Silicon nanowire arrays as thermoelectric material for a power

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Abstract: Silicon nanowires (Si NWs) have been implemented in microfabricated structures to develop planar thermoelectric microgenerators (μTEGs) ...
SILICON NANOWIRE ARRAYS AS THERMOELECTRIC MATERIAL FOR A POWER MICROGENERATOR D. Dávila1*, A. Tarancón1,2, M. Fernández-Regúlez1, D. Kendig3, N. Sabaté1, M. Salleras1, C. Calaza1, A. San Paulo1, A. Shakouri3, L. Fonseca1 1 Instituto de Microelectrónica de Barcelona – Centro Nacional de Microelectrónica (IMB-CNM, CSIC), Campus UAB, Bellaterra, 08193, Barcelona, Spain 2 Department of Advanced Materials for Energy Applications, Catalonia Institute for Energy Research (IREC), Josep Pla 2, B2, planta baixa, 08019, Barcelona, Spain 3 Jack Baskin School of Engineering, University of California, Santa Cruz, CA 95064, USA *Presenting author: [email protected] Abstract: Silicon nanowires (Si NWs) have been implemented in microfabricated structures to develop planar thermoelectric microgenerators (�TEGs) monolithically integrated in silicon. The purpose is to convert into electric energy the heat flow originated by thermal gradients naturally present in the environment. The compatibility of typical microfabrication technologies and the vapor-liquid-solid growth mechanism (VLS) for growing silicon nanowires has been evaluated. Low-thermal mass suspended structures have been designed, simulated and microfabricated on Silicon On Insulator substrates to passively generate thermal gradients and operate as microgenerators using silicon nanowires as thermoelectric material. Electrical measurements to evaluate the connectivity of the nanowires and thermoreflectance imaging to determine the heat transfer along the device have been employed. Keywords: Microgenerator, silicon nanowires, thermoelectricity, harvesting

INTRODUCTION The increasing demand of miniaturized systems with long-lasting operation is driving the development of new technologies to achieve efficient energy generation. Alternative power sources based on energy harvesting are promising candidates to substitute batteries due to their ability to extract unlimited power from the environment or secondary processes as well as to attain fully autonomous systems without periodical human intervention. Due to the large amount of residual heat yielding from the current energy generation technology based on fossil fuels, thermoelectric energy harvesters have received special attention in recent years [1]. However, although particularly interesting for portable devices, thermopower generators have not been successfully integrated because of the poor thermoelectric properties of the materials traditionally used in microelectronics, e.g. silicon. Nevertheless, low-dimensional structures have been recently discovered to be a promising approach for enhancing the thermoelectric properties of semiconductors [2]. Independent works of Boukai et al. [3] and Hochbaum et al. [4] showed an astonishing enhancement of the thermoelectric properties for the particular case of silicon nanowires (Si NWs), opening fresh perspectives for the integration of thermoelectric generators in microtechnology. In this work, silicon nanowires have been implemented in microfabricated structures to develop unileg thermoelectric microgenerators (�TEGs) as a

way to turn into electric energy the heat flow originated by the presence of thermal gradients in the environment [5]. A chemical vapor deposition (CVD) growth based on the vapor-liquid-solid mechanism has been employed for growing the silicon nanowires [6]. In order to characterize the device, electrical characterization and thermoreflectance imaging [7, 8] have been used to observe the heat transfer along the device.

EXPERIMENTAL The designed device consists of a suspended silicon platform connected to a silicon mass (in contact with a heat source) through silicon nanowires, which act as the thermoelectric material for power generation (Figure 1). This design allows passive generation of thermal gradient through the low-thermal mass platform and the bulk in contact with a heat source. Nevertheless, a heater was integrated on top of the suspended platform to heat the device for an easier thermal characterization. Simulations were performed in order to determine the geometry of the suspended and isolated square microplatform of the device. Square microplatforms of 500, 1000 and 2000 µm and trenches of 10, 15, 20, 30 and 50 µm width (Si NWs lengths) were initially considered. The measurements reported in this work correspond to a device of 500 µm platform and 10 µm trench. Figure 2 shows the microfabrication process flow of the device. Silicon nanowire growth was achieved by using the vapor-liquid-solid (VLS) mechanism. The

galvanic displacement method was used to deposit the Au catalyst nanoparticles needed for the VLS synthesis. This method ensures the selective growth of Si NWs only at silicon-exposed areas [6]. Silicon nanowires with 90 nm of average diameter and 10 µm long were grown in an atmospheric pressure CVD reactor at 750-800 ºC with 10% H2/Ar as both the diluent and carrier gas. The carrier gas was circulated through liquid SiCl4 (Si NWs growth precursor) and BBr3 (in situ p-doping) bubblers kept at 0 ºC in order to maintain a constant vapour pressure.

Fig. 2: Main steps of the microfabrication process. Fig. 1: Basic concept of the device. The thermoreflectance imaging technique has been used to observe the heat transfer distribution along the device. The images are used to determine the temperature difference achieved across the thermoelectric elements (Si NWs) due to an applied electric current to the heater on the suspended silicon platform. Measurements were performed using a 50x, NA = 0.5 microscope objective and a white lightemitting diode (LED) as the illumination source. A 30Hz charged-couple device (CCD) camera was used to obtain two-dimensional thermal images. The heater excitation was a 10Vpp and 3.75Hz sinusoidal voltage driving a current of 35mA through the heater. Calibration of the thermoreflectance coefficient (Cth) was performed for silicon and platinum; however, a precise Cth for each material was difficult to accomplish due to the roughness that the device acquire after the silicon nanowire growth process.

