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porous Si
Sensors and Actuators B 146 (2010) 53–60
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Investigation of hydrogen sensing properties and aging effects of Schottky like Pd/porous Si F. Razi a , A. Iraji zad a,b,∗ , F. Rahimi a a b
Department of Physics, Sharif University of Technology, Tehran, P.O. Box 11155-9161, Iran Institute for Nanoscience and Technology (INST), Sharif University of Technology, Tehran, P.O. Box 11155-8639, Iran
a r t i c l e
i n f o
Article history: Received 31 July 2009 Received in revised form 4 January 2010 Accepted 23 January 2010 Available online 1 February 2010 Keywords: Pd Electroless Porous silicon Hydrogen gas sensor Schottky like based gas sensor
a b s t r a c t We prepared porous silicon samples coated by continuous palladium layer in electroless process. Scanning electron microscopy (SEM) showed cauliflower-shape Pd clusters on the surface. I–V curves of Schottky like Pd/porous Si samples were measured in air and in hydrogen. These measurements showed a metal–interface–semiconductor configuration rather than an ideal Schottky diode. Variations of the electrical current in the presence of diluted hydrogen at room temperature revealed that the samples can sense hydrogen in a wide range of concentration (100–40,000 ppm) without any saturation behavior. Hydrogen sensing properties of these samples were investigated at room temperature for a duration of nine months. Sample sensitivity (response time) decreased (increased) to a saturated value after 45 days. We discussed sensing and Schottky contact properties of the fresh and aged Pd/porous Si samples by variation of structure and chemical composition using SEM, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) data. © 2010 Elsevier B.V. All rights reserved.
1. Introduction In recent years, hydrogen has received considerable attention due to its applications in various industries. Since it burns with zero carbon emissions, hydrogen can be regarded as a clean and green fuel especially for the future. Thus, production, storage, transportation and finally detection and monitoring of this odorless, colorless and combustive gas have received more attention. Today there are many hydrogen gas sensors based on different mechanisms, most of them use palladium to trap hydrogen [1]. A wide branch of Pd–hydrogen sensors work on the basis of change in the work function of the Pd and electrical properties of the interface between palladium and a semiconductor (or oxide/semiconductor) as reviewed in our earlier published paper [2]. Due to the high surface area of porous silicon, capability of being a substrate in electroless process for deposition of palladium and finally the easy oxidation of this substrate in foregoing process, in our previous studies we used palladium nanoparticles over SiOx /porous silicon samples as a hydrogen sensor at room temperature [2–5]. We showed that the change in the Schottky barrier
∗ Corresponding author at: Institute for Nanoscience and Technology (INST), Sharif University of Technology, Tehran, P.O. Box 11155-8639, Iran. Tel.: +98 21 66164513; fax: +98 21 66072636. E-mail addresses: razias [email protected] (F. Razi), [email protected] (A. Iraji zad), [email protected] (F. Rahimi). 0925-4005/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2010.01.047
between palladium and silicon oxide has the most significant role in hydrogen sensing in those samples. A reliable gas sensor should have some characteristics like high sensitivity, linear response in wide gas concentration, good stability, and low response and recovery times. Low power consumption is another critical factor to be considered recently. To gain these characteristics and reach high performance, selecting a proper material is the first step. In addition, morphology, structure and chemical composition have a significant role on the operation of this material. Moreover gas sensors have many possibilities of faults which degrade reliability due to aging. Many changes can occur in Pdsemiconductor sensors that are exposed to room air such as loss of hydrogen sensitivity, together with a greatly lengthened response time. Also aging can change particle morphology, chemical composition and interfacial structure. Aging depends heavily on the environmental storage, operating conditions and also on how well the sensor materials are isolated from the environment [6]. In porous silicon (PS) based sensors non-oxidized samples show high sensitivity only for fresh devices, which reduces with the aging of the sample. Oxidation of the PS layer improves the electrical performance of sensors, in terms of stability, recovery and response time [7]. In this study, we produced porous silicon samples and coated them by continuous palladium layer by an electroless process. Hydrogen sensing properties of these samples and their change by aging were measured at room temperature. The morphology, structure and chemical composition of Pd/Porous silicon samples
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Fig. 1. Schematic representation of electrode geometry for electrical measurements.
was investigated and the effects of these factors on the hydrogen sensing properties were discussed. 2. Materials and methods Macroporous Si substrates were obtained by electrochemical etching of p-type silicon (1 0 0) wafers in a solution of 95 vol% DMF which diluted by HF. The current density and time of etching are 3.9 mA/cm2 and 30 min, respectively. Pd deposition was performed by the electroless technique immediately after porous layer formation. For this process diluted PdCl2 in water in the presence of HCl was used. The concentrations of PdCl2 and HCl (37 wt%) in the solution were 2 g l−1 and 10 ml l−1 , respectively. Plating was performed at room temperature for 5 min. Scanning electron microscopy (SEM) and energy-dispersive Xray analysis (EDX) were obtained by Philips XL30. For the X-ray photoelectron spectroscopy (XPS), an Al anode X-ray source was employed with a concentric hemispherical analyzer (Specs model EA10 plus) to analyze the surface composition. The chamber pressure during the XPS experiment was 10−9 mbar. The XRD patterns were recorded using a Philips X’pert instrument operating with ´˚ at 40 kV/40 mA. CuK␣ radiation ( = 1.54178 A) Hydrogen sensing characteristics of Pd/PS/p-type Si samples was measured at room temperature. For this purpose, they were placed in a stainless steel chamber of 10 ml volume, with several electrical feed-troughs, one gas inlet and one gas outlet. The hydrogen concentration in air carrier was from about 0.01% (100 ppm) to 4%. Schematic drawing of the electrode geometry for sensing measurement is shown in Fig. 1. The total flow rate of feed gas through the chamber was always kept at 500 sccm by using Brooks mass flow controllers. The Schottky contacts were produced by thermal evaporation of Al and Pd electroless-plating in the back of the silicon and on the PS surface, respectively. The detailed descriptions for the hydrogen detection system were presented in our previous works [2]. The response, S, is defined as: S = (Iair /Igas ) − 1 where Iair and Igas are currents of sample in dry air and in a fixed hydrogen concentration, respectively. The response time (recovery time) is defined as the time required for the sample conductance to reach (return) 90% (60%) of the total change. 3. Results 3.1. SEM and EDAX Fig. 2(a) and (b) show SEM images from top and cross-section of the fresh porous silicon after electroless deposition of palladium. A homogenous distribution of agglomerated Pd clusters covering the porous matrix is observed. The composition of these clusters was confirmed by EDAX experiment. Fig. 2(b) shows that Pd particles are partially diffused into the pores. The higher resolution images as
Fig. 2. SEM images from the surface (a) and cross-section (b) of porous silicon samples after electroless deposition of palladium. Insets in (a) and (b) show the images with higher resolution.
insets in parts (a) and (b) demonstrate cauliflower-like growth for Pd layer. The SEM data from the aged samples (after two months) presented similar images to the fresh samples.
3.2. I–V measurements Current–voltage (I–V) characteristics of the Pd/porous Si samples are shown in Fig. 3. As illustrated in Fig. 3a, the curve in air shows a rectifying behavior. We observed that after exposure to 1% hydrogen, the rectifying behavior is preserved, however the forward current decreases clearly. For a typical sample which is shown in Fig. 3, the forward current decreases by a factor of 100 under a fixed forward voltage of 1 V at 295 K. A part of these I–V curves in air and in hydrogen (between 0.1 and 0.6 V) are shown in Fig. 3b with more details. These curves are linear in small voltages; however deviation from linearity is seen by increasing voltage especially for the sample in diluted hydrogen gas. In the ideal Schottky contact sensors the I–V relationship for forward bias (V > 3kT) based on thermionic-field emission, is given by Ref. [8]:
I = Io exp
qV nkT
−1
(1)
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Fig. 4. A typical change in electrical current for a fresh sample in 1% H2 diluted with dry air, at room temperature.
metal–interface–semiconductor configuration rather than an ideal Schottky diode. (b) ˚BP of these samples compared with reported values such as those by Lundstrom et al. (0.5 eV for 94 ppm H2 ) [11], and Ruths et al. (0.21 eV for 154 ppm H2 ) [9] in Pd–SiO2 –Si structures, is very low. The reasons of this observation will be discussed in Section 4.
