Doped poly-Si Research Articles

Current status and challenges for hole-selective poly-silicon based passivating contacts

Doped polysilicon (poly-Si) passivating contacts have emerged as a key technology for the next generation of silicon solar cells in mass production, owing to their excellent performance and high compatibility with the existing passivated emitter and rear cell technology. However, the current solar cell architecture based on a rear-side electron-selective (n+) poly-Si contact is also approaching its practical limit (∼26%) in mass production. The full potential of doped poly-Si passivating contacts can only be realized through incorporation of both electron-selective and hole-selective (p+) poly-Si contacts. While studies of both p+ and n+ poly-Si contacts commenced simultaneously, significant performance differences have arisen. Phosphorus-doped poly-Si contacts consistently outperform boron-doped counterparts, displaying typically lower recombination current density (J0) values (1–5fA/cm2 vs 7–15fA/cm2). This discrepancy can be attributed to inadequate optimization of p+ poly-Si contacts and fundamental limitations related to boron doping. The poorer passivation of p+ poly-Si contacts can be at least partly attributed to boron segregation into the interfacial oxide layers, compromising the interfacial oxide integrity and reducing the chemical passivation effectiveness. This review critically examines the progress of p+ poly-Si contacts characterized by cell efficiency and J0 values, delves into existing challenges, identifies potential solutions, and explores some potential solar cell architectures to enhance efficiency by incorporating p+ poly-Si contacts.

AFORS-HET-based numerical exploration of tunnel oxide passivated contact solar cells incorporating n- and p-type silicon substrates.

The development of a tunnel oxide interfacial layer capped by a highly doped poly-Si layer is considered one of the most promising methods to reduce charge carrier recombination and improve the performance of conventional PERC devices. The thickness and doping concentration of emitters and BSF layers greatly influence the tunnelling current in TOPCon devices. In this research, we evaluated the performance of tunnel oxide passivated contact (TOPCon) solar cells by conducting an in-depth analysis of various key parameters. The parameter include the type of silicon substrate (n or p-type); the thickness and doping density (N a/N d) of n, n+, p, and p+ layers; and surface recombination velocity (front/rear), which were analyzed using AFORS-HET simulation software. A comparative analysis of performance demonstrates that the highest efficiency is achieved in the n-TOPCon solar cell with the following values: V oc = 660.2 mV, J sc = 45.05 mA cm-2, FF = 82.87%, and PCE = 25.74%. In the optimized p-TOPCon solar cell, the open circuit voltage (V oc) and fill factor (FF) exhibit improvements of 35.9 mV and 0.39%, respectively. However, the values of J sc and PCE decrease by 6.44 mA cm-2 and 2.2%, respectively, in p-TOPCon solar cells. Furthermore, photo-electroluminescence analysis reveals that the n-TOPCon solar cells exhibit a higher maximum photon flux (front/rear) than p-TOPCon solar cells.

Characterization of solar cell passivating contacts using time-of-flight elastic recoil detection analysis

The unambiguous detection of hydrogen in solar cell contact structures is critical to understanding passivation and degradation phenomena. Deuterium is often used to depict the distribution of hydrogen more clearly. However, experimental noise and artifacts can hinder the clear identification of species. This work provides a report of time-of-flight elastic recoil detection (ToF-ERD) analysis to identify H/D contents in a thin poly-Si/SiOx passivating contact. The structure contained a 1.3 nm interfacial SiOx and an n+ doped poly-Si layer with a partly deuterated SiNx coating. The samples were annealed to release H/D, and ToF-ERD was used to detect H/D in monatomic, singly charged forms, without the detection artifacts associated with conventional secondary ion mass spectroscopy. Chlorine ions were used to recoil surface species, which were analyzed to clearly and unambiguously resolve H and D. Depth profiles for the recoiled Si, N, O, D, and H atoms were calculated from the energy and velocity information registered after scattering events, which enabled the analysis of the structure of the multilayer stack. Even though the surface roughness and experimental limitations cause visible broadening of the profiles, which can hinder clear detection at the interfacial oxide, the ability to resolve hydrogen-related species makes ToF-ERD a significant and promising tool for studying the role of hydrogen in the performance and degradation of solar cell passivating contacts.

Thermoelectric transport properties of Si, SiGe, and silicide CMOS-compatible thin films.

