HfO2 Stacks

Article pubs.acs.org/JPCC

Oxygen Transport and Incorporation in Pt/HfO2 Stacks Deposited on Germanium and Silicon Guilherme Koszeniewski Rolim,† Angelo Gobbi,‡ Gabriel Vieira Soares,§ and Cláudio Radtke*,∥ †

Programa de Pós Graduaçaõ em Microeletrônica, UFRGS, Porto Alegre-RS, Brazil Laboratório de Microfabricaçaõ , LNNano/CNPEM, Campinas-SP, Brazil § Instituto de Física, UFRGS, Porto Alegre-RS, Brazil ∥ Instituto de Química, UFRGS, Porto Alegre-RS, Brazil ‡

S Supporting Information *

ABSTRACT: Ge is a promising material to improve transistor performance. However, finding an efficient passivation strategy for this semiconductor is still a challenge. Annealing in O2 of metal/dielectric stacks prepared on Ge can improve the electrical properties of the final structure. However, excessive Ge oxidation cannot take place. O isotopic tracing in conjunction with subnanometric depth profiling of 18O were used to investigate oxygen transport and incorporation in Pt/HfO2/(Ge or Si) stacks. The supply of atomic oxygen (able to diffuse through HfO2) is a function of temperature and the number of available O2 dissociation sites in HfO2. A Pt top layer promotes a more efficient O2 dissociation, resulting in a higher O exchange in HfO2 and a higher O supply at the HfO2/semiconductor interface. The different nature of the native oxides of Si and Ge has a direct influence on the resulting physicochemical modifications of the stacks prepared on these semiconductor materials.

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INTRODUCTION The continuous downscaling of complementary metal oxide semiconductor (CMOS) transistors demands new materials in order to overcome physical and electrical limits which stem further device evolution. Germanium (Ge) is a promising material in this field, presenting the highest p-type mobility among all known semiconductor materials.1 However, Ge surface passivation constitutes one of the main tasks to the use of this material as a replacement for silicon (Si).2 Ge can be thermally oxidized to grow a germanium oxide (GeO2) layer which efficiently passivates its surface.3 Despite the good GeO2/Ge interface quality, GeO2 is not compatible with usual device processing steps in the microelectronics industry. In contrast to SiO2, GeO2 is a polymorph. The hexagonal and amorphous phases are water-soluble, preventing usual wet chemistry processing. Moreover, GeO2 is not stable on Ge above 400 °C:4 oxygen vacancies are generated from the interfacial redox reaction between GeO2 and Ge. Since the equilibrium concentration of oxygen vacancies is much higher in the interfacial region than in the GeO2 bulk, they diffuse toward the GeO2 surface, promoting desorption of the oxide layer.5 Different passivation strategies were already employed in order to obtain a stable dielectric layer (with the highest possible dielectric constant) forming an interface with the Ge substrate with low density of electrically active defects. These strategies usually involve the formation of a thin layer on the top of Ge (usually Ge oxides or oxynitrides) followed by the deposition of a high dielectric constant material (HfO2, for example).6−8 The resulting stack can be further annealed in © XXXX American Chemical Society

order to heal defects formed during the deposition process. Postdeposition annealing (PDA) in oxygen is commonly used for this purpose.9 This annealing can be at the same time beneficial (passivating electrically active defects) and/or deleterious (forming a low dielectric interfacial layer, lowering the overall capacitance of the stack). Thus, tailoring these effects is mandatory to modify conveniently the physicochemical and, consequently, electrical properties of the resulting structure. PDAs can be performed after the formation of metallic electrodes on the top of the dielectric which can modify the reactivity of the annealing gas. These electrodes are formed by lithography of deposited layers in the current top-down approach employed in the semiconductor industry. Pt/HfO2/ Ge stacks evidenced reduced interface trap densities following oxygen thermal treatments.10 The Pt layer thickness was varied, evidencing an influence on the resulting electrical properties. Comparison with bare samples annealed in oxygen (without the Pt layers) confirmed the role played by the electrode. Pt is known to act as an oxygen dissociation agent, supplying oxygen species with higher reactivity than molecular O2.11,12 An example is the oxidation of metals, which are catalyzed by Pt films sputter deposited on their surfaces.13 This effect was attributed to the increased inward transport of dissociated oxygen and an enhanced oxide growth near the metal substrate. In the present study, these species may reach HfO2 and interact Received: November 6, 2014 Revised: February 5, 2015

