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CdS SELF-ELECTRO-OPTIC EFFECT DEVICE
M. Wegener, A. Witt, C. Klingshirn, D. Gnass, Y. Iyechika, D. Jäger
To cite this version:
M. Wegener, A. Witt, C. Klingshirn, D. Gnass, Y. Iyechika, et al.. CdS SELF-ELECTRO- OPTIC EFFECT DEVICE. Journal de Physique Colloques, 1988, 49 (C2), pp.C2-109-C2-112.
�10.1051/jphyscol:1988224�. �jpa-00227641�
JOURNAL D E PHYSIQUE
Colloque C2, Supplement au n 0 6 , Tome 49, juin 1988
CdS SELF-ELECTRO-OPTIC EFFECT DEVICE
M. WEGENER, A. WITT, C. KLINGSHIRN, D. GNASS*
,
Y. IYECHIKA** and D.JXGER*
Fachbereich Physlk der Unlversitbt, Erwin Schrtidinger Str. 46, 0-6750 Kaiserslautern, F.R.G.
'~nstitut fifr Angewandte Physik der Universitdt, Corrensstr. 2-4, 0-4400 Milnster, F.R.G.
* * ~ a k a t Suki Research Laboratory, Sumi Torno Chemical Company, Japan
Abstract
-
We present a CdS-SEED based on photothennal induced absorption optical bistability at room temperature. It is demonstrated, that this element may controll (a) photonics with photonics (whereby e.g. the gain of an optical transistor can be engineered via the applied voltage). (b) pho- tonics with electronics (with big contrast ratio and voltages which are compatible with usual electronic logic levels) and (c) electronics with photonics with reasonable contrast ratio.INTRODUCTION.
O ~ t i c a l data communication via fiber ootics needs good electro-o~tic and ovto-electric interconnects. It has been shown1 that the so-called self-electro-optic effect device (SEED) is capable t i fullfill both tasks. Further on optical bistability has been obtained with such a device1, which allows to controll photonics with photonics. These SEED devices are based on the quantum confined Stark effect2, which is observed in quantum well materials. On the other hand also photothermal mechanisms in bulk semiconductor platelets may be used. The platelet is contacted with two metal contacts forming a small slit of width w (we100-200pm) to transmit a light beam. Such SEED de- vices have been demonstrated in Si and InP3. In the present paper we present a CdS-SEED which works with the green (514nm) line of an AT+-laser. The results and specific advantages of this device are presented as follows: in I we characterize our element and compare the results with model calculations in I1 we demonstrate its capability to serve as an optical transistor with gain adjustable by an external voltage and. finally, in IIl the opto-electric proper- ties are emphasized.
I: CHARACTERIZATION OF THE ELEMENT
If CdS single crystal platelets are illuminated with the green line (514nm) of an Ar+-laser photothermal all optical bistability may be observed'. Here the absorption of photons leads to a heating of the laser-excited spot which in turn increases the absorption via the well known Urbach-rule behaviour of the absorption edge. This is the feed- back mechanism of intrinsic induced absorption optical bistability. If now an additional voltage is applied to the contacts (as described above) a second feedback mechanism is introduced. The absorption of photons now also leads to a photocurrent and via the absorbed electrical power the laser spot is heated too, which again in turn increases the optical absorption. This can be seen in Fig.l(a), where the optical switching threshold is lowered with increasing voltage V, applied. Pin (Pout) are the optical input (output) powers and j is the current through the element. The switching processes can also be seen in the current through the element Fig.l(c). On the other hand at constant Pin the transmitted power Pout can be controlled by V,, which is shown in Fig.l(b) together with the corresponding bistability in the current (Fig.l(d)). These four dependences characterize together the SEED element.
Idealized model calculations (for details see Refs. 5.6) (only photocurrent, no dark current and no resistor serial to the element - neither external nor intrinsic by the contacts) exhibits discrepancies especially in the jumps of the current (Fig.2). If a small dark current and, more important, a serial resistor of 3kfl are taken into account (Fig.3) the experimental characteristics are well described. This leads to the conclusion that some serial resistance, may be due to the metal contacts, is present in our element.
CdS SEED elements can also be produced by using epitaxial or evaporated layers instead of single crystal platelets.
This has advantages from a technological point of view and will be described in a postdeadline paper to this mee- ting.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1988224
JOURNAL
DE
PHYSIQUE\a1
4 t V o w = Ibl
t
P,, = 7 . wO O 5 10 i5 0
incident power P,, (nH1 applied voltage Vo Wl
Fig.1: Experiment: the four characteristics of the CdS-SEED.
v r o o m temperature, w=100pm, contacts are Au, sample thickness L=5pm, X=514.5nm.
incident power Pi, IMI applied voltage V, (V)
Fig.2: Theory, adiabatic limit, the four characteristics of the CdS-SEED.
No dark conductivity, serial resistor R 4 , for details see Ref.5.
la)
io? Y o M - IDi P,, (.HI =
incident power P, (DM) applied voltage Yo M Fig.3: Theory, adiabatic limit, the four characteristics of the CdS-SEED.
-
Small dark conductivity and serial resistor R=3kn in contrast to Fig.2.i n c i d e n t power P,, (mW)
Fig.4: Experiment, the CdS-SEED as an optical transistor, the gain of which can be adjusted by V,.
Also see Fig.l(a) (experiment) and Fig.3(a) (theory).
V,,=l.lV gives a small signal gain of a factor of 8. Contact material is In, other parameters as Fig.].
incident power P,, (smI
ro
'0 1 B 12 16inciaent power P,, Inn)
Fig.5:
Left part experiment, the upper part shows the all optical bistability.
It induces a photovoltage Vph if the laser spot is situated near one of the Schottky contacts.
Contact material is Au. Parameters as Fig.1.
Right part theory, Vph is assumed to be proportional to the number of created carriers.
C2-112 JOURNAL DE PHYSIQUE
II: OPTICAL TRANSISTOR WITH ADJUSTABLE GAIN
As can already be seen in Fig.l(a) the optical bistability disappears if V, exceeds a certain voltage. At this point the dependence Pout=f(Pin) changes from bistable behaviour to steep negative slopes. If a working point is chosen in the middle of the negative slope regime. a small modulation in Pin is amplified in Pout In Fig.4 this is demon- strated experimentally. Here the gain is about a factor of 8.
III: FURTHER OPTO-ELECTRIC PROPERTIES
In Fig.l(c) it is already shown, that it is possible to control1 electronics by photonics. Nevertheless the jumps in the photocurrent are not yet satisfying for practical purposes. In Fig.5 we plot the photovoltage V h as a function of Pin without applied external voltage. The observed Vph is equal to zero if the laser is centereain the slit between the contacts. If the laser spot is situated near one Schottky-contact a positive voltage is obtained at this contact with respect to the other one. The voltage changes sign if the laser spot is moved over the middle of the slit.
By detecting the photovoltage instead of the photocurrent much more satisfying contrast ratios are obtained (in Fig.5 about a factor of 3).
CONCLUSION:
We have demonstrated that a CdS-SEED based upon photothermal induced absorption optical bistability offers an electro-optic and opto-electric interconnect with good contrast ratios in both directions. The voltages necessary to modulate light with this element are as low as about 5V which is compatible with usual electronic logic levels.
Beside this it is possible to use this element as an optical transistor with a gain which is adjustable by the voltage applied.
This work is supported by the DFG through the Sonderforschungsbereich 185 and by the Stimulating Action of the European Community.
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