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Photoresponse of MIMIM triodes
Thin Solid Films, 38 (1976) 61-66
61
© Elsevier Sequoia S.A., Lausanne -- Printed in Switzerland
PHOTORESPONSE
O F MIMIM T R I O D E S
K. H. GUNDLACHand J. KADLEC Max-Planck-Institut fiir Physik,und Astrophysik, 8 Miinchen 40 (F.R.G.)
(Received February 22, 1976; accepted April 13, 1976)
The investigation of the reversal o f the photocurrent polarity with changing p h o t o n energy in MIM diodes is extended to MIMIM threeelectrode structures. The spectral dependence of the photocurrent between the different pairs of electrodes of a triode is shown to be strongly controlled by the electrode thickness and the direction of illumination. Interesting results are observed when the electrode thickness is below 200 A.
1, INTRODUCTION Recently we reported on the spectral dependence of the photoresponse in MIM structures 1.2. It was shown that they can exhibit reversal of the photocurrent polarity with changing p h o t o n energy and t h a t this effect is controlled by the electrode thickness and the direction of illumination. To understand the p h e n o m e n o n both theoretical and further experimental work is necessary. An extensive theory of internal photoemission in sandwich structures has been given recently 3. In the present paper we describe experimental investigations on AI-(A1 oxide)-Al-(A1 oxide)-Al sandwich triodes. The thickness of the A1 electrodes was varied from triode to triode but the thickness of the oxide layers was always held at about 35 A. The photoresponse between the different pairs of electrodes of the triode structures was measured w i t h o u t applying an external voltage. It will be shown that thicker electrodes give rise to the usual unipolar photocurrent but sufficiently thin electrodes can cause sign reversal of the current as the p h o t o n energy is changed.
2. E X P E R I M E N T A L
DETAILS
The arrangement o f the triode is illustrated in Fig. 1. First an A1 film was evaporated onto a glass microscope slide. We refer to this film as the base electrode b. N e x t an A1 oxide layer of about 35 A was formed on the base electrode by plasma oxidation. Then the middle electrode m was evaporated and its surface was plasma-oxidized to about 35 A. Finally the top electrode t was evaporated. The thickness of the oxide layer was obtained
62
K.H. GUNDLACH, J. KADLEC / / /
x/D: /y/
1.5 mm
ZZ
U
=
(a)
/ z / /
(b)
(c)
/ / / /
/ / / /
(d)
Fig. 1. Fabrication of MIMIM triodes: (a) deposition of-the base electrode b and oxidation; (b) deposition of the middle electrode m and oxidation; (c) deposition of the top electrode t; (d) view through the glass substrate. The blank areas in (c) and (d) illustrate the light spot when the top and the base electrode, respectively, are illuminated.
/
/ Light
%
(~tb bottom of the insulator conduction band
Photoemissionl.--w" ~
,rom t .
Ir
t~,.ct,o,,
l.
/ II
_
II
F.,m~-
~,.ct,on
Fig. 2. Illustration of the cross section of the triode system and the notation of the photoresponse between different pairs of electrodes when the top electrode t and the base electrode b are illuminated. The lower part shows schematically the band model of the triode.
f r o m c a p a c i t a n c e m e a s u r e m e n t s assuming a relative dielectric c o n s t a n t o f 8.4 f o r t h e A1 o x i d e 4. T h e c a p a c i t a n c e thickness is, o f course, n o t an e x a c t value o f t h e actual i n s u l a t o r thickness b u t provides a reasonable i n d i c a t i o n f o r the c o n s t a n c y o f t h e thickness f r o m sample to sample. T h e d e p o s i t i o n o f t h e e l e c t r o d e s w i t h a desired thickness was c o n t r o l l e d b y a q u a r t z ~ r y s t a l m o n i t o r . T h e a c c u r a c y o f this thickness m e a s u r e m e n t was p r o b a b l y b e t t e r t h a n _+ 10 A, as e s t i m a t e d f r o m optical i n t e r f e r e n c e calibrations o f the q u a r t z monitor.
