Thin-film Thermoelectric Materials Research Articles

Copper-Enriched Nanostructured Conductive Thermoelectric Copper(I) Iodide Films Obtained by Chemical Solution Deposition on Flexible Substrates

The objects of our research are flexible thin-film thermoelectric materials with nanostructured CuI layers 0.5–1.0 μm thick, fabricated by the chemical solution method Successive Ionic Layer Adsorption and Reaction (SILAR) on flexible polyethylene terephthalate and polyimide substrates. These cubic γ-CuI films differ from films obtained by other chemical solution methods, such as spin-coating, sputtering, and inject printing, in their low resistivity due to acceptor impurities of sulfur and oxygen introduced into CuI from aqueous precursor solutions during SILAR deposition. Energy barriers at the boundaries of 18–22 nm CuI nanograins and a large number of charge carriers inside the nanograins determine the transport properties in the temperature interval 295–340 K characterized by transitions from semiconductor to metallic behavior with increasing temperature, which are typical of nanostructured degenerate semiconductors. Due to the resistivity of about 0.8 mΩ· m at 310 K and the Seebeck coefficient 101 μV/K, the thermoelectric power factor of the CuI film 1.0 μm thick on the polyimide substrate is 12.3 μW/(m · K2), which corresponds to modern thin-film p-type thermoelectric materials. It confirms the suitability of CuI films obtained by the SILAR method for the fabrication of promising inexpensive non-toxic flexible thermoelectric materials.

Nanoscale Si fishbone structures for manipulating heat transport using phononic resonators for thermoelectric applications

Thermoelectric materials have the potential to convert waste heat into electricity, but their thermoelectric efficiency must be improved before they are effective and economically viable. One promising route to improving thermoelectric efficiency in thin-film thermoelectric materials is to reduce the material’s thermal conductivity through nanopatterning the surface. In this work nanoscale phononic resonators are introduced to the surface, and their potential to reduce thermal conductivity is explored via coupled experimental and theoretical techniques. Atomistic modelling is used to predict the dependence of the thermal conductivity on different design parameters and used to guide the design and fabrication of silicon fishbone nanostructures. The nanostructure design incorporates a variation on design parameters such as barb length, width and spacing along the shaft length to enable correlation with changes in thermal conductivity. The thermal characteristics of the nanostructures are investigated experimentally using the spatial resolution of scanning thermal microscopy to correlate changes in thermal conductivity with the changes in the structure parameters. The method developed uses a microheater to establish a temperature gradient along the structure which will be affected by any local variations in thermal conductivity. The impact on the thermal gradient and consequently on the tip temperature is modelled using finite element computer simulations. Experimental changes as small as 7.5% are shown to be detectable in this way. Despite the experimental technique being shown to be able to detect thermal changes far smaller than those predicted by the modelling, no modifications of the thermal conductivity are detected. It is concluded that in order to realise the effects of phononic resonators to reduce thermal conductivity, that much smaller structures with a greater ratio of resonator to shaft will be needed.

Study on the electrodeposition of Bi2Te3 thin film thermoelectric material in dimethyl sulfoxide

In this paper, the electrochemical behaviors of Bi3+ and Te4+ ions in dimethyl sulfoxide (DMSO) organic solvent were studied by testing the cyclic voltammetry curves and cathodic polarization curves. The diffusion coefficients of Bi3+ and Te4+ ions were investigated and calculated, and reduction difficulty of them was compared. Then, bismuth telluride thermoelectric thin films were prepared using potentiostatic electrodeposition mode in the electrolytes with different compositions at different potentials. The structure, morphology and compositions of the prepared thermoelectric materials were characterized and analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), and X-ray energy dispersive spectroscopy (EDS). The results show that the redox processes of Bi, Te and bismuth telluride on Au electrode are all completely irreversible processes. The diffusion coefficient of Bi3+ ion has an order of magnitude of approximately 10−7 while that of Te4+ ion is about 10−5. The reduction of Bi3+ ion is more difficult than that of Te4+ ion in dimethyl sulfoxide organic solvent. The prepared thin film thermoelectric materials are bismuth telluride alloy, and the electrolyte composition and deposition potential have great influence on the structure, morphology and composition of the films. Increasing the concentration of Bi3+ ions in the electrolyte can greatly increase the content of element Bi in the deposits. In the electrolyte containing 20 mol·L−1 Bi(NO3)3·5H2O and 10 mol L−1 TeCl4, the bismuth telluride film electrodeposited at − 1.2 V has uniform grain size and compact structure, shows good surface morphology, and the atomic fraction ratio of Bi/Te element in the film is 1:1.80.

