Workshop of Teaching Photonics 18 May 99, Cairo, Egypt

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Microwave High Power MSM PD at 1.55 pm With Optimal Gradual Heterojunction

Iman S . Ashour

National Telecommunication Institute Electronic Departement

5 El Mokaym El Daym Street, Nasr City, Cairo, Egypt

Key words: Modelling Photodetectors, Metal-semiconductor-metal photodiode, High power, Saturation.

Abstract:

Using a 2-D physical model we present a theoretical study of the parameters that affect the saturation condition of an MSMPD under high optical power at 1.55 pm wavelength modulated with 40GHz. We also report that, with an MSM operating at 40 GHz, we can expect a saturation limit with a maximum microwave power of 17dBm due to space charge effect.

I. Introduction

The improvement of the frequency response of optoelectronic components and consequently performance of optical fiber links, made it now possible to transmit microwave signals through an optical fiber cable. This leads to possible new applications, at the interface between microwave and optics. For these applications, such as antenna remoting, phase array antennas, etc. the transmission of relatively high power microwave signals through the optical links could simple the systems. To reach such a goal, it is necessary to fabricate high speed photodetectors with high saturation optical power.

Over the last few years, numerous researches have been carried out in th is field, a lot of work has been made to study the effect of high power illumination on high speed PIN photodetectors [l-51.

Although PIN photodiodes are. generally used as high speed photodetectors due to their superior quantum efficiency over MSM photodiode, MSM photodetectors can be attractive devices for microwave and millimetre wave applications, due to reduced capacitance and planar structure which can lead to easy monolithic integration.

Planar interdigitated MSM Schottky barrier photodetectors are well established components in integrated GaAs based receivers [6],[7]. For longer wavelengths 1.3p and 1 . 5 5 ~ which are fiber bands of importance for long distance communications, the lower bandgap material, InGaAs grown lattice-matched on InP substrate, is under intensive investigation [S-1 11.

Regardless of the s t r u w of the device (planar topillumination or waveguide illumination), very high-frequency operation needs a small active region. In this case, we can expect spacecharge effect under high illumination. Indeed, high optical power absorption in such a small device can perturb the electrical field which governs the carriers drift, by spacecharge effects, and then introduce saturation of theelectrical response.

Workshop of Teaching Photonics 18 May 99, Cairo, Egypt

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Two' dimensional modeling becomes necessary in studymg and optimizing different parameters of the devices, predicting their performance, and examining their intenal operation early in the product development cycle.

II. Physical Model

We have previously studied the effect of high operating optical power on a direct heterojunction AlWGaInAs MSM PD operating at 1.3 pm wavelength and 20 GHz modulating frequency [12].The 1.55 p window is becoming more and more interesting with its minimal fibex attenuation and minimal dispersion with dispersion shifted fibers we present in this paper a study of the effect of high optical power at 1.55 Fm wavelength on an AlInAdGaInAs MSM PD operating at 40GHz using an optimum gradual heterojunction, with the aid of the 2-D physical model previously reported and tested with experimental comparision [lo] talang into account spaaxharge , breakdown, and biasing voltage. The study of the optimumgradual heterojunUion at the AlInAs/GaInAs interface has been covered in detail previously to improve the device performance [ 131,

The device under study is an MSM AlInAdGaInh topilluminated photodiode with GaInAs absorbing layer of thickness 0.4p.m, ah AlInAs barrier enhancement layer of thickness 500&electrode spacing of 0.25 pm, electrode width of 0 . 2 5 ~ 100% modulation depth, and an optimum gradual heterojunction of 0 . 1 ~ thickness is used [13] (taking into account that the optimum aspect ratio between the electrode Spacing and the end of the absorbing layer thickness is approximately 50% [lo]). Trapping effects which occur in low frecruency domain, will not be considered in this paper.

To understand the behaviour of photo-generated carriers and electric fields in the MSM PD's, Poisson's equation and the currentcontinuity equations are numerically solved in two dimensions [lo].