3.3. Hydrogen sensing measurements at room temperature Fig. 3. I–V characteristics of the typical sample at T = 295 K under atmospheric condition in air and in 1% hydrogen in air; (a) between −4 V and 2 V in linear plot (b) between 0.1 V and 0.6 V in semi-logarithmic plot. Inset in part (a) shows the plot with higher resolution.
where saturation current, Io, is given by Ref. [8]: Io = aA∗∗ T 2 exp
−q BP
kT
(2)
where a, A** and ˚BP are the active area, the effective Richardson constant and the barrier height, respectively. The ideality factor, n, for an ideal diode should be nearly equal to unity and sensitivity of this diode would be large, if ˚BP = ˚BP (air) − ˚BP (H2 ) is great. By increasing forward bias voltage, deviation from Eq. (1) happens due to voltage drop in the bulk [9]. The voltage drop is attributed to the effect of these two factors (a) interface states and (b) series resistance associated with the interface layer, substrate and ohmic back contact. In both cases, deviation increases by increasing the applied voltage. Thus, the voltage across the diode can be expressed in terms of the total voltage drop across the diode and the resistance Rs . This is accounted for by replacing the voltage V by V–IRS in Eq. (1) [8]. nkT dV = IRS + q d ln(I)
(3)
Drawing dV/dln(I) vs. I for the Pd/porous Si samples in air ambient results in a low value for RS (∼90 ). The low value of series resistance indicating clearly the good performance of the Schottky diodes. By deriving Io from Eq. (2), considering 32 Acm−2 K−2 for ptype Si [10] for effective Richardson constant and 3.67 cm2 for the active area (from SEM images) the ideality factor and ˚BP = BP (air) − BP (H2 ) would be 3.75 and 1 meV, respectively. There is two noticeable points: (a) for an ideal diode, the ideality factor (n) should be nearly equal to unity. But in a real situation, it may increase when some effects such as series resistance or leakage current come into play [8]. Thus Pd/porous Si samples obey
As shown in Fig. 3a the electrical current variation by hydrogen exposure in forward bias voltage is higher than in reversed bias. Thus, for hydrogen sensing characteristics, 1 V forward bias voltage is used. At higher voltages, although forward current shows more variation but deviation from diodic behavior is also observed. Fig. 4 shows a typical transient response of fresh Pd/porous Si sample to 1% H2 in dry air at room temperature. Region A shows the sample current in dry air. Introduction of hydrogen decreases the forward current and makes it to approach saturation (region B). By cutting off the hydrogen flow, the current increases and reaches its initial value (region C). For instance, at a fixed forward voltage, the forward current decreased from 3.95 × 10−3 A (in air) to 3.89 × 10−4 A (in 1% H2 in air), as is shown in Fig. 4. The response and recovery time for this sample is about 7 s and 27 min, respectively. The response of the fresh samples to different hydrogen concentrations at room temperature is reported in Fig. 5. It exhibits an increase in response with increasing hydrogen concentration without any saturation trend even in 40,000 ppm H2 in air for both samples (fresh and aged). In addition, samples can detect hydrogen down to level of 10 ppm (which has not shown). The Pd/SiO2 /Si based sensor was previously used for hydrogen detection at room temperature but the behavior was rather poor [12]. Therefore, increasing the temperature of Pd–SiO2 –Si structure to 150 ◦ C, make it ability to detect 1 ppm H2 in air with response and recovery times of the order of a few seconds. However, the sensitivity of this device tends to saturate at hydrogen concentration above 1000 ppm [13]. If ˚BP as a function of hydrogen concentration follows a Langmuir adsorption isotherm, change in the energy barrier will represent a linear dependency on PH2 , which PH is the partial pressure of hydrogen [11]. Therefore, by using Eq. (2), be linearly proportional to
1
1 ln(Io /Ig )
would
. Where Ig is the saturation cur-
PH
2
rent in hydrogen contained environment. The insert plot in Fig. 5 shows a linear relation between ln(I 1/I ) and 1 over the intero
g
PH
2
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Fig. 5. The response of a fresh and aged sample (after two months) as a function of hydrogen concentration in dry air measured at room temperature. Insert graph for fresh and aged sample. The corshows the linear behavior of ln(I 1/I ) vs. 1 o
g
Fig. 7. Variations of response time in different hydrogen concentrations vs. aging times.
PH
2
relation coefficients (R2 ) of these lines for fresh and aged samples are 0.9977 and 0.9894, respectively.
mediate hydrogen concentrations in fresh samples. Thus hydrogen detection follows Langmuir adsorption model. The effect of aging on hydrogen sensing properties of Pd/porous Si samples was studied. As illustrated in Fig. 5 and Fig. 6 response of samples to hydrogen decreases by aging and reaches steady states after about 45 days. As illustrated in insert of Fig. 5, the linear behavior of ln(I 1/I ) vs. 1 repeats in the aged samples. Since o
g
PH
2
the smaller intercept value with larger slope of this linear curve represents a weaker hydrogen adsorption reaction [14], weaker hydrogen detection for aged samples are expected. The values of response time to different hydrogen concentrations and their variation by aging are illustrated in Fig. 7. It presents a reduction by increasing hydrogen concentration and an increase by aging. However, response times approach to the steady states values after about 45 days in all foregoing hydrogen concentration. 3.4. XRD results
sian/Lorentzian combination peak shape by variation in peak full width at half maximum (FWHM), position and height. The raw spectra and results of these fitted data are illustrated in Fig. 8(a–d). The XRD spectrum of the fresh sample (Fig. 8a) shows a relatively strong Si (1 0 0), face centered cubic (fcc) Pd (2 0 0), Pd(1 1 1) and Pd3 Si (1 2 1) reflection peaks which is in conformity with reference data (JCPDS numbers 271402, 050681, 011310 and 360932, respectively). The existence of both (2 0 0) and (1 1 1) crystal faces reveals a polycrystalline nature of Pd lattice. This observation confirms the presence of grain boundaries and vacancies in this sample. The values of lattice parameter, d, (by Bragg formula), for the (1 1 1) and (2 0 0) reflections of the Pd layer in fresh samples are equal to a1 1 1 = 3.856 Å and a2 0 0 = 3.888 Å which are slightly smaller than the standard value for bulk Pd (a1 1 1 = 3.89 Å, a2 0 0 = 3.95 Å). Since the smaller d value gives compressive stress along the given hkl phase [15], the fresh Pd layer has a compressive stress along (1 1 1) and (2 0 0) directions. A noticeable point in Fig. 8a is the existence of palladium silicide in the fresh sample. It is well known that a silicon oxide layer between silicon and palladium prevent migration of Si into Pd or vice versa. Otherwise formation of palladium silicide in the metal-semiconductor structures can occur without heat treatment [16]. As is shown in our previous work, deposi-
To understand variation of sensing properties as a function of aging time and the saturation behavior, we studied the sample’s structure by X-ray diffraction (XRD) method. XRD experiments were carried on the fresh (electroless-deposited Pd on the porous silicon immediate after preparation) and aged (two months after synthesis) samples. Deconvolution of the spectra was achieved by an iterative program. In this program data are fitted with a Gaus-
Fig. 6. Variations of response in different hydrogen concentrations vs. aging times.
Fig. 8. X-ray diffractograms of Pd thin films deposited on porous silicon by electroless technique after manufacturing and two months later in (a) survey mode, (b) range: 2 = 39–42◦ , (c) range: 2 = 45–48◦ and (d) range: 2 = 68–70◦ .
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Fig. 9. Grazing XRD patterns of Pd thin films deposited on porous silicon by electroless technique after manufacturing and two months later.
tion of palladium on porous silicon in the electroless process takes place simultaneously with silicon oxidation [2]. Thus the observation of Pd3 Si (in fresh samples) may have arisen from the presence of hydrofluoric acid-etched Si surface before palladium deposition [17], the insufficient thickness of oxide as mentioned by Yalcin and Avci [18] or the existence of discontinuous oxide layer between Pd and Si. The XRD spectrum for the aged sample (Fig. 8a) has moderately strong Si (1 0 0) and polycrystalline fcc Pd in (2 0 0) and (1 1 1) directions. As the XRD data shows, aging produces the following changes in the sample structure: First, the Pd3 Si (1 2 1) reflection peak that appeared in the fresh sample, disappeared in the aged one. However, the different stoichiometries of PdSi becomes visible in deconvolution of Pd and Si peaks as demonstrated in Fig. 8b–d. Second, the aging process increases the lattice parameters of Pd along the (1 1 1) and (2 0 0) directions slightly (a1 1 1 = 3.881 Å and
57
a2 0 0 = 3.892 Å) and makes them nearer to the standard value for bulk Pd. Thus, the compressive stress along these two directions decreases [15]. Third, the results of deconvolution of Pd and Si peaks show that the full width at half maximum (FWHM) of the characteristic peaks of the Pd reduces in the aged samples. It means that the grain size of palladium increases in the aged samples. Finally, the ratio of SiO2 /Si and PdSi/Pd after two months which deduced from these fitted data, demonstrates an increasing from 4.5 × 10−4 to 1256 × 10−4 and 1.48 to 153.67, respectively. It illustrates that the conversion of silicon to silicon oxide and palladium to palladium silicide takes place even at room temperature. In summary, aging decreases the strength and the area of Si and Pd peaks. This reduction is perhaps due to reduction of crystallinity of silicon and palladium after oxidation or silicide formation, respectively. To gain more information from the aging effects, grazing XRD (GXRD) spectrum was also obtained from the fresh and aged samples. We observed only Pd on the surface of the fresh sample and no evidence for Si (Fig. 9). Since the X-ray beam impacts on the surface with a grazing angle, information of GXRD belongs on top of the pores. Therefore, absence of Si peaks shows top of the pores is completely covered by palladium with at least 20 nm average thicknesses. Decline of the Pd peak intensities in the aged samples may have arisen from diffusion of Pd in Si substrate or decrease of palladium crystallinity. Since the XRD data shows that the grain size of palladium increases in the aged samples, decreasing in crystallinity may be due to oxygen reaction with palladium structure. 3.5. XPS results To confirm change in surface composition we performed Xray photoelectron spectroscopy (XPS) experiments. These trials were carried on fresh and aged (as defined in Section 3.4) samples. Deconvolution of the spectra was achieved by fitting the data with a Gaussian/Lorentzian combination peak shape as explained in the previous section. The survey and deconvolution spectra for fresh and aged samples are illustrated in (Fig. 10a–c).
