Organic passivation of silicon through multifunctional polymeric interfaces

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Highlights

  • We demonstrate a novel solvent-free, low temperature (<25–170 °C) approach using chemical vapor deposition (CVD) grafting and polymerization processes to passivate the unpaired electrons or “dangling” bonds on the surface of silicon.

  • Our low temperature passivation demonstrated several orders of magnitude improvement in minority carrier lifetime (>2 ms) compared to bare silicon (~30 μs).

  • The highly reproducible passivation quality achieved at low temperatures approaches that of SiNx (deposited at temperatures between 400–800 °C) and remained stable in air for >200 h.

  • The passivation processes are shown to reduce surface recombination chemically by reducing density of surface states and electrostatically by altering band bending at the silicon interface. Passivation quality improved on grafting using aliphatic monomers compared to aromatic ones suggesting a reduction in steric effects in the former.

  • The ability to create multifunctional dielectric and electronically conducting passivating layers creates a direct interface between traditional silicon microelectronics and organic electronics. Applications include dielectric antireflective coatings and patterned conducting polymer current collector grids in silicon solar cells, “lab-on-a-chip” biosensors, and light emitting diodes (LEDs).

Abstract

In this work, we demonstrate a solvent-free, low temperature (<25–170 °C) approach using chemical vapor deposition (CVD) grafting and polymerization processes to passivate the unpaired electrons or “dangling” bonds on the surface of silicon. The multifunctional (dielectric and electronically conducting) passivating layers described here achieved several orders of magnitude improvement in minority carrier lifetime (>2 ms) compared to bare silicon (~30 μs), and remained stable in air for over 200 h. These values approach that of SiNx films deposited at significantly higher temperatures. The polymer passivation processes and materials are shown to significantly reduce surface recombination rates: chemically by reducing density of surface states and electrostatically by altering band bending at the silicon interface. Passivation quality also improved on grafting using aliphatic monomers compared to aromatic ones suggesting a reduction in steric effects in the former, helping us posit design rules for polymer based surface passivation of silicon. Finally, the ability to use an electrically conducting polymer for passivation creates a direct interface between traditional silicon microelectronics and organic electronics. Beyond microelectronics and photovoltaics, these processes can enable the fabrication of hybrid or multifunctional devices, such as biosensors and light emitting diodes.

Graphical abstract

Low temperature chemical vapor deposition (CVD) grafting and polymerization achieves several orders of magnitude improvement in minority carrier lifetime (>2 ms) compared to bare silicon (~30 μs). Passivation quality improved on grafting using aliphatic monomers compared to aromatic ones, providing design rules for organic surface passivation of silicon.

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Introduction

While silicon atoms inside the bulk of a silicon wafer have all their valence electrons engaged in covalent bonding, atoms at the surface can have unpaired electrons. These so-called “dangling” bonds can behave as charge carrier recombination centers, thereby lowering the overall operating efficiency of silicon devices [1], [2], [3], [4], [5]. Passivating these electronic traps on the surface of silicon wafers is thus an essential consideration for applications including photovoltaics, microelectronics, and sensors [1], [2], [5]. Inorganic silicon nitride (SiNx) films of thicknesses on the order of 100–200 nm deposited using chemical vapor deposition (CVD) processes are currently the industrial standard for realizing passivated silicon surfaces. The nitride passivation can provide high minority carrier lifetimes (several ms for single-crystalline silicon) and surface recombination velocity (SRV) values as low as 1–10 cm/s [6], [7], [8], 9]. However, substrate processing temperatures approaching 400–800 °C during the SiNx deposition can both damage the substrates and increase operating costs [10], [11]. Room-temperature processed organic passivation layers using solution-based techniques like chlorination/alkylation, have indeed produced high quality surface passivation with SRV values comparable to those obtained in SiNx. However, widespread deployment of these techniques is impeded by complex processing requirements, poor reproducibility, and inability to grow functional passivation layers beyond a few monolayers in thickness [12], [13], [14], [15].

