This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Appl. Phys. Lett. 111, 243508 (2017) and may be found at https://doi.org/10.1063/1.5003288.The static and dynamic performance of vertical-cavity surface-emitting lasers (VCSELs) used as light-sources for optical interconnects is highly influenced by temperature. We study the effect of temperature on the performance of high-speed energy-efficient 980 nm VCSELs with a peak wavelength of the quantum well offset to the wavelength of the fundamental longitudinal device cavity mode so that they are aligned at around 60 °C. A simple method to obtain the thermal resistance of the VCSELs as a function of ambient temperature is described, allowing us to extract the active region temperature and the temperature dependence of the dynamic and static parameters. At low bias currents, we can see an increase of the −3 dB modulation bandwidth f−3dB with increasing active region temperature, which is different from the classically known situation. From the detailed analysis of f−3dB versus the active region temperature, we obtain a better understanding of the thermal limitations of VCSELs, giving a basis for next generation device designs with improved temperature stability

Bimberg, Dieter

Jia, Xiaowei

Li, Hui

Lott, James A.

Wolf, Philip

Applied Physics Letters

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Appl. Phys. Lett. 111, 243508 (2017); https://doi.org/10.1063/1.5003288 111, 243508© 2017 Author(s).Thermal analysis of high-bandwidth andenergy-efficient 980 nm VCSELs withoptimized quantum well gain peak-to-cavityresonance wavelength offsetCite as: Appl. Phys. Lett. 111, 243508 (2017); https://doi.org/10.1063/1.5003288Submitted: 05 September 2017 . Accepted: 30 November 2017 . Published Online: 14 December 2017Hui Li , Philip Wolf , Xiaowei Jia, James A. Lott , and Dieter BimbergARTICLES YOU MAY BE INTERESTED INRelative intensity noise of temperature-stable, energy-efficient 980 nm VCSELsAIP Advances 7, 025107 (2017); https://doi.org/10.1063/1.4974258 Heat dissipation effect on modulation bandwidth of high-speed 850-nm VCSELsJournal of Applied Physics 121, 133105 (2017); https://doi.org/10.1063/1.4979532 Microwave extraction method of radiative recombination and photon lifetimes up to 85 °C on50 Gb/s oxide-vertical cavity surface emitting laserJournal of Applied Physics 120, 223103 (2016); https://doi.org/10.1063/1.4971978Thermal analysis of high-bandwidth and energy-efficient 980nm VCSELswith optimized quantum well gain peak-to-cavity resonance wavelengthoffsetHui Li,1,a) Philip Wolf,2 Xiaowei Jia,1,3 James A. Lott,2 and Dieter Bimberg2,41College of Mathematical and Physical Sciences, Qingdao University of Science and Technology,266061 Qingdao, China2Institut f€ur Festk€orperphysik and Zentrum f€ur Nanophotonik, Technische Universit€at Berlin,Hardenbergstrasse 36, 10623 Berlin, Germany3State Key Lab of Luminescence and Applications, Changchun Institute of Optics, Fine Mechanicsand Physics, Chinese Academy of Sciences, 130033 Changchun, China4King Abdul-Aziz University, 21589 Jeddah, Kingdom of Saudi Arabia(Received 5 September 2017; accepted 30 November 2017; published online 14 December 2017)The static and dynamic performance of vertical-cavity surface-emitting lasers (VCSELs) used aslight-sources for optical interconnects is highly influenced by temperature. We study the effect oftemperature on the performance of high-speed energy-efficient 980 nm VCSELs with a peak wave-length of the quantum well offset to the wavelength of the fundamental longitudinal device cavitymode so that they are aligned at around 60 C. A simple method to obtain the thermal resistance ofthe VCSELs as a function of ambient temperature is described, allowing us to extract the activeregion temperature and the temperature dependence of the dynamic and static parameters. At lowbias currents, we can see an increase of the 3 dB modulation bandwidth f3dB with increasing activeregion temperature, which is different