Low and high temperature TiO2 oxygen sensors

Sensors and Actuators, BI (1990) 103-107

Low

103

and High Temperature TiO, Oxygen Sensors

U. KIRNER, K. D. SCHIERBAUM and W. G6PEL Institute of Physical and Theoretical Chemistry of the Unioersity of Ttibingen, D- 7400 Ttiingen (F.R.G.) B. LEIBOLD, N. NICOLOSO and W. WEPPNER Max-Plan&-institute

of Solid State Research, D-7ooO Stuttgart (F.R.G.)

D. FISCHER and W. F. CHU Batelle-Institute e.V., D-6ooo Frankfurt (F.R.G.)

Abstract We have studied two types of TiO,-based oxygen sensors operating at different temperatures with different detection principles. At high temperatures, TiO, devices can be used as thermodynamically controlled bulk defect sensors to determine oxygen over a large range of partial pressures. Their intrinsic behaviour can be controlled by carefully directed doping with tri- or pentavalent cations. At low temperatures, we find that Pt/TiO, Schottky diodes make extremely sensitive oxygen detection possible. The latter show reversible shifts of current-voltage curves, which are determined by interface states formed by chemisorbed oxygen. 1. Intmduction Various oxides with ionic, mixed ionic and electronic conductivity may in principle be use.d to design oxygen sensors operating at high temperatures [ 11. In contrast to the often-used ionic conducting zirconia sensors, which require reference phases with constant oxygen activity in both amperometric and potentiometric devices [2], we investigated TiOz as a prototype system for a binary metal oxide with electronic conductivity for detecting O2 in the gas phase without a reference [3]. We started with undoped material in order to minimize the number of device parameters to be controlled in adjusting well-defined electronic and geometric studies [4]. In addition to these high temperature devices, the present study also suggests the use of platinum/TiO, Schottky diodes as oxygen sensors operating at low temperatures with their currentvoltage (I- V) characteristics changing sensitively upon changes in the oxygen partial pressures. The detection principles of both types of sen09254005/90/$3.50

sors make use of specific interaction mechanisms that involve intrinsic and extrinsic defect reactions in the bulk of TiOz and at the interface between Pt and TiOz. 2. Experimental We prepared TiO, thin ti on sapphire by r.f. sputtering, electron beam, thermal and ion cluster beam (ICB) deposition techniques with fihn thicknesses between 30 and 1000 mn. For comparative studies, we used single crystals with ( 110) orientation. Various spectroscopic methods (XPS, UPS, SIMS and ISS) were applied to characterize the atomistic structure, i.e., chemical composition, core level binding energies and the valence band structure [ 51. Details are given elsewhere [a]. Current-voltage measurements and four-point conductivity measurements were applied to characterize the electrical properties at different temperatures (403 < T d 1073 K) and oxygen partial pressures (10-i’
3. Re.sults and Discudoa 3.1. TiOl Bulk Defect Sensors At high temperatures, TiOz devices act as typical bulk defect sensors, providing oxygen partial pressure measurements over a large range between lo-” and lo5 Pa. Typical results are shown in Fig. 1. In comparative studies, we have measured the specific electronic conductivities, Q, of single 0 Elsevier Sequoia/Printed

in The Netherlands

104

h’l=[A

2 lVJ= IA’ 1

i‘.

,,.J ‘., ,:’ ..-.._, -..

log(p@J nICB fibn(d=XKlnm) oEB film Id =28&m) oRF film Cd =lWrm) 0 cemmic l single crystal

log (p#d

-

Fig. 1. Specific bulk conductivities, u, of TiO, single crystals [A, ceramics, ion-cluster-deposited (ICB), electron-beam evaporated (EB) and r.f. sputtered (RF) t3lms with thicknesses d between 100 and 300nm as a function of oxygen partial pressure, p(O,), at a constant temperature T = 1073 K in a log/log plot. Slopes are indicated for characteristic ranges of P(O,).

crystals, polycrystalline ceramics [7j and differently prepared thin films as a function of the oxygen partial pressure p( 0,). At low oxygen partial pressures, characteristic slopes of - l/6 in Fig. 1 may be attributed to intrinsic defect equilibria between doubly ionized oxygen vacancies, If& and oxygen in the gas phase 0, [8] V;j+2e’+fO,*O&

-

Fig. 2. Oxygen partial pressure., p(O,), dependence of the relative concentrations of oxygen vacancies [Vo], of electrons [e] and of defect electrons [/I’] at a given acceptor concentration [A’] in doped binary oxides. At a certain oxygen partial pressure [V;;] remains constant, leading to a change in the conductivity dependence upon ~(0,) from u -p(0,)-“6 to p(0,) - u4.

Only at high temperatures does complete ionization of defects occur due to large differences between their ionization potentials and the Fermi level EF, with occupation probabilities of the defects determined by Fermi statistics [4]. We observed no significant enrichment of intrinsic and extrinsic defects at the surface [6] except at the interface between Ti02 and the platinum ohmic contacts (see Section 3.2). Therefore, flat-band conditions may be assumed with negligible surface contribution to the overall conductivities. Under these conditions, the temperature dependence of the bulk concentration of conduction band electrons is determined mainly by the formation enthalpies of oxygen vacancies. As indicated in Fig. 3, we find that the enthalpies

Here, e’ and 06 denote conduction band electrons and lattice oxygen, respectively, in the KriigerVink notation. In impurity-controlled regimes at higher p(O,), in contrast, the concentration of oxygen vacancies [V;;] is controlled thermodynamically by the concentration of acceptor-type impurities [A’] (Fig. 2). Assuming singly charged trivalent acceptors, A ‘, such as iron or aluminium ions, electroneutrality requires 2[V;;] = [A’] and hence CJ-p(O,) -‘j4 holds [9]. Depending on the sample preparation and hence the total concentration of acceptors, transitions from n-type to p-type semiconducting TiOz were observed for oxygen partial pressures about lo* Pa. As a result, specific minima occur in the overall conductivities and hence non-equivocal correlations between ~(0,) and ~7occur. Differences in the absolute values of specific conductivities are caused by different acceptor concentrations.

