AVS2009 Session EM2-TuM: Complex and Multifunctional Oxides

Hafnium oxide (HfO₂), which exhibits a very high dielectric constant (k) and large bandgap, is considered as a next-generation high-k material for application in complementary metal-oxide-semiconductor (CMOS) technology. However, growth of high-quality HfO₂ layers on Si without the formation of interfacial compounds poses a significant challenging problem. The objective of the present work was to optimize the conditions to grow high-quality HfO₂ nanolayered films on Si(100). In our work, HfO₂ films were grown by RF sputtering of HfO₂ ceramic target at various substrate temperatures (T_s= 30-500 ^oC) and studied their structure and electrical properties. Al/HfO₂/Si capacitive structures were fabricated to obtain a metal-oxide-semiconductor (MOS) configuration to mimic the gate stack of CMOS technology and study the electrical properties. Grazing incidence x-ray diffraction (GIXRD) and X-ray photoelectron spectroscopy (XPS) measurements indicate that the effect of T_s is significant on the microstructure. HfO₂ films grown at T_s<200 ^oC are amorphous. An amorphous-to-crystalline transition occurs at T_s=200 ^oC. Nanocrystalline HfO₂ films crystallized in a monoclinic structure with a particle size ~20 nm. XPS measurements indicate the high chemical quality of HfO₂ films grown at T_s=30-500 ^oC. The capacitance-voltage characteristics of the Al/HfO₂/Si devices indicate that HfO₂ films grown (or post-deposition annealed) at 400 ^oC exhibit the expected monoclinic-HfO₂ characteristics. HfO₂ films exhibit a direct correlation with the microstructure. The results obtained are presented and discussed in detail.

As the MOSFET SiO₂-based gate dielectric layer approaches its fundamental physical limits, the investigation of high-k oxides is ongoing in order to determine which oxides can best continue the scaling of the MOSFET. HfO₂, hafnium silicates and nitrided hafnium silicates are leading candidates due to their relatively large band gaps, thermal stability in proximity to Si and relatively high dielectric constants.

The band offsets between the high-k oxide layers and Si in high-k/SiO₂/Si films stacks are important parameters in that the gate leakage current depends strongly on them. Recent studies by various groups have also shown that the threshold voltage in MOSFETs with high-k gate oxides can be altered by the presence of an additional oxide layer such as La₂O₃ or Al₂O₃.^1-3 In the case of a La₂O₃ interface layer, the flatband voltage shift has been correlated with a shift in the energy level alignment in the high-k gate stack.⁴

We have used a combination of x-ray photoemission spectroscopy (XPS) and spectroscopic ellipsometry (SE) to measure the valence and conduction band offsets (VBO and CBO, respectively) between high-k layers and Si substrates. We will report VBO and CBO values for HfO₂, hafnium silicate and nitrided hafnium silicate films with Si. In addition, we will report measurements of the HfO₂-Si band offsets from HfO₂/La₂O₃/SiO₂/Si film stacks, noting the effect of the La₂O₃ layer.

Analysis of XPS spectra of the Si 2p spectra from HfO₂/SiO₂/Si film stacks will also be reported, which shows that the composition of the SiO₂ layer and the energy level alignment between the SiO₂ and Si layers were affected by the growth of the HfO₂ layer and the annealing of the film stack. These results will be discussed with respect to the band offset measurements for HfO₂ film stacks.

Finally, using angle resolved XPS (ARXPS) data we constructed non-destructive compositional depth profiles using a maximum entropy algorithm. A comparison between thickness values extracted from these depth profiles and thicknesses measured with SE will be presented.

References:

1. P. D. Kirsch et al., Appl. Phys. Lett., 92, 092901 (2008).

2. V. Narayanan et al., Symp. VLSI Tech., 178 (2006)

3 K. Iwamoto et al., Appl. Phys. Lett., 92, 132907 (2008)

4. K. Kakushima et al., Appl. Surf. Sci., 254, 6106 (2008).

The use high dielectric constant materials (high-κ) as gate dielectric in complementary metal-oxide-semiconductor devices based on Si and other higher carrier mobility semiconductors has been focus of intense research. So far, hafnium oxide is among the high-κ dielectrics with higher dielectric constant and higher technologic interest. The most common phase of HfO₂ thin films after deposition and processing is monoclinic with a dielectric constant of κ~ 16-20. However, its dielectric constant can be improved by increasing it in its tetragonal (κ~ 30-35) or cubic phase (κ~ 20-25), which are its thermodynamic equilibrium phases at higher temperatures

We report here on the investigation of Ge doping HfO₂ films and its role in stabilizing the tetragonal phase of hafnium oxide phase. The films were deposited by remote plasma enhanced chemical vapor deposition on both Si(001) and Ge(001) substrates. Hf-t-butoxide and GeH₄ were used as source for Hf and Ge, while O₂ as the oxidizing agent. Germanium concentration in the films was determined by Auger electron spectroscopy. Interface reaction was hindered by nitridation of the substrates before the deposition of the films. Conventional x-ray diffraction analysis is not suited to investigate the crystallographic order in very thin films (< 10 nm), as well as those of interest for device applications. Therefore, we probed the local symmetries by x-ray absorption spectroscopy using the beam line 10 in SSRL.

