2 3 4 5 6 7 8 9 IEWELL, J.L., HARBISON, I.P., SCHERER, A., LEE, Y.H., and FLOREZ, LT.: ‘Vertical-cavitysurface-emitting lasers: design, growth, fabrication, characterisation’, IEEE J. Quantum Electron.. 1991, QEn,pp. 1332-1346 MATTHEWS,J.w., and BLMSLEE, A.E.: ‘Defects in epitaxial multilayers’, J. Crystal Growth, 1974, 27, pp. 118-125 WANG, c.A., GROVES. s.H., REINHOLD, J.H.,and CALAWA, D.R.:‘Critical layer thickness of strained-layer InGaAdGaAs multiple quantum wells determined by double crystal X-ray diffraction’, J. Electron. Mater., 1993, 22, pp. 1365-1368 BENDER, G., LARKINS, E.c., SCHNEIDER, H., RAMON, I.D., and KOIDL,P.: ‘Strain relaxation in high speed p-i-n photodetectors with In&a, ,AdGaAs multiple quantum wells’, Appl. Phys. Lett., 1993, 63, pp. 292tL2922 GCQSSEN, K.W., CLTNNINGHAM, J.E., and IAN, W.Y.: ‘Electroabsorption in ultranarrow-bamer GaAdAIGaAs multiple quantum well modulators’, Appl. Phys. Lett., 1994, 64, pp. 1071-1073 FEWSTER, P.F.: ‘X-ray diffraction from low-dimensional structures’, Semicond. Sri. Technol., 1993, 8, pp. 1915-1934 GREY, R., DAVID,I.P.R., CLAXTON,P.A., GONZALEZ SANZ, F., and WOOOHEAD, I.: ‘Relaxation of strain within multilayer InGaAd GaAs pseudomorphic structnres’, J. Appl. Phys., 1989.66, pp. 975977 MILLER, D.A.B., CHEMLA, D.S, DAMEN. T.C., GOSSARD. A.C., WIEGMANN, w., WOOD,T.H., and BURRUS,C.A.: ‘Electric field dependence of optical absorption near the band gap of quantum well structures’,Phys. Rev. B., 1985, 32, pp. 1043-1060 Monolithic integration of optoelectronic smart pixels U. Kehrli, D. Leipold, K. Thelen, H.P. Schweizer, P. Seitz and B.D. Patterson Indexing t e r n : MMICs, Light entitring diodes. Photodiodes An A l W G a A s layer structure, grown in a single step, and a fabrication process has been developed for the monolithic integration of cascadable optoelectronic smart pixels. Metal semiconductor field-effect transistors (MESFETs), light-emitting diodes (LEDs) and photodiodes PDs) are uad for the integration. As an example a threshold circuit consisting of a dual-photodiode input and a current balanced output containing an LED is presented. The circuit shows a switching energy of 2pJ and a minimum switching power of 3nW. The maximum light output of the LED is 30mW with a contrast ratio =. 1000. The overall power dissipation is I5mW. LED ~ Experiment: The layer structure, which is grown in a single step by metal organic chemical vapour deposition (MOCVD), is shown schematically in Fig. 1. On an n+-doped GaAs substrate, first an LED structure is grown, starting with an n-doped lower cladding of 9oonm AL,,G%,,As. Next an 8nm undoped active &.mGa,.~As quantum well (QW) is sandwiched between 6Onm thick undoped &,Ga,.,As layers. As the upper cladding, a 480nm thick p-doped A&,Ga,,,As layer is used. The LED structure ends with a 400nm thick p + a n t a c t layer. On top of the LED the PD/MESFET layers are grown. They consist of a Ipm thick GaAs p--buffer/absorber layer, which is weakly doped by Zn diffused from the p+-contact layer, a 200nm thick 10”m-3 ndoped GaAs channel, a lOnm thick etch-stop layer of &,,Ga,,,As and a GaAs n + a n t a c t layer. ELECTRONICS LElTERS 24th November 1994 PD rn Fig. 1 Schematic layer sequence of cascodoble optoelectronic smart pixel The fabrication process requires nine photolithographic steps and is based on mesa isolation. The backside of the n+-substrate is used as the ohmic contact, formed by G e - A N i A u , to the n-side of all LEDs. For the ohmic contacts to the MESFETs and the PDs, an Ni/Ge-Au/Ni/AuPt [5] metallisation is evaporated on the top n+-contact layer. Both contacts are annealed in the same step at 440°C in a forming gas ambient. The individual MESFETs and PDs are separated by magnetic field enhanced reactive ion etching (MIE). After the deposition of a l00nm thick SIN, dielectric film by plasma enhanced chemical vapour deposition (PECVD), contact areas to the n-ohmic contacts of the MESFETs and PDs as well as to the p-ohmic contact areas of the LEDs and the PDs are opened by reactive ion etching (RIE). The gate areas of the MESFETs are then opened by RIE. The dielectric film is used as a mask for a selective, recess wet etch of these areas using the Ab,Gq,As layer as an etch-stop [a]. The wet etch produces an undercut of -2OOnm under the dielectric film. In the next step, a Ti/Pt/Au metallisation is applied by a liftoff technique. This metallisation is used as a Schottky metal for the gates of the MESFETs, as an ohmic tunnelling contact to the p+-layer and as a fust wiring level. The LEDs, PDs and MESFETs are then isolated by a second MIE mesa etch step. A 3OOnm dielectric film isolates the main wiring level, which is made by the deposition of TdAI over the whole wafer. These metals are patterned by wet etching. $04 0 so 2 a00 Introduction: Monolithic integrated optoelectronic smart pixels are of current interest in the field of parallel optical interconnects, optical computing and optoelectronic neural networks [l, 21. To integrate different functional devices such as detectors, transistors and light sources, the compatibility of the materials and the whole structure must be considered. Approaches to this integration have included selective area growth, regrowth or vertical integration [3, 41. In this Letter we present a layer structure which uses vertical integration and a fabrication process which allows the integration of cascadable optoelectronic smart pixels with different electrooptical functions. MESFET 0 ~ 2 4 6 drain voltage, V 450 600 750 goo wavelength,nm Fig. 2 PD quantum efficiency and LED emission spectrum against wavelength a Drain current-voltagecharacteristics for MESFET vertical scale is normalised with respect to gate width (mA/mm) Results: The characteristics were measured for devices processed on the same wafer. The MESFET shows a maximum transconductance of 80mS/mm and a threshold voltage of -1.7V. The drain current-voltage characteristics are shown in Fig. 2 left. A transit frequency of f r = 1.2GHz and a maximum oscillation frequency fmaX = 2GHz were measured at a gate source voltage of OV. The peak emission wavelength of the LED was set to be 790nm, where the quantum efficiency of the PD is 0.85. Therefore the emitted light can eficiently be detected on the same wafer (Fig. 2, right). The PD capacitance was measured to be 100nF/cmz at a reverse voltage of A V . The LED has an efficiency of 0.008W/ A and a cutoff frequency of 150MHz at a current of IO&. The fabricated cascadable optoelectronic smart pixels are threshold circuits. The example is schematically drawn in Fig. 3, and consists of a dual-photodiode differential input (PDI, PD2) c o ~ e c t e dto a balanced LED driver (MI,M2, LED). The LED is turned off when the input power of the switching beam (P_I,cJ Vol.30 No.24 2069 exceeds that of the reference beam (P,</). The switching threshold can thus be controlled by the reference beam. The LED driver is current-balanced: if the LED is off, the current flows through MESFET MI, and the LED is turned on by pinching off MI. f-? Fig. 3 Schematic diagram of threshold circuit including parasitic LEDs connected to MESFETs and PDs WOODWARD, T.K., LENTINE, A.L., CHIROVSKY, L.M.F., and ASARO, L.AD.: ‘GaAdAIGaAs FET-SEED receiverltransmitters’. OSA Proc. Photonics in Switching, 1994, Vol. 16 T., JIMBO, T., and UMENO, M.: ‘Monolithic integration of AlGaAsJGaAs MQW laser diode and GaAs MESFET grown on Si using selective regrowth’, IEEE Photonics Technol. Lett., 1992, 4, pp. 612414 CHENG, J , mou, P., SUN, s.z., HERSEE, s., MYERS, D.R.,ZOLPER, J., and VAWTER, G.A.: ‘Surface-emitting laser-based smart pixels for twodimensional optical logic and reconfigurable optical interconnections’, IEEE J. Quantum Electron., 1993, QE29, pp. 741-756 SHIH, Y.-c., MURAKAMI, M., WILKIE, E.L., and CALLEGARI, A.c.: ‘Effects of interfacial microstructureon the uniformity and thermal stability of AuNiGe ohmic contact to n-type GaAs’, J. Appl. Phys., 1987,62, pp. 582-590 DESALVO,G.C., TSENG,W.F.,and COMAS,I.:‘Etch rates and selectivities of citric acidhydrogen peroxide on GaAs, AI,,,Ga.,As, IQ,Ga.,As, In&.3q.4As, and InF”, J. Electrochem. Soc.. 1992, 139, (3), pp. 831-835 EGAWA, The dimensions of the MESFET gates in the diagram are given by length/width The output power is 30pW in the ON state, and a contrast ratio greater than IO00 has been estimated (Fig. 4). The minimnm switching power is measured to be 3nW at vanishing reference power. At a differential input power of 5OnW between Pref and PlWlrrh, the circuit has a switching delay of 4 3 p , which corresponds to a switching energy of 2 pJ. The photodetector area is twice 50 x S o p 2 and the whole circuit, consisting of two PDs, two MESFETs and an LED, occupies 200 x 2 0 0 p 2 . The power dissipation remains constant at 15mW. Noise and small-signal performance of three different monolihic InP-based 10Gbit/s photoreceiver OElCs D. Kaiser, F. Besca, H. GroBkopf, I. Gyuro, J.-H. Reemtsma and W. Kuebart Indexing terms: Integrated optoelectronics, Optical receivers, Semiconductor device noise Three circuit concepts (high impedance, common gate, and transimpedance) for a 10GbiUs monolithic receiver OEIC consisting of an InGaAsiInP pin photodiode and InAlAsflnGaAsJ InP HEMTs are compared in terms of noise and small-signal performance using on-wafer measurements. A total equivalent input noise current of 13.5pNdHz within the bandwidth of the transimpedance circuit is the lowest value ever reported for a monolithic InP-based IOGhiUs reiver OEIC. Ught input power, n W 171(111 Fig. 4 Switching behaviour of threshold circuit P m f = IlSnW, V,, = 4V, Vss = 2-V, V, = 1.5V Conclusion: We have developed a layer structure grown in a single step and a fabrication process for the monolithic integration of cascadable optoelectronic smart pixels. A threshold circuit with low switching power and switching energy has been demonstrated. The performance of such circuits is suitable for application in the fields of parallel optical interconnects and optoelectronic neural networks. Acknowledgments: The authors respectfully acknowledge the discussions with J.E. Epler, K.H. Gulden and M. Moser and for the support of W. Bachtold, W. Kiindig and J. Mlynek. For long-haul lightwave communication systems there is an increasing interest in IOGbit/s components. However, it has not been clear to date which concept will be successful in providing a high-performance but lowcost optical receiver. Monolithically integrated receiver front ends offer lower parasitics than hybrid solutions, which is essential for obtaining the highest bit rates. However, obviously the noise of the integrated devices is still too high. Today the best monolithic result reported is a 7.4GHz bandwidth circuit with a sensitivity of -17.3dBm at a bit error ratio of Ik9for a lOGbit/s NRZ signal [I]. The sensitivity was traced hack to a total medium input noise current of 25 pA/dHz. We present on-wafer measurements on three different circuit concepts for an InP-based monolithic photoreceiver using an InGaAdInP pin photodiode and InAIAs/InGaAflnP HEMTs. The circuits with increasing complexity, from a simple high impedance OEIC over a common gate circuit with source follower up to a transimpedancecascode circuit, are shown in Fig. 1 and are compared in terms of bandwidth, responsivity, electrical output matching, and noise current. 0 IEE 1994 23 September 1994 Electronics Letters Online No: 19941380 U. Kehrli, D. Leipold, K. Thelen, H.P. Schweizer, P. Seitz and B. D. Patterson (Paul Scherrer Institute, Baaherstr 569, Ch-8048 Zurich, Switzerlad References 1 BROWN,J.J., GARDNER, IT., and FORREST,S.R.: ‘An integrated, optically powered, optoelectronic ‘smart’ logic pixel for interconnection and computing applications’, IEEE J. Quantum Electron., 1993, QE29, pp. 715-726 2070 HI GO TI lbiiiil Fig.1 Circuit diagrams of three different realised receiver-OEIC concepts ELECTRONICS LETTERS 24th November 7994 Vol. 30 No. 24
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