Si-based Substrates Research Articles

Enhancement of the 90° domain-wall mobility in sol-gel (Pb,La)TiO3 thin films prepared by multiple deposition and crystallization

Sol-gel lanthanum-modified lead titanate thin films deposited onto Si-based substrates with two different processing conditions have been studied. The films have been processed with a single crystallization or with a multiple deposition and crystallization process. A preferential ⟨001⟩,⟨100⟩-mixed orientation is present in the two types of films. The dependence of the dielectric permittivity on frequency and field amplitude at subswitching fields, permittivity versus temperature curves, and ferroelectric hysteresis loops have been measured in the films. Both types of films present Rayleigh-type behavior with no differences between them, which suggests a similar density of defects in both films. Higher values of remanent polarization and dielectric permittivity, when approaching the transition temperature, have been achieved for the film prepared with the multiple deposition and crystallization process when compared with the obtained ones in the film prepared with a single crystallization. This is due to a lower clamping effect and an enhancement of the 90° domain-wall mobility in the former, as a consequence of the lower residual stress present in this film, achieved by means of the multiple deposition and crystallization process.

Investigations of High-Performance GaAs Solar Cells Grown on Ge–Si<tex>$_1-xhbox Ge_ x$</tex>–Si Substrates

High-performance p/sup +//n GaAs solar cells were grown and processed on compositionally graded Ge-Si/sub 1-x/Ge/sub x/-Si (SiGe) substrates. Total area efficiencies of 18.1% under the AM1.5-G spectrum were measured for 0.0444 cm/sup 2/ solar cells. This high efficiency is attributed to the very high open-circuit voltages (980 mV (AM0) and 973 mV (AM1.5-G)) that were achieved by the reduction in threading dislocation density enabled by the SiGe buffers, and thus reduced carrier recombination losses. This is the highest independently confirmed efficiency and open-circuit voltage for a GaAs solar cell grown on a Si-based substrate to date. Larger area solar cells were also studied in order to examine the impact of device area on GaAs-on-SiGe solar cell performance; we found that an increase in device area from 0.36 to 4.0 cm/sup 2/ did not degrade the measured performance characteristics for cells processed on identical substrates. Moreover, the device performance uniformity for large area heteroepitaxial cells is consistent with that of homoepitaxial cells; thus, device growth and processing on SiGe substrates did not introduce added performance variations. These results demonstrate that using SiGe interlayers to produce "virtual" Ge substrates may provide a robust method for scaleable integration of high performance III-V photovoltaics devices with large area Si wafers.

Composition and thickness distribution of HgCdTe molecular beam epitaxy wafers by infrared microscope mapping

A technique based on infrared microscopy and the automatic fitting of the transmissivity curves has been developed to measure the composition and thickness distributions of HgCdTe wafers grown by molecular beam epitaxy (MBE). This technique has the ability to handle many measuring points, provides high spatial resolution, and can be used for the routine characterization of the uniformity of HgCdTe grown on CdZnTe and Si-based substrates. The mapping measurements both are helpful for infrared detector fabrication and can help MBE growers adjust growth conditions and optimize equipment configurations to grow large area uniform HgCdTe material. The reason that limits the enhancement of uniformity was also discussed.

(Pb,La)TiO3∕(Pb,Ca)TiO3 ferroelectric heterostructures for nonvolatile memories

The existence of voltage shifts in the ferroelectric loops measured in (Pb,La)TiO3∕(Pb,Ca)TiO3 heterostructures onto Si-based substrates after poling at 150°C has been studied. Results have been compared with the measured ones in (Pb,La)TiO3 and (Pb,Ca)TiO3 single-component films, which have been prepared in identical conditions. Lower voltage shifts have been obtained in the heterostructures. The reduced number of VO–VPb defect-dipoles present in the heterostructures as a consequence of the mechanism of stress relaxation via plastic deformation through vacancies diffusion that takes place in these films results in a reduced electron trapping, and thus, in a lower voltage shift of the loop.

