Article pubs.acs.org/cm
Atomic Layer Deposition of Gd2O3 and Dy2O3: A Study of the ALD Characteristics and Structural and Electrical Properties Ke Xu,† Ramdurai Ranjith,‡ Apurba Laha,‡ Harish Parala,† Andrian P. Milanov,† Roland A. Fischer,† Eberhard Bugiel,‡ Jürgen Feydt,§ Stefan Irsen,§ Teodor Toader,∥ Claudia Bock,∥ Detlef Rogalla,⊥ Hans-Jörg Osten,‡ Ulrich Kunze,∥ and Anjana Devi*,† †
Inorganic Materials Chemistry, Ruhr-University Bochum, 44780 Bochum, Germany Institute of Electronic Materials and Devices, Leibniz University of Hannover, 30167 Hannover, Germany § Center of Advanced European Studies and Research (Caesar), 53175 Bonn, Germany ∥ Institute of Electronic Materials and Nanoelectronics, Ruhr-University Bochum, 44780 Bochum, Germany ⊥ Dynamitron Tandem Laboratory (DTL) of RUBION, Ruhr-University Bochum, 44780 Bochum, Germany ‡
ABSTRACT: Gd2O3 and Dy2O3 thin films were grown by atomic layer deposition (ALD) on Si(100) substrates using the homoleptic rare earth guanidinate based precursors, namely, tris(N,N′-diisopropyl-2-dimethylamidoguanidinato)gadolinium(III) [Gd(DPDMG)3] (1) and tris(N,N′-diisopropyl-2dimethylamido-guanidinato)dysprosium(III) [Dy(DPDMG)3] (2), respectively. Both complexes are volatile and exhibit high reactivity and good thermal stability, which are ideal characteristics of a good ALD precursor. Thin Gd2O3 and Dy2O3 layers were grown by ALD, where the precursors were used in combination with water as a reactant at reduced pressure at the substrate temperature ranging from 150 °C to 350 °C. A constant growth per cycle (GPC) of 1.1 Å was obtained at deposition temperatures between 175 and 275 °C for Gd2O3, and in the case of Dy2O3, a GPC of 1.0 Å was obtained at 200− 275 °C. The self-limiting ALD growth characteristics and the saturation behavior of the precursors were confirmed at substrate temperatures of 225 and 250 °C within the ALD window for both Gd2O3 and Dy2O3. Thin films were structurally characterized by grazing incidence X-ray diffraction (GI-XRD), atomic force microscopy (AFM), and transmission electron microscopy (TEM) analyses for crystallinity and morphology. The chemical composition of the layer was examined by Rutherford backscattering (RBS) analysis and Auger electron spectroscopy (AES) depth profile measurements. The electrical properties of the ALD grown layers were analyzed by capacitance−voltage (C−V) and current− voltage (I−V) measurements. Upon subjection to a forming gas treatment, the ALD grown layers show promising dielectric behavior, with no hysteresis and reduced interface trap densities, thus revealing the potential of these layers as high-k oxide for application in complementary metal oxide semiconductor based devices. KEYWORDS: atomic layer deposition, rare earth oxides, structure, morphology, electrical properties
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INTRODUCTION Over the last decades, rare earth (RE) oxide based materials (e.g., Gd2O3, Dy2O3, etc.) have been extensively studied because of their wide range of applications in various fields of technology such as in optical devices, microelectronics, and magnetic devices.1−4 The application of RE oxides as a high-k dielectrics in complementary metal oxide semiconductor (CMOS) devices has been investigated in detail, as a result of their many promising properties (e.g., high dielectric constant, better thermodynamical stability on Si, and large band gap).5,6 RE oxides, such as Gd2O3 and Dy2O3, are considered to be promising candidates for alternative gate oxide insulating materials as a result of the high dielectric constant (k[Gd2O3] = 12−17, k[Dy2O3] = 14−18), optimum band offset values with respect to silicon (Eg[Gd2O3] ≈ 6.0 eV, Eg[Dy2O3] ≈ 4.9 eV), and high thermodynamic stability.7 In addition, the Gd2O3 and Dy2O3 have a close lattice match to Si, © 2012 American Chemical Society
