A titanium dioxide-based MOS hydrogen sensor

Solid-StareElecrronicsVol. 33, No. IO, pp. 1229-1234, 1990 Printedin Great Britain.All rightsreserved

A TITANIUM

$3.00 + 0.00 0038-I lOI/ Copyright0 1990 PergamonPressplc

DIOXIDE-BASED

MOS HYDROGEN

SENSOR

LALLAN YADAVA, R. DWIVEDI and S. K. SRIVASTAVA Department of Electronics Engineering, Centre for Research in Microelectronics, Institute of Technology, Banaras Hindu University, Varanasi-221005, India

(Received

3 October

1989; in revised form 20 March

1990)

Abstract-This paper deals with a Pd-gate MOS hydrogen sensor using titanium dioxide as an insulator. The Pd-gate MOS capacitor was fabricated on p-type (111) silicon wafer having resistivity 3-5 &cm. The titanium dioxide thickness in such a sample was about 0.5pm. The capacitance and conductance vs voltage characteristics of the fabricated device have been studied. Such titanium dioxide based MOS capacitor showed significant change in capacitance value and also parallel shifting of conductance peak at room temperature when exposed to hydrogen. The device exhibits better sensitivity to hydrogen at zero gate bias. The fixed surface state density in such a device is found to increase linearly upon exposure to hydrogen.

1. INTRODUCTION

The palladium-gate widely been studied

MOS hydrogen sensors have by several workers[l-61. In such

MOS structures, silicon dioxide has been used as insulator. The thickness of such insulators have been varied from 30 to 1000 A. These devices, when exposed to hydrogen exhibit change in capacitance and conductance with voltage. Shivraman et a/.[71 and Ruths et a[.[81 have attributed these changes in the characteristics of MOS devices as due to the decrease in the work function of Pd upon exposure to H,. Lunstrom[9] has explained such changes in more detail. He suggested that the hydrogen molecules are first dissociated on the catalytic metal surface and then the atoms are adsorbed on to metal. Subsequently, some of the hydrogen atoms diffuse through the palladium film and are adsorbed at the metal-insulator interface; these atoms become polarized and give rise to a dipole layer which effectively changes the work function of the palladium. Bahman and Zemel[lO] have investigated the Pdthin SiO,-Si diode and its suitability for the detection of hydrogen. However, they have envisaged the stability and reliability problems for the Pd-thin SiOzSi as a hydrogen detector. Hydrogen detector based on non-SiO, has also been investigated. Harris[l 11 has reported on the titanium dioxide hydrogen detector. He has used a sandwiched structure consisting of titanium-titanium dioxide-platinum and found that the structure is a highly selective detector for hydrogen. The detection of hydrogen is based on change in conductance of titanium-dioxide. Yamamoto et af.[12] have also investigated the Pd/TiO, diode as a hydrogen sensor. They have stated that O2 is chemisorbed in the terms of anions

(e.g. 0, 0, and O-‘) on the Pd surface. The chemical bond between Pd and these oxygen anions is through an electrical double layer at the Pd surface and thus effects the work function of the metal. The decrease in work function on exposure to hydrogen is attributed to the reaction between hydrogen and the adsorbed anions. Johnson et a1.[13] have studied the diffusion of hydrogen in TiO, and have tried to explain the results based on high polarizability of TiO, lattice. Due to the high polarizability of TiO, lattice, a point charge placed in the lattice promptly (lo-‘* - lo-” s) induces a polarization cloud around itself. The diffusibility of hydrogen in titanium dioxide and its high polarizability causes it to be a potential hydrogen sensor and needs further investigation for its suitability as an alternative to SiOZ in MOS structure for hydrogen detection. This has the importance, particularly with reference to the technological problems such as pin holes, ionic contamination etc. associated with the growth of ultra-thin SiOZ layers. Keeping this in view, in the present work a hydrogen detector based on a Pd-TiO,-Si MOS structure has been investigated. The MOS structure using TiO, as an insulator for hydrogen detection is reported for the first time. The C-Y and G-V characteristics of the fabricated device with varying hydrogen concentration have been studied. The fixed surface state density has also been evaluated and it is observed that the fixed surface state density increases with hydrogen gas concentration. The observed change in capacitance due to hydrogen exposure by an earlier worker[14] is between (16-19%) of its initial value for a fixed and low concentration of Hz. However, in the present MOS structure the observed change in capacitance is about 47% for a varying hydrogen concentration of 2-g% in N, ambient.

1229

LALLANYADAVAet al.

