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On the electroluminescence from evaporated thin gold films
Volume 42A, number 7
PHYSICS LETTERS
29 January 1973
ON THE ELECTROLUMINESCENCE FROM EVAPORATED THIN GOLD FILMS H. MORGENTHALER and H. PAGNIA Fachbereich Physik, Technische Hochschule, Darmstadt, Germany Received 3 October 1972 Au films with island structure show a spot-like electroluminescence (EL) under a low dc or ac voltage. I-V characteristics and their relation to the EL, and some EL features were investigated.
Since the first observation of an electroluminescence (EL) from thin metal films with island structure published by Borzjaket a!. [1] in 1965 there was given no further information on this topic of principal interest. The interpretation of the phenomenon was field heating of electrons tunneling from island to island and subsequent radiative transitions at the end of the free paths. The distances of the quantized electron energy levels in the small particles did not exceed kT at r.t. [2] Thus, in the EL no influence from discrete transitions should be observable. Independent from the real structure of the films, the maximum photon energy should be hVm~= eV, with V the voltage drop over the whole film, and the probability for lower energy transitions should then be strongly increased. We shall report here on our first results from studies on thIs type of EL; but the interpretation is at the time rather obscured. EL behaviour was found on thin gold films evaporated on glas or quartz substracts in a vacuum of about 10—6 torr. Electron micrographs of carbon-platinum replici of typical films show, that the islands are randomly distributed in either size (mean values about 400 A), shape, and distances (mean values about 50 A). EL behaviour is only observable in a small region of film structures lying between discontinously, well isolated particles and continous films. Electrically, this corresponds to a few decades above the inset of conductance (as a function of mean thickness) due to quantum mechanical tunneling. Typical current densities were 2 under a voltage of 1V, estimated l0~to l0~A/cmthickness laying between 70 and with a weighing 90 A. A static I-V characteristic is shown in fig. 1. Al measurements were done in a vacuum, and at r.t.
50
~
1Au_J-L-10
I I
/
/
110 ..., 5
T 18fF?!!)
I
~
~ 50
20~30pm~
Sliver Paint Au Island Strijcture Film
/.c5.-cz-.stOrtif~Tof visible EL “°~-°-~‘—--°
7’
.
RI
it) 1 Voltage IV)
Fig. 1. Del-V-characteristic of an Au island film. Insets: the exponential current rise at low voltages, and the sample arrangement.
Au - - L - 11 EL ON MAX OFF
t
.~‘
-
-
~~rZ 1’)
—
5Ocps -Sine R. I.
4 6 8 Voltage0 2 IV)
Fig. 2. Reproduction of ac osdillograms: the numbers mark endpoints of arbitrarily chosen I-V-characteristics.
529
Volume 42A, number 7
PHYSICS LETTERS
The rise at low voltages is by good approximation exponentially in agreement with theoretical assumptions for this type of conductors [3] The deviation from the exponentail shape was only found in the case of films showing EL. Films are often non-stable after a too high current has passed, curves shapes are then changed (see the return curve in fig. 1). Possibly, the island structure is by this way irreversibly altered. The decrease of the current at higher voltages is assumingly at least partially a temperature effect. Dynamic I-V characteristics are shown in fig. 2. No maximum is seen, but the curves flatten by leaps from a certain critical voltage V~with increasing voltage, and the envelope is similar shaped like the dc characteristic. Generally, below V~a recovery of the initial shape is observed within the order of 10 s. The EL starts usually at about 2 to 3 V, and the intensity increases exponentially (fig. 3). It seems that EL starting is under dc operation correlated to the deviations of the I-V curve from the exponential shape. The following strong EL increase is then correlated to a current decrease. The electrical power input seems thus split off in a light power output and heating of the sample. Unter ac operation the EL intensity shows a maximum and switches off at a high voltage. This behaviour is marked by arrows in fig. 2. The EL shows some characteristic features. The sources of the light are single and usually isolated spots randomly distributed nearly along a line (inset in fig. 3) which lies about middle between the electrodes. We never saw more than one spot in the current direction, But the spots are also to find on their line in the inhomogenous field region outside the edges of the electrodes when the latter are smaller than the spread of the evaporated area. In a triangular electrode arrangement with a distance ranging from 15 to 250 ~zm EL spots were present up to 200 j.tm, respectively a mean field strength of 5 X 102 V/cm. In the large distance region we found fluctuations of parts of the spot lines between parallel positions in about some tenth micron distances. It is furtherregion to mention thatabout EL is10—6 observable wide pressure between torr (orin a
29 January 1973 +
-.
