EFFECT OF VARIATIONS IN THE DOPING PROFILES ON THE PROPERTIES OF DOPED MULTIPLE-QUANTUM-WELL AVALANCHE PHOTODIODES

The purpose of this study is to use both theoretical and experimental evidence to determine the impact of doping imbalance and symmetry on t he physical and electrical characteristics of doped multiple quantum w ell avalanche photodiodes. Theoretical models have been developed to c alculate the electric field, valence and conduction bands, capacitance -voltage (CV), and carrier concentration versus depletion depth profil es. The models showed a strong correlation between the p- acid n-dopin g balance inside the GaAs wells and the number of depleted stages and breakdown voltage of the APD. A periodic loping imbalance in the wells has been shown to result in a gradual increase (or decrease) in the e lectric field profile throughout the device which gave rise to partial ly depleted devices at low bias. The MQW APD structures that we modele d consisted of a 1 mu m top p(+)- doped (3 x 10(18) cm(-3)) GaAs layer , followed by a 1 mu m region of alternating layers of GaAs (500 Angst rom) and Al0.42Ga0.58As (500 Angstrom), and a 1 mu m n(+) back layer ( 3 x 10(18) cm(-3)). The GaAs wells were doped with p-i-n layers placed at the center of each well. The simulation results showed that in an APD with nine doped wells, and where the 50 Angstrom p-doped layer is off by 10% (p = 1.65 x 10(18) cm(-3), n = 1.5 x 10(18) cm(-3)), almost half of the MQW stages were shown to be undepleted at low bias which was a result of a reduction in the electric field near the p(+) cap la yer by over 50% from its value in the balanced structure. Experimental CV and IV data on similar MBE grown MQW structures have shown very si milar depletion and breakdown characteristics. The models have enabled us to better interpret our experimental data and to determine both th e extent of the doping imbalances in The devices as well as the overal l p- or n-type doping characteristics of the structures.