Heterojunction blocking contacts in MOCVD grown Hg1-xCd xTe long wavelength infrared photoconductors

The theoretical and experimental performance of Hg_1-xCd _xTe long wavelength infrared (LWIR) photoconductors fabricated on two-layer heterostructures grown by in situ MOCVD has been studied. It is shown that heterojunction blocking contact (HBC) photoconductors, consisting of wider bandgap Hg_1-xCd_x Te on an LWIR absorbing layer, give improved responsivity, particularly at higher applied bias, when compared with two-layer photoconductors incorporating n⁺/n contacts. An extension to existing device models is presented, which takes into account the recombination rate at the heterointerface and separates it from that occurring at both the contact-metal/semiconductor and passivant/semiconductor interfaces. The model requires a numerical solution to the continuity equation, and allows the device responsivity to be calculated as a function of applied electric field. Model predictions indicate that a change in bandgap across the heterointerface corresponding to a compositional change of Δx⩾0.04 essentially eliminates the onset of responsivity saturation due to minority carrier sweepout at high applied bias. Experimental results are presented for frontside-illuminated n-type Hg_1-xCd_xTe photoconductive detectors with either n⁺/n contacts or heterojunction blocking contacts. The devices are fabricated on a two-layer in situ grown MOCVD Hg_1-xCd_xTe wafer with a capping layer of x=0.31 and an LWIR absorbing layer of x=0.22. The experimental data clearly demonstrates the difficulty of forming n ⁺/n blocking contacts on LWIR material, and indicates that heterojunctions are the only viable technology for forming effective blocking contacts to narrow bandgap semiconductors