Electrical Properties of Screen Printed PZT Thick Films Infiltrated with Photo-Sensitive Sol Compared with Normal Sol for Cantilever Type Biochip

Much attention has been paid to PZT thick films on Si-based substrate since they began to be used as piezoelectric actuators and sensors with introduction of MEMS technology and screen printing method. However, the thick films made just by screen printing method have high porosity as compared with bulk product, and the PZT thick films on Si-based substrate have problems regarding degradation of active materials and interface properties owing to inter-diffusion or reaction between Si substrate and PZT materials at high temperature for sintering. Thus we have fabricated screen printed PZT thick films on Si substrate using screen printing method and sol infiltration for enhancing densification. New ethanol based photo-sensitive (self-pattern-able) sol and conventional diol based sol were used in order to compare influence of patterning process and investigate adoptability in device process for biochip. Thick films with relative high densities at low temperature, 800°C and without inter-diffusion and reaction between the layers and thick film were accomplished. And it was revealed that the PZT thick film treated by ethanol based self pattern-able sol showed better electrical properties as well as better pattern-ability, and consequently it was applicable to cantilever based biochip without additional PZT film patterning process.

Reduced dielectric dispersion in ferroelectric (Pb,La)TiO3/(Pb,Ca)TiO3 thin-film multilayer heterostructures due to a mechanical stress relaxation mechanism

The dielectric dispersion of (Pb,Ca)TiO3/(Pb,La)TiO3/(Pb,Ca)TiO3 and (Pb,La)TiO3/(Pb,Ca)TiO3/(Pb,La)TiO3 ferroelectric thin-film multilayer heterostructures onto Si-based substrates has been studied and compared with that of identically prepared (Pb,La)TiO3 and (Pb,Ca)TiO3 films. Grazing incidence x-ray diffraction analysis reveals that the tetragonal distortion of the heterostructures is higher than the ones for the single-component films, which evidences a mechanical stress relaxation mechanism in the heterostructures. A lower dielectric dispersion has been obtained in the heterostructures with respect to the single-component films. This is also a consequence of the stress relaxation by plastic deformation through vacancy diffusion present in the former. Vacancy diffusion decreases the number of VPb–VO defect-dipoles, which could contribute to the dielectric permittivity at low frequencies in the heterostructures.

Er-LiYF4 coating of Si-based substrates by pulsed laser deposition

We have produced by pulsed laser deposition thin films of Er-doped LiYF4 onto silicon. Substrate coating turns out efficient, due to the strong coupling between the UV pulsed laser radiation and the fluoride target. Samples have been analyzed by fluorescence emission spectroscopy upon pulsed excimer laser excitation and spectra compared to those of the bulk target. Furthermore, upconversion effect has been verified in the thin film with near-infrared excitation. The results, suggesting the occurrence of nonlinear phenomena, are encouraging in view of possible applications in optoelectronics and photonics devices and demonstrate the feasibility of pulsed laser deposited Er-doped fluoride films.

Ferroelectric Properties of PbZr0.40Ti0.60O3/ LaxSr1−xCoO3 Heterostructures Prepared by Chemical Solution Routes

PbZr0.40Ti0.60O3/La x Sr1 − x CoO3 (PZT/LSCO) heterostructures have been grown on Si-based substrates by chemical solution routes. X-ray diffraction analysis showed that PZT on LSCO are polycrystalline with (110) orientation and entirely perovskite phase. The resistance of LSCO is strongly affected by the ratio of La/Sr and the annealing atmosphere for the LSCO thin films. The ferroelectricity of PZT is strongly dependent on LSCO electrode, the P-E hystersis loops of PZT deposited on LSCO which annealed in air is more asymmetry than that annealed in oxygen. At the applied electric field of 300 kV/cm, all PZT films have the resisitivity larger than 1010 Ω · cm. The fatigue resistance is strongly affected by the resistance of the LSCO electrode. The ferroelctric properties are strongly improved by LSCO buffer layer between PZT and Pt bottom electrode.