PHOTORESPONSE OF MIMIM TRIODES
63
The experiments consisted of illuminating the base or top electrode with monochromatic light chopped at a frequency of 45 Hz, and detecting the photoresponse with a lock-in amplifier. The amplifier (including the load resistor) was connected to the pair of electrodes across which the response was measured b u t there was no external connection b e t w e e n these t w o electrodes and the remaining third electrode. For illumination of the top electrode (Fig. 2), the photoresponse between middle and base, top and middle, and t o p and base electrodes is denoted by (1)rob, (1)tin and (1)tb, respectively. The symbols (2)rob, (2)tm and (2)tb correspond to the analogous cases with the base electrode illuminated. We follow the usual practice s-7 and plot the square root o f the photocurrent per incident p h o t o n (in arbitrary units) as a function o f p h o t o n energy hr. This facilitates the comparison o f the present data with other published photoresponse curvesS-S; in addition, one can immediately see the energy range over which the photocurrent follows the Fowler relation J ~ (hv -- ~)2 where ~ is the barrier height for carrier injection from the electrodes into the oxide layer. In the figures, positive values o f J ~ refer to electron current through the oxide layer from the top to the middle and from the middle to the base electrode. Negative values of J~ indicate an electron current in the opposite direction.
3. RESULTS i n the first three experiments to be described the thickness o f the base electrode was k e p t almost constant ( 1 6 0 - 1 8 0 A) whilst the thicknesses of the middle and t o p electrodes were consecutively decreased from the 200 A to the 100 A range in such a w a y that both electrodes always had nearly the same thickness. In the experiment of Fig. 3 the base electrode was 170 A and the middle and top electrodes were 200 A thick. The resulting spectral response was as expected 5-s. Illumination of the t o p electrode gave a signal which corresponded to electron flow through the insulator from the top to the middle electrode (curve (1)tin) and from the middle to the base electrode (curve (1)rob). The response between the directly illuminated (top) electrode and the middle electrode was larger and its onset was at somewhat lower p h o t o n energy than the response between the middle and the base electrode. Analogous results were obtained on illuminating the base electrode. This induced an electron current through the insulator from the base to the middle electrode (curve (2)rob) and from the middle to the top electrode (curve (2)tin)- Here again the response between the directly illuminated and the middle electrode was larger and its onset was at somewhat lower p h o t o n energy than between the middle and the top electrode. Note that in the experiment o f Fig. 3 a variation o f the p h o t o n energy gave no reversal o f the photocurrent polarity in the measured range. This remained true even when thicker electrodes were evaporated. Next we turn to measurements on triodes with thinner electrodes. In Fig. 4 the thicknesses of the top and middle electrodes were decreased to
64
K.H. GUNDLACH, J. KADLEC
7O 3½
,v,,~.,~Cd, i~gltt V,,~;.? .... ..
4O
r///J
20
10
-10 -20
-40 -50
5O ,
40
m
1
I
30 20
•
I
°,,~,,.o.o!s5.<,.. ° 3,0 3.5 hv /,V] "~o,o, ". . . . . . . . ° l ~ . . . . • k, ~'~')t m
k
~_
10
0
-10 o~
-30
,0
t'///I
~
3O
0
V///~
-20
"°',o, "q.
"~)~
°'"°" "°" b
-30 -40
.t ;
--+-J]' . . . . . . . . . . // 1.5
.+~I~'~... -, . . . . . . . . . .
"oo,
.~ "~ .o"" :o" ......
(~tm o
~, "~., ~'o
"'~-,-~b ~,
Fig. 3. Square root of the photocurrent J per incident photor~ (in arbitrary units) us.
photon energy hP. The labels (1)t m and (1)m b ((2)t m and (2)rob) refer to the photoresponse across top and middle and middle and base electrode, respectively, when the top (base) electrode is illuminated. The inset shows the cross section of the triode; the electrode thicknesses and the direction of illumination are also displayed. Fig. 4. As Fig. 3 but with a thickness of all three electrodes of about 160 A.
