Phosphorus-doped Films Research Articles

In Situ Investigation of Biomaterial Surfaces Using LDH and ELISA

Lactate dehydrogenase (LDH) and enzyme-linked immunosorbent assay (ELISA) have attracted much attention recently for the evaluation of blood compatibility of biomaterials due to their convenience and quantifiability. In this paper, the use of LDH and ELISA is described for in situ investigation of platelet behavior on biomaterial surfaces, including quantification of platelet adhesion and platelet activation, after suitable testing conditions have been established. The material samples investigated in these tests included low temperature isotropic carbon (LTIC), Ti-O films, and phosphorus- and aluminum-doped TiO2 films. The evaluation results show that the lowest platelet adhesion and activation are observed on phosphorus-doped TiO2 films while the highest platelet adhesion and activation are observed on LTIC. In addition, conventional platelet adhesion experiments were performed for comparison, and yielding similar evaluation results as LDH and ELISA. It is suggested that LDH and ELISA tests can be successfully applied to evaluate the blood compatibility of biomaterials and can show many advantages, such as quantification, reliability and objectivity, compared with conventional platelet adhesion test.

Characteristics of phosphorus-doped diamond-like carbon films synthesized by plasma immersion ion implantation and deposition (PIII and D)

Plasma immersion ion implantation and deposition (PIII and D) is an effective approach to synthesize high quality thin films for different industrial applications. This dual implantation and deposition process can produce a graded layer that mitigates delamination and poor adhesion. In this work, phosphorus-doped diamond-like carbon (DLC) films were synthesized by PIII and D. Thin DLC films with various phosphorus concentrations were produced by using different experimental parameters. The chemical composition was determined by X-ray photoelectron spectroscopy (XPS) and electron energy loss spectroscopy (EELS). Micrographs obtained by cross-sectional transmission electron microscopy (X-TEM) reveal good adhesion between the films and substrates. The biocompatibility was evaluated using platelet adhesive tests. The results show that the sample doped with an optimal amount of phosphorus exhibits less platelet adhesive and activation, and the overall results are better than that observed on low-temperature isotropic pyrolytic carbon (LTIC). Phosphorus-doped DLC films thus have potential applications in blood contacting medical devices.

Doped Wide Bandgap Materials and Devices from Semiconducting Boron Carbide

Semiconducting boron carbide is showing promise as a refractory and robust material with a tunable bandgap. Thin films and devices are easily grown with standard chemical vapor deposition techniques using a variety of boron and carbon precursors including the closo-dodecacarboranes (C2B10H12). Although undoped boron carbide films grown from orthocarborane (1,2- C2B10H12) are p-type, those grown from metacarborane (1,7- C2B10H12) and paracarborane (1,12-C2B10H12) are n-type allowing the production of all boron carbide diodes. In this experimental study, new precursors are used that contain phosphorus and yield new doped n-type films on deposition. The films were characterized by a variety of techniques using UV-Vis spectroscopy (optical bandgap), photoemission (PES) and inverse photoemission (IPES) (electronic structure) and FTIR spectroscopy. Performance evaluation of these new materials will be compared with previous materials.

Dopant effect on in situ doped metal-induced lateral crystallization of amorphous silicon films

Dopant (boron or phosphorus) effect on metal-induced lateral crystallization (MILC) of amorphous silicon (a-Si) films has been systematically studied by using scanning electron microscopy (SEM) and X-ray diffraction (XRD). The MILC was realized by depositing a-Si film onto an oxidized Si wafer and then annealing in a N 2 atmosphere. The doped amorphous films have been developed by plasma-enhanced chemical vapor deposition (PECVD). Various dopant gases and annealing times were investigated to study dopant effects on MILC films. It has been found that the un-doped poly-Si film has a MILC length larger than that of samples doped with boron or phosphorus. The phosphorus-doped films exhibit larger grain size as compared with un-doped and boron-doped films. Larger MILC length is obtained by increasing annealing time, but the MILC rate is decreased. In addition, nickel-silicides were found in the doped samples by XRD. Physical mechanisms for these experimental results are proposed.

