Heterojunction Blocking Contacts in MOCVD Grown Hg1-xCdXTe Long-Wavelength Infrared Photoconductors

The theoretical and experimental performance of Hg1-xCdxTe 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 Hg1-xCdxTe 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 Delta x greater than or equal to 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 Hg1-xCdxTe photoconductive detectors with either n(+)/n contacts or heterojunction blocking contacts, The devices are fabricated on a two-layer in situ grown MOCVD Hg1-xCdxTe wafer with a capping layer of x = 0.31 and an LWIR absorbing layer of x = 0.22, The measured responsivity, at an applied bias of 10 V/cm, of the HBC photoconductor is almost double that of devices with nonblocking n(+)/n contacts, The corresponding values for the effective contact recombination velocity extracted from the device model are 250 cm/s for the HBC photoconductor, and greater than 10(4) cm/s for two-layer devices with n(+)/n contacts, 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.