which allows the possibility to grow epitaxial thin films on Si substrates.2 Special attention has been devoted to doping the rare-earth elements (Gd, Dy, Er, etc.) into HfO2 to stabilize the cubic and tetragonal phase in order to obtain high-k values for HfO2.8−10 Other promising classes of materials for high-k applications are the rare earth scandates (GdScO3, DyScO3) where Gd and Dy were used as dopants for Sc2O3.11−16 The RE oxide thin films have been deposited mainly via physical vapor deposition (PVD) techniques such as ultrahigh vacuum vapor deposition,17 electron beam evaporation,18 and molecular beam epitaxy (MBE),19 as well as chemical methods such as metal-organic chemical vapor deposition (MOCVD). Atomic layer deposition (ALD) has been preferred to PVD Received: July 21, 2011 Revised: October 20, 2011 Published: February 10, 2012 651
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Thin Film Characterization. The growth per cycle (GPC) and the thickness of the Gd2O3 and Dy2O3 films were determined by spectroscopic ellipsometry (SE) and the measurements were performed on a J. A. Woollam Co. ellipsometer from 283.5 to 751.8 nm using an incident angle of 75°. Film crystallinity was investigated by grazing incidence X-ray diffraction (GI-XRD) analysis in a Bruker AXS D8 Discover diffractometer, using Cu Kα radiation (1.5418 Å). Film morphology was studied with atomic force microscopy (AFM, NanoScope 5, Veeco instruments GmbH) and high resolution transmission electron microscopy (HRTEM). The AFM measurements were performed in the tapping mode. Film composition was determined by Auger electron spectroscopy (AES) and Rutherford backscattering (RBS) analysis. AES was carried out on a scanning Auger spectrometer (PHI-AES 690, Physical Electronics) with a beam energy of 10 kV and 10 nA. For the AES depth profiles, Ar+ sputtering was carried out at 0.5 kV and 500 mA (1.5 × 1.5 mm2 scanning size). Atomic composition was calculated using the software package Multipak (Physical Electronics). RBS analysis was performed using an instrument from the Dynamitron Tandem Laboratory (DTL) in Bochum using 2.0 MeV 4He+ ions. A beam intensity of approximately 20−40 nA incident to the sample at a tilt angle of 7° was used. The backscattered particles were measured at an angle of 160° by a Si detector with a resolution of 20 keV. The stoichiometry of the films was calculated using the program RBX.29 Platinum metal dots with a diameter of 360 μm and a thickness of 25 nm were deposited through a shadow mask on top of the ALD grown Gd2O3 and Dy2O3 layers to characterize the layers in a metal-oxide-semiconductor (MOS) configuration. The Pt metal was evaporated using electron beam heating in an ultrahigh vacuum chamber. The capacitance−voltage (C−V) and leakage current (I−V) measurements were carried out using an impedance analyzer (Agilent 4294A) and semiconductor Hewlett-Packard parameter analyzer (4155A), respectively. To improve the electrical behavior and reduce the interface trap densities, the wafers were cleaved into two halves and one-half of the wafer was subjected to a forming gas (FG) annealing at 450 °C for 15 min. Following the FG treatment, Pt dots (360 μm) were deposited to study the electrical behavior, and the samples were compared with the pre-FG-treated samples. Though the electrical studies were performed on layers of varying thickness, the electrical behavior of the lower thicknesses is more important in terms of the downscaling features, and hence, only the low thickness samples of Gd2O3 and Dy2O3 are presented. However, the observation of generic behavior was common, and the details specific to samples are discussed wherever needed.