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DOI: 10.1021/jp511127c J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

a Cu Kα source. X-ray photoelectron spectroscopy (XPS) was performed with Al Kα radiation in an Omicron-SPHERA station.

with this layer. Density functional theory calculations of oxygen incorporation and diffusion in monoclinic hafnia (HfO2) demonstrate that atomic oxygen incorporation is favored and that O2− is the most stable species. Oxygen interstitials diffuse via exchange with lattice oxygen sites in hafnia, where O− species have the smallest diffusion barrier.14 Formation of native oxides during PDA on the top of the semiconductor substrate can play an important role in further oxygen transport and incorporation. The interplay of oxides in the resulting stack was already investigated by annealing HfO2/ SiO2/Si structures in oxygen enriched in the 18O isotope.15 Depth profiling of this isotope revealed that bare SiO2 samples accumulate 18O at the SiO2/Si interface as expected.16 Samples with HfO2 presented a homogeneous incorporation throughout the HfO2 layer. Oxidation in HfO2/SiO2/Si stacks proceeds slower than samples without HfO2, despite the fast atomic oxygen exchange occurring in the HfO2 layer during annealing. The process occurs by molecular O2 diffusion throughout the entire stack. Such a diffusion process is limited by the low O2 solubility in HfO2. In the present work, we investigated O incorporation in Pt/ HfO2/Ge stacks aiming at understanding the role of the Pt layer in the physicochemical modifications of this structure following PDA in O2. The oxygen reactivity with Ge and the stability of the formed oxide are different from those of Si, which certainly influence oxygen incorporation in such structures. PDAs were performed with oxygen enriched in the 18 O isotope. Depth profiling of this isotope with subnanometric depth resolution enabled the identification of incorporation sites and transport mechanisms.

Figure 1. (Left) Experimental excitation curves (symbols) of the 18 O(p,α)15N nuclear reaction for HfO2 films deposited on Ge (upper part) and on Si (lower part). The corresponding simulations are shown by lines. Data correspond to a reference Hf18O2 film deposited on both substrates (open symbols) and to HfO2/semiconductor structures submitted to annealing in 1 atm of 18O2 for 15 min at the indicated temperatures (solid symbols). (Right) 18O profiles assumed in the simulation of the respective nuclear reaction data. Profiles of the reference samples are shown as gray boxes. a.u. stands for arbitrary units.

EXPERIMENTAL METHODS Ge(100) p-type epiready wafers doped with Ga (Umicore), with a resistivity of 0.24−0.47 Ω.cm, were used in this work. Ge substrates were first cleaned in an ultrasonic acetone bath and etched in a 1:4 HCl aqueous solution. Following this step, samples were immersed in 20% H2O2 aqueous solution and then etched again in the HCl solution.17 This procedure was repeated three times. Samples were finally rinsed in deionized water and dried with N2. Si samples were first cleaned in a mixture of H2O2 and H2SO4, followed by etching with HF solution. HfO2 films (5 nm thick) were deposited using pulsed DC reactive magnetron sputtering with a hafnium (Hf) target. Deposition conditions were set aiming at a stoichiometric hafnia layer as checked by Rutherford backscattering spectrometry (RBS). RBS was performed using 2 MeV He+ ions and a detection angle of the backscattered particles of 165°. This technique was also used to determine Hf areal densities. Pt films that were 5 and 20 nm thick were deposited by DC magnetron sputtering. PDA in oxygen was performed in a static atmosphere furnace. Samples were annealed in 1 atm of oxygen enriched to 97% in the 18O isotope (hereafter termed 18 O2). Depth distributions of 18O (natural abundance of 0.2%) were obtained by nuclear reaction profiling (NRP) using the resonance at 151 keV in the cross-section curve of the 18 O(p,α)15N nuclear reaction.18 18O areal densities were determined by nuclear reaction analysis (NRA) using the plateau region in the cross-section curve of the same reaction (730 keV incident protons).19 For ion beam analyses we used a 3 MV HVEE Tandetron ion accelerator and a 500 kV HVEE ion implanter. X-ray reflectivity (XRR) analyses were performed on PANalytical X’Pert3 Powder equipment, using