Understanding the Effects of Primary and Secondary Doping via Post‐Treatment of P‐Type and N‐Type Hybrid Organic–Inorganic Thin Film Thermoelectric Materials

AbstractHybrid organic/inorganic materials have emerged as promising thermoelectric (TE) materials since they inherit the individual strengths of each component, enabling rational materials design with enhanced TE performance. The doping of hybrid TE materials via post‐treatment processes is used to improve their performance, but there is still an incomplete understanding of the elicited effects. Here, the impact of different doping methods on the thin film TE performance of p‐type Te/poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and n‐type Ag2Te/PEDOT:PSS hybrid materials is investigated. Primary doping through acid–base and charge transfer processes using H2SO4 and tetrakis(dimethylamino)ethylene, respectively, and the effects of secondary doping using ethylene glycol is examined. Through a combination of Hall effect measurements, hard X‐ray photoelectron spectroscopy, and Raman spectroscopy, variations in the charge carrier concentration, mobility, and overall TE performance are related to the morphological and chemical structure of the hybrid materials. This study provides an improved understanding of the effects that different post‐treatments have on hybrid materials and shows that the impact of these post‐treatments on pure PEDOT:PSS does not always apply to hybrid systems. These new insights into post‐treatment effects on hybrid materials is expected to facilitate further enhancement of their performance as electronic materials in general and thermoelectric materials in particular.

Synergetic optimization of thermoelectric properties in SnSe film via manipulating Se vacancies

Thin film thermoelectric materials have attracted much attention due to the application prospect in nanoscale devices. However, the commonly used optimization strategy in bulk materials, such as defect engineering, is hard to be realized in films. In this work, we synthesized SnSe films by the magnetron sputtering method and manipulated the Se vacancy concentration simply by controlling the sputtering time. The carrier transport properties were systematically investigated by combining theoretical simulation and experimental measurement. In the 130 nm-thick ultrathin film, Se vacancies were formed due to the difference in atomic mass between Sn and Se. The electrical conductivity and carrier mobility are improved by the weak electron-phonon coupling strength and small indirect band gap. Meanwhile, the thermal conductivity of the vacancy structure has an average reduction of 33.8% compared with the pristine SnSe films due to the strong vacancy scattering. The Seebeck coefficient maintains above 300 μV K-1 due to the relatively large direct band gap. A high ZT of 0.6 was finally achieved at 700 K in the film sample with Sn:Se ratio of 1.08, 50% improvement over the pristine SnSe film. However, a transition to the metallic characteristic occurs when Se vacancy concentration further increases, resulting in deterioration of ZT at high temperature. The present work demonstrates a guiding principle of manipulating vacancy concentration for high thermoelectric performance and reveals how vacancies influence energy transfer.

Characterization of Magnetron Sputtered BiTe-Based Thermoelectric Thin Films.

Thermoelectric (TE) technology attracts much attention due to the fact it can convert thermal energy into electricity and vice versa. Thin-film TE materials can be synthesized on different kinds of substrates, which offer the possibility of the control of microstructure and composition to higher TE power, as well as the development of novel TE devices meeting flexible and miniature requirements. In this work, we use magnetron sputtering to deposit N-type and P-type BiTe-based thin films on silicon, glass, and Kapton HN polyimide foil. Their morphology, microstructure, and phase constituents are studied by SEM/EDX, XRD, and TEM. The electrical conductivity, thermal conductivity, and Seebeck coefficient of the thin film are measured by a special in-plane advanced test system. The output of electrical power (open-circuit voltage and electric current) of the thin film is measured by an in-house apparatus at different temperature gradient. The impact of deposition parameters and the thickness, width, and length of the thin film on the power output are also investigated for optimizing the thin-film flexible TE device to harvest thermal energy.