methods because of the possibility of large area deposition, precise thickness control and uniform step coverage, even on 3D device geometries.20,21 Despite the distinct advantages of this technique, there are very limited true ALD processes for RE oxides in general, especially with H2O as a coreactant. One of the main reasons for this is the lack of suitable precursors for RE elements. ALD of Gd2O3 was reported using [Gd(thd)3]/ O3, with a low growth rate of ca. 0.3 Å/cycle, and the thermal decomposition of the precursor was observed in the case of the [Gd(CpCH3)3]/H2O process.22 Kukli et al. reported the use of Gd alkoxide [Gd(DMB)3] (DMB = 2,3-dimethyl-2-butoxide) in ALD with H2O as coreactant, and the ALD characteristics were poor.23 To date, there have not been many reports in the literature exclusively on the ALD of binary Dy2O3, although ALD of DyScO3 was reported using the Dy-diketonate class of precursors with O3.24 In our earlier publications, we reported the successful development of a promising class of precursors for a series of RE elements using the guanidinate ligand system. These monomeric RE tris-guanidinates have been tested for the MOCVD of RE oxides and effectively used as a single source precursor for RE nitrides.25,26 It has also been shown that these ligands offer necessary reactivity, volatility, and good thermal stability, which are essential for an ALD precursor.27 H2O assisted ALD of Gd2O3 thin films using tris(N,N′-diisopropyl-2dimethylamido-guanidinato) gadolinium(III) [Gd(DPDMG)3] with preliminary results on the characterization of the ALD grown films were published as a communication earlier.28 The current work demonstrates that the thin films of Gd2O3 and Dy2O3 can be grown efficiently by ALD using guanidinate based complexes. The homoleptic RE−guanidinate complexes tris(N,N′-diisopropyl-2-dimethylamido-guanidinato) gadolinium(III) [Gd(DPDMG)3] (1) and tris(N,N′-diisopropyl-2-dimethylamido-guanidinato)dysprosium(III) [Dy(DPDMG) 3 ] (2) 27 were used as ALD precursors in combination with water. The ALD grown Gd2O3 and Dy2O3 thin films were analyzed using a series of characterization techniques to study the structure, morphology, composition, interface quality, and electrical properties relevant for high-k applications.
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RESULTS AND DISCUSSION Precursor Evaluation. Gd and Dy precursors were characterized for their volatility and thermal stability, as well as their reactivity (before using the precursors for ALD experiments), by different techniques. The thermal properties of [Gd(DPDMG)3] (1) and [Dy(DPDMG)3] (2) were investigated by thermogravimetry and differential thermal analysis (TG/DTA), isothermal TG analysis, and NMR decomposition studies. The compounds were found to be volatile exhibiting constant sublimation rates over a long period of time (>10 h). Half-life evaluation via NMR decomposition experiments shows that [Dy(DPDMG)3] (2) is stable for more than one year at 140 °C (typical evaporation temperature in an ALD process).27 The NMR decomposition experiments could not be carried out for [Gd(DPDMG)3] (1) because of its extremely high paramagnetic character. We assume that properties similar to Dy complex can be expected for Gd complex because of the analogous coordination environment. Because of the presence of six basic metal−nitrogen bonds, which could involve rapid nucleophilic substitution of OH group into the metal centers, high reactivity of these precursors toward water can be expected during the ALD experiments in addition to higher thermal stability.
EXPERIMENTAL SECTION
Precursor Synthesis and Characterization. The RE complexes [Gd(DPDMG)3] (1) and [Dy(DPDMG)3] (2) were synthesized following the procedure published earlier27 and characterized in detail (TG/DTA and NMR decomposition studies) and scaled up to large batches of ca. 10 g for ALD experiments. ALD of Gd2O3 and Dy2O3. Gd2O3 and Dy2O3 thin films were grown in a commercial flow-type hot-wall ALD reactor (F-120; ASM Microchemistry Ltd., Finland) on 2 in. p-type Si(100) substrates (SIMAT) using [Gd(DPDMG)3] (1) and [Dy(DPDMG)3] (2) as precursors and H2O as coreactant. The substrates were ultrasonically cleaned in acetone and ethanol, rinsed with deionized water (Millipore Water Purification System), and dried under argon stream. The native SiO2 layer was not removed prior to deposition. The precursors were handled in a glovebox, and inserted into the