Organic passivation of silicon through multifunctional polymeric interfaces
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.
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 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 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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