Preparation and characterization of ferroelectric SrBi2Ta2O9 thin films on Si using Al2O3 buffer layers

SrBi₂Ta₂O₉ (SBT) thin films were prepared on p-type Si(100) substrates with Al₂O₃ buffer layers. Both the SBT films and the Al₂O₃ buffer layers were deposited by a pulsed laser deposition technique using a KrF excimer laser. An Al prelayer was used to prevent Si surface oxidization in the initial growth stage. It is shown that Al₂O₃ buffer layers effectively prevented interdiffusion between SBT and Si substrates. Furthermore, the capacitance–voltage (C-V) characteristics of the SBT/Al₂O₃/Si heterostructures show a hysteresis loop with a clockwise trace, demonstrating the ferroelectric switching properties of SBT films and showing a memory window of 1.6 V at 1 MHz. Received: 17 July 2000 / Accepted: 16 August 2000 / Published online: 30 November 2000     

Film structure and ferroelectric properties of in situ grown SrBi2Ta2O9 films

Ferroelectric SrBi₂Ta₂O₉ (SBT) films were grown by pulsed-laser deposition (PLD) at different substrate temperatures and fluences. A correlation between film structure and ferroelectric properties is established. The dielectric function ε^′ of thin SBT films shows a Curie–Weiss behavior well below the peak temperature T_max and relaxor-like behavior in the vicinity of T_max. Domain walls have a strong influence on the dielectric and ferroelectric properties and on the polarization fatigue of SBT films below 100 °C. The formation of ferroelectric phases is favored at lower substrate temperatures by incorporating Bi₂O₃ template layers into the structure. Received: 18 March 1999 / Accepted: 19 March 1999 / Published online: 5 May 1999     

Improvement of metal–ferroelectric–silicon structures without buffer layers between Si and ferroelectric films

Si-based metal–ferroelectric–semiconductor (MFS) structures without buffer layers between Si and ferroelectric films have been developed by depositing SrBi₂Ta₂O₉ (SBT) directly on n-type (100)-oriented Si. Some effective processes are adopted to improve the electrical properties of these MFS structures. Contrary to the conventional MFS structures with top electrodes directly on ferroelectrics, our MFS structures have been developed with thin dense SiO₂ films deposited between ferroelectric films and top electrodes. Due to the SiO₂ films, the leakage current densities of MFS structures are reduced to 2×10^-8 A/cm² under the bias of 5 V. The C-V electrical properties of the MFS structures are greatly improved after annealing at 400 °C in N₂ ambient for 1 h. The C-V memory windows are increased to 3 V, which probably results from the decrease of the interface trap density at the Si/SBT interface. Received: 7 September 1999 / Accepted: 24 November 1999 / Published online: 2 August 2000     

Voltage shift of hysteresis loops of SrBi2Ta2O9 thin films under unipolar stress

Voltage shifts of hysteresis loops of metalorganic decomposition (MOD)-derived SrBi₂Ta₂O₉ (SBT) thin films, known as imprint, have been observed after exposing the thin-film capacitors to unipolar pulses. The voltage shift changes with cumulative total time at maximum voltage, following a relationship with no pulse-width dependence. The origin of the voltage shift is briefly discussed in terms of an internal bias field induced by injected electrons trapped at positive polarity. The pulse-measurement responses are greatly affected by the internal bias field, even though no imprint failure was observed up to 10¹⁰ unipolar pulses. The voltage shift and asymmetric properties can be removed easily by applying bipolar pulses of saturation amplitude. Received: 27 June 2000 / Accepted: 16 August 2000 / Published online: 5 October 2000     

Growth of completely (110)-oriented Pt film on Si (100) by using MgO as a buffer by pulsed laser deposition

(110)-textured MgO films were grown on Si (100) with etching and without etching by pulsed laser deposition. The deposited MgO films were shown to be droplets-free. The MgO film was used as a buffer layer to further grow Pt film on Si (100). A completely (110)-oriented Pt film was obtained on such a buffer layer and its surface is very smooth with a roughness of about 7.5 nm over 5×5 μm. This can be used as a new oriented Pt electrode on silicon for devices. Received: 23 January 2001 / Accepted: 27 April 2001 / Published online: 27 June 2001     

Texture control and its impact on the properties of SrBi2Ta2O9 thin films prepared by pulsed laser deposition

