On the electron emission from evaporated thin Au films

Volume 62A, n u m b e r 7

PHYSICS LETTERS

ON THE ELECTRON

3 October 1977

EMISSION FROM EVAPORATED

THIN AU FILMS

M. BISCHOFF, R. HOLZER and H. PAGNIA Institut fiir Angewandte Physik, Teehnische Hoehsehule Darmstadt, Germany Received 7 April 1977 Revised manuscript received 9 June 1977 Au films with island structure simultaneously show electroluminescence and electron emission. One c o m p o n e n t of the latter orginate from h o t electrons, the other from field effect.

In the course of investigations on thin Au films with island structure we found elecXroluminescence spectra with an intensity W which may be characterized by the equation W ~ exp(-hv/kTel), where Tel is the temperature of a hot electron gas localized in some tens of spot-like regions [1]. It was our ambition to reconsider our interpretations with the help of a more direct method, the energy analysis of the emitted electrons. On the integral electron emission from those films Borzjak et al. [2] have already reported. The sample preparation has been described elsewhere [1,3], sample arrangement is inserted in fig. 1. After placing the quartz substrates in the vacuum chamber, evaporation of these island films, forming processes and all measurements were realized without breaking the vacuum. In fig. 1 the typical d.c. I s - Vs characteristic of a formed island film diode is shown, also the integral visible electroluminescence intensity W and the integral electron emission current I e. The latter was measured with a simple two electrodes arrangement with 10 mm distance between island film and collector plate, a bias voltage of 100 V, and a Keithley model 602 electrometer. The similarity of the three curves up to about 6 V is evident. The following descending current I s was interpreted to be due to interrupture of the filamentary current paths [ 1 ]. We suppose that at the higher voltages the electron emission is controlled by field emission from Au-islands in the locally very strong field between the islands. A dependence in the form of the Fowler-Nordheim equation I e ~ U 2 × exp(-a/Us) is to observe in this region. To analyse the electron energy spectrum a retarding potential method was used. Part of the experimental arrangement is shown in fig. 2. The steel grid electrodes El, E 2, E 3 had 250 #m meshes and a wire diam512

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eter of 25 gin. The aperture of the front electrode of the collector tube was alternatively between 0.3 and 2 mm. The electron detector at the end of the tube was a channel electron multiplier (channeltron), operated at 4.5 kV. The collector tube was mounted on an X - Y coordinate guide sledge for adjusting in an optimum position. Furthermore fig. 2 shows the electron emission current pulses counted by multi-channel analyser where the channel number is dependent on the voltage Vg between the grid and the cathode of the sample. Thus, ~p= Vg + (~bFe -- q~Au)/q is the height of the retarding potential barrier with respect to the vacuum level of the sample-cathode. Here the qTs are the work functions of the Fe-grid electrodes and of the Au-

Volume 62A, number 7

PHYSICS LETTERS

3 October 1977

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Fig. 2. Shown are: Electron emission current pulses (in arbitrary units) versus retarding voltage Vg between the Fermi levels of the retarding grid (Fe) and the sample cathode (Au), v;ith the bias voltage on the samples Vs a parameter, for clearness curves are shifted vertically. Duration of each run: 200-600 s, linear ramp voltage with a repetition frequency of about 10 Hz. Given are also the grid voltages of the electron collector and the principal arrangement of the collector tube with the geometrical data. sample-electrode material. Work functions from real surfaces are known to be about 4.6 eV for Fe, and lie between 4 and 4.9 eV for Au films. The zero point of than should be shifted by less than +0.3 V to - 0 . 6 V compared with the Vg scale. Especially from the 4 V curve, the existence of electrons with energies greater than the vacuum level of the cathode is evident. This means, that there were obviously some hot electrons with an energy much higher than the Fermi level o f the negative electrode of the Au-island film. On the basis of our earlier studies we proposed a model of discrete current paths including small regiops with a high field gradient in about the middle between the electrodes. The hot electron gas, is then created by field acceleration and impacts and can reach temperatures up to about 6000 K. The appearance o f hot electrons in the emission current agrees with this model. Unfortunately it is not possible to get a higher resolution in the energy distribution of the electrons. The reason is the high electrical field in the film plane perpendicular to the normal direction in which the

collector is positioned. The electrons gain parallel impuls components which cannot be treated quantitatively, and which are dependent on the direction and on the transient time through the strongly inhomogenous field of the small emission areas. Experimentally, we found strongly deformed emission characteristics controllable by the bias voltage of the sample and by the voltage of the collector arrangement. Considering this complexity it is clear, that it is impossible to get any quantitatively exact energy spectrum from these measured dates, which in other cases commonly are obtained from the differentiated I e - Vg characteristic. Nevertheless, we have estimated electron temperatures Tel from the greatest slopes of the logarithmically drawn high energetical parts of the Ie Vg curves. One example is: Tel ~ 6000 K from the Vs = 4 V curve in fig. 2, in agreement with our interpretation [1]. The shape of the higher voltage curves show the predominance of field emission beyond the shoulder near about Vg = - 5 V. -

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Volume 62A, number 7

PHYSICS LETTERS

References [1] M. Bischoff and H. Pagnia, Thin Solid Films 29 (1975) 303.

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3 October 1977

[2] P.G. Borzjak, I.A. Sarbey and R.D. Fedorowitsch, Phys. Star. Sol. 8 (1965) 55. [3] H. Morgenthaler and H. Pagnia, Phys. Lett. 42A (1973) 529.