Electrochemical and Solid-State Letters, 12 4 G13-G15 2009
1099-0062/2009/12 4 /G13/3/$23.00 © The Electrochemical Society

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Atomic Layer Deposition of Lanthanum-Based Ternary Oxides
Hongtao Wang,a Jun-Jieh Wang,b Roy Gordon,b,*,z Jean-Sébastien M. Lehn,c Huazhi Li,c Daewon Hong,c and Deo V. Shenaic
a School of Applied Science and Engineering and bDepartment of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts 02138, USA c Rohm and Haas Electronic Materials, North Andover, Massachusetts 01845, USA

Lanthanum-based ternary oxide LaxM2−xO3 M = Sc, Lu, or Y ﬁlms were deposited on HF-last Si substrates by atomic layer deposition. Both LaScO3 and LaLuO3 ﬁlms are amorphous while the as-deposited LaxY2−xO3 ﬁlms form a polycrystalline layer/ amorphous layer structure on Si. Transmission electron microscopy and electrical analysis show the absence of interfacial layers. The dielectric constants for LaScO3, LaLuO3, and La1.23Y0.77O3 ﬁlms are 23, 28 1, and 17 1.3, respectively, with leakage current density up to 6 orders of magnitude lower than that of thermal SiO2 with the same effective oxide thickness. Conformal coating thickness is demonstrated on holes with aspect ratio 80:1. © 2009 The Electrochemical Society. DOI: 10.1149/1.3074314 All rights reserved. Manuscript submitted November 30, 2008; revised manuscript received January 5, 2009. Published January 29, 2009.

Hafnium oxide has been widely studied as an alternative gate dielectric to replace silicon dioxide for metal-oxide-semiconductor ﬁeld-effect transistors MOSFETs and dynamic random access memories. In 2007, Intel Corporation announced its accomplishment of integrating HfO2 into MOSFETs with the physical gate length of 45 nm.1 However, pure HfO2 is readily crystallized at temperatures as low as 500°C.2 Amorphous dielectrics with high thermal stability are still preferred because they have no intrinsic defects, such as grain boundaries, and show homogeneous electrical properties, provided they still have the advantages of HfO2, such as high dielectric constant 22 to 23 , wide bandgap Eg = 5.5 eV , and low leakage.3 Recent reports show that lanthanum-based ternary oxides, such as lanthanum scandate LaScO3 and lanthanum lutetium oxide LaLuO3 , can meet all these requirements. These materials were grown by molecular beam deposition,4 pulsed laser deposition,5 or atomic layer deposition ALD .6 However, these lanthanide oxide ﬁlms had nanometer-thick interfacial layers when deposited on Si substrates, which made it impossible to scale the effective oxide thickness EOT to the subnanometer range. Previously, we found that interfacial layers could be avoided when ternary rare earth oxide GdScO3 ﬁlms were deposited on Si by ALD from metal amidinate precursors and H2O.7 This research shows that ALD-deposited LaScO3 and LaLuO3 thin ﬁlms have desirable structural and electrical properties, and are free of interfacial layers. Lanthanum yttrium oxide thin ﬁlms were also deposited by ALD under the similar condition, in order to show the importance of the choice of the element combination. Here we report ALD of LaScO3 and LaLuO3 thin ﬁlms that are free of interfacial layers, while retaining the desirable high- and amorphous properties. Thus, we obtained ﬁlms with EOT values 1 nm and very low leakage. Polycrystalline lanthanum yttrium oxide thin ﬁlms were also deposited by ALD under similar conditions, showing the importance of the proper choice of element combinations. Experimental The oxide ﬁlms were deposited in a ﬂow-type ALD reactor with water vapor alternating with vapors of metal amidinate precursors: lanthanum tris N,N -di-iso-propylformamidinate , scandium lutetium tris N,N -diethyltris N,N -diethylacetamidinate , formamidinate , and yttrium tris N,N -di-iso-propylacetamidinate . The ternary oxide ﬁlms LaxM2−xO3 M = Sc, Lu, or Y were deposited by repeatedly growing m-layers of La2O3 followed by n-layers of M2O3 with m, n = 1 or 2. The deposition temperature was 300°C for LaScO3 and LaLuO3, and 280°C for LaxY2−xO3. n-Type Si 100 ,