SrBi₂Ta₂O₉ (SBT) ferroelectric thin films with different preferred orientations were deposited by pulsed laser deposition (PLD). Several methods have been developed to control the preferred orientation of SBT thin films. For SBT films deposited directly on Pt/TiO₂/SiO₂/Si substrates and in situ crystallized at the deposition temperature, the substrate temperature has a significant impact on the orientation and the remnant polarization (Pr) of the films; a higher substrate temperature benefits the formation of (115) texture and larger grain size. The films deposited on Pt/TiO₂/SiO₂/Si substrates at 830 °C are (115)-oriented and exhibit 2Pr of 6 μC/cm². (115)- and (200)-predominant films can be formed by using a La_0.85Sr_0.15CoO₃ (LSCO) buffer layer or by annealing amorphous SBT films deposited on Pt/TiO₂/SiO₂/Si substrates at 450 °C using rapid thermal annealing (RTA). These films exhibit good electric properties; 2Pr of the films are up to 12 μC/cm² and 17 μC/cm², respectively. The much larger 2Pr of the films deposited on the LSCO buffer layer and of the films obtained by RTA than 2Pr of the films deposited on Pt/TiO₂/SiO₂/Si substrates at 830 °C is attributed to a stronger (200) texture. Received: 30 January 2001 / Accepted: 30 May 2001 / Published online: 25 July 2001     

Microstructure and dielectric properties of La2O3 doped amorphous SiO2 films as gate dielectric material

As a potential gate dielectric material, the La₂O₃ doped SiO₂ (LSO, the mole ratio is about 1:5) films were fabricated on n-Si (0 0 1) substrates by using pulsed laser deposition technique. By virtue of several measurements, the microstructure and electrical properties of the LSO films were characterized. The LSO films keep the amorphous state up to a high annealing temperature of 800 °C. From HRTEM and XPS results, these La atoms of the LSO films do not react with silicon substrate to form any La-compound at interfacial layer. However, these O atoms of the LSO films diffuse from the film toward the silicon substrate so as to form a SiO₂ interfacial layer. The thickness of SiO₂ layer is only about two atomic layers. A possible explanation for interfacial reaction has been proposed. The scanning electron microscope image shows the surface of the amorphous LSO film very flat. The LSO film shows a dielectric constant of 12.8 at 1 MHz. For the LSO film with thickness of 3 nm, a small equivalent oxide thickness of 1.2 nm is obtained. The leakage current density of the LSO film is 1.54 × 10⁻⁴ A/cm² at a gate bias voltage of 1 V.     

Ferroelectric SrxBa1-xNb2O6 optical waveguiding thin films on SiO2-coated Si(100) substrates

x Ba_1-xNb₂O₆ (x=0.5) films (abbreviated as SBN:0.5) on SiO₂-coated Si substrates are potential components for the application of integrated electro-optics devices. SBN:0.5 optical waveguiding thin films on SiO₂-coated Si substrates with a very thin MgO diffusion buffer have been successfully prepared by pulsed laser deposition. The as-grown films have a refractive index of 2.28, which is close to that of bulk SBN. X-ray analysis showed that the as-grown films have a single-phase tetragonal tungsten bronze structure. The SBN:0.5 thin films prepared by PLD exhibit favorable ferroelectric and optical waveguiding properties. The composition and the morphology of the films were also examined by XPS and by SEM, respectively. Ferroelectric SBN:0.5 optical waveguiding thin films on SiO₂-coated Si substrates are expected to be used in integrated electro-optic devices. Received: 27 February 1997/Accepted: 17 October 1997     

Growth of stoichiometric and textured LiNbO3 films on Si by pulsed laser deposition

Laser ablation of single-crystal LiNbO₃ in a gas environment is used to grow films on (100) Si substrates heated to 650 °C. The film composition and crystallinity are studied as a function of the nature (reactive, O₂, or inert, Ar) and pressure of the gas environment applied during deposition and cooling-down processes, the laser energy density and the target–substrate distance. Experimental results show that a gas pressure close to 1 mbar is required to produce stoichiometric films in either O₂ or Ar. The modification of the laser energy density and the target–substrate distance allows us to improve the crystallinity of the films that become textured along the (006) direction. The influence of the experimental parameters on the film properties is discussed in the frame of the formation of a blast wave, that leads to the focusing of the expanding Li species and thus, to the increase of the Li content in the films. Received: 8 February 2001 / Accepted: 9 February 2001 / Published online: 3 May 2001     

