Boron-doped Silicon Films Research Articles

Low-thermal-budget electrically active thick polysilicon for CMOS-First MEMS-last integration

Low-thermal-budget, electrically active, and thick polysilicon films are necessary for building a microelectromechanical system (MEMS) on top of a complementary metal oxide semiconductor (CMOS). However, the formation of these polysilicon films is a challenge in this field. Herein, for the first time, the development of in situ phosphorus-doped silicon films deposited under ultrahigh-vacuum conditions (~10−9 Torr) using electron-beam evaporation (UHVEE) is reported. This process results in electrically active, fully crystallized, low-stress, smooth, and thick polysilicon films with low thermal budgets. The crystallographic, mechanical, and electrical properties of phosphorus-doped UHVEE polysilicon films are studied. These films are compared with intrinsic and boron-doped UHVEE silicon films. Raman spectroscopy, X-ray diffraction (XRD), transmission electron microscopy (TEM) and atomic force microscopy (AFM) are used for crystallographic and surface morphological investigations. Wafer curvature, cantilever deflection profile and resonance frequency measurements are employed to study the mechanical properties of the specimens. Moreover, resistivity measurements are conducted to investigate the electrical properties of the films. Highly vertical, high-aspect-ratio micromachining of UHVEE polysilicon has been developed. A comb-drive structure is designed, simulated, fabricated, and characterized as an actuator and inertial sensor comprising 20-μm-thick in situ phosphorus-doped UHVEE films at a temperature less than 500 °C. The results demonstrate for the first time that UHVEE polysilicon uniquely allows the realization of mechanically and electrically functional MEMS devices with low thermal budgets.

Excellent surface passivation of p-type TOPCon enabled by ozone-gas oxidation with a single-sided saturation current density of ∼ 4.5 fA/cm2

High-quality p-type tunnel oxide passivated contact (p-type TOPCon) is a feasible technical solution to further improve the efficiency of TOPCon silicon solar cells. Plasma-enhanced chemical vapor deposition (PECVD) technology route could deposit boron-doped amorphous silicon film in-situ and thus becomes one of the most promising industry routes to prepare the TOPCon structure. However, the passivation quality of p-type TOPCon by PECVD is not satisfactory till now. In this work, we develop the high-performance p-type TOPCon technology by integrating the ozone-gas oxidation to prepare the ultra-thin SiOx film, which shows excellent passivation and contact properties. The experimental results suggest that the double-sided p-type TOPCon passivated samples with p-type Si substrates receive a maximal implied open-circuit voltage (iVoc) of ∼ 734 mV together with a minimum single-sided saturation current density (J0,s) of ∼ 4.5 fA/cm2. Correspondingly, the contact resistivity is less than 5 mΩ·cm2, yielding a high selectivity S10 of 15.6, which is one of the best values for p-type TOPCon technology. As a result, the precursor cell manifests an excellent iVoc of 717 mV, and the p-type silicon solar cell with rear-sided p-type TOPCon passivating contact receives a high efficiency of 22.23%. Generally, this work provides a promising technology for preparing the high-quality p-type TOPCon passivating contact for industrial application.

Boron Detection Technique in Silicon Thin Film Using Dynamic Time of Flight Secondary Ion Mass Spectrometry

The impurity concentration is a crucial parameter for semiconductor thin films. Evaluating the impurity distribution in silicon thin film is another challenge. In this study, we have investigated the doping concentration of boron in silicon thin film using time of flight secondary ion mass spectrometry in dynamic mode of operation. Boron doped silicon film was grown on i) p-type silicon wafer and ii) borosilicate glass using hot wire chemical vapor deposition technique for possible applications in optoelectronic devices. Using well-tuned SIMS measurement recipe, we have detected the boron counts 101~104 along with the silicon matrix element. The secondary ion beam sputtering area, sputtering duration and mass analyser analysing duration were used as key variables for the tuning of the recipe. The quantitative analysis of counts to concentration conversion was done following standard relative sensitivity factor. The concentration of boron in silicon was determined 1017~1021 atoms/㎤. The technique will be useful for evaluating distributions of various dopants (arsenic, phosphorous, bismuth etc.) in silicon thin film efficiently.

