Nonresonant optical bistability in InP:Fe seed devices

Volume 62, number 3

OPTICS COMMUNICATIONS

1 May 1987

N O N R E S O N A N T OPTICAL BISTABILITY IN InP:Fe SEED DEVICES

F. FORSMANN, D. J~,GER and W. NIESSEN Institut fiir Angewandte Physik, Universitiit Mfinster, D-4400 Mfinster, Fed. Rep. Germany

Received 5 January 1987

At the nonresonant wavelengthof 1.06/tm, optical bistability is observed in an etalon of semi-insulatingInP:Fe using the selfelectrooptic effect.The mechanismof photocarrier generation is traced back to electronictransitions from the deep lyingimpurity centers to the conduction band. The nonlinearity of the device is attributed to the dispersive thermooptical properties, largely enhanced by the induced electrical power.

1. Introduction

Very recently, SEED devices have been proposed as a new class of integrated hybrid, optically bistable semiconductor elements [ 1 - 5 ]. The underlying selfelectrooptic effect which is based on combined optoelectronic photodetector and electro-optical modulator characteristics, leads to large effective nonlinearities allowing optical processing even at microwatt optical input powers and therefore the use of laserdiodes. In particular, in 1984 GaAs/GaAIAsMQW devices [ 1,2] have been shown to exhibit cavityless optical bistability due to induced excitonic absorption tuned by an external voltage. In the same year, dispersive optical bi- and multistability have been observed in a silicon SEED [3-5] due to a thermal nonlinearity. Besides, most experimental results on optical bistability have been achieved in the past using resonant wavelengths, i.e. photon energies near the bandgap where the optical nonlinearities are large. Only a few measurements on the basis of nonresonant mechanisms have become known, as for example the twophoton absorption in InSb at 10.6 #m [ 6 ] or the twostep optical transitions assumed to be relevant in GaAs at 1.15/~m [ 7]. Furtheron, a resonator-free optical bistability in InP has been observed at 1.06 /tm which is attributed to an avalanche breeding of heated free carders [8]. Obviously, efficient nonresonant mechanisms seem to be advantageous since optical bistability can be achieved over a broad spec0 030-4018/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

tral range which is also true for the recently proposed BEAT (Bistable etalon with absorbed transmission) device as described in ref. [ 9 ]. In this paper we present an InP:Fe SEED device which shows optical, optoelectronic and electrical bistability at the nonresonant wavelength of 1.06/zm at milliwatt optical input power. The device consists of a Fabry-Perot interferometer with additional electrical contacts and acts both as a photodetector and an integrated modulator. Here the photocurrent at the nonresonant wavelength of 1.06/lm results from band tailoring due to the trapping centers [ 10 ]. The nonlinearity is based upon thermal effects which are largely increased by the self-electrooptic effect as described in more detail in refs. [ 3-5 ].

2. SEED device on semi-insulating InP:Fe

The SEED device used in our experiments is sketched in fig. 1. It consists of an InP:Fe etalon with polished surfaces. Additional electrical contacts are formed by evaporated Au layers, where on one side a gap is provided for the optical input as shown in fig. 1. The back metallic contact is used as a mirror. The resistivity of the semi-insulating InP:Fe is about 10 7 ~'2cm as a result of the deep lying Fe centers which behave as trapping centers for free electrons. The back Au mirror of the sample is connected to the heat sink with temperature To controlled externally. To characterize the fundamental behaviour of our 193

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-t--

u

d

_t_

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Fig. 1. Sketch of the InP:Fe SEED; the experimental values are 2o= 1.06 ktm, d = 150/~m, and R~=2.35 k.Q. The width of the gap at the optical input is 500/an. To is the temperature of the heat sink.

device, a first experiment is carried out where the temperature dependence of the reflected power P~en, the dark current Id and the photo-current Iph are measured under small-signal conditions, see fig. 2. On the basis of the Airy formalism, from the form of P~n as a function of temperature one immediately coneludes that the index of refraction depends on temperature. From the experimental results in fig. 2, n ' = d n / d T = 2 . 9 7 × l O -4 K -1 is obtained at room temperature. In a further step the value R~ = 0.267 of the reflectivity at the front mirror is taken from the literature [ 11 ], and the ratio of the maximum and minimum values of P~n are determined from fi~. 2. Now the Airy formalism yields the product x/R2 T~ of the reflectivity R2 of the back Au-InP mirror and