1 6 0 A w h i l s t t h e t h i c k n e s s o f t h e base e l e c t r o d e w a s a l m o s t t h e s a m e as in Fig. 3. S u c h a m o d i f i c a t i o n had n o qualitative e f f e c t o n t h e curves (1)tin, (1)m b and (2)rob, as c a n be seen in Figs. 3 and 4. Curve (2)tin, h o w e v e r , e x h i b i t s sign reversal near 3 . 3 5 eV. N o t e t h a t t h e e f f e c t w a s o b s e r v e d b e t w e e n t h e m i d d l e e l e c t r o d e and t h e o u t e r e l e c t r o d e t h a t w a s n o t directly illuminated. In t h e s p e c i m e n o f Fig. 5, the t h i c k n e s s e s o f t h e t o p and m i d d l e e l e c t r o d e s w e r e further decreased t o 1 2 0 A w h i l s t t h e t h i c k n e s s o f t h e base e l e c t r o d e w a s nearly t h e s a m e as in Figs. 3 and 4. This h a d t w o e f f e c t s o n t h e p h o t o r e s p o n s e curves: first, t h e z e r o current p o i n t o f c u r v e (2)t m Was s h i f t e d f r o m a b o u t 3 . 3 5 e V t o 2.9 eV; s e c o n d , curve (1)m b also e x h i b i t e d sign reversal w i t h t h e z e r o c u r r e n t at a b o u t 3 . 1 5 eV. As s e e n in Fig. 6, a decrease o f t h i c k n e s s o f all three e l e c t r o d e s t o t h e 1 0 0 A range shifted t h e e n e r g y at w h i c h curves (2)tin and (1)m b passed t h r o u g h z e r o t o a still l o w e r p h o t o n energy. S u m m a r i z i n g t h e results so far, a s i m u l t a n e o u s decrease o f t h e t h i c k n e s s o f all three e l e c t r o d e s f r o m t h e 2 0 0 A range (Fig. 3) t o t h e 1 0 0 A range (Fig. 6) h a d n o qualitative e f f e c t o n t h e p h o t o r e s p o n s e curves m e a s u r e d across t h e m i d d l e e l e c t r o d e and t h e directly i l l u m i n a t e d o u t e r e l e c t r o d e ,
PHOTORESPONSE OF MIMIM TRIODES
65
/
:)I/~
?/
1oo
$0
50
60
40
I-A/[ i"A ~I f/A/A'/1
~
10 tm
20
o'"
d
i
-I0
o -50
Qb
"~"
-20
b -40
o
/~)mb ,¢ R
"~,. -100
Fig. 5. As Fig. 3 but with the thickness of the middle and top electrodes decreased to about 120 A. Fig. 6. As Fig. 3 but with the thickness o f all three electrodes in the 100 ~ range.
but it induced a sign reversal o f the photoresponse curve measured across the middle electrode and the outer electrode that was not directly illuminated. The photon energy at which the current passed through zero could be controlled by appropriately varying the electrode thickness. For a given triode, the zero current point occurred at lower energy when the base electrode was illuminated or, in other words, when it was observed between top and middle electrodes. A further ~ was to study under what conditions the photocurrent between the directly illuminated electrode and the middle electrode exhibits sign reversal. It was found that a sufficiently thin middle electrode can cause such an effect. Figure 7 shows the example of a triode with the middle electrode only about 50 A thick. The top electrode was illuminated. (The results were qualitatively similar when the base electrode was illuminated.) In contrast to Figs. 4 and 6, it is seen in Fig. 7 that curve (1)tin exhibits sign reversal while curve (1)rob is unipolar. We also show the response measured between top and base electrodes (curve (1)tb) because this curve changed polarity twice, near 2.4 eV and 3.05 eV. This result is expected when one adds the response between middle and top and middle and base electrodes. It is apparent from what has been described so far that an appropriate variation of the "electrode thicknesses produces a wide variety of photoresponse curves between the different pairs of electrodes. The explanation
66
K.H. GUNDLACH, J. KADLEC
j½ 6O 5O 40
7/
30 20 10
0 -10
\,
-20 -30 -40
Fig. 7. Photoresponse curves when the middle electrode is very thin. The signal between
top and base electrodes exhibits sign reversal twice.