Er-LiYF 4 coating of Si-based substrates by pulsed laser deposition

We have produced by pulsed laser deposition thin films of Er-doped LiYF 4 onto silicon. Substrate coating turns out efficient, due to the strong coupling between the UV pulsed laser radiation and the fluoride target. Samples have been analyzed by fluorescence emission spectroscopy upon pulsed excimer laser excitation and spectra compared to those of the bulk target. Furthermore, upconversion effect has been verified in the thin film with near-infrared excitation. The results, suggesting the occurrence of nonlinear phenomena, are encouraging in view of possible applications in optoelectronics and photonics devices and demonstrate the feasibility of pulsed laser deposited Er-doped fluoride films.

Crystalline silicon thin films with porous Si backside reflector

Thin film crystalline Si solar cells on cheap Si-based substrates have a large potential in PV technology. Optical light confinement is a very crucial point of such thin film structures. Porous Si (PS) as a perfect light diffuser could be used as a backside reflector if its multi-layer structure would be preserved during the deposition of a thin Si film. That is why low-energy plasma enhanced chemical vapor deposition (LEPECVD) was chosen to deposit a thin Si film on a PS multilayer structure at low temperature and high deposition rate. This technique allows one to deposit a Si film with epitaxial quality on the top of PS without destroying its multi-layer structure as revealed by high-resolution X-ray diffraction and cross-sectional transmission electron microscopy (TEM). The epi-layers of 10 μm are grown at very high deposition rates (approx. 3 nm/s) at 590°C. TEM-analysis reveals that during the deposition a high density of defects forms at the PS/epi-Si interface and spreads through the whole epi-layer. The defect density is decreased when the deposition temperature is increased to 645°C. LEPECVD appears to be an appropriate deposition technique to grow thin Si films on cheap Si based substrates with PS reflector.

High Pyroelectric Coefficients of Ca-Modified Lead Titanate Sol-Gel Thin Films Obtained by Multiple Deposition and Crystallization

Ferroelectric thin films of (Pb 0.76 Ca 0.24 )TiO 3 nominal composition have been obtained, by multiple spin-on deposition on Si-based substrates and RTP crystallization, from precursor solutions processed by the diol-based sol-gel method. A mixed <001>/<100> or <111> preferential orientations were promoted by appropriate choice of the substrate, as determined by XRD measurements. The effect of substrate annealing and crystallized film annealing on the pyroelectric coefficient of the films was determined and discussed. Poling of the films at room temperature and at 150 °C was tested. The highest pyroelectric coefficient achieved was n ∼ 40.0 2 10 m 9 Ccm m 2 K m 1 .

Ca and La-Modified Lead Titanate Sol-Gel Thin Films by UV-Assisted Processing for Piezoelectric Sensors

A diol-based sol-gel method was used to prepare UV sensitive precursor solutions in the processing of modified lead titanate ferroelectric thin films. Films were deposited by spin-coating onto platinized Si-based substrates. A <111> preferential orientation was promoted by appropriated choice of the substrate and multiple deposition and crystallization steps. Processing under UV radiation was carried out in a novel RTP equipment including an arrangement of high intensity UV excimer lamps. To obtain the desired ferroelectric perovskite structure, moderate temperatures and times were required, with respect to those of the processing without UV irradiation. XRD measurements were carried out to determine the degree of orientation of the films. Ferroelectric characterization was made to investigate the optimum poling conditions of the films. Piezoelectric coefficients, e 31 and d 33 , were determined in order to evaluate the feasibility of the use of the films in piezoelectric sensors.

