Solid State Communications,
Vol. 107. No. 8, pp. 391-393,
@ 1998 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0038-1098/98 $19.00 + .OO
VACUUM EMISSION OF ELECTRONS FROM (Pb,Ca)TiOs THIN FILMS K. Biedrzycki * and L. Markowski Institute of Experimental Physics, University of Wroclaw, Max Born Sq. 9, 50-204 Wroclaw, Poland (Received 27 April 1998; accepted 26 May 1998 by G Bastard)
Electron emission of Pbo.~&ao,24TiOs (PTC) thin films, deposited on a Pb/TiO#iO#i( 100) substrate, induced under ac electric field driving, have been investigated. The influence of temperature and top Au electrode on time and energy distribution measurements, for field-induced electron emission from a PTC sample, are presented and discussed from the point of view of electron transport in a perovskite-oxide based thin layer. @ 1998 Elsevier Science Ltd. All rights reserved Keywords: A. semiconductors, A. surface and interfaces, D. electronic transport.
In recent years, much attention has been devoted to the perovskite-oxide based thin films because of their potentiality to the wide application in the rapidly expanding field of integrated microelectronic, optic and optoelectronic devices such as nonvolatile memories, infrared sensors, low-threshold-voltage electroluminescent devices, etc. [ 1,2]. On the other hand, electron emission phenomena occurring at the free surface of ferroelectric materials under electric field excitation, open new areas for application, especially ceramic and thin film materials, in electronic devices such as electron guns (cold cathodes), flat pane1 displays, fast switches, etc. [3-51. This electron emission can be produced under IO-’ ’ to 1O* A/cm* current density levels. Moreover, for perovskiteoxide based materials, this field-induced electron emission can be observed at temperatures below as well as above their phase transition temperatures [6-81. In this case, the semiconducting properties of the emitting sample surface seem to play an important role in the electron emission processes mentioned above. It is also known that electron emission experiments on these materials can provide new data about their surface structure, which seems to be important, for example, in the case of integrated ferroelectric systems or ferroelectric-based electron emitters. The studies of electronic surface structure, impurity or defect states, * Corresponding
electron transport in the material are, therefore, crucial for new potential application of these materials. For example, high field electron transport in thin, about a few (m thick, semiconducting as well as dielectric layers such as ZnS:Mn and GaAs, can be examined by means of vacuum emission of hot and ballistic electrons, as reported by Fitting et al. [9, lo]. Recently, electron emission of Ca modified PbTiOs (PTC) semiconducting thin films, deposited on a Pb/Ti02/Si02/ Si( 100) sandwich structure, induced under drive (1 O4 V/cm) ac electric field, was observed [l 11. The PTC sample was covered with hollow, vacuum evaporated Au electrodes. The diameter of a bare emitting aperture, located in the centre of each top electrode, was equal to about 0.5 mm. Under this condition, the electron emission processes were strongly affected by temperature of a PTC sample. This is illustrated in Fig. 1 by time distribution measurements for electron emission from PTC sample, monitored during a cycle of the applied ac (200 Hz) electric field. The sample was maintained at 208 (a), 278 (b) and 338 K (c); amplitude oftheexciting acfield was 5.7x lo4 (a), 5.5~10~ (b) and 5.1 x lo4 V/cm (c). The details about electron emission experiment and data acquisition board are described in Refs [12,13]. This unipolar electron emission from PTC sample consists of two temperature dependent current components. The first of them (component 1) is predominant at higher temperatures [Fig. l(c)] and shows an energy peak at about 2 to 3 eV [ll]; the
VACUUM EMISSION OF ELECTRONS
FROM THIN FILMS
Vol. 107, No. 8
z4 $0 z >" -4
/’ j 0
4 0 -4 012345 Time[ms]
Fig. 1, Time distribution of electron emission for PTC thin sample covered with a hollow Au top electrode, monitored during a cycle of ac (200Hz) electric field, as a function of temperature. Temperature of the sample is 208 (a), 278 (b) and 338 K (c). Amplitude of the exciting ac field is 5.7~10~ (a), 5.5~10~ (b) and 5.1 x lo4 V/cm (c). The time window (5 ms) is divided into 250 time channels. second one (component II) with energy tail depending on time scale of the drive ac voltage, was more yielded at lower temperatures, i.e., at 208 and 278 K [Fig. l(a) and l(b)]. If now PTC sample is supplied with the top, about 10 nm thick penetrable for electrons Au electrode, the time distributions of this electron emission are only slightly dependent on temperature. Examples of the time distribution measurements for electron emission produced under ac electric field driving on PTC sample covered with ultrathin top Au electrode are presented by curve I and 3 in Fig. 2(a) and 2(b), respectively. Three-dimensional time and energy distribution of this electron emission, determined for instance at 338 K is shown in Fig. 3. For PTC sample covered with penetrable for electrons Au electrode, electron current emitted into vacuum is mainly determined by component I. This component lies at the beginning and at the end of emissively active field half-period, i.e., when the accelerating for electrons electric field is directed towards the emitting sample
2 3 Time [ms]
Fig. 2. Time distributions of electron emission from PTC sample covered with penetrable for electrons Au electrode, monitored during a cycle of ac (200 Hz) electric field at 243 (a) and 338 K (b). Amplitude of the applied ac field is 5.4~10~ (a) and 5.2~10~ V/cm (b). Curves 1 and 3 represent total electron current emitted into vacuum; curves 2 and 4 the contribution of hot electrons into this electron current. The time window (5 ms) is divided into 250 time channels.