Characterization of thermoelectric transport properties for temperature sensing, cooling, and energy harvesting applications is necessary for a reliable device performance in progressively minimized computer chips. In this contribution, we present a fully automated thermovoltage and sheet resistance measurement setup, which is calibrated and tested for the production of silicon- and silicon-germanium-doped as well as silicide complementary metal-oxide-semiconductor-compatible thin films. A LabVIEW-programmed software application automatically controls the measurement and recording of thermovoltages at individually defined temperature set points. The setup maps average temperature and temperature differences simultaneously in the regime from 40 to 70 °C. The Seebeck coefficient calculated by means of the inversion method was used to eliminate the offset voltage influence. Finally, we present and discuss the Seebeck coefficient as well as the sheet resistance for application-specific different temperature set points of several doped poly-Si, poly-SiGe, and silicides.

Assessing the stability of p+ and n+ polysilicon passivating contacts with various capping layers on p-type wafers

Polysilicon (poly-Si)-on-oxide passivating contact structures (POLO/TOPCon) enable high-efficiency solar cells as they simultaneously provide a very high level of surface passivation and a high conductance for either electrons or holes. The ease of incorporation with existing manufacturing lines and their tolerance for high-temperature processing has increased the wide acceptance of this structure in the PV industry. In this report, we explore the effects of short high-temperature annealing required for effective hydrogenation and formation of ohmic screen-printed contacts across a wide temperature range (636 °C–846 °C) on the stability of passivating contact structures. We study this on p-type c-Si substrates with phosphorus-doped (n-type) or boron-doped (p-type) polysilicon contacts capped with either an AlOx or SiNx coating. Our experimental results show that irrespective of the poly-Si doping type, AlOx-capped samples suffer a loss in surface passivation across the investigated temperature range, while SiNx-capped samples show an improvement at lower annealing temperatures. Above 744 °C, severely ruptured blisters occur for the samples coated with a SiNx layer, leading to lift-off of the poly layer in extreme cases, and in all cases, significant surface passivation losses, up to 99%. A study of the long-term stability of these fired samples under 1-sun illumination @ 140 °C shows that they suffer from both bulk and surface-like instabilities. Two degradation cycles were observed: the first, a boron-oxygen light-induced degradation (BO-LID) observed after 5 min, with capture cross-section ratios of 15.8–19.2, and a slower secondary degradation, similar to light and elevated temperature-induced degradation (LeTID), with maximum degradation reached after ∼ 14 days. The presence of a silicon nitride layer does not appear to influence the kinetics of post-degradation recovery. Our results suggest that the effect of firing may be influenced by the polarity of the bulk c-Si or perhaps the chemistry of the SiNx film and highlight that passivating contact structures based on p-type c-Si may offer better long-term stability than those based on n-type c-Si.

Characterization of tunnel oxide passivated contact fabricated by sputtering and ion implantation technique

Tunnel oxide passivated contact (TOPCon) structures using highly doped n-type polycrystalline silicon were fabricated using facing target sputtering and ion implantation techniques for a SiH4-free fabrication process of high-efficiency silicon solar cells. We investigated the structural and electrical properties of the highly doped n-type poly-Si layers to optimize the ion implantation process. We also investigated the surface passivation quality of our TOPCon structure. An effective carrier lifetime of 2.01 ms and an implied open circuit voltage of 704 mV were obtained for our sample annealed at 950 °C. The sample also exhibits a low contact resistance of 3.22 × 10−3 Ω cm−2. Our results open the way for SiH4-free fabrication of silicon solar cells with a TOPCon structure.

Process Considerations for selective doping of poly-Si thin films with spin-on dopants and nickel silicide formation for planar thermoelectric devices

The formation of highly doped p-type (boron-doped) and n-type (phosphorus-doped) poly-Si thin films with spin-on dopants on the same wafer poses unique challenges. This work evaluates the pros and cons of different approaches to dopant diffusion, residue removal, and diffusion mask selection when working with spin-on dopants. The experiments have been carried out to optimize the formation of highly doped poly-Si thin films that will form the active layers of a micro-thermoelectric generator (μ-TEG). Building upon previous work on residue removal methods, we report that the residue formation is also dependent on the oxide-silicon surface with a PECVD oxide-Si interface not requiring a post-dopant diffusion nitric acid treatment to achieve a hydrophobic surface. Furthermore, comparing the direct dispense and proximity diffusion methods verifies the existing reports of enhanced diffusion in the latter case. Besides, we observe that doping first with Boron is more conducive than doping first with phosphorus to reduce sheet resistance. We also optimize the contact formation with Ti/Ni thin films and report the mean contact resistivity. Notably, contact annealing precludes the nitric acid treatment requirement even if the surface is hydrophilic after removing dopant residue. Additionally, the sequence of silicidation is observed to be dependent on the doping of the underlying areas.