from the classically known situation. From the detailed analy-sis of f3dB versus the active region temperature, we obtain a better understanding of the thermal lim-itations of VCSELs, giving a basis for next generation device designs with improved temperaturestability. Published by AIP Publishing. https://doi.org/10.1063/1.5003288Vertical-cavity surface-emitting lasers (VCSELs) arethe typical light sources used for high-speed optical intercon-nects (OIs) for data communication based on multi-modeoptical fibers. The goals for advanced VCSEL-based OIsinclude low energy consumption, low cost, efficient opticalcoupling, and large direct current modulation frequency.1–3The high bit rate, energy efficiency, and temperature stabilityof OIs are achieved by the direct current modulation of smalloxide-aperture diameter VCSELs that are biased at moderatecurrents with simple non-return-to-zero on-off keyingdata coding.4–6 Both the static and dynamic performanceof VCSELs and OI systems are strongly influenced by anincrease in the ambient temperature. In a VCSEL, a tempera-ture increase can be caused by an ambient temperatureincrease and, more importantly, by the internal heating of theactive region. Different approaches have been demonstratedto decrease the internal VCSEL temperature by, for example,a reduction of the thermal resistance, by minimizing the opti-cal absorption, and by minimizing the series resistance.7–9The thermal resistance and temperature sensitivity ofVCSELs are related to their structure and thermal conductiv-ity of the constituent materials. The use of high thermal con-ductivity materials for the distributed Bragg reflector (DBR)mirrors and device layouts is helpful to achieve a high ther-mal performance for VCSELs.Heat transport in semiconductor lasers can be simulatedby numerically solving the heat conduction equation usingfinite-element methods.10 The heat equation readsqC@T@Tþ D  ðkDTÞ ¼ Q; (1)where T (K) is the temperature, Q (W/m3) is a heat source, q(kg/m3) is the material density, C (J/kgK) is the specific heatcapacity, and k (W/mK) is the thermal conductivity of themedium. The thermal behavior of semiconductor lasers isvery complex, since many parameters are functions of tem-perature. The heat sources in VCSELs are more numerousthan those in edge-emitting lasers, where the non-radiativerecombination of charge carriers in the active region is thedominate heat source. The important heat sources in VCSELsare non-radiative recombination, reabsorption of spontaneousemission in the active region, and Joule heating. The heatflux distribution in the device is related to the voltage dropacross the active region and the DBRs.11 The voltage dropacross the active region is 1.26V for 980 nm VCSELs (asestimated from the real-space bandgap energy). The voltagedrop across the DBRs is caused by interface barriers, whichincrease the resistance and the threshold voltage.12 The heattransfer equation is solved by defining an initial constantvalue of temperature T0 for all VCSEL materials.The device structure of our VCSELs used in this workis described in detail in Ref. 13. This structure employsa15 nm room temperature quantum well (QW) gain peakwavelength [based on a room temperature photolumines-cence (PL) measurement of a QW active region calibrationsample] to etalon wavelength offset to improve ourVCSELs’ elevated temperature static and dynamic perfor-mance.14 A contour plot of the temperature distribution in across-sectional plane (r-z plane) of, for example, a 2.5 lma)Electronic mail: lihui6526@qust.edu.cn0003-6951/2017/111(24)/243508/4/$30.00 Published by AIP Publishing.111, 243508-1APPLIED PHYSICS LETTERS 111, 243508 (2017)oxide-aperture diameter 980 nm VCSEL is given in Fig. 1.The continuous wave (CW) injection current is 1mA, thevoltage is 2.064V, and the output power is 0.233 mW. Thetotal dissipated heat in the VCSEL is 1.831 mW. Based onthe voltage drop across the active region and DBRs, theactive region heat source is calculated