ZnO Schottky diodes and their use for hydrogen sensing

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Semimetal graphite/ZnO Schottky diodes and their use for hydrogen sensing R. Yatskiv

a,* ,

J. Grym a, K. Zdansky a, K. Piksova

b

a

Institute of Photonics and Electronics, Academy of Science of the Czech Republic, Chaberska 57, 18251, Prague 8, Czech Republic Department of Physical Electronics, Faculty of Nuclear Sciences and Physical Engineering, Czech Technical University in Prague, Brehova 7, 115 19, Prague 1, Czech Republic

b

A R T I C L E I N F O

A B S T R A C T

Article history:

A method is described for the fabrication of highly rectifying Schottky contacts on n-type

Received 17 February 2012

ZnO (O and Zn polar face) single crystals, both bare and partially covered with Pt nanopar-

Accepted 13 April 2012

ticles, coated with mechanically deposited colloidal graphite. A layer of Pt nanoparticles

Available online 20 April 2012

deposited by in situ pulsed electrophoretic deposition from isooctane colloid solutions is inserted between the graphite and the ZnO surface serves to dissociate hydrogen molecules in hydrogen sensing elements based on the highly rectifying Schottky barriers. The sensing elements are sensitive to gas mixtures with a low hydrogen concentration down to 10 ppm and show an extremely fast response above 1000 ppm.  2012 Elsevier Ltd. All rights reserved.

1.

Introduction

In recent years increased interest in the application of ZnO in new optoelectronic and microelectronic devices has emerged [1]. One of the crucial areas, which has come into play, is understanding and control of its electrical contact properties. Schottky contacts to ZnO were first reported in 1965 on vacuum cleaved n-type surfaces [2]. Since then different transition metals such as Pd, Pt, Au, Ag and Ir have been reported to form relatively high Schottky barriers of 0.6–0.9 eV [3]. The barrier height for a particular metal strongly varies depending on the crystal quality [4], the polarity [5], the surface preparation, and condition under which the contact was formed [6,7]. Recently, it has been demonstrated that highly oriented pyrolytic graphite (HOPG) forms high-quality Schottky barriers on various n-doped semiconductors such as Si, GaAs, SiC [8], InP, and GaN [9]. The semimetal graphite/semiconductor Schottky barriers were well described by thermionic emis-

sion model and extracted values of the barrier height followed the Schottky–Mott relation. As one of the first gas sensing materials, ZnO has many advantages compared with other materials, such as low cost, facile synthesis and simple integration on cheap and flexible substrates. To date, various types of ZnO hydrogen gas sensors of resistivity type [10–12], with Schottky barrier [13], or heterojunction-based [14] were reported. Compared to the resistivity type sensor, the Schottky structure with catalytic metal electrode contact provides a higher sensitivity and shorter response time. We report on the preparation of highly rectifying graphite based Schottky contact (SC) prepared on O-face ZnO substrate with the ideality factor and barrier height of 1.0 and 0.86 eV, respectively. We demonstrate that the barrier quality strongly depends on kind of the substrate face. Moreover, we verify that a sensitive hydrogen sensor element with a fast response can be fabricated by inserting Pt nanoparticles (NPs) between the graphite SC and the ZnO substrate. These sensors are

* Corresponding author. E-mail address: [email protected] (R. Yatskiv). 0008-6223/$ - see front matter  2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.carbon.2012.04.047

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capable of the detection of ppm concentrations of hydrogen in nitrogen at room temperature.

2.