Artificially nanostructured Cu:Al2O3 films produced by pulsed laser deposition

The processes leading to the formation of Cu:Al₂O₃ composite films on Si (001) with a well defined nanostructure by alternate pulsed laser deposition (a-PLD) in vacuum are investigated. Alternately amorphous Al₂O₃ layers and Cu nanocrystals nucleated on the Al₂O₃ surface are formed, according to the PLD sequence. The Al₂O₃ deposited on the Cu nanocrystals fills in the space between them until they are completely buried, and subsequently, a continuous dense layer with a very flat surface (within 1 nm) is developed. The nucleation process of the nanocrystals and their resulting oblate ellipsoidal shape are discussed in terms of the role of the energetic species involved in the PLD process and the metal–oxide interface energy. Received: 4 July 2000 / Accepted: 5 July 2000 / Published online: 13 September 2000     

Structural and ferroelectric properties of Bi4Ti3O12 thin films on IrO2 prepared by rf magnetron sputtering

Bismuth titanate, Bi₄Ti₃O₁₂, thin films were grown on IrO₂/SiO₂/Si substrates by radio-frequency magnetron sputtering. Crystallinity and microstructure of the films were characterized over a wide range of oxygen mixing ratio (OMR) during deposition. X-ray fluorescence spectra reveal that the cation content of the films is dependent upon the OMR, suggesting that control of Bi to Ti ratio is possible by the oxygen content in the sputtering atmosphere. Rutherford backscattering spectrometry measurements also show that oxygen content of the BTO film grown with an OMR = 0.5is close to a stoichiometric phase. In addition, Bi–O bonding chemistry is studied by X-ray photoelectron spectroscopy. Polarization vs. voltage loop shows that remnant polarization, P_r, is +12 μC/cm². Received: 26 November 1999 / Accepted: 26 November 1999 / Published online: 9 August 2000     

Ferroelectric polarization-controlled two-dimensional electron gas in ferroelectric/AlGaN/GaN heterostructure

The control effect of the ferroelectric polarization on the two-dimensional electron gas (2DEG) in a ferroelectric/AlGaN/GaN metal–ferroelectric–semiconductor (MFS) structure is theoretically analyzed by a self-consistent approach. With incorporating the hysteresis nature of the ferroelectric into calculation, the nature of the control effect is disclosed, where the 2DEG density is depleted/restored after poling/depoling operation on the MFS structure. The orientation of the ferroelectric polarization is clarified to be parallel to that of the AlGaN barrier, which, based on an electrostatics analysis, is attributed to the pinning effect of the underlying polarization. Reducing the thickness of the AlGaN barrier from 25 nm to 20 nm leads to an improved control modulation of the 2DEG density from 36.7% to 54.1%.     

Enhanced adhesion of diamond-like carbon films with a composition-graded intermediate layer

Received: 15 September 1998 / Accepted: 3 November 1998     

Microstructures and dielectric properties of compositionally graded (Ba1-xSrx) TiO3 thin films prepared by pulsed laser deposition

Compositionally graded (Ba_1-xSr_x)TiO₃ (BST) (x:0.0∼0.25) thin films were grown on Pt (111)/TiO₂/SiO₂/Si (100) substrates using layer-by-layer pulsed laser deposition in the temperature range 550–650 °C. Both downgraded (Ba/Sr ratio varying from 100/0 at the bottom surface to 75/25 at the top surface) and upgraded (Ba/Sr ratio varying from 75/25 at the bottom surface to 100/0 at the top surface) BST films were prepared. Their microstructures were systematically studied by X-ray diffractometry and scanning electron microscopy. A grain morphology transition from large ‘rosettes’ (>0.30 μm) to small compact grains (70–110 nm) was observed in the downgraded BST films as the deposition temperature was increased from 550 to 650 °C. No such grain morphology transition was detected in the upgraded BST films. Dielectric measurements with metal electrodes revealed an enhanced dielectric behavior in the downgraded films. This enhancement is mainly attributed to the large compressive stress field built up near the interface between the downgraded film and substrate. Furthermore, the BaTiO₃ layer in the downgraded BST films not only serves as a bottom layer but also as an excellent seeding layer for enhancing the crystallization of the subsequent film layers in the downgraded films. Received: 10 December 2001 / Accepted: 12 March 2002 / Published online: 19 July 2002 RID="*" ID="*"Corresponding author. Fax: 86-25/359-5535, E-mail: xhzhu@public1.ptt.js.cn     

Structural and optical properties of Bi<Subscript>3.25</Subscript>La<Subscript>0.75</Subscript>Ti<Subscript>3</Subscript>O<Subscript>12</Subscript> ferroelectric thin films prepared by chemical solution methods