Characteristics of solid-phase diffused ultra-shallow junction using phosphorus doped silicon oxide films for fabrication of sub-100 nm SOI MOSFET

Strenuous efforts are being focused on further scaling of metal-oxide-semiconductor field-effect transistors (MOSFETs) into the sub-100 nm regime. Complementary MOS devices (CMOS) fabricated on silicon-oninsulator (SOI) wafers have attracted a lot of attention because of the advantages of full dielectric isolation and reduced junction capacitance over those on bulk silicon wafers in regard to integrated circuit (IC) applications for low stand-by power and high performance. Moreover, the fabrication technique involved in SOI MOSFET can remove or relax some of the process and material constraints on the MOSFET scaling, because it provides a low leakage current, latch-up elimination, high soft error immunity, smaller subthreshold swing, and a simple device structure and process. However, one of the major challenges to overcome is the ultra-shallow junction formation method, which prevents short channel effects for deep sub-100 nm devices [1, 2]. The conventional approach for junction formation is ion implantation with low-energy ion beams. The crystal defects associated with the energetic ion beam bombardment are unavoidable and act as the primary source of junction leakage currents. The junction leakage degrades the device performance and leads to an increase in power consumption. Therefore, an alternative method is required to make use of the advantages of the SOI devices. Doping using solid-phase diffusion (SPD) is an effective method to form damage-free ultra-shallow junctions. In this paper, we report the doping properties of the SPD method and the electrical characteristics of sub-100 nm SOI MOSFETs fabricated using the SPD technique for ultra-shallow source and drain junction formation. The phosphorus doped oxide films used as the SPD source are usually prepared by chemical vapor deposition (CVD), but we used a liquid-state dopant source containing silicon, oxygen and phosphorus. A wafer was coated with the dopant source using a spin coating technique. After baking at 250 ◦C for 20 min, the liquid source was converted to a solid phosphorus doped silicon oxide layer on the top of the wafer. The refractive index and density of this oxide layer were 1.42 and 2.05 g/cm3, respectively. Then the coated wafer was placed in a rapid thermal annealing (RTA) system and a PN junction was created after the phosphorus had diffused into the silicon. In this way, N+P junction diodes and SOI MOSFETs were fabricated. Fig. 1 shows the secondary ion mass spectrometry (SIMS) depth profile of phosphorus for various RTA process temperatures. The RTA process was performed for 30 s in N2 ambient. The diffusion depth of the phosphorus increases with RTA temperature. The surface concentration of phosphorus also increased with the RTA temperature due to the enhanced solid solubility of phosphorus atoms. In nano-scale MOSFET devices, a channel impurity concentration of 1018 cm−3 is required to suppress the short channel effect. It is clear that a shallow junction, less than the 50 nm deep, defined at 1 × 1018 cm−3 can be obtained when the RTA temperature is below 925 ◦C. Fig. 2 shows the relationship between the sheet resistance (Rs) measured using the four-point probe and RTA temperature. The Rs monotonically decreases as the RTA temperature and junction depth increase. The Rs is lower than 1 k Ohm/cm2 at 925 ◦C, which is the maximum drain extension sheet resistance required for 32 nm-technology [3]. Since the surface dopant concentration increased as the RTA temperature increased, more improvements can be expected with control of RTA duration time. Fig. 3 shows the current-voltage (I-V) characteristics of N+P diodes fabricated on SOI substrates by SPD methods. In order to verify the effectiveness of the SPD process, the I-V characteristics of N+P diodes fabricated by the plasma doping method, which is often used in ultra-shallow junction formation, are also shown in Fig. 3 for comparison. It is observed that the forward bias current depends on the RTA temperature. As the RTA temperature increased, the forward bias current increased, while the reverse bias current remained almost constant. The increase in forward bias current could be attributed to the reduction of sheet resistance resulting from high temperature RTA. In the case of the plasma doping sample, the forward bias cur-

