Finite Difference Analysis of thermal characteristics of Continuous Wave operation 850 nm lateral current injection and implant-apertured VCSEL with flip-chip bond design

R. Mehandru¹, G. Dang¹, B. Luo¹, S. Kim¹, F. Ren¹, S. J. Pearton², W. S. Hobson³, J. Lopata³,

M. Tayahi⁴, W. Chang⁵ and H. Shen⁵

1. Department of Chemical Engineering, University of Florida, Gainesville FL 32611

2. Department of Material Science and Engineering, University of Florida, Gainesville FL 32611

3. Multiplex Inc., South Plainfield, NJ 07974

4. Bell Labs, Lucent Technologies, Holmdel, NJ07733

5. Army Research Laboratory, Adelphi, MD 207835

mehandru@che.ufl.edu, gdang@che.ufl.edu, benluo@ufl.edu, skim@che.ufl.edu, ren@che.ufl.edu, spear@mse.ufl.edu, whobson@multiplexinc.com, jlopata@multiplexinc.com, moncef@lucent.com, wchang@arl.army.mil, pshen@arl.army.mil

Abstract: We used a finite difference simulation to quantitatively estimate the effect of flip-chip bonding on the thermal characteristics of high speed 850nm vertical-cavity surface-emitting lasers (VCSELs) with TiO₂/SiO₂ based top mirror, implanted-aperture and lateral current injection.

© 1999 Optical Society of America

OCIS codes: (250.7260) Vertical cavity surface emitting lasers; (140.5960) Semiconductor lasers

The VCSELs of this work are based on an implanted-aperture, index-guided, lateral current-injected, top dielectric mirror GaAs quantum well 850 nm design. The VCSELs were tested with the light signal propagated through 300 meters of optical fiber and a propagation rate of 11.5 Gb/s and bit error rates below 10^-14 were obtained[1, 2]. The thermal simulation employs Quasi-three dimensional finite difference analysis to calculate the temperature, T(r, z), thermal resistance and the rise time of temperature at a fixed bias to calculate the non-uniform heat source distribution. Carrier diffusion and distributed heat sources inside the active layer were also included. In addition, we have also simulated temperature distributions in the different layers comprising the lasers and estimated the rise time of temperature at a fixed bias.

Fig. 1. Eye diagram generated at 11.5 Gb/s via 300 m Lazrspeed fiber by a pseudorandom bit sequence (PRBS) of 2³¹-1.

The main heat generation, (85%) in the device occurs due to Joule heating due to the resistance of semiconductor layers. Joule heating is given by w(r, z) = r(r, z)j²(r, z) where j is the current density flowing in the device and r is the electrical resistivity of the materials for the VCSEL layers. A resistance model was also developed and the resistances for metal contacts, epi-layers for different current flow directions (vertical or lateral) and the Joule heat generations were estimated for each of the unit cells in our model. The other major heat source, (15%), is nonradiative recombination in the active region. Non-radiative recombination is governed by Auger recombination and Shockley-Read-Hall recombination. This portion of the heat generation was determined from the measured LIV (Power Current Voltage) characteristics of the laser diode and the estimated Joule heat generations.

Two-dimensional temperature-distribution of VCSELs with and without flip-chip bonding are illustrated in Figure 2. The maximum temperature rise is 60 °C for the structure without flip-chip bonding and it is 42 °C for the flip-chip bonded VCSEL. The result is in excellent agreement with the measured temperature rise of 1.96 °C/mW for 30 mW input power. The flip-chip bonded structure differs from the other due to the fact that the contacts (n- and p-) act as a heat sink. The maximum temperature occurred in the core of the cylindrical VCSEL near the top, because the center-top of VCSEL is covered by the dielectric mirror, which has a low thermal conductivity. The temperature difference between flip-chip bonded devices and bottom bonded devices increased from 15 °C at low current injection level to > 20 °C at high current injection level. This clearly demonstrates the efficiency of heat dissipation of flip-chip bonded devices.

Figure 2. Two-dimensional temperature-distribution of VCSELs (a)with and (b)without flip-chip bonding.

To investigate the temperature rise when device is turning on, a dynamical model was investigated. The transient-temperature in the different layers during turn-on of a VCSEL was also investigated. Figure 3 illustrates the temperature in the active region as a function of time. The time needed for the device to reach the steady-state temperature was in the range of a few tenths of a milli-second, which is orders of magnitude larger than the electrical or optical switch time. Flip-chip bonding will improve the shift of the wavelength, peak power, threshold current and slope efficiency during VCSEL operation.

Figure 3. Temperature changes as a function of time in the active region.

REFERENCES

1. W.S. Hobson, J. Lopata, L.M.F. Chirovsky, S.N.G. Chu, G. Dang, B. Luo, F. Ren, M. Tahayi, D.C. Kilper and S.J. Pearton, “Small and large signal performance and gain-switching of intra-cavity contacted, shallow implant apertured VCSELs,” Solid-State Electron 45, 1639 (2001).

2. G. Dang, W.S. Hobson, L.M.F. Chirovsky, J. Lopata, M. Tayahi, S.N.G. Chu, F. Ren and S.J.Pearton, “High-speed modulation of 850-nm intracavity contacted shallow implant-apertured vertical-cavity surface-emitting lasers,” IEEE Photonics Techn. Lett. 13, 924 (2001).