to be 4.02 1015 W/m3. So, 1mA of CW injection current causes the activeregion temperature to increase by 3.427 C.The simulation results are just as expected. The activeregion temperature is much higher than the ambient tempera-ture. Since the static and high-speed modulation performanceof VCSELs is decisively influenced by the active region tem-perature, an accurate knowledge of the heat generation insidethe device is required. A precise determination of the activeregion temperature Tactive is very important for a good under-standing of the device and to provide insight for the nextgeneration of device designs. A common method to obtainTinternal is the determination of the thermal resistance fromthe emission spectra. According to Ref. 15, the thermal resis-tance Rth can be determined from Rth¼ (Dk/DPdiss)/(Dk/DT).From the fundamental mode peak emission wavelength, wedetermine the cavity resonance wavelength shift rate versusthe dissipated power Dk/DPdiss, where Pdiss¼ IVPout,and the heat-sink temperature variation is (Dk/DT). Then, wecan calculate the thermal resistance. This is an approximateapproach, as we ignore the bias current effect on the internalheating upon obtaining Dk/DT. The thermal resistance canbe more precisely calculated from Rth¼DT/DPdiss, whichrelates the average temperature change DT in the laser cavityto a change DPdiss of the dissipated power. For this purpose,the VCSEL is typically driven with electrical pulses up to afew hundred nanoseconds, which are shorter than the thermaltime constant which is in the range of some microseconds.With this method, no internal heating of the VCSEL by thedissipated power arises and thereby no thermal wavelengthshift appears. In this work, we determine the cold cavitypeak wavelength by linearly extrapolating the peak wave-length change with the CW dissipated power emission wave-length k0 of the fundamental mode to the point of no internalheating at Pdiss¼ 0 mW. We start with emission spectrameasurements at different bias currents at an ambient tem-perature Tambient¼ 25 C using our 2.5 lm oxide-aperturediameter 980 nm VCSELs, from which we extract the funda-mental mode peak emission wavelength. For each biascurrent, we calculate the dissipated power via Pdiss¼ IVPout. Then, we plot the fundamental mode peak emis-sion wavelength change versus the dissipated power. Byusing simple linear extrapolation, we identify the peak emis-sion wavelength k0 of the fundamental mode without internalheating at Pdiss¼ 0 mW. This extrapolated cold cavity wave-length from a CW measurement is identical to a measure-ment of the peak wavelength in pulsed operation. We repeatthis procedure for different temperatures Tambient, resulting inthe plot in Fig. 2 to determine the ambient temperaturedependence of the active region and extract the k0 values atdifferent ambient temperatures, as shown in Fig. 3. We caneasily see that k0 linearly increases with ambient temperaturewith a slope of 0.065 nm/K, as shown in Fig. 3. This analysisgives us accurate values of the cavity resonance wavelengthshift rate versus the heat-sink temperature Dk/DT.As can be seen in Fig. 2, at all investigated ambienttemperatures Tambient, the fundamental mode peak emissionwavelength kP increases linearly with the dissipated electri-cal power Pdiss but with different slopes. No curvature isseen even for high dissipated power Pdiss. The slope of thekP versus Pdiss curve is given by the cavity resonanceFIG. 1. The simulated temperature distribution T(r,z) of our 2.5lm oxide-aperture diameter 980 nm VCSELs at a continuous wave (CW) bias currentof 1mA.FIG. 2. The measured fundamental mode peak emission wavelength kP ver-sus the CW dissipated electrical power Pdiss for different ambient tempera-tures Tambient for 980 nm VCSELs. The points marked with circles are theextrapolated wavelengths k0 of the fundamental mode peak emission wave-lengths without internal heating where Pdiss¼ 0 mW at the given ambienttemperature.FIG. 3. The wavelength k0 of the fundamental mode peak emission wave-length without internal heating at Pdiss¼ 0 mW, determined by simple linearextrapolation for different ambient temperatures Tambient from the data inFig. 