Functionalized vertical InAs nanowire arrays for gas sensing

Sensors and Actuators B 161 (2012) 1144–1149

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Functionalized vertical InAs nanowire arrays for gas sensing P. Offermans ∗ , M. Crego-Calama, S.H. Brongersma imec-nl/Holst Centre, High Tech Campus 31, 5656 AE Eindhoven, The Netherlands

a r t i c l e

i n f o

Article history: Received 18 July 2011 Received in revised form 7 November 2011 Accepted 25 November 2011 Available online 6 December 2011 Keywords: InAs Nanowire Metalloporphyrin Nitric oxide NO2

a b s t r a c t Vertical InAs nanowires are contacted in situ using an air-bridge construction and functionalized with a metalloporphyrin (Hemin). The response of bare and functionalized vertical InAs nanowire arrays to ppb-level concentrations of NO2 and NO is demonstrated. Hemin enhances the response to NO whereas NO2 is also detected without functionalization. Transconductance measurements are used to investigate the effect of gas exposure on current density and electron mobility. Furthermore, the signal-to-noise ratio is shown to increase with the number of contacted nanowires, underlining the application of dense vertical nanowire arrays for sensing. Nanowire self-heating strongly enhances gas desorption, decreasing the recovery time. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Nanowire-based devices show great promise for next generation label-free (bio)chemical sensors as they offer a large surface-to-volume-ratio which enables the effective modulation of their electrical properties by interactions at their surface. They can be formed from a large variety of materials: for example, metal oxides have been known for a long time for their gas sensing properties and their use in crystalline nanowire-based devices has been thoroughly explored [1–4]. Also carbon nanotubes [5–8] and silicon nanowires [9] have been investigated for gas detection. III–V-based nanowires, however, have only recently attracted attention for their interaction with gases and vapors [10]. InAs is a promising sensing material as it exhibits an electron accumulation layer at the surface [11,12], which renders it sensitive to accumulated charges or dipoles [13]. At the same time, InAs allows a relatively easy fabrication of ohmic contacts and offers a high mobility by which changes in electron density can be effectively amplified in the device current. Recently, we demonstrated the fabrication of vertically integrated InAs nanowire arrays for gas sensing [14]. A vertical device geometry eliminates the need for pick and place methods as it allows the controlled contacting of nanowires in their as-grown positions [15]. In situ contacting has become especially attractive, since the catalyst-free growth of III–V nanowires in ordered arrays was demonstrated on silicon substrates [16].

∗ Corresponding author. Tel.: +31 404020517; fax: +31 404020699. E-mail address: [email protected] (P. Offermans). 0925-4005/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2011.11.069

Here, we focus on nanowire surface modification towards the selective binding of a variety of gases. We aim to functionalize the vertical InAs nanowire arrays with metalloporphyrins to enhance their sensitivity and selectivity for gas sensing. Metalloporphyrins are a class of heterocyclic aromatic compounds that contain a metal atom in the center and are known to interact with gaseous molecules [17,18]. We specifically investigated the effect of functionalization with an iron-containing porphyrin, Hemin, on the response of vertical InAs nanowire arrays to ppb-levels of NO2 and NO in dry N2 . A low detection limit of these gases is interesting for the future use of these devices for application in areas such as air quality monitoring [19] and asthma detection [20]. Hemin is known to form a charge transfer complex in the presence of adsorbed NO molecules [21], which is expected to influence the nanowire conductivity by dipole formation at the surface [22]. We demonstrate the detection of NO and NO2 concentrations at the ppb-level using Hemin-functionalized vertical InAs nanowire arrays. Noise measurements are performed to determine the relation between array size and their signal-to-noise ratio for sensing. Finally, we investigate the feasibility of nanowire self-heating [23,24] to enhance the gas desorption from the nanowires, thereby reducing the recovery time.

2. Experimental InAs nanowires were grown on n-doped epitaxy-ready InP(1 1 1)B wafers using low-pressure metal-organic-vapor-phase epitaxy (LP-MOVPE) with trimethylindium (TMI) and arsine (AsH3 ) as precursor materials, transported in a flow of H2 gas. Before growth, a 1.2 nm thin SiOx initiation layer [25] was evaporated

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Fig. 1. (a) Catalyst-free as-grown InAs nanowires on an InP(1 1 1)B substrate. The nanowires were covered with a conformal layer of SiNx to improve stability during processing. (b) TEM image of the tip of a nanowire directly after growth. A 2 nm thick Inx Oy layer can be distinguished at the surface. (c) Vertical InAs nanowire arrays with varying sizes contacted using an air bridge construction (inset). (d) Close-up of a contacted nanowire. The non-contacted part of the nanowire is still covered with SiNx .