Experimental

Schottky diodes were fabricated on O-face and Zn-face of ntype ZnO substrates grown by hydrothermal method (supplied by MTI Corporation). The free electron concentration of 1.8 · 1014 cm3 and the Hall mobility of 140 cm2/V s were determined by Hall measurements at room temperature (RT). RT photoluminescence (PL) measurements on both faces were performed using He–Cd laser with 325 nm wavelength as the excitation source. Schottky contacts were created by painting colloidal graphite at RT on bare ZnO substrates and on ZnO substrates partly covered with Pt NPs. The Pt interlayer was created by electrophoretic deposition (EPD) of Pt NPs (for details see our previous paper [15]) prepared in isooctane by the reverse micelle technique [16]. Pt nanoparticles in colloid solutions were previously characterized by transmission electron microscopy (showing an average size of 10 nm) and by optical absorption spectra (two peaks at 260 and 236 nm due to surface plasmons of Pt nanoparticles, and a peak associated with AOT absorption were observed) [17]. As was presented in our previous work [15] for Pd NPs, changes in deposition time resulted in a variation of the surface coverage. High deposition times not only increased the amount of Pd NPs on semiconductor substrate but also increased the amount of surfactant (AOT). As a result, the diodes with higher coverage showed low values of the rectification ratio, barrier height, and sensitivity response. Similar effect was observed for SD with Pt NP on InP and GaN. Optimum deposition parameters – based on the experience with Pt(Pd)/InP(GaN) [9,15,17,18] – were used for the deposition onto ZnO. The surface morphology of the Pt NPs and of the graphite layer was characterized by scanning electron microscopy (SEM). An ohmic contact on the back side was formed by rubbing liquid gallium with a tin rod. Schottky diodes (SD) on a bare and partly covered with Pt ZnO substrate were investigated by the measurement of cur-

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rent–voltage (I–V) characteristics and further tested for their sensitivity to hydrogen in a cell with a through-flow gas system. The measuring system consisted of a test chamber, a sensor holder, a Keithley current/voltage source measure unit, mass flow controllers (Bronkhorst High-Tech B.V.) and of a data acquisition system (LabView, National Instruments).

3.

Results and discussion

3.1.

Semimetal graphite schottky diodes

The advantage of graphite contacts applied at room temperature, as compared with conventional deposition methods, is that it causes minimal damage at a metal/semiconductor interface. SEM investigation of the graphite layer deposited onto the ZnO substrate showed that it does not form a solid layer but it consists of irregular particles of sizes in the range of about 1 lm with openings among them (Fig. 1c). I–V characteristics of the SDs in a dark chamber at room temperature are given in Fig. 2 and show significant polarity effect with higher quality barriers achieved on O-face. In the case of O-face the current transport through the Schottky barrier can be described by thermionic emission model (TEM). According to TEM the forward I–V relationship of a Schottky diode at V > 3kT/q can be expressed as: Is ¼ Io exp ðqV=gkTÞ where Io ¼ A T2 exp ðq/B =kTÞ where A** is the Richardson constant, which has theoretical value of 32 Acm2K2, T is the absolute temperature, k is the Boltzmann constant, uB is the barrier height and g is the ideality factor. By fitting the forward I–V curve (voltage in the interval of 0.1–0.3 V), the barrier height and ideality factor were determined to be 0.89 eV and 1.0, respectively. The barriers on the Zn-polar face were poorly rectifying with apparent uB = 0.6 eV and g > 2. A summary of the basic electrical parameters calculated from I–V characteristics is presented in Table 1. The dependence of the surface polarity can be explained by

Fig. 1 – Schematic cross-section of the graphite/ZnO (a) and graphite/Pt NP/ZnO (b) Schottky diodes. SEM image of the graphite contact (c); SEM image of Pt NPs deposited on the Zn-face ZnO substrate by EPD (d).

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Fig. 2 – Current voltage characteristics of the graphite Schottky diodes fabricated on O-face (a) and Zn-face (b) ZnO substrate.