o f this effect is probably complex. It should include the energy dependence o f the optical absorption and o f electron scattering in the electrodes. We believe that the theory o f photoresponse in sandwich structures given by Kadlec s and applied recently to the calculations of the excitation function in MIM diodes 9 can provide a base for the understanding of the photocurrent curves o f the MIMIM triodes under consideration.
ACKNOWLEDGMENT
The authors are indebted to Mr. and Mrs. Lindner for sample preparation and technical assistance.
REFERENCES K . H . Gundlach and J. Kadlec, Thin 8olid Films, 28 (1975) 107. K. H. Gundlach and J. Kadlec, Appl. Phys. Lett., 27 (1975) 429. J. Kadlec, Phys. Rep., 26C (1976) 69. W. I. Bernard and J. W. Cook, J. Electrochem. 8oc., 106 (1959) 643. A.M. Goodman, J. Electrochem. 8oc., 115 (1968) 276c. R. Williams, in R. K. Willardson and A. C. Beer (eds.), Semiconductors and Semimetals, Vol. 6, Academic Press, New York, London, 1970, p. 97. 7 G. Lewicki, J. Maserjian and C. A. Mead, J. Appl. Phys., 43 (1972) 1764. 8 J. Kadlec and K. H. Gundlach, Phys. Status 8olidiA, 37 (1976) 11. 9 J. Kadlec and K. H. Gundlach, J. Appi. Phys., 47 (1976) 672. 1 2 3 4 5 6
61
© Elsevier Sequoia S.A., Lausanne -- Printed in Switzerland
PHOTORESPONSE
O F MIMIM T R I O D E S
K. H. GUNDLACHand J. KADLEC Max-Planck-Institut fiir Physik,und Astrophysik, 8 Miinchen 40 (F.R.G.)
(Received February 22, 1976; accepted April 13, 1976)
The investigation of the reversal o f the photocurrent polarity with changing p h o t o n energy in MIM diodes is extended to MIMIM threeelectrode structures. The spectral dependence of the photocurrent between the different pairs of electrodes of a triode is shown to be strongly controlled by the electrode thickness and the direction of illumination. Interesting results are observed when the electrode thickness is below 200 A.
1, INTRODUCTION Recently we reported on the spectral dependence of the photoresponse in MIM structures 1.2. It was shown that they can exhibit reversal of the photocurrent polarity with changing p h o t o n energy and t h a t this effect is controlled by the electrode thickness and the direction of illumination. To understand the p h e n o m e n o n both theoretical and further experimental work is necessary. An extensive theory of internal photoemission in sandwich structures has been given recently 3. In the present paper we describe experimental investigations on AI-(A1 oxide)-Al-(A1 oxide)-Al sandwich triodes. The thickness of the A1 electrodes was varied from triode to triode but the thickness of the oxide layers was always held at about 35 A. The photoresponse between the different pairs of electrodes of the triode structures was measured w i t h o u t applying an external voltage. It will be shown that thicker electrodes give rise to the usual unipolar photocurrent but sufficiently thin electrodes can cause sign reversal of the current as the p h o t o n energy is changed.