Strain-enhanced thermoelectric properties of Nb-doped SrTiO<sub>3</sub> thin films

The development of high-performance thermoelectric materials can help solve the energy crisis in the future. Thin-film thermoelectric materials can meet the requirement for flexibility of wearable devices while supplying electrical power to them. In this study, high-quality Nb-doped SrTiO<sub>3</sub> films (Nb:STO) with different thickness are prepared on SrTiO<sub>3</sub> (STO) and La<sub>0.3</sub>Sr<sub>0.7</sub>Al<sub>0.65</sub>Ta<sub>0.35</sub>O<sub>3</sub> (LSAT) substrates by pulsed laser deposition. The surface morphologies, crystal structures, and thermoelectric performances of the films are characterized. The results show that the thermoelectric performance of the strain-free film increase with thickness increasing. The power factor at room temperature increases by 187%. The Seebeck coefficient of the 144 nm-thick Nb:STO/LSAT sample with strain is greatly improved to <inline-formula><tex-math id="M2">\begin{document}$265.95\;{\text{μ}}{\rm{V}}/{\rm{K}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="9-20222301_M2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="9-20222301_M2.png"/></alternatives></inline-formula> at room temperature, which is likely to be due to the strain induced changes in the energy band of the thin film. The improvement of the thermoelectric performances of Nb:STO thin films by strain engineering provides a new approach to improving the thermoelectric properties of oxide thin films.

Flexible elemental thermoelectrics with ultra-high power density

Flexible thermoelectrics have attracted great attention due to their potential to convert waste heat to electricity to power wearable devices and IoT (Internet of things) sensors. Though organic thin-film thermoelectric materials are popular choice for preparing flexible thermoelectrics, they suffer from low thermoelectric performances due to the low figure of merit zT. In this work, we demonstrate a new strategy to design high-performance thermoelectrics by highlighting the influence of power factors instead of zT. To verify our claim, thermoelectric is prepared through sputtering high-power factor but low zT elemental thin films of Sb (p-type) and Ni (n-type) on a polyimide substrate. In this system, ascribed by the high-power factor (up to 7 mW/mK2), a high power density of up to 4.7 mW/cm2 at a temperature gradient of 50 K can be achieved, even with the low zT (<0.1). In addition, finite element method (FEM) analysis shows that the benefit of high-power factor increases with decreasing film thicknesses. Our results show that in the case of organics and inorganics thin films, maximizing power factor is more important than maximizing zT. This finding serves to guide the design paradigm of thermoelectrics for wearable devices and is useful for the broad organic electronics community.

A flexible spring-shaped architecture with optimized thermal design for wearable thermoelectric energy harvesting

Despite the rapid advancements made for thin-film thermoelectric materials, the rational design of corresponding thermoelectric devices (TEDs) raises the urgent need for thermal optimization to meet the requirement of efficient utilization of the temperature gradient vertical to the device plane. Herein, a three-dimensional spring-like thermoelectric device (S-TED) with fundamantal dual elastomer layers and air gaps is proposed. The novel device architecture not only inherits excellent flexibility and compressibility, but also enables the capability of harvesting waste heat under vertical temperature gradient. Detailed analysis of thermal modeling indicates that optimization is considered in three aspects compared to conventional TEDs, including the strengthened contact for more efficient interfacial heat transfer, the hindered heat transfer across the device body via the construction of thermally-insulating air gaps, and the effective heat sink that facilitates the rapid heat transfer from device to the ambient. The as-fabricated S-TED delivers a high output power of 749.19 nW under a vertical gradient of 30 K with only three pairs of p-n couple, corresponding to high output power of 416.22 nW cm -2 compared to other reported flexible TEDs. The design strategy conceived in this work will promote the development of high-performance TEDs for flexible and wearable applications. • A 3D spring-shaped architecture for flexible thermoelectric devices. • Thermal optimization ensures the effective building up of temperature gradient. • Based on in-plane properties, novel devices utilize vertical temperature gradient. • Design strategy can be extended to most thin-film thermoelectric materials. • High potential in flexible and wearable applications.