RESULTS AND DISSCUSION Figure 3a shows the as-fabricated microdevice before the nanowires growth. Homogeneous and welldefined electrical Ti/Pt pads for both heater and current collectors were observed for the whole batch of devices. However, several compatibility problems of conventional microfabrication technologies and the Si NWs growth arose during the development of this work. Delamination of the passivation layer on top of the heater occurred during the silicon nanowire growth process due to the thermal mismatch forced during the high temperature treatment (Figure 3b).

This delamination produces unwanted exposed metal areas and therefore irregular silicon NW growth that induces changes in the concentration of precursors in the atmosphere inside the CVD tube furnace. In addition, it is possible to observe the formation of hillocks and delamination of the platinum strip itself. This is likely due to the reaction with the passivation layer and the oxidation/reoxidation cycles taking place during the fabrication process. According to Puigcorbé et al. [9], Ti from the adhesion layer and N from the Si3N4 passive layer probably migrate into the Pt film reaching the Pt surface and modifying the heater electrical resistivity. Additionally, the intrinsic and thermal stresses of the Ti/Pt layer led to hillocks formation on the heater surface, which has been previously observed for annealing temperatures above 650ºC [10]. These problems caused a heater more resistive than anticipated but still operative. In any case, it must be noticed that the above problems related to the heater fabrication, which is a self-test accessory part, should not compromise the intrinsic operation of the device as a thermal scavenger. Different tests in order to define a well suited metal for the device that can withstand the aggressive conditions of the silicon NWs growth mechanism are being performed. Some of the metals that are being currently considered for this purpose are TaSi/Pt, Ti/W, W, TiW and Ta/Pt. Resistance along the silicon nanowire array was measured to be ca. 300�, which indicates that the silicon nanowires are electrically connected to both the suspended platform and the surrounding silicon mass. Figure 3c shows the silicon NW array bridging the suspended mass and the bulk.

the silicon nanowires in response to this temperature gradient was measured to be 30mV, these data yields a preliminary Seebeck coefficient of 1500 µV/K which is much higher than the one obtained in previous studies [3, 4]. This discrepancy may arise from a different Si NWs morphology or dopant concentration. However, a possible overestimation of the real thermal gradient due to the difficult thermoreflectance calibration of the real surface condition of the device is under study. Finite-elements simulations of the structure (non-presented here) showed that such temperature differences are feasible for the geometry employed and for the dissipated power values in the heater. Nevertheless, finer temperature and electrical measurements will be carried out after solving compatibility problems in the fabrication.

Fig. 3: Scanning electron microscopy images of the microfabricated thermoelectrical generator (a) before and (b) after growing the silicon nanowires. (c) Detail of the silicon nanowire region connecting the suspended mass with the bulk. A thermal image of the device when heated up using the platinum strip is shown in figure 4b. Heat transfer from the suspended hot platform to the cold silicon bulk through the Si NWs array is observed. The image confirms that even though silicon nanowires electrically connect the suspended platform to the surrounding silicon mass the thermal transport across them is poor pointing to a promising thermoelectric behavior. Figure 4c shows a thermal profile of the device, the rows indicate the zone where the profiles (100 lines) were measured and averaged. A temperature gradient of ca. 20°C across the silicon nanowires was measured for a 10Vpp sinusoidal signal applied to the heater. The Seebeck voltage induced by

Fig. 4: (a) Optical image and (b) thermal image of the device in ΔT. (c) Thermal profile along the arrows in (b), 100 lines were measured and averaged. The dotted lines of the thermal profile correspond to the silicon nanowires region. Figure 5a shows the Seebeck voltage and resistance measured at the edges of the silicon nanowires while heating the base of the device whereas figure 5b shows Seebeck voltage of the silicon nanowires measured while applying a DC current sweep to the heater. These preliminary measurements can be considered a proof-of-concept of the device under study.

ACKNOWLEDGEMENTS This investigation has been supported by the Spanish Ministry of Science and Innovation (TEC2008-03255-E) and the “Generalitat de Catalunya” (Advanced Materials for Energy Network (XaRMAE), 2009-SGR-35). A. Tarancón, N. Sabaté and C. Calaza would like to thank the financial support of the Ramon y Cajal postdoctoral program of the Spanish Ministry of Science and Innovation. D. Dávila would like to thank the financial support granted by the Spanish MAEC-AECID fellowship program.

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Fig. 5: (a) Seebeck voltage and resistance measured at the edges of the Si NWs while heating the base of the device. (b) Seebeck voltage of the Si NWs while applying a DC current sweep to the heater.

CONCLUSION Dense ordered arrays of Si NWs grown by the VLS technique have been monolithically integrated in a thermoelectric conversion microdevice for the first time. The device consists of a suspended silicon platform laterally isolated from a surrounding silicon rim by a trench that is bridged by closely packed and well oriented Si NWs. The Si NWs ensemble has been shown to be a good thermal barrier and high electrical conductivity path between the hot and cold silicon parts of the device. The thermoreflectance imaging technique has been used to observe the heat transfer distribution along the active part of the device. Although some processing fine-tuning and better thermoreflectance coefficients calibration are still needed, this preliminary characterization points to the promising practical application of the improved thermoelectric behavior of single Si NWs anticipated in the literature.

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