P+ polycrystalline silicon growth via hot wire chemical vapour deposition for silicon solar cells

Hot wire chemical vapour deposition (HWCVD) is explored as a way of growing boron-doped silicon for photovoltaic devices. Deposition temperatures are measured using a custom-built monitoring system for two different filament configurations within the HWCVD tool. A refined fabrication process is presented, using an altered filament arrangement, that currently provides a maximum deposition temperature of 535 °C, for growing boron-doped silicon films via HWCVD, with the inclusion of a short post-deposition anneal at 800 °C for 2 min. Transmission electron microscopy reveals improvements in interfacial quality, as well as larger grains, present after post-annealing treatments. In addition, re-crystallisation of as-deposited amorphous Si films under a short anneal is confirmed using Raman spectroscopy. The enhancements in morphology translate to a boost in current rectification based on dark current-voltage measurements. This is further supported by secondary-ion mass spectrometry analysis, presenting p+ properties with uniform doping in the 1021 cm−3 region.

Synthesis of Boron-Doped Silicon Film Using Hot Wire Chemical Vapor Deposition Technique

Boron-doped polycrystalline silicon film was synthesized using hot wire chemical vapor deposition technique for possible application in photonics devices. To investigate the effect of substrate, we considered Si/SiO2, glass/ITO/TiO2, Al2O3, and nickel tungsten alloy strip for the growth of polycrystalline silicon films. Scanning electron microscopy, optical reflectance, optical transmittance, X-ray diffraction, and I-V measurements were used to characterize the silicon films. The resistivity of the film was 1.3 × 10−2 Ω-cm for the polycrystalline silicon film, which was suitable for using as a window layer in a solar cell. These films have potential uses in making photodiode and photosensing devices.

Thermoelectric Properties of Nanocrystalline Silicon Films Prepared by Hot-Wire and Plasma-Enhanced Chemical-Vapor Depositions

We report thermoelectric measurements over a temperature range of 80 K to 300 K of heavily boron-doped nanocrystalline silicon films prepared by hot-wire and plasma-enhanced chemical-vapor depositions. The nanocrystalline silicon films were doped by either gaseous deposition precursors or post-deposition ion implantation, resulting in boron concentrations ranging from 1–2×1020 cm−3 to 3 × 1021 cm−3. Reasonable values of the Seebeck coefficient and electrical conductivity were obtained at 300 K, comparable to many other research work. We also report thermal conductivity measurements on these films before doping, which we use to estimate their prospective thermoelectric efficiency. These measurements show values as low as 0.76 W/mK at 300 K which depend highly upon the grain sizes of the nc-Si films. We find that post-deposition doping by ion-implantation is more effective at enhancing the power factor than gaseous doping, and the power factor is only weakly dependent upon doping concentration for the films doped by ion implantation. We conclude that improvements of the thermoelectric efficiency of nc-Si films may depend more on a reduction of their thermal conductivity than doping optimization. The small grain sizes and the low thermal conductivity of the undoped nc-Si films accomplished in this work are therefore encouraging developments.

Effect of the Annealing on the Low-Temperature Charge Transport Properties of Heavily Boron-Doped Nanocrystalline Silicon Films for Thermoelectric Applications

Abstract Silicon is the reference material of microelectronics, is readily available, relatively unexpensive, and its use may take profit of a fantastic technology. This may explain why a substantial effort has focused on improving its thermoelectric efficiency, either by top-down nanostructuring or through suitable processing. In this paper we report an analysis of the electronic transport properties of heavily boron-doped nanocrystalline silicon films. High-temperature thermal treatments are confirmed to remarkably increase its thermoelectric power factor. Electrical conductivity and Hall effect measurements were carried out over the temperature range 20–300 K along with Seebeck coefficient measurements. We provide evidence of the occurrence of low-temperature hopping conduction between impurity subbands. Dopant ionization was studied as a function of temperature. Freeze-out temperature was found to correlate with the Seebeck coefficient in agreement with Pisarenko equation. This brings to the conclusion that, while untreated samples are weakly degenerate, the thermal processing reverts them into non-degenerate semiconductors, in spite of the high doping level.