Ferroelectric Bi_3.25La_0.75Ti₃O₁₂ (BLT) thin films have been grown on Pt/Ti/SiO₂/Si substrates by chemical solution methods. X-ray diffraction analysis shows that BLT thin films are polycrystalline with (171)-preferential orientation. Atomic force microscopy investigation shows that they have large grains about 120 nm in size. A Pt/BLT/Pt capacitor has been fabricated and showed excellent ferroelectricity, with a remnant polarization and coercive field of 24 μC/cm² and 116 kV/cm, respectively. The capacitor shows no polarization fatigue up to 10⁹ switching cycles. The optical constants (n,k) of the BLT thin films in the wavelength range 0.35–1.7 μm were obtained by spectroscopic ellipsometry measurements, and the band-gap energy was found to be about 3.25 eV. Received: 16 October 2001 / Accepted: 6 January 2002 / Published online: 3 June 2002 RID="*" ID="*"Corresponding author. Fax: +86-21/65830-734, E-mail: gswang@mail.sitp.ac.cn     

Capacitance analysis for a metal–insulator– semiconductor structure with an ultra-thin oxide layer

Applied Physics A - We have studied theoretically the capacitance characteristics of a metal–insulator–semiconductor structure with an ultra-thin oxide layer by self-consistently...     

Pulsed laser deposition of ZnO as conductive buffer layer of (001)-LiNbO3 thin films

1 (LO) mode peak at 579 cm^-1, indicating oxygen deficiency in the films. Significant dependence of the electrical property for the films on substrate temperature was shown. The I–V relation of the films exhibited non-ohmic behavior. Whereas the films deposited above 500 °C showed a metal-like property, at a lower substrate temperature semiconducting thin films were achieved. The (001)-oriented LiNbO₃/ZnO heterostructure was successfully prepared on quartz fused and (001) sapphire plates. Received: 6 April 1998/Accepted: 26 May 1998     

Microstructural investigations of Y2O3 thin films deposited by laser ablation on MgO

The epitaxial relationships and the microstructure of thin Y₂O₃ film, in situ grown by laser ablation at 700 °C under 0.5 mbar oxygen pressure on MgO substrate, are studied by X-ray and HRTEM investigations. X-ray φ-scanexperiments show that Y₂O₃ exhibits two different crystallographic growing directions <111> and <100> on (100) single-crystalline MgO substrate. The <111> growing direction appears with four different epitaxial relationships where the <110> Y₂O₃ in-plane direction is parallel to the <110> in-plane direction of MgO. The <100> growing direction appears with one epitaxial relationship, cube Y₂O₃ on cube MgO. The HRTEM experiments show the columnar aspect of the Y₂O₃ thin film. Both (111) and (100) Y₂O₃-oriented grains are observed in good agreement with the X-ray experiments. Received: 16 May 2000 / Accepted: 22 May 2000 / Published online: 13 September 2000     

Structure and electrical properties of HfO<Subscript>2</Subscript> high-<Emphasis Type="Italic">k</Emphasis> films prepared by pulsed laser deposition on Si (100)

High-k gate dielectric hafnium dioxide films were grown on Si (100) substrate by pulsed laser deposition at room temperature. The as-deposited films were amorphous and that were monoclinic and orthorhombic after annealed at 500°C in air and N₂ atmosphere, respectively. After annealed, the accumulation capacitance values increase rapidly and the flat-band voltage shifts from −1.34 V to 0.449 V due to the generation of negative charges via post-annealing. The dielectric constant is in the range of 8–40 depending on the microstructure. The I–V curve indicates that the films possess of a promising low leakage current density of 4.2×10⁻⁸ A/cm² at the applied voltage of −1.5 V.     

The orientation-selective growth of LaNiO3 films on Si(100) by pulsed laser deposition using a MgO buffer

Highly (100)-oriented, (110)-oriented and polycrystalline LaNiO₃ (LNO) films were successfully prepared on Si(100) using an oriented MgO film as a buffer. It was somewhat surprising to find that that the orientation relation between the LNO film and the corresponding MgO buffer was: LNO(100)\MgO(110), LNO(110)\MgO(111) and LNO(polycrystalline)\MgO(100). The crystalline quality of the LNO films was shown to be sensitive to the preparation conditions of the MgO buffer. The film surface was very smooth, without micrometer-sized droplets being observed. All LNO films were of metallic conductivity, with a room-temperature resistivities of approximately 250, 280 and 420 μΩ cm for the (110)-oriented, (100)-oriented and polycrystalline LNO, respectively. Received: 2 April 2001 / Accepted: 23 October 2001 / Published online: 3 June 2002