Fig. 10. XPS spectra for the fresh and aged sample in (a) survey mode, (b) Si2p peaks, (c) Pd3d peaks.
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The surface of fresh samples consisted of very small silicon, chlorine, and carbon contamination, oxygen and large amounts of palladium (Fig. 10a). Deconvolution of Si2p peak in Fig. 10b confirms the existence of SiO2 and Si simultaneously. This is due to the oxidation process in the electroless technique [2]. Because of the grazing angle in GXRD, this method cannot show the interior of the pores and therefore we guess this silicon peak arises from the bottom of pores. Deconvolution of Pd3d peaks in Fig. 10c shows that the observed binding energy (BE) peak at 336.08 eV in the fresh sample corresponds to the Pd (3d5/2 ) in the oxidized state. Oxidation of the metallic palladium to PdOx may have arisen from the presence of the dissolved oxygen in water in the electroless process or rinsing with deionised water after this process [19]. Whereas XRD data did not show any sign of PdO crystalline, we suggest that PdO thickness is very small or in amorphous state. Although the survey XPS spectrum of the aged sample shows a similar materials as fresh one but deconvolution of the Si2p (Fig. 10b) and Pd3d (Fig. 10c) peaks demonstrates that some changes take place. First, the Pd/Si atomic ratio decreases from 4.68 to 2.35 in the aged sample. The formation of both silicide compounds in interface and oxide phase of palladium can reduce the Pd/Si atomic ratio. Second, SiO2 /Si atomic ratio increases from 0.68 to 0.77. Finally, Pd/PdO (3d5/2 ) atomic ratio decreases from 0.79 to 0.62. 4. Discussion According to XRD and XPS results, we consider the schematic representation of fresh Pd/porous silicon samples as shown in Fig. 11a. SiO2 , PdSix , Pd, PdOx and surface contaminations cover the porous silicon, respectively. Due to the granularity of the palladium layer (SEM results) and polycrystallinity of each grain (XRD results), this layer consists of many grain boundaries. Low temperature coating of Pd (room temperature) and polycrystallinity of Pd layer (XRD results) ensure us to have many defects and vacancies in each granule of Pd (insert of part a in Fig. 11). Since our I–V measurements show that Pd/porous Si samples obey the metal–interface–semiconductor configuration rather than an ideal Schottky diode, equivalent circuit of these samples is considered as part b in Fig. 11. This means that the electrical current passes through the Pd layer then silicide zone (with together
as R1 ) and traverses from the Schottky barrier (as D1 ) formed between palladium silicide and Si and finally passes through the Si substrate. When hydrogen molecules are introduced to the samples, they are dissociately adsorbed on the Pd surface and penetrate to the bulk. Generally, there are three places for atomic hydrogen population at Pd bulk: First, octahedral interstitial sites in the fcc lattice [20,21]; hydrogen atom at these sites form a solid solution of Pd–H at low (␣-phase) and a hydride (-phase) at high hydrogen pressures [22]. Phase transitions (␣ → ) (∼1%H2 at sea level and room temperature for bulk Pd [23,24]) are accompanied by changes of palladium volume. At -phase, hydrogen absorption can induce internal stresses being maintained in dislocation-free crystals of palladium [25]. In addition, these atoms of hydrogen in octahedral interstitial sites increase the Pd electrical resistance [24]. Second, trapping sites at defects [26,27]; this trapping decreases the hydrogen diffusion flux especially at room temperature [26]. Finally deep adsorption sites near the Pd–Si interface [28–30]. In the presence of Pdx Si, the Schottky contact is formed in the junction of Pdx Si and Si (Pdx Si has high electrical conductivity [31]). Therefore, in these cases the third sites for hydrogen adsorption are those near the palladium silicide/silicon oxide interface [10]. To capture those sites, hydrogen atoms should transit from the Pd to Pdx Si zone and finally reach the Pdx Si/SiO2 interface. The hydrogen atoms at Pdx Si/SiO2 interface form a dipole layer that decreases the work function of the palladium silicide and consequently the Schottky barrier’s height (˚BP ) at this interface increases [10]. Resistivity enhancement of the serried resistance in the equivalent circuit (part b in Fig. 11) and especially increase in the Schottky barrier height by introduction of hydrogen decreases the current at forward bias as illustrated in Fig. 3 and Fig. 4. The oxygen and other contaminations on the surface of palladium block the surface sites thereby prevent adsorption of H2 [32,33]. Since, the rate-limiting step for hydrogen permeation through palladium is mainly reactions that take place on the Pd surface, when other gases are introduced together with hydrogen [32]; hydrogen permeation is declined by surface contamination. In agreement with representations illustrated in Fig. 11 and foregoing discussion in the previous paragraphs, hydrogen sensing properties of Pd/porous Si samples are justified as below:
Fig. 11. (a) Schematic representation and (b) equivalent circuit for fresh sample. Insert in part (a) is a granule of Pd contains vacancies, defects and grain boundaries.
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Fig. 12. (a) Schematic representation and (b) equivalent circuit for aged sample. Insert in part (a) is a granule of Pd contains vacancies, defects and grain boundaries.
A series resistance due to the existence of different layers (surface contamination, PdOy , Pd, Pdx Si, . . .) besides the existence of many defects in these structures comes into play (part b in Fig. 11) and makes the ideality factor greater than unity (Section 3.2). Due to the existence of oxygen and other contaminations at the surface and defects in the structure of the deposited layer, hydrogen diffusion flux through Pd decreases. In addition the diffusion coefficient of hydrogen on palladium silicide is approximately one order of magnitude lower than those obtained for metallic Pd [34]. All foregoing phenomena retard the reach of hydrogen atoms to the Pdx Si/SiO2 interface. Thus, the response time increases especially at low hydrogen concentrations (parts of data in Fig. 7). On the other hand, existence of the defect in the structure of the deposited layer can decrease the number of hydrogen atoms that reach the Pdx Si/SiO2 interface and reduce the variation of Schottky barrier height, ˚BP (Section 3.2). By cutting off the hydrogen gas flow, all reactions take place in the reverse direction and finally hydrogen atoms desorbs as H2 molecules. As a result of these reactions the electrical current approaches to its initial value. However, two factors delay this phenomenon: (1) defects in the deposited layer which trap hydrogen atoms and retard the returning of resistivity (serried resistance) to the initial value and (2) especially the defects at the Pdx Si/SiO2 interface trap hydrogen atoms and prevent returning of the Schottky barrier height to the initial value. These factors increases the recovery time as illustrated in Fig. 4. As discussed in Sections 3.4 and 3.5, aging makes some changes on Pd/porous Si samples: (1) increase in oxygen and other contaminations on the surface, (2) increase in SiO2 and Pdx Si values at the interface between palladium and silicon and (3) decrease in the crystallinity of Pd layers. Decreasing the crystallinity increases the defects in the structure of deposited layer. Additional defects were generated in the palladium layer in the ␣ ↔  phase transition (∼1%H2 at sea level and room temperature for bulk Pd) following successive addition and removal of hydrogen, respectively [26]. These changes are illustrated schematically for aged samples in Fig. 12. By considering the foregoing changes in the aged samples and regarding the discussions of previous paragraphs about the fresh ones, it is acceptable that the sensitivity decreases and the response
times increase by aging. However, after about 45 days all changes take place and samples reach a steady state. 5. Conclusion In the present paper macroporous silicon samples were coated by a continuous palladium layer in the electroless process. Our I–V measurements show that samples obey the metal–interface–semiconductor configuration rather than an ideal Schottky diode. Schottky like contacts of Pd/PS/p-type Si samples can detect hydrogen in a wide range of concentration (10–40,000 ppm) at room temperature. The sensitivity of the samples increases with hydrogen concentration and does not saturate. However, response time in low hydrogen concentration and recovery time in all foregoing hydrogen concentration are long. Characterization of the fresh samples shows existence of palladium oxide, chlorine and carbon contamination at the surface. In addition SiO2 and Pdx Si are in the metal-semiconductor interface. Aging decreases the hydrogen sensitivity and increases the response time due to increases in the amounts of oxides, carbon contamination, Pdx Si and defects. Nevertheless, sensitivity parameters reach to steady states after about 45 days. We attributed the low variation of the Schottky barrier height in hydrogen sensitivity and high response time in fresh sensors to: (i) work at room temperature, (ii) presence of trap states and defects in Pd lattice with high binding energy and (iii) existence of oxygen and other impurities on the Pd surface. We ascribed the losses of hydrogen sensitivity together with a greatly lengthened response time in the aged samples to an increase of: carbon contaminations and PdO at the surface, defects in palladium film and silicide layer formation at the interface. Acknowledgements This work was supported by the Sharif University of Technology research department. The authors would like to thank Mr. Rezaie and Mrs. Fardindust for performing SEM images and XRD analysis, respectively and Mr. Rafiee for doing XPS analysis.