In‐situ observation of chemical vapor deposition using SiHCl3 and BCl3 gases

AbstractThe lowest temperature for initiating the film deposition was evaluated by in situ monitoring using a highly sensitive langasite crystal microbalance (LCM) in order to produce a thin boron doped silicon film using trichlorosilane gas and boron trichloride gas at low temperatures. The thin films of silicon, boron and boron‐doped silicon were individually formed from the trichlorosilane gas, boron trichloride gas and their gas mixture, respectively, on the LCM. The silicon film deposition occurred between 570 and 600 °C, while the boron film deposition occurred between 470 and 530 °C. The film deposition from both gases occurred at temperatures higher than 530 °C. Based on secondary ion mass spectrometry measurements, the boron‐doped silicon film was determined to be formed on the silicon substrate at 570 °C. Thus, the LCM could be useful for developing a thin film formation process including that for doping. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Influence of doping and microstructure on electrical properties of doped silicon thin films

In this work, phosphorus and boron doped silicon films were deposited on monocrystalline silicon substrates by plasma enhanced chemical vapour deposition. Microstructure and net doping concentration were investigated using Raman spectroscopy and secondary ion mass spectrometer, and the dark conductivity, carrier concentration and mobility of the films were measured using semiconductor characterisation system. The results indicated that the structure of the films were all amorphous despite of the different doping gas flow ratios, and the dark conductivity and carrier density changed by only one order of magnitude with the increase in [PH3]/[SiH4] and [B2H6]/[SiH4] gas flow ratios. The best flow ratios of [PH3]/[SiH4] and [B2H6]/[SiH4] were obtained to be 1·5 and 1% respectively, and then the optimised samples were annealed. After annealing, the film expressed a tendency of transition to crystalline from amorphous according to the Raman results, and the dark conductivity and carrier concentration increased by two to three orders of magnitude for both doped film series. The structure has more significant influence on the electrical properties of doped silicon thin film than the doping concentration.

Paradoxical Enhancement of the Power Factor of Polycrystalline Silicon as a Result of the Formation of Nanovoids

Hole-containing silicon has been regarded as a viable candidate thermoelectric material because of its low thermal conductivity. However, because voids are efficient scattering centers not just for phonons but also for charge carriers, achievable power factors (PFs) are normally too low for its most common form, i.e. porous silicon, to be of practical interest. In this communication we report that high PFs can, indeed, be achieved with nanoporous structures obtained from highly doped silicon. High PFs, up to a huge 22 mW K−2 m−1 (more than six times higher than values for the bulk material), were observed for heavily boron-doped nanocrystalline silicon films in which nanovoids (NVs) were generated by He+ ion implantation. In contrast with single-crystalline silicon in which He+ implantation leads to large voids, in polycrystalline films implantation followed by annealing at 1000°C results in homogeneous distribution of NVs with final diameters of approximately 2 nm and densities of the order of 1019 cm−3 with average spacing of 10 nm. Study of its morphology revealed silicon nanograins 50 nm in diameter coated with 5-nm precipitates of SiB x . We recently reported that PFs up to 15 mW K−2 m−1 could be achieved for silicon–boron nanocomposites (without NVs) because of a simultaneous increase of electrical conductivity and Seebeck coefficient. In that case, the high Seebeck coefficient was achieved as a result of potential barriers on the grain boundaries, and high electrical conductivity was achieved as a result of extremely high levels of doping. The additional increase in the PF observed in the presence of NVs (which also include SiB x precipitates) might have several possible explanations; these are currently under investigation. Experimental results are reported which might clarify the reason for this paradoxical effect of NVs on silicon PF.

Absence of boron aggregates in superconducting silicon confirmed by atom probe tomography

Superconducting boron-doped silicon films prepared by gas immersion laser doping (GILD) technique are analyzed by atom probe tomography. The resulting three-dimensional chemical composition reveals that boron atoms are incorporated into crystalline silicon in the atomic percent concentration range, well above their solubility limit, without creating clusters or precipitates at the atomic scale. The boron spatial distribution is found to be compatible with local density of states measurements performed by scanning tunneling spectroscopy. These results combined with the observations of very low impurity level and of a sharp two-dimensional interface between doped and undoped regions show that the Si:B material obtained by GILD is a well-defined random substitutional alloy endowed with promising superconducting properties.