High temperature processing of TiO2 thin films for application in silicon solar cells

Low temperature deposited coatings have the potential to reduce the production costs of silicon (Si) photovoltaic devices, such as the buried-contact (BC) solar cell. This work investigates the potential of titanium dioxide (TiO2) films to replace silicon dioxide (SiO2) or nitride coatings currently implemented in the BC fabrication sequence, which requires films to be stable at temperatures up to 1000 °C in a variety of gas atmospheres. The authors demonstrate that: (i) TiO2 films do not reduce the bulk minority carrier lifetime of the silicon wafer after lengthy high temperature processing, however chemical reduction of the TiO2 film can occur if samples are not loaded in an oxygen-containing ambient; (ii) a thin SiO2 passivation layer can be formed at the TiO2:Si interface by performing a brief oxidation after TiO2 film deposition; (iii) while TiO2 coatings function as a phosphorus diffusion barrier, reactions between TiO2 and the phosphorus source result in irreversible damage to the TiO2 film; (iv) phosphorus-doped TiO2 films can act as an n-type dopant source, however further reactions with phosphorus limit the usefulness of the film. Thus, TiO2 films are compatible with the high temperature processing required for BC solar cells, but not with any phosphorus diffusion steps.

Transport properties of phosphorus-doped ZnO thin films

The doping behavior of phosphorus in ZnO thin films grown by pulsed laser deposition is examined. The transport properties of epitaxial ZnO films doped with 1–5 at. % P were characterized via room temperature Hall measurements. As-deposited films doped with phosphorus are highly conductive and n type. The origin of the shallow donor level appears to be either substitution of P on the Zn site or formation of a donor complex. Annealing these phosphorus-doped films significantly reduces the carrier density, transforming the transport from highly conducting to semi-insulating. These results indicate that the phosphorus-related donor defect is relatively unstable, and suggests the formation of a deep level upon annealing. The latter is consistent with phosphorus substitution on the O site yielding a deep level in the gap.

Effect of hydrogen on stability of amorphous silicon thin films

This paper presents results of the investigation of hydrogen influence on the stability of low pressure chemical vapour deposition a-Si films. We measured boron- or phosphorus-doped films post-hydrogenated by ion implantation with different hydrogen doses. The dark conductivity after fast quenching and slow cooling and the isothermal relaxation were measured at different annealing temperatures. It was found that higher hydrogen concentration causes greater metastable changes but shorter relaxation time of defects.

Formation of Polycrystalline Silicon Films Using Electrical-Current-Induced Joule Heating

ABSTRACTElectrical current induced joule heating method was applied to crystallization of 50 nm-thick amorphous silicon films formed on glass substrates. Voltages were applied to silicon films connected with a capacitance in parallel. Irradiation with 5 ns-pulsed-laser melted films partially and caused electrical current induced joule heating because of reduction of electrical resistance. Lateral grain growth with 12.6 µm was realized using the tapered-shaped-electrodes to apply voltages to silicon films with a capacitance of 0.33 µF. For 2.6×1017cm−3phosphorus-doped films, analysis of temperature change in the electrical conductivity gave that defect states at grain boundaries was low of ∼1017cm−3.

Electrical properties of in situ phosphorus- and boron-doped polycrystalline SiGeC films

The sheet resistance, effective carrier concentration, and Hall mobility of in situ boron- and phosphorus-doped polycrystalline Si0.82−yGe0.18Cy films are presented for carbon contents between 0% and 4%. Phosphorus and boron doping levels of 4×1019 and 2×1020 cm−3 were achieved for the n- and p-type layers, respectively, and remained largely unaffected by carbon content. The phosphorus-doped films showed a dramatic increase in sheet resistivity and a corresponding drop in effective carrier concentration and Hall mobility. In contrast, the boron-doped films showed only a minor increase in resistivity. This is attributed to interstitial carbon increasing the defect density and also shifting the defect energy levels at the grain boundaries towards the valence band. This causes an increase in the grain-boundary energy barrier in n-type layers, but leaves the p-type layers largely unaffected.