differences in the defect nature of the Zn-face and O-face surfaces. The better quality of the O-face was supported by PL measurements at room temperature (Fig. 3). Both faces exhibit identical near-band gap emission at around 378 nm known as due to superposition of the zero phonon free exciton line with its multiple LO phonon replica [19] and a broad composite line in the visible part of the spectrum with components related predominantly to structural defects [19]. The inset in Fig. 3 shows a fitting of the broad PL band with five Gaussians using the labels adopted in [20]. The obtained peak positions and FWHM values are in good agreement with the data summarized in Ref. [20]. The dominant PL component in the broad band of the spectrum obtained from the Zn-face of the substrate peaks at 2.24 eV and is identified as due to internal transitions in oxygen vacancies. The so called ‘‘orange band’’ with maximum at 2 eV is established as due to transitions from a shallow donor to a deep acceptor [20]. In view of the increased intensity of these two components in the PL spectrum obtained from the Zn-face polarity as compared to the O-face one may expect increased concentration of oxygen vacancies as well as variation in the shallow donor concentration across the samples thickness. We presume that the density of oxygen vacancies is of primary importance in the formation of Schottky contact to ZnO [21]. The PL measurements showed that the Zn face of our samples suffered from a higher oxygen vacancy concentration and from a shallow donor concentration variation; both having negative impact on the ideality factor. The thermionic

Fig. 3 – PL spectra of a bare Zn-face and O face ZnO substrate measured at room temperature.

emission is thus no longer the dominant transport mechanism. Several recent papers have dealt with the influence of the ZnO surface polarity on the luminescence [22], and Schottky barrier quality [5,23]. Some of these results are contradictory and strongly depend on the ZnO growth method, manufacturer or the substrate surface treatment.

3.2. Semimetal graphite Schottky nanoparticles for hydrogen sensing

diodes

with

Pt

The sensing mechanism of semiconducting oxide materials is surface controlled; the grain size, surface states and oxygen absorption play an important role [24]. Concerning ZnO, oxygen molecules absorbed on the surface extract electrons from the conduction band of ZnO to form O, O2. This process leads to the formation of a depletion region with reduced carrier concentration near the sample surface resulting in its higher resistivity. When exposed to hydrogen, chemisorbed oxygen species react with hydrogen, the extracted electrons are released to the conduction band, and the resistivity decreases [25]. To enhance the detection sensitivity, the ZnO surface can be covered with catalytic metals (Pt, Pd), which increase dissociation efficiency of molecular hydrogen to the more reactive atomic form [26]. The high quality SD formed by colloidal graphite is a good starting point for the fabrication of a high-sensitivity hydrogen sensor. A catalytic metal was deposited electrophoretically in the form of individual NPs or nanoparticle clusters

Table 1 – Electrical parameters calculated from I–V characteristics for the graphite based Schottky diodes. Samples Graphite/ZnO (O-face) Graphite/ZnO (Zn-face) Graphite/Pt NP*/ZnO (O-face) Graphite/Pt NP*/ZnO (Zn-face) * Nanoparticles.

Rectification ration at 1.5 V

Ideality factor, g

Schottky barrier high, /b (eV)

Sensitive response at 0.1 V S, (in 0.1% H2/air)

3.5E7 4.6E2 7.7E4 1.0E3

1.0 2.53 1.3 2.91

0.89 0.60 0.86 0.61

– – 2.7E4 1.2E3

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Fig. 4 – Current–voltage (a) and current–transient (b) characteristics of the graphite-Pt NP/ZnO (O- and Zn-face) Schottky diodes. (i) Reverse bias, (ii) forward bias.