1230

Pd aate small

(wit;

decreases. However, the reduction in the value of capacitance is maximum at zero gate bias. It is further observed that, the change in capacitance at lower voltage is more when the gate is negative in comparison with that of positive. The variation of capacitance of the device with varying concentration of hydrogen is shown in Fig, 4 at a different gate bias. The results of Fig. 4 are derived from Fig. 3. The sensitivity of the device is defined as

holes)

TiOZ Silicon Al

metal

Fig. 1. Structure of the fabricated device. 2. EXPERIMENTAL PROCEDURE

The Pd-MOS structure with titanium dioxide as an insulator was fabricated on a p-type (111) silicon wafer having resistivity 3-5 R-cm. The wafer was properly cleaned using standard technological procedures used in silicon technology. The structure of the device was completed by evaporating titanium dioxide over a silicon wafer and subsequent palladium front (with circular holes) and aluminium back metallizations. The oxide thickness was about 0.5 pm. The necessary lithography using lift-off technique was performed to have the proper dimension of MOS structure. The structure of the fabricated device is shown in Fig. 1. The C-V and G-V characteristics of the fabricated MOS capacitor with exposure of hydrogen in nitrogen ambient have been studied using the experimental set-up shown in Fig. 2. The hydrogen concentration in N, ambient has been varied from 2 to 8% which is sufficient to saturate the characteristics of the device. The device showed full recovery at room temperature. The observed recovery time was about 1 h 30min.

S=$

1AC/C L&J/ where C, is capacitance and C is gas concentration. Figure 5 represents the sensitivity of the device at different gate voltages for varying concentration of hydrogen. The maximum sensitivity is observed at zero gate bias. Figure 3 also reveals a shift in flatband voltage of the MOS capacitor in the presence of hydrogen. The observed shift in V,, with hydrogen concentration is plotted in Fig. 6. It is evident from this figure, that for nitrogen ambient, the VFe has a positive value. However, when the device is exposed to hydrogen VFBbecomes zero at 2% of hydrogen in N, and, with further increase of hydrogen concentration, VFe becomes negative and attains a value -0.3V at 8% of H, in N, ambient. The observed change in VFB, has been used for determining the fixed surface state density of the device. Figure 7 shows the variation of fixed surface state density with concentration of H2 which increases with the increase of hydrogen concentration. Figure 8 shows the variations of conductance of the device with gate voltage with varying concentration of hydrogen in nitrogen ambient. It is observed that the presence of hydrogen results in parallel shifting of the conductance peak. The shift in conductance peak lies within f0.2 V of gate bias. There is a slight change in the conductance peak value with increasing hydrogen gas concentration. The variational behaviour of the conductance of the device derived from Fig. 8 is represented in

3. RESULTS

The variation of high frequency (1 MHz) capacitance with gate voltage for the fabricated MOS capacitor in nitrogen ambient and with the subsequent addition of hydrogen (2-8%) in nitrogen, are shown in Fig. 3. It is evident from this figure that, as the concentration of hydrogen increases, capacitance Fan

I

+ I

r

Testing

c Device

I

I

I I

I~~+----Vacuum

chamber

EG C-V

and park plotter

G

__c -

1

pump

Fig. 2. Block diagram of measurement system.

X-Y recorder

TiO, based hydrogen sensor

I

I

-5

I

-4

I

-3

-2

I -1

0

1231

I

I

I

I

I

1

2

3

4

5

IV)

Bias voltage

Fig. 3. Capacitance-voltage response at 1 MHz. Fig. 9. It is evident from this figure that, initially the peak value of conductance decreases as the hydrogen concentration increases and later it increases with further increase of hydrogen concentration. 4.

DISCUSSION

It is known that metal-oxide-semiconductor (MOS) structures based on silicon/silicon dioxide with palladium top electrode when exposed to hydrogen gas, show a lateral shift in flat-band voltage of the device. This results in a lateral shift in the capacitance-voltage

(C-V) and conductance voltage (G-V) responses of the MOS device and is the prime principle of detection of a low concentration of hydrogen[l5]. The change in VFr, of the device is attributed to the change in work function of the palladium because of adsorption of H2 on Pd and subsequent diffusion through metal; forming a dipole layer at the Pd/SiO, interface. Some workers have suggested[S, 10,16-181 that hydrogen alters the density of the carrier trapping centre at the silicon/silicon dioxide interface. Other workers[ 191 have envisaged the change in flat-band voltage because of both, i.e. change in metal work function and 0.7

u -

x_:----_.

-.-.a2sv

-x-x-xOx)v

0

0.6 I

0 l.OOV 100 NP

I 2

I 4 H,

gas

concentration

I 6

I 6

(%I

Fig. 4. H, gas concentration vs capacitance at different gate voltage.