Large crystalline grain growth using current-induced Joule heating

Electrical-current-induced Joule heating was applied to crystallization of 60-nm-thick amorphous silicon films formed on glass substrates. 3-μs-pulsed voltages were applied to silicon films connected with a capacitance in parallel. Coincident irradiation with 28-ns-pulsed excimer laser melted films partially and reduced its resistance. Complete melting for 12 μs and a low cooling rate at 1.1×108 K/s were achieved by Joule heating from electrical energy accumulated in the capacitance at 0.22 μF. For 7.4×1017 cm-3 phosphorus-doped films, analysis of temperature change in the electrical conductivity gave that the density of defect states localized at grain boundaries was 1.5×1012 cm-2. Formation of 3.5-μm-long crystalline grains was observed by transmission electron micrograph. Preferential crystalline orientation was (110).

High-quality epitaxial growth of in situ phosphorus-doped Si films by promoting dispersion of native oxides in α-Si

Two-step growth in reduced pressure chemical vapor deposition has been successfully developed to achieve in situ phosphorus-doped silicon epitaxial layers, and the characteristic evolution of their microstructures has been investigated using scanning electron microscopy, transmission electron microscopy, and secondary ion mass spectroscopy. The two-step growth, which employs crystallization of amorphous silicon (α-Si) layer grown at low temperature, offers crucial advantages in manipulating crystal structures of in situ phosphorus-doped silicon. Our experimental results suggest that an epitaxial or polycrystalline film is readily obtained by initiating growth either without or with annealing to crystallize the α-Si layers. Kinetic processes for crystallization were accompanied by stimulated dispersion of native oxides in α-Si buffers. We also observed that a high phosphorus doping level plays an important role in promoting the dispersion of native oxides, and that the shape of doping profiles depends upon the evolution of crystal structures with V-shaped defects. Thus, the phosphorus doping concentration remains uniform in high-quality single-crystalline Si films obtained by the two-step growth.

KrF excimer laser crystallization of silicon thin films

A KrF excimer laser operating at 249 nm has been employed to crystallize silicon thin films deposited by electron cyclotron resonance plasma-enhanced chemical vapor deposition (ECR PECVD) and by RF magnetron sputtering on Corning glass and SiO2. All films display a substantial improvement in crystallinity after ELC with the optimum laser fluence for as-deposited ECR films being higher than for sputtered films. This is probably related to the presence of Si−Hx bonds in the former. A pronounced bimodality in the Raman spectra of some amorphous, as-deposited ECR samples has been observed after laser crystallization where, in addition to the peak at 520cm−1, a strong peak at 509cm−1 is also present. Such behavior has not been reported previously to our knowledge in ELC silicon films. Interestingly, the XRD spectra of these samples do not exhibit any peaks suggesting the films are composed of nano-grain material. The dehydrogenation of ECR films by ELC has been demonstrated to be substantial, the hydrogen content typically decreasing from ∼30 at % in an as-deposited film to ∼10 at % after a single low fluence laser shot. Raman spectroscopy has shown that the film bonding changes from predominantly Si−H2 to Si−H after ELC. Electrical resistivity measurements of phosphorus-doped films show a controllable and repeatable change with laser fluence. The results in this paper show that it is possible to crystallize and controllably change the electrical characteristics of ECR PECVD produced silicon thin films by ELC.