Simulation of Boron-doped Silicon Film Thickness within Holes on the Wafer

A reaction model is discussed for in-situ boron-doped silicon chemical vapor deposition that uses Si2H6 and B2H6. First, the elementary reactions were studied by using an ab initio molecular orbital method and the first-principle molecular dynamics method. These showed that the B2H6 decomposes into BH3 easily and that BH3 has a high probability of sticking to silicon film. Therefore, the boron was doped in the silicon film by BH3. The desorption energy of H2 terminated silicon film surface was lower due to the boron doping in the silicon film. Then, an increase in the boron concentration of the silicon film increased the sticking probability of the Si2H6. Next, a deposition model was made on the basis of these results, and reaction constants were determined by using experimental results. Finally film thickness distribution was simulated by using a flow-diffusion-reaction analysis and an integrated method on the above deposition model. The result shows that our reaction model can replicate experimental step coverage.

Laser fired contacts applied to the rear surface of heterojunction silicon solar cells

In this work, we fabricate heterojunction silicon solar cells on p-type substrates whose rear surface configuration is based on dielectric passivation and laser fired contacts (LFC cells). This is an alternative to boron-doped amorphous silicon film, with which we also fabricate solar cells for direct comparison (HJ cells). As substrates, 3.5 and 0.8 Ω cm p-type double-side polished FZ c-Si wafers are used. Regarding surface passivation for highly doped substrates, LFC configuration has some advantage due to the higher difficulty in creating an efficient amorphous back surface field. Additionally, those substrates are also more advantageous in terms of carrier injection when the rear surface is locally contacted. Thus LFC cells made on 0.8 Ω cm substrates reach V oc values up to 680 mV, in the same range as that of their HJ cell counterpart, with better FF demonstrating that LFC configuration is a feasible alternative for highly doped substrates. Focusing on the impact of the distance between rear contacts on cell performance, we found a trade-off between open circuit voltage V oc and fill factor FF. Finally electroluminescence characterization and the dependence of V oc on pitch, modeled by Fischer's equation, indicate that the depassivated area due to the laser processing of the contacts is bigger than the contacted area.

Study on Solid Phase Crystallization of Silicon Films Deposited on Different Substrates

Porous silicon were prepared by electrochemical corrosion. Undoped and boron doped silicon films were deposited on quartz substrate、porous silicon and silicon substrate by PECVD,and were solid phase crystallized at different temperature and different hours. The microstructure of films before and after annealing were studied by Raman and XRD. The results show that:the crystallization of films deposited on porous silicon and monocrystalline silicon substrate are better than quartz substrate; The substrate which has silicon crystal lattice play an important role of seed crystal in the solid phase crystallization, the same grain orientation film can be grown on certain condition.

The Superconducting Transition in Boron Doped Silicon Films

We report on a detailed analysis of the superconducting properties of boron-doped silicon films grown along the 001 direction by gas immersion laser doping. This technique is proved to be a powerful technique to dope silicon in the alloying range 2-10 at.% where superconductivity occurs. The superconducting transitions are sharp and well defined both in resistivity and magnetic susceptibility. The variation of Tc on the boron concentration is in contradiction with a classical exponential dependence on superconducting parameters. Electrical measurements were performed in magnetic field on the sample with cB = 8 at.% (400 laser shots) which has the highest Tc (0.6 K). No hysteresis was found for the transitions in magnetic field, which is characteristic of a type-II superconductor. The corresponding upper critical field was on the order of 1000 G at low temperatures, much smaller than the value previously reported. The temperature dependence of Hc2 is very well reproduced by the linearized Gorkov equations neglecting spin effects in the very dirty limit. These measurements in magnetic field allow an estimation of the electronic mean-free path, the coherence length, and the London penetration depth within a simple two-band free electron model.

Low-temperature transition to a superconducting phase in boron-doped silicon films grown on (001)-oriented silicon wafers

We report on a detailed analysis of the superconducting properties of boron-doped silicon films grown along the 001 direction by Gas Immersion Laser Doping. The doping concentration cB has been varied up to approx. 10 at.% by increasing the number of laser shots to 500. No superconductivity could be observed down to 40mK for doping level below 2.5 at.%. The critical temperature Tc then increased steeply to reach 0.6K for cB = 8 at%. No hysteresis was found for the transitions in magnetic field, which is characteristic of a type II superconductor. The corresponding upper critical field Hc2(0) was on the order of 1000 G, much smaller than the value previously reported by Bustarret et al. in Nature (London) 444, 465 (2006).

