The influence of YSZ interlayer on microstructures and dielectric properties of BST thin films prepared by RF magnetron sputtering

(Ba_{1 − x} Sr _x )TiO₃ (BST) thin films were deposited on Pt/Ti/SiO₂/Si and YSZ/Pt/Ti/SiO₂/Si substrates by radio frequency (RF) magnetron sputtering. The influence of YSZ interlayer on microstructures and dielectric properties of BST thin films were investigated by X-ray diffraction, atomic force microscopy, scanning electron microscopy and dielectric frequency spectra. It was found that the preferred orientation of BST thin films could be tailored by insertion of YSZ interlayer and adjusting the thickness of YSZ interlayer. The BST thin films deposited on YSZ interlayer exhibited a more compact and uniform grain structure than that deposited directly on Pt electrode. Dielectric measurement revealed that the BST thin films deposited on 10 nm YSZ interlayer have the largest dielectric constant and a low dielectric loss tangent. The enhanced dielectric behavior is mainly attributed to the YSZ interlayer which serves as an excellent seeding layer to enhance the crystallization of subsequent BST films layer, and a smaller thermal stress field built up at the interface between YSZ interlayer and BST film layer.     

The behavior of Ti silicidation on Si/SiGe/Si base and its effect on base resistance and fmax in SiGe hetero-junction bipolar transistors

The relation between Ti silicidation and base resistance in SiGe hetero-junction bipolar transistors (HBT) was investigated. The Ti layer deposited on the Si/SiGe/Si base converted to Ti silicide during two-step annealing. The thickness of the Ti silicide, which was identified as the Ti(Si_1-xGe_x) phase of uniform composition, abruptly increased over the annealing temperature of 650/850 °C, and as a result it accomplished a very low extrinsic base resistance. The Ti silicidation affected the base resistance of real devices (R_B), which was extracted from simulating the electrical data of SiGe HBTs such as I –V curves, forward Gummel plots, forward current gain curves, and s-parameter plots. It was shown that the R_B was compatible with the theoretical relation which included the small-signal unity-gain frequency (f_T), the maximum oscillation frequency (f_max) and R_B. f_max varied more sensitively with R_B than f_T, which was due to the inherent property of f_max being inversely proportional to R_B. The f_max of the SiGe HBT reached 47.4 GHz when Ti silicidation was performed at the annealing temperature of 650/850 °C. This silicidation condition is thought to be an appropriate temperature for Ti silicidation applicable to SiGe HBT fabrication. © 2001 Kluwer Academic Publishers     

Studies of BaTiO3 thin films on different bottom electrode

BaTiO₃ (BT) thin films were prepared on Pt/Ti/SiO₂/Si and Ru/Ti/SiO₂/Si substrates by a modified sol-gel technique. The microstructure of the films was characterized by atomic force microscopy (AFM), X-ray diffraction (XRD) and Raman spectroscopy. The results showed that BT thin films crystallized with perovskite structure. Compared to BT film on Pt/Ti/SiO₂/Si substrate, BT thin film deposited on Ru electrode has similar dielectric constant, while it has higher dielectric loss. C–E curve for BT film on Pt/Ti/SiO₂/Si was more symmetrical around zero-bias field than C–E curve for BT film on Ru/Ti/SiO₂/Si substrate. The tunability was 52.02% for BT film on Pt electrode, which was 33.42% on Ru electrode, at 275 kV/cm and room temperature. The leakage current density of BT on Pt electrode was about an order of magnitude lower than BT film on Ru electrode at the applied electrical field below 150 kV/cm. The leakage conduction mechanism was investigated.     

Determination of carrier mobility vs resistivity relation in Czochralski-grown n- and p-type Si x Ge1−x (0.93 < x < 0.96) single crystals

Undoped and impurity-doped single crystals of Si _x Ge_1−x with the composition 0.93 < x < 0.96 were grown by the Czochralski technique. The Hall electron and hole mobilities in undoped or lightly impurity-doped SiGe were somewhat lower than those in Si, while in heavily impurity-doped SiGe were comparable to those in Si. The alloy disordered scattering and charged impurity scattering may govern the carrier transport process in undoped or lightly impurity-doped SiGe and in heavily impurity-doped SiGe, respectively. The so-called Irvin’s curve of the practical relation between resistivity and carrier concentration was obtained for SiGe with the composition 0.93 < x < 1.     

Studies on growth mechanism of HgCdTe epilayer on Si grown by HWE

In this paper, the mechanism of Hg_1−xCd_xTe/Si heterojunction grown by HWE (Hot Well Epitaxy) was studied. Opitical characterizations were shown with FTIR, the composition x = 0.39 was deduced by using MIR transmittance, the absorbing peak at 319.4 cm⁻¹ was measured by FIR transmittance, 319.4 cm⁻¹ confirmed the existence of Si–Te bond of at Si/HgCdTe interfacial layer. The I-V characteristics at both room temperature and 77 K of HgCdTe (n-type)/Si (p-type) heterojunction show that the good p-n heterojunction properties was obtained by using HWE. XRD study confirmed the formation of (111) oriented HgCdTe on (211) Si. Morphology of a cross section observed using EPMA indicates the columnar growth of HgCdTe. An analysis of interfcial layer by EPMA indicated presence of three layers composed of Si + Te, Si + Te + Hg and Si + Te + Cd + Hg. Among them, the most important one is the first layer. The problem of lattice mismatch and the difference of thermal expansion coefficient between Si and CdTe or HgTe may be improved by formation of Si–Te stable chemical bond through bybridization orbital bonding between Si and Te. The second and third layers are formed by evaporation-interdiffusion. Formation of the whole interfacial layer provides the appetency for the growth of (111) Hg_1−xCd_xTe epilayer on (211) Si substrate.     

RF sputtered piezoelectric zinc oxide thin film for transducer applications

This paper demonstrates the substrate dependency of the c-axis zinc oxide growth in radio-frequency sputtering system. Different deposition conditions were designed to study the influences of Si, SiO₂/Si, Au/Ti/Si, and Au/Ti/SiO₂/Si substrates on the piezoelectric and crystalline qualities of the ZnO thin films. Experimental results showed that the multilayer of Au/Ti/SiO₂/Si-coated silicon substrate provided a surface that facilitated the growth of ZnO thin film with the most preferred crystalline orientation. The 1.5 μm-thick thermally grown amorphous silicon dioxide layer effectively masked the crystalline surface of the silicon substrate, thus allowing the depositions of high-quality 20 nm-thick titanium adhesion layer followed by 150 nm-thick of gold thin film. The gold-coated surface allowed deposition of highly columnar ZnO polycrystalline structures. It was also demonstrated that by lowering the deposition rate at the start of sputtering by lowering RF power to less than one-third of the targeted RF power, a fine ZnO seed layer could be created for subsequent higher-rate deposition. This two-step deposition method resulted in substantially enhanced ZnO film quality compared to single-step approach. The influence of stress relaxation by annealing was also investigated and was found to be effective in releasing most of the residual stress in this layered structure.     

Fabrication of TiO<Subscript>x</Subscript>–Si photoanode and its energetic photoelectrochemical performance

Pristine Si is oxidized to insulative SiO₂ when it comes in contact with air and water. Covering it with a protection layer inhibits passivation of Si and significantly improves its photoelectrochemical performance. In this study, TiO_x with gradient change of oxygen stoichiometry ratio (TiO_x) was designed as a protection layer and fabricated via a chemical vapour deposition process in Ar flow under 400 °C for 1 min. The anaerobic atmosphere and short heating duration synergistically produced the ratio of O and Ti lower than two in the prepared film. XPS analysis suggested the existance of TiO₂ only at the surface of TiO_x film and Ti³⁺ and Ti²⁺ appeared successively with the increase of distance to the surface. The first advantage of lower-valence-state Ti and oxygen deficiency was to inhibit the oxidation of Si and to reduce electric resistance of the interface and the protection layer. The second advantage was to create a defect energy level under the conduction band of TiO₂ which provided the possibility for holes in the valence band of Si to be transferred to this defect level. This tunnel like transfer enhanced the photogenerated charge separation and redox ability of TiO_x–Si which brought a 3.25 folds enhancements in photocurrent density compared to that of stoichiometric TiO₂–Si at 0 V (SCE) under simulated sunlight. This study highly motivates further research on transparent and conductive protection layer of Si photoelectrode.     

Interfacial reactions for the non-stoichiometric TiB x /(100)Si system

In order to evaluate the interfacial reactions in the TiB _x /(100)Si system and the thermal stability of non-stoichiometric TiB _x films (0 B/Ti 2.5), TiB _x /Si samples prepared by a co-evaporation process were annealed in vacuum at temperatures between 300 and 1000°C. The solid phase reactions were investigated by means of sheet resistance, X-ray diffraction, transmission electron microscopy, X-ray photo-electron spectroscopy, and stress measurement. For TiB _x samples with a ratio of B/Ti 2.0, an apparent structural change is not observed even after annealing at 1000°C for 1 h. For samples with a ratio of B/Ti < 2.0, however, there are two competitive solid phase reactions: the formation of a titanium silicide layer at the interface and the formation of a stoichiometric TiB₂ layer at the surface, indicating the salicide process. The sheet resistance and the film stress in the Ti/Si and TiB _x /Si systems are well explained by the solid phase reactions.     

Fabrication and characteristics of La,Cd and Sn doped BST thin films by sol–gel method

Ba_1−x Sr _x TiO₃ (BST) thin films doped with La, Cd and Sn were prepared by sol–gel method on the Pt/Ti/SiO₂/Si substrate. X-ray diffraction analysis and atomic force microscopy showed that dopant La and Cd causes decreased grain size of BST thin films obviously, and Sn-doped BST thin films was similar to BST films in grain size. La and Cd doped decreased the tunability of BST thin films and Sn doped increased it, which may be explained by stress, electronegativity and oxygen vacancies factors. All the doped BST thin films improved the leakage current characteristic.     

Characterization of Sn-doped BST thin films on LaNiO<Subscript>3</Subscript>–coated Si substrate

Sn-doped (Ba,Sr)TiO₃(BSTS) thin films have been deposited on highly (200) oriented LaNiO₃(LNO) thin films by sol-gel method. The atomic force microscope (AFM) images exhibited that the dopant Sn did not decrease the crystalline grain size of BST thin films. The structure of the BST film, determined by X-ray diffraction (XRD), presented the higher intensity (110) and (200) peaks, while the latter was distinctly induced by LNO layer. Evidently, Sn-doped BST thin films on LNO/Si substrate were found to decrease the dielectric constant and the dielectric loss, which is favourable to potentially improve the figures of merits (F _D ) of pyroelectric materials. The BSTS thin films on LNO layer also displayed an excellent leakage current property comparing with the BST thin film on Pt/Ti/SiO₂/Si and LNO/Si substrates.     

Structural and dielectric properties of Ba0.7Sr0.3TiO3 thin films grown on thin Bi layer-coated Pt(111)/Ti/SiO2/Si substrates

In this work, the growth and study of dielectric properties of Ba_0.7Sr_0.3TiO₃ (BST) thin films grown on thin Bi layer coated Pt(111)/Ti/SiO₂/Si substrates, depending on thin Bi layer thickness is reported. The BST thin film (thickness 180 nm) grown on 10-nm-thick Bi layer exhibited more improved structural and dielectric properties than that grown on bare Pt(111)/Ti/SiO₂/Si substrate. The 10-nm-thick Bi layer in optimum configuration was effective for the grain growth of BST phase and suppressed the formation of the oxygen-deficient layer at the interface between the BST thin film and bottom electrode, which resulted in an increase in dielectric constant and a decrease in leakage current density of the Pt/BST thin film/Pt capacitor.     

Thickness scaling and electric properties of highly (111) oriented Nb-doped Pb(Zr,Ti)O<Subscript>3</Subscript> thin film prepared at low temperature by a sol–gel route

A series of highly (111) oriented Pb(Nb_0.01Zr_0.2Ti_0.8)O₃ (PNZT) thin films of variant thickness were successfully achieved on Pt/Ti/SiO₂/Si substrate by a sol–gel route. By introducing Pb_0.8Ca_0.1La_0.1Ti_0.975O₃ (PLCT) layer between the PNZT film and Pt electrode, the PNZT film could be crystallized at as low as 500 °C. When a maximum applied voltage is 3 V, it was found that the PNZT film with PLCT seed layer possessed higher remnant polarization (22 μC/cm²) as film thickness was scaled down to 50 nm. It was also found that enhanced pyroelectric properties could be observed in 50-nm thickness PNZT thin film due to its relatively low dielectric constant. The results demonstrated that the film thickness could be scaled down for low voltage operations using lattice matched interface between PNZT film and PLCT seed layer on Pt/Ti/SiO₂/Si substrate, and this interface optimization is the key technology for synthesizing thin PNZT films at low temperature with good insulating and electric properties.     

Microstructure, optical and dielectric properties of compositional graded (Ba,Sr)TiO3 thin films derived by RF magnetron sputtering

Thin films of compositional graded Ba_1−x Sr _x TiO₃ (BST) (x decreasing from 0.3 to 0) were prepared on fused quartz and Pt/Ti/SiO₂/Si substrates by RF magnetron sputtering. The microstructure of the graded BST thin films was characterized by X-ray diffraction (XRD). It indicates that the films were crystallized with peroveskite structure and (100) + (111) preferred orientation. The refractive index and the band gap were determined at room temperature in the wavelength 200–1100 nm from spectrophotometric measurements of the transmittance. The average value of the refractive index is found to be 2.17 for the graded BST films in the wavelength 400–1000 nm. The optical band gap of the graded BST film was 3.77 eV. The dielectric measurement showed that the dielectric constant and loss factor of the graded BST film was 318.04 and 0.028 at 100 KHz and room temperature.     

Microstructure and texture development in single layered and heterolayered PZT thin films

Single-layered Pb(Zr_0.7Ti_0.3)O₃ and Pb(Zr_0.3Ti_0.7)O₃ thin films and heterolayered Pb(Zr_1−x Ti _x )O₃ thin films consisting of alternating Pb(Zr_0.7Ti_0.3)O₃ and Pb(Zr_0.3Ti_0.7)O₃ layers were studied for their microstructure and texture development. The texture in the single-layered PZT films is affected by the Zr/Ti ratio as they have different crystallization behavior depending on the Zr/Ti ratio. With increasing film thickness, the average grain size of Pb(Zr_1−x Ti _x )O₃ increases. An unusually large grain size of 1–3 μm together with a strong (001)/(100) preferred orientation were observed for the heterolayered PZ₇₀T₃₀, whereby Pb(Zr_0.7Ti_0.3)O₃ was used as the seeding layer, for film as thin as 150 nm. The film microstructure is refined drastically when the stacking sequence is changed, i.e., when Pb(Zr_0.3Ti_0.7)O₃ is employed as the seeding layer. Thermal treatment of the PZ₇₀T₃₀ seeding layer also plays an important function in the microstructure development of the heterolayered PZ₇₀T₃₀ film. The formation of the large-grained film is correlated to the lowered nucleation energy of crystallizing Pb(Zr_0.7Ti_0.3)O₃ by the top Pb(Zr_0.3Ti_0.7)O₃. The Pb(Zr_0.3Ti_0.7)O₃ layer facilitated the nucleation and crystallization of the Pb(Zr_0.7Ti_0.3)O₃ amorphous seeding layer, whereby the overall microstructure of the heterolayered thin film was dictated by the Pb(Zr_0.7Ti_0.3)O₃ seeding layer leading to the growth of larger PZT grains.     

Polycrystalline Si1-x Ge x thin film deposition by rapid thermal chemical vapor deposition

Polycrystalline silicon germanium (poly-Si_1−xGe_x) thin films on a-Si film have been deposited by rapid thermal chemical vapor deposition (RTCVD) with SiH₄–GeH₄–H₂. Effect of GeH₄/SiH₄ and deposition temperature on stoichiometry (x), Si-Ge binding character, composition, hydrogen configuration, crystallinity, preferred orientation, grain size, and surface roughness of poly-Si_1−xGe_x films has been investigated. Poly-Si_1−xGe_x deposited on the substrate with amorphous silicon buffer layer on oxide shows better crystallinity and contains the less amount of oxygen than the one deposited directly on the oxide surface. At low temperature region, the Ge–H bond with the small amount of Si–H₂ bond is dominant but all hydrogen bonds are desorbed at high temperature. All films have polycrystalline phase and the grain size and (111) orientation increased with increasing deposition temperature in which Ge content also increases at the fixed gas flow rate of GeH₄ to total source gas. Poly-Si_1−xGe_x/Si thin film transistors (TFT) are fabricated and hydrogen during post-hydrogenation process preferentially is attached to Ge dangling bond and the TFT characteristics could be improved.     