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References [1] C. Christofides, A. Mandelis, Solids-state sensors for trace hydrogen gas detection, J. Appl. Phys. 68 (1990) R1–R30. [2] F. Rahimi, A. Iraji zad, Characterization of Pd nanoparticle dispersed over porous silicon as a hydrogen sensor, J. Phys. D: Appl. Phys. 40 (2007) 7201–7209. [3] F. Rahimi, A. Iraji zad, F. Razi, Characterization of porous poly-silicon impregnated with Pd as a hydrogen sensor, J. Phys. D: Appl. Phys. 38 (2005) 36–40. [4] F. Rahimi, A. Iraji zad, Effective factors on Pd growth on porous silicon by electroless-plating: response to hydrogen, Sens. Actuators B 115 (2006) 164–169. [5] F. Rahimi, A. Iraji zad, F. Razi, Palladium plating on macroporous/microporous silicon: application as a hydrogen sensor, Synthesis React. Inorg. Metal-Organ. Nano-Metal Chem. 37 (2007) 377–380. [6] X. Wang, M. Ye, Hysteresis and nonlinearity compensation of relative humidity sensor using support vector machines, Sens. Actuators B 129 (2008) 274–284. [7] G. Barillaro, A. Diligenti, A. Nannini, L.M. Strambini, E. Comini, G. Sberveglieri, Low-concentration NO2 detection with an adsorption porous silicon FET, IEEE Sens. J. 6 (2006) 19–23. [8] K. Bourenane, A. Keffous, G. Nezzal, A. Bourenane, Y. Boukennous, A. Boukezzata, Influence of thickness and porous structure of SiC layers on the electrical properties of Pt/SiC-pSi and Pd/SiC-pSi Schottky diodes for gas sensing purposes, Sens. Actuators B 129 (2008) 612–620. [9] P. Ruths, S. Ashok, S.J. Fonash, J.M. Ruths, A study of Pd/Si MIS Schottky barrier diode hydrogen detector, IEEE Trans. Electron Dev. ED-28 (1981) 1003–1009. [10] S. Chand, J. Kumari, Current–voltage characteristics of Pd2 Si based Schottky diodes on p-type (1 1 1) silicon and evaluation of their barrier heights, SolidState Electron. 38 (1995) 1103–1104. [11] I. Lundstrom, T. DiStefano, Hydrogen induced interfacial polarization at Pd–SiO2 interfaces, Surf. Sci. 59 (1976) 23–32. [12] I. Lundström, S. Shivaraman, C. Svensson, L. Lundkvist, A hydrogen-sensitive MOS field-effect transistor, Appl. Phys. Lett. 26 (1975) 55–57. [13] L. Stiblert, C. Svensson, Hydrogen leak detector using a Pd-gate MOS transistor, Rev. Sci. Instrum. 46 (1975) 1206–1208. [14] C.-C. Cheng, Y.-Y. Tsai, K.-W. Lin, H.-I. Chen, C.-T. Lu, W.-C. Liu, Hydrogen sensing characteristics of a Pt–oxide–Al0.3 Ga0.7 As MOS Schottky diode, Sens. Actuators B 99 (2004) 425–430. [15] R. Chan, PhD Thesis, Interfacial electrochemistry and surface characterization Hydrogen terminated Silicon, electrolessly deposited Palladium and Platinum on pyrolyzed photoresist films and electrodeposited Copper on Iridium, 2003, University of North Texas, Texas. [16] L.L. Tongson, B.E. Knox, T.E. Sullivan, S.J. Fonash, Comparative study of chemical and polarization characteristics of Pd/Si and Pd/SiO2 /Si Schottky barrier-type devices, J. Appl. Phys. 50 (1979) 1535–1537. [17] H. Kobayashi, H. Kawa, T. Yuasa, Y. Nakato, K. Yoneda, Palladium-promoted oxidation of Si at low temperatures, Appl. Surf. Sci. 113/114 (1997) 590–594. [18] S. Yalcin, R. Avci, Characterization of PdAu thin films on oxidized silicon wafers: interdiffusion and reaction, Appl. Surf. Sci. 214 (2003) 319–337. [19] E. Natividad, E. Lataste, M. Lahaye, J.M. Heintz, J.F. Silvain, Chemical and morphological study of the sensitisation, activation and Cu electroless-plating of Al2 O3 polycrystalline substrate, Surf. Sci. 557 (2004) 129–143.
[20] M. Eriksson, L.-G. Ekedahl, Hydrogen adsorption states at the Pd/SiO2 interface and simulation of the response of a Pd metal-oxide-semiconductor hydrogen sensor, J. Appl. Phys. 83 (1998) 3947–3951. [21] L.L. Jewell, B.H. Davis, Review of absorption and adsorption in the hydrogen–palladium system, Appl. Catal. A: Gen. 310 (2006) 1–15. [22] M.K. Kumar, M.S. Ramachandra Rao, S.R. Amaprabhu, Structural, morphological and hydrogen sensing studies on pulsed laser deposited nanostructured palladium thin films, J. Phys. D: Appl. Phys. 39 (2006) 2791–2795. [23] G. Kaltenpoth, P. Schnabel, E. Menke, E.C. Walter, M. Grunze, R.M. Penner, Multimode detection of hydrogen gas using palladium-covered silicon -channels, Anal. Chem. 75 (2003) 4756–4765. [24] F.A. Lewis, The Palladium–Hydrogen System, Academic Press, New York, 1968. [25] W.S. Zhang, Z.L. Zhang, X.W. Zhang, Effects of self-stress on the hydrogen absorption into palladium hydride electrodes of plate form under galvanostatic conditions, J. Electroanal. Chem. 474 (1999) 130–137. [26] R.V. Bucur, Effect of trapping on the solubility and diffusivity of hydrogen in palladium (␣-phase), J. Mater. Sci. 22 (1987) 3402–3406. [27] J. Will Medlinl, A.E. Lutz, R. Bastasz, A.H. McDaniel, The response of palladium metal-insulator-semiconductor devices to hydrogen–oxygen mixtures: comparisons between kinetic models and experiment, Sens. Actuators B 96 (2003) 290–297. [28] A. Salehi, A. Nikfarjam, D. Jamshidi Kalantari, Pd/porous-GaAs Schottky contact for hydrogen sensing application, Sens. Actuators B 113 (2006) 419–427. [29] I. Lundstrom, Hydrogen-sensitive MOS-structures part 1. Principles and applications, Sens. Actuators 1 (1981) 403–426. [30] I. Lundstrom, D. Soderberg, Hydrogen-sensitive MOS-structures part 2. Characterization, Sens. Actuators 2 (1981/1982) 105–138. [31] T. Utsumi, Keynote address microelectronics: what’s new and exciting, IEEE Trans. Electron Dev. 38 (1991) 2276–2283. [32] H. Amandusson, L.-G. Ekedahl, H. Dannetun, The effect of CO and O2 on hydrogen permeation through a palladium membrane, Appl. Surf. Sci. 153 (2000) 259–267. [33] J.W. Medlin, A.H. McDaniel, M.D. Allendorf, R. Bastasz, Effects of competitive carbon monoxide adsorption on the hydrogen response of metal–insulator–semiconductor sensors: the role of metal film morphology, J. Appl. Phys. 93 (2003) 2267–2274. [34] V.A. Paganin, A.A. Tanaka, L.A. Avaca, Voltametric behavior of amorphous Pd–Si alloys in acid and alkaline solutions, J. Braz. Chem. Soc. 3 (1992) 106–111.
Biographies F. Razi is a PhD student in Physics Department, Sharif University of Technology. Her research interests are thin films and surface physics. She is working on gas sensors. A. Iraji zad, professor in the Physics Department and head of the Institute for Nanoscience and Nanotechnology, Sharif University of Technology. Main interests are surface science and nanophysics. F. Rahimi received her PhD degree in thin film gas sensors at Physics Department, Sharif University of Technology, Tehran, Iran in 2006. Her research interests are thin films and surface physics.