on the substrates. Pinholes in this layer facilitate nanowire nucleation and thus eliminate the need for gold catalyst particles. The substrates were heated to the growth temperature of 600 ◦ C in a H2 atmosphere and the nanowires were grown in 90 s. After growth, the nanowires were covered with a conformal 80 nm thick SiNx PECVD layer, which was used as a protection layer for subsequent processing. As shown in Fig. 1a, the nanowires appear untapered and have a length of about 3 ␮m with a diameter of 50–100 nm. The nanowires grown in this manner have a predominantly zincblende crystal structure with wurtzite segments and were found to be covered by a 2 nm thick In-rich amorphous oxide layer (Inx Oy ) as observed by transmission electron microscopy (Fig. 1b). The nanowires were patterned into arrays ranging in size from 30 ␮m × 30 ␮m to 100 ␮m × 100 ␮m (Fig. 1c), using the SiNx layer as a hard mask. The SiNx layer was patterned by reactiveion etching in a CF4 plasma, after which the nanowires outside the arrays were removed by 30 s etching in a piranha solution (3 H2 SO4 :1 H2 O2 30%) cooled down to 30 ◦ C. The nanowire arrays were then contacted using an air bridge construction (Fig. 1d) as outlined in Ref. [14]. Basically, the air bridge construction relies on the use of a sacrificial resist layer, which enables contacting of only the tips of the nanowires, leaving the largest part of their surface available for subsequent surface functionalization. Prior to the gas measurements and functionalization, the protective SiNx at the nanowire surface was removed by a CF4 plasma. Electrical characterization and noise measurements were performed on devices with the SiNx layer still intact. Noise spectra were taken using an Agilent Technologies 35665A Dynamic Signal Analyzer. The gas response was measured in a custom build flow chamber equipped with electrical feed-through using a Keithley 2400 Source Meter. NO2 and NO were supplied from a certified permeation tube (KIN-TEK, emission rate 502 ng/min at 45 ◦ C) and gas cylinder (Praixair, 2500 ppm in N2 ), respectively, and mixed with N2 as the carrier gas using mass flow controllers (Brooks Instrument). The mixed gas was pumped through the flow chamber at a constant flow of 300 sccm.

Back-gated nanowire structures were fabricated by drop casting InAs nanowires after sonification of grown samples in isopropanol, on top of a heavily doped n-type Si wafer (Si-Mat) acting as a common gate electrode. A 500-nm thick thermally oxidized SiO2 layer was used as bottom gate dielectric. Interdigitated electrodes were used to contact to the randomly distributed nanowires. The contacts were made by stepper lithography and lift-off after deposition of 10 nm Ti followed by 100 nm Au, resulting in finger transistors with 1 ␮m channel length. Prior to metal deposition, the oxide was removed from the nanowire surface in the contact area by a short HF dip. Transconductance measurements were performed using an Agilent Technologies B1500 Device Analyzer with a sweep rate of 3 V/s and a drain bias of 0.2 V. Functionalization with Hemin was performed by submerging the wire bonded devices overnight in a 5 mM solution of Hemin (Sigma–Aldrich) in dimethylformamide (DMF). Hemin binds directly to the oxide at the nanowire surface by its two carboxylic side groups [26]. After functionalization the devices were rinsed with DMF and dried in isopropanol vapor. 3. Results and discussion In the fabrication of vertical air bridge-contacted nanowire arrays many process variations exist, which will ultimately affect device performance. To quantify these variations, we measured the device current of five sets of vertical nanowire arrays with increasing array size. The current was measured by probing a pair of nanowire arrays simultaneously at their air bridge contacts, thus effectively measuring the resistance of two arrays in series. As shown in Fig. 2a, the device current scales linearly with array size, which indicates that it is proportional to the number of contacted nanowires. The error bars indicate the standard deviation for the five sets of devices. We attribute the spread in the data to a variation in the number of contacted nanowires, due to fluctuations in the nanowire density and diameter across the surface. The offset in the data is caused by a misalignment of the top contacts to the nanowire arrays, resulting in a lower number of contacted nanowires. From

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(b)

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Fig. 2. (a) Device current as function of device area (the offset in device area is caused by the misalignment of the air bridge contact to the nanowire array leading to smaller contact areas than assumed), averaged over 5 pairs of devices with the same area. Inset: relative response as function of device area to saturating concentrations of NO2 and NO for bare vertical InAs nanowire devices. The average response is indicated by the dashed lines. (b) Minimum measureable relative change in resistance determined from noise spectra as function of device resistance. Inset: noise spectrum of a 100 ␮m × 100 ␮m nanowire array showing 1/f behavior.