High thermoelectric power factors in polycrystalline germanium thin films

The high potential of polycrystalline Ge as a thin-film thermoelectric material was demonstrated. We synthesize a polycrystalline Ge layer on an insulating substrate at 450 °C via advanced solid-phase crystallization and control its carrier concentration through the solid-phase diffusion of various p- and n-type dopants. The heating deposition (150 °C) of the amorphous precursor considerably improves the crystal quality of the polycrystalline Ge layer as well as the doping properties. The solid-phase diffusion of Ga and P dopants onto the Ge layers allows for control of the carrier concentration in the ranges of 1017–1020 cm−3 for p-type and 1018–1019 cm−3 for n-type, respectively, by modulating the diffusion annealing temperature and time. Because of the high electrical conductivities reflecting the carrier mobilities and carrier concentrations, the maximum power factors reach a value of 1080 μW m−1 K−2 for p-type and 2300 μW m−1 K−2 for n-type at room temperature. These power factors are higher than those of most polycrystalline semiconductor thin films formed at temperatures below 1000 °C. Therefore, this study serves as a milestone toward high-performance and reliable thin-film thermoelectric generators based on an environmentally friendly semiconductor.

Study of thermoelectric properties in the PEDOT:PSS/Te double-layer thin film devices

The research on the organic hybrid composites have been regarded as important developing direction in the field of thermoelectric fields, which combine the advantages of organic thermoelectric materials and inorganic thermoelectric materials. In this paper, the thermoelectric properties of Tellurium (Te) and Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) hybrid composite thin film and double-layer structured of PEDOT/Te thin film are studied, separately. We found that the PEDOT:PSS/Te double-layer structured thin film thermoelectric devices can greatly improve conductivity and Seebeck coefficient of PEDOT:PSS compared with the hybrid structure of PEDOT:PSS thermoelectric devices. We thought that the semi-metallicity of PEDOT:PSS and Te forms an ohmic contact interface, which is advantageous to charge transfer, so the double-layer structure of thin film devices possess a better carrier transport channels by the interface effect than hybrid structure devices. On the other hand, the C–V results also confirmed that the interfacial charge accumulation in the PEDOT:PSS/Te double-layered thin-film device, when the temperature rises, the Fermi level close to the conduction band as the temperature increases, leading to the number of high-energy carriers increase at the interface, which effectively improves the Seebeck coefficient of PEDOT:PSS. Our research provide experimental and theoretical support for the development of thin-film thermoelectric materials and devices.

Features for the design of a specialized information-measuring system for the study of thermoelectric properties of semiconductors

Methods for studying thermoelectric parameters of semiconductors that are optimal for the implementation of software and hardware have been analyzed and selected. It is based on the Harman method and its modifications, adapted for pulse measurements, which are convenient to implement on a modern element base. An important advantage of these methods is the absence of the need for accurate measurements of heat fluxes, which greatly simplifies and reduces the time for conducting experimental research. The required operating ranges for the voltage 10 µV–1 V, for the current 10 µA–300 mA and the element base performance at the processing level of 40–200 million samples per second have been determined. Structural and electrical circuits, as well as software for a specialized computer system for studying thermoelectric parameters of both bulk and thin-film thermoelectric materials, and express analysis of the operational characteristics of finished modules have been developed. It has been shown that the proposed scheme copes well with the task. And the use of FPGA and 32-bit microcontrollers provide sufficient processing speed up to 200 MSPS and the necessary synchronization modes for the implementation of the Harman pulse method even when studying films of nanometer thickness. Experimental studies of both bulk thermoelectric modules based on Bi2Te3 and thin-film thermoelectric material based on PbTe have been carried out. The effectiveness of the developed tools and techniques has been shown, which made it possible to more than halve the time for sample preparation and experiment. Based on the presented models, all the main thermoelectric and operational parameters have been determined, in particular, electrical conductivity, Seebeck coefficient, thermal conductivity, thermoelectric figure of merit. As a result of the development of specialized computer tools, it was possible to reduce the labor intensity of the process of measuring the main electrical and operational parameters of semiconductor thermoelectric materials and energy conversion modules based on them, as well as to automate the process of defects identification of thermoelectric modules. The labor intensity of the research process has decreased not only due to the automation of the measurement process, but also due to an optimized technique that allows research on a sample of one configuration, since the manufacture and preparation of samples are the most laborious

A Robust System for Thermoelectric Device Characterization

Due to the large reduction in fossil fuel reservoirs, the consequent cost increase of deepwater extraction, and the emission of pollutants, there is a constant search for alternative ways to obtain clean energy at a lower cost. Among those sources, we focus on the energy produced by thermoelectric materials. In this work, we present a new system for the characterization of thermoelectric generation devices. Such a system performs measurements of electrical resistivity, Seebeck coefficient, and thermal conductivity in a single setup. With this, it is possible to reduce the systematic errors in the figure of merit $ZT$ and the cost of the equipment. Our equipment, together with the developed software, presented excellent results and analyses, and with that, it proves to be a robust alternative for the characterization of commercial thermoelectric devices, and of laboratory thin film thermoelectric materials.

