B-doped poly-Si Research Articles

Blistering-free polycrystalline silicon carbide films for double-sided passivating contact solar cells

Polysilicon (poly-Si) passivating contacts have attracted considerable attentions in the academic community and photovoltaic industry due to their remarkable advantages of outstanding passivation quality and low contact resistivity. Plasma enhanced chemical vapor deposition (PECVD), as one of the widely adopted techniques to prepare doped poly-Si films, is usually limited by the occurrence of blistering, especially for boron-doped (B-doped) poly-Si films. In this work, we present a study of PECVD preparation of B-doped polycrystalline silicon carbide (poly-SiC x ) films with a blistering-free appearance by incorporating carbon (C) and optimizing the annealing process. It demonstrates that a thick poly-Si deposited on polished c-Si substrates with a low surface roughness tends to blister, which thus leads to a poor passivation performance. Moreover, the investigation of C content and annealing condition suggests that increasing C content or lowering incipient annealing temperature is beneficial to suppress the blistering. However, the C content needs to be well controlled to balance the passivation and contact properties, because the excessive CH 4 flow during the film deposition would degrade the contact performance with a high contact resistivity (>100 mΩ cm 2 ). Finally, the proof-of-concept devices featuring the double-sided passivating contacts (DPPCs) were fabricated with a front n-type poly-Si and rear p-type poly-SiC x , and presented an open-circuit voltage of 695 mV and an efficiency of 19.82%. The results clarify the correlation of C contents, c-Si substrate roughness and annealing process with the blistering levels and the passivation/contact properties, providing a valuable guidance for fabricating high-efficiency DPPC solar cells. • A path way to prepare blistering-free Boron-doped PECVD poly-Si films has been revealed. • A 90 nm thick PECVD prepared B-doped poly-Si film with a blistering-free appearance has been proposed. • A proof-of-concept double-sided poly-Si passivating contact solar cells with an efficiency of 19.82% has been presented.

A Unique Approach to Generate Self-Aligned T-Gate Transistors in Counter-Doped Poly-Si With High Etching Selectivity and Isotropy

A unique approach for fabricating poly-Si thin-film transistors (TFTs) with self-aligned T-shaped gate (T-gate) structure is reported. A counter-doped poly-Si process comprises an in-situ doped ${n}^{+}$ poly-Si deposition followed by a subsequentshallow implantation of BF $_{{2}}^{+}$ . Both high etching isotropy in ${n}^{+}$ poly-Si and high etching selectivity between ${n}^{+}$ poly-Si and B-doped poly-Si in a Cl2-based plasma process are the key enablers for the fabrication of our T-gate structures. Thanks to good control in the shape and deformation of our T-gate structure, sidewall air-gap spacers in combination with self-aligned Ni silicided gate and source/drain were established. High-performance sub-micron poly-Si TFTs are evidenced by superior transfer characteristics measured on TFTs with effective gate length of $0.15~\mu \text{m}$ . The unique T-gate structure provides an effective way for possible production of poly-Si radio-frequency TFTs viable for emerging new applications.

Investigating the effects of microstructure on optical properties of different kinds of polysilicon thin films

Abstract Flat, B-doped, low-stress and non-stress polysilicon (poly-Si) thin films were deposited by low pressure chemical vapor deposition (LPCVD). X-ray diffraction (XRD), combined with atomic force microscopy (AFM) and Raman system, UV-Raman measurements and spectroscopic ellipsometry, was used to study the microstructure, the morphology and optical properties of the films. The results indicate that the surface roughness is related to the grain size and the change in the microstructure. The B-doped poly-Si shows a tensile stress, while the flat poly-Si shows a high compressive stress. The bandgap of the B-doped poly-Si is the narrowest because of its most disordered microstructure.

Study of Nitrogen Effect on the Boron Diffusion during Heat Treatment in Polycrystalline Silicon/Nitrogen-Doped Silicon Thin Films

The present paper studies the boron (B) diffusion in nitrogen (N) doped amorphous silicon (a-Si) layer in original bi-layer B-doped polycrystalline silicon (poly-Si)/in-situ N-doped Si layers (NIDOS) thin films deposited by low pressure chemical vapor deposition (LPCVD) technique. The B diffusion in the NIDOS layer was investigated by secondary ion mass spectrometry (SIMS) and Fourier transform infrared spectroscopy (FTIR) analysis. A new extended diffusion model is proposed to fit the SIMS profile of the bi-layer films. This model introduces new terms which take into account the effect of N concentration on the complex diffusion phenomena of B atoms in bi-layer films. SIMS results show that B diffusion does not exceed one third of NIDOS layer thickness after annealing. The reduction of the B diffusion in the NIDOS layer is due to the formation of complex B–N as shown by infrared absorption measurements. Electrical measurements using four-probe and Hall effect techniques show the good conductivity of the B-doped poly-Si layer after annealing treatment.

