MSM Photodetector with an Integrated Microlens Array for Improved Optical Coupling H.F.B. Ozelo a, L.E.M. d e Barros Jra, B. Nabetb, L. G. Neto', M.A. Romeroc, J.W. Swart'
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Abstract In this paper we describe the fabrication of an array of integrated cylindrical microlenses on top of a single GaAs MSM photodetector. Preliminary experimental data already shows an iwrease of about 11% on the photocurrent of the MSM photodiode as a result of the improved optical coupling efficiency. However ray-tracing calculations indicate that this increase can reach values as high as 50% with a more carefuI flensdesign
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iKey words integrated optics, photodetectors, GaAs.
I. INTRODUCTION Planar photodeteitbrs are used in almost all photodetection applications and typically consist of interdigitated patterns in which one pad and the associated fingers form the cathode and the other forms the anode. Photoconcjuctors, PIN photodiodes and MetalSemiconductor- Metal (MSM) photodetectors, for example, employ such pattern. Light reaching the area between fingers is absorbed and produces pairs which are transported to the fingers and collected. This planar travel is time and advantageous because Of reduced compatibility with microelectronic semiconductor devices. Among the fotodetectors mentioned above' MSM structures have attracted a great deal of attention in recent years. A conventional MSM consists essentially of two backto-back Schottky contacts deposited over an undoped substrate 111. Thev offer electrical bandwidth beyond 10 Gbitsh, low-dark current and are easy to fabricate-[2-4]. A major prohlem, however, exists. The metal surface of the fingers reflects light qnd hence a large portion of the incident photons, typically from 30-70%, is lost. Attempts to solve this problem include the use of transparent IT0 (Indium Thin Oxide) films [ 5 ] or highly doped semiconductor material [6] to form the electrode fingers. The drawback, however, is that both IT0 contacts and bighly doped semiconductor layers are neither as conductive Manuscript received on April 18, 1999 This work was supported in part by FAPESP (Silo Paul0 State Research Foundation). University of Campinas - Unicamp, Laborat6rio de Pesquisa em Dispositivos - LPD. Campinas, SP,Brazil, E-mail. Lubarros@ifi unicamp br Drexel University, Electrical and Computer Engineering Department. 32'Id and Chestnut Streets, 19104, Philadelphia, PA, USA, E-mail.
[email protected] edu ' University pf S o Paulo, Escola de Engenharia de SHo Carlos, Dpto. de Engenhana Eletrica, AV Dr. Carlos Botelho, 1465, 13560 - 250, SHo Carlos, SP, Brapl E-mail Muriloa@sel eesc sc usp br
as metal nor as rectifying as a Schottky barrier. Hence, devices fabricated using these techniques have relatively large leakage currents. In this paper we propose a novel technique to improve the coupling efficiency of planar photodetectors. Previous work used dome-shaped lenses cover the entire active area of the detector [7]. Here we propose an optical microlens array where each lens is cylindrical-shaped and seats on top of the uncovered semiconductor area between the MSM metal fingers. The microlenses redirect light, that would normally be deflected from metal-covered areas of the detector, to photoabsorptive areas, with no associated penalty in any figures of merit, inciuding the speed of response. It is worth mentioning that, since the microlens array are fabricated from photoresist, the proposed technique is entirely processcompatible with ordinary microfabrication technology.
11. MSM STRUCTURE AND FABRICATION PROCESS The developed MSM detectors employ an interdigital finger pattern, as shown in ~ i ~ .ne 1 . devices are fabricated on top of a 1 p GaAs semi-insulating mesa structure and require only two iithographic steps, namely, the definitionand contact patterning. All the MSM designed have active area of 50x50 pm2 and are labeled FxS according to the finger width (F) and space between fingers (s).ne metalization sequence used was 10nm of Ti followed by 2oo nm of Al. metal /---
Fig.1 Schematic diagram and photograph of MSM device. The photo shows a 3x2 (top) and a 2x2 MSM. The device characterization was performed on-wafer using a probe station connected to a semiconductor parameter analyzer. Laser light, at a wavelength of 650 nm, was brought in through the station's microscope optics, which allowed some flexibility on spot size and position. All experimental data reported in this work refer to 4x4 MSMs, with expected reach-through voltage of 9 volts. The measurements were
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calibrated by using a large area Si detector placed at the same height of the device under test (DUT) and are a result of at least 4 different readings with distinct devices of the same wafer. The obtained results are summarized on Fig.2, which shows the Current-Voltage characteristics of the MSM under three different excitation conditions, namely, dark, microscope white light and laser light:
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Fig. 3 Schematic diagram of MSM with a polymer lens on
top. The improvement on couplhg efficieixy or &ris gain can be measured by the geometrical relationship below:
Fig. 2 Current-Voltage characteristics of 4x4 MSM under dark conditions and excited by microscope white light and laser light.
where 6 is the length over the electrode for which incident rays can still be focused into the semiconductor (see Fig.3). The polymer used was the Shippley's which is fully compatible with standard techniques. The polymer is spun at 4200 rpm, patterned with the metal layout and developed. Then the resist is baked at 130 "C,and as a result, it spreads over the metal fingers, in a process known as reflow [7,9]. The angle of the reflow, a, is controlled by the baking temperature and duration. Figure 4 displays a SEM photograph of the fabricated lens. ,
Satisfactory behavior is observed under dark conditions with dark current (Idalk)lower than 5.2nA up to 15 volts. The detector shows excellept sensitivity under laser illudnation reaching 5 orders of magnitude difference between currents under illumination and under dark conditions. The resulting responsivity at saturated levels (Vblas> 10 volts) is 0.2 A N , similar to devices previously reported in the literature [8].
111. THE ARRAY OF INTEGRATED MICROLENSES In order to improve sensitivity of the detector we have designed a polymer-bgsed microlens on top of each exposed semiconductor region between the device's fingers. The lens stretches out on top of the metal finger so that light rays that would be normally blocked by the metal on a perpendicular path can be focused inside the semiconductor. The schematic diagram of a cross section of the MSM of the whole structure is shown in Fig.3. Lens design is performed assuming spherical lens approximation and by using ray optics, since the wavelength (650 nm) is much smaller than the lens diameter (6 pm).
Fig. 4 SEM picture of thelphotoresist micro-lens on top af the MSM The micro-lenses were fabricated on the same MSM measured before and the results of I vs.V characterization are shown in FigS. In this case, F = S = 4 pm and d = 1 pm, which yields a calculated improvement of 12% with respeqt to the conventional device with no lens.
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IV. DISCUSSION Dark current of MSM
Ideally, it is desirable to generate the photocarriers in a high electric field region, in such way that the current transport occurs mainly by drift. This makes carrier recombination negligible and assures maximization of the photocurrent. Therefore it is crucial to operate the detector in a bias condition where the active layer is fully depleted. Now, one notes that the width of the depletion region depends on the inverse of the square root of the carrier
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density (1 [ll]. Hence, in order to explain the results observed in Fig.5, one notes that the effect of the microlens array is to increase the number of optically injected carriers in comparison to the conventional MSM. As a consequence, for the same bias point, the depletion width in the lensed devices is smaller than the one in the conventional MSM. For bias voltages well above reach-through, even with this reduction, the depletion width still extends through all the active region and an increase in the number of generated carriers will result in a proportional increase in the photocurrent. However, for the voltage range from zero to slightly above reach-through this shrinking will allow greater rate of carrier generation in a low field region, increasing recombination and decreasing the photocurrent, when compared to the values obtained for the non-lensed structures. Therefore the results displayed in Fig.5 for voltages below reach-through are not contradictory with the improved coupling efficiency offered by the lenses. They are, in fact, caused by it. A more detailed quantitative analysis of this behavior is being carried out and will be published elsewhere. For a detailed characterization of this effect refer to Liou et al. [7]
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Fig.5 I vs. V curves of the MSM in dark (a) and under illumination with and without the micro-lens axray (b). As expected there is no significant difference in dark current although the MSM with the lens showed dark current slightly lower than the conventional detector. Investigation is under way to verify if this lowering can be explained by passivation effects. Also, we observe that, at flat-band condition (high bias voltages), well above reach-through voltage (i.e. the potential for which the depletion regions of the two Schottky contacts begin to overlap), the results show an improvement on optical coupling efficiency near 12%, in excellent agreement with the theoretical predictions. However, for bias voltages ranging from zero to values slightly above reach-through (9 volts) the lensed devices displays a reduction on the measured photocurrent. Specifically, the cross-over point for the 4x4 device is around 10.5 volts. The reason for this phenomenon is the optical field modulation of the depletion region width as discussed as follows.
In order to improve the optical coupling efficiency of our lens we have carried out the design of a pedestal structure using ray optics and assuming the same lens shape and material described before. Optical coupling efficiency is enhanced by 50%, compared to the conventional device, at a pedestal height of 2.5pm. Figure 6 shows the computer generated ray-tracing results for the lens-pedestal arrangement. The fabrication steps follow the same procedure described in [7,9] and it is still compatible with integrated circuit technology. Moreover, careful choice of the material may bring other benefits besides enhancement of coupling efficiency. We are currently working on a special dielectric with high index of refraction and excellent passivation properties [ 121. Such features are interesting for two purposes: 1) reduction of dark current due to passivation of surface states of the GaAs substrate and 2) index matching between air and GaAs forming a anti-reflection layer which can potentially reduce reflection loss to zero.
