Enhanced ferroelectric properties of multilayer SrBi2Ta2O9/SrBi2Nb2O9 thin films for NVRAM applications

Ferroelectric SrBi₂Ta₂O₉/SrBi₂Nb₂O₉ (SBT/SBN) multilayer thin films with various stacking periodicity were deposited on Pt/TiO₂/SiO₂/Si substrate by pulsed laser deposition technique. The X-ray diffraction patterns indicated that the perovskite phase was fully formed with polycrystalline structure in all the films. The Raman spectra showed the frequency of the O–Ta–O stretching mode for multilayer and single layer SrBi₂(Ta_0.5Nb_0.5)₂O₉ (SBNT) samples was 827–829 cm⁻¹, which was in between the stretching mode frequency in SBT (813 cm⁻¹) and SBN (834 cm⁻¹) thin films. The dielectric constant was increased from 300 (SBT) to 373 at 100 kHz in the double layer SBT/SBN sample with thickness of each layer being 200 nm. The remanent polarization (2P_r) for this film was obtained 41.7 μC/cm², which is much higher, compared to pure SBT film (19.2 μC/cm²). The coercive field of this double layer film (67 kV/cm) was found to be lower than SBN film (98 kV/cm).     

Characterization of fluorine-doped silicon dioxide film by Raman spectroscopy

We have measured Raman spectra of fluorine-doped SiO₂ (SiOF) films and quartz glass. From a comparison between Raman spectra of the SiO₂ film and quartz glass, it has been found that the SiO₂ film is under compressive stress and that it has more threefold ring defects than quartz glass. Raman bands from threefold and fourfold ring defects in SiOF films become week as the fluorine/oxygen (F/O) ratio increases and as the stress decreases. The decrease of intensities of these Raman bands shows that ring defects in SiOF films decrease as the F/O ratio increases and as the stress decreases. The triply degenerated ω₄ mode at 460 cm⁻¹ becomes sharp as the F/O ratio increases and as the stress decreases. Furthermore, the peak-frequency of ω₁ mode around 820 cm⁻¹ decreases with a decrease of stresses whereas that of ω₃ mode around 1065 cm⁻¹ increases. These results can be well understood in terms of a decrease of O-Si-O bonding angle caused by relaxation of stresses.     

Al2O3 formation on Si by catalytic chemical vapor deposition

Catalytic chemical vapor deposition (Cat-CVD) has been developed to deposit alumina (Al₂O₃) thin films on silicon (Si) crystals using N₂ bubbled tri-methyl aluminum [Al(CH₃)₃, TMA] and molecular oxygen (O₂) as source species and tungsten wires as a catalyzer. The catalyzer dissociated TMA at approximately 600 °C. The maximum deposition rate was 18 nm min⁻¹ at a catalyzer temperature of 1000 °C and substrate temperature of 800 °C. Metal oxide semiconductor (MOS) diodes were fabricated using gates composed of 32.5-nm-thick alumina film deposited at a substrate temperature of 400 °C. The capacitance measurements resulted in a relative dielectric constant of 7.4, fixed charge density of 1.74×10¹² cm⁻², small hysteresis voltage of 0.12 V, and very few interface trapping charges. The leakage current was 5.01×10⁻⁷ A cm⁻² at a gate bias of 1 V.     

Preparation of highly c-axis-oriented Bi4Ti3O12 thin films and their crystallographic, dielectric and optical properties

Bismuth titanate (Bi₄Ti₃O₁₂) thin films with a high c-axis orientation up to 99% were prepared on (100)-oriented silicon wafers by r.f. planar magnetron sputtering using a Bi₂TiO₅ ceramic target at a substrate temperature of 600 °C. From the Auger electron spectroscopy depth profile of the film, there is no evidence of interdiffusion of a specific element between the film and the substrate. Relative dielectric constant of these films depends on film thickness. The behavior was explained assuming a low-dielectric-constant interface layer. Using this assumption, the relative dielectric constant of Bi₄Ti₃O₁₂ film was estimated to be approximately 140. This value is close to that along the c axis in a bulk form. The remanent polarization and the coercive field were 0.8 μC cm⁻² and 20 kV cm⁻¹, respectively.     

High concentration erbium doping of silicon-rich SiO2 thin films on silicon

In this work, high concentration erbium doping in silicon-rich SiO₂ thin films is demonstrated. Si plus Er dual-implanted thermal SiO₂ thin films on Si substrates have been fabricated by using a new method, the metal vapor vacuum arc ion source implantation with relatively low ion energy, strong flux and very high dose. X-Ray photoelectron spectroscopy measurement shows that very high Er concentrations on the surfaces of the samples, corresponding to 10 at.% or the doping level of 10²¹ atoms cm⁻³, are achieved. This value is much higher than that obtained by using other fabrication methods such as the high-energy ion implantation and molecular beam epitaxy. Reflective high-energy electron diffraction, atomic force microscopy and cross-section high-resolution transmission electron microscopy observations show that the excess Si atoms in SiO₂ matrix accumulate to form Si clusters and then crystallize gradually into Si nanoparticles embedded in SiO₂ films during dual-ion implantation followed by rapid thermal annealing. Er segregation and precipitates are not formed. Photoluminescence at the wavelength of 1.54 μm exhibits very weak temperature dependence due to the introduction of Si nanocrystals into the SiO₂ matrix. The 1.54-μm light emission signals from annealed samples decrease by less than a factor of 2 when the measuring temperature increases from 77 K to room temperature.     

Influence of Si doping level on the Raman and IR reflectivity spectra and optical absorption spectrum of GaN

The optical absorption (hν) and Raman and Infra Red (IR) spectra of Si doped GaN layers deposited on sapphire through buffer layers have been recorded for electron concentrations from 5×10¹⁷ to 5×10¹⁹ cm⁻³. The (hν) values deduced from photothermal deflection spectroscopy (0.5–3.5 eV) and IR absorption (0.15–0.5 eV) vary between 50 and 10⁴ cm⁻¹ showing doping dependant free electron absorption at low energy, doping independant band gap at high energy, and slowly doping dependant defect absorption in the medium energy range. In our micro Raman geometry, maxima appear or can be deduced near the frequency expected for either the A₁(LO⁻) or the A₁(LO⁺) modes split from the A₁(LO) mode by plasmon phonon interaction. There is a large systematic evolution in the expected way for the IR reflectivity.     

Application of decomposed species generated by a heated catalyzer to ULSI fabrication processes

In this paper we report on surface nitridation of SiO₂ and on photoresist removal by using species decomposed from NH₃ or H₂ and generated by a heated tungsten catalyzer. The top surface of SiO₂/Si(100) is nitrided at temperatures as low as 300 °C using species decomposed from NH₃ species. The removal of heavy-doped, as high as 1×10¹⁶ cm⁻², ion-implanted photoresist is realized using atomic hydrogen.     

Structural, electrical, and optical studies on rapid thermally processed ferroelectric BaTiO3 thin films prepared by metallo-organic solution deposition technique

Polycrystalline BaTiO₃ thin films having the perovskite structure were successfully produced on platinum coated silicon, bare silicon, and fused quartz substrate by the combination of the metallo-organic solution deposition technique and post-deposition rapid thermal annealing treatment. The films exhibited good structural, electrical, and optical properties. The electrical measurements were conducted on metal-ferroelectric-metal (MFM) and metal-ferroelectric-semiconductor (MFS) capacitors. The typical measured small signal dielectric constant and dissipation factor at a frequency of 100 kHz were 255 and 0.025, respectively, and the remanent polarization and coercive field were 2.2 μC cm⁻² and 25 kV cm⁻¹, respectively. The resistivity was found to be in the range 10¹⁰–10¹² Ω·cm, up to an applied electric field of 100 kV cm⁻¹, for films annealed in the temperature range 550–700 °C. The films deposited on bare silicon substrates exhibited good film/substrate interface characteristics. The films deposited on fused quartz were highly transparent. An optical band gap of 3.5 eV and a refractive index of 2.05 (measured at 550 nm) was obtained for polycrystalline BaTiO₃ thin film on fused quartz substrate. The optical dispersion behavior of BaTiO₃ thin films was found to fit the Sellmeir dispersion formula well.     

The effect of thermal annealing on aerosol-gel deposited SiO2 films: a FTIR deconvolution study

Aerosol-gel process has been used for the deposition of SiO₂ thin films. Layers were deposited from a solution with pH = 3.5 and water to TEOS molar ratio (rw) 2.2 and then treated at various temperatures ranging from room temperature to 700°C. As-prepared thin films have been characterized by FTIR spectroscopy. Spectra were acquired in transmission at 65° angle of incidence or at perpendicular incidence. Characteristic absorption bands of the SiO₂ sol-gel system have been studied with respect to the posttreatment temperature. Bands located at 1250–1000 cm⁻¹ and around 960 cm⁻¹ have been deconvoluted in several peaks. The origin and temperature dependence of these peaks are discussed.     

Measurement of the current transient in Ta2O5 films

Current vs. time (I−t) measurements were performed on Ta₂O₅-based devices. Charge build-up at the Ta₂O₅/SiO₂ interface was used to explain the transient. The interfacial charge density was calculated from the I−t curve and the maximum was found to be 398 nC cm^-2 and 317 nC cm^-2 for Al/Ta₂O₅/Si and Al/Ta₂O₅/SiO₂/Si capacitors respectively. The value for MTOS was comparable with the value obtained by quasi-static measurements.     

Semiconducting and structural properties of CrSi2 A-type epitaxial films on Si(111)

Chromium disilicide (CrSi₂) films 1 000 Å thick have been prepared by molecular beam epitaxy on CrSi₂ templates grown on Si(111) substrate. The effect of the substrate temperature on the structural, electrical and optical properties of CrSi₂ films has been studied by transmission and scanning electron microscopies, optical microscopy, electrical resistivity and Hall effect measurements and infrared optical spectrometry. The optimal temperature for the formation of the epitaxial A-type CrSi₂ film have been found to be about 750°C. The electrical measurement have shown that the epitaxial A-type CrSi₂ film is p-type semiconductor having a hole concentration of 1 × 10¹⁷cm⁻³ and Hall mobility of 2 980 cm² V⁻¹ s⁻¹ at room temperature. Optical absorption coefficient data have indicated a minimum, direct energy gap of 0.34 eV. The temperature dependence of the Hall mobility (μ) in the temperature range of T = 180–500 K can be expressed as μ = 7.8 × 10¹⁰T⁻³cm²V⁻¹s⁻¹.     

Optical and structural characterizations of Cu-doped KCl films

The production of highly Cu⁺-doped KCl films and the properties of their 266 nm absorption band, which has an off-center property in doped single crystals, open the possibility of application of these films as ultra-violet optical filters. The investigated films, of approximately 1 μm thickness, were prepared by resistive co-evaporation of KCl and CuCl powders on different substrates of CaF₂, Al₂O₃, SiO₂, KCl and Si. The Cu⁺ concentration, as determined by energy-dispersive X-ray, ranges from 10²⁰ to 10²¹ cm⁻³, for 1–15% CuCl nominal mole percent concentration. Structural and optical properties were investigated through scanning electron microscopy, X-ray diffraction, ellipsometry, optical absorption and transmission. The films are polycrystalline, and the gain size decreases with increasing Cu⁺ impurity concentration, yielding an increase of visible transmission to a limited CuCl concentration. These films show a 6.295 Å lattice parameter with a f.c.c. structure and an index of refraction of 1.53 at 266 nm. When the Cu⁺ concentration is increased, the UV band position remains the same and no clusters are evidenced even to the high 15% CuCl concentration investigated, which differs very much from single crystals samples grown by the Kyropoulos-Czochralski method. For a Cu⁺ concentration of 8×10²⁰ cm⁻³ the film shows a transmission better than 88% at 350 nm wavelength.     

Effect of N2O/SiH4 ratio on the properties of low-temperature silicon oxide films from remote plasma chemical vapour deposition

Silicon oxide films have been deposited with remote plasma chemical vapour deposition (RPCVD) at low temperatures (<300 °C) from SiH₄---N₂O. The effect of a gas-phase reaction on the SiO₂ film properties and Si/SiO₂ interface was investigated. As the partial pressure ratio was increased above N₂O/SiH₄ = 4, a gas-phase reaction with powder formation was observed, which degraded film properties, increased surface roughness, and decreased deposition rate. When N₂O/SiH₄ <4, there was no detectable IR absorption in the film associated with hydrogen-related bonds (Si---OH and Si---H) but when N₂O/SiH₄ >4, the incorporation of Si---OH bond became significant and Si¹⁺, intermediate state silicon at the interface, was more abundant. The oxide fixed charge, interface trap density, surface roughness and leakage current were increased when there was powder formation in the gas phase. High plasma power also favoured the powder formation in the gas phase. C---V and I---V characteristics were measured and it was shown that these electrical properties were directly related to the process condition and material properties of the oxide and the Si/SiO₂ interface.     

Temperature-dependent photoluminescence study of As-doped p-type (Zn0.93Mn0.07)O layer

The temperature-dependent photoluminescence (PL) properties of the As-doped (Zn_0.93Mn_0.07)O layer, which showed the stable p-type conductivity with carrier concentration of  10¹⁸ cm^− 3 and carrier mobility of  10 cm² V^− 1 s^− 1, were investigated. From fitting of the Bose–Einstein approximation using extracted emission parameters from temperature-dependent PL spectra, it was found that the interaction between exciton and phonon in p-(Zn_0.93Mn_0.07)O:As is higher than that in host material ZnO. This result was confirmed as resulting from the increase of disorder-activated phonon scattering which is attributed to the increase of free carriers donated from implanted As dopants.     

Doping and electrical characteristics of in-situ heavily B-doped Si1−x−yGexCy films epitaxially grown using ultraclean LPCVD

Doping and electrical characteristics of in-situ heavily B-doped Si_1−x−yGe_xC_y (0.22<x<0.6, 0<y<0.02) films epitaxially grown on Si(100) were investigated. The epitaxial growth was carried out at 550°C in a SiH₄–GeH₄–CH₃SiH₃–B₂H₆–H₂ gas mixture using an ultraclean hot-wall low-pressure chemical vapor deposition (LPCVD) system. It was found that the deposition rate increased with increasing GeH₄ partial pressure, and only at high GeH₄ partial pressure did it decrease with increasing B₂H₆ as well as CH₃SiH₃ partial pressures. With the B₂H₆ addition, the Ge and C fractions scarcely changed and the B concentration (C_B) increased proportionally. The C fraction increased proportionally with increasing CH₃SiH₃ partial pressures. These results can be explained by the modified Langmuir-type adsorption and reaction scheme. In B-doped Si_1−x−yGe_xC_y with y=0.0054 or below, the carrier concentration was nearly equal to C_B up to approximately 2×10²⁰ cm⁻³ and was saturated at approximately 5×10²⁰ cm⁻³, regardless of the Ge fraction. The B-doped Si_1−x−yGe_xC_y with high Ge and C fractions contained some electrically inactive B even at the lower C_B region. Resistivity measurements show that the existence of C in the film enhances alloy scattering. The discrepancy between the observed lattice constant and the calculated value at the higher Ge and C fraction suggests that the B and C atoms exist at the interstitial site more preferentially.     

Optical properties of non-stoichiometric SiO2 as a function of excess silicon content and thermal treatments

The infrared (IR) absorption spectra and the behavior of the refraction index of a two-phase non-stoichiometric SiO₂ film with excess Si have been studied as a function of the excess of Si and post-deposition thermal treatment. The oxides were deposited by low-pressure chemical vapor deposition using SiH₄ and N₂O as reactant gases at a substrate temperature in the range of 650 to 750 °C. Some of the films were given a final annealing treatment at temperatures ranging from 700 to 1100 °C in N₂ for 30 min. Both annealed and as-deposited oxides have IR absorption peaks associated with the bending, rocking and stretching modes of the Si-O-Si bonds in SiO₂, although the exact location of these peaks is different for different contents of excess silicon and it also depend on the post-deposition thermal treatment given to the oxides. Unannealed samples present a shift of the stretching peak towards low wavenumbers as the excess of Si is increased. The samples annealed at 1000 °C on the other hand do not present this shift. Unannealed samples with large content of Si also present an absorption peak at 890 cm⁻¹ that could be associated with partially oxidized Si. It is suggested that at least part of the excess Si in the as-deposited samples is present in the form of an SiO_x phase while in the annealed samples a clear separation occurs between a Si and a SiO₂ phase. The behavior of the refraction index is similar for both types of sample, increasing as the excess silicon is increased.     

Structural, chemical, and electrical characterisation of reactively sputtered WSx thin films

WS_x films were sputter-deposited on Si, SiO₂/Si, and glass substrates from a WS₂ target in an Ar/H₂S atmosphere. Their structure, morphology, chemical composition, and electrical properties were investigated as a function of deposition parameters such as working pressure and H₂S fraction. Films could be grown in the composition range WS_0.3−WS_3.5. Crystallisation was achieved at substrate temperatures T_s > 70 °C and compositions 0.7 ≤ x ≤ 1.95. While the first 5–50 nm near the interface exhibited a basal orientation (c), further growth resulted in the formation of edge-oriented platelets (c) giving rise to a porous, lamellar microstructure. The crystalline structure was mainly turbostratic, while some degree of ordered stacking was present in samples grown at high substrate temperature (600 °C). Resistivity measurements showed a semiconductor-type temperature dependence characterised by activation energies up to 95 meV. Sheet resistance was found to be nearly independent of film thickness, suggesting that the main carrier transport takes place in an interfacial layer of about 20 nm in thickness.     

MBE growth and characterization of buried silicon oxide films on Si(100)

Silicon molecular beam epitaxy (Si-MBE) has been used to produce silicon oxide (SiO_x) films by evaporating Si on a heated Si(100) substrate in an ultra high vacuum system with an O₂ pressure of 10⁻⁶ to 10⁻⁴ mbar. Then the SiO_x films were overgrown with pure Si. The influence of the substrate temperature, the O₂ pressure and the Si deposition rate on the oxygen content in the SiO_x films and on the crystalline quality of the Si top-layer was investigated by Rutherford backscattering spectrometry and ion channeling. Epitaxial growth of the Si top-layer was observed up to a maximum concentration of ≈20 at.% oxygen content in the SiO_x film. Cross-sectional transmission electron microscopy shows that the structure of the SiO_x film changes duringa subsequent annealing procedure. Electron energy loss spectrometry proves that amorphous SiO₂ is formed and the development of holes indicates that the density of the as-grown SiO_x film is much lower than that of SiO₂. The specific for the as-grown SiO_x films was determined by I–V measurements.     

Improved tribological properties of sputtered MoS2 films through N implantation

The effect of N⁺ implantation on the microstructural and tribological properties of r.f.-sputtered MoS₂ films was studied. The cross-section scanning electron micrographs show that, after N⁺ implantation, the loose column structure of the sputtered MoS₂ films increases in density. A decrease in film thickness of about 50% is also observed. The results of X-ray diffraction analysis show that N⁺ bombardment enhances the (100) edge plane orientation of the MoS₂ crystal in the film. The scratch test indicates an improved film-substrate adherence. The tribological test results indicate that N⁺ implantation yields a distinct enhancement in the wear life of the sputtered MoS₂ films. Compared with the as-deposited MoS₂ film, the wear life of the sputtered MoS₂ films implanted with 150 keV N⁺ at 1 × 10¹⁶ N⁺ cm⁻² shows a threefold increase in a relative humidity of 60%–70% and a twofold increase in a vacuum of 5 × 10⁻³ Pa. However, N⁺ implantation inreases the friction coefficient. The lubrication model of the N⁺-modified film is given.     

Atomic-layer doping in Si by alternately supplied PH3 and SiH4

Atomic-layer doping of P in Si epitaxial growth by alternately supplied PH₃ and SiH₄ was investigated using ultraclean low-pressure chemical vapor deposition. Three atomic layers of P adsorbed on Si(100) are formed by PH₃ exposure at a partial pressure of 0.26 Pa at 450°C. By subsequent SiH₄ exposure at 220 Pa at 450°C, Si is epitaxially grown on the P-adsorbed surface. Furthermore, by 12-cycles of exposure to PH₃ at 300–450°C and SiH₄ at 450°C followed by 20-nm thick capping Si deposition, the multi-layer P-doped epitaxial Si films of average P concentrations of 10²¹ cm⁻³ are formed. The resistivity of the film is as low as 2.4×10⁻⁴ Ω cm. By annealing the sample at 550°C and above, it is found that the resistivity increases and the surface may become rough, which may be due to formation of SiP precipitates at 550°C and above. These results suggest that the epitaxial growth of very low-resistive Si is achieved only at a very low-temperature such as 450°C.