with resistivity 0.5–1 cm, was selected as the substrate. All substrates were treated in UV/ozone to remove surface organic contamination, then dipped into 10% aqueous HF solution for 5 s and rinsed with deionized water right before deposition. Metal-oxidesemiconductor MOS capacitors were made to measure the electrical properties. Tungsten nitride WN was deposited in the same ALD reactor as a top metal electrode.8 Platinum dots were ﬁnally deposited by evaporation and liftoff and used as hard masks during the removal of exposed WN by reactive ion etching CF4 + Ar . Results and Discussion The ﬁlm thickness and the number of ALD cycles have a linear relation with zero intercept Fig. 1a , showing that growth begins immediately on H-terminated Si surfaces. For m, n = 1 or 2, the ternary oxide growth rate is approximately the summation of m times of the growth rate of La2O3 1.3 Å/cycle and n times of the growth rate of M2O3 1.1 Å/cycle for Sc2O3, 1.2 Å/cycle for Lu2O3, and 0.8 Å/cycle for Y2O3 . The impurity contents, including carbon and nitrogen, are below the detection limit 1% of X-ray photoelectron spectroscopy XPS Fig. 1b . The ﬁlm composition by Rutherford backscattering not shown depends on both the ratio m/n and the metal precursors. For m = n = 1, the ternary oxide ﬁlms were determined to be LaSc1.02 0.07O3, LaLu1.00 0.05O3, and La1.23Y0.77O3, respectively. The compositions of LaxY2−xO3 ﬁlms for various m and n show a linear relationship between x/2 and m/ m + 0.63n with unit slope, which implies that the growth rate for each material is independent of the composition of the substrate that it is growing on. On the basis of this observation, LaYO3 ﬁlms can be obtained by setting m = 2 and n = 3. Figure 2a shows a sharp interface between amorphous LaScO3 and crystalline Si in a stack of WN/LaScO3 /Si. Similar results were found for LaLuO3 and LaxY2−xO3 ﬁlms Cross-sectional transmission electron microscope XTEM images not shown . The step coverage is close to 100% in holes with an aspect ratio of 80:1. Figure 2b shows that a 12 nm LaLuO3 ﬁlm has a uniform thickness from the top to the bottom of the hole. Despite the fact that all the as-deposited binary oxides M2O3 are polycrystalline bodycentered-cubic phases determined by electron diffraction, both LaScO3 and LaLuO3 ﬁlms are amorphous and homogeneous. In contrast, as-deposited LaxY2−xO3 ﬁlms show a polycrystalline layer over an amorphous layer on Si by XTEM. The lattice incompatibility between these oxides and Si increases the activation energy barrier for nucleating crystalline phases adjacent to Si, resulting in an amorphous lower layer of LaxY2−xO3. After the growth of a thin amorphous layer 3–7 nm , the mismatch is relaxed so that a polycrystalline layer of LaxY2−xO3 can grow on the top. MOS capacitors were made to measure the electrical properties. Figure 3a shows the high-frequency 1 MHz capacitance-voltage C-V curves of LaLuO3, LaScO3, and La1.23Y0.77O3 ﬁlms with no

* Electrochemical Society Active Member.
z

E-mail: gordon@chemistry.harvard.edu

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Electrochemical and Solid-State Letters, 12 4 G13-G15 2009

Figure 1. Color online a The thickness vs ALD cycle plot. The thickness was measured by both X-ray reﬂectivity XRR and variable angle scanning ellipsometer VASE . b The XPS spectrum for LaLuO3.

Figure 3. Color online a C-V curves measured at 1 MHz. The lines are simulated curves with MISFIT by assuming no interface traps. b The EOT plots as a function of the physical thickness.

Figure 2. a XTEM image of a WN/LaScO3 stack on Si substrate. The white line along the interface is caused by transmission electron microscope aberration. It consists of discrete spots, which are an extension of the Si lattice. b A 12 nm LaLuO3 ﬁlm deposited in holes with aspect ratio 80:1. The hole has an elliptical cross section with semi-long axes 75 nm and semi-short axes 35 nm. Its depth is 7.2 m. On the right-hand side are three higher magniﬁcation images for comparing the ﬁlm thickness in the top, middle, and bottom parts of the trench.