Reverse recovery of Si/Si<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Ge<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> heterodiodes fabricated by direct bonding

We have studied the process of reverse recovery of Si/Si_{1 − x} Ge _x heterodiodes fabricated by direct bonding of (111)-oriented n-type single crystal silicon wafers with p-type Si_{1 − x} Ge _x wafers of the same orientation containing 4–8 at. % Ge. An increase in the germanium concentration N _Ge in p-Si_{1 − x} Ge _x layer is accompanied by a decrease in the reverse recovery time of heterodiodes. The presence of a sharp drop in the reverse current on the diode recovery characteristic can be explained by the existence of a narrow region with decreased minority carrier lifetime at the bonding interface (compared to carrier lifetime in the bulk), which is caused by the accumulation of misfit dislocations (generated by bonding (in this region). The results demonstrate the principal possibility of creating fast-recovery diodes based on the Si/Si_{1 − x} Ge _x heterosystem for high-power semiconductor devices manufactured using the direct bonding technology.     

Structure and ferroelectric properties of stoichiometric and Sr-deficient–SrBi<Subscript>4</Subscript>Ti<Subscript>4</Subscript>O<Subscript>15</Subscript> thin films

SrBi₄Ti₄O₁₅ (SBTi) and Bi-excess and Sr-deficient SBTi (Sr-deficient SBTi, Sr_0.8Bi_4.13Ti₄O₁₅) thin films were deposited on Pt/Ti/SiO₂/Si (100) substrates using a sol–gel method. Structure and electric properties were investigated systematically. These films were random oriented. The remnant polarization (2P _r) of SBTi film was about 25.3 μC/cm², which was larger than the reported value of SBTi thin film. The film with Sr-deficient and Bi-excess composition showed a very large remnant polarization of 36.6 μC/cm². The capacitance–voltage (C–V) characteristics of both the films showed normal ferroelectric behavior. The Curie temperatures of the same Sr-deficient and Bi-excess component ceramics sample increased slightly in comparison with that of SBTi. More importantly, the Sr-deficient and Bi-excess SBTi thin film showed high fatigue resistance against continuous switching up to 4.4 × 10¹⁰ cycles.     

Atomistic structure and strain relaxation in Czochralski-grown Si<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Ge<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript> bulk alloys

The local atomistic structure in bulk Si_xGe_1−x alloys in the whole composition range 0 < x < 1 was investigated experimentally and theoretically. By extended X-ray absorption fine structure measurements in Czochralski-grown bulk SiGe crystals it is found that bulk SiGe is a random mixture and that the Ge–Ge, Ge–Si and Si–Si bond lengths maintain distinctly different lengths and vary in a linear fashion as a function of alloy composition across the entire composition range 0 < x < 1, in good agreement with expectations derived from the ab-inito electronic structure calculations. The results indicate that SiGe is a typical disorder material and that the bond lengths and bond angles are distorted with alloy composition in SiGe.     

The effects of oxygen partial pressure on BST thin films deposited on multilayered bottom electrodes

Ba_0.5Sr_0.5TiO₃ (BST) thin films have been deposited by r.f. magnetron sputtering on silicon and platinum-coated silicon substrates with different buffer and barrier layers. BST films deposited on Si/SiO₂/SiN/Pt and Si/SiO₂/Ti/TiN/Pt multilayer bottom electrode have been used for the fabrication of capacitors. XRD and SEM studies were carried out for the films. It was found that the crystallinity of the BST thin film was dependent upon oxygen partial pressure in the sputtering gas. The role of multilayered bottom electrode on the electrical properties of Ba_0.5Sr_0.5TiO₃ films has been also investigated. The dielectric properties of BST films were measured. The results show that the films exhibit pure perovskite phase and their grain sizes are about 80–90 nm. The dielectric properties of the BST thin film on Si/SiO₂/Ti/TiN/Pt electrode was superior to that of the film grown on Si/SiO₂/SiN/Pt electrode.     

Effects of rapid thermal treatments on Ti/TiN/Si structures for electrical contacts on silicon

Abstract

A possible new technique for metallisations in Si microelectronics technology has been prepared and characterised. Bilayers of TiN_x/Ti were deposited by sputtering over a Si substrate. The samples were annealed in a rapid thermal processing system, and further analysed using Auger electron spectroscopy and electrical measurements (Schottky barrier height and sheet resistance). Significant differences from the more usual silicidiltion process of a Ti/Si structure have been observed. The silicidation process of the TiN_x/Ti structures is mainly controlled by the presence of the intermediate TiN layer. The final structure was determined to be Si/TiSi_x/TiN_y/TiSi_z.

MST/3336