The x-ray absorption spectra for 2 and 5 nm thick HfO₂ films deposited on Si with 0, 5, or 15 at.% Ge, as well as the second derivative of these spectra was studied. Based on previous studies of HfO₂ films on Si, the shape of the transitions to the E_g and T_2g states of the 5 nm thick film without Ge doping corresponds to a monoclinic phase with a different Jahn-Teller distortion than the trigonal. A significant change in these features is observed when the film is doped with 5 at.% Ge. In this case the E_g and T_2g transitions correspond to a tetragonal symmetry. Higher germanium doping leads to a mixture of monoclinic and tetragonal phases causing a broadening of the absorption spectra. There is not significant difference in the absorption spectra for the 2 nm thick films with different doping, indicating that the Ge stabilization of a tetragonal phase can only be achieved above a critical thickness.

The mechanism that stabilizes the tetragonal phase of HfO₂ using a tetravalent dopant is significantly different from that associated with trivalent impurities as yttrium, which involves oxygen vacancies. This mechanism results in decrease of the c/a ratio with no vacancies involved.

Electron tunneling underlies numerous devices relevant to information technology and has been proposed in future energy harvesting and quantum computing applications. Replacing a conventional insulator in the tunnel junction with an electronically correlated material can yield new types of electronic functionality. In one such concept, dubbed ferroelectric tunneling, the tunneling barrier height is controlled by the polarization of a ferroelectric oxide, enabling non-volatile conduction states that can be switched with electric field. Although ferroelectric tunneling has been thoroughly theorized, a convincing experimental demonstration of this phenomenon is still lacking. The key challenge is to find a material system that simultaneously satisfies the dimensional constraints for tunneling and ferroelectricity, as well as to assure that the conductance is not dominated by extrinsic effects of charge injection and filamentary conduction, which is ubiquitous in complex oxides.

In this talk we will demonstrate a highly reproducible polarization control of local electron transport through epitaxial Pb(Zr_0.2Ti_0.8)O₃ films. Despite being 30-50 nm thick, conductive atomic force microscopy revealed that the films possessed spatially and temporally reproducible local conductivity in the regime of Fowler-Nordheim electron tunneling. This is likely due to a strong electric field in the sub-surface region (excess of 10⁶ V/cm) created by the relatively sharp metal tip. Local I-V characteristics exhibited strong hysteretic behavior across the surface. By combining conducting atomic force microscopy with piezoresponse force microscopy, we have, for the first time, directly correlated local events of ferroelectric and resistive switching [1]. The large spontaneous polarization of PZT produced as strong as 500-fold enhancement of FN-tunneling conductance upon ferroelectric switching, sufficient to demonstrate a local non-volatile memory function. The physical mechanism of the observed effect was traced to the polarization-dependence of the height and possibly width of the metal-ferroelectric Schottky barrier.

By observing the role of inherent disorder in ferroelectrics and comparing films grown on different electrode materials, we have shown that the switching voltage and the magnitude of conductance hysteresis are subject to electrostatic control via ferroelectric switching. Variable-temperature measurements and local effects due to dielectric non-linearities will also be discussed.

[1] P. Maksymovych, S. Jesse, P. Yu, R. Ramesh, A. P. Baddorf, S. V. Kalinin, Science (2009) in press.

There is considerable interest in complex oxides comprised of transition metal, TM, and lanthanide rare earth, LRE, atoms with room temperature magnetic properties that can modulated by electrical input. Ferro- and ferri-magnetic properties require strongly correlated spin sub-bands derived from TM(LRE) d(f)-states, and have been reported in elemental oxides: CrO₂ and Fe₃O₄, and EuO₂. Strongly correlated bands are created by a double exchange mechanism requiring a transition to a metallic state. The incorporation of tetravalent Ti into a d⁰ complex oxide with trivalent Gd and Sc, requires that Ti be in a formal trivalent valence state, Ti³⁺. Alloying with Ti then introduces a d¹ occupied state into the complex oxide host in direct proportion to the Ti content.

Gd(Sc_1-xTi_x)O₃ alloys > 5 nm thick with x = 0.0, 0.01, 0.05, 0.18 and 0.25 were deposited at room temperature in an UHV system onto (i) LaAlO₃ substrates for epitaxial growth, and (ii) superficially oxidized Si(001) to produce nano-grain films. As-deposited nano-grain dimensions are 2-2.5 nm, and are too small for Jahn-Teller distortions and spin ordering of Ti alloy atoms. The correlation exchange energy for spin correlated bands is obtained from room temperature films on Si is approximately equal to the energy difference between the localized Ti impurity state energy and Sc band edge d-state.

