We report on the first integration of a resonant tunneling diode and an optical communications laser operating at around 1.5 /spl &#956;m. We demonstrate its low-frequency bistable operation and model its electrical characteristics

Farmer, C.D.

Hopkinson, M.

Ironside, C.N.

Slight, T.J.

Stanley, C.R.

English

Slight, TJ

Ironside, CN

Stanley, CR

Hopkinson, M

Farmer, CD

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Integration of a resonant tunneling diode and an optical communications laser

T.J.  Slight

C.N. Ironside

C.R. Stanley

M. Hopkinson

C.D. Farmer

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      Slight, T.J. and Ironside, C.N. and Stanley, C.R. and Hopkinson, M. and Farmer, C.D. (2006) Integration of a resonant tunneling diode and an optical communications laser. IEEE Photonics Technology Letters 18(14):pp. 1518-1520.        http://eprints.gla.ac.uk/3892/  Deposited on: 11 February 2008   Glasgow ePrints Service http://eprints.gla.ac.uk 1518 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 18, NO. 14, JULY 15, 2006Integration of a Resonant Tunneling Diode and anOptical Communications LaserT. J. Slight, C. N. Ironside, Senior Member, IEEE, C. R. Stanley, M. Hopkinson, and C. D. FarmerAbstract—We report on the first integration of a resonanttunneling diode and an optical communications laser operatingat around 1.5 m. We demonstrate its low-frequency bistableoperation and model its electrical characteristics.Index Terms—Driver circuits, integrated optoelectronics, opticalbistability, optical fiber communication, resonant tunneling diodes(RTDs), semiconductor lasers.I. INTRODUCTIONI N THIS letter, we demonstrate the use of a resonant tun-neling diode (RTD) as a driver for a semiconductor laser.This is a novel alternative to traditional transistor-based drivercircuits [1]. The RTD acts as a voltage controlled switch for thelaser and causes the device (the RTD-LD) to become electricallybistable, making it particularly convenient for nonreturn-to-zero(NRZ) digital modulation. Digital signals with low power canbe employed to switch the device between ON and OFF statesand the high-speed modulation characteristics of the laser willbe improved by the smaller drive powers required. Furthermore,integration of a RTD fits well with the vertical stack arrangementof the epitaxial layers that make up a semiconductor laser, andso it is easily achieved by introducing two barrier layers closeto the contact layer.Previous light emitting devices utilizing the resonant tun-neling diode/effect include the resonant tunneling light-emittingdiode (RTLED) [2], the resonant tunneling effect quantum welllaser [3], and the resonant tunneling injection laser [4]. Graveet al. [5] have demonstrated an integrated RTD/laser in theGaAs–AlGaAs material system and have demonstrated itsapplication as an optical two state memory. In this work, weare targeting optical communications applications with the em-phasis on increased functionality and an emission wavelengthof around 1.55 m. We have characterized the low-frequencyoperation of the RTD-LD, demonstrated optical/electrical bista-bility, and modeled its electrical characteristics using SPICE[6].II. DEVICE LAYOUTA suitable wafer for the device was grown by molecular beamepitaxy and consisted of a resonant tunneling diode grown aboveManuscript received February 16, 2006; revised April 12, 2006. The work ofT. J. Slight was supported by the Engineering and Physical Sciences ResearchCouncil (EPSRC).T. J. Slight, C. N. Ironside, C. R. Stanley, and C. D. Farmer are with theDepartment of Electronics and Electrical Engineering, University of Glasgow,Glasgow G12 8LT, U.K.M. Hopkinson is with the Department of Electronic and Electrical Engi-neering, University of Sheffield, Sheffield, U.K.Digital Object Identifier 10.1109/LPT.2006.877574Fig. 1. Cross section (not to scale) of layout of ridge waveguide RTD laser.Cavity length is 500 m, giving an RTD active area of 1500 m . Alongside isschematic circuit representation of device.a multiple quantum well laser. This vertical integration of RTDand laser is straightforward as the RTD section requires onlythree extra epilayers of growth in addition to the basic laserstructure.The laser section was based on a design presented in [7]and consisted of six 6.7-nm-thick InGaAs quantum wells with10-nm InGaAlAs barriers. The RTD consisted of a 5-nm In-GaAs quantum well sandwiched by strained 2-nm AlAs bar-riers. Devices were fabricated based on the ridge waveguidelaser design shown in Fig. 1. In order to control its active area,the RTD was situated in the waveguiding ridge, between thelaser section of the device and the n-type contact. The laser hada cavity length of 500 m, and so with the 3- m ridge widththe RTD had an active area of 1500 m . The laser active areawas a minimum of 1500 m with the actual area dependent onthe extent of current spreading beneath the ridge. RTDs usuallyoperate with n-type material, and therefore in this design it wasmore convenient to use a p-type InP substrate.III. PRINCIPLES OF OPERATIONTo understand how the series combination of