Available online at ScienceDirect. Procedia Engineering 168 (2016 ) th Eurosensors Conference, EUROSENSORS 2016

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1 Available online at ScienceDirect Procedia Engineering 168 (2016 ) th Eurosensors Conference, EUROSENSORS 2016 Site-selectively grown p-type Ge NWs as a gas sensor J. Samà a,*, G. Domènech-Gil a, I. Gràcia b, J. Santander b, C. Cané b, M. Seifner c, S. Barth c, A. Romano-Rodríguez a a Universitat de Barcelona (UB), MIND-Departament of Engineering and Institute of Nanoscience and Nanotechnology (IN2UB) Barcelona, Spain b Consejo Superior de Investigaciones Científicas (CSIC), Institut de Microelectrònica de Barcelona-Centro Nacional de Microelectrónica, Bellaterra, Spain c Vienna University of Technology (TUW), Institute of Materials Chemistry, 1060 Vienna, Austria Abstract Ge NWs have been selectively grown directly on top of the suspended microhotplates, with heater and interdigitated electrodes, via Au catalyzed Vapor Liquid Solid (VLS) mechanism through a low-power consumption CVD-like process. The so-fabricated devices, with a p-type semiconductors behavior, have been characterized towards water vapor in synthetic air, keeping the temperature at 100ºC in order to achieve a stable oxide. The results show reproducible responses towards H2O Published The Authors. by Elsevier Published Ltd. This by Elsevier is an open Ltd. access article under the CC BY-NC-ND license ( Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference. Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference Keywords: Nanowires; Germanium; Microhotplate; Humidity sensor 1. Introduction Significant efforts have been focused in the last years on the use of NWs as building blocks for electronic applications as gas sensors, due to the large surface to volume ratio [1]. Especially monocrystalline materials are important to achieve effective and known interactions of their surface in several applications. The integration of NWs on a final electronic device in an easy, straight and low time-consuming process is challenging, and constant research is being developed in order to improve and carry out the synthesis of the nanowires on top of the electronic devices Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license ( Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference doi: /j.proeng

2 J. Samà et al. / Procedia Engineering 168 ( 2016 ) In this direction, here we present a direct growth of inorganic germanium nanowires on top of CMOS compatible microhotplates and micromembranes with buried heater in order to provide the thermal energy necessary to synthesize the nanowires [2,3]. Germanium is a material that has been rarely used a gas sensor, due to the poor chemical stability of the oxide form. Despite it was the semiconductor employed on the fabrication of the first transistor [4], few works are found concerning the fabrication of a gas sensor based on Ge structured material [5]. However, the surface properties of Ge, especially the chemisorption process of oxygen has been both theoretically [6,7] and experimentally studied [8,9], due to the interest in the formation steps of the germanium oxide, and its chemical reactivity towards other gases like oxygen or water vapor [10,11]. This work presents a humidity sensor based on a network of Ge NWs grown on top of suspended microhotplates, by means of a vapor-liquid-solid (VLS) mechanism, using Au as a catalyst. 2. Experimental results Ge NWs are site-specifically grown on top of microhotplates through a CVD-like method. Microhoplates are free-suspended membranes (100x100µm size) fabricated by surface micromachining, with two separated Pt electrodes on top of the silicon dioxide membrane and a Pt buried heater embedded in a silicon nitride layer which is underneath. [12] An thin and non-continuous Au layer is sputtered on the top of the chip, which will act as a catalyst of the precursor dissociation and growth process. The sample, mounted on a TO-8 package, is placed within a quartz reactor, and the heaters from the suspended membranes are electrically accessed when the precursor is flowing and the growth process is carried out. Diphenylgermane is used as the gas precursor, which dissociates and nucleates site-selectively where the heater provides the required temperature for the growth of Ge NWs, about C. The growth is a fast (5 min) and requires very low (3 mw). The nanowires are grown majoritarily between the electrodes, creating multiple contacts between them and also to the electrodes. These electrodes can also be externally accessed and can be used to verify that there is a correct electrical connection between them and to decide to quench the growth process. Once the growth is carried out, no other lithography step or contacting process is required since the network of nanowires can already be measured. The same heater used to carry out the thermolysis of the precursor is employed to heat the nanowires for gas sensing operation. Fig. 1. SEM image of a suspended microhotplate on which the Ge NWs network has been grown. The electrodes on top of the membrane are also shown.