obtained from HfO2 deposited on Si and Ge substrates following annealing in 18O2 at the indicated temperatures. 18O profiles assumed in the simulations are shown in the right side of the same figure. Profiles obtained from HfO2 films deposited using 18O2 are used to find the position of the dielectric/ semiconductor interface as well as to set the vertical concentration axes. Samples annealed at 350 °C presented an 18 O incorporation in the first nanometers beneath the HfO2 surface. Following annealing at 450 °C, 18O incorporation takes place throughout the HfO2 layer, reaching ∼40% of the oxygen concentration of stoichiometric HfO2. A further increase in the annealing temperature to 550 °C raises the 18O concentration to ∼50%. 18O profiles also reach deeper regions of the samples. This oxygen exchange dependence with temperature was previously observed by Goncharova et al.20 Their results indicate that atomic oxygen diffusion via oxygen lattice exchange is the predominant diffusion mechanism in hafnia. Despite the intense O exchange observed in bulk HfO2, 18O incorporation beyond the HfO2/semiconductor interface reaches slightly deeper regions in both Ge- and Si-based samples: ∼ 0.1 and ∼0.4 nm, respectively. These values scale with the transition layer thickness obtained by XRR analyses of as-deposited samples (see the Supporting Information (SI)). The thinner transition region observed for Ge is a result of differences in the initial stages of sputter deposition of HfO2 on Ge and Si.21 The Hf chemical environment close to the semiconductor substrate was probed with XPS in conjunction with Ar sputtering. Components with binding energies lower than HfO2 were observed (see the SI), confirming the formation of a transition region. 18O profiles evidence that no pronounced oxidation of the semiconductor substrate is taking

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RESULTS AND DISCUSSION Figure 1 (left side) presents measured (symbols) and simulated (lines) excitation curves of the 18O(p,α)15N nuclear reaction

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oxygen exchange in hafnia since their amount is no longer a function of the concentration of oxygen dissociation sites in this material. Since the diffusivity of atomic oxygen is higher than that of the molecular species in HfO2, a higher supply of oxygen is also provided to the HfO2/semiconductor interface. Thus, substrate oxidation is more pronounced in this case. The difference observed in the extent of substrate oxidation while comparing Si and Ge is a result of the higher reactivity of the latter. The low diffusivity of atomic oxygen in the formed SiO2 can also hamper further Si oxidation. The formation of GeO2 in the case of Ge does not seem to hamper O transport as in the case of SiO2. The more defective nature of this oxide4,24 in contact to Ge at this temperature creates transport and incorporation paths for incoming oxygen. Oxygen dissociated by Pt interacts with the underlying dielectric layer promoting exchange with oxygen previously present in HfO2. This inward O flux is partially consumed at the dielectric/semiconductor interface, resulting in substrate oxidation. Oxygen also leaves the sample through the Pt layer. This picture applies to Ge-based samples, where a pronounced Ge oxidation is observed (see Figure 2). In the case of Si (more resistant to oxidation) a higher outward O flux through Pt should take place, resulting in a higher O exchange in hafnia. In order to check these assumptions, we prepared a new set of samples: HfO2 was sputter deposited with 18O2 followed by Pt deposition. The resulting stacks were then annealed in natural O2. Nuclear reaction data regarding these samples are shown as lines in Figure 2. The sample prepared on Ge presents a lower 18 O loss in comparison with its Si counterpart. Moreover, 18O is present in deeper regions in the former case. Both observations evidence the higher O consumption at the HfO2/Ge interface and the consequent lower O loss. The role of Pt in supplying atomic oxygen to the HfO2/ semiconductor interface can be confirmed by varying the flux of O originated by this metal layer. This was accomplished by depositing a thicker Pt layer (20 nm), increasing the diffusion path of oxygen. Nuclear reaction data of these samples are shown in Figure 3. Almost no 18O was detected at the region corresponding to HfO2 following PDA at 350 °C. Samples annealed at 450 °C evidence oxygen exchange in HfO2. At 550 °C, 18O incorporation is more intense. Nevertheless, the trailing edges of the excitation curves for both Ge and Si samples correspond to those of the respective reference samples, evidencing that substrate oxidation does not take place, at least not to the same extent as the counterpart samples of Figure 2. Analysis of the total amount of 18O incorporated in samples covered with Pt confirms the oxidation of the semiconductor substrate in specific PDA temperatures and Pt layer thickness. Figure 4 shows 18O concentrations obtained by NRA as a function of the annealing temperature for samples covered with Pt. The amount of oxygen in the as-deposited sample is indicated by a dashed line. This value corresponds to twice the Hf concentration of as-deposited samples determined by RBS. The actual O concentration may be slightly lower due to the formation of the transition region between HfO2 and the substrate, which does not have the same stoichiometry from the bulk oxide. The sample prepared on a Ge substrate with a 5 nm thick Pt layer has a higher 18O concentration than the O content of the as-deposited HfO2 layer. Since no considerable amount of this isotope is incorporated inside Pt (see Figure 2), Ge substrate oxidation accounts for this additional oxygen amount. The same sample prepared on Si shows a lower 18O concentration, which confirms that Si is not as oxidized as Ge.