RTD and laserwill behave electrically and optically, it is worthwhile to con-sider the graphical technique of load line analysis. This methodis used to determine the operating point of the series combina-tion of two devices. The current–voltage characteristic of onedevice is plotted on the same axes as the load line of the second.The load line is simply the current through the second deviceas a function of the voltage across the first. The point where theload line intersects the current–voltage characteristic gives theoperating point of the series combination.Fig. 2 illustrates the principles of operation of the RTD-LDusing the load line technique. The bold plot gives a simulatedcurrent–voltage characteristic for a RTD, which was based ona model presented in [8]. The RTD exhibits negative differen-tial resistance (NDR), i.e., as the RTD voltage rises above athreshold value the RTD current drops steeply from its1041-1135/$20.00 © 2006 IEEESLIGHT et al.: INTEGRATION OF RESONANT TUNNELING DIODE AND OPTICAL COMMUNICATIONS LASER 1519Fig. 2. Illustration using load line analysis to explain operation of series com-bination of RTD and laser diode. Bold plot represents I–V characteristic of RTD,and three lines represent load lines of laser diode with series resistance at threedifferent bias voltages. Dashed horizontal line represents laser threshold current.peak value to a lower value before rising again.The RTD current–voltage characteristic is overlaid with a hor-izontal line to represent the laser threshold current (thecondition needs to be met for the de-vice to operate as described) and three load lines to represent thelaser diode combined with any series resistance attributableto the laser diode itself or the external circuit (one load line foreach of three different bias voltages— , and ). The loadline illustrates the diode voltage drop followed by a period ofexponential growth before tending to the gradient .When the bias voltage is increased from 0 V to , the loadline will reach point (a) upon which the current will reachand the laser will switch on. As the voltage increases further to, the load line will reach point (b) and the current will jumpfrom point (b) to point (b’) below and the laser will switchoff. Now decreasing the voltage to will cause the currentto jump from point (c) to point (c’) and the laser will switchon again. Clearly, the current–voltage characteristic will differdepending on whether the voltage is increasing or decreasing,resulting in electrical/optical hysteresis in the device. This hys-teresis occurs if (as in this example) the load line slopeis not as steep as the NDR region of the RTD current–voltagecharacteristic. Lower values of series resistance would reduceand eventually remove the hysteresis.IV. RESULTS AND DISCUSSIONThe dc electrical and optical characteristics of the deviceare shown in Figs. 3 and 4. For epilayer up mounting of theRTD-LD, to obtain cw laser operation, it was necessary tooperate the device at a temperature of 130 K. We anticipatewith some improvement of the laser design and epilayer downmounting, room temperature operation will be obtainable.From Fig. 3, we can see that the device is electricallybistable and thereby optically bistable, confirmed by Fig. 4.Fig. 3. Current versus voltage curve for RTD-LD at 130 K showing clear hys-teresis and electrical bistability. Dashed horizontal line represents threshold cur-rent of laser. With RTD active area of 1500 m , RTD peak and valley currentdensities are 11947 and 980 Acm respectively, giving a peak-to-valley cur-rent ratio of 12. SPICE model data is given by dotted curves.Fig. 4. Optical power versus voltage curve for RTD-LD at 130 K showing hys-teresis and bistability.The threshold current of the laser is represented by a dashedline in Fig. 3 and was taken from the optical power versuscurrent curve in Fig. 5. The width of the hysteresis loop is 1.3 Vand so when dc biased to the middle of the loop a 0.65-Vpulse will switch the laser from ON to OFF states and a 0.65-Vwill switch the laser from OFF to ON states. There is an opticalextinction ratio of 20 dB between the two states, so a 0.65-Vpulse can provide 20 dB of modulation. Furthermore, with abistable device NRZ modulation is more easily obtained.Bistability in RTDs can arise for one of two reasons. Theintrinsic bistability—where charge storage allows two differentstates of band bending for one applied bias—has been observedby Alves et al. [9] and explained theoretically in [10] and [11].The extrinsic bistability is illustrated in the load line analysis ofFig. 