2. E X P E R I M E N T A L
DETAILS
The arrangement o f the triode is illustrated in Fig. 1. First an A1 film was evaporated onto a glass microscope slide. We refer to this film as the base electrode b. N e x t an A1 oxide layer of about 35 A was formed on the base electrode by plasma oxidation. Then the middle electrode m was evaporated and its surface was plasma-oxidized to about 35 A. Finally the top electrode t was evaporated. The thickness of the oxide layer was obtained
62
K.H. GUNDLACH, J. KADLEC / / /
x/D: /y/
1.5 mm
ZZ
U
=
(a)
/ z / /
(b)
(c)
/ / / /
/ / / /
(d)
Fig. 1. Fabrication of MIMIM triodes: (a) deposition of-the base electrode b and oxidation; (b) deposition of the middle electrode m and oxidation; (c) deposition of the top electrode t; (d) view through the glass substrate. The blank areas in (c) and (d) illustrate the light spot when the top and the base electrode, respectively, are illuminated.
/
/ Light
%
(~tb bottom of the insulator conduction band
Photoemissionl.--w" ~
,rom t .
Ir
t~,.ct,o,,
l.
/ II
_
II
F.,m~-
~,.ct,on
Fig. 2. Illustration of the cross section of the triode system and the notation of the photoresponse between different pairs of electrodes when the top electrode t and the base electrode b are illuminated. The lower part shows schematically the band model of the triode.
f r o m c a p a c i t a n c e m e a s u r e m e n t s assuming a relative dielectric c o n s t a n t o f 8.4 f o r t h e A1 o x i d e 4. T h e c a p a c i t a n c e thickness is, o f course, n o t an e x a c t value o f t h e actual i n s u l a t o r thickness b u t provides a reasonable i n d i c a t i o n f o r the c o n s t a n c y o f t h e thickness f r o m sample to sample. T h e d e p o s i t i o n o f t h e e l e c t r o d e s w i t h a desired thickness was c o n t r o l l e d b y a q u a r t z ~ r y s t a l m o n i t o r . T h e a c c u r a c y o f this thickness m e a s u r e m e n t was p r o b a b l y b e t t e r t h a n _+ 10 A, as e s t i m a t e d f r o m optical i n t e r f e r e n c e calibrations o f the q u a r t z monitor.
PHOTORESPONSE OF MIMIM TRIODES
63
The experiments consisted of illuminating the base or top electrode with monochromatic light chopped at a frequency of 45 Hz, and detecting the photoresponse with a lock-in amplifier. The amplifier (including the load resistor) was connected to the pair of electrodes across which the response was measured b u t there was no external connection b e t w e e n these t w o electrodes and the remaining third electrode. For illumination of the top electrode (Fig. 2), the photoresponse between middle and base, top and middle, and t o p and base electrodes is denoted by (1)rob, (1)tin and (1)tb, respectively. The symbols (2)rob, (2)tm and (2)tb correspond to the analogous cases with the base electrode illuminated. We follow the usual practice s-7 and plot the square root o f the photocurrent per incident p h o t o n (in arbitrary units) as a function o f p h o t o n energy hr. This facilitates the comparison o f the present data with other published photoresponse curvesS-S; in addition, one can immediately see the energy range over which the photocurrent follows the Fowler relation J ~ (hv -- ~)2 where ~ is the barrier height for carrier injection from the electrodes into the oxide layer. In the figures, positive values o f J ~ refer to electron current through the oxide layer from the top to the middle and from the middle to the base electrode. Negative values of J~ indicate an electron current in the opposite direction.