place at the PDA conditions of the present work. These observations confirm the results of Ferrari et al.15 that HfO2 is a diffusion barrier for molecular oxygen. A much higher oxidation rate of the Ge substrate was expected at 550 °C,22 confirming the diffusion barrier role of HfO2 in this case. Moreover, the concentration of oxygenic species that may reach the substrate is not enough to promote a substantial oxidation during annealing. As shown in the following, this picture is completely modified by the deposition of a Pt layer on HfO2. PDAs in 18O2 were performed with HfO2/Ge and HfO2/Si samples covered by a 5 nm Pt layer. Nuclear reaction data regarding these samples are shown in Figure 2. α particle yield

Figure 2. Experimental excitation curves (symbols) of the 18O(p,α)15N nuclear reaction for Pt(5 nm)/HfO2 stacks deposited on Ge (upper part) and on Si (lower part). Data correspond to a reference Pt/Hf18O2 stack deposited on both substrates (open symbols) and to Pt/HfO2/semiconductor structures submitted to annealing in 1 atm of 18 O2 for 15 min at the indicated temperatures (solid symbols). Lines correspond to nuclear data obtained from Pt/Hf18O2/semiconductor structures submitted to annealing in 1 atm of natural O2 for 15 min at 550 °C. Shaded areas are the energy window corresponding to the HfO2 layer. a.u. stands for arbitrary units.

in the figure is proportional to 18O concentration, and depth in the sample scales with proton energy. The curves correspond to the actual concentration versus depth information convoluted with instrumental and proton energy loss functions.23 Pt/HfO2 and HfO2/semiconductor interface depths were estimated by the energy position of the leading and the trailing edges of the excitation curves obtained from reference samples (Hf18O2 deposited with 18O2), respectively. Samples annealed at 350 °C do not evidence oxygen exchange in the HfO2 layer as no product of the nuclear reaction is detected within the proton energy range of the shaded area (this energy window corresponds to the HfO2 layer). At 450 °C oxygen exchange is observed predominantly in HfO2 for both substrates. Following annealing at 550 °C, different 18O distributions are observed while comparing Si- and Ge-based samples. In both cases, 18O is detected in deeper regions than the HfO2/ semiconductor interface depth, evidencing substrate oxidation. This effect is more pronounced in the case of Ge. Moreover, excitation curves indicate a much stronger oxygen exchange taking place in HfO2 in comparison with the samples without Pt. These observations evidence the role of Pt in supplying oxygenic species, most probably atomic oxygen, to the underlying HfO2 layer. These species promote an intense C


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oxidation since HfO2 constitutes a diffusion barrier for this species. Annealing of HfO2/Ge samples in O2 promotes oxygen exchange within HfO2 without significant substrate oxidation since the amount of O that reaches Ge is a function of the available sites in HfO2 for O2 dissociation. On the other hand, Pt overlayer supplies atomic oxygen to HfO2, raising the concentration of this species beyond that obtained with bare HfO2 for a given temperature. Consequently, there is a higher O concentration at the HfO2/Ge interface. Following this reasoning, longer annealings should increase the amount of 18O incorporated in Pt/HfO2/Ge stacks, while a less pronounced influence of time should be observed with HfO2/Ge counterparts. The results of such a comparison are shown in Figure 5:

Figure 3. Experimental excitation curves (symbols) of the 18O(p,α)15N nuclear reaction for Pt(20 nm)/HfO2 stacks deposited on Ge (upper part) and on Si (lower part). Data correspond to a reference Pt/Hf18O2 stack deposited on both substrates (open symbols) and to Pt/HfO2/semiconductor structures submitted to annealing in 1 atm of 18 O2 for 15 min at the indicated temperatures (solid symbols). Shaded areas are the energy window corresponding to the HfO2 layer. a.u. stands for arbitrary units.

Figure 5. Experimental excitation curves of the 18O(p,α)15N nuclear reaction for HfO2 films deposited on Ge covered (lower part) or not (upper part) by a Pt layer. Samples were submitted to annealing in 1 atm of O2 for 15 min (circles) and 2 h (triangles) at 450 °C. a.u. stands for arbitrary units. Figure 4. 18O areal densities as a function of the oxidation temperature in 18O2 for Ge- and Si-based samples with different Pt layer thicknesses. The dashed line corresponds to the 18O areal density of the as-deposited sample. This value is twice the Hf concentration of asdeposited samples determined by RBS.

HfO2/Ge samples with and without Pt overlayers were annealed in 18O2 for 15 and 120 min at 450 °C. 18O profiles of samples without Pt are superposable. Pt-covered samples presented a strong time influence: the longer the time, the higher the 18O incorporation. This result evidences the higher supply of oxygenic species provided by Pt which diffuse through HfO2, oxidizing the Ge substrate. In Figure 6, a sketch of the phenomena taking place at each layer of the samples investigated in the present work and the net flux of atomic O at the interfaces is presented.

Lower PDA temperatures and thicker Pt layers (20 nm) result in a lower 18O incorporation which is almost entirely related to oxygen exchange within HfO2. Besides the formation of a GeO2 interlayer in specific annealing conditions, additional modifications of the stacks may take place. Delabie et al.25 evidenced that HfO2 layers deposited on Ge and Si crystallize during thermal annealing at temperatures as low as 400 °C. With the increase of temperature, the transformation from the cubic to monoclinic phase is observed. The main difference for Ge and Si substrates is that no epitaxial crystallization was observed on Si. Soares et al.26 also observed crystallization of HfO2 during thermal annealings, further evidencing the influence of the annealing atmosphere. In the present work, we performed grazing incidence X-ray diffraction (GIXRD) aiming at indentifying HfO2 crystallization (results not shown). No diffraction peaks were observed. The sensitivity of the technique was probably not sufficient to detect a limited amount of crystalline phase. Oxygen incorporation in Pt/HfO2/Ge stacks seems to be governed by the amount of atomic oxygen supplied by the Pt layer which diffuses through HfO2 and reacts with Ge. Molecular oxygen does not contribute significantly to Ge

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CONCLUSIONS In summary, oxygen transport and incorporation in Pt/HfO2/ Ge stacks were investigated. HfO2 deposited on Ge constitutes a diffusion barrier for molecular O2, preventing pronounced substrate oxidation. The supply of atomic oxygen (able to diffuse through HfO2) is a function of temperature and the number of available O2 dissociation sites in HfO2. O transport promotes exchange of this element in the HfO2 lattice. The extent of oxidation of the underlying Ge depends on the amount of O reaching the HfO2/Ge interface. Deposition of a top Pt layer promotes a more efficient O2 dissociation, raising the net flux of incoming O in HfO2. This process is followed by a higher O exchange in HfO2 and a higher O supply at the HfO2/semiconductor interface. In the case of Ge, the more defective nature of its oxide (in comparison with SiO2) promotes a higher consumption of O. Thus, oxidation of the D


