MBE GROWTH AND CHARACTERIZATION OF DOPED MULTIPLE-QUANTUM-WELL AVALANCHE PHOTODIODES

In the present study, how certain growth, processing and surface treat ment techniques can be used to dramatically lower the dark current beh avior of APDs by several orders of magnitude and improve the depletion characteristics of the built-in structures is shown. Through careful dopant calibration, the devices could be grown such that they were ess entially fully depleted as grown, even though doping levels in the ava lanche region were in the low 10(18) cm(-3). The full depletion charac teristics of the APDs were verified using theoretical modeling conduct ed for devices with similar structure and doping profiles. After proce ssing the devices into mesa diodes, various surface passivation treatm ents were investigated These include both O-2 plasma and sodium sulfid e treatments. Through the application of such treatments, a decrease i n the reverse bias dark current by as much as a factor of 1000 was ach ieved. Dark currents as low as 1 pA were obtained near zero bias. In s ome APDs, the dark currents increased to only 12 pA at 20% of breakdow n. In addition, these devices exhibited extremely high gains, which ex ceeded 35 000 in some APDs. The effect of the dopant profile in the av alanche region was also investigated. It was found that by the proper choice of dopant density and separation the break down voltage of the devices could be reduced by as much as 25%.