50
.
530
:
Au -I -N- 1 ThickFitm Au f(~Spots
I
sUt,~c~
.~
+
.~
10 Visible EL .~
R.T.
~
0
Volta e
IV)
15
Fig. 3. Electro-Luminescence (EL) intensity detected with a photomultiplier with no spectral selection. The inset sketches the spot-like EL behaviour.
lower [1]) and 1 torr, and no direct correlation to the gas pressure was to observe, whilst some changes occure like in the long time operations with high current densities, but at fixed pressures. The most features of the EL spots are in contrast to what one should assume from the model of electron heating. Mean free paths are shorter than the half distance between the electrodes. It is further not to see why spots are localized on a line, and why at large enough voltages there is no more than one spot alonge a current path. Usually the spots emit a white light sometimes we saw a few red points even at 2 V. That means, photons with energies hi-’ > eV, too, are emitted. We are now looking for the spectral photon energy distribution using a photon counter of high sensitivity, and hope to get more information and to find a better interpretation of this EL phenomenon. -
References
[11 P.G.
Borzjak, 1G. Sarbej and R.D. Fedorowitsch, Phys. Stat. Sol. 8 (1965) 55. [2] P.M. Tomchuk and RD. Fedorowich, Soviet Phys. State 8 (1966) 226. [31 Solid R.M. Hill, Proc. Roy. A309 (1969) 377.
PHYSICS LETTERS
29 January 1973
ON THE ELECTROLUMINESCENCE FROM EVAPORATED THIN GOLD FILMS H. MORGENTHALER and H. PAGNIA Fachbereich Physik, Technische Hochschule, Darmstadt, Germany Received 3 October 1972 Au films with island structure show a spot-like electroluminescence (EL) under a low dc or ac voltage. I-V characteristics and their relation to the EL, and some EL features were investigated.
Since the first observation of an electroluminescence (EL) from thin metal films with island structure published by Borzjaket a!. [1] in 1965 there was given no further information on this topic of principal interest. The interpretation of the phenomenon was field heating of electrons tunneling from island to island and subsequent radiative transitions at the end of the free paths. The distances of the quantized electron energy levels in the small particles did not exceed kT at r.t. [2] Thus, in the EL no influence from discrete transitions should be observable. Independent from the real structure of the films, the maximum photon energy should be hVm~= eV, with V the voltage drop over the whole film, and the probability for lower energy transitions should then be strongly increased. We shall report here on our first results from studies on thIs type of EL; but the interpretation is at the time rather obscured. EL behaviour was found on thin gold films evaporated on glas or quartz substracts in a vacuum of about 10—6 torr. Electron micrographs of carbon-platinum replici of typical films show, that the islands are randomly distributed in either size (mean values about 400 A), shape, and distances (mean values about 50 A). EL behaviour is only observable in a small region of film structures lying between discontinously, well isolated particles and continous films. Electrically, this corresponds to a few decades above the inset of conductance (as a function of mean thickness) due to quantum mechanical tunneling. Typical current densities were 2 under a voltage of 1V, estimated l0~to l0~A/cmthickness laying between 70 and with a weighing 90 A. A static I-V characteristic is shown in fig. 1. Al measurements were done in a vacuum, and at r.t.
50
~
1Au_J-L-10
I I
/
/
110 ..., 5
T 18fF?!!)