dZN dt dE
Time [ins] Fig. 3. Time and energy distribution of electron emission, monitored during a cycle of ac electric field, for PTC sample maintained at 338 K. The sample was supplied with ultrathin ( about 10 nm thick) penetrable for electrons Au electrode. Amplitude of the applied ac field is 5.2x lo4 V/cm. surface, and originates from surface donor states produced in PbTiOl-based materials by oxygen vacancies. However, it is seen from Fig. 3 that the so-called component II discussed above can also be distinguished. If we take into account only more energetic electrons, i.e. the electrons constituting the energy tail in the observed energy spectrum, the contribution of component II into the total current is evident [curve 2 and 4 in Fig. 2(a) and (b)]. We can see that intensity of this
Vol. 107, No. 8
VACUUM EMISSJON OF ELECTRONS
FROM THIN FILMS
component, involving the electrons heated up to 6.5 eV under about 5x IO4 V/cm field transport, increases at lower temperatures. It is known that electron transport phenomena are governed by temperature dependent electron dispersion processes, much more efficient at higher temperatures. Additionally, part of these hot electrons dissipate their energies during interaction with sample-electrode interface, and after thermalization contribute to component I. In conclusion, electron emission from a semiconducting thin PTC sample into a vacuum, induced under drive ac electric field, is observed only during the other half-period of the exciting ac electric field. This unipolar electron emission depends on the temperature of the sample and on the kind of top Au sample electrode. The energy of electrons, determined with regard to the potential of the emitting sample surface. can exceed even 6 eV.
2. Okuyama, M. and Hamakawa, Y., ht. .I Engng. Sci., 29, 1991, 391. 3. Riege, H., Nucl. Znstrum.Methods, A340. 1994.80. 4. Rosenman, G., Shur, D. and Skliar, A., 1 Appf. Phys., 79, 1996, 740 1. 5. Gundel, H., Ferroefectrics, 184, 1996, 89. 6. Biedrzycki, K., Aboura, H. and Le Bihan, R.. Phys. Stat. Sol. (a), 140, 1993, 257. 7. Biedrzycki, K., Ferroelectrics, 192, 1997, 269. 8. Okuyama, M., Asano, J. and Hamakawa, Y., Jpn. J Appl. Phys.. 33, 1994, 5506. 9. Fitting, H.J., Miiller, G.O., Mach, R., Reinsperger, G.U., Hingst. Th. and Schreiber, E., Phys. Stat. Sol.. 121, 1990, 305. IO. Fitting, H.J.. Hingst, Th., Schreiber, E. and Geib, E., J Vat. Sci. Technol., B14, 1996, 2087.
12. Biedrzycki, K. and Markowski, L., Ferroelectrics,
11. Biedrzycki, K., Markowski, L., Calzada, M.L. and Mendiola, J.. Microelectronic Engineering, 29, 1995,265.
supported by University of Wroclaw under the contract No. 2016/W/IFD/97. REFERENCES
1. Xu, Y., Ferroelectrics Materials and Their Applications, North-Holland Pub]. Corn., Amsterdam,
172, 1995,405. 13. Biedrzycki, K., Markowski, L. and Czapla, Z., Phys. Stat. Sol. (a), 165, 1998, 283.