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.

Evaluation of stability and robustness of poly-Si resistors with different dopant concentrations

The stability and robustness of lightly and highly doped poly-Si resistors were evaluated. These resistors exhibited distinct electrical resistance properties and temperature dependences, which can be explained through the grain and grain boundary conduction mechanisms. The resistance shift saturated under the low current stress condition, but continued to increase under the high current stress condition. A novel carrier trapping density model was proposed to explain this behavior. A generalized free energy model that considered stress temperature and stress current dependences was proposed to account for the stability lifetime of a poly-Si resistor based on the resistance shift criterion. Robustness evaluation with transmission line pulse test revealed that the breakdown current exhibited a pulse width dependence which was further explained by a thermal-conduction energy model.

Progress with passivation and screen-printed metallization of Boron-doped monoPoly™ layers

In this work we have analyzed the doping, passivation and contact properties of boron-doped (p+) polysilicon (poly-Si) layers to understand the two key limiting factors for industrial adoption of p+ poly-Si based passivated contacts: challenges with diffusion and challenges with screen-printed, fire-through (FT) metallization. Investigation of test samples with ex-situ doped poly-Si layers of varying thickness and surface morphologies revealed that sheet resistance (Rsheet) and passivation quality in ex-situ doped poly-Si are limited by the maximum solid solubility of boron in silicon, and possible damage to the interfacial silicon oxide (iOx) layer and increased Auger recombination at high diffusion temperatures, respectively. An interesting correlation is found between the maximum dopant density at the interface and the J0,pass, with the latter increasing with increasing dopant density. The contacts to p+ poly-Si layers are formed with commercially available FT Ag/Al pastes by screen printing process. Metal contact properties strongly depend on the surface morphology and thickness of poly-Si layer. Lower contact resistivity was achieved on textured surface with thickest poly-Si layers, while lowest recombination under metal contacts was achieved with planar surface with thickest poly-Si layer. Further microstructure analysis of metallized interfaces using Scanning electron microscope shows partial etching of thick (220 nm) poly-Si layers and near complete etching of thin (20 nm) poly-Si layers by FT Ag-Al pastes. Using optimized doping and contact conditions, excellent surface passivation in unmetallized regions (J0,pass of 3.8 fA/cm2), contact resistivities as low as 1.2 ± 0.8 mΩ-cm2 and recombination current densities under the metal contacts of 155 ± 10 fA/cm2 are achieved on textured and planar samples respectively.

Impact of the gate fabrication process of light emitting capacitors based on silicon-rich oxide: Low voltage electroluminescence

We study the effect of the gate fabrication process on the electro-optical properties of metal-oxide-semiconductor (MOS) capacitors with a silicon-rich oxide (SRO) film and n+ polysilicon (Poly-Si) layer as the active layer and gate electrode, respectively. Two sets of capacitors were fabricated changing the Poly-Si doping process. The first set was doped using a mixture of N2, O2 and PH4 at 1000 °C for 15 min. The second set was similarly doped but an additional re-diffusion process using only N2 and O2 at 1000 ᵒC for 20 min was carried out. The doping step causes diffusion of silicon (Si) atoms from a Poly-Si gate towards the SRO which binds to its microstructure forming Si0, Si2+, Si3+ states while the re-diffusion also causes a diffusion of phosphorus (P) atoms, forming P–Si bonds and generating a restructuring of the SRO. These changes determine the electro-optical properties of devices based on SRO. Capacitors with doped emit full area white electroluminescence (EL) observed a naked eye at ∼27 V (∼7 MV/cm) while capacitors with doped + re-diffusion emit orange-red EL spots at ∼ 6 V (∼1.5 MV/cm).