2 and is caused by the presence of the laser diode and seriesresistance.SPICE, the general purpose analog circuit simulator, wasused to model the RTD-LD and show whether the observedbistability in Figs. 3 and 4 is explainable in terms of the ex-trinsic effect. The SPICE RTD model presented by Brown [8]was modified to replicate the observed peak and valley RTDcurrents of the RTD-LD. A simple diode model was used to1520 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 18, NO. 14, JULY 15, 2006Fig. 5. Optical power output versus drive current for RTD-LD at 130 K. Laserthreshold current is 77 mA.Fig. 6. Emission spectrum of RTD-LD at 130 K (0.2-nm resolution).represent the electrical characteristics of the laser diode. Usinga method presented in [12], whereby the gradient of the RTDI-V characteristic is measured at higher biases as it tends to thevalue , the series resistance of the RTD-LD was found tobe 3 . This series resistance was added to the simulation. Afteradjusting the diode model parameters of saturation current andideality factor, the best fit obtainable with the SPICE RTD-LDmodel is given in Fig. 3. The fit agrees with the experimentaldata at low voltages but fails to replicate the increase in currentas the bias increases above . This is a consequence ofthe RTD model I–V not matching the the actual device RTDsection I–V at biases above . The experimental hysteresiswidths of the RTD-LD (1.3 V) could be replicated in the SPICEmodel and therefore the RTD-LD optical/electrical bistabilitywas attributed to the extrinsic load line effect of the laser diodeand series resistance.The optical spectrum at 130 K for the device is given in Fig. 6;the central peak is at a wavelength of 1.476 m. The RTD-LDoperated as a laser at room temperature with a pulsed drive cur-rent, and under these conditions the emission wavelength was1.54 m. The 130 K operating temperature shifted the emissionpeak to a shorter wavelength.The device can be improved by refinement of the laser sectiondesign in combination with epilayer down mounting to enableroom temperature operation and a lower threshold current. Inaddition, decreasing and with an improved RTD de-sign will reduce the hysteresis width according to the simulationresults and match a lowered laser threshold current. The reducedhysteresis loop width will lower the voltage of drive signals re-quired by the device. Further investigation of the RTD-LD willinclude characterizing the high-speed operation.REFERENCES[1] P. R. Berger, N. K. Dutta, D. L. Sivco, and A. Y. Cho, “GaAsquantum-well laser and heterojunction bipolar-transistor integrationusing molecular-beam epitaxial regrowth,” Appl. Phys. Lett., vol. 59,no. 22, pp. 2826–2828, 1991.[2] C. Vanhoof, J. Genoe, R. Mertens, G. Borghs, and E. Goovaerts, “Elec-troluminescence from bipolar resonant tunneling diodes,” Appl. Phys.Lett., vol. 60, no. 1, pp. 77–79, 1992.[3] Y. Kawamura, H. Asai, and H. Iwamura, “Fabrication of resonant-tun-neling optical bistable laser-diodes,” Electron. Lett., vol. 30, no. 3, pp.225–227, 1994.[4] C. R. Lutz, F. Agahi, and K. M. Lau, “Resonant-tunneling injectionquantum-well lasers,” IEEE Photon. Technol. Lett., vol. 7, no. 6, pp.596–598, Jun. 1995.[5] I. Grave, S. Kan, G. Griffel, S. Wu, A. Saar, and A. Yariv, “Monolithicintegration of a resonant tunneling diode and a quantum-well semicon-ductor-laser,” Appl. Phys. Lett., vol. 58, no. 2, pp. 110–112, 1991.[6] G. W. Roberts and A. S. Sedra, SPICE, 2nd ed. Oxford: Oxford Univ.Press, 1997.[7] M. Jain, “An investigation of broad gain spectrum InGaAs/InAlGaAsquantum well lasers latticed matched to InP,” Ph.D. dissertation, Univ.Glasgow, Glasgow, U.K., 2002.[8] E. R. Brown, O. B. McMahon, L. J. Mahoney, and K. M. Molvar,“SPICE model of the resonant-tunnelling diode,” Electron. Lett., vol.32, no. 10, pp. 938–940, 1996.[9] E. S. Alves, L. Eaves, M. Henini, O. H. Hughes, M. L. Leadbeater, F.W. Sheard, G. A. Toombs, G. Hill, and M. A. Pate, “Observation ofintrinsic bistability in resonant tunnelling devices,” Electron. Lett., vol.24, no. 18, pp. 1190–1191, 1988.[10] F. W. Sheard and G. A. Toombs, “Space-charge buildup and bistabilityin resonant-tunneling double-barrier structures,” Appl. Phys. Lett., vol.52, no. 15, pp. 1228–1230, 1988.[11] M. Rahman and J. H. Davies, “Theory of intrinsic bistability in a res-onant tunneling diode,” Semiconductor Sci. Technol., vol. 5, no. 2, pp.168–176, 1990.[12] M. J. Deen, “Simple method to determine series resistance and its tem-perature-dependence in AlAs/GaAs/AlAs double barrier resonant tun-neling diodes,” Electron. Lett., vol. 28, no. 13, pp. 1195–1197, 1992.

An investigation of broad gain spectrum InGaAs/InAlGaAs quantum well lasers latticed matched to InP,”

Electroluminescence from bipolar resonant tunneling diodes,”

Fabrication of resonant-tunneling optical bistable laser-diodes,”

GaAs quantum-well laser and heterojunction bipolar-transistor integration using molecular-beam epitaxial regrowth,”

Monolithic integration of a resonant tunneling diode and a quantum-well semiconductor-laser,”

Observation of intrinsic bistability in resonant tunnelling devices,”

Resonant-tunneling injection quantum-well lasers,”

Simple method to determine series resistance and its temperature-dependence in AlAs/GaAs/AlAs double barrier resonant tunneling diodes,”

Space-charge buildup and bistability in resonant-tunneling double-barrier structures,”

Theory of intrinsic bistability in a resonant tunneling diode,”

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Integration of a resonant tunneling diode and an optical communications laser

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