Study of Nitrogen Effect on the Boron Diffusion during Heat Treatment in Polycrystalline Silicon/Nitrogen-Doped Silicon Thin Films

The present paper studies the boron (B) diffusion in nitrogen (N) doped amorphous silicon (a-Si) layer in original bi-layer B-doped polycrystalline silicon (poly-Si)/in-situ N-doped Si layers (NIDOS) thin films deposited by low pressure chemical vapor deposition (LPCVD) technique. The B diffusion in the NIDOS layer was investigated by secondary ion mass spectrometry (SIMS) and Fourier transform infrared spectroscopy (FTIR) analysis. A new extended diffusion model is proposed to fit the SIMS profile of the bi-layer films. This model introduces new terms which take into account the effect of N concentration on the complex diffusion phenomena of B atoms in bi-layer films. SIMS results show that B diffusion does not exceed one third of NIDOS layer thickness after annealing. The reduction of the B diffusion in the NIDOS layer is due to the formation of complex B–N as shown by infrared absorption measurements. Electrical measurements using four-probe and Hall effect techniques show the good conductivity of the B-doped poly-Si layer after annealing treatment.

Effect of grain boundary on electrical characteristics in B- and P-doped polycrystalline Si1−x−yGexCy film deposited by ultraclean LPCVD

The effect of grain boundary on electrical characteristics in B- and P-doped polycrystalline (poly) Si 1− x − y Ge x C y films was investigated. Poly-Si 1− x − y Ge x C y films were deposited on thermally oxidized Si(100) at 500–650 °C in a SiH 4 –GeH 4 –SiH 3 CH 3 –H 2 gas mixture by an ultraclean hot-wall low-pressure chemical vapor deposition. B and P were doped into the films by ion implantation and diffusion by heat-treatment. The electrical properties are characterized by grain size, width of disordered region near grain boundaries, carrier trap density and the amount of impurity segregation at grain boundaries. In the B-doped poly-Si 1− x − y Ge x C y films heat-treated at 900 °C, the increase of carrier concentration n poly and the decrease of resistivity ρ poly with Ge addition are caused by the narrowing of the width of disordered regions, i.e., crystallization of disordered regions induced by Ge atoms. The decrease of n poly and the increase of ρ poly with C addition are explained by the suppression of crystallization of disordered region due to C atom segregation at grain boundaries. In the P-doped poly-Si 1− x − y Ge x C y films, it is found that n poly and ρ poly are influenced by P atom segregation at grain boundaries due to lowering solid solubility of P in grain by the existence of Ge.

Distinction of the irreversible and reversible actuation regions of B-doped poly-Si based electrothermal actuators

Polycrystalline-Si microactuators based on electrothermal principles exhibit many interesting features but their practical use is severely limited by permanent damage that may occur due to accidental overheating. Under these conditions, polycrystalline-Si structures will display irreversible structural changes ranging from slight geometrical deformations to complete damage. In this paper, an approach is presented to avoid permanent structural deformation of B-doped polycrystalline-Si based electrothermal actuators by overheating. The method allows us to distinguish reversible and irreversible actuation conditions and is demonstrated under environmental and vacuum conditions. It enables full utilization of the capabilities of B-doped polycrystalline-Si based electrothermal actuators with reproducible performance.

Effects of mechanical stress on polycrystalline-silicon resistors

Results are presented of a study on the mechanical stress dependence of the resistance of polycrystalline silicon (Poly-Si) films doped with different atomic species, namely, with boron and phosphorus ions. B-doped films of 400 nm and P-doped films of 250 nm thickness were deposited by low-pressure chemical vapor deposition at 620 °C on thermally oxidized silicon wafers. A controlled amount of external stress was applied to the silicon wafers in order to investigate the impact on the electrical performance of the implanted Poly-Si resistors. The resistance of the B-doped Poly-Si films is shown to increase by the mechanical stress, while the resistance of the P-implanted Poly-Si films remained unchanged. It is concluded that this difference is related to the structural differences between Poly-Si films implanted with boron and phosphorus, respectively.

Mechanical stress of the electrical performance of polycrystalline-silicon resistors

Results are presented of a study on the mechanical stress dependence of the resistance of polycrystalline silicon (Poly-Si) films doped with different atomic species. Two types of Poly-Si film implanted with boron and phosphorus ions were studied, namely, B-doped films of 400 nm and P-doped films of 250 nm thickness, which were deposited by low-pressure chemical vapor deposition at 620 °C on thermally oxidized silicon wafers. The film doping was done by ion implantation at 50 keV, with a dose of boron and phosphorus of 2 × 1014 and 5.3 × 1014 cm−2, respectively. The Poly-Si films were annealed in a N2 ambient at 1000 °C for 20 min to activate the implanted atoms. A controlled amount of external stress was applied to the silicon wafers to study the impact on the electrical performance of the implanted Poly-Si resistors. The resistance of the B-doped Poly-Si films is shown to increase by the mechanical stress, while the resistance of the P-implanted Poly-Si films remained unchanged. It is concluded that this difference is related to the structural differences between Poly-Si films implanted with boron and phosphorus, respectively.

