Enhanced thermoelectric performance of holey silicon thin films using F₄TCNQ surface doping

Highlights

• Demonstrated three-dimensional surface doping scheme in organic-inorganic systems.

• Improved thermoelectric power factor and figure of merit by two orders of magnitude by 3D surface doping.

• Design of hybrid organic-inorganic silicon thermoelectric thin-films for integrated electronics and wearable applications.

Abstract

Silicon thin films have great potential as chip-integrated Peltier micro-coolers and thermoelectric power generators due to their industry compatibility and cost effectiveness. Improving the thermoelectric figure of merit, zT, and therefore the device efficiency can be achieved by increasing the power factor while decreasing the thermal conductivity. In this work, we study single crystalline silicon thin films with patterned nano-holes with sizes comparable to the phonon mean free path to suppress thermal conductivity. The holey silicon thin films are then surface doped with organic molecules F₄TCNQ to create a three-dimensional modulation doping scheme. As the dopants are outside the host material, there is less impurity scattering, which improves carrier mobility and the overall power factor. We fabricate silicon thin films with periodic arrays of nano-sized holes, with a fixed pitch size of 300 nm. By changing the hole diameters, we vary the neck size from 169 nm to 22 nm. The in-plane thermal conductivity, measured using the heat diffusion imaging method, demonstrates an order of magnitude reduction compared to bulk silicon and a change from 26 Wm⁻¹K⁻¹ to 5 Wm⁻¹K⁻¹ at room temperature. The films with large hole diameters allow space for the relatively large F₄TCNQ molecules and hence effective surface doping, which is evident by orders of magnitude improvement in the electrical conductivity and power factor.

Introduction

Silicon thin films are potential candidates for thermoelectric applications such as on-chip thermal management [1] and power generation for wearable electronics [2], due to their industrial process compatibility and cost-effectiveness. It is shown that silicon thin-films are flexible and upon transfer to flexible substrates, they can be used for wearable electronic applications[3,4]. The efficiency of thermoelectric power generators and the coefficient of performance of Peltier coolers are an increasing function of the thermoelectric figure of merit, zT = σS²T/κ, where σ is the electrical conductivity, S is the Seebeck coefficient and κ is the thermal conductivity. Improving the zT of a material can be achieved by increasing the power factor (PF = σS²) while decreasing the thermal conductivity κ.

An effective route to reducing the thermal conductivity of Si is through the addition of nanostructures [5 ,6] or increasing surface roughness [7], which induces more frequent phonon scattering and limits the phonon mean free path (MFP). This has been implemented on Si thin films, by patterning arrays of nano-sized holes [[8], [9], [10], [11], [12], [13]]. These silicon thin films with an array of holes are referred to as holey Si thin films or sometimes as phononic crystals. Research has shown that the thermal conductivity depends on the neck size, i.e., the distance between adjacent holes [5 ,6]. The reduction in thermal conductivity can be one or almost two orders of magnitude compared to thin film without holes, depending on the neck size and the temperature [10,14]. In these structures, an optimum design is to have neck sizes in between the electron and the phonon MFP to selectively limit phonon diffusion with minimum effect on electron mobility, enabling zT improvement [[10], [11], [12], [13]].

Most thermoelectric materials are made out of heavily doped semiconductors. Bulk silicon demonstrates the maximum thermoelectric power factor at high doping levels (around 10²⁰ cm⁻³) [15]. Conventional doping techniques insert dopant atoms into the host materials. These dopants introduce strong Coulomb repulsion and reduce carrier mobility. In comparison, surface charge transfer doping is a clean and effective alternative, where charges exchange across the interface between the surface dopants and the host material due to energy band misalignment [16]. The ionized atoms are therefore distanced from the mobile charge carriers and a space-charge layer is formed at the interface. As the dopants are outside the host material, the extra space between the dopants and the carriers lowers the ionized impurity interactions significantly. This is similar to the modulation doping idea used in transistors. The electron mobility in GaAs, for instance, has been enhanced by 4 orders of magnitudes using modulation doping strategy at below 10 K [17].

The organic molecule, F₄TCNQ (2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane), is a strong electron acceptor with an electron affinity as high as E_a = 5.24 eV [18] and is often used as a p-type surface dopant for organic [19,20] and inorganic [[21], [22], [23]] materials. First-principles calculations predict that physisorbed F₄TCNQ monolayer can efficiently dope Si [24], achieving a surface hole concentration as high as 10¹³ cm⁻². An experimental study on the transport properties at F₄TCNQ-Si interface demonstrated a resistance reduction by a factor of 10 compared to undoped film and a thermoelectric PF enhancement by 75% [25].

In this work, we extend the surface doping to three dimensions (3D). In a 2D geometry, while the thermoelectric power factor can be extremely large [26], one has to deal with an electron gas with a large thermal conductivity. The modulation doping concept has been extended to 3D in the past and improved zT has been shown in SiGe nanostructures [27,28]. The improvement, in that case, was limited due to the grain-boundary scattering. Here, we propose the usage of holey silicon thin films to demonstrate 3D surface doping in organic-inorganic structures. The holey silicon films are clean with minimal scattering due to the periodic nature of holes and clean thin-film fabrication process, compared to the nanostructured bulk samples [27,28] with irregular grain boundaries and impurities. They provide a possible geometry for a 3D network of organic dopants and effective doping not only from the top surface but also from within the film.

We fabricate single crystalline Si thin films with square lattice arrays of nanosized holes of different diameters and surface-dope them with F₄TCNQ. We report improvement in electrical conductivity by a factor as high as 350 and in the thermoelectric power factor by a factor as high as 200 after the deposition of F₄TCNQ, demonstrating the effectiveness of 3D surface doping. This is combined with the low thermal conductivity of the silicon films due to the presence of the holes to enhance the zT by 2 orders of magnitudes.