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(4) Kamata, Y. High-k/Ge MOSFETs for Future Nanoelectronics. Mater. Today 2008, 11, 30−38. (5) Wang, S. K.; Kita, K.; Lee, C. H.; Tabata, T.; Nishimura, T.; Nagashio, K.; Toriumi, A. Desorption Kinectics of GeO from GeO2/ Ge Structure. J. Appl. Phys. 2010, 108, 054104_1−054104_8. (6) Seo, J. W.; Dieker, Ch.; Locquet, J.-P.; Mavrou, G.; Dimoulas, A. HfO2 high-k Dielectrics Grown on (100)Ge with Ultrathin Passivation Layers: Structure and Interfacial Stability. Appl. Phys. Lett. 2005, 87, 221906_1−221906_3. (7) Minoura, Y.; Kasuya, A.; Hosoi, T.; Shimura, T.; Watanabe, H. Design and Control of Ge-based Metal-Oxide-Semiconductor Interfaces for High-mobility Field-effect Transistors with Ultrathin Oxynitride Gate Dielectrics. Appl. Phys. Lett. 2013, 103, 033502_1− 033502_5. (8) Lu, C.; Lee, C. H.; Zhang, W.; Nishimura, T.; Nagashio, K.; Toriumi, A. Enhancement of Thermal Stability and Water Resistance in Yttrium-doped GeO2/Ge Gate Stack. Appl. Phys. Lett. 2014, 104, 092909_1−092909_4. (9) Wilk, G. D.; Wallace, R. M.; Anthony, J. M. High-κ Gate Dielectrics: Current Status and Materials Properties Considerations. J. Appl. Phys. 2001, 89, 5243−5275. (10) Henkel, C.; Bethge, O.; Abermann, S.; Puchner, S.; Hutter, H.; Bertagnolli, E. Pt-assisted Oxidation of (100)-Ge/high-k Interfaces and Improvement of their Electrical Quality. Appl. Phys. Lett. 2010, 97, 152904_1−152904_3. (11) Hultquist, G.; Hornlund, E.; Dong, Q. Platinum-induced Oxidation of Chromium in O2 at 800 °C. Corros. Sci. 2003, 45, 2697− 2703. (12) Yang, Z.; Jinlong, W.; Xiaohu, Y. Density Functional Theory Studies on the Adsorption, Diffusion and Dissociation of O2 on Pt(111). Phys. Lett. A 2010, 374, 4713−4717. (13) Dong, Q.; Hultquist, G.; Sproule, G. I.; Graham, M. J. Platinumcatalyzed High Temperature Oxidation of Metals. Corros. Sci. 2007, 49, 3348−3360. (14) Foster, A. S.; Shluger, A. L.; Nieminen, R. M. Mechanism of Interstitial Oxygen Diffusion in Hafnia. Phys. Rev. Lett. 2002, 89, 225901_1−225901_4. (15) Ferrari, S.; Fanciulli, M. Diffusion Reaction of Oxygen in HfO2/ SiO2/Si Stacks. J. Phys. Chem. B 2006, 110, 14905−14910. (16) Baumvol, I. J. R. Atomic Transport During Growth of Ultrathin Dielectrics on Silicon. Surf. Sci. Rep. 1999, 36, 1−166. (17) Okumura, H.; Akane, T.; Matsumoto, S. Carbon Contamination Free Ge(100) Surface Cleaning for MBE. Appl. Surf. Sci. 1998, 125, 125−128. (18) Driemeier, C.; Miotti, L.; Pezzi, R. P.; Bastos, K. P.; Baumvol, I. J. R. The Use of Narrow Nuclear Resonances in the Study of Alternative Metal-Oxide-Semiconductor Structures. Nucl. Instrum. Methods Phys. Res., Sect. B 2006, 249, 278−285. (19) Pitthan, E.; Corrêa, S. A.; Soares, G. V.; Radtke, C.; Stedile, F. C. Synthesis and Applications of 18O Standards for Nuclear Reaction Analysis. Nucl. Instrum. Methods Phys. Res., Sect. B 2014, 332, 56−59. (20) Goncharova, L. V.; Dalponte, M.; Starodub, D. G.; Gustafsson, T.; Garfunkel, E.; Lysaght, P. S.; Foran, B.; Barnett, J.; Bersuker, G. Oxygen Diffusion and Reactions in Hf-based Dielectrics. Appl. Phys. Lett. 2006, 89, 044108_1−044108_3. (21) Kita, K.; Kyuno, K.; Toriumi, A. Growth Mechanism Difference of Sputtered HfO2 on Ge and on Si. Appl. Phys. Lett. 2004, 85, 52−54. (22) da Silva, S. R. M.; Rolim, G. K.; Soares, G. V.; Baumvol, I. J. R.; Krug, C.; Miotti, L.; Freire, F. L., Jr.; da Costa, M. E. H. M.; Radtke, C. Oxygen Transport and GeO2 Stability during Thermal Oxidation of Ge. Appl. Phys. Lett. 2012, 100, 191907_1−191907_4. (23) Amsel, G.; Nadai, J. P.; D’Artemare, E.; David, D.; Girard, E.; Moulin, J. Microanalysis by the Direct Observation of Nuclear Reactions Using a 2 MeV Van de Graaff. J. Nucl. Instrum. Methods 1971, 92, 481−498. (24) Wang, S. K.; Kita, K.; Nishimura, T.; Nagashio, K.; Toriumi, A. Kinetic Effects of O-Vacancy Generated by GeO2/Ge Interfacial Reaction. Jpn. J. Appl. Phys. 2011, 50, 10PE04_1−10PE04_4.