Crystalline silicon thin films with porous Si backside reflector (abstract only)

Thin film crystalline Si solar cells on cheap Si-based substrates have a large potential in PV technology. Optical light confinement is a very crucial point of such thin film structures. Porous Si (PS) as a perfect light diffuser could be used as a backside reflector if its multi-layer structure would be preserved during the deposition of a thin Si film. That is why low-energy plasma enhanced chemical vapor deposition (LEPECVD) is chosen to deposit a thin Si film on a PS multilayer structure at low temperature and a high deposition rate. This technique allows one to deposit a Si film with an epitaxial quality on the top of PS without destroying its multilayer structure as revealed by high-resolution X-ray diffraction and cross-sectional transmission electron microscopy (TEM). The epilayers of 10 μm are grown at very high deposition rate (around 3 nm/s) at 590°C. TEM-analysis reveals that during the deposition a high density of defects forms at the interface PS/epi-Si and spreads through the whole epilayer. The defect density is decreased when the deposition temperature is increased to 645°C. LEPECVD appears to be an appropriate deposition technique to grow thin Si films on cheap Si based substrates with PS reflector.

Spontaneous pyro- and piezoelectricity of sol-gel La-modified lead titanate thin films

Polycrystalline La-modified lead titanate thin films have been processed, by a diol-based sol-gel route, on Si-based substrates. Multiple deposition and crystallization steps were carried out in order to promote heterogeneous nucleation and, as a consequence, a preferred crystallographic orientation and self polarization. Crystallization was achieved by rapid thermal processing at 650°C for 50s, at a heating rate of ∼30°C/s. The films developed either a <111> or a mixed <100>/<001> preferred orientation, which were determined by quantitative texture analysis of X-ray pole figures. Hysteresis cycles of the piezoelectric d33 coefficient versus electric field were obtained by a double-beam laser interferometric technique and found to strongly depend on the film texture. <111> oriented films present a thickness-dependent spontaneous pyroelectric coefficient and field endurance of the piezoelectric coefficient. These films are thus, appropriate to be used in non-switching applications, such as pyroelectric infra-red detectors and integrated piezoelectric sensors.

Growth mechanism of sputter deposited Ta and Ta–N thin films induced by an underlying titanium layer and varying nitrogen flow rates

Tantalum (Ta) and nitrogen-contained tantalum (Ta–N) thin films are sputter deposited on Si-based substrates with and without a titanium adhesion layer. The impact of varying the nitrogen flow rate and the underlying titanium on the phase formation process is investigated using X-ray diffraction analysis, resistivity measurement and scanning electron microscopy. Our results indicate that the titanium layer inhibits the formation of high-resistivity tetragonal β-Ta, and leads to the deposition of low-resistivity cubic α-Ta arising from its epitaxial orientation on the underlying titanium. Consequently, the electrical properties and microstructures of the Ta-based films are significantly changed.

Detailed characterization of anodic bonding process between glass and thin-film coated silicon substrates

Anodic bonding between Si-based and glass substrates has been characterized in detail. The effects of magnitude of the applied voltage, surface properties (coating of Si substrate), and surface cleanliness (pre-bonding cleaning procedure) on the time required for complete bonding were thoroughly studied. First, the generic bonding time versus applied voltage plot was found to be concave in shape (viewed from the origin). For bonding between p-type Si substrate and Corning 7740 glass pre-cleaned with acetone, the time required was cut down from 38 to 4 min if the applied voltage was increased from 200 to 500 V. Second, the bonding time required for five Si-based substrates in ascending order was determined to be Si (p-type), polysilicon, silicon nitride, silicon oxide and then Si (n-type). Third, the bonding between p-type Si substrate, pre-cleaned with H 2SO 4–H 2O 2 and HF, and Corning 7740 glass was completed within 1 min, which was much faster than that pre-cleaned with acetone (4 min). Finally, from bonding point of view, Corning 7740 glass was superior to Corning 7059 glass and Fisher slide due to its thermal coefficient of expansion matching with the underlying Si substrate and the presence of significant amount of sodium ions in the glass.

Preparation of Pb(Zr, Ti)O3 Thin Films on Glass Substrates

Lead-zirconate-titanate (PZT) thin films were prepared on non-alkaline glass substrates widely used in liquid crystal display (LCD) devices, by plasma-assisted magnetron RF sputtering with an immersed coil. After preparation of the PZT thin film, the glass was available for use in LCD device processing. No mutual diffusion of the elements was recognized between the glass substrate and the bottom electrode. The PZT layer had a dense film structure with rectangular and columnar grains, and only its perovskite phase was crystalline. PZT thin films on a glass substrate had leakage current densities of about 10-8 A/cm2, acceptable hysteresis loop shapes with the remanent polarization (Pr) of 45 µC/cm2 and the coercive field (Ec) of 90 kV/cm. Ferroelectric properties on a glass substrate almost conform with those on a Si-based substrate.