Plasma processing in the fabrication of amorphous silicon thin-film-transistor arrays

Plasma deposition and etching are two critical processes in the fabrication of the arrays of thin-film transistors (TFTs) in active-matrix liquid crystal displays. The processes fulfill three important production requirements: suitability for use at relatively low substrate temperatures, suitability for large substrates, and high throughput capability. We present an overview of our recent studies on the plasma-processing aspects of the fabrication of large-area arrays of TFTs in which use is made of thin films of amorphous silicon containing hydrogen (a-Si:H). Subjects covered include the plasma-enhanced chemical vapor deposition (PECVD) of a-Si:H films, SiN <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</inf> films, and heavily phosphorus-doped Si films; the influence of the films on TFT characteristics; associated plasma-etching and edge-profile aspects; plasma-etching-induced damage to TFTs and repair of the damage; and equipment aspects.

Probing of microvoids in high-rate deposited a-Si: H thin films by variable energy positron annihilation spectroscopy

In this paper, positron annihilation measurements have been carried out on a-Si: H thin films deposited by plasma-enhanced chemical vapor deposition (PECVD) at high and low rates by means of the variable energy positron beam Doppler-broadening technique. The depth profiles of microvoids in the films grown under different conditions have been determined. We found a smaller void fraction in the surface region of all films compared to the bulk, and a smaller void fraction in low rate than in high growth rate films. By plotting S and W parameters in the (S, W) plane, we have shown that the vacancies in all of the high-rate and low-rate deposited intrinsic samples, and in differently doped low-rate samples are of the same nature, although there appears to be a higher density of defects in the boron than phosphorus doped films. The depth profiles of the microvoid-like defects in the a-Si: H films are extracted by use of the vepfit program.

Morphology and semiconducting properties of homoepitaxially grown phosphorus-doped (1 0 0) and (1 1 1) diamond films by microwave plasma-assisted chemical vapor deposition using triethylphosphine as a dopant source

Phosphorus-doped (1 0 0) and (1 1 1) diamond films were homoepitaxially formed on nondoped diamond films, which had been also formed homoepitaxially on type-Ib (1 0 0) and (1 1 1) diamond substrates, respectively, by microwave plasma-assisted chemical vapor deposition. Methane and triethylphosphine (TEP, P(C 2H 5) 3) were used as the carbon and dopant sources, respectively. When the P/C ratio in the gas phase was in the range of 10 −2–10 −1 and the methane concentration was 0.5%, smooth homoepitaxial (1 0 0) diamond films with a thickness of approximately 800 nm were obtained at 1123 K. Raman spectroscopy showed that the P-doped (1 0 0) diamond films formed with gas phase P/C ratios higher than 4×10 −2 contained sp 2 carbon. Phosphorus was found to be uniformly incorporated in the films, as evidenced by secondary ion mass spectroscopy, and the phosphorus concentration in the doped (1 0 0) diamond films was estimated to be (2–8)×10 18 cm −3. All P-doped (1 0 0) diamond films showed insufficient ohmic contacts for a Hall-effect determination, and no n-type conduction was confirmed. However, a homoepitaxial (1 1 1) diamond film, which was formed at 1173 K using a gas-phase P/C ratio of 5×10 −3 and a methane concentration of 0.1% on a nondoped homoepitaxial (1 1 1) diamond layer at 1123 K exhibited n-type conduction at temperatures higher than 485 K. The carrier concentration and Hall mobility at 500 K were 3.8×10 16 cm −3 and 38 cm 2/V s, respectively. Phosphorus was uniformly incorporated in the film at a concentration of 1×10 20 cm −3.

Fabrication of on-chip barium strontium titanate capacitors by metallo-organic decomposition

Metallo-organic thin film decomposition (MOD) was used in forming barium strontium titanate (BST) thin film capacitors on phosphorus doped polysilicon films deposited on 4 in. silicon wafers. A single step deposition process yielded highly uniform, crack-free BST films ranging up to 0.25 εm in thickness and having various step heights and dimensional area. Scanning electron microscopy (SEM) showed very good step coverage and planarization of the BST. The capacitors had capacitance densities above 200 nF/cm2, leakage current densities less than 1.55 εA/cm2 at a bias voltage of 10 V, and a dielectric breakdown field above 1 MVycm. Small temperature coefficients of capacitance and dissipation (tan δ) were also observed. Frequency response measurements were made using the BST capacitors and on-chip resistors in low pass and high pass circuit configurations. A plot of relative gain and phase angle versus frequency showed excellent agreement with predicted results.