III. Results and Discussion A. Cutoff Frequency at Low Power Levels

The cutoff frequency of the MSM photodetector under study is limited by two factors. The first is due to carrier transient time and this is calculated by the aid of the physical model described earlier, and using the empirical relation fc = 1 . l h where z is the 10-90% rise time of the impulse response. Fig.( 1) shows the device cutoff fresuency versus biasing potential under low optical power density where it is clear the dependance of cutoff fiquency on the applied biasing potential where we reach a maximum cutoff frequency of 50 GHz for biasing votage greater than or equal to 5V . These results can be easily understood by considering the increasing field strength in the Inch+ in conjunction with the field - dependent carrier velocities. On the other hand as the applied bias voltage increases above 5V we reach a saturation condition where the carriers travel from one electrode to the other with the saturation velocity. When Operating highbiasingpotential (>5V), the breakdown potential which is mainly due to tunnelling effect 114-161 must be studied. The maximum acceptable electric field before the appearance of the tunnelling effect is approximately equal to 800 KV/cm for AUnAs [ 161. Fig.2 gives the Xamponent (a) and the Yamponent (b) of the electric field for 6 volt applied bias. It is clear that with maximum electric field approximately equal to 650 KV/cm at the AUnAs we have not reached breakdown condition.

The second limiting factor of the device cutoff fresuency is the circuit time constant whichis calculated using an empirical relation (17 for the MSM capacitance and assuming a 50n load resistance. Fig431 shows the dependance of the cutoff frequency on device surface area where it can be shown that a device having a surEace a m of (20pmY results in a cutoff fresuency due to circuit time constant is approximately equal to 43GHz.

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. . The effective cutoff frequency of the MSM detector is then limited by the lower cutoff frequency of

these two cutoff frequencies depending on bias voltage and device surface area.

B. Effect of High Power on Cutoff Frequency:

Fig.(.la) shows the effect of increasing optical power on device cutoff fresuency due to the space charge which results in a reduction of the electric field and consequently the carrier velocity. We can notice that with SV biasing potential the cutoff frequency decreases from SOGHz at low power to approximately 30GtIz at 120mW. The figure also shows that increasing biasing potential decreases space charge effect and so increases device cutoff frequency at high operating power levels. We can see from Fig.(4a) also that for example an optical lower level of lOOmW results in decreasing the device cutoff frequency to 34GHz at SV bias while the cutoff firequency decreases to 48GHz at 7V bias when applying the same optical power level (lOOmW).

Fig(4b) gives the evaluation of the maximum transverse electric field as a function of applied optical power on the device under test for different biasing potentials. We can notice the increase of the electric field as the applied optical power increases. For such high electric fields we can expect the appearance of tunneling effect explained earlier, and breakdown Condition. We can see that a breakdown condition is reached for an optical power of 1 lOmW at 7V biasing potential.

Fig.(4c) indicates the electric field reduction at mid point between the twoeleclrodesduetothe increased optical power, which results in lower carrier velocity between the two electrodes and so the saturation condition. Merent CUIV~S are given for various biasing potentials.

Fig.(Sa) represents the simulated linear response at low optical power levels of the photocurrents at 6V biasing potential and 40GHz modulating sinusoidal optical power, where we can notice thesimilar responses of electron and hole currents. On the other hand Fig.(Sb)presentsthesimulatedreponseofthe saturated photocurrents due to high optical power levels (39mW).We can notice the Merent behaviour of hole and electron responses as the electric field between electrodes decreases due to the different mobilities of holes and electons.

C. Microwave Saturation Power

Fig.(6) gives the microwave output power as a function of the optical input power of 40GHz modulating frequency and with 6V biasing potential, where it is clear that we star& to have a saturation condition at approximately 200mW and a following breakdown condition at 400mW due to space charge effect . The curve shows a maximum microwave saturation power of approximately 17dBni.

IV. Conclusion

Indeed, a large optical power absorption in a small device (microwave MSM photodiode) can perturb, by spacecharge effects, the electric field which governs the carrier transport, and then introduce a nonlinear electrical response depending on optical power density and device biasing potential. This papr shows that for a small surface area MSM pllotodiode depending on applied bias we can reach a break-down condition at high operating power densities. It is found also that an MSM operating at 40GHz modulating fresuency and in the 1 . 5 5 ~ wavelength window could reach a maximum microwave power level of approximatelyl7dBm due to space charge effect.

Workshop of Teaching Photonics 18 May 99, Cairo, Egypt

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. - ACKNOWLEDGEMENT

The author would like to thank gratefully Prof. A. Ammar from NTI for his guidance and sincere help.