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1 May 1987

of the transmission Tr= exp( - a d ) . From experiments, x ~ 2 Tr=0.93 is calculated to yield the absorption coefficient a = 3 cm -I, where R2=0.95 from refraction and absorption data at 1.06 #m of InP [12] and Au [13] is used. It should be noted, however, that the unknown value of RE is a critical parameter to determine a, but the measurements define at least upper and lower limits of a and R2, respectively. Nevertheless, a = 3 cm-1 is in accordance with the results in ref. [ 10]. Additionally, from the extrema of Pren (T) in fig. 2 one recognizes that Tr depends on temperature, leading to or' = d a / d T ~ 0.05 (cm K) - 1 at room temperature. As can be seen from the dotted lines in fig. 2, the device also acts as a photodetector where the photocurrent is related to the absorption optical power Pa through Iph = qPa. Taking Pa from the Airy formula, the photosensitivity q is estimated to be 1.3 × 10 -2 A/W in the present case at V0= 30 V. The observed photocurrent establishes that the absorbed optical power generates free charge carriers which can be traced back to electronic transitions from the trapping centers. On the other hand, since the temperature change in the sample depends not only on the absorbed optical power, but also on the generated electrical power due to current flow, the device exhibits the properties of an electro-optic modulator which together with the photodetector characteristics leads to a self-electrooptic effect. Consequently the resulting overall optical nonlinearity is greatly enhanced and can be controlled by the electrical power input which is described in more detail in ref. [4].

o 3. Nonresonant bistability

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30 /-,0 50 TEMPERATURE ,To(°C) Fig. 2. Experimental results of reflected power (solid line) and current In (dotted line (2)) versus temperature, linear regime of sample of fig. I at Vo=30 V and Pi,=27 roW. Additionally, the temperature dependence of the dark current Ia at Iio= 30 V is shown (dotted line (1)), Io=Ia+I~h.

194

In the following, the results of typical experiments performed on the InP SEED device (fig. 1 ) are presented. The measurements are carried out using a Nd:YAG laser beam focussed to a radius of about 30 /~m. The external voltage is applied in such a way that the back Au contact is positive. The resistance Rv which introduces an additional nonlinearity [ 3 ] is used to protect the sample in case of electrical break down. In a first experiment (fig. 3(a)) the reflected power is measured by changing the optical input power at

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OPTICS COMMUNICATIONS

1 May 1987

-1

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Fig. 3. Bistabilityof the InP:Fe SEED device at To=298 K. (a) Reflected power against optical input power, parameter is Vo = 0 V (1), 220 V (2), and 250 V (3). (b) Current Io against optical input power at Vo= 220 V. The dark current is 32 pA. different bias voltages. As can be seen, at Vo= 0 V the devices exhibits a linear characteristic, whereas at Vo= 220 V and 250 V a distinct optical bistability is observed where switch down occurs at Pi, = 67 mW and 58 mW, respectively. Clearly, the optical switching power depends critically on the applied voltage, but the characteristics of the sample are almost not changed by Vo in the range of smaller optical input powers. In an additional step, the corresponding current lo is measured as a function of the optical input power. The result in fig. 3(b) also shows a clear bistability with a large contrast ratio between the two different states. For example, the current at the optical switch down point changes from 0.36 to 0.96 mA. Hence, the experiments elucidate that the optical bistability is directly connected with the hysteresis of the current through the device, in agreement with the underlying self-electrooptic effect. Accordingly, the critical optical power level for switching decreases with increasing voltage, since the total necessary power has to be constant. Additional detail can be judged from fig. 4 where the current-voltage characterestics at different optical input powers are plotted. First, it can be seen that the dark current is always very small, here below 20 /zA (curve (1)). Second, curve (2) shows that the photocurrent is proportional to the applied voltage which is the characteristic of a photoconductor. The internal photosensitivity is estimated to be ~ 0.2 A/W where Pa~ 1 m W is determined from the device parameters. Third, a clear electrical bistability is abserved in curves (3) and (4), the critical powers for switching up for example being 77 m W in both cases. It is concluded therefore, that according to the

(2)

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VOLTAGE,Vo(V) Fig. 4. Current-voltage characteristics of the device in fig. 1 at different optical input powers. Pi,=0; 34.5; 58.6 and 72.4 mW for curves (1) to (4), respectively; To=298 K. SEED mechanism the electrical power mainly determines the point of bistability. In other words, the obsorbed optical power is only used to generate the photocurrent, and the corresponding, electrically generated heat then induces the instability.

4. Conclusion In summary, the InP:Fe SEED device shows pronounced optical and optoelectronic bistability and also multistability [ 14] at milliwatt optical input powers, very similar to the Si SEEDs [ 3 - 5 ]. The main features are summarized as follows. Due to the deep impurity centers, nonresonant wavelengths can be used. And it can be foreseen that such devices may basically operate over a broad spectrum. This is especially true, since the density of the traps and therefore the absorption can be adjusted during common technological steps. It is also possible to realize inhomogeneous doping or hetero structures like InGaAs/InP in order to seperate areas of layers for optical absorption from the optical cavity with high finesse, cf. the BEAT devices as proposed in ref. [ 19 ]. This is another important property of our device: the optical absorption can be kept very small, it only serves to generate some photocarriers so that the photocurrent produces the necessary heat. Consequently, the fmesse can be large and the optical power levels small. The use of an avalanche process would further be helpful. On the other side, the nonlinearity 195

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of the device is always large and can be varied by the applied voltage. In the present case for example, the effective nonlinearity of the index of refraction increases with the square of the voltage [4]. The measurements have also pointed out, that in case of compensated material the dark current is very small. As a result, the detuning of the cavity and hence the bistability are not changed by the voltage, cf. fig. 4. Finally, it should be mentioned that semi-insulating InP:Fe has been used as a material for four-wave mixing at 1.06 #m [ 10 ], the underlying photorefracrive effect may also play an important role in future SEED devices.

References [ 1] D.A.B. Miller, D.S. Chemla, T.C. Damen, A.C. Gossard, W. Wiegmann, T.H. Wood and C.A. Burrus, Appl. Phys. Lctt. 45 (1984) 13. [ 2 ] D.A.B. Miller, D.S. Chemla, T.C. Damen, T.H. Wood, C.A. Bun'us, A.C. Gossard and W. Wiegrnann, IEEE J. Quantum

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Electron. QE-21 (1985) 1463. [3] B. Wedding and D. Jiiger, Proc. SPIE, ECOOSA'84, 492 (1984) 391. [4] D. J~iger, F. Forsmann, and B. Wedding, IEEE J. Quantum Electron. QE-21 (1985) 1453. [ 5] D. J/iger, F. Forsmann, Solid-St. Electron., (in press). [6] A.K. Kar, J.G.H. Mathew, S.D. Smith, B. Davis and W. Prett, Appl. Phys. Lett 42 (1983) 334. [7] B.S. Ryvkin and M.N. Stepanova, Sov. Tech. Phys. Lett. 8 (1982) 413. [8] B.G. Faradzkev, I.P. Areshev, M.I. Stepanova and V.K. Subashiev, Sov. Tech. Phys. Lett. 11 (1985) 313. [9] A.C. Walker, Optics Comm. 59 (1986) 145. [ 10] A.M. Glass, A.M. Johnson, D.H. Olson, W. Simpson and A.A. BaUmann, Appl. Phys. Lett. 44 (1984) 948. [ 11 ] M. Cardona, J. Appl. Phys. 36 (1965) 2181; 32 (1961) 958. [ 12 ] G.D. Petit and W.J. Turner, J. Appl. Phys. 36 ( 1965 ) 2081. [ 13 ] G. Hass and K. Hadley, Optical properties of metals, American Institute of Physics Handbook (Mc Graw-Hill Book company, 1972), Chap. 63. [ 14] D. J§ger and F. Forsmann, in: Nonlinear optical properties of semiconductors, ed. H. Haug (Academic Press, in press); D. J~iger and F. Forsmann, in: From optical bistability towards optical computing - the EJOB report, eds. P. Mandel, S.D. Smith and B.S. Wherrett (North-Holland, in press).