noticeable stretching or shoulders. The small hysteresis 0–10 mV indicates very few bulk traps in the ﬁlms. The 10 and 100 kHz C-V curves not shown are closely aligned to 1 MHz ones with frequency dispersion less than 2–3% of the accumulation capacitance. Small shoulders appear in the weak inversion region of C-V curves measured at 10 and 100 kHz, which indicate the existence of some slowly responding interface states. The EOT was obtained by ﬁtting the C-V data to ideal simulation curves using the Metal-InsulationSemiconductor CV Fitting MISFIT program with charge quantization effect Fig. 3a .9 By linearly ﬁtting the EOT vs physical thickness plot in Fig. 3b, the dielectric constants, extracted from the slopes, are 28 1 and 17 1.3 for LaLuO3 and La1.23Y0.77O3 ﬁlms, respectively. The nearly zero intercept for LaLuO3 ﬁlms indicate the absence of any interfacial layer, consistent with the sharp interfaces observed by high-resolution XTEM. The dielectric constant for LaScO3 is 23, which is estimated by = 3.9tphysical /EOT. Both LaScO3 and LaLuO3 ﬁlms have higher dielectric constants than those of their binary oxide components, i.e., 19 ,10 Lu2O3 16 , and Sc2O3 17 . These results imply La2O3 that the amorphous ternary oxides form new microscopic structures, rather than simple mixtures of the two binary oxides. In view of the continuous random network theory,11 it is possible that locally –O– La–O– La3+ radius 103 pm develops frames of polyhedrons with the smaller ions Sc3+ radius 75 pm or Lu3+ 86 pm caged inside. The Sc–O or Lu–O bonds are softened due to their smaller metal ion sizes, and the polarizability is therefore enhanced by the bond soft-

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Electrochemical and Solid-State Letters, 12 4 G13-G15 2009

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measurement show linear behaviors in the range of 0.3–1.5 V. The dynamic refraction index calculated from the slopes is 1.9 to 2.0, which is comparable to the optical refraction index measured at wavelength of 630 nm. The leakage currents also obey the Arrhenius law at different ﬁxed voltages not shown . Combining these two observations, we conclude that 1/2 J = cV exp − /kBT , which is exactly the Poole– B − PFV Frenkel formula. The extracted trap depth B is 0.3–0.4 eV. Conclusions In summary, LaxM2−xO3 M = Sc, Lu, or Y ﬁlms were deposited by ALD with metal amidinate precursors and H2O. Both LaScO3 and LaLuO3 ﬁlms are amorphous and free of interfacial layers. Besides the structural beneﬁts, both oxides have high dielectric constants 23 for LaScO3 and 28 1 for LaLuO3 , low leakage current density, and very few bulk traps, and are scalable to EOT 1 nm. La1.23Y0.77O3 ﬁlms have polycrystalline structures with moderately high = 17 1.3 and low leakage current. The Poole– Frenkel mechanism is veriﬁed in the ternary oxide ﬁlms by studying temperature dependence of the leakage current. Acknowledgments This work was supported in part by Rohm and Haas Electronic Materials and performed, in part, at Harvard University’s Center for Nanoscale Systems, a member of the National Nanotechnology Infrastructure Network, supported by the U.S. National Science Foundation under award no. ECS-0335765. We also thank Professor Ramanathan for helping in the leakage current measurement.
Harvard University assisted in meeting the publication costs of this article.

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Figure 4. Color online a Leakage current density at Vg − VFB = 1 V and b Poole–Frenkel plot of the leakage current density of a LaScO3 ﬁlm EOT = 0.9 nm at various temperatures.

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ening, which can be more than enough to make up for an adverse effect caused by the relatively larger molar volume in the amorphous ﬁlms.12 Figure 4a shows the leakage current density scaling of our ALD ﬁlms compared to that of thermal SiO2 ﬁlms with the same EOT. The current density at 1 V gate bias Vg − VFB = 1 V is up to 6 orders of magnitude lower than that of thermal SiO2 for both LaScO3 and LaLuO3 ﬁlms and 2–4 orders of magnitude lower for La1.23Y0.77O3 ﬁlms. All ternary oxide ﬁlms have the same leakage current-voltage J-V behaviors. Figure 4b shows the J-V curves of a LaScO3 ﬁlm with 0.9 nm EOT at temperatures from room temperature to 200°C. The Poole–Frenkel plots Fig. 3b of this J-V

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