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driver and photoreceiver for 20 Gb/s optic-fiber links”, IEEE/OSA Journal of Lightwave Technology, Vol. 16, no. 8, pp. 1491-1497, August 1998. [3] E. Droge, E.H. Bottcher, D. Bimberg, 0. Reimann and R. Steingruber, “70 GHz InGaAs metal-semiconductor-metal photodetectors for polarisation-insensitive operation”, Electronic Letters, Vol. 34, no. 14, pp. 1421-1422, July 1998. [4] M. Loken, L. Kappius, S. Manti and C. Buchal, “Fabrication of ultrafast Si based MSM photodetector”, Electronic Letters, Vol. 34, no. 10, pp. 1027-1028, May 1998. [5] J.W. Seo, A.A. Ketterson, D.G. Ballegeer, K.Y. Cheng, A. Adesida, X.N. Li and T. Gessert, “Comparative Study of Metal-Semiconductor-Metal Photodetectors on GaAs with Indium Tin Oxide and TUAu Electrodes”, IEEE Photonics Technology Letters, Vol. 4, no. 8, pp. 888-890, August 1992. [6] R.B. Darling, B. Nabet, J.E. Samaras, S. Ray, E.L. Carter, “Epitaxial n+ Layer GaAs Mesa Finger Interdigital Surface Photodetectors”, IEEE Electron Device Letters, Vol. 10, no. 10, pp. 461-463, October 1989. [7] Z.D. Popovic, R.A. Sprague and G.A. Neville-Connel, “Technique for Monolithic Fabrication of Microlens Array”, Applied Optics, Vol. 27, no. 7, April 1988. [8] W.A. Wohlmuth, High-speed and high sensitivity metalsemiconductor-metal photodetectors for optoelectronic integrated circuit applications, Ph.D. Thesis dissertation, Univ. of Illinois, USA, 1997 191 A C.S. Ramos, et al., Analysis of via hole opening by plasma etching in polyimide for electrode access”, Proc. Of the XI Conference of the Brazilian Microelectronics Society, pp.235-240, 1996 [lo] M. Shur, Physics of Semiconductor Devices, Prentice Hall, 1990. [ 111L.C.Liou and B. Nabet, “Simple analytical model of bias dependence of the photocurrent of metal-semiconductormetal photodetectors”, AppOptics, Vo1.35, No. 1, pp. 1523,1996 [12] J.A.Diniz, L.E.M.de Barros Jr., R.T.Yoshioka, G. S. Lujan, I.Danilov, J.W. Swart, “One-step silicon nitride passivation by ECR-CVD for heterostructure transistors and MIS devices”, Proc. Of the 1999 Materials Research Society spring meeting, San Francisco, CA, USA.
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Fig.6 Computer generated ray-tracing diagram of the optimized design of the cylindrical microlens. The area from -2 to 2 pm corresponds to the area between electrodes. V. CONCLUSIONS We have demonstrated a new approach to enhance optical coupling efficiency to MSM photodetectors by fabricating a photoresist microlens array on top of the semiconductor area between each MSM finger. The modulation of the depletion region by the incident optical field caused reduction of photocurrent in lensed devices for biases slightly above reach-through and below. However, at high bias voltages the optical power is not sufficient to undeplete any of the absorption region and an enhancement of 12% on coupling efficiency is readily observed. There is very good agreement of experimental and theoretical results, demonstrating that such improvement is due solely to better focusing and not to index matching. An optimized lens design was also discussed. It is shown that a lens-pedestal arrangement can potentially enhance optical coupling up to 50% better than conventional devices. Acknowledgments The authors would like thank Mi. A.C.S. Ramos for the SEM pictures and valuable discussions.
REFERENCES [I] S.M. Sze, D.J. Coleman Jr. and A. Loya, “Current Transport in Metal-Semiconductor-Metal (MSM) Structures”, Solid-state Electronics, Vol. 14, pp. 1209-1218, 1971. [2] Z.H. Lao, V. Hurm, A. Thied, M. Berroth, M. Ludwig, H. Lienhart, M. Schlechtweg, J. Hornung, J, W. Bronner, K. Kohler, A. Hulsmann, G. Kaufel and T. Jakobus, “Modulator
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