3. RESULTS i n the first three experiments to be described the thickness o f the base electrode was k e p t almost constant ( 1 6 0 - 1 8 0 A) whilst the thicknesses of the middle and t o p electrodes were consecutively decreased from the 200 A to the 100 A range in such a w a y that both electrodes always had nearly the same thickness. In the experiment of Fig. 3 the base electrode was 170 A and the middle and top electrodes were 200 A thick. The resulting spectral response was as expected 5-s. Illumination of the t o p electrode gave a signal which corresponded to electron flow through the insulator from the top to the middle electrode (curve (1)tin) and from the middle to the base electrode (curve (1)rob). The response between the directly illuminated (top) electrode and the middle electrode was larger and its onset was at somewhat lower p h o t o n energy than the response between the middle and the base electrode. Analogous results were obtained on illuminating the base electrode. This induced an electron current through the insulator from the base to the middle electrode (curve (2)rob) and from the middle to the top electrode (curve (2)tin)- Here again the response between the directly illuminated and the middle electrode was larger and its onset was at somewhat lower p h o t o n energy than between the middle and the top electrode. Note that in the experiment o f Fig. 3 a variation o f the p h o t o n energy gave no reversal o f the photocurrent polarity in the measured range. This remained true even when thicker electrodes were evaporated. Next we turn to measurements on triodes with thinner electrodes. In Fig. 4 the thicknesses of the top and middle electrodes were decreased to
64
K.H. GUNDLACH, J. KADLEC
7O 3½
,v,,~.,~Cd, i~gltt V,,~;.? .... ..
4O
r///J
20
10
-10 -20
-40 -50
5O ,
40
m
1
I
30 20
•
I
°,,~,,.o.o!s5.<,.. ° 3,0 3.5 hv /,V] "~o,o, ". . . . . . . . ° l ~ . . . . • k, ~'~')t m
k
~_
10
0
-10 o~
-30
,0
t'///I
~
3O
0
V///~
-20
"°',o, "q.
"~)~
°'"°" "°" b
-30 -40
.t ;
--+-J]' . . . . . . . . . . // 1.5
.+~I~'~... -, . . . . . . . . . .
"oo,
.~ "~ .o"" :o" ......
(~tm o
~, "~., ~'o
"'~-,-~b ~,
Fig. 3. Square root of the photocurrent J per incident photor~ (in arbitrary units) us.
photon energy hP. The labels (1)t m and (1)m b ((2)t m and (2)rob) refer to the photoresponse across top and middle and middle and base electrode, respectively, when the top (base) electrode is illuminated. The inset shows the cross section of the triode; the electrode thicknesses and the direction of illumination are also displayed. Fig. 4. As Fig. 3 but with a thickness of all three electrodes of about 160 A.
1 6 0 A w h i l s t t h e t h i c k n e s s o f t h e base e l e c t r o d e w a s a l m o s t t h e s a m e as in Fig. 3. S u c h a m o d i f i c a t i o n had n o qualitative e f f e c t o n t h e curves (1)tin, (1)m b and (2)rob, as c a n be seen in Figs. 3 and 4. Curve (2)tin, h o w e v e r , e x h i b i t s sign reversal near 3 . 3 5 eV. N o t e t h a t t h e e f f e c t w a s o b s e r v e d b e t w e e n t h e m i d d l e e l e c t r o d e and t h e o u t e r e l e c t r o d e t h a t w a s n o t directly illuminated. In t h e s p e c i m e n o f Fig. 5, the t h i c k n e s s e s o f t h e t o p and m i d d l e e l e c t r o d e s w e r e further decreased t o 1 2 0 A w h i l s t t h e t h i c k n e s s o f t h e base e l e c t r o d e w a s nearly t h e s a m e as in Figs. 3 and 4. This h a d t w o e f f e c t s o n t h e p h o t o r e s p o n s e curves: first, t h e z e r o current p o i n t o f c u r v e (2)t m Was s h i f t e d f r o m a b o u t 3 . 3 5 e V t o 2.9 eV; s e c o n d , curve (1)m b also e x h i b i t e d sign reversal w i t h t h e z e r o c u r r e n t at a b o u t 3 . 1 5 eV. As s e e n in Fig. 6, a decrease o f t h i c k n e s s o f all three e l e c t r o d e s t o t h e 1 0 0 A range shifted t h e e n e r g y at w h i c h curves (2)tin and (1)m b passed t h r o u g h z e r o t o a still l o w e r p h o t o n energy. S u m m a r i z i n g t h e results so far, a s i m u l t a n e o u s decrease o f t h e t h i c k n e s s o f all three e l e c t r o d e s f r o m t h e 2 0 0 A range (Fig. 3) t o t h e 1 0 0 A range (Fig. 6) h a d n o qualitative e f f e c t o n t h e p h o t o r e s p o n s e curves m e a s u r e d across t h e m i d d l e e l e c t r o d e and t h e directly i l l u m i n a t e d o u t e r e l e c t r o d e ,
PHOTORESPONSE OF MIMIM TRIODES
65
/
:)I/~
?/
1oo
$0
50
60
40
I-A/[ i"A ~I f/A/A'/1
~
10 tm
20
o'"
d
i
-I0
o -50
Qb
"~"
-20
b -40
o
/~)mb ,¢ R
"~,. -100
Fig. 5. As Fig. 3 but with the thickness of the middle and top electrodes decreased to about 120 A. Fig. 6. As Fig. 3 but with the thickness o f all three electrodes in the 100 ~ range.
