The Experimental Determination of Impact Ionization Coefficients in Gallium-Arsenide and Indium Phosphide

213 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1983.Impact ionization coefficients are important parameters for the understanding of high field transport in semiconductors. Previous determinations of the electron and hole impact ionization coefficients, (alpha) and (beta) respectively, in both GaAs and InP have been limited to narrow ranges of electric fields and are not in agreement. The results of extensive photocurrent multiplication measurements on both materials over a very wide electric field range are reported.The electroabsorption in the depletion region of recombination radiation generated in thick heavily doped contacts has been shown to produce a mixed photocurrent injection condition in InP devices. This process was observed in a device structure conventionally used for ionization measurements and will be important for measurements on any direct band gap material. A special device structure eliminating this process has been developed which permits simultaneous injection of pure electron and hole primary photocurrents needed for accurate impact ionization coefficient measurements.The impact ionization coefficients in (100) InP have been determined from photomultiplication measurements performed on specially fabricated p('+)n and n('+)p devices permitting both electron and hole injection. The ratio of (beta)/(alpha) is found to vary from 4.0 to 1.3 as the electric field increases from 2.5 to 7.7 x 10('5) V/cm. Avalanche noise measurements performed at 30 MHz are in general agreement with the experimental ionization coefficient data.Similar (100) GaAs p('+)n device structures have been fabricated, and the impact ionization coefficients determined from devices having heavy p('+) doping indicate the effects of a finite electron threshold energy. A method is devised to include this effect in the calculation of (alpha) and (beta), and agreement is obtained among these same devices by assuming a 1.7 eV electron threshold energy. The corrected ionization data is found to also be in agreement with data derived from devices not requiring this correction. The measured ratio of (alpha)/(beta) in GaAs is found to vary from 2.5 at 2.2 x 10('5) V/cm to 1.3 at 6.25 x 10('5) V/cm. Avalanche noise measurements performed on the same devices confirm these results. Devices fabricated on (111) GaAs exhibit a periodic variation in multiplication across the active region that is associated with terracing produced during the LPE growth process.U of I OnlyRestricted to the U of I community idenfinitely during batch ingest of legacy ETD