Thermal and Mechanical Analyses of Compliant Thermoelectric Coils for Flexible and Bio-Integrated Devices

AbstractThree-dimensional coil structures assembled by mechanically guided compressive buckling have shown potential in enabling efficient thermal impedance matching of thermoelectric devices at a small characteristic scale, which increases the efficiency of power conversion, and has the potential to supply electric power to flexible bio-integrated devices. The unconventional heat dissipation behavior at the side surfaces of the thin-film coil, which serves as a “heat pump,” is strongly dependent on the geometry and the material of the encapsulating dissipation layer (e.g., polyimide). The low heat transfer coefficient of the encapsulation layer, which may damp the heat transfer for a conventional thermoelectric device, usually limits the heat transfer efficiency. However, the unconventional geometry of the coil can take advantage of the low heat transfer coefficient to increase its hot-to-cold temperature difference, and this requires further thermal analysis of the coil in order to improve its power conversion efficiency. Another challenge for the coil is that the active thin-film thermoelectric materials employed (e.g., heavily doped Silicon) are usually very brittle, with the fracture strain less than 0.1% in general while the overall device may undergo large deformation (e.g., stretched 100%). Mechanical analysis is therefore necessary to avoid failure/fracture of the thermoelectric material. In this work, we study the effect of coil geometry on both thermal and mechanical behaviors by using numerical and analytical approaches, and optimize the coil geometry to improve the device performance, and to guide its design for future applications.

An Origami Structure Combined with a Thin Film Thermoelectric Material for High-power Output (About the Use of the JSME Specification Template File)

An Origami Structure Combined with a Thin Film Thermoelectric Material for High-power Output (About the Use of the JSME Specification Template File)

Use of biomass for a development of nanocellulose-based biodegradable flexible thin film thermoelectric material

In this work, we used solar energy converted via photosynthesis into chemical energy of the biomass of the fast-growing perennial herb Miscanthus × giganteus for the manufacture of nanocellulose (NC) films, which are biodegradable alternative to common petroleum-based polymer substrates used in flexible electronics. To create the NC substrates, we applied an environmentally friendly method of organosolv delignification of plant raw materials carried out at a low temperature and in a relatively short time. Then by means of the low-temperature cheap and scalable method Successive Ionic Layer Adsorption and Reaction (SILAR) we deposited copper iodide (CuI) film of 0.72 µm thickness on both sides of the 12 µm thick NC substrate, and thus obtained light-weight and flexible biodegradable nontoxic thermoelectric material CuI/NC. Crystal structure, morphology, chemical composition, and optical, electrical and thermoelectric properties of the CuI/NC have been researched. Studies have shown that nanostructured p-type semiconductor CuI film in the CuI/NC TE material is quite dense and completely covers the NC surface. It has typical optical direct band gap ≈ 3.0 eV, is single-phase γ-CuI with crystallite sizes in the 19–25 nm range, with moderate dislocation density of (1.6–2.8) × 1015 lines/m2, and tolerable microstrains ε of (4–9) × 10−3 a.u. The determined value of the Seebeck coefficient S is ~228 μV K−1, at that, S is constant in the temperature range 290–335 K. Together with the thermoelectric power factor ≈ 36 μW·m−1·K−2it is favorable for the use of CuI/NC as new thermoelectric material for an in-plane design of biodegradable flexible thin film thermoelectric generator (TEG). At temperature gradient of 50 K, the single p-CuI thermoelectric leg made from CuI/NC strip of 3 cm long and 0.5 cm wide generates open circuit voltage 8.4 mV, short circuit current 0.7 µA and maximum output power 1.5 nW. It corresponds to the output power density 10 µW/m2, and thus confirms the suitability of CuI/NC to obtain electricity by the harvesting the waste environmental heat.

