Room temperature synthesized highly conducting B-doped nanocrystalline silicon thin films on flexible polymer substrates by ICP-CVD

Highlights

• Room-temperature growth of p-nc-Si films on optically transparent flexible substrate, using ICP-CVD.

• Maintaining high conductivity, adequately wide optical band gap, and superior crystallinity together in p-nc-Si film.

• Stress-induced crystallization on polymer substrate via local heating by high-density plasma-precursors/atomic-H.

• Evolution of columnar-like growth induced by dominant 〈2 2 0〉 orientation in unc-Si triggered at nano-bends on PET substrate.

• Dopant substitution by the $B_4^{-}$ atoms in Si-nc core causes high-density of free-carriers and boosts electrical conductivity (∼1 S cm⁻¹).

Abstract

Boron-doped nc-Si thin films were developed on inexpensive, optically-transparent, and flexible PET-substrates at ambient-temperature (∼30 °C) using (SiH₄ + B₂H₆)-plasma, without additional H₂-dilution, by optimizing the gas pressure in inductively-coupled low-pressure plasma-CVD. An amorphous and monohydride-Si dominated partially-nanocrystalline network was obtained at pressure, p ≤ 15 mTorr. At higher pressure, the local plasma heating on the polymer-substrate by the high-density plasma-precursors and over-abundant atomic-H causes plasma-induced nano-bends, which trigger the stress-induced-crystallization of the network on the flexible substrate even at ambient-temperature. Although, the enhanced atomic-H reactivity on the growing-surface at very-high pressures causes the formation of the polyhydride-dominated defective grain-boundary, which acts as the electron-trapping-center and eventually reduces the dark-conductivity. At an optimum pressure of ∼30 mTorr, the transport of ample SiH₃-radicals and atomic-H from the plasma appears instrumental in producing the p-nc-Si thin film with high-crystallinity (∼77%) in columnar-like growth morphology and adequately wide optical bandgap (∼1.88 eV). Substitution of the Si-atoms, primarily residing in the lattice within the core of Si-nanocrystals, by the electrically-active four-fold-coordinated B-atoms ($B_4^{-}$) generates high-density of free charge-carriers and contributes to high electrical-conductivity (∼1 S cm⁻¹) of the room-temperature grown p-nc-Si thin film on the flexible PET-substrate, which seems suitable for the fabrication of low-cost flexible-electronics, including Si solar-cells.

Introduction

In recent years, mechanically flexible electronics and energy harvesting devices have gained significant interest because of their potential in providing a cost-effective solution to large area photovoltaics, rollable displays, sensors, and portable electronics [1], [2], [3]. For a long time, nanocrystalline silicon (nc-Si) thin film, a mixed phase material containing Si nanocrystals surrounded by the amorphous network, has been extensively used in photovoltaic devices. Boron doped nc-Si retains higher mobility, superior doping efficiency, and reduced light-induced degradation compared to its doped amorphous silicon (a-Si) counterpart. Besides, it reduces the production cost relative to the crystalline silicon (c-Si) while demonstrating similar electrical characteristics to a large extent, which altogether recognizes it as the material of choice for electronics applications [4], [5], [6], [7]. However, the nc-Si technology does not permit easy integration with flexible electronics. A low softening temperature of the polymer substrates makes it challenging to obtain device-grade material with high crystallinity and electrical conductivity [8], [9]. Recently, sincere efforts have been paid to achieve high-quality nc-Si thin films on the flexible substrates at low temperatures, using plasma-enhanced chemical vapour deposition (PECVD). However, several attempts on polymer substrates at temperatures < 150 °C resulted in a-Si or polymorphous Si with low crystallinity, carrier mobility, and conductivity, which diminished their suitability in high-performance devices [10], [11]. In conventional capacitively coupled PECVD, generally elevated H₂-dilution to the silane (SiH₄) precursors is maintained at high applied RF power, and in addition, d.c. electrical bias is applied to the substrates to sustain nanocrystallinity in the doped Si thin film at low temperatures [12], [13], [14], [15]. However, high H₂-dilution lowers the deposition rate that retards the cost-effective production; and high electrical power or bias results in surface damage at the film growing interface with the substrate, which altogether hinders the technical effectiveness and commercial viability of the process [16], [17], [18]. To overcome these drawbacks, inductively coupled plasma (ICP) CVD, a high-density plasma source, has been employed wherein the higher dissociation efficiency of the plasma produces high atomic H density even at a lower H₂-dilution of the precursor gas and can lead to easier crystallization even at low temperature (<150 °C), which is crucial for growing B-doped nc-Si (p-nc-Si) films on flexible substrates. The present investigation concerns developing the p-nc-Si thin films on polyethylene terephthalate (PET) substrate, particularly maintaining crystallinity at ambient temperature, through an intricate control of the ICP-plasma by varying the gas pressure in the reactor. The novelty of this work lies in achieving superior crystallinity, significant dark conductivity, and adequately wide optical bandgap in the p-nc-Si thin films, grown on transparent, flexible, and low-cost PET substrates at ambient temperature, using low-pressure and high-density ICP-plasma produced from SiH₄ and B₂H₆ gases without any additional H₂-dilution.