Thermal activation of hydrogen for defect passivation in poly-Si based passivating contacts

Hydrogenation of poly-Si based passivating contacts (TOPCon) is an essential process to achieve a very high level of surface passivation, especially on textured surface. This contribution is dedicated to improve the understanding of the hydrogenation mechanism. We compare different hydrogen sources with regard to their ability to chemically passivate defects at the Si/SiO x interface and presumably in the doped poly-Si layer as well as their thermal stability in a low and high temperature range. To this end, hotplate annealing series were performed on textured n -type TOPCon structures and Al 2 O 3 /SiN x multi-layer stacks were exposed to fast-firing processes. Very distinct activation characteristics were detected. It was also observed that the Al 2 O 3 capping layers enable a higher level of surface passivation and higher thermal stability compared to SiN x . When implemented in multi-layer stacks, Al 2 O 3 acts as a hydrogen diffusion barrier und prevents effusion from the TOPCon structure that deteriorates the passivation quality irreversibly. • Hydrogenation of poly-Si based passivating contacts. • Thermal activation of hydrogenation. • Hydrogenation of TOPCon using AlOx/SiNx stacks. • Thermal stability of hydrogenation upon fast-firing.

Impact of firing temperature on fire-through metal contacts to P-doped (n) and B-doped (p) poly-Si

Screen-printed contacts to doped polysilicon (poly-Si) layers with high-temperature fire-through pastes is one of the key enablers for widespread industrial acceptance of passivating contact solar cells. In recent years, rapid progress has been made in the development of pastes for contacting doped poly-Si layers. However, there are still gaps in understanding the behavior of fire-through contacts, especially regarding damage to poly-Si layers. In this work we present a comprehensive study of metal contacts formed with fire-through pastes to both phosphorus-doped ( n + ) poly-Si and boron-doped ( p + ) poly-Si layers. The contacts are fired at different temperatures and characterized by evaluating specific contact resistivity and recombination current density under the metal contacts. The metal crystallites formed at the interface between metal fingers and poly-Si is quantitatively analyzed to determine their area fraction. It is observed that the area fraction of the crystallites tends to saturate at higher temperatures for n + poly-Si, however such behavior is not observed for p + poly-Si within the range of firing temperatures relevant for industrial solar cells. Metal crystallites appear to agglomerate at higher temperatures for contacts to both n + and p + poly-Si layers which explains increased recombination due to aggressive etching of poly-Si. For Ag contacts to n + poly-Si best results are achieved when the metal crystallites covered 50% of the surface area at the optimum firing temperature. For Ag–Al contacts to p + poly-Si the crystallites covered 30% of the area, but resulted in higher recombination under the contacts. n-type bifacial solar cells are fabricated with n + poly-Si passivating contacts on the rear side and fired at different temperatures. The solar cell results followed the trends shown by the contacts and efficiency reached maximum for an optimized firing temperature. However, the change in efficiency was dominated by the front side contact characteristics. The results obtained on solar cells validate the observations about contact formation and metal recombination. Etch pits on p + poly-Si observed under SEM at different peak firing temperatures. Different splits represent the peak firing temperatures. A) 710 B) 730 C) 750 D) 770 E) 790. • In depth analysis of fired-through contacts formed to 150 nm doped poly-Si layers at different temperatures. • Contact resistivity improved with increasing temperature, attributed to increased number of metal crystallites. • Excellent contact resistivity ≈0.5 mΩ/cm 2 and ≈4 mΩ/cm 2 achieved for Ag and Ag–Al fired through contacts respectively. • The J 0,metal increased with increasing temperature due to increasingly aggressive etching of the poly-Si layer. • An ideal metal paste should result in optimum number of crystallites that do not extensively etch the poly-Si layer.

Understanding the impurity gettering effect of polysilicon/oxide passivating contact structures through experiment and simulation

Polysilicon/oxide (poly-Si/SiOx) passivating contacts are a promising technology for the next-generation of high-efficiency silicon solar cells. The structure can be realised by a range of fabrication techniques, which can induce very different impurity gettering effects during the formation process. Understanding the different gettering effects will enable tailored solutions to optimise the gettering efficiency in device fabrication. This paper demonstrates a method to separately quantify the impact of each component on the overall gettering effect of the poly-Si/SiOx passivating contact structures. These components consist of the heavily doped poly-Si layer, in terms of its gettering strength; the SiOx interlayer, regarding its potential blocking effect for slowing down the diffusion of impurities; and the dopant in-diffused surface regions of the silicon wafer bulk directly below the SiOx interlayer, which may have a small additional gettering effect due to heavy doping. Phosphorus in-situ doped poly-Si layers from plasma-enhanced chemical vapour deposition (PECVD), coupled with SiOx interlayers from different growth techniques, were used to demonstrate the method. The experimental and simulation results confirm that the heavily doped poly-Si layer acts as the main gettering sink and the presence of different SiOx interlayers determines the overall gettering rate. For the ultrathin SiOx interlayers studied in this work, which have a similar thickness but different stoichiometry, a standard thermally grown SiOx demonstrates the strongest blocking effect, followed by a chemically grown SiOx from hot nitric acid, and a thermal SiOx of a reduced stoichiometry (grown in a pure nitrogen ambient) demonstrates practically no blocking effect.

