Impact of CdTe/CdZnTe substrate resistivity on performance degradation of long-wavelength n(+)-on-p HgCdTe infrared photodiodes

For realizing HgCdTe focal plane arrays on alternate substrates (Si or GaAs ), CdTe buffer layers are essential. Bulk CdTe/CdZnTe substrates are also u sed for LPE growth of HgCdTe, A model for the effect of the n-CdTe substrat e resistivity on the quantum efficiency, eta, and the dynamic resistance-ar ea product, R(d)A, of a n(+)-on-p HgCdTe backside illuminated photodiode ha s been developed, taking into account the effect of the graded heterointerf ace between CdTe/CdZnTe and HgCdTe on the homojunction photodiode. The issu e of how Low the substrate/buffer layer resistivity can be, without degradi ng the performance of the photodiode, has been addressed. For low substrate resistivities, the RdA can drop by about 50%, white the quantum efficiency decreases by about 5%, It has been found that as low as 2 Omega-cm for lon g wavelength IR photodiodes (cutoff wavelength 14 mu m) is acceptable. To o btain the R(d)A and eta from the band profile, a linear approximation has b een used in which the interface barrier region has a constant electric fiel d, while the bulk of the epilayer has no electric field. General expression s have been derived for the R(d)A and eta in this two-region model. Our sol utions are valid for both high and low electric fields, unlike previously d erived solutions in the literature valid either in the one-region low-field case or the two-region, high-field approximation.