3 1058 J. Samà et al. / Procedia Engineering 168 ( 2016 ) Fig. 2 Field Effect measurement of Ge NWs using the heater of the membrane as gate electrode. Inlet: drain-source I-V curves when forcing different gate voltages. Both measurements indicate the p-type behaviour of Ge NWs. 3. Results and discussion An SEM image of the resulting growth of NWs network on the microsized membrane is shown in Fig. 1. The growth occurs not only at the center but also on part of the arms closer to the microhotplate, while further away, the temperature is not high enough to allow thermolysis of the precursor. The semiconductor behavior of the Ge NWs has been studied through Field Effect measurements using the same structure previously detailed. The platinum heater is used as a bottom gate electrode, and both surface electrodes act as a drain and source of the FET. The drain current flowing through the NWs, when keeping a constant drain-tosource voltage of 10 mv, is measured as a function of the gate voltage (transfer curve) and is represented in Error! No se encuentra el origen de la referencia., where the decrease in current for positive gate-source voltages demonstrates univocally the p-type behavior of the Ge NWs. The inset image demonstrates also the reduction of the drain-to-source current for increasing gate voltages for different I-V curves of the nanowires. High resolution transmission electron microscopy (HRTEM) analysis showed monocrystal germanium nanowires, but surrounded by an amorphous GeO x layer. Contrarily to silicon, germanium oxide which is formed due to oxidation of germanium in air, is not stoichiometric and shows an increase of the oxygen concentration from the interface Ge/GeO x towards the nanowire surface. The nanowires have been studied as gas sensor in a temperature range between 50 and 120 C. The upper value was chosen to minimize the oxidation of the germanium surface due to the presence of oxygen in the atmosphere. A severe drifting of the resistance baseline has also been observed after some tests at the higher temperatures. Probably this higher temperature causes a further oxidation of the outer GeOx layer, giving rise to a non-uniform surface of the nanowires. Furthermore, the high temperatures may also suppress sensitivity towards the surrounding gas species. The response of the Ge nanowires towards water vapour concentrations in the range of % of relative humidity at room temperature has been tested. The resistance variation in the presence of three different pulses from 20 to 80% of relative humidity in synthetic air are shown in Fig. 3. It can be seen that with the introduction of the gas, the resistance increases by about 29% for 60% of RH and that the change is increases with increasing relative humidity. From Fig. 3 the response time (variation from 10% to 90%) of the signal is also obtained, which is in the range of 10 minutes.

4 J. Samà et al. / Procedia Engineering 168 ( 2016 ) Fig. 3. Change in resistance of Ge NWs towards different pulses of water vapour in synthetic air at 100ºC. 4. Conclusions Locally grown Ge NWs have been implemented through a robust, low-consumption and fast fabrication process on top of microhotplates. The NW-based devices have been characterized as humidity sensors operating at 100ºC to avoid an irreversible further oxidation of the nanowires that may severely change the sensing properties of the Ge nanostructures. The results show a variation of the sensor s resistance with relative humidity values between 20 and 80%. A response time of 10 minute has been measured. Acknowledgements This work has been partially funded by the Spanish Ministry of Economy and Competitivity, through project TEC C6 (TEMIN-AIR) and the Spanish Ministry of Science and Innovation, through the ICTS program Access to Integrated Micro-Nanofabrication Cleanroom, projects NGG118 and NGG199. References [1] S. Barth, F. Hernandez-Ramirez, J.D. Holmes, A. Romano-Rodriguez, Synthesis and applications of one-dimensional semiconductors, Prog. Mater. Sci. 55 (2010) doi: /j.pmatsci [2] S. Barth, R. Jimenez-Diaz, J. Samà, J. Daniel Prades, I. Gracia, J. Santander, et al., Localized growth and in situ integration of nanowires for device applications, Chem. Commun. 48 (2012) doi: /c2cc30920c. [3] J. Samà, S. Barth, G. Domènech-Gil, J.-D. Prades, N. López, O. Casals, et al., Site-selectively grown SnO2 NWs networks on micromembranes for efficient ammonia sensing in humid conditions, Sensors Actuators B Chem. 232 (2016) doi: [4] J. Bardeen, W.H. Brattain, The Transistor, A Semi-Conductor Triode, Phys. Rev. 74 (1948) [5] D.M.A. Mackenzie, S.A. Brown, Germanium nano-cluster films as humidity and hydrogen sensors, J. Appl. Phys. 112 (2012) doi: [6] G.A. Shah, M. Radny, P. Smith, Initial Stages of Oxygen Chemisorption on the Ge (001) Surface, J. Phys.. (2014). [7] X.L. Fan, W.M. Lau, Z.F. Liu, Nondissociative Adsorption of O2 on Ge(100), J. Phys. Chem. C. 113 (2009) doi: /jp809480c. [8] L. Surnev, M. Tikhov, Oxygen adsorption on a Ge(100) surface: I. Clean surfaces, Surf. Sci. 123 (1982) doi: [9] D.A. Hansen, J.B. Hudson, David A. Hansen a* and John B. Hudson b* * B, Surf. Sci. 254 (1991)

5 1060 J. Samà et al. / Procedia Engineering 168 ( 2016 ) [10] B. Bernstein, Effects of Thick Oxides on GerArxanium Surface Properties ~, 105 (1956). [11] J.M. Soon, C.W. Lim, K.P. Loh, N.L. Ma, P. Wu, Initial-stage oxidation mechanism of Ge(100)2 1 dimers, Phys. Rev. B - Condens. Matter Mater. Phys. 72 (2005) 1 6. doi: /physrevb [12] J. Samà, G. Domènech-Gil, M.S. Seifner, J. Santander, C. Calaza, I. Gràcia, et al., Low temperature humidity sensor based on Ge nanowires selectively grown on suspended microhotplates. Manuscript submitted for publication, (2016).

Parallel Alignment of Nanowires for Fast Fabrication of Nanowire Based Gas Sensors

Parallel Alignment of Nanowires for Fast Fabrication of Nanowire Based Gas Sensors R. Jiménez-Díaz 1, J.D. Prades 1 F. Hernández-Ramírez, J. Santander 3 C. Calaza 3, L. Fonseca 3, C. Cané 3 A. Romano-Rodriguez