Innovative passivating contact using quantum well at poly-Si/c-Si interface for crystalline silicon solar cells

Herein, we propose a quantum well comprising a stack of SiOx/nanocrystalline silicon (nc-Si)/SiOx layers between a polycrystalline silicon (poly-Si) layer and a crystalline silicon wafer (c-Si) for enhancing the passivation quality through the poly-Si/c-Si passivating contact for c-Si solar cells. A large number of dopants can diffuse deeply into c-Si from a heavily doped poly-Si layer during high-temperature annealing, reducing the surface passivation quality. The quantum well with the double SiOx layer plays a significant role as a double barrier, thus suppressing this. The formation of the nc-Si phase sandwiched between the SiOx layers can support carrier transport, i.e., the tunnelling of carriers through the SiOx layers for collection. Moreover, a highly doped poly-Si/quantum well/c-Si contact exhibits considerable improvement in the passivation quality, achieving a high iVoc of 742 mV and low Jo of 1.1 fA/cm2, compared with a reference poly-Si/c-Si passivating contact (iVoc = 707 mV, Jo = 32.5 fA/cm2). The results indicate the effectiveness of the innovative passivating contact concept and pave the way for further improvements in the performance of c-Si solar cells.

Role of oxygen in the UV-ps laser triggered amorphization of poly-Si for Si solar cells with local passivated contacts

In recent years, poly-Si based passivated contacts elevated the conversion efficiencies of crystalline Si solar cells to levels of 26%abs due to their outstanding electrical surface passivation performance and current transport characteristics. A major associated challenge, however, is the large parasitic light absorption within the doped poly-Si, regardless if the contacts are applied on the front and/or on the rear side of the solar cell. It, therefore, might be beneficial to confine the passivated contacts to local regions underneath the metal contacts. We present an effective and flexible laser-based approach to structure the poly-Si layer after its full-area deposition. Laser pulses with a pulse duration of 9 ps and a wavelength of 355 nm trigger an amorphization of the poly-Si surface. The minimum threshold fluence for amorphization is between 89 and 129 mJ/cm2. The a-Si layer, which is laterally homogeneous and up to (33 ± 4) nm in thickness, works as an etch barrier in an alkaline solution. The most robust barrier corresponding to the maximum thickness of the a-Si layer is found for a fluence of (270 ± 30) mJ/cm2. Besides the impact of the laser fluence on the etch resistiveness of the modified poly-Si layer, we study the role of oxygen during the laser process. We find that oxygen becomes incorporated into the material for certain laser fluences, which results in a more robust etch barrier. The amount of oxygen incorporated is below 3 wt. %. Eventually, we present a phenomenological model of our findings.

Polysilicon passivated junctions: The next technology for silicon solar cells?

<h2>Summary</h2> Despite the maturity of crystalline silicon photovoltaics (c-Si PV), the last 6 years have seen a string of efficiency improvements, most of which are centered around reducing the losses related to the directly metallized, heavily doped regions found in conventional c-Si solar cells. Among these advancements, polysilicon (poly-Si) passivated junctions, formed by embedding a thin silicon oxide (SiO₂) layer between the c-Si wafer and a highly doped poly-Si layer, are emerging as one of the most promising alternatives, and efficiencies above 26% have already been demonstrated. The excellent performance of this junction architecture has been found to be remarkably independent of the deposition and/or doping technique used—even extending to techniques already prevalent in industry. This greatly reduces the capital and retraining expenditure needed to integrate the new technology into mainstream production lines, allowing it to be an evolutionary, rather than disruptive advancement. This has led to the rapid demonstration of large-area cells featuring poly-Si contacts by multiple PV manufacturing companies, with efficiencies above 24.5%. Although a bright future for poly-Si junctions is anticipated, as supported by the predictions of the International Technology Roadmap of Photovoltaics, several issues remain to be resolved, including those associated with the cost of and damage to the poly-Si layers due to the cell's metallization process. This paper provides a perspective of the remaining challenges and potential of poly-Si junctions to transform the PV industry.