Nanostructural effect on thermoelectric properties in Si films containing iron silicide nanodots

Thin film thermoelectric materials have drawn much attention for realizing one-chip stand-alone power sources of Internet of Things devices. Here, we fabricate two types of the nanostructured Si films with high crystallinity: Si films containing β-FeSi2 nanodots with a wider nanodot size distribution of ∼5–120 nm and Si films containing α-FeSi2 nanodots with a narrow size distribution of ∼5–20 nm. The thermal conductivity of these films is lower than those of Si–silicide nanocomposite bulks. Interestingly, Si films containing β-FeSi2 nanodots show about two times lower thermal conductivity than Si films containing α-FeSi2 nanodots. This is because the widely-size-distributed β-FeSi2 nanodots can effectively work as phonon scattering centers due to hierarchical architectures. These films also exhibited a high power factor due to the small amount of point defects and single crystalline epitaxial interfaces, regardless of the iron silicide phase of nanodots. These detailed investigations will open a road for realizing high-performance thin film thermoelectric materials.

Electrodeposition preparation and optimization of fan-shaped miniaturized radioisotope thermoelectric generator

In view of the current energy demand for miniaturized equipment in extreme environmental fields, such as in deep space exploration. A new fan-shaped radioisotope thermoelectric generator is innovatively presented and designed. Thin-film thermoelectric materials used for miniaturized radioisotope thermoelectric generators are first prepared by electrochemical methods. The prepared fan-shaped radioisotope thermoelectric generator has a volume of 5.75 cm3 and consists of 8 thermoelectric modules and 32 thermoelectric legs. The study finds that when a 1.5 W heat source is loaded, the temperature difference of the device is 54.8 K, the output voltage and the maximum output power is 174.88 mV and 333.20 nW, respectively. On this basis, the number and size of the modules are optimized by the finite element method. When the thermoelectric leg size is optimized to 9 × 2 mm2 and the number of modules is 8, the maximum output power can be up to 369.02 nW. The corresponding experimental verification work is further developed and discussed. This work provides a novel solution for the energy supply problem of small-volume devices in extreme space environments.

Recent development and application of thin-film thermoelectric cooler

Recently, the performance and fabrication of thin-film thermoelectric materials have been largely enhanced. Based on this enhancement, the thin-film thermoelectric cooler (TEC) is becoming a research hot topic, due to its high cooling flux and microchip level size. To fulfill a thin-film TEC, interfacial problems are unavoidable, as they may largely reduce the properties of a thin-film TEC. Moreover, the architecture of a thin-film TEC should also be properly designed. In this review, we introduced the enhancement of thermoelectric properties of (Bi,Sb)2(Te,Se)3 solid solution materials by chemical vapor deposition, physical vapor deposition and electrodeposition. Then, the interfacial problems, including contact resistance, interfacial diffusion and thermal contact resistance, were discussed. Furthermore, the design, fabrication, as well as the performance of thin-film TECs were summarized.

Growth Optimization of Multi-layer Thin Film Thermoelectric Materials based on Bi2Te3 / WS2 superlattice Structure

Heat is the most ubiquitous form of energy on planet Earth. Every day, the sun continuously strikes the Earth’s surface with 120,000 Terawatts of energy. This solar energy is more than 10,000 times the amount of energy produced worldwide. With the scarcity of fossil fuels looming on the horizon and its adverse effect on the environment many researchers, from academia to industry, are exploring cleaner, greener and more efficient renewable energy technologies. Thermoelectricity can provide an alternative to hazardous fossil fuels as its electricity is produced directly from heat with no moving parts or working fluid. The efficiency of any thermoelectric material is given by a quantity called the figure of merit ZT. For thermoelectric (TE) devices to be competitive with fluid-based and other energy related devices, ZT greater than 2 is usually sought. Here, we report on the fabrication of thin film thermoelectric materials based on Bi2Te3/WS2 superlattice layer structure using RF magnetron sputtering deposition method. Quantum confinement in these low dimensional and ultrathin superlattices can enhance the density of states near the fermi level resulting in higher ZT value. The thermoelectric figure of merit can be enhanced by controlling the layer thickness close to the phonons mean free path. This way heat carrying phonons with different wavelengths can be scattered efficiently resulting in lower lattice thermal conductivity.