High-Frequency Impedance of Proton-Bombarded Injection Lasers

Injection lasers are inherently suited for high-frequency (>50 MHz) applications.1,2 In the Bell System, the commonly used GaAs doubleheterostructure (DH) stripe geometry laser is obtained by either shallow- or deep-proton bombardment.3,4 For deep bombardment, the active stripe is well defined electrically by the high-resistivity material that confines it. In the case of shallow bombardment, as well as oxide stripe geometry lasers, current can flow laterally, beyond the stripe, through the conductive cladding layer.5"8 Previously, the impedances of several laser geometries were measured at high frequencies, and the results interpreted in terms of phenomenological equivalent circuits.9-13 In this paper, we will restrict the analysis and measurements to zero-biased shallow-bombarded laser diodes, for simplicity. We will show that, for shallow-bombarded stripe geometry lasers, the low-level (radio frequency) RF current at 463 high frequencies is confined under the stripe. The interpretation of measurements in terms of the fundamentally derived analytical expressions provides a useful measure of relevant material properties. Finally, although the main emphasis is on stripe geometry lasers, the analysis is applicable to other optoelectronic devices. Thus, the approach can be applied to predict the small-signal, unbiased impedance of lightemitting diodes (LEDs) and photodetectors operating at high frequencies.