Status and prospective of high-efficiency c-Si solar cells based on tunneling oxide passivation contacts

Current photovoltaic market is dominated by crystalline silicon (c-Si) solar modules and this status will last for next decades. Among all high-efficiency c-Si solar cells, the tunnel oxide passivated contact (TOPCon) solar cell has attracted much attention due to its excellent passivation and compatibility with the traditional c-Si solar cells. The so-called tunnel oxide passivated contact (TOPCon) consists of an ultra-thin silicon oxide layer less than 2 nm in thickness and a heavily doped poly-Si layer, which is used for implementing effective passivation and selective collection of carriers. This TOPCon solar cell has some advantages including no laser contact opening, no light-induced degradation and no elevated temperature-induced degradation because of N-type c-Si wafer, compatibility with high temperature sintering and technical scalability. This paper first introduces the basic structure and principles of TOPCon solar cells, then compares the existing methods of preparing ultra-thin silicon oxide layer and heavily doped poly-Si layer, and finally points out the future research direction of this cell based on the analysis of the current research status.

Evolution of the surface passivation mechanism during the fabrication of ex-situ doped poly-Si(B)/SiOx passivating contacts for high-efficiency c-Si solar cells

Passivating the contacts of crystalline silicon (c-Si) solar cells (SC) with a poly-crystalline silicon (poly-Si) layer on top of a thin silicon oxide (SiOx) is currently sparking interest for reducing carrier recombination at the interface between the metal electrode and the c-Si substrate. However, due to the interrelation between different mechanisms at play, a comprehensive understanding of the surface passivation provided by the poly-Si/SiOx contact in the final SC has not been achieved yet. In the present work, we report on an original ex-situ doping process of the poly-Si layer through the deposition of a B-rich dielectric layer followed by an annealing step to diffuse B dopants in the layer. We propose an in-depth investigation of the passivation scheme of the resulting B-doped poly-Si/SiOx contact by first comparing the surface passivation provided by ex-situ doped and intrinsic poly-Si/SiOx contacts at different steps of the fabrication process. The excellent surface passivation properties obtained with the ex-situ doped poly-Si(B) contact (iVoc = 733 mV and J0 = 6.1 fA cm−2) attests to the good quality of this contact. We then propose further STEM, ECV and ToF-SIMS characterizations to assess: i) the evolution of the microstructure and B-doping profile through ex-situ doping and ii) the diffusion profile of hydrogen in the poly-Si contact. Our results show a gradual filling of the poly-Si layer with active B dopants with increasing annealing temperature (Ta), which strengthens the field-effect passivation and enables an iVoc increase after annealing up to 800 °C. We also observe a diffusion of O from the SiON:B doping layer to the interfacial SiOx layer during annealing, that likely enhances the passivation stability of our ex-situ doped poly-Si contact with increasing Ta. Finally, we conclude that the mechanism dominating the surface passivation changes during the fabrication process of the poly-Si/SiOx contact from field-effect passivation after annealing (performed for B-diffusion in the contact) to chemical passivation after following hydrogenation of the samples (performed by depositing a H-rich silicon nitride layer).

CMOS-Compatible Ex-Situ Incorporated Junctionless Enhancement-Mode Thin Polysilicon Film FET pH Sensor

In this article, a junctionless enhancement-type field-effect-transistor based pH sensor is proposed and fabricated through low-cost bulk CMOS aligned process. The thin device layer consists of uniformly ex-situ doped p-type polySi, yielding a junctionless doping profile throughout the active region. A thin Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> layer is deposited through atomic layer deposition as a sensing membrane, providing high site binding density for pH solutions. The effect of channel width (W <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">si</sub> ) ranging from 5 to 15 μm on threshold voltage and drain current sensitivities (ΔV <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</sub> /ΔpH}, ΔI/ΔI <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">d</sub> ) respectively, have been investigated for different pH buffer solutions. For Ws of 5 μm, a maximum ΔV <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</sub> /ΔpH sensitivity of 66.7mV/pH and current sensitivity of 98.9% was observed. Further, the drain current of ≃65$ nA at V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">gs</sub> = 0V and V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ds</sub> = -4V, yields low leakage operation of the proposed sensor. Furthermore, the repeatability of the proposed sensor is demonstrated by repeating the drain current measurements thrice with an interval of 10 minutes between corresponding measurements. A maximum shift of 30.9% among all measurements at pH = 6 which corresponds to 0.1 μA was observed, implying excellent repeatability of the proposed sensor.