the linear dependence, a resistance of 20 k for a single nanowire is obtained, which is comparable to that of gold catalyzed MOVPE grown nanowires reported by Thelander et al. [15]. Next, we tested the gas sensing performance for the differently sized nanowire arrays by recording their response to concentrations of NO2 and NO in dry N2 at atmospheric pressure and a temperature of 21.3 ◦ C. As we are specifically interested in the response to NO and NO2 , and the effects of surface functionalization, we used dry N2 as a carrier gas to prevent any possible influence of oxygen and humidity on the detection of NO and NO2 [34]. It has been shown before that InAs nanowires are sensitive to air and humidity [10]. Prior to gas exposure, the nanowire arrays were wire bonded to a 24-pin DIL package and the protective SiNx layer was removed from the nanowire surface. First the relative response R/R to saturating concentrations of NO and NO2 (>10 ppm) was investigated as function of device area. The resistance of the nanowires increases upon gas exposure and saturates in about 10 min. As shown in the inset of Fig. 2a, we find that the relative saturated response is independent of device area and averages to about 75% for exposure to NO2 and about 10% for exposure to NO. Thus, without functionalization the nanowires are significantly more sensitive to NO2 than to NO. The gas sensing performance of the nanowires does not only depend on their relative change in resistance upon gas exposure, but also on their inherent noise level, as both of these determine the signal-to-noise ratio. Therefore, for the detection of lower concentrations of the gases, i.e. at the ppb-level, it is interesting to determine how the array size influences the noise level of the nanowires. Noise in semiconductor structures may be characterized by their resistor noise spectra. Such spectra show the various components of the total noise as function of frequency. We measured the noise in the low frequency range of 0.7–1600 Hz, by

700

IB2

[2 /Hz],

(1)

R =

˛ 2 [ /Hz], f

(2)

with ˛ a fitting parameter varying with the size of the nanowire arrays. By integrating the resistor noise over the applied frequency range, the minimum measurable relative change in resistance (Rmin )2 was obtained as function of nanowire device area. Defining the sensor response as R/R, i.e. the relative change in resistance, the quantity Rmin /R represents a sensor response equivalent to the noise level. We find that (Rmin /R)2 scales inversely proportional with R as shown in Fig. 2b. Having established that the relative response to gas is independent of array size, this indicates that the highest signal-to-noise ratio may be expected from the largest nanowire arrays. In this respect, the use of vertically oriented nanowire arrays is advantageous compared to horizontally contacted nanowires, as their density can be much larger. Thus, we selected a set of the largest nanowire arrays (100 ␮m × 100 ␮m) to measure the response to ppb-levels of NO2 and NO. First, the response was measured for the bare, nonfunctionalized nanowire arrays. As shown in Fig. 3a, the device resistance increases during gas exposure, and slowly recovers by flushing with pure N2 . At these concentrations, the response does

(b)

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NO 2 1400

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V

where  R is the noise spectral density per unit Hertz. The normalized nanowire noise spectra show a 1/f behavior (Fig. 2b, inset) which could be fitted using

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R (Ω)

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NO2

R =

R (Ω)

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forcing a bias current IB through a pair of arrays such that the voltage drop over the nanowire arrays is 0.2 V. The resistor noise  R was then calculated as function of array size using

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Fig. 3. Resistance change of (a) a bare InAs nanowire array and (b) a Hemin-functionalized InAs nanowire array upon exposure to varying concentrations of NO2 and NO in N2 . The resistance increases during gas exposure and decreases during flushing with pure N2 .

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Fig. 4. I–Vg curves of Hemin-functionalized InAs nanowires before and after exposure to (a) 10 ppm NO2 and (b) 50 ppm NO. The insets show the same for bare nanowires.