(NPCs) (Fig. 1d) between the graphite SC and ZnO (O and Znface) substrate. Different thickness of the graphite contact gives identical I–V and I–t characteristics of the graphite/ ZnO and graphite-Pt NP/ZnO SD with and without exposure to hydrogen. The highly porous graphite contact allows easy penetration of hydrogen molecules regardless the layer thickness. Similar behavior was observed in graphite based SD formed on Si, GaAs and 4H-SiC surface [8,27]. I–V characteristics of SDs with Pt NP interlayer are given in Fig. 4a. A summary of the basic electrical parameters calculated from I–V characteristics are presented in Table 1. The ideality factor moderately deviates from g = 1, when the Pt NP interlayer is inserted between the graphite contact and the O-face ZnO substrate. This phenomenon can be explained by the increased number of interface states, which evoke increased reverse current on I–V characteristics. Two types of SD with and without Pt NP interlayer were tested for their sensitivity to hydrogen in a cell with a through-flow gas system. First experiments were performed with a mixture of H2/N2 containing 1000 ppm of hydrogen. (Fig. 4b). Rapid current increase characterized by the sensing response S = 2.7*104 (O-face ZnO) and S = 1.3*103 (Zn-face ZnO) is observed for the graphite-Pt NP/ZnO SD (S=[JHJair]/Jair, where JH is a saturation current density under exposure to hydrogen and Jair is the same for air). No response to pure nitrogen gas was observed for either type of SD (with and without Pt NPs), which confirms the high selectivity of these diodes. We assume that the formation of a depleted region due to the presence of oxygen species on the metal oxide surface is of secondary importance in the case of Schottky-based ZnO sensors. Recently, a strong dependence of sensing response on ZnO thickness has been reported [11]. We claim that in our case bulk conductivity of ZnO dominates, and that the sensing properties cannot be described by the mecha-

nisms mentioned above (presence of oxygen species and subsequent formation of the depletion region). This suggestion is supported by the measurement of I–V characteristics of graphite/ZnO(O- and Zn-face) SD with and without exposure to 1000 ppm H2 in N2. No current change was observed. The hydrogen sensing mechanism of graphite-Pt NPs/ZnO is based on the creation of a polarized layer near the Pt/ZnO interface by the H atom assisted by the diffusion through the catalytic metal, and the resultant change in the work function [28–30]. The Schottky barrier changes due to the

Fig. 5 – Current-transient characteristics of the graphite/PtNPs/ZnO (O-face) Schottky-based sensing element measured at various hydrogen concentrations in N2: (a) 1000 ppm, (b) 500 ppm, (c) 100 ppm, (d) 50 ppm, (e) 10 ppm. (i) H2 in N2, (ii) air.

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reduction of metal work function and this change can be evaluated by the measurement of I–V, and I–t characteristics. Fig. 5 shows the time dependence of current change of graphite-Pt/ZnO(O-face) SD as the gas ambient is switched from air to various concentration of H2 in N2 (10–1000 ppm) and then back to air. The sensing elements are sensitive to gas mixtures with a low hydrogen concentration down to 10 ppm and show extremely fast response above 1000 ppm (for 1000 ppm, response time is equal 10 s). Much longer time is required for hydrogen atoms to diffuse out of the Pt surface and then to recombine together to form hydrogen molecules or react with other species. The time needed to completely flush hydrogen out of the test chamber is a minor source of longer recovery time. The recovery time can be reduced at elevated temperatures [31]. We suggest that a reduction in NP size could be also helpful.

4.

Conclusions

We discussed the formation of a Schottky contact by painting colloidal graphite on O- and Zn-face ZnO substrate. A significant face effect with higher quality barriers achieved on Oface compared to Zn-face was observed. The extracted values of the barrier height and of the ideality factor from I–V characteristics were 0.89 and 1.0 eV for O-face and apparent 0.60 and 2.53 eV for Zn-face respectively. This significant variation may be explained by differences in the defect nature of the Zn-face and O-face surfaces of ZnO. A high quality Schottky barrier is a good starting point for the fabrication of a high-sensitivity hydrogen sensor element. To dissociate hydrogen molecules, a Pt NP interlayer was inserted between the graphite and the ZnO surface. The Pt NP interlayer was created by EPD and had a form of separated NPs or NPCs on the ZnO surface. The porous graphite contact ensures easy penetration of hydrogen gas to the Pt/ZnO interface. The hydrogen sensing characteristics of the graphitePt/ZnO SDs under different-concentration of hydrogen gases were studied. These SD are capable of the detection of 10 ppm concentration of hydrogen at room temperature with extremely fast response (ta = 10 s) above 1000 ppm.

Acknowledgements This work has been supported by the project COST OC10021 of the Ministry of Education CR. We would like thank Prof. P. Gladkov from the Institute of Photonics and Electronics, ASCR for providing PL measurements and for useful discussions.

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