Susanne Hoffmann-Eifert

Sr diffusion in undoped and La-doped SrTiO3 single crystals under oxidizing conditions

Strontium titanate SrTiO3(100), (110), and (111) single crystals, undoped or donor doped with up to 1 at% La, were isothermally equilibrated at temperatures between 1523 and 1773 K in synthetic air followed by two different methods of Sr tracer deposition: ion implantation of 87Sr and chemical solution deposition of a thin 86SrTiO3 layer. Subsequently, the samples were diffusion annealed under the same conditions as before. The initial and final depth profiles were measured by SIMS. For strong La-doping both tracer deposition methods yield similar Sr diffusion coefficients, whereas for weak doping the tracer seems to be immobile in the case of ion implantation. The Sr diffusivity does not depend on the crystal orientation, but shows strong dependency on the dopant concentration supporting the defect chemical model that under oxidizing conditions the donor is compensated by Sr vacancies. A comparison with literature data on Sr vacancy, Ti, and La diffusion in this system confirms the concept that all cations move via Sr vacancies. Cation diffusion is several orders of magnitude slower than oxygen diffusion.

Advanced chemical deposition techniques - from research to production

Abstract Chemical deposition techniques show distinct advantages for research tasks as well as for microelectronic production lines. This review will summarize the state of the art of the Chemical Solution Deposition (CSD) and of the Metal-Organic Chemical Vapor Deposition (MOCVD) processes for high-permittivity, ferroelectric, and other complex electronic oxide thin films as well as their applications in integrated capacitors, FeRAMs, etc Emphasis will be laid on the selection and the properties of the precursors, the microscopic mechanisms of film deposition, and the tailoring of the microstructure by the deposition parameters. In the main part of the review, topics of current interest will be specifically discussed: (1) the deposition of ultrathin films, (2) device integration issues, and (3) lateral nanostructuring and the formation of single grains. The techniques will be evaluated for their potential use in a future nanotechnology environment. Besides these research-oriented topics, both CSD and MOCVD show features which render the techniques highly suitable for large-wafer, high through-put production. The migration path from R&D systems to production tools will be outlined.

Grain-Boundary Effect on the Curie-Weiss Law of Ferroelectric Ceramics and Polycrystalline Thin Films: Calculation by the Method of Effective Medium

The influence of grain boundaries on the dielectric properties of ferroelectric ceramics and polycrystalline thin films is described theoretically by the method of effective medium. Grain boundaries are modeled by low-permittivity (“dead”) layers, which do not exhibit ferroelectric instability. The effective permittivity of a polycrystalline material is calculated in the paraelectric regime above the transition temperature. The calculations are based on the solution of electrostatic problem for a spherical dielectric inclusion separated from the surrounding dissimilar matrix by a low-permittivity interface layer. For isotropic bulk ceramics, an analytic expression is derived for the effective permittivity as a function of the grain size, dead-layer thickness, and its permittivity. Temperature dependence of the aggregate dielectric response is calculated for BaTiO3 (BT) ceramics of different grain sizes and found to be in good agreement with measurements. It is shown that grain boundaries not only renormalize the Curie-Weiss temperature and constant, but may also cause deviations from the Curie-Weiss law. For BT polycrystalline thin films grown on dissimilar substrates, numerical calculations of the effective dielectric constants are performed, taking into account both the grain-boundary and substrate effects on the film anisotropic dielectric response. Theoretical predictions are compared with the grain size dependence of the permittivity of BT films grown on Pt-coated Si.

Influence of charge compensation mechanisms on the sheet electron density at conducting LaAlO3/SrTiO3-interfaces

The equilibrium conductance of LaAlO3/SrTiO3 (LAO/STO)-heterointerfaces was investigated at high temperatures (950?K-1100?K) as a function of ambient oxygen partial pressure (pO2). Metallic LAO/STO-interfaces were obtained for LAO grown on STO single crystals as well as on STO-buffered (La,Sr)(Al,Ta)O3 substrates. For both structures, the high temperature sheet carrier density nS of the LAO/STO-interface saturates at a value of about 1?×?1014?cm?2 for reducing conditions, which indicates the presence of interfacial donor states. A significant decrease of nS is observed at high oxygen partial pressures. According to the defect chemistry model of donor-doped STO, this behavior for oxidizing conditions can be attributed to the formation of Sr-vacancies as charge compensating defects.

Sharp ferroelectric phase transition in strained single-crystalline SrRuO3/Ba0.7Sr0.3TiO3/SrRuO3 capacitors

Single-crystalline all-perovskite SrRuO3/Ba0.7Sr0.3TiO3/SrRuO3 thin-film capacitors epitaxially grown on SrTiO3 exhibit a sharp paraelectric-to-ferroelectric phase transition at 350 K with a maximum permittivity of about 6660. This value is comparable to that of bulk ceramics and exceeds by several times the highest values reported for Ba0.7Sr0.3TiO3 thin film capacitors. The observed thickness dependence of the dielectric response is analyzed with the aid of a thermodynamic theory. It is shown that a weak decrease of the permittivity with the Ba0.7Sr0.3TiO3 thickness decreasing from 200 to 10 nm can be explained solely by the thickness-dependent strain relaxation in epitaxial films without assuming the presence of low-permittivity layers at the film/electrode interfaces.