T/K

-

(1)

Km933m

-16

8al

fibnti=280runl fiiti=300mJ

aE0 0

ml

RF

I

1

12

1.L

103K/T

16

r

18

-

Fig. 3. Specific conductivities, Q, of TiO, single crystals [lo], electron-beam evaporated lilms, and r.f. sputtered TiO, films as a function of reciprocal temperature.

105

P(O*) 101Pa

IOPa

YOLPa+

Pt/TiO,-diode T=403K

nW

Pa ~(0,) 0 W Pa + IF Pa * lO-’ Pa

-3

0

5

10

15

20

time (min)

Fig. 4. Time dependence of relative changes of signals of ion-cluster-beam deposited TiO, tihns (ICB) and a zirconia I-probe as determined under UHV conditions after exposure to oxygen at 1073 K.

of the oxygen vacancy formation have the same values in single crystals and in polycrystalline films. The values are determined from temperature-dependent conductivity measurements, with details published earlier [ 111. We also determined response times of our thin-film TiO, devices in a flow test chamber equipped with a zirconia I2-probe operating at 973 K to monitor ~(0,). Typical results are shown in Fig. 4. Within experimental error, no significant differences are observed in the response times.

-2

-1

0

1

2

voffwe /v/

l-

Fig. 5. Current-voltage curves of Pt/TiO, Schottky diodes with zirconium as ohmic backside contact after exposure to different partial pressures of oxygen at constant temperature.

Fig. 6. For ideal Pt/TiO* junctions, we expect the densities of intrinsic interface states to be negligible [12]. As a result, the Schottky barrier height is obtained by aligning the two Fermi levels EFp( and EFTio2. This leads to electron transfer from the semiconducting TiOz to platinum and hence to band bending eAV, at the interface equal to the contact potential difference. As a result, depletion of electrons in the space charge layer and Schottky barrier formation occur. Under equilibrium conditions, the Schottky barrier height &, is given by ~SB=eAV,+E,-EE,=~,--TiO,

(2)

with XTio2denoting the electron affinity of TiOz 3.2. PtlTiO, Schottky Diode Type Sensors At low temperatures, Schottky diode characteristics were observed at our Pt/TiOz devices. We observe Z-V curves to show large shifts of forward voltage upon exposure to oxygen (Fig. 5). A similar effect has been observed for Pd/ TiO, Schottky diodes after exposing them to hydrogen/air gas mixtures [ 111. During operation in the constant-voltage mode, we find remarkable sensitivities but limited detection ranges for oxygen. In the constantcurrent mode we find an almost logarithmic dependence of the voltage on ~(0,). The sensor properties of these diodes can be explained by applying the band model to the Pt/TiOz junctions. Modifications result from extrinsic states at the interface, as indicated in

[131* During chemisorption of oxygen, extrinsic interface states are formed at the porous interface by trapping of electrons in localized oxygen species such as 0~ or O-. As a result, the barrier height C& changes and hence a decrease of the electron concentration in the depletion layer occurs, which then induces changes in the Z-V curve of the diode. With increasing temperature, the Schottkytype behaviour of Pt/TiO, interfaces changes irreversibly. Interdiffusion then leads to the formation of ohmic contacts. This phenomenon has been studied in detail by ultraviolet-photoemission (UPS) measurements in the valence band region of TiOz single crystals covered with submonolayers of Pt. Significant concentrations

106

of Ti3+ donor states are observed at the Fermi level. As a result, we observe accumulation layers at the interface, and hence ohmic properties of the contacts [ 61. 4. conelnsions As we have deduced from the bulk defect thermodynamics of Ti02, sensitivities and absolute values of specific conductivities of the hightemperature TiOz oxygen sensors are strongly influenced by small amounts of dopants introduced from impurities. No significant differences in signal-to-oxygen pressure ratios were observed for the different TiOz samples, whether they were prepared as polycrystalline thin-film or singlecrystal material. We find similar response times of the TiOl films compared with the L-probe. The low-temperature Pt/Ti02 Schottky barrier devices allow extremely sensitive oxygen detection to take place. Because of the possible chemisorption of other gases, such as CO, and NO,, at the Pt/TiO, interface, other chemical sensors based upon this detection principle may now be developed.

PI (a) E

Acknowledgement

Pt

li0,

This work was supported by the Btmdesminister fiir Forschung und Technologie of the Federal Republic of Germany.

@I E

Fig. 6. Band model of PtjTiO, junctions before (a) and after (b) formation of the contact. Due to different work functions I$ of Pt and TiO, referred to the vacuum level E,,, electron transfer occurs and depletion layers are formed in TiO,. (c) The Schottky barrier height &s is in addition influenced by chemisorbed Or- at the interface. Further explanations are given in the text.

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