2. The slope (0.06543 nm/K) of the linear fit gives the value of the cav-ity resonance wavelength shift rate versus temperature Dk/DT.243508-2 Li et al. Appl. Phys. Lett. 111, 243508 (2017)wavelength shift rate versus dissipated power Dk/DPdiss,which varies for each given ambient temperature. At ambienttemperatures Tambient of 25, 35, 45, 55, 65, 75, and 85C, thecavity resonance wavelength shift rate versus dissipatedpower Dk/DPdiss is 0.360, 0.369, 0.385, 0.396, 0.412, 0.426,and 0.440 nm/mW, respectively. These data allow us to derivethe dependence of the thermal resistance on the ambient tem-perature. We can calculate now the thermal resistance usingRth¼ (Dk/DPdiss)/(Dk/DT). The extracted values of Rth areshown in Table I. The high thermal resistance of our smalloxide-aperture diameter VCSELs results in large active regiontemperatures and negatively impacts the high-speed modula-tion performance. By using our ambient temperature-dependent thermal resistance data, we can perform a basicthermal effects analysis based on the VCSEL’s active regiontemperature at different bias currents.From the ambient-temperature-dependent thermal resis-tance, we extract the active region temperature Tactive for eachpoint in a given set of L-I-V curves within the linear L-Iregions. The active region temperature Tactive change withincreasing bias current can be estimated for the linear L-Iregion using Tactive¼TambientþRth (IVPout).15 Thisgives us the possibility to perform a detailed analysis of thethermal effects and extract the internal temperature-dependentVCSEL parameters. Furthermore, we can exclude the influenceof internal heating in the examination of the L-I-V curves. Bymeasuring the L-I-V curves at certain ambient temperatures,we obtain the output powers at certain currents (for example, at1mA). We can next calculate the active region temperatures byrepeating this procedure at other ambient temperatures. Finally,the optical output power Pout as a function of Tactive for eachindividual bias current can be determined, as shown in Fig. 4.Usually, the optical output power decreases with increasingTactive. Instead, our VCSELs show a clear first increase in opti-cal output power with increasing active region temperatureTactive. The Pout reaches a maximum as Tactive increases but thenPout decreases with further increases in Tactive. This is due toour VCSEL design where the peak PL gain from our quantumwell active region is offset from the cavity resonance wave-length by about15nm. This offset improves our VCSELs’high temperature performance in both static and high-speedmodulation operations.Figure 5 shows the3 dB modulation bandwidth f3dBchange with the active region temperature Tactive at differentbias currents. At low bias currents, we can see an increase off3dB with increasing internal active region temperature,which is contrary to typical VCSEL behavior. Again, thisincrease in f3dB is a result of the15 nm gain peak-to-cav-ity wavelength offset, which leads to a partial increase of thedifferential gain with an increase in the active region temper-ature. With an increase in the bias current, we find that ther-mal effects mainly limit the bandwidth. At currents largerthan 2.4mA and an active region temperature larger than95 C, the3 dB bandwidth shows no increase with increas-ing bias current. This means that the modulation bandwidthis limited thermally at large currents. Improving the heat dis-sipation by, for example, using novel VCSEL packagingmethods will lead to further improvement of the modulationbandwidth f3 dB at large bias.In conclusion, we studied numerically and experimen-tally thermal effects on the device performance of high-speed energy-efficient VCSELs with an optimized quantumwell PL peak-to-cavity wavelength offset. We used a simplemodel to obtain the VCSEL thermal resistance. The ambienttemperature-dependent thermal resistance in-turn enables usto extract the internal active region temperature versus biascurrent. A detailed analysis of the thermal effects allows usto extract the temperature dependence of the internalVCSEL parameters. From the modulation bandwidth f3dBversus the internal active region temperature, we gain insightTABLE I. Thermal resistance Rth for 2.5 lm oxide-aperture diameter980 nm VCSELs at different ambient temperatures.Ambienttemperature (C)Thermal resistanceRth (K/mW)25 5.49635 5.64345 5.87855 6.0565 6.2975 6.5185 6.72FIG. 4. Output power of the 2.5lm oxide-aperture diameter VCSEL versusthe active region temperature Tactive for different bias currents.FIG. 5. 3 dB small signal modulation bandwidth of the 2.5lm oxide-aperture diameter VCSEL versus the extracted active region temperatureTactive for different bias currents.243508-3 Li et al. Appl. Phys. Lett. 111, 243508 (2017)into the thermal limitations of our high-speed VCSELs. Thisinformation is extremely useful for improving the devicedesigns of our next generation VCSELs to achieve furtherimproved temperature stability.This work was supported by (1) the German ResearchFoundation via the Collaborative Research Center 787; (2)the National Natural Science Foundation of China (via GrantNo. 11647169); (3) the Natural Science Foundation ofShandong Province (via Grant No. ZR2016FB05); (4) theNatural Science Foundation of Qingdao (via Grant No. 16-5-1-8-jch); and (5) the Open Fund of the State Key Laboratoryof Luminescence and Applications.1A. Larsson, “Advances in VCSELs for communication and sensing,”IEEE J. Sel. Top. Quantum Electron. 17(6), 1552–1567 (2011).2A. Kasukawa, “VCSEL technology for green optical interconnects,” IEEEPhotonics J. 4(2), 642–646 (2012).3D. Bimberg, A. Larsson, and A. Joel, “Faster, more frugal, greenerVCSELs,” Compd. Semicond. 22, 34–39 (2014).4P. Moser, P. Wolf, G. Larisch, H. Li, J. A. Lott, and D. Bimberg, “Energy-efficient oxide-confined high-speed VCSELs for optical interconnects,”Proc. SPIE 9001, 900103 (2014).5H. Li, P. Wolf, P. Moser, G. Larisch, A. Mutig, J. A. Lott, and D.Bimberg, “Energy-efficient and temperature-stable 980 nm VCSELs for 35Gb/s error-free data transmission at 85 C with 139 fJ/bit dissipated heat,”IEEE Photonics Technol. Lett. 26(23), 2349–2352 (2014).6E. Haglund, P. Westbergh, J. S. Gustavsson, E. P. Haglund, A. Larsson,M. Geen, and A. Joel, “30 GHz bandwidth 850 nm VCSEL withsub-100 fJ/bit energy dissipation at 25–50 Gbit/s,” Electron. Lett. 51(14),1096–1098 (2015).7A. Al-Omari and K. Lear, “VCSEL with a self-aligned contact andcopper-plated heatsink,” IEEE Photonics Technol. Lett. 17(9), 1767–1769(2005).8D. Mathine, H. Nejad, D. Allee, R. Droopad, and G. Maracas, “Reductionof the thermal impedance of vertical-cavity surface-emitting lasers afterintegration with copper substrates,” Appl. Phys. Lett. 69(4), 463–464(1996).9K. L. Lear and R. P. 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QuantumElectron. 21(6), 405–413 (2015).14H. Li, P. Wolf, P. Moser, G. Larisch, A. Mutig, J. A. Lott, and D.Bimberg, “Impact of the quantum well gain-to-cavity etalon wavelengthoffset on the high temperature performance of high bit rate 980 nmVCSELs,” IEEE J. Quantum Electron. 50(8), 613–621 (2014).15R. Michalzik, “VCSEL fundamentals,” in VCSELs: Fundamentals,Technology and Applications of Vertical-Cavity Surface-Emitting Lasers,edited by R. Michalzik (Springer-Verlag, Berlin, Germany, 2013), vol.166, pp. 19–75.243508-4 Li et al. Appl. Phys. Lett. 111, 243508 (2017)

Thermal analysis of high-bandwidth and energy-efficient 980 nm VCSELs with optimized quantum well gain peak-to-cavity resonance wavelength offset

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Thermal analysis of high-bandwidth and energy-efficient 980 nm VCSELs with optimized quantum well gain peak-to-cavity resonance wavelength offset

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