Enhanced thermoelectric performance of holey silicon thin films using F4TCNQ surface doping
Graphical abstract
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 Ea = 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.
Section snippets
Material and methods
The holey silicon thin-film devices were fabricated from silicon-on-insulator (SOI) wafers, which had an active Si layer of 220 nm thickness on top of a 3 μm-thick SiO2 layer. We used laser lithography and reactive ion etching (RIE) to define device areas of 200 × 30 μm2. Subsequently, e-beam lithography and RIE patterned the nano-sized hole arrays onto each device. The holes were kept at a fixed pitch distance of a = 300 nm, and the neck size n of each device varied from 22 nm to 169 nm.
Results and discussion
The in-plane thermal conductivity of the holey silicon thin film depends on the neck size. Fig. 2 shows that as the neck size goes down from 169 nm to 22 nm, the room temperature thermal conductivity decreases from 26.4 Wm−1K−1 to 4.7 Wm−1K−1, nearly 5 times. Smaller neck size suppresses the phonon transport more and leads to smaller thermal conductivity values. The trend is consistent with those summarized in the literature [5 ,6]. In comparison, the room temperature thermal conductivity of a
Conclusion
In conclusion, we fabricated Si thin films with periodic nm-sized holes from SOI wafers and performed surface charge transfer doping with F4TCNQ organic molecules. The in-plane thermal conductivity depends on the neck size and changed from 26 Wm−1K−1 to 5 Wm−1K−1 as the neck size decreased from 169 nm to 22 nm. The F4TCNQ molecules were deposited via thermal evaporation. The sample with a larger hole diameter allowed more space for the F4TCNQ molecules and the surface doping was more effective.
Credit author statement
Tianhui Zhu: Investigation, Data curation, Validation, Formal analysis, Writing-original draft. Yunhui Wu: Resources, Investigation, Writing - Review & Editing. Shuai Li: Resource, Visualization, Investigation. Farjana F. Tonni: Data curation, Masahiro Nomura: Conceptualization, Formal analysis, Resources, Writing - Review & Editing, Supervision, Funding acquisition Mona Zebarjadi: Conceptualization, Validation, Formal analysis, Resources, Writing - Review & Editing, Supervision, Funding
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
We acknowledge the support of NSF grant number 1653268 and CREST JST grant Number JPMJCR19Q3. The authors thank Ryoto Yanagisawa and Junichiro Shiomi for the discussions on sample fabrication.
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Cited by (2)
- 1
T.Z. and Y.W. contributed equally to this work.