In this work, we demonstrate a low-temperature, scalable approach using CVD polymerization processes to passivate the surface of silicon using multifunctional polymer films. Indeed, we demonstrate the ability to use an electrically conducting polymer for passivation, hence creating a direct interface between traditional silicon microelectronics and organic electronics, with its capabilities for processing flexibility, as well as chemical and biological specificity. Our CVD approach covalently grafts the polymer film directly onto the silicon substrate by initiating a chemical reaction between the surface hydride bonds on silicon and the reactive groups of the monomer; thereby satisfying surface dangling bonds and thus “passivating” electrically active interface states [16], [17], [18], [19].

While the CVD process used for depositing inorganic passivating films like SiNx requires high temperature (>400 °C), the CVD grafting and polymerization processes demonstrated here offers a solvent-free, low temperature (<25–170 °C) alternative. These mild processes help retain delicate organic functionalities in the monomers, enabling the subsequent growth of both insulating/dielectric and conducting polymer layers on top, potentially serving diverse functionalities. Examples in solar cells include dielectric antireflective coatings [20], [21] and patterned conducting polymer grids to replace the expensive silver metallization [22], [23]. Exciting applications can also be found for these conducting passivating coatings as current collectors in silicon “lab-on-a-chip” biosensors [24] and silicon based light emitting diodes (LEDs) [25]. The low-temperature CVD polymer passivation processes reported here achieve several orders of magnitude improvement in minority carrier lifetime (>2 ms at carrier injection levels of Δn=1×1015 cm−3) compared to bare silicon (~30 μs), and remained stable in air for over 200 h. This significantly improved passivation quality approaching that of SiNx [6], [7], [8], 9] and the ability to create electronically conducting passivating layers distinguishes the current results from our previous work where we first reported CVD-based organic passivation [26].

Section snippets

Overview of passivation process

Fig. 1 schematically depicts our organic passivation procedure carried out on double side polished <100> oriented p-type silicon wafer samples (B-doped, 80–120 Ω-cm, 750±25 µm thickness) with SiNx deposited on the backside as described in Supplementary information (Section 1). Samples were etched in 1% hydrofluoric acid (HF) for 2 min to remove the native silicon oxide layer and obtain a H-terminated Si surface. The samples were then immediately transferred into a vacuum chamber to begin the

Results

Two figures-of-merit were used to determine the quality of the passivation layer: minority carrier effective lifetimes (τeff) and surface recombination velocities (SRV). The former determines how long a surface minority carrier survives before encountering a recombination site. The SRV values quantify the surface current arising from these recombination events and are calculated by measuring (τeff) values using a combination of quasi-steady state photoconductance (QSSPC) and transient

Discussion

Following the first step of the iCVD process, monomers (DD, EGDA, MDEB) get grafted onto radical sites created on silicon by abstracting H from Si-H; using methyl radicals generated from the initiator (TBPO) through the process of β-scission [36]. In order to verify the success of this step, we performed x-ray photoelectron spectroscopy (XPS) on these surfaces immediately afterwards. As seen in Fig. 3, the XPS spectrum at the Si 2p core level reveals the Si 2p doublet peaks corresponding to the

Conclusions

In this work, we demonstrate a low-temperature, scalable approach using CVD grafting and polymerization processes to passivate the surface of silicon using multifunctional polymer films. Dielectric iCVD polymer films impart a degree of chemical passivation to the silicon surface by satisfying the dangling bonds on the surface. Our novel electrically conducting, air-stable passivation using oCVD PEDOT, which exhibits remarkably high minority carrier lifetimes (>2 ms) and champion SRV value of 32.3

Acknowledgements

The authors acknowledge the support from the ENI-MIT Alliance. The authors are thankful to Sin-Cheng Siah and Mallory Ann Jensen for guidance on the surface recombination velocity calculations, and Carlos del Cañizo Nadal for his guidance in photovoltage modeling.

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