but it induced a sign reversal o f the photoresponse curve measured across the middle electrode and the outer electrode that was not directly illuminated. The photon energy at which the current passed through zero could be controlled by appropriately varying the electrode thickness. For a given triode, the zero current point occurred at lower energy when the base electrode was illuminated or, in other words, when it was observed between top and middle electrodes. A further ~ was to study under what conditions the photocurrent between the directly illuminated electrode and the middle electrode exhibits sign reversal. It was found that a sufficiently thin middle electrode can cause such an effect. Figure 7 shows the example of a triode with the middle electrode only about 50 A thick. The top electrode was illuminated. (The results were qualitatively similar when the base electrode was illuminated.) In contrast to Figs. 4 and 6, it is seen in Fig. 7 that curve (1)tin exhibits sign reversal while curve (1)rob is unipolar. We also show the response measured between top and base electrodes (curve (1)tb) because this curve changed polarity twice, near 2.4 eV and 3.05 eV. This result is expected when one adds the response between middle and top and middle and base electrodes. It is apparent from what has been described so far that an appropriate variation of the "electrode thicknesses produces a wide variety of photoresponse curves between the different pairs of electrodes. The explanation
66
K.H. GUNDLACH, J. KADLEC
j½ 6O 5O 40
7/
30 20 10
0 -10
\,
-20 -30 -40
Fig. 7. Photoresponse curves when the middle electrode is very thin. The signal between
top and base electrodes exhibits sign reversal twice.
o f this effect is probably complex. It should include the energy dependence o f the optical absorption and o f electron scattering in the electrodes. We believe that the theory o f photoresponse in sandwich structures given by Kadlec s and applied recently to the calculations of the excitation function in MIM diodes 9 can provide a base for the understanding of the photocurrent curves o f the MIMIM triodes under consideration.
ACKNOWLEDGMENT
The authors are indebted to Mr. and Mrs. Lindner for sample preparation and technical assistance.
REFERENCES K . H . Gundlach and J. Kadlec, Thin 8olid Films, 28 (1975) 107. K. H. Gundlach and J. Kadlec, Appl. Phys. Lett., 27 (1975) 429. J. Kadlec, Phys. Rep., 26C (1976) 69. W. I. Bernard and J. W. Cook, J. Electrochem. 8oc., 106 (1959) 643. A.M. Goodman, J. Electrochem. 8oc., 115 (1968) 276c. R. Williams, in R. K. Willardson and A. C. Beer (eds.), Semiconductors and Semimetals, Vol. 6, Academic Press, New York, London, 1970, p. 97. 7 G. Lewicki, J. Maserjian and C. A. Mead, J. Appl. Phys., 43 (1972) 1764. 8 J. Kadlec and K. H. Gundlach, Phys. Status 8olidiA, 37 (1976) 11. 9 J. Kadlec and K. H. Gundlach, J. Appi. Phys., 47 (1976) 672. 1 2 3 4 5 6