150 nm in diameter with a film thickness of 160 nm. In order to identify the phase and stoichiometry of the films, x-ray photoelectron spectroscopy ~XPS! and Rutherford backscattering spectroscopy ~RBS! were used. As given in the inset of Fig. 2, the 3/2 and 1/2 spin-orbit doublet components of the Mn 2p photoemission were located at 654

FIG. 3. ~a! Temperature dependent C – V characteristics of the Au/YMnO3 /Si MFS structure measured at 1 MHz (C o 577 nF/cm2 at RT!. ~b! C – V characteristics after bias aging at RT. The flatband shifts are due to the drift motion of the mobile ionic charges by electrical stress at RT.

and 666 eV, respectively. The XPS result shows that the YMnO3 thin film has a single phase with a Mn31 valence state,8 which is different from the mixed Mn valence state in the orthorhombic phase of YMn2O5. Analysis of the RBS spectrum showed that a stoichiometric YMnO3 film was grown with an interfacial oxide layer suspected to be SiOx . Capacitance–voltage (C – V) characteristics of the films were measured by using an HP4280A in a MFS structure with a circular gold electrode (731024 cm2) as the gate. On the back surface of the n-type Si~100! with a resistivity of 5210V cm, gold was evaporated. The gate voltages were swept in the negative direction and then in the positive direction completing a cycle at a constant sweep rate of 1.5 V/s. Probing ac signal frequency and amplitude were 1 MHz and 30 mV, respectively. The temperature dependences of C – V characteristics are shown in Fig. 3~a!. From the capacitances in accumulation, the dielectric constant of the YMnO3 film was estimated to be about 14 at RT and about 10 at 86 K. Below 100 K, a counterclockwise hysteresis in C – V curves could be observed. At low temperatures, the hysteresis due to the mobile ionic charges can be suppressed because the mobility becomes very low. Thus, the observed hysteresis is attributed to the ferroelectric polarization. In the temperature range between 100 K and RT, the hysteresis in C – V curves could not be observed. This result can be explained by the charge effects ~see below!. As the temperature was increased, the flatband voltage (V FB) of the C – V curves shifted toward the negative voltage direction. The V FB at 86 K was 0.5 V while that at RT was 20.1 V. This flatband voltage shift is thought to be due to the charge effects. The V FB is often described by the formula V FB5 f MS2

Qi , Co

~1!

where f MS is the work function difference between metal and semiconductor, C o the capacitance of the oxide layer,

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Appl. Phys. Lett., Vol. 73, No. 7, 17 August 1998

FIG. 4. Gate voltage dependences of the C – V characteristics at 86 K (C o 554 nF/cm2).

and Q i the effective interface charge.9 Q i is sensitive to the distribution of charges in the film. The V FB at 86 K ~0.5 V! is compatible with the work function difference between Au and Si ~0.57 V! indicating that the charge effects are negligible at low temperature. With increasing temperature, mobile positive ionic charges seem to cause the flatband shift toward the negative voltage direction. The ionic charges, presumably originated from the ionic impurities in YMnO3 film, are immobile at low temperature but can move under high field and high temperature operations.10 C – V measurements after 63 V bias aging for 1, 4, and 8 min were carried out at RT with gate voltage sweep directions as 63 V→73 V→63 V. The results, given in Fig. 3~b!, show that the flatband voltages were shifted in both positive and negative directions due to the drift motion of the charges depending on the gate bias polarity. The mobile ionic charge density at RT was calculated to be about 38 nC/cm2 from the flatband voltage shift after bias aging. This value is much larger than the effective surface charge density, 5.4 nC/cm2, which was calculated from the hysteresis loop attributed to the ferroelectric polarization at 86 K. Thus, the mobile ionic charge can result in the screening of the ferroelectricity of the film at RT. However, as the starting gate voltage (V gs) to be swept was increased from 6 to 12 V, a counterclockwise hysteresis in C – V curves was observed at RT. Also, the hysteresis loop width increased as V gs was increased. The same behaviors were observed more clearly at 86 K, as shown in Fig. 4, with the effective surface charge density increased from 5.4 nC/cm2 at V gs53 V to 27 nC/cm2 at V gs59 V. The increase in the effective surface charge density is also attributed to the ferroelectric remanent polarization which increases as the maximum applied gate voltage increases. Moreover, the increase of the hysteresis loop width in C – V curves was caused mainly by the shift of the V FB in the negative voltage direction as the gate voltage was swept in the negative voltage direction. This result implies that the ferroelectric polarization in the film may have a preferential direction from gate to semiconductor (n-Si). Alignment of the polarization along a preferential direction in the MFS structure has been suggested in previous reports.11,12

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For the C – V characteristics at RT, as can be seen in Fig. 3~a!, a similar behavior to the low frequency C – V characteristics was observed. This result may be due to the existence of an inversion layer beyond the gate. The inversion layer beyond the gate affects minority carrier response time and the C – V characteristics in inversion even at high frequencies.13 Oxygen anion vacancies in the interfacial SiOx layer are suspected to invert the silicon surface beyond the gate. Effect of the oxygen anion vacancies on the V FB may be compensated by the mobile positive ionic charges. Additional current–voltage (I – V) measurement at RT gave the leakage current density of 331026 A/cm2 at 3 V showing a leaky property of the film. In conclusions, highly ~0001!-oriented stoichiometric YMnO3 films were grown directly on Si substrates by CSD. Low temperature C – V characteristics showed a counterclockwise hysteresis loop, which was attributed to the ferroelectric polarization of YMnO3 film. The ferroelectric polarization seems to have a preferential direction from gate to semiconductor (n-Si). The temperature dependences of the C – V characteristics showed that the charge effects should be considered to understand the electrical property of the YMnO3 film. The ferroelectricity of the YMnO3 thin film at RT could be screened by charge effects and large leakage current. Since the MFS structure is one of the most promising candidates for realizing MFSFET nonvolatile memory devices, attempts to fabricate a YMnO3 film of high quality are necessary in order to reduce the charge effects. The authors thank Kwang-Hyun Cho for access to the XPS equipment and discussion. This work was supported in part by the Korea Science and Engineering Foundation ~KOSEF! through the Research Center for Dielectric and Advanced Matter Physics ~RCDAMP! at Pusan National University and in part by the academic research fund of the Ministry of Education, Republic of Korea.

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