I
~
~ 50
20~30pm~
Sliver Paint Au Island Strijcture Film
/.c5.-cz-.stOrtif~Tof visible EL “°~-°-~‘—--°
7’
.
RI
it) 1 Voltage IV)
Fig. 1. Del-V-characteristic of an Au island film. Insets: the exponential current rise at low voltages, and the sample arrangement.
Au - - L - 11 EL ON MAX OFF
t
.~‘
-
-
~~rZ 1’)
—
5Ocps -Sine R. I.
4 6 8 Voltage0 2 IV)
Fig. 2. Reproduction of ac osdillograms: the numbers mark endpoints of arbitrarily chosen I-V-characteristics.
529
Volume 42A, number 7
PHYSICS LETTERS
The rise at low voltages is by good approximation exponentially in agreement with theoretical assumptions for this type of conductors [3] The deviation from the exponentail shape was only found in the case of films showing EL. Films are often non-stable after a too high current has passed, curves shapes are then changed (see the return curve in fig. 1). Possibly, the island structure is by this way irreversibly altered. The decrease of the current at higher voltages is assumingly at least partially a temperature effect. Dynamic I-V characteristics are shown in fig. 2. No maximum is seen, but the curves flatten by leaps from a certain critical voltage V~with increasing voltage, and the envelope is similar shaped like the dc characteristic. Generally, below V~a recovery of the initial shape is observed within the order of 10 s. The EL starts usually at about 2 to 3 V, and the intensity increases exponentially (fig. 3). It seems that EL starting is under dc operation correlated to the deviations of the I-V curve from the exponential shape. The following strong EL increase is then correlated to a current decrease. The electrical power input seems thus split off in a light power output and heating of the sample. Unter ac operation the EL intensity shows a maximum and switches off at a high voltage. This behaviour is marked by arrows in fig. 2. The EL shows some characteristic features. The sources of the light are single and usually isolated spots randomly distributed nearly along a line (inset in fig. 3) which lies about middle between the electrodes. We never saw more than one spot in the current direction, But the spots are also to find on their line in the inhomogenous field region outside the edges of the electrodes when the latter are smaller than the spread of the evaporated area. In a triangular electrode arrangement with a distance ranging from 15 to 250 ~zm EL spots were present up to 200 j.tm, respectively a mean field strength of 5 X 102 V/cm. In the large distance region we found fluctuations of parts of the spot lines between parallel positions in about some tenth micron distances. It is furtherregion to mention thatabout EL is10—6 observable wide pressure between torr (orin a
29 January 1973 +
-.
50
.
530
:
Au -I -N- 1 ThickFitm Au f(~Spots
I
sUt,~c~
.~
+
.~
10 Visible EL .~
R.T.
~
0
Volta e
IV)
15
Fig. 3. Electro-Luminescence (EL) intensity detected with a photomultiplier with no spectral selection. The inset sketches the spot-like EL behaviour.
lower [1]) and 1 torr, and no direct correlation to the gas pressure was to observe, whilst some changes occure like in the long time operations with high current densities, but at fixed pressures. The most features of the EL spots are in contrast to what one should assume from the model of electron heating. Mean free paths are shorter than the half distance between the electrodes. It is further not to see why spots are localized on a line, and why at large enough voltages there is no more than one spot alonge a current path. Usually the spots emit a white light sometimes we saw a few red points even at 2 V. That means, photons with energies hi-’ > eV, too, are emitted. We are now looking for the spectral photon energy distribution using a photon counter of high sensitivity, and hope to get more information and to find a better interpretation of this EL phenomenon. -
References
[11 P.G.
Borzjak, 1G. Sarbej and R.D. Fedorowitsch, Phys. Stat. Sol. 8 (1965) 55. [2] P.M. Tomchuk and RD. Fedorowich, Soviet Phys. State 8 (1966) 226. [31 Solid R.M. Hill, Proc. Roy. A309 (1969) 377.