reactor and evaporated from an open crucible maintained at 130 °C. Approximately 200 mg of the precursor was used for each ALD experiment. Deionized water evaporated from a stainless steel container kept at 25 °C was employed as the oxygen source. High-purity nitrogen (99.9999%) was used as both carrier and purging gas. The reactor pressure was 1−3 mbar during the deposition experiments. The ALD growth of Gd2O3 and Dy2O3 were studied at substrate temperatures ranging from 150 to 350 °C using the following standard pulsing sequence (ALD growth cycle): 3 s of metal precursor pulse, followed by 20 s of nitrogen purge, 1 s of water pulse, and finally 10 s of nitrogen purge. 652
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ALD Growth and Self-Limiting Behavior. The selflimiting character and the saturation behavior of compounds 1 and 2 were evaluated in terms of the ALD window, saturated growth characteristics, and linear dependence between the number of ALD cycles and film thickness. These are the ideal characteristics of a good ALD precursor required for an efficient ALD process. The GPC of the RE oxide thin films deposited using the [Gd(DPDMG)3]/H2O and [Dy(DPDMG)3]/H2O precursor combinations were first investigated as a function of deposition temperature in the range 150−350 °C. In the case of Gd2O3, no film growth was observed below 160 °C. As shown in Figure 1a,
behavior has been previously observed for ALD of RE oxide (e.g., Sc2O3 growth utilizing [Sc(Cp)3]/H2O).33 In the case of ALD of Dy2O3, as illustrated in Figure 1b, the ALD window ranges 200−275 °C, with a constant GPC of ca. 1.0 Å. In comparison with Gd2O3, the ALD window, as well as GPC, for Dy2O3 is almost in the same range. (175−275 °C vs 200−275 °C, 1.1 Å/cycle vs 1.0 Å/cycle) This observation could probably be attributed to the similar chemical environment of both metal centers involved with similar chemical reactivity and thermal properties.27 In the case of Dy2O3 ALD, unlike in the Gd2O3 case, films could be grown at 150 °C. In the growth temperature range 150−175 °C, the GPC of [Dy(DPDMG)3]/H2O process is about 0.9 Å. However, in the case of Dy2O3, no precursor condensation was observed in the ALD growth temperature range 150−175 °C, which led to a higher GPC of 1.7 Å for Gd2O3 (at 160 °C). This may be due to the slight increase in the reactivity of compound 2 when compared to compound 1 toward water at lower temperatures. At higher deposition temperatures (>275 °C), the increase in the GPC could be due to the CVD contribution similar to the [Gd(DPDMG)3]/H2O process observed as discussed earlier. The surface controlled self-limiting ALD growth for [Gd(DPDMG)3]/H2O and [Dy(DPDMG)3]/H2O at two different temperatures (225 °C, 250 °C) was studied in order to estimate the influence of the thin film growth rate on the precursor pulse length. As shown in Figures 2a and 3a, a fully
Figure 1. Growth per cycle of (a) Gd2O328 and (b) Dy2O3 films deposited from [Gd(DPDMG)3] and [Dy(DPDMG)3] as a function of deposition temperature, using a pulse sequence of 3 s of metal precursor pulse, 20 s of nitrogen purge, 1 s of water pulse, and 10 s of nitrogen purge.
a nearly constant GPC of 1.1 Å was achieved in the temperature range 175−275 °C for [Gd(DPDMG)3] (1), thus indicating a surface controlled growth with a good ALD window.30,31 A significantly higher GPC of ca. 1.7 Å obtained at 160 °C could be attributed to the condensation of the reactants.30 Beyond 275 °C, the GPC first slightly increased to 1.5 Å at 285 °C and subsequently decreased to 0.3 Å at 300 °C. The small increase in GPC is possibly due to a minor CVD-like contribution. This matches very well with the temperature of decomposition (ca. 270 °C) of compound 1, according to the TG experiment.27 The rapid decrease in GPC beyond 300 °C is probably due to either re-evaporation of the chemisorbed precursor species, dehydroxylation of the film surface, or reduction of the precursor flux owing to self- decomposition before it reaches the substrate surface.30−32 Similar growth
Figure 2. (a) Growth per cycle of Gd2O3 at 225 °C28 and 250 °C as a function of the [Gd(DPDMG)3] pulse length; (b) dependence of the Gd2O3 film thickness on the number of deposition cycles for the [Gd(DPDMG)3]/H2O process at 225 °C28 and 250 °C. 653
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Figure 4. GI-XRD patterns of (a) Gd2O328 and (b) Dy2O3 deposited on Si(100) as a function of deposition temperature.