not saturate within the exposure time of 1 h. However, the onset of the response is almost immediate, and NO2 concentrations as low as 75 ppb can be detected within 5 min with a signal-to-noise ratio >3. Although also NO concentrations as low as 40 ppb can clearly be detected, the response to NO is considerably lower than that to NO2 . To improve the response to NO, the nanowires were functionalized with Hemin, which directly binds to the native oxide at the nanowire surface by its two carboxylic side groups (Fig. 3b, inset) [26]. After functionalization, the devices where stabilized in N2 and exposed to the exact same sequence of NO and NO2 exposure as used before functionalization, as shown in Fig. 3b. Interestingly, we find that after functionalization the response to NO is strongly enhanced and comparable to that to NO2 . We investigated the sensing mechanism by performing transconductance measurements on nanowires using a horizontal geometry with back-gating. Transconductance measurements give information on the changes in charge density and carrier mobility that occur during gas exposure [10,27–29]. After growth, InAs nanowires were transferred to a doped silicon wafer and contacts were made to the randomly distributed nanowires using interdigitated electrodes. The doped silicon wafer was used as the back-gate. Id –Vg curves were measured before and after exposure to saturating concentrations of NO (50 ppm) and NO2 (10 ppm) in N2. Fig. 4a and b shows the Id –Vg curves for NO2 and NO after functionalization with Hemin. The corresponding Id –Vg curves before functionalization are shown in the insets. A clear hysteresis is observed, which is attributed to the presence of interface state charges at the InAs/Inx Oy interface [27]. A similar hysteresis was observed in Ref. [10] for InAs nanowires. It was shown that the hysteresis could be removed by neutralizing surface states by charging the nanowires with an electron beam. This resulted in a reduced sensitivity to adsorption of molecules on the surface, underlining the importance of the surface accumulation layer and surface states in the response of InAs nanowires to chemisorptions [10]. We thus expect that the InAs/Inx Oy interface states play an important role in the detection mechanism. We find that both the current and the apparent electron field-effect mobility, which can be estimated from dI/dVg , decrease in the presence of NO2 . This is in contrast to the findings in Ref. [10] for the sensing of various saturated vapors, where an increase in apparent electron mobility was observed. By using I2 n2 2 = I1 n1 1

(3)

where n1 (n2 ) indicates the electron density and 1 (2 ) the apparent electron mobility before (after) gas exposure, the relative change in electron density due to the presence of NO2 can be derived. At Vg = 0 V, we find that after NO2 exposure, I2 /I1 = 54% and 2 /1 = 64%, resulting in a reduction of the electron density to 85%

for the positive sweep direction. The reduced field-effect mobility and electron density can be attributed to a reduced surface electron accumulation, and increased charging of and scattering at InAs/Inx Oy interface states [29]. This is in agreement with the expected role of NO2 as a strong electron acceptor: after its adsorption, NO2 attracts negative charge to the nanowire surface leading to a reduced band-bending and, hence, a reduced electron accumulation below the surface. The high density of surface states at the InAs/Inx Oy interface are known to be critical in the formation of the electron accumulation layer and can facilitate electron transfer to adsorbate molecules [10]. The reduced response to NO for bare nanowires (Fig. 4b, inset), may be explained by the fact that it is a weaker electron acceptor than NO2 [35]. After functionalization with Hemin, the change in transconductance upon NO2 exposure was significantly more pronounced with 2 /1 = 19% and n2 /n1 = 78% at Vg = 0 V (Fig. 4a). Similar results were found for NO after functionalization (of a different set of nanowires) as shown in Fig. 4b with 2 /1 = 45% and n2 /n1 = 41% at Vg = 0 V. Before functionalization, the response to NO was very small (Fig. 4b, inset), in agreement with the measurements on the vertical devices. Note that these measurements do no clarify the exact role of Hemin in the sensing mechanism [30]. It is hypothesized that Hemin facilitates the interaction of NO2 and NO with the nanowire surface, by providing active adsorption sites and possibly mediating the charge transfer to the gas molecule. The enhanced electron affinity of Hemin by the attachment of electron-withdrawing NO molecules, would favor the electron transfer from the nanowire to the Hemin [31], leading to a faster and higher response. Alternatively, the enhanced sensitivity may be attributed to surface dipoles induced by the charge transfer between Hemin and the absorbed gas molecule, modifying the surface polarization and the surface state distribution [36] and thus changing the band bending at the nanowire surface [22]. Finally, Hemin may catalyze the formation of NO2 from NO [37] which implies that effectively NO2 is detected by the nanowire instead of NO. To reduce the recovery time of the nanowire devices after gas exposure, the feasibility of nanowire self-heating [23,24,38] was investigated. Self-heating may be achieved by applying a voltage on the nanowire array, large enough to cause heating by resistive power dissipation, and is expected to induce enhanced desorption of adsorbed gas molecules, by changing the desorption kinetics [39]. Nanowire self-heating has been demonstrated for a variety of nanowire materials [23,24,38] and may be regarded as Joule heating at the nanoscale [40], featuring a reduced lattice thermal conductivity compared to the bulk material due to scattering of phonons at the surfaces of the nanowire [41]. The temperature increase by self-heating depends on the balance between the dissipated Joule power and the power losses via heat transfer to the contacts and, since we operate the devices at atmospheric pressure, to the gas flow [24]. Fig. 5 shows the resistance of a vertical nanowire array