High temperature conductance characteristics of LaAlO3/SrTiO3-heterostructures under equilibrium oxygen atmospheres

The interface conductance of LaAlO3/SrTiO3 heterostructures was investigated under high temperature oxygen equilibrium. The dependence of the heterostructure’s conductance on oxygen partial pressure (from 10−22 to 1 bar) and temperature (800 to 1100 K) was compared to the characteristic of SrTiO3 single crystals, which is described in terms of a defect chemistry model. Up to 950 K the equilibrated heterostructures reveal an additional influence of a metallic-like conduction path with a very slight dependence on the oxygen partial pressure. Donor-type interface states which may result from either lattice distortions or A-site cation intermixing during processing are discussed as a possible origin for the exceptional interface conduction of LaAlO3/SrTiO3 heterostructures.

Electrical conductivity and segregation effects of doped SrTiO3 thin films

Abstract The potential of perovskite type thin films for applications in electronic devices or oxygen sensors strongly relies on a detailed understanding of electrical properties. While the conduction mechanisms of single crystal SrTiO 3 , frequently referred to as a model system, have been widely investigated and are usually described in terms of point defect chemistry, the conductivity behavior of thin films has not been studied in detail. In this paper we report on investigations on doped SrTiO 3 thin films under varying oxygen partial pressures and at temperatures between 700 and 1000°C. The electrical measurements show remarkable results, where a sharp drop and a plateau region are the dominant characteristics in log( σ )–log( p O 2 ) plots. Recent investigations of the structural behavior of SrTiO 3 single crystal surfaces under certain atmospheric conditions suggested accompanying examinations by SEM and AFM, primarily addressing the morphological phenomena of the films.

Stoichiometry dependence and thermal stability of conducting NdGaO3/SrTiO3 heterointerfaces

The structural and electrical properties of conducting NdGaO3/SrTiO3 (NGO/STO) heterostructures grown at various deposition temperatures were investigated. X-ray diffraction and X-ray photoelectron spectroscopy reveal a strong impact of the growth temperature on both crystallinity and cation stoichiometry of the NGO thin films. This stoichiometry variation significantly affects the electrical properties of the NGO/STO interface. High temperature conductance measurements under oxygen equilibrium conditions show a distinct conductance contribution of the NGO/STO interface up to 1000 K and exclude a conduction effect caused by a mere reduction of the STO substrate. Above 1000 K, the interface conduction is degrading due to a thermal instability. Both stoichiometry variation in as-grown films and thermal instability are attributed to the preferential evaporation of gallium from the NGO thin films at elevated temperatures.

Growth of ternary PbTiOx films in a combination of binary oxide atomic layer depositions

Ternary PbTiOx films were deposited at 240°C on Pt-covered Si substrates using a combination of liquid injection atomic layer depositions of binary TiOx and PbO films. Ti(OC3H7)2(C11H19O2)2 [Ti(Oi−Pr)2(DPM)2] and Pb(C11H19O2)2 [Pb(DPM)2] dissolved in ethylcyclohexane and H2O were used as source materials. The deposition rates of Pb and Ti were enhanced in the ternary process compared to their binary processes under comparable deposition conditions. The Pb∕Ti ratio of PbTiOx films saturated with an increase in Ti precursor input, while it continued to increase with an increasing Pb precursor input. The self-regulated growth nature of the Pb–O layer in the binary film growth was lost in the mixed PbTiOx process as a result of interaction with the predeposited Ti–O layer. It was confirmed that for the PbTiOx film to grow on Pt substrates, an initial incubation period is required. Both Pb–O and Ti–O layers shared a common incubation period of up to ten sequences. The incubation period was shortened by increas...

Growth Behavior of Atomic-Layer-Deposited Pb ( Zr , Ti ) O x Thin Films on Planar Substrate and Three-Dimensional Hole Structures

Quaternary Pb(Zr,Ti)O x (PZT) films were deposited at 240°C by a combination of liquid-injection atomic layer deposition (ALD) of binary PbO, TiO x , and ZrO x thin films. In preliminary work, binary ZrO x films were deposited at 240°C by ALD. Two solutions of Zr(C 9 H 1S O 2 ) 4 [Zr(DIBM) 4 ] and Zr(C 11 H 19 O 2 ) 4 [Zr(DPM) 4 ] dissolved in ethylcyclohexane (ECH) were prepared, and it was found that the Zr(DIBM) 4 solution provides a three-times-higher deposition rate for the ZrO x films than the Zr(DPM) 4 solution. We focused the study on the set of precursors which offers the highest degree of flexibility for adjusting the Zr/(Zr + Ti) ratio in the PZT films: Pb(C 11 H 19 O 2 ) 2 [Pb(DPM) 2 ], Ti[OCH(CH 3 ) 2 ] 4 [Ti(Oi-Pr) 4 ], and Zr(DIBM) 4 dissolved in ECH, and water as the oxidant. This set of solutions contributed to increasing the Zr/(Zr + Ti) ratio in the deposited PZT films to more than 0.2, which remained below 0.1 in the ALD-PZT using Pb(DPM) 2 , Ti(Oi-Pr) 4 , and Zr(DPM) 4 . The Zr/(Zr + Ti) ratio was further increased to 0.5 by modifying the sequence of the discrete-source gas pulses. A polarization-voltage hysteresis loop was observed for a 70 nm thick PZT film deposited on a planar substrate after postannealing for crystallization. To assess the feasibility of ALD as a tool for coating three-dimensional (3D) structures uniformly, PZT films were deposited on submicrometer 3D structures. Asdeposited amorphous PZT films as well as crystallized PZT films were both free of any gradient in the cation composition over the structure. The present work reports interesting interactions on stacking different binary-oxide layers by ALD and demonstrates why the multiprecursor ALD process is a promising approach for uniformly coating 3D nanostructures with complex oxide materials.