Ultra-shallow junctions formed by quasi-epitaxial growth of boron and phosphorous-doped silicon films at 175 °C by rf-PECVD

In this paper, we use in-situ and ex-situ spectroscopic ellipsometry as an optical tool to optimize the process conditions that lead to epitaxial growth of undoped and doped silicon films in a standard radio-frequency Plasma Enhanced Chemical Vapor Deposition (rf-PECVD) reactor at temperatures below 200 °C. The influence of the plasma conditions (such as pressure, inter-electrode distance, hydrogen dilution and dopant precursor gas concentration) on the nature of the films is studied. This optimization allows us to achieve epitaxial growth of phosphorous-doped and boron-doped silicon films at temperatures as low as 175 °C and 140 °C, respectively. The epitaxial films possess thicknesses on the order of a few tens of nanometers, and sheet resistance values below 150 Ω/□. Annealing in air at 275 °C helps to further improve the conductivity of boron-doped layers. Four-point probe sheet resistance measurements and secondary ion mass spectrometry profiles are used to assess the dopant profile in the epitaxial layers. The junctions thus obtained are also characterized through their application in a solar cell, resulting in a fill factor over 76% and an efficiency exceeding 14%.

Boron-doped silicon film as a recombination layer in the tunnel junction of a tandem solar cell

Boron-doped hydrogenated silicon films with different gaseous doping ratios (B2H6/SiH4) were deposited in a plasma-enhanced chemical vapor deposition (PECVD) system. The microstructure of the films was investigated by atomic force microscopy (AFM) and Raman scattering spectroscopy. The electrical properties of the films were characterized by their room temperature electrical conductivity (σ) and the activation energy (Ea). The results show that with an increasing gaseous doping ratio, the silicon films transfer from a microcrystalline to an amorphous phase, and corresponding changes in the electrical properties were observed. The thin boron-doped silicon layers were fabricated as recombination layers in tunnel junctions. The measurements of the I–V characteristics and the transparency spectra of the junctions indicate that the best gaseous doping ratio of the recombination layer is 0.04, and the film deposited under that condition is amorphous silicon with a small amount of crystallites embedded in it. The junction with such a recombination layer has a small resistance, a nearly ohmic contact, and a negligible optical absorption.

Physicochemical Characterization of Annealed PolySi/NIDOS/SiO2 Structures

ABSTRACT Physicochemical characterizations of polysilicon thin multilayer films have been studied after annealing. In-situ heavily boron-doped polysilicon (poly/Si) and nitrogen-doped silicon (NIDOS) films were deposited by low-pressure chemical vapor deposition (LPCVD) onto an oxidized monocrystalline silicon substrate. Secondary ion mass spectrometry (SIMS) profiles showed a boron diffusion reduction in the NIDOS layer. In addition, Fourier transform infrared spectroscopy (FTIR) analysis clearly showed an evolution of the absorption picks related to the nitrogen atom in the complex form leading to boron atom activation in the films. Therefore, this multilayer structure can be largely used for metal-oxide-semiconductor (MOS) devices.

Anomalous Increase in Resistivity of Boron-Doped Silicon Film after Solid Phase Crystallization

After the furnace annealing of heavily boron-doped Si film below 650 °C, the electronic properties and relating crystallinity of the Si film were evaluated. The sheet resistance obtained using four-point probe decreases as a result of solid phase crystallization (SPC) at 650 °C for 1 h. With efficient solidified activation, the sheet resistance for the Si film of 50 nm thickness with implantation at a B+ dose of 5 ×1015 cm-2 decreased below 900 Ω/□. By extending annealing time for more than 1 h, resistance increased anomalously. The increase in resistance is mainly due to boron redistribution in the Si film and at the Si/SiO2 interface and to boron deactivation as well, as determined from the secondary ion mass spectrometry (SIMS) profile, which is related to a decrease in carrier concentration.