Influence of Mechanical Stress on The Electrical Performance of Polycrystalline-Silicon Resistors

ABSTRACTResults are presented of a study on the mechanical stress dependence of the resistance of polycrystalline silicon (Poly-Si) films, doped with different atomic species. Two types of Poly-Si film implanted with boron and phosphorus ions were studied, namely, B-doped films of 400 nm and P-doped layers of 250 nm thickness, which were deposited by LPCVD at 620 °C on thermally oxidized silicon wafers. Film doping was done by ion implantation at 50 keV, with a dose of boron and phosphorus of 2 × 1014 and 5.3 × 1014 cm−2, respectively. The Poly-Si films were annealed in a H2 ambient at 1000 °C for 20 min to activate the implanted atoms. A controlled amount of external stress was applied to the silicon wafers in order to study the impact on the electrical performance of the implanted Poly-Si resistors. The resistance of the B-doped Poly-Si films is shown to increase by the mechanical stress, while the resistance of the P-implanted Poly-Si films remained unchanged. It is concluded that this difference is related to the structural differences between Poly-Si films implanted with boron and phosphorus, respectively.

In Situ Tem Analysis of TiSi2 C49-C54 Transformations During Annealing

AbstractIn situ transmission electron microscope (TEM) observations of TiSi2 C49-to-C54 phase transformations were recorded on video tape and photographic negatives, and transformation front velocities (vTF) were measured from the data. The samples studied in this work include: C49-TiSi2 blanket films and 0.2 μm wide, 7 mm long lines on undoped poly-Si and 0.13 μm wide, 10 μm long lines on either B- or As-doped poly-Si. The in situ TEM analysis showed that blanket TiSi2 films on undoped poly-Si fully transformed at 830°C and had an average vTF of 0.5 ± 0.2 μm/s. The transformation occurred from a sparse nucleation density of ˜0.1 site/μm2. The 0.2 μm wide lines transformed at temperatures greater than 885°C, and the average vTF was 1.1 ± 0.2 μm/sec. Agglomeration started for both the blanket film and the 0.2 μm wide lines at temperatures above 900°C. In situ x-ray diffraction (XRD) analyses of C49-TiSi2 on B- and As-doped poly-Si showed that blanket films completely transformed to C54-TiSi2 at T = 835–843°C, but 0.13 μm wide, varying length lines did not fully transform, even when rapid thermal annealed to 1025°C. From in situ TEM analysis of 0.13 μm wide, 10 μm long C49-TiSi2 lines on B-doped poly-Si, a distinct transformation was not observed. Instead, the lines slowly agglomerated, and electron diffraction of the agglomerated regions showed that the film had transformed to C54-TiSi2. An individual line either 1) completely transformed and agglomerated or 2) remained as C49-TiSi2 and did not agglomerate. Approximately 85% of all lines transformed. In situ TEM analysis of 0.13 μm wide, 10 μm long lines of C49-TiSi2 on As-doped poly-Si also showed an indistinct transformation to the C54 phase along with agglomeration, and 100% of the lines transformed.

Resistivity of Heavily Doped Polycrystalline Silicon Subjected to Furnace Annealing

We investigated the dependence of the resistivity of heavily doped polycrystalline silicon (poly-Si) on the doping concentration and annealing conditions. The resistivity of As-doped poly-Si with equal sized grains depends on the annealing temperature when the doping concentration is less than 5×1020 cm-3. This dependence is related to As segregation to the grain boundaries; that is, more As segregates to the grain boundaries as the temperature decreases. When the doping concentration exceeds 5×1020 cm-3, the As concentration in the grain reaches saturation and remains almost constant with increasing temperature. This means that the solid solubility limit is insensitive to the annealing temperature, and the resistivity remains almost constant with increasing annealing temperature. The resistivity of B-doped poly-Si does not depend on the annealing temperature when the doping concentration is less than the solid solubility limit. This indicates that B does not segregate significantly at the grain boundaries. When the doping concentration exceeds the solid solubility limit, the resistivity depends on the annealing temperature, because the solid solubility limit of B depends significantly on the annealing temperature. The solubility limit of B in poly-Si increases from 6×1019 cm-3 at 800° C to 1.5×1020 cm-3 at 1100° C. We proposed an empirical model that explains the experimental data.

Effect of high-temperature H 2-anneals on the slow-trapping instability of MOS structures

It is shown that the slow-trapping instability upon negative bias-temperature aging of MOS structures (poly-Si and Al-gate) can be significantly reduced by a high temperature (800–900°C) H 2-anneal prior to Al metallization. Additional data are presented on the effects of high (700–900°C) and low temperature (450°C) H 2 annealing of MOS structures containing n-(111) Si, dry HCl oxide and B-doped poly-Si. The midgap N ss (range: 8×10 11 to 1.5×10 10 cm −2 eV −1) is reduced by both high and low temperature H 2 anneals whereas the Q ss (range: 7×10 11 to 1×10 11 cm −2 eV −1) is reduced mainly by the high-temperature H 2-anneals. Presence of B near the interface is believed to cause an abnormal voltage asymmetry of the slow-trapping drift, i.e. ΔV FB (- BT) < ΔV FB (+ BT). These effects are discussed in the light of Deal's model of the structure of the Si/SiO 2 interface.