0

4

2 H2

gas

concentration

6

8

(%I

Fig. 5. Sensitivity vs H, gas concentration at different gate bias.

LALLAN YADAVA et al.

1232

due to the hydrogen

trapping

in the oxide

layer

itself.

H, gas concentration

Fig. 6. Variation

In the present investigation, the observed change in C-V and G-V responses of MOS capacitor based on silicon-titanium dioxide and palladium metal as a top electrode are analogous to the variational behaviour of the Pd-SiO,--Si system in the presence of hydrogen (Figs 3 and 8). However, the observed change in capacitance upon exposure to the higher concentration of hydrogen in TiO, based devices is about 47% which is about three times the change in capacitance in the Pd-SiO,-Sib) system[ 141when exposed to hydrogen. Similar results are obtained in VFB as is evident from Fig. 6. This response of the Pd-TiOZ-Si structure for hydrogen appears to be basically due to the high polarizability[l3] of Ti02 in addition to the usual work function change of palladium metal. It has been reported that the presence of metallic electrodes on a TiO, crystal induces a redistribution of mobile charged ions in the crystal and a layer, adjacent to the surface, is depleted of ions, because of the difference in work function of the TiO, and the electrode material[20]. When the device is exposed, the hydrogen is dissociated on the Pd top layer into H atoms, some okwhich cross the interface into TiO,. The diffusion of Hz into TiO_, has already been reported[ 131.Once inside the TiO,, these atoms ionize to produce a conduction electron and intersitial proton. During this, some carriers may get trapped inside Ti02 resulting in an increase in fixed surface state density. Also, due to the high polarizability of

(%)

of flat-band voltage concentration.

with

H, gas

Y

E f0 x z! g

1.4c

1.2-

L? 0.8.

I

I

2

0

I

4

1

6

Hz gas concentration

8

P/J

Fig. 7. Variation of fixed surface state density with H, gas concentration.

698.1

I -5

I

I

I

I

-4

-3

-2

-1 Bias

1

I

0 voltage

Fig. 8. Conductance-voltage

1

I 2

(VI

response at 1MHz.

I

I

I

3

4

5

TiO, based hydrogen sensor 660 r

Y

650

z a 640

I 1

I

I

I

I

NZ

2

4

6

6

H, gas

concentration

(%I

Fig. 9. Variation of conductance peak value with H, gas concentration.

TiO,, the rate of dipole formation upon exposure to hydrogen will be more in the Pd-TiO,-Si system in comparison to the Pd-SiOr-Si system. The increase fixed surface state density which effects the depletion layer in silicon and higher dipole formation in the TiO, layer, upon exposure to hydrogen, appears to be responsible for a better hydrogen response of the device based on TiOz. To have the estimation of fixed surfaces state density, the same has been evaluated using a technique reported earlier[21]. The flat-band capacitance (C,,), is given by U,= 1.16+2.*741n(&-f) 2=

l/[&(&

(1) 1)+ 11,

(2)

where Ci is the insulator capacitance, C,, is the semiconductor capacitance under strong inversion state. C,, is the semiconductor capacitance in the flat-band condition. U, is the normalized Fermi potential and A is gate area. The value of C,, is fitted to the experimental C-V curve (Fig. 3) for evaluating flat-band voltage V,,. Once VrB is known Q, is calculated by, .

I’r, = r/Wns- $ I

(3)

It is observed that the fixed surface state density increases linearly as the concentration of hydrogen increases (Fig. 7). The increase in surface state density in Pd-SiO,--Si system upon exposure to hydrogen has also been reported for oxide thickness < 12 nm[12]. However, we have observed that the increase in fixed surface state density of Pd-TiO,-Si system, even when the TiO, thickness is upto about 0.5 pm. Therefore, the change in fixed surface state density is again attributed to the high diffusibility and polarizability of hydrogen in TiOr. The variational behaviour of capacitance of PdTiO,-Si system upon exposure to hydrogen is further