Figure 6. Sketch of the mechanisms taking place during O2 annealing of the investigated structures. The net oxygen fluxes at the Pt/HfO2 interface and HfO2/semiconductor interface are represented by arrows. The size of the arrow indicates the flux intensity. O2 molecules are dissociated by Pt. O diffuses through HfO2 promoting exchange with O from the hafnia lattice. The intensity of O exchange and substrate oxidation are determined by the presence of Pt and by the semiconductor material (Ge or Si).

Ge substrate is higher than that of bare HfO2 samples. On the other hand, in the case of Si, the lower diffusivity of charged O species in interfacial Si oxide prevents pronounced oxidation. Since oxygen consumption at this region is not as high as in Ge, O loss from the sample is more pronounced. The different nature of thermally grown oxides on Si (amorphous with low density of defects in the bulk) and on Ge (polymorph with oxygen vacancies produced at the GeO2/Ge interface) has a direct influence on the resulting physicochemical modifications of the stacks prepared on these semiconductor materials. These results constitute important benchmarks to the choice of PDA conditions and Pt electrode thickness aiming at Pt/HfO2/Ge stacks with the desired characteristics. They also evidence the complex interplay between materials during processing of this stacked structure which can be part of many electronic devices.

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ASSOCIATED CONTENT

S Supporting Information *

XRR and XPS characterization of as-deposited samples. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +55 51 33086204. Fax: +55 51 33087304. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to thank the financial support of INCT Namitec, INCT INES, MCT/CNPq, CAPES, and FAPERGS. REFERENCES

(1) Pillarisetty, R. Academic and Industry Research Progress in Germanium Nanodevices. Nature 2011, 479, 324−328. (2) Gupta, S.; Gong, X.; Zhang, R.; Yeo, Y.; Tagaki, S.; Saraswat, K. C. New Materials for post-Si Computing: Ge and GeSn Devices. MRS Bull. 2014, 39, 678−686. (3) Matsubara, H.; Sasada, T.; Takenaka, M.; Tagaki, S. Evidence of Low Interface Trap Density in GeO2/Ge Metal-Oxide-Semiconductor Structures Fabricated by Thermal Oxidation. Appl. Phys. Lett. 2008, 93, 032104_1−032104_3. E


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The Journal of Physical Chemistry C (25) Delabie, A.; Puurunen, L.; Brijs, B.; Caymax, M.; Conard, T.; Onsia, B.; Richard, O.; Vandervorst, W.; Zhao, C.; Heyns, M. M.; et al. Atomic Layer Deposition of Hafnium Oxide on Germanium Substrates. J. Appl. Phys. 2005, 97, 064104_1−064104_10. (26) Soares, G. V.; Feijó, T. O.; Baumvol, I. J. R.; Aguzzoli, C.; Krug, C.; Radtke, C. Thermally-driven H Interaction with HfO2 films Deposited on Ge(100) and Si(100). Appl. Phys. Lett. 2014, 104, 042901_1−042901_4.

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HfO2 Stacks

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