Figure 3. (a) Growth per cycle of Dy2O3 at 225 and 250 °C as a function of the [Dy(DPDMG)3] pulse length; (b) dependence of the Dy2O3 film thickness on the number of deposition cycles for the [Dy(DPDMG)3]/H2O process at 225 and 250 °C.
tapping mode (Figures 5 and 6), and the films were found to be very smooth. As expected for surface controlled ALD-type
saturated ALD GPC was first achieved with ca. 2−2.5 s metal precursor pulse at 225 and 250 °C. Further increase in the precursor pulse duration to 3−5 s had no effect on the GPC, which confirms the self-limiting character of the ALD growth. Another characteristic feature of ALD, namely, the proportional relationship between the number of deposition cycles and film thickness, was also verified in the present study (Figures 2b and 3b). For the ALD experiments carried out at two different deposition temperatures (225 and 250 °C), the thicknesses of the deposited films were linearly dependent on the number of ALD cycles, as expected for an ideal ALD process. Crystallinity and Morphology of Gd2O3 and Dy2O3 Films. The crystallinity of the ALD grown Gd2O3 and Dy2O3 thin films (film thickness; 20−25 nm) deposited on the Si(100) was analyzed using grazing incidence X-ray diffraction (GIXRD; Figure 4). Both Gd2O3 and Dy2O3 thin films deposited in the temperature range 160−250 °C and 150−300 °C, respectively, were polycrystalline in the cubic phase. In the case of Gd2O3, reflections of 222, 400, 440, and 622 can be indexed according to the GI-XRD pattern (JCPDS-43-1014). However, only two predominant reflections of 222 and 400 were observed for Dy2O3 films, corresponding to the database (JCPDS-43-1006). In comparison with Gd2O3, Dy2O3 thin films are poorly crystalline with the appearance of low intensity diffraction peaks of 222 and 400, without other recognizable reflections. The surface morphology of the Gd2O3 and Dy2O3 films deposited at 175−250 °C was further analyzed by AFM in the
Figure 5. AFM micrographs of Gd2O3 thin films deposited by the [Gd(DPDMG)3]/H2O process at 175 °C (24 nm) and 225 °C (23 nm), respectively.
growth, the surface roughness of the films was low and did not vary significantly as a function of the deposition temperature. The rms roughness estimated for a 24 nm thick Gd2O3 film and a 19 nm thick Dy2O3 film deposited at 175 °C was 0.3 nm (rms/thickness = 1.3%) and 0.4 nm (rms/thickness = 2.1%), respectively. A further increase in the deposition temperature slightly affected this value, and for the thin films deposited at 225 °C (Gd2O3, 23 nm; Dy2O3, 21 nm), rms roughness values 654
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here) has shown the cubic structure of the crystallites in both cases, in agreement with the XRD results. The interface layers seen for both the oxides might stem from the native oxide present on Si substrates as the wafers were not etched with HF dip as the last step prior to ALD growth. However, the detailed HRTEM investigation of pre-FG-treated layers (not shown) shows that no further growth of an interfacial layer was observed upon subjecting the layer to a FG annealing treatment. From the HRTEM images, more nanocrystalline domains of Gd2O3 (Figure 7a) can be observed compared to Dy2O3 (Figure 7b) deposited at 225 °C supporting the GI-XRD data indicating the poor crystalline nature of Dy2O3 layers. From TEM investigations, in dark field mode (not shown here), we can conclude that the crystallites in the Gd2O3 are slightly greater. Composition of Gd2O3 and Dy2O3 Thin Films. Chemical composition of the Gd2O3 and Dy2O3 thin films were investigated using RBS (Figure 8) and AES depth profile
Figure 6. AFM micrographs of Dy2O3 thin films deposited by the [Dy(DPDMG)3]/H2O process at 175 °C (19 nm) and 225 °C (21 nm), respectively.
of 0.7 and 0.5 nm were obtained, corresponding to rms/ thickness ratios of 3.0% and 2.4%, respectively. The slightly increased rms/thickness ratios in both cases could be attributed to the increase in the film crystallinity at the corresponding temperatures. The nature of the interface and the crystallinity of the films were further studied using the cross sectional HRTEM analysis. Parts a and b of Figure 7 show a typical cross sectional view of
Figure 8. RBS spectra of Gd2O3 (a) and Dy2O3 (b) thin films grown at 200, 225, and 275 °C.
experiments (Figure 9). According to the RBS analysis illustrated in Figure 8, the metal (Gd or Dy) and the oxygen signals are clearly evident, whereas other signals such as nitrogen and carbon could not be detected, which are below the RBS detection limit (
