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We study the impact of device parameters, such as inner-aperture diameter and cavity photon lifetime, on thermal rollover mechanisms in 850-nm, oxide-confined, vertical-cavity surface-emitting lasers (VCSELs) designed for high-speed operation. We perform measurements on four different VCSELs of different designs and use our empirical thermal model for calculating the power dissipated with increasing bias currents through various physical processes such as absorption within the cavity, carrier thermalization, carrier leakage, spontaneous carrier recombination, and Joule heating. When reducing the top mirror reflectivity to reduce internal optical absorption loss we find an increase of power dissipation due to carrier leakage. There is therefore a trade-off between the powers dissipated owing to optical absorption and carrier leakage in the sense that overcompensating for optical absorption enhances carrier …