Edward Kwei Wei Huang

High operating temperature midwave infrared photodiodes and focal plane arrays based on type-II InAs/GaSb superlattices

The dominant dark current mechanisms are identified and suppressed to improve the performance of midwave infrared InAs/GaSb type-II superlattice photodiodes at high temperatures. The optimized heterojunction photodiode exhibits a quantum efficiency of 50% for 2 μm thick active region without any bias dependence. At 150 K, R0A of 5100 Ω cm2 and specific detectivity of 1.05×1012 cm Hz0.5/W are demonstrated for a 50% cutoff wavelength of 4.2μm. Assuming 300 K background temperature and 2π field of view, the performance of the detector is background limited up to 180 K, which is improved by 25 °C compared to the homojunction photodiode. Infrared imaging using f/2.3 optics and an integration time of 10.02 ms demonstrates a noise equivalent temperature difference of 11 mK at operating temperatures below 120 K.

Background limited long wavelength infrared type-II InAs/GaSb superlattice photodiodes operating at 110 K

The utilization of the P+-π-M-N+ photodiode architecture in conjunction with a thick active region can significantly improve long wavelength infrared type-II InAs/GaSb superlattice photodiodes. By studying the effect of the depletion region placement on the quantum efficiency in a thick structure, we achieved a topside illuminated quantum efficiency of 50% for an N-on-P diode at 8.0 μm at 77 K. Both the double heterostructure design and the application of polyimide passivation greatly reduce the surface leakage, giving an R0A of 416 Ω cm2 for a 1% cutoff wavelength of 10.52 μm, a Shot–Johnson detectivity of 8.1×1011 cmHz/W at 77 K, and a background limited operating temperature of 110 K with 300 K background.

Band edge tunability of M-structure for heterojunction design in Sb based type II superlattice photodiodes

We present theoretically and experimentally the effect of the band discontinuity in type II misaligned InAs∕GaSb superlattice heterodiodes. Calculations using the empirical tight binding method have shown the great flexibility in tuning the energy levels of the band edge in M-structure superlattice as compared to the standard InAs∕GaSb superlattice. Through the experimental realization of several p-π-M-n photodiodes, the band discontinuity alignment between the standard binary-binary superlattice and the M-structured superlattice was investigated via optical characterization. The agreement between the theoretical predictions and the experimental measurement confirms the capability of controlling the M-structure band edges and suