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A FIELD EMISSION STUDY OF SILICON
V. Binh, M. Chaouch
To cite this version:
V. Binh, M. Chaouch. A FIELD EMISSION STUDY OF SILICON. Journal de Physique Colloques,
1989, 50 (C8), pp.C8-443-C8-448. �10.1051/jphyscol:1989875�. �jpa-00229973�
COLLOQUE DE PHYSIQUE
Colloque C8, Suppl6ment au noll, Tome 50, novembre 1989
A FIELD EMISSION STUDY OF SILICON
V.T. BINH and M. CHAOUCH
Ddpartement de Physique des Mat6riaux (UA CNRS), Universitd Claude Bernard Lyon 1, F-69622 Villeurbame. France
Abstract: Electron field emission experiments on silicon tips are presented. In the first part, FEM patterns and current-voltage characteristics are reported for an atomically clean surface, and thus for different values of heating and field emission temperatures. The thermal end forms show the formation of high index facets, whose dimensions are functions of the annealing temperature. The temperature dependance of the I-V characteristics is observed, and contributions of the electrons from the different bands (conduction, surface states, valence) are then deduced. In the second part, the oxidation of Si is followed starting from an exposure of a few Langmuirs. The oxidation is initiated at the vicinal regions and propagates inside the facets until the saturation. This progression is accompanied by a translation of the Fowler-Nordheim characteristics until the saturation. Finally and for both cases, the high stability of the current in function of the time is presented and discussed.
1. I n t r o d u c t i o n
An exact k~owledge of the field emission current from a single semiconductor is important not only to the understanding of the electronic behaviour of its surface, but also for its applications. We present here some results concerning the field emission of silicon from a clean surface and also from a surface after reaction with oxygen. In this work the silicon tip is prepared exclusively from a thermal sharpening technique in a vacuum of Torr,l and the initial single crystal rods (0.5x0.5 mm2 cross section) are cut from a (111) wafer (100 Q.cm, p-type Boron) with selected orientations. In order to prevent contamination by diffusion along the shank during thermal treatments, the tips are held only mechanically by clamps in Ta which are always at relatively low temperature during heat treatments by electron bombardment directly on the tip end. The sharpening temperature is around 1600K, so the final radii of the tips are about a hundred nanometers.
2.Clean S i surface
Field emission patterns of clean Si thermal end forms obtained after annealing at 1565 K for two orientations of the tip axis are presented in fig. 1. The observed planes of the thermal end forms are in agreement with former and are practically the same for annealing temperatures from 1200 K to near the melting point. The salient feature of these eqyilibrium form planes is the presence of high index surfaces in particular in the region between {110} and (111). Identification of these surfaces is obtained by comparison with a stereographic projection, and are reported in fig. 1.a for the principal orientations and in fig. 1.b for high index facets, they are given here with all the limitations proper to this technique. Recently, such high index facets were also observed by FIM,s'6 or by combined FIMIFEM observations.' For annealing temperatures under or equal 1150 K we observed a modification of the thermal end form characterized by an increase of the dimensions of some of the facets to the detriment of others, and most of them are now separated by sharp emission lines which we interpreted as monoatomic boundaries. We notice also the formation of the {001} plane which is absent for higher annealing temperatures.
These differences could be set out by comparing the clean patterns of fig. 4.a obtained
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1989875
Figure 1: Field emission patterns of the clean Si thermal end form obtained after annealing to 1565 K in UHV, with indications of the facets of the equilibrium form. (a)
Near <Ill>-tip axis. (b) <Oll>-tip axis.
Figure 2: Current-voltage characteristics measured with the same tip and for two different field emission temperatures, 300 K and 77 K.
after an annealing at 1565 K and fig. 4.b for 1130 K. Although the equilibrium end form is a function of the temperature, the presence of a critical temperature around 1150 K is not yet completely understood.
Total emission currents versus the inverse of the applied voltage are plotted in fig. 2.
The experimental points obtained after an annealing at 1565 K could be fitted by straight lines, and all our observations for different annealing conditions indicate also the same behaviour in the range from 10-lOA to 10-*A. By cooling the tip from 300K to 77K, the emission currents decrease without any noticeable modification of the emission pattern. An example of such a temperature dependance of the emission current is illustrated in fig. 2.
The theory of field emission from a semiconductor has been thoroughly analyzed by Strattone, and the total current has been calculated as the sum of three currents J,, J,, and ,J coming respectively from the conduction band, the surface states band, and the valence band. Several a ~ t h o r s * . ~ have also proposed other models to fit the experimental data at their disposal. However, from these theories and in order to interpret and to fit our experimental data, we are led to introduce a model which is based on the following assumptions:
-
The conduction band is not degenerated in the presence of the electric field in the field emission range, as consequence of a screening by surface states.Vacuum level I
Valence band
Silicon Vacuum
b
Figure 3: (a) Calculated current-voltage characteristics by using our model.The values of kb is determined from Tsong's calculations6, 8,. = 0.18 eV and
a,,
= 0 . 4 0 eV; the twolate values give the best fit with the experimental data in fig. 2. (b) Diagram illustrating electron field emission at 300 K from Si and for an annealing temperature of
1565 K, according to our assumptions.
-
The surface states are made up from the dangling bonds of the surface atoms. They will form a band which overlaps the valence band edge, is fully occupied and gives rise to a p-type space charge region. We have used here the model of overlapping surface states and valence bands which has been originally suggested by HandlerXo.The energy interval between the top of the valence band and the top of the surface states band inside the forbidden band, a,,, is a parameter which is characteristic of the surface structure.
- The thickness of this space charge region is taken equal to around 108,*" this value is also roughly equal to the field penetration depthxX. The screening effect on the band bending at the surface is then expressed by
where Qbr is the actual band bending,
abb
is the calculated band bending for an intrinsic siliconXX anda , ,
a parameter which is related to the screening effect.This type of approximate expression has been originally put forward by TsonglX for a p-type or a n-type impurity doping. Both L, and
a,,
depend on the surface structure and are the two parameters which will be determined by a fit with the experimental data, in particular on the variation of the emission current with the temperature.-
From the field emission point of view, due to the overlapping of the surface states band and the valence band, the electrons emitted from either band could not be distinguished. The sum J,+
J, is then expressed by the following equation:12where J, and J, are in Alcm2, F is the applied field in Vlcm, 8 = (0,
- a,,),
t(y) and v(y) are tabulated Fowler-Nordheim field emission functionsx3, and y = 3.488 F ~ I ~ / @ ;-
The current density J, emitted from the non-degenerate conduction band will be given by:where J, is in Alcm2, E, is the forbidden gap, 8, is the width of the conduction band in eV9 GO is a slowly varying function of (F,T,E,,ab,,O,) which could be evaluated by numerical integratione, and k, is given by the eq. (1).
Figure 4: FEM patterns of Si thermal end form during the oxidation. (a) The annealing temperature is 1565 K , the last pattern corresponds to 1000 L exposure (saturation). (b) The annealing temperature is 1130 K. no saturation is observed for an exposure of 2500 L.
Within these assumptions the total currents are then calculated for 300 K and 77 K, the results are plotted in fig. 3.a and are in good agreament with the experimental data presented in fig. 2 for an annealing temperature of 1565 K and a cooling at room temperature. The surface states band parameter a,, is found equal to 0.18 eV under these conditions. This model gives also a good fit for other experimental data, and results show a variation of ,@, for different annealing conditions.
One of the main reasons behind the success of silicon stems from the outstanding qualities of silicon oxides which can be used as a passivation layer as well as a gate insulator.
Silicon oxidation is then a domain which is intensely studied. However, FEM, with its lateral resolution in the order of 3 nm and the possibility to follow the reaction Langmuir after Langmuir, is a valuable tool in particular to investigate the first steps of the oxidation of Si. In the present study, oxidation of silicon is done at room temperature and field free, that means that FEM observations are done only in vacuum better than Torr, usually 5 Torr. Oxygen is introduced with a background vacuum of Torr, and the oxidation pressure is in the range of to Torr. The reaction of Si to oxygen is revealed by the formation of bright spots on the field emission pattern, the dimension of these spots is around 3 nm. These spots appeared in the vicinal regions around the facets from the very first Langmuir exposure. This reveals that the oxidation starts from the vicinal regions which are rich in surface defects, and progresses towards the center of the facets for further exposures.
In fig. 4.a FEM patterns for a thermal end form obtained after annealing at 1565 K are shown for clean surface and after 1000 L exposure. The oxidation covers all the surface when the exposure is around lo3 L, further exposure does not modify the emission pattern showing therefore that oxygen adsorption comes to a halt or at least becomes considerably slower. The progression of the oxidation over the surface is accompanied by a translation of the Fowler-Nordheim straight lines until the saturation, the slope value staying practically the same. Such variation of the I-V characteristics is illustrated in fig. S.a, and in this experiment the I-V plot for 1500 L, for example, merged with the 1000 L straight line.
For a stronger facetted crystal, obtained by annealing at temperature under 1150 K, we also observe the formation of bright spots at the vicinal regions and their progression towards the center of the different facets in function of the exposure. However, after a 2500 L exposure for some facets this progression is not complete, and this behaviour is
Silicon + Oxygen
- t O [ - . - . . . - . - - - . ' - I.+ I 0 (.a 1 2 2 2 4
'
Figure 5: Variations of the I-V characteristics during the oxidation of silicon at room temperature. The corresponding FEM patterns are in fig. 4. (a) The initial thermal end form is obtained after annealing at 1565 K ; the 1000 L exposure straight line corresponds to the saturation. (b) The initial thermal end form is obtained after annealing at 1130 K;
no saturation is observed for the exposure of 2500 L.
also related to the I-V curve translation which does not come to an halt. This is illustrated in fig.4.b where the last FEM pattern which corresponds to a 2500 L exposure exhibits always well defined facets. Concerning the I-V characteristics, the oxidation of this equilibrium form is also accompanied by a continuous translation of the Fowler- Nordheim plots (fig. 5.b) without noticeable change of the slope.
As conclusion for this section, the upper results on oxidation of Si lead to several questions which are still open. For example first, do the bright spots consecutive to the oxidation of silicon reflect an atomic reconstruction of the surface or only a modification of the electronic structure of this surface? Second, what is the correlation between the oxidation speed and the atomic structure of the facets? And finally, what is the mechanism responsible of the translation, without slope change, of the F.N. plots during the oxidation?
4.Current s t a b i l i t y
Field electron emission source is of high interest in practical applications due to its high brightness and small source size. However, one important drawback is the stability of the field emission current which is very bad in particular for metallic (W) tips at room temperature.z4 The change in the total field emission current is mainly the consequence of the adsorption and the migration of the species from the residual gas; consequently, the stability will be enhanced if the sticking coefficient is lowered.
Figure 6: Variations of the total emission.current versus time measured at room temperature. The pressure indications are the vacuum pressure during the field emission
process. (a) Initial clean silicon surface after annealing at 1565 K. (b) The tip is annealled at 1130 K, exposed to 2500 L of oxygen and 10 hours in a residual vacuum of
Torr.
Total currant I bi nA I
,f a
m . 4 0 .
w B 0
v
0 l ) Z o X ) ~ M U
Ihmhl
As the order of magnitude of the sticking coefficient of clean Si surface, or recovered by an oxide layer, is in most case very small (in the order of loe2 to it is then straight away to predict a much better stability of the emission current from Si tips than from the W tips.
This prediction is verified experimentally, and in fig. 6 the recorded variations of the total current emission from clean and oxidized Si tips display a relative decrease of ~ 6 % in one hour and =5% in two hours respectively. In the last case; the relative fluctuation of the current is well under 1%. Complementary studies for applications as FE source are needed, but with these prelimiliary results we can forecast that Si tips are competitive with regards to metallic tips as field emission sources.
References
1. Vu Thien Binh, Springer Proc. Phys. 22(1987)126 2. L.A.dfAsaro, 3 . Appl. Phys. a(1958)33
3. F.A. Allen, J. Phys. Chem.,Solids &(1959)119
4. G. Bush and T. Fisher, Phys. Kondens. Materie. 1(1963)367
5. H.M. Liu, T.T. Tsong and Y. Liou, Phys. Rev. Letters 2(1987)1535 6. T.T. Tsong, D.L. Feng and H.M. Liu, Surface 6ci. =9(1988)421 7. W.A. Schmidt and M. Naschitzki, Surface Sci. 206(1988)244 8. R. Stratton, Proc. Phys. Soc. @(1955)746; Phys. Rev. =(1962)67 9. A. Modinos ,' Surface Sci. G(1974)205
10. P. Handler, J. Phys. Chem. Solids E(1960)l
11. T.T. Tsong, Surface Sci. 8l(1979)28; Surface Sci. g(1979)l
12. In our calculations the following data are used for Si: Electron affinity
QC = 4.05 eV; Band gap E, = 1.12 eV; a, = 5.17 eV; Dielectric constant, E = 11.9 (from J.M. Sze in Physics of Semiconductor Devices J. Wiley h Sons Ed. (1985)), and they are reported in fig. 3.b. We do not take in account the variations of these parameters with the temperature.
13. R.E. Burgess, H. Kroemer and J.M. Houston, Phys. Rev. 90(1953)515
14. S . Yamamoto, S. Fukuhara, N. Saitou and H.Okano, Surface Sci. f?_1(1976)535; N.
Saitou and S. Yamamoto, Appl. Surface Sci.A(1980)374
15. for example A. Many, Y. Goldstein,and N.B. Grover, ltSemiconductor Surfaces"
(1976) North-Holland Publ. Co. Amsterdam
