Equilibrium Barrier Formation In p-on-N And P-on-n Graded HgCdTe Heterojunctions

We present results of our calculations of the equilibrium barrier formation in graded Hg1_xCdxTe heterojunctions using a highly accurate numerical model. Results for wide gap-p on narrow gap-n structures (Pn) are presented together with a review of our earlier results on narrow gap-p on wide gap-n (pN) heterojunctions [J. Appl. Phys. 62, 3267 (1987); 64, 6373 (1988)1 in which the barrier forms only in the conduction band. All band profiles are calculated with and without (common anion rule) a valence band offset; clear trends are observed. In the case of the narrow gap-p on wide gap-n heterostructures, the band profiles calculated with and without the valence band offset do not differ significantly. On the other hand, for the wide gap-p on narrow gap-n heterostructures, and using the common anion rule, the valence band tends to bow down on the n-side for the larger grading widths. The band bowing acts as a potential barrier for the minority carriers. When the valence band offset is included, though, the existence of a barrier to minority carriers depends upon the grading width: for the most narrowly graded junctions, a potential well for the minority carriers is present. In our calculations, we assume a valence band offset of 300 meV for HgTe:CdTe. We cannot make general predictions with regard to conditions needed to support the formation and growth of a barrier either in the conduction or valence band; instead; we find the band profiles to be a complex function of all the junction design parameters.