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Investigation of hydrogen sensing properties and aging effects of Schottky like Pd/porous Si F. Razi a , A. Iraji zad a,b,∗ , F. Rahimi a a b
Department of Physics, Sharif University of Technology, Tehran, P.O. Box 11155-9161, Iran Institute for Nanoscience and Technology (INST), Sharif University of Technology, Tehran, P.O. Box 11155-8639, Iran
a r t i c l e
i n f o
Article history: Received 31 July 2009 Received in revised form 4 January 2010 Accepted 23 January 2010 Available online 1 February 2010 Keywords: Pd Electroless Porous silicon Hydrogen gas sensor Schottky like based gas sensor
a b s t r a c t We prepared porous silicon samples coated by continuous palladium layer in electroless process. Scanning electron microscopy (SEM) showed cauliflower-shape Pd clusters on the surface. I–V curves of Schottky like Pd/porous Si samples were measured in air and in hydrogen. These measurements showed a metal–interface–semiconductor configuration rather than an ideal Schottky diode. Variations of the electrical current in the presence of diluted hydrogen at room temperature revealed that the samples can sense hydrogen in a wide range of concentration (100–40,000 ppm) without any saturation behavior. Hydrogen sensing properties of these samples were investigated at room temperature for a duration of nine months. Sample sensitivity (response time) decreased (increased) to a saturated value after 45 days. We discussed sensing and Schottky contact properties of the fresh and aged Pd/porous Si samples by variation of structure and chemical composition using SEM, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) data. © 2010 Elsevier B.V. All rights reserved.
1. Introduction In recent years, hydrogen has received considerable attention due to its applications in various industries. Since it burns with zero carbon emissions, hydrogen can be regarded as a clean and green fuel especially for the future. Thus, production, storage, transportation and finally detection and monitoring of this odorless, colorless and combustive gas have received more attention. Today there are many hydrogen gas sensors based on different mechanisms, most of them use palladium to trap hydrogen [1]. A wide branch of Pd–hydrogen sensors work on the basis of change in the work function of the Pd and electrical properties of the interface between palladium and a semiconductor (or oxide/semiconductor) as reviewed in our earlier published paper [2]. Due to the high surface area of porous silicon, capability of being a substrate in electroless process for deposition of palladium and finally the easy oxidation of this substrate in foregoing process, in our previous studies we used palladium nanoparticles over SiOx /porous silicon samples as a hydrogen sensor at room temperature [2–5]. We showed that the change in the Schottky barrier
∗ Corresponding author at: Institute for Nanoscience and Technology (INST), Sharif University of Technology, Tehran, P.O. Box 11155-8639, Iran. Tel.: +98 21 66164513; fax: +98 21 66072636. E-mail addresses: razias [email protected] (F. Razi), [email protected] (A. Iraji zad), [email protected] (F. Rahimi). 0925-4005/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2010.01.047
between palladium and silicon oxide has the most significant role in hydrogen sensing in those samples. A reliable gas sensor should have some characteristics like high sensitivity, linear response in wide gas concentration, good stability, and low response and recovery times. Low power consumption is another critical factor to be considered recently. To gain these characteristics and reach high performance, selecting a proper material is the first step. In addition, morphology, structure and chemical composition have a significant role on the operation of this material. Moreover gas sensors have many possibilities of faults which degrade reliability due to aging. Many changes can occur in Pdsemiconductor sensors that are exposed to room air such as loss of hydrogen sensitivity, together with a greatly lengthened response time. Also aging can change particle morphology, chemical composition and interfacial structure. Aging depends heavily on the environmental storage, operating conditions and also on how well the sensor materials are isolated from the environment [6]. In porous silicon (PS) based sensors non-oxidized samples show high sensitivity only for fresh devices, which reduces with the aging of the sample. Oxidation of the PS layer improves the electrical performance of sensors, in terms of stability, recovery and response time [7]. In this study, we produced porous silicon samples and coated them by continuous palladium layer by an electroless process. Hydrogen sensing properties of these samples and their change by aging were measured at room temperature. The morphology, structure and chemical composition of Pd/Porous silicon samples
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Fig. 1. Schematic representation of electrode geometry for electrical measurements.
was investigated and the effects of these factors on the hydrogen sensing properties were discussed. 2. Materials and methods Macroporous Si substrates were obtained by electrochemical etching of p-type silicon (1 0 0) wafers in a solution of 95 vol% DMF which diluted by HF. The current density and time of etching are 3.9 mA/cm2 and 30 min, respectively. Pd deposition was performed by the electroless technique immediately after porous layer formation. For this process diluted PdCl2 in water in the presence of HCl was used. The concentrations of PdCl2 and HCl (37 wt%) in the solution were 2 g l−1 and 10 ml l−1 , respectively. Plating was performed at room temperature for 5 min. Scanning electron microscopy (SEM) and energy-dispersive Xray analysis (EDX) were obtained by Philips XL30. For the X-ray photoelectron spectroscopy (XPS), an Al anode X-ray source was employed with a concentric hemispherical analyzer (Specs model EA10 plus) to analyze the surface composition. The chamber pressure during the XPS experiment was 10−9 mbar. The XRD patterns were recorded using a Philips X’pert instrument operating with ´˚ at 40 kV/40 mA. CuK␣ radiation ( = 1.54178 A) Hydrogen sensing characteristics of Pd/PS/p-type Si samples was measured at room temperature. For this purpose, they were placed in a stainless steel chamber of 10 ml volume, with several electrical feed-troughs, one gas inlet and one gas outlet. The hydrogen concentration in air carrier was from about 0.01% (100 ppm) to 4%. Schematic drawing of the electrode geometry for sensing measurement is shown in Fig. 1. The total flow rate of feed gas through the chamber was always kept at 500 sccm by using Brooks mass flow controllers. The Schottky contacts were produced by thermal evaporation of Al and Pd electroless-plating in the back of the silicon and on the PS surface, respectively. The detailed descriptions for the hydrogen detection system were presented in our previous works [2]. The response, S, is defined as: S = (Iair /Igas ) − 1 where Iair and Igas are currents of sample in dry air and in a fixed hydrogen concentration, respectively. The response time (recovery time) is defined as the time required for the sample conductance to reach (return) 90% (60%) of the total change. 3. Results 3.1. SEM and EDAX Fig. 2(a) and (b) show SEM images from top and cross-section of the fresh porous silicon after electroless deposition of palladium. A homogenous distribution of agglomerated Pd clusters covering the porous matrix is observed. The composition of these clusters was confirmed by EDAX experiment. Fig. 2(b) shows that Pd particles are partially diffused into the pores. The higher resolution images as
Fig. 2. SEM images from the surface (a) and cross-section (b) of porous silicon samples after electroless deposition of palladium. Insets in (a) and (b) show the images with higher resolution.
insets in parts (a) and (b) demonstrate cauliflower-like growth for Pd layer. The SEM data from the aged samples (after two months) presented similar images to the fresh samples.
3.2. I–V measurements Current–voltage (I–V) characteristics of the Pd/porous Si samples are shown in Fig. 3. As illustrated in Fig. 3a, the curve in air shows a rectifying behavior. We observed that after exposure to 1% hydrogen, the rectifying behavior is preserved, however the forward current decreases clearly. For a typical sample which is shown in Fig. 3, the forward current decreases by a factor of 100 under a fixed forward voltage of 1 V at 295 K. A part of these I–V curves in air and in hydrogen (between 0.1 and 0.6 V) are shown in Fig. 3b with more details. These curves are linear in small voltages; however deviation from linearity is seen by increasing voltage especially for the sample in diluted hydrogen gas. In the ideal Schottky contact sensors the I–V relationship for forward bias (V > 3kT) based on thermionic-field emission, is given by Ref. [8]:
I = Io exp
qV nkT
−1
(1)
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55
Fig. 4. A typical change in electrical current for a fresh sample in 1% H2 diluted with dry air, at room temperature.
metal–interface–semiconductor configuration rather than an ideal Schottky diode. (b) ˚BP of these samples compared with reported values such as those by Lundstrom et al. (0.5 eV for 94 ppm H2 ) [11], and Ruths et al. (0.21 eV for 154 ppm H2 ) [9] in Pd–SiO2 –Si structures, is very low. The reasons of this observation will be discussed in Section 4.
3.3. Hydrogen sensing measurements at room temperature Fig. 3. I–V characteristics of the typical sample at T = 295 K under atmospheric condition in air and in 1% hydrogen in air; (a) between −4 V and 2 V in linear plot (b) between 0.1 V and 0.6 V in semi-logarithmic plot. Inset in part (a) shows the plot with higher resolution.
where saturation current, Io, is given by Ref. [8]: Io = aA∗∗ T 2 exp
−q BP
kT
(2)
where a, A** and ˚BP are the active area, the effective Richardson constant and the barrier height, respectively. The ideality factor, n, for an ideal diode should be nearly equal to unity and sensitivity of this diode would be large, if ˚BP = ˚BP (air) − ˚BP (H2 ) is great. By increasing forward bias voltage, deviation from Eq. (1) happens due to voltage drop in the bulk [9]. The voltage drop is attributed to the effect of these two factors (a) interface states and (b) series resistance associated with the interface layer, substrate and ohmic back contact. In both cases, deviation increases by increasing the applied voltage. Thus, the voltage across the diode can be expressed in terms of the total voltage drop across the diode and the resistance Rs . This is accounted for by replacing the voltage V by V–IRS in Eq. (1) [8]. nkT dV = IRS + q d ln(I)
(3)
Drawing dV/dln(I) vs. I for the Pd/porous Si samples in air ambient results in a low value for RS (∼90 ). The low value of series resistance indicating clearly the good performance of the Schottky diodes. By deriving Io from Eq. (2), considering 32 Acm−2 K−2 for ptype Si [10] for effective Richardson constant and 3.67 cm2 for the active area (from SEM images) the ideality factor and ˚BP = BP (air) − BP (H2 ) would be 3.75 and 1 meV, respectively. There is two noticeable points: (a) for an ideal diode, the ideality factor (n) should be nearly equal to unity. But in a real situation, it may increase when some effects such as series resistance or leakage current come into play [8]. Thus Pd/porous Si samples obey
As shown in Fig. 3a the electrical current variation by hydrogen exposure in forward bias voltage is higher than in reversed bias. Thus, for hydrogen sensing characteristics, 1 V forward bias voltage is used. At higher voltages, although forward current shows more variation but deviation from diodic behavior is also observed. Fig. 4 shows a typical transient response of fresh Pd/porous Si sample to 1% H2 in dry air at room temperature. Region A shows the sample current in dry air. Introduction of hydrogen decreases the forward current and makes it to approach saturation (region B). By cutting off the hydrogen flow, the current increases and reaches its initial value (region C). For instance, at a fixed forward voltage, the forward current decreased from 3.95 × 10−3 A (in air) to 3.89 × 10−4 A (in 1% H2 in air), as is shown in Fig. 4. The response and recovery time for this sample is about 7 s and 27 min, respectively. The response of the fresh samples to different hydrogen concentrations at room temperature is reported in Fig. 5. It exhibits an increase in response with increasing hydrogen concentration without any saturation trend even in 40,000 ppm H2 in air for both samples (fresh and aged). In addition, samples can detect hydrogen down to level of 10 ppm (which has not shown). The Pd/SiO2 /Si based sensor was previously used for hydrogen detection at room temperature but the behavior was rather poor [12]. Therefore, increasing the temperature of Pd–SiO2 –Si structure to 150 ◦ C, make it ability to detect 1 ppm H2 in air with response and recovery times of the order of a few seconds. However, the sensitivity of this device tends to saturate at hydrogen concentration above 1000 ppm [13]. If ˚BP as a function of hydrogen concentration follows a Langmuir adsorption isotherm, change in the energy barrier will represent a linear dependency on PH2 , which PH is the partial pressure of hydrogen [11]. Therefore, by using Eq. (2), be linearly proportional to
1
1 ln(Io /Ig )
would
. Where Ig is the saturation cur-
PH
2
rent in hydrogen contained environment. The insert plot in Fig. 5 shows a linear relation between ln(I 1/I ) and 1 over the intero
g
PH
2
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Fig. 5. The response of a fresh and aged sample (after two months) as a function of hydrogen concentration in dry air measured at room temperature. Insert graph for fresh and aged sample. The corshows the linear behavior of ln(I 1/I ) vs. 1 o
g
Fig. 7. Variations of response time in different hydrogen concentrations vs. aging times.
PH
2
relation coefficients (R2 ) of these lines for fresh and aged samples are 0.9977 and 0.9894, respectively.
mediate hydrogen concentrations in fresh samples. Thus hydrogen detection follows Langmuir adsorption model. The effect of aging on hydrogen sensing properties of Pd/porous Si samples was studied. As illustrated in Fig. 5 and Fig. 6 response of samples to hydrogen decreases by aging and reaches steady states after about 45 days. As illustrated in insert of Fig. 5, the linear behavior of ln(I 1/I ) vs. 1 repeats in the aged samples. Since o
g
PH
2
the smaller intercept value with larger slope of this linear curve represents a weaker hydrogen adsorption reaction [14], weaker hydrogen detection for aged samples are expected. The values of response time to different hydrogen concentrations and their variation by aging are illustrated in Fig. 7. It presents a reduction by increasing hydrogen concentration and an increase by aging. However, response times approach to the steady states values after about 45 days in all foregoing hydrogen concentration. 3.4. XRD results
sian/Lorentzian combination peak shape by variation in peak full width at half maximum (FWHM), position and height. The raw spectra and results of these fitted data are illustrated in Fig. 8(a–d). The XRD spectrum of the fresh sample (Fig. 8a) shows a relatively strong Si (1 0 0), face centered cubic (fcc) Pd (2 0 0), Pd(1 1 1) and Pd3 Si (1 2 1) reflection peaks which is in conformity with reference data (JCPDS numbers 271402, 050681, 011310 and 360932, respectively). The existence of both (2 0 0) and (1 1 1) crystal faces reveals a polycrystalline nature of Pd lattice. This observation confirms the presence of grain boundaries and vacancies in this sample. The values of lattice parameter, d, (by Bragg formula), for the (1 1 1) and (2 0 0) reflections of the Pd layer in fresh samples are equal to a1 1 1 = 3.856 Å and a2 0 0 = 3.888 Å which are slightly smaller than the standard value for bulk Pd (a1 1 1 = 3.89 Å, a2 0 0 = 3.95 Å). Since the smaller d value gives compressive stress along the given hkl phase [15], the fresh Pd layer has a compressive stress along (1 1 1) and (2 0 0) directions. A noticeable point in Fig. 8a is the existence of palladium silicide in the fresh sample. It is well known that a silicon oxide layer between silicon and palladium prevent migration of Si into Pd or vice versa. Otherwise formation of palladium silicide in the metal-semiconductor structures can occur without heat treatment [16]. As is shown in our previous work, deposi-
To understand variation of sensing properties as a function of aging time and the saturation behavior, we studied the sample’s structure by X-ray diffraction (XRD) method. XRD experiments were carried on the fresh (electroless-deposited Pd on the porous silicon immediate after preparation) and aged (two months after synthesis) samples. Deconvolution of the spectra was achieved by an iterative program. In this program data are fitted with a Gaus-
Fig. 6. Variations of response in different hydrogen concentrations vs. aging times.
Fig. 8. X-ray diffractograms of Pd thin films deposited on porous silicon by electroless technique after manufacturing and two months later in (a) survey mode, (b) range: 2 = 39–42◦ , (c) range: 2 = 45–48◦ and (d) range: 2 = 68–70◦ .
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Fig. 9. Grazing XRD patterns of Pd thin films deposited on porous silicon by electroless technique after manufacturing and two months later.
tion of palladium on porous silicon in the electroless process takes place simultaneously with silicon oxidation [2]. Thus the observation of Pd3 Si (in fresh samples) may have arisen from the presence of hydrofluoric acid-etched Si surface before palladium deposition [17], the insufficient thickness of oxide as mentioned by Yalcin and Avci [18] or the existence of discontinuous oxide layer between Pd and Si. The XRD spectrum for the aged sample (Fig. 8a) has moderately strong Si (1 0 0) and polycrystalline fcc Pd in (2 0 0) and (1 1 1) directions. As the XRD data shows, aging produces the following changes in the sample structure: First, the Pd3 Si (1 2 1) reflection peak that appeared in the fresh sample, disappeared in the aged one. However, the different stoichiometries of PdSi becomes visible in deconvolution of Pd and Si peaks as demonstrated in Fig. 8b–d. Second, the aging process increases the lattice parameters of Pd along the (1 1 1) and (2 0 0) directions slightly (a1 1 1 = 3.881 Å and
57
a2 0 0 = 3.892 Å) and makes them nearer to the standard value for bulk Pd. Thus, the compressive stress along these two directions decreases [15]. Third, the results of deconvolution of Pd and Si peaks show that the full width at half maximum (FWHM) of the characteristic peaks of the Pd reduces in the aged samples. It means that the grain size of palladium increases in the aged samples. Finally, the ratio of SiO2 /Si and PdSi/Pd after two months which deduced from these fitted data, demonstrates an increasing from 4.5 × 10−4 to 1256 × 10−4 and 1.48 to 153.67, respectively. It illustrates that the conversion of silicon to silicon oxide and palladium to palladium silicide takes place even at room temperature. In summary, aging decreases the strength and the area of Si and Pd peaks. This reduction is perhaps due to reduction of crystallinity of silicon and palladium after oxidation or silicide formation, respectively. To gain more information from the aging effects, grazing XRD (GXRD) spectrum was also obtained from the fresh and aged samples. We observed only Pd on the surface of the fresh sample and no evidence for Si (Fig. 9). Since the X-ray beam impacts on the surface with a grazing angle, information of GXRD belongs on top of the pores. Therefore, absence of Si peaks shows top of the pores is completely covered by palladium with at least 20 nm average thicknesses. Decline of the Pd peak intensities in the aged samples may have arisen from diffusion of Pd in Si substrate or decrease of palladium crystallinity. Since the XRD data shows that the grain size of palladium increases in the aged samples, decreasing in crystallinity may be due to oxygen reaction with palladium structure. 3.5. XPS results To confirm change in surface composition we performed Xray photoelectron spectroscopy (XPS) experiments. These trials were carried on fresh and aged (as defined in Section 3.4) samples. Deconvolution of the spectra was achieved by fitting the data with a Gaussian/Lorentzian combination peak shape as explained in the previous section. The survey and deconvolution spectra for fresh and aged samples are illustrated in (Fig. 10a–c).
Fig. 10. XPS spectra for the fresh and aged sample in (a) survey mode, (b) Si2p peaks, (c) Pd3d peaks.
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The surface of fresh samples consisted of very small silicon, chlorine, and carbon contamination, oxygen and large amounts of palladium (Fig. 10a). Deconvolution of Si2p peak in Fig. 10b confirms the existence of SiO2 and Si simultaneously. This is due to the oxidation process in the electroless technique [2]. Because of the grazing angle in GXRD, this method cannot show the interior of the pores and therefore we guess this silicon peak arises from the bottom of pores. Deconvolution of Pd3d peaks in Fig. 10c shows that the observed binding energy (BE) peak at 336.08 eV in the fresh sample corresponds to the Pd (3d5/2 ) in the oxidized state. Oxidation of the metallic palladium to PdOx may have arisen from the presence of the dissolved oxygen in water in the electroless process or rinsing with deionised water after this process [19]. Whereas XRD data did not show any sign of PdO crystalline, we suggest that PdO thickness is very small or in amorphous state. Although the survey XPS spectrum of the aged sample shows a similar materials as fresh one but deconvolution of the Si2p (Fig. 10b) and Pd3d (Fig. 10c) peaks demonstrates that some changes take place. First, the Pd/Si atomic ratio decreases from 4.68 to 2.35 in the aged sample. The formation of both silicide compounds in interface and oxide phase of palladium can reduce the Pd/Si atomic ratio. Second, SiO2 /Si atomic ratio increases from 0.68 to 0.77. Finally, Pd/PdO (3d5/2 ) atomic ratio decreases from 0.79 to 0.62. 4. Discussion According to XRD and XPS results, we consider the schematic representation of fresh Pd/porous silicon samples as shown in Fig. 11a. SiO2 , PdSix , Pd, PdOx and surface contaminations cover the porous silicon, respectively. Due to the granularity of the palladium layer (SEM results) and polycrystallinity of each grain (XRD results), this layer consists of many grain boundaries. Low temperature coating of Pd (room temperature) and polycrystallinity of Pd layer (XRD results) ensure us to have many defects and vacancies in each granule of Pd (insert of part a in Fig. 11). Since our I–V measurements show that Pd/porous Si samples obey the metal–interface–semiconductor configuration rather than an ideal Schottky diode, equivalent circuit of these samples is considered as part b in Fig. 11. This means that the electrical current passes through the Pd layer then silicide zone (with together
as R1 ) and traverses from the Schottky barrier (as D1 ) formed between palladium silicide and Si and finally passes through the Si substrate. When hydrogen molecules are introduced to the samples, they are dissociately adsorbed on the Pd surface and penetrate to the bulk. Generally, there are three places for atomic hydrogen population at Pd bulk: First, octahedral interstitial sites in the fcc lattice [20,21]; hydrogen atom at these sites form a solid solution of Pd–H at low (␣-phase) and a hydride (-phase) at high hydrogen pressures [22]. Phase transitions (␣ → ) (∼1%H2 at sea level and room temperature for bulk Pd [23,24]) are accompanied by changes of palladium volume. At -phase, hydrogen absorption can induce internal stresses being maintained in dislocation-free crystals of palladium [25]. In addition, these atoms of hydrogen in octahedral interstitial sites increase the Pd electrical resistance [24]. Second, trapping sites at defects [26,27]; this trapping decreases the hydrogen diffusion flux especially at room temperature [26]. Finally deep adsorption sites near the Pd–Si interface [28–30]. In the presence of Pdx Si, the Schottky contact is formed in the junction of Pdx Si and Si (Pdx Si has high electrical conductivity [31]). Therefore, in these cases the third sites for hydrogen adsorption are those near the palladium silicide/silicon oxide interface [10]. To capture those sites, hydrogen atoms should transit from the Pd to Pdx Si zone and finally reach the Pdx Si/SiO2 interface. The hydrogen atoms at Pdx Si/SiO2 interface form a dipole layer that decreases the work function of the palladium silicide and consequently the Schottky barrier’s height (˚BP ) at this interface increases [10]. Resistivity enhancement of the serried resistance in the equivalent circuit (part b in Fig. 11) and especially increase in the Schottky barrier height by introduction of hydrogen decreases the current at forward bias as illustrated in Fig. 3 and Fig. 4. The oxygen and other contaminations on the surface of palladium block the surface sites thereby prevent adsorption of H2 [32,33]. Since, the rate-limiting step for hydrogen permeation through palladium is mainly reactions that take place on the Pd surface, when other gases are introduced together with hydrogen [32]; hydrogen permeation is declined by surface contamination. In agreement with representations illustrated in Fig. 11 and foregoing discussion in the previous paragraphs, hydrogen sensing properties of Pd/porous Si samples are justified as below:
Fig. 11. (a) Schematic representation and (b) equivalent circuit for fresh sample. Insert in part (a) is a granule of Pd contains vacancies, defects and grain boundaries.
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Fig. 12. (a) Schematic representation and (b) equivalent circuit for aged sample. Insert in part (a) is a granule of Pd contains vacancies, defects and grain boundaries.
A series resistance due to the existence of different layers (surface contamination, PdOy , Pd, Pdx Si, . . .) besides the existence of many defects in these structures comes into play (part b in Fig. 11) and makes the ideality factor greater than unity (Section 3.2). Due to the existence of oxygen and other contaminations at the surface and defects in the structure of the deposited layer, hydrogen diffusion flux through Pd decreases. In addition the diffusion coefficient of hydrogen on palladium silicide is approximately one order of magnitude lower than those obtained for metallic Pd [34]. All foregoing phenomena retard the reach of hydrogen atoms to the Pdx Si/SiO2 interface. Thus, the response time increases especially at low hydrogen concentrations (parts of data in Fig. 7). On the other hand, existence of the defect in the structure of the deposited layer can decrease the number of hydrogen atoms that reach the Pdx Si/SiO2 interface and reduce the variation of Schottky barrier height, ˚BP (Section 3.2). By cutting off the hydrogen gas flow, all reactions take place in the reverse direction and finally hydrogen atoms desorbs as H2 molecules. As a result of these reactions the electrical current approaches to its initial value. However, two factors delay this phenomenon: (1) defects in the deposited layer which trap hydrogen atoms and retard the returning of resistivity (serried resistance) to the initial value and (2) especially the defects at the Pdx Si/SiO2 interface trap hydrogen atoms and prevent returning of the Schottky barrier height to the initial value. These factors increases the recovery time as illustrated in Fig. 4. As discussed in Sections 3.4 and 3.5, aging makes some changes on Pd/porous Si samples: (1) increase in oxygen and other contaminations on the surface, (2) increase in SiO2 and Pdx Si values at the interface between palladium and silicon and (3) decrease in the crystallinity of Pd layers. Decreasing the crystallinity increases the defects in the structure of deposited layer. Additional defects were generated in the palladium layer in the ␣ ↔  phase transition (∼1%H2 at sea level and room temperature for bulk Pd) following successive addition and removal of hydrogen, respectively [26]. These changes are illustrated schematically for aged samples in Fig. 12. By considering the foregoing changes in the aged samples and regarding the discussions of previous paragraphs about the fresh ones, it is acceptable that the sensitivity decreases and the response
times increase by aging. However, after about 45 days all changes take place and samples reach a steady state. 5. Conclusion In the present paper macroporous silicon samples were coated by a continuous palladium layer in the electroless process. Our I–V measurements show that samples obey the metal–interface–semiconductor configuration rather than an ideal Schottky diode. Schottky like contacts of Pd/PS/p-type Si samples can detect hydrogen in a wide range of concentration (10–40,000 ppm) at room temperature. The sensitivity of the samples increases with hydrogen concentration and does not saturate. However, response time in low hydrogen concentration and recovery time in all foregoing hydrogen concentration are long. Characterization of the fresh samples shows existence of palladium oxide, chlorine and carbon contamination at the surface. In addition SiO2 and Pdx Si are in the metal-semiconductor interface. Aging decreases the hydrogen sensitivity and increases the response time due to increases in the amounts of oxides, carbon contamination, Pdx Si and defects. Nevertheless, sensitivity parameters reach to steady states after about 45 days. We attributed the low variation of the Schottky barrier height in hydrogen sensitivity and high response time in fresh sensors to: (i) work at room temperature, (ii) presence of trap states and defects in Pd lattice with high binding energy and (iii) existence of oxygen and other impurities on the Pd surface. We ascribed the losses of hydrogen sensitivity together with a greatly lengthened response time in the aged samples to an increase of: carbon contaminations and PdO at the surface, defects in palladium film and silicide layer formation at the interface. Acknowledgements This work was supported by the Sharif University of Technology research department. The authors would like to thank Mr. Rezaie and Mrs. Fardindust for performing SEM images and XRD analysis, respectively and Mr. Rafiee for doing XPS analysis.
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References [1] C. Christofides, A. Mandelis, Solids-state sensors for trace hydrogen gas detection, J. Appl. Phys. 68 (1990) R1–R30. [2] F. Rahimi, A. Iraji zad, Characterization of Pd nanoparticle dispersed over porous silicon as a hydrogen sensor, J. Phys. D: Appl. Phys. 40 (2007) 7201–7209. [3] F. Rahimi, A. Iraji zad, F. Razi, Characterization of porous poly-silicon impregnated with Pd as a hydrogen sensor, J. Phys. D: Appl. Phys. 38 (2005) 36–40. [4] F. Rahimi, A. Iraji zad, Effective factors on Pd growth on porous silicon by electroless-plating: response to hydrogen, Sens. Actuators B 115 (2006) 164–169. [5] F. Rahimi, A. Iraji zad, F. Razi, Palladium plating on macroporous/microporous silicon: application as a hydrogen sensor, Synthesis React. Inorg. Metal-Organ. Nano-Metal Chem. 37 (2007) 377–380. [6] X. Wang, M. Ye, Hysteresis and nonlinearity compensation of relative humidity sensor using support vector machines, Sens. Actuators B 129 (2008) 274–284. [7] G. Barillaro, A. Diligenti, A. Nannini, L.M. Strambini, E. Comini, G. Sberveglieri, Low-concentration NO2 detection with an adsorption porous silicon FET, IEEE Sens. J. 6 (2006) 19–23. [8] K. Bourenane, A. Keffous, G. Nezzal, A. Bourenane, Y. Boukennous, A. Boukezzata, Influence of thickness and porous structure of SiC layers on the electrical properties of Pt/SiC-pSi and Pd/SiC-pSi Schottky diodes for gas sensing purposes, Sens. Actuators B 129 (2008) 612–620. [9] P. Ruths, S. Ashok, S.J. Fonash, J.M. Ruths, A study of Pd/Si MIS Schottky barrier diode hydrogen detector, IEEE Trans. Electron Dev. ED-28 (1981) 1003–1009. [10] S. Chand, J. Kumari, Current–voltage characteristics of Pd2 Si based Schottky diodes on p-type (1 1 1) silicon and evaluation of their barrier heights, SolidState Electron. 38 (1995) 1103–1104. [11] I. Lundstrom, T. DiStefano, Hydrogen induced interfacial polarization at Pd–SiO2 interfaces, Surf. Sci. 59 (1976) 23–32. [12] I. Lundström, S. Shivaraman, C. Svensson, L. Lundkvist, A hydrogen-sensitive MOS field-effect transistor, Appl. Phys. Lett. 26 (1975) 55–57. [13] L. Stiblert, C. Svensson, Hydrogen leak detector using a Pd-gate MOS transistor, Rev. Sci. Instrum. 46 (1975) 1206–1208. [14] C.-C. Cheng, Y.-Y. Tsai, K.-W. Lin, H.-I. Chen, C.-T. Lu, W.-C. Liu, Hydrogen sensing characteristics of a Pt–oxide–Al0.3 Ga0.7 As MOS Schottky diode, Sens. Actuators B 99 (2004) 425–430. [15] R. Chan, PhD Thesis, Interfacial electrochemistry and surface characterization Hydrogen terminated Silicon, electrolessly deposited Palladium and Platinum on pyrolyzed photoresist films and electrodeposited Copper on Iridium, 2003, University of North Texas, Texas. [16] L.L. Tongson, B.E. Knox, T.E. Sullivan, S.J. Fonash, Comparative study of chemical and polarization characteristics of Pd/Si and Pd/SiO2 /Si Schottky barrier-type devices, J. Appl. Phys. 50 (1979) 1535–1537. [17] H. Kobayashi, H. Kawa, T. Yuasa, Y. Nakato, K. Yoneda, Palladium-promoted oxidation of Si at low temperatures, Appl. Surf. Sci. 113/114 (1997) 590–594. [18] S. Yalcin, R. Avci, Characterization of PdAu thin films on oxidized silicon wafers: interdiffusion and reaction, Appl. Surf. Sci. 214 (2003) 319–337. [19] E. Natividad, E. Lataste, M. Lahaye, J.M. Heintz, J.F. Silvain, Chemical and morphological study of the sensitisation, activation and Cu electroless-plating of Al2 O3 polycrystalline substrate, Surf. Sci. 557 (2004) 129–143.
[20] M. Eriksson, L.-G. Ekedahl, Hydrogen adsorption states at the Pd/SiO2 interface and simulation of the response of a Pd metal-oxide-semiconductor hydrogen sensor, J. Appl. Phys. 83 (1998) 3947–3951. [21] L.L. Jewell, B.H. Davis, Review of absorption and adsorption in the hydrogen–palladium system, Appl. Catal. A: Gen. 310 (2006) 1–15. [22] M.K. Kumar, M.S. Ramachandra Rao, S.R. Amaprabhu, Structural, morphological and hydrogen sensing studies on pulsed laser deposited nanostructured palladium thin films, J. Phys. D: Appl. Phys. 39 (2006) 2791–2795. [23] G. Kaltenpoth, P. Schnabel, E. Menke, E.C. Walter, M. Grunze, R.M. Penner, Multimode detection of hydrogen gas using palladium-covered silicon -channels, Anal. Chem. 75 (2003) 4756–4765. [24] F.A. Lewis, The Palladium–Hydrogen System, Academic Press, New York, 1968. [25] W.S. Zhang, Z.L. Zhang, X.W. Zhang, Effects of self-stress on the hydrogen absorption into palladium hydride electrodes of plate form under galvanostatic conditions, J. Electroanal. Chem. 474 (1999) 130–137. [26] R.V. Bucur, Effect of trapping on the solubility and diffusivity of hydrogen in palladium (␣-phase), J. Mater. Sci. 22 (1987) 3402–3406. [27] J. Will Medlinl, A.E. Lutz, R. Bastasz, A.H. McDaniel, The response of palladium metal-insulator-semiconductor devices to hydrogen–oxygen mixtures: comparisons between kinetic models and experiment, Sens. Actuators B 96 (2003) 290–297. [28] A. Salehi, A. Nikfarjam, D. Jamshidi Kalantari, Pd/porous-GaAs Schottky contact for hydrogen sensing application, Sens. Actuators B 113 (2006) 419–427. [29] I. Lundstrom, Hydrogen-sensitive MOS-structures part 1. Principles and applications, Sens. Actuators 1 (1981) 403–426. [30] I. Lundstrom, D. Soderberg, Hydrogen-sensitive MOS-structures part 2. Characterization, Sens. Actuators 2 (1981/1982) 105–138. [31] T. Utsumi, Keynote address microelectronics: what’s new and exciting, IEEE Trans. Electron Dev. 38 (1991) 2276–2283. [32] H. Amandusson, L.-G. Ekedahl, H. Dannetun, The effect of CO and O2 on hydrogen permeation through a palladium membrane, Appl. Surf. Sci. 153 (2000) 259–267. [33] J.W. Medlin, A.H. McDaniel, M.D. Allendorf, R. Bastasz, Effects of competitive carbon monoxide adsorption on the hydrogen response of metal–insulator–semiconductor sensors: the role of metal film morphology, J. Appl. Phys. 93 (2003) 2267–2274. [34] V.A. Paganin, A.A. Tanaka, L.A. Avaca, Voltametric behavior of amorphous Pd–Si alloys in acid and alkaline solutions, J. Braz. Chem. Soc. 3 (1992) 106–111.
Biographies F. Razi is a PhD student in Physics Department, Sharif University of Technology. Her research interests are thin films and surface physics. She is working on gas sensors. A. Iraji zad, professor in the Physics Department and head of the Institute for Nanoscience and Nanotechnology, Sharif University of Technology. Main interests are surface science and nanophysics. F. Rahimi received her PhD degree in thin film gas sensors at Physics Department, Sharif University of Technology, Tehran, Iran in 2006. Her research interests are thin films and surface physics.