Dependence of functional properties on processing in sol-gel (Pb,La)TiO3 thin films

Pb0.88La0.08TiO3 films were processed on Si-based substrates by a diol-based sol-gel route from solutions with variable content of PbO excess. Crystallisation was performed at heating rates of 10 °C min-1 and higher than 500 °C min-1 (rapid heating). The pyroelectric coefficient was measured after poling the samples by two methods: applying a sinusoidal wave and applying a train of square pulses, with the latter showing a higher poling efficiency. The piezoelectric d33 coefficient was determined by double-beam interferometry. Strain vs. field measurements provided evidence of 90° domain orientation in these films. Those crystallised at 10 °C min-1 showed the highest functional properties (γ=1.7×10-2 μC cm-2 K-1 and d33=57 pm V-1). This is a consequence of the higher stability of the 90° domains oriented during poling, caused by the lower tensile stress arising during preparation. The voltage responsivity of these films also benefited from the lower permittivity arising from their higher porosity. These films are good candidates for applications in infrared detectors and microelectromechanical devices.

Effect of dislocations on performance of LWIR HgCdTe photodiodes

The epitaxial growth of HgCdTe on alternative substrates has emerged as an enabling technology for the fabrication of large-area infrared (IR) focal plane arrays (FPAs). One key technical issue is high dislocation densities in HgCdTe epilayers grown on alternative substrates. This is particularly important with regards to the growth of HgCdTe on heteroepitaxial Si-based substrates, which have a higher dislocation density than the bulk CdZnTe substrates typically used for epitaxial HgCdTe material growth. In the paper a simple model of dislocations as cylindrical regions confined by surfaces with definite surface recombination is proposed. Both radius of dislocations and its surface recombination velocity are determined by comparison of theoretical predictions with carrier lifetime experimental data described by other authors. It is observed that the carrier lifetime depends strongly on recombination velocity; whereas the dependence of the carrier lifetime on dislocation core radius is weaker. The minority carrier lifetime is approximately inversely proportional to the dislocation density for densities higher than 105 cm−2. Below this value, the minority carrier lifetime does not change with dislocation density. The influence of dislocation density on the R0A product of long wavelength infrared (LWIR) HgCdTe photodiodes is also discussed. It is also shown that parameters of dislocations have a strong effect on the R0A product at temperature around 77 K in the range of dislocation density above 106 cm−2. The quantum efficiency is not a strong function of dislocation density.

Oriented growth of sol-gel-modified PbTiO3 thin films on Si-based substrates

A sol–gel Pb0.76Ca0.24TiO3 solution was deposited by spin-coating onto Pt/Ti/SiO2/Si(100), Ti/Pt/Ti/SiO2/ Si(100) and Pt/TiO2/SiO2/Si(100) substrates. As-prepared substrates and those subjected to a thermal treatment in air at 650 °C for 1800 s were used. The wet films were dried at 350 °C for 60 s and crystallized in air by rapid thermal processing (RTP) at 650 °C for 50 s, with a heating rate of ∼30 °C s−1. This procedure was repeated four times to obtain crystalline films with a thickness of ∼250–300 nm. Perovskite films with different textures were obtained after crystallization. This texture was a consequence of the multilayer structure of the substrate. The formation of Pb0.76Ca0.24TiO3 films with a preferred orientation in the 〈 111〉 direction of the perovskite was observed when Ti was present at the top surface of the Pt. Texture of the films was studied by x-ray diffraction (XRD) and the structure of the substrate surfaces was studied by grazing incidence x-ray diffraction (GIXRD). Spontaneous pyroelectric coefficients were measured in the films. Copyright © 2000 John Wiley & Sons, Ltd.