1233

reflected in Fig. 6 which represents the variation of flat-band voltage (I’,,) with hydrogen concentration. It is evident from this that VFr,, which is initially positive, decreases with increase in hydrogen concentration and attains a value -0.3 V. The V,, is a parameter which depends on ms and Q,. It has already been stated and represented in Fig. 7 that Q, in the present case linearly increases as hydrogen concentration increases. The increasing tendency of fixed surface state density upon exposure to hydrogen and modification of work function of Pd appears to be basically controlling the variation of VFr,. Initially, flat-band voltage which is positive for N, ambient decreases towards negative value when the device is exposed to hydrogen. Total shift in VFB is about 4OOmV. The initial change in flat-band voltage of the device could be due to the change in metal work function where fixed surface state density is low but as the device is exposed to a higher concentration of hydrogen gas (over 2%) the change in Vra appears to be dominated by fixed surface states where its density is linearly increasing with hydrogen concentration. The fabricated devices based on TiO, respond significantly when exposed to hydrogen. However, in such devices, practically no change in its characteristics (C-V, G-V) is observed if the concentration of hydrogen approaches to about 8% in N, ambient. This predicts that such devices will be more appropriate for detecting of low concentration of hydrogen. It is an established fact that the surface state density decides the depletion layer position in silicon and thereby the capacitance of the MOS structure. In the presence of higher fixed surface state density, even without any external bias, the silicon surface could be inverted and as a result, no change in capacitance is observable. It is also evident from the C-V characteristics of the device that the external voltage required to invert the surface, reduces as the concentration of hydrogen increases. In fact, it is true that the capacitance of MOS structure does not vary after inversion and therefore, saturation of the response of the MOS capacitor could be again attributed to the increase in the fixed surface state density upon exposure to hydrogen. 5. CONCLUSION It is observed that Pd/TiO,/Si -p MOS sensor is an effective detector for hydrogen gas particularly for low concentration. The observed change in the capacitance due to exposure in hydrogen is about 47% which is much higher than the change of capacitance in the Pd/SiO,/Si --p system. The fixed surface state density increases linearly with hydrogen concentration, when the device is exposed to hydrogen in N2 ambient. The higher change in capacitance in the Pd/TiOrSi system appears to be basically due to the high polarizability of TiOz.

LALLAN YADAVA et al.

1234

Further, silicon dioxide based devices require an ultra-thin layer of Si02 which, technologically is more difficult to achieve compared to the titanium dioxide based devices which can work well at much higher thickness (0.5 pm). Thus, it may be concluded that hydrogen detector based on the Pd-TiO,-Si MOS structure could be technologically a better choice than the SiOz based device.

8.

9. 10. 11. 12. 13. 14.

REFERENCES

1. I. Lundstrom, Sensor Actuators 1, 426 (1981). 2. I. Lundstrom and T. Distfano, Solid St. Commun. 19, 871 (1976). 3. I. Lundstrom and D. Soderberg, Sensors Actuators 2, 105 (1981). 4. T. L. Poteat and B. Lalevic. IEEE Trans. EZectron Devices ED-29, 123 (1982). 5. B. Keramati and J. Zemel, J. uppl. Phys. 53, 1100 (1982). 6. T. L. Poteat, B. Lalevic, B. Kuliyev, M. Yousuf and M. Chem, J. Electron. Muter. 12, 181 (1983). 7. M. S. Shivaraman, I. Lundstrom, C. Svensson and H. Hammarsten, Electron. Z&f. 12, 983 (1976).

P. F. Ruths, S. Ashok, S. J. Fonash and J. M. Ruths, Trans. Electron. Deuices ED-B, 1003 (1981). I. Lundstrom, Sensors Actuators 1, 403 (1981). B. Keramati and J. N. Zemel, J. appl. Phys. 53, 1091 (1982). L. A. Haris, J. Electrochem. Sot. Solid St. Sci. Techn. 127, 2657 (1980). N. Yamamoto, S. Tonomura, T. Matsuoka and H. Isubomura, Surface Sci. 92, 400 (1980). 0. W. Johnson, S. H. Pack and J. W. Deford, J. appl. Phys. 46, 1026 (1975). G. Jorden Maclav, IEEE Trans. Electron Devices ED-32, 1158 (1985). -. L. Stilbert and C. Svensson, Rev. Sci. Znstrum. 46, 1206 (1975). M. C. Petty, Electron. Left. 18, 34 (1982). D. Diligenti, M. Stagi and V. Cinti, Solid St. Commun. 45, 347 (1983). M. C. Petty, Solid-St. Elecfron. 29, 89 (1986). C. Nylander, M. Armgarth and C. Svensson, J. appl. Phys. 56, 1177 (1984). 0. W. Johnson, J. W. Deford and S. Myhra, J. appl. Phys. 43, 807 (1972). A. Jakubowski and Krzysztof, Iniewski, Solid-St. Electron. 26, 755 (1983). N. J. Evans, M. C. Petty and G. G. Roberts, Sensors Actuators 9, 165 (1986).

IEEE

15. 16. 17. 18. 19. 20.

21. 22.