Shallow-Etch Mesa Isolation of Graded-Bandgap “W”-Structured Type II Superlattice Photodiodes

Shallow-etch mesa isolation (SEMI) of graded-bandgap “W”-structured type II superlattice (GGW) infrared photodiodes provides a powerful means for reducing excess dark currents due to surface and bulk junction related processes, and it is particularly well suited for focal-plane array fabrication. In the n-on-p GGW photodiode structure the energy gap is increased in a series of steps from that of the lightly p-type infrared-absorbing region to a value typically two to three times larger. The wider gap levels off about 10 nm short of the doping-defined junction, and continues for another 0.25 μm into the heavily n-doped cathode before the structure is terminated by an n+-doped InAs top cap layer. The increased bandgap in the high-field region near the junction helps to strongly suppress both bulk tunneling and generation–recombination (G–R) current by imposing a much larger tunneling barrier and exponentially lowering the intrinsic carrier concentration. The SEMI approach takes further advantage of the graded structure by exposing only the widest-gap layers on etched surfaces. This lowers surface recombination and trap-assisted tunneling in much the same way as the GGW suppresses these processes in the bulk. Using SEMI, individual photodiodes are defined using a shallow etch that typically terminates only 10 nm to 20 nm past the junction, which is sufficient to isolate neighboring pixels while leaving the narrow-gap absorber layer buried 100 nm to 200 nm below the surface. This provides for separate optimization of the photodiode’s electrical and optical area. The area of the junction can be reduced to a fraction of that of the pixel, lowering bulk junction current, while maintaining 100% optical fill factor with the undisturbed absorber layer. Finally, with the elimination of deep, high-aspect-ratio trenches, SEMI simplifies array fabrication. We report herein results from SEMI-processed GGW devices, including large-area discrete photodiodes, mini-arrays, and a focal-plane array. Current–voltage data show strong suppression of side-wall leakage relative to that for more deeply etched devices, as well as scaling of dark current with junction area without loss of quantum efficiency.

[1]  Manijeh Razeghi,et al.  Background limited long wavelength infrared type-II InAs/GaSb superlattice photodiodes operating at 110 K , 2008 .

[2]  William W. Bewley,et al.  Lifetimes and Auger coefficients in type-II W interband cascade lasers , 2008 .

[3]  Martin Walther,et al.  InAs/GaSb type-II superlattices for single- and dual-color focal plane arrays for the mid-infrared spectral range , 2006 .

[4]  Jerry R. Meyer,et al.  Type‐II quantum‐well lasers for the mid‐wavelength infrared , 1995 .

[5]  Vishnu Gopal Variable-area diode data analysis of surface and bulk effects in HgCdTe photodetector arrays , 1994 .

[6]  Frank Fuchs,et al.  Investigation of trap-assisted tunneling current in InAs/(GaIn)Sb superlattice long-wavelength photodiodes , 2002 .

[7]  Jeffrey H. Warner,et al.  Graded band gap for dark-current suppression in long-wave infrared W-structured type-II superlattice photodiodes , 2006 .

[8]  Darryl L. Smith,et al.  Proposal for strained type II superlattice infrared detectors , 1987 .

[9]  L. Esaki,et al.  Two-dimensional electronic structure in InAs-GaSb superlattices☆☆☆ , 1978 .

[10]  L. Esaki,et al.  A new semiconductor superlattice , 1977 .

[11]  S. Chuang,et al.  Midinfrared type-II InAs∕GaSb superlattice photodiodes toward room temperature operation , 2008 .

[12]  Martin Walther,et al.  Passivation of InAs∕(GaIn)Sb short-period superlattice photodiodes with 10μm cutoff wavelength by epitaxial overgrowth with AlxGa1−xAsySb1−y , 2005 .

[13]  Mark Johnson Spin injection and accumulation in mesoscopic metal device structures , 2007, SPIE OPTO.

[14]  Manijeh Razeghi,et al.  Surface leakage reduction in narrow band gap type-II antimonide-based superlattice photodiodes , 2009 .

[15]  Yajun Wei,et al.  Ammonium sulfide passivation of Type-II InAs/GaSb superlattice photodiodes , 2004 .

[16]  Jerry R. Meyer,et al.  AUGER LIFETIME ENHANCEMENT IN INAS-GA1-XINXSB SUPERLATTICES , 1994 .

[17]  Yajun Wei,et al.  Near bulk-limited R0A of long-wavelength infrared type-II InAs/GaSb superlattice photodiodes with polyimide surface passivation , 2007 .

[18]  Manijeh Razeghi,et al.  Dark current suppression in type II InAs∕GaSb superlattice long wavelength infrared photodiodes with M-structure barrier , 2007 .