Metastability of Phosphorus- or Boron Doped a-Si:H Films

ABSTRACTThe effects of light-soaking on either phosphorus- or boron-doped a-Si:H films were studied as functions of the doping level and the temperature. In the case of boron-doped films, the most important effect is the improvement of the conductivity during light-soaking, which is related to the activation of boron. On the contrary, phosphorus-doped films present a remarkable stability, although lightly phosphorus-doped ones show a decrease of their conductivity by five orders of magnitude when light-soaking is performed below 40 °C. This effect is attributed to the formation of P-H complexes which are stable at low temperature only. Our results suggest that in both types of doped a-Si:H films the creation of metastable defects is a second order effect with respect to the activation or passivation of dopants, which results from their interaction with hydrogen.

Structural, electrical and optical characterization of N- and P-type SiC:H films deposited using ECR-CVD

Hydrogenated silicon carbide films (SiC:H) were deposited using the electron cyclotron resonance chemical vapour deposition (ECR-CVD) method from a mixture of methane, silane and hydrogen, and using diborane and phosphine as doping gases. The effects of changes in the diborane and phosphine levels on the optical bandgap and conductivity were investigated. In the case of boron-doped films, there is evidence from Raman scattering analysis to show that films deposited at a low microwave power of 150 W were largely amorphous and the bandgap decreases as the diborane levels are highly conductive and contains the whereas films deposited at a high microwave power of 800 W at low diborane levels are highly conductive and contains the silicon microcrystalline phase. These films become amorphous as the diborane level is increased, while the optical bandgap remains relatively unaffected throughout the entire range of diborane levels investigated. In the case of phosphorus-doped films, Raman scattering analysis showed that the deposition conditions strongly influence the structural, optical and electrical properties of the SiC:H films. Unlike boron doping, doping with phosphorus can have the effect of increasing the silicon microcrystalline phase in the SiC:H films which were prepared at low (150 W) and high (600 W) microwave powers. Films prepared at high microwave power showed only small variations in the optical bandgap, suggesting that good phosphorus doping efficiency can be achieved in films which contain the silicon microcrystalline phase (mc-SiC:H).

Preparation of n-type SiC:H using ECR—CVD: some effects of microcrystallite silicon formation induced by phosphorus doping and microwave power

This paper reports the deposition of hydrogenated silicon carbide (SiC:H) films using the electron cyclotron resonance chemical vapour deposition (ECR—CVD) technique. Using this technique, SiC:H films were prepared from a mixture of methane, silane and hydrogen, with phosphine as the doping gas. The effects of changing the phosphine fraction on the optical bandgap, activation energy and conductivity were investigated in films deposited at two different microwave powers of 150 and 600 W, respectively. Additionally, the effects of changes in the microwave power (from 150 to 900 W) on the characteristics of the phosphorus-doped films were also investigated. Raman scattering analysis shows that increase in the microwave power and high phosphorus doping fractions have the effect of enhancing the formation of the silicon microcrystalline phase in the films. Films having a strong silicon microcrystalline phase exhibit relatively small changes in the optical bandgap. The film conductivity increased rapidly followed by saturation as the microwave power or phosphorus doping fraction is increased. Correspondingly, the activation energy decreased and saturated suggesting an effect due to dopant saturation. The results showed that good phosphorus doping efficiency can be obtained in SiC:H films deposited at high microwave power. Results from IR absorption measurements suggest that hydrogen is bonded to silicon in the Si-H stretching mode and to carbon in the sp 3 CH n rocking/wagging and bending modes in films deposited at high microwave powers.