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Time (min) Fig. 5. Enhanced recovery by self-heating. The resistance of an InAs nanowire array is plotted as function of time during exposure to 10 ppm NO2 at an applied voltage of 0.2 V. During recovery, a voltage of 3.4 V was applied for 800 s to induce self-heating leading to faster recovery.

plotted as function of time during exposure to 10 ppm NO2 while applying a voltage of 0.2 V. During recovery, a voltage of 3.4 V was applied for 800 s to induce self-heating. At this voltage, the resistance is strongly enlarged due to a reduced scaling of the current with voltage, which may be attributed to electron velocity saturation. Note that increasing the voltage above this point might lead to breakdown of the nanowire array. The strong decrease of resistance during application of the higher voltage is attributed to the desorption of adsorbed NO2 molecules [42]. Indeed, the nanowire array shows an enhanced recovery from the gas exposure as the resistance quickly reverts to the baseline upon reducing the voltage back to 0.2 V. The possibility of enhanced recovery by the simple application of a higher voltage eliminates the need for external or integrated micro-heaters, which simplifies sensor fabrication while reducing power consumption. 4. Conclusion We have shown that surface modification with Hemin influences the response of vertical InAs nanowires arrays to ppb-levels of NO and NO2 . Before functionalization, the response to NO2 is significantly larger than that to NO, whereas after functionalization, the response to NO is strongly enhanced and comparable to that of NO2 . Thus, by combining non-functionalized and functionalized nanowire arrays in the same device, NO and NO2 concentrations can be selectively detected at the ppb-level. Selectivity and sensitivity towards other gases may be achieved by modifying the nanowire surface with different metalloporphyrins as the metal (e.g. Fe, Zn, Cu, Ni, and Co) plays a large role in their interaction with gases [17,18]. Surface passivation [32,33] may improve response times and decrease the noise level enabling a lowering of the limit of detection. Finally, we demonstrated enhanced recovery by the simple application of a higher voltage. Thus, functionalized nanowire-based devices form a promising platform for high performance nanosensors that employ direct electrical readout. References [1] F. Hernandez-Ramirez, J.D. Prades, A. Tarancon, S. Barth, O. Casals, R. JimenezDiaz, E. Pellicer, J. Rodriguez, J.R. Morante, M.A. Juli, S. Mathur, A. RomanoRodriguez, Insight into the role of oxygen diffusion in the sensing mechanisms of SnO2 nanowires, Adv. Funct. Mater. 18 (2008) 2990–2994. [2] E. Comini, C. Baratto, G. Faglia, M. Ferroni, A. Vomiero, G. Sberveglieri, Quasione dimensional metal oxide semiconductors: preparation, characterization and application as chemical sensors, Prog. Mater. Sci. 54 (2009) 1–67.

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Biographies Peter Offermans completed his doctoral research at the Technical University of Eindhoven (The Netherlands) in 2005 with a Ph.D. thesis about cross-sectional

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Scanning Tunneling Microscopy of III–V semiconductor nanostructures. Currently, he is a researcher in the Sensors and Actuators group at Holst Centre/IMEC, where he is involved in the Wireless Autonomous Transducer Solutions program. He works on the development of new technologies for chemical sensing, more specifically on optical sensors and electrical sensors based on III–V semiconductors. Mercedes Crego Calama received her Ph.D. in Chemistry (1995) on design and synthesis of artificial enzymes in Salamanca, Spain. In 1995, she moved to the University of Pittsburgh (USA), to work on oligopeptide conformation and synthetic antibodies. In 1997 she moved to the University of Twente (The Netherlands) where she worked on self- assembly and dynamic combinatorial libraries. Since 2000, she held a tenured position as associate professor. In November 2006, she started to work in the Sensors and Actuators group within the Wireless Autonomous Transducer Solutions program at Holst Centre/IMEC, where she is currently a principal researcher. Sywert H. Brongersma obtained his Ph.D. at the Free University of Amsterdam in the field of superconductivity. After a postdoc at the University of Western Ontario (Canada) concerning clustering phenomena on semiconductor surfaces, he joined the Advanced Silicon Processing division of IMEC (Leuven, Belgium) in 1998. Since 2004 he was a principal scientist in both the Cu/Low-k integration and the Nanotechnology industrial affiliation programs. In 2006, he transferred to the Wireless Autonomous Transducer Systems program at Holst Centre/IMEC, where he is a currently a principal researcher in the Sensors and Actuators group.