A compositionally dependent insulator/metal transition is reported for the first time in Gd(Sc_1-xTi_x)O₃ alloys for x > 0.16, and is attributed with a correlated Ti d-state band. This insulator to metal transition is identified by X-ray absorption spectroscopy, XAS, extending into the pre-edge regime for X-ray energies <530 eV. Annealing at 900°C in Ar increases grain size enabling Jahn-Teller distortions, and results in an insulator to metal transition evidenced by opening of a gap between oppositely directed Ti spin states. This insulator to metal transition is also observed in as-deposited epitaxial films as well, and consistent with percolation theory, only if the Ti concentration exceeds a critical concentration of ~16.5 %, as in the 18 and 25 % alloys. Differentiation of XAS spectra indicates ferrimagnetism, with a second partially occupied spin band. As predicted by theory, the separation of these spin bands is the same as the correlation exchange energy obtained from the Ti impurity band energy.

Ferroelectric/semiconductor heterostructures are desirable for multifunctional devices using the charge of a ferroelectric material to manipulate the conductivity of a semiconductor. The quality of the ferroelectric/semiconductor interface is critical for maintaining a significant ferroelectric polarization charge density, and coupling this charge density into the semiconductor. Therefore, materials must have excellent chemical and structural compatibility. ZnO and LiNbO₃ may provide the desired characteristics based on the crystalline compatibility of the materials, excellent semiconducting properties of ZnO, and excellent ferroelectric properties of LiNbO₃. In this work, the structural and electrical characteristics of ZnO thin films deposited on z-cut LiNbO₃ substrates by pulsed laser deposition will be presented and compared to ZnO thin films on c-plane sapphire substrates. In all experiments, preferentially-oriented c-plane ZnO thin films were obtained based on x-ray diffraction measurements.

Hall effect measurements demonstrate a background carrier concentration in ZnO of n=2. 6x10¹⁷cm^-3 for ZnO/LiNbO₃, and an order of magnitude decrease of n=3.0x10¹⁸cm^-3 for ZnO/sapphire. Similarly, an improved electron mobility of μ=36 cm²/Vs is observed for ZnO/LiNbO₃ in comparison to μ=21 cm²/Vs for ZnO/sapphire. The reduced carrier concentration and improved mobility are attributed to a depletion layer at the ZnO/LiNbO₃ interface induced by polarization charge. The temperature dependence of electron transport in ZnO thin films will also be presented to examine the influence of polarization charge induced by the pyroelectric effect in LiNbO₃.

High quality SiO₂ film formation is important for many applications as electrical insulator films, protective films for semiconductor, alkali-dissolution barrier films and antireflection films on the glass. In some of applications, the electrical property of the films plays an important role. Various techniques have been used thus far in preparing SiO₂ films. Especially, in various techniques, sol-gel technique has significant advantage compared with other techniques with uniformity of thickness, damage free, easy coating of large surfaces, homogeneous multi-component oxide films, controllability of compositions, and potential industrial application. So, in this work, we have evaluated the electrical and structural characteristics of SiO₂ film using organosiloxane-based silica precursor. And we have evaluated the influence of additives to organosiloxane-based silica precursor, such as TiO₂, HfO₂, Ta₂O₅, ZrO₂ and several kinds of metal oxide components on the electrical and structural property of these SiO₂ base insulator films.

The organosiloxane-based sol-gel material investigated here is derived from the mixture of tetraethoxysilane (TEOS) and methyltrimethoxysilane (MTMS) by changing the molar ratio of TEOS/MTMS. In the case of the addition of metal oxide to SiO₂ matrix, such as TiO₂, HfO₂, Ta₂O₅, ZrO₂ and several kinds of metal oxide, metal alkoxides such as tetraisopropoxytitanate (TPOT) were added in the concentration ranging from 1 to 15 mol %. In order to evaluate the electrical property of films, we fabricated MOS (Metal-Oxide-Silicon) devices.

By controlling mixing molar ratio between TEOS and MTMS in the sol-gel precursor, the dielectric constant of films can be controlled. This sol-gel precursor was prepared not only of perfect silica film but also a dense film through optimized conditions such as the pre-baking for 5 min at 130˚C, N₂ ambience low pressure gas removing process continuously increasing the temperature up to 900˚C, and oxidation process using O₂/H₂O at 900˚C. In this study, the influence of additives to the sol-gel precursor was examined on the structural and electrical property of the SiO₂ based films derived from the sol-gel precursor. The breakdown voltages of the resultant films were effectively improved by adding a small amount of metal oxide such as TiO₂, HfO₂, Ta₂O₅ and ZrO₂. It was found that the SiO₂ based film derived from the sol-gel precursor added with metal oxide components, such as TiO₂, HfO₂, Ta₂O₅ and ZrO₂, had excellent uniformly-structured Si-O-Metal bond and that the electrical insulation property of the film was improved by the concentration of metal oxide additives.