REFERENCES

[ 11 M.Dentan and B. de Crernoux," Numerical simulation of the non-linear response of a PIN photodetector under high illumination", IEEE J. Lightwave Technology, vol.JLT 8, pp. 1137-1144, 1990 [2] RR Hayes and D.L. F%whini,"Nonlinearity of PIN photodetectors", IEEE Photon. TechJett. Vol.5,pp. 70- 72 Jan. 1983. [3] kHarari, F.Journet, O.Rabii, G.Jin, J.P.Vilcot, D.Decoster,"Modeling of waveguide PIN photodett@or under very high optical power," IEEE Trans.Microwave Theory and Tech., vo1.43, pp.1357-1361 [4] J.Harari, F . J w e t ,G.Jin, J.Vandeccasteele, J.P.Vdcot, D.Decoster,"M&lling of microwave top illuminated PIN photodetector under very high optical power,"IEEE Trans. Microwave Theory and Tech.,

[5] J.Harari, G Jin, J.P.Vilcot, D.Dec&aer,"Theoretical study of PIN photodetectors power limitations from 2.5-6OGHzn, IEEE Trans. On Microwave Theory and Tech.,vol45,No.8, August 1997 [6] B.J. Ban zeghbreock, W. Patrick, J.M. Halbout, and P. Vettiger," 105 GHz bandwidth metal- semiconductor-metal photodiode", IEEE Electron Dev. Lett.., vol. EDL-9, pp.527-529,1988 [7] L.F.-" High speed optical devices and their integration with transistors", lntegrated Oplics and Ogtoelectronics, vol. CR 45, OELASE'93,SPIE meeting, pp. 452-458, 21-23 Jan. 1993, Los Angeles, California, USA [8] M.Z. Martin, F.K. Oshital, M. Matloubian, H.R Fetterman, L. Shaw, and K.L. Tan,"High-speed optical response of pseudomorphic In- high electron mobility transistors", IEEE Photonic Tech. Lett., v01.m- 4,

191 Julian B. D. Soole and Herman Schumacher," Transit-time limited fresuency response of InGaAs MSM photodetectors", IEEE Trans. Electron Devices, vol.ED 37, No. 11, pp. 2285-2291, Nov. 1990 [ 101 I.S. Ashour, et al.. "Cutoff frequency and responsivity limitation of AlInAs/GaInAs MSM PD using a two dimensional bipolar physical model ", IEEE Trans. on Electronic Devices, vo1.42, No.2, pp 23 1-238, Feb. 1995 [ 1 11 Iman S. Ahour et al..,"Comparision Between GaAs and AlIWGaInAs MSM PDs for Microwave and Millimeter-Wave Applications Using a Two Dimensional Bipolar Physical Model", Microwave and Optical Technology Letters, Vo1.9, No.1, May 1995 [ 121 Iman S. Ashour et al.,'"@ Optical Power Nonlinear Dynamic Response of AlIWGaInAs MSM Photodiode", IEEE Transactions on Electronic Devices, Vol. 42, No.5, May 1995 1131 Iman S . Ashour,"Optimization Study of Gradual Heterojunction in AlWGaInAs MSM PD for

Microwave and Millimeter-Wave Applications Using a Two Dimensional Bipolar Physical Model ", IEEE conference, ECS , Cairo, Egypt, Dec. 1997. I141 S.R Forrest, M. Didomenico, RG. Smith, and H.J. Stocker,"Evidence for tunnelling in reverse-biased 111- V photodetector diodes,"Appl. Phys. Lett.,36,(7), pp.580-582, 1980, 1151 S.R Forrest, O.K. Kim," Analysis of the dark current and photoresponse oflnGaAs/lnPavalanche photodiodes,* Solid State Elect., 26, (lo), pp. 951-968, 1983 [16] J. Harari, D. Decoster, J.P. Vilcot, B. Kramer, C. Oguey, P. Salsac, G. Ripoche,"Numerical simulation of avalanche photodiodes with guard ring," IEE proceedings -3, Vol. 138, (3), pp. 211-217, June 1991 [ 171 K C. Gupta, R Garg, and R Chadha, "Computer-aided design of microwave circuits", Artech: Dedham, 1981

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Workshop of Teaching Photonics 18 May 99, Cairo, Egypt

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(c) Masimum inter-electrodes electric field versus applied optical power for different biasing potential @) Masirnun electric field versus applied optical power for Werent biasing potential.

Workshop of Teaching Photonics 18 May 99, Cairo, Egypt

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Figure (5) For an optical signal of 4OGHz modulating frequency, 100% modulation index, for MSM PD under study and 6V applied bias, (i) Total photocurrent f (ii) Electrons current , (iii) Holes current (a) 7.8mW

maximum applied optical power @) 390mW maximum applied optical power,

Figure (6) Output microwave power versus optical power of 40GHz modulating frequency, 100% modulation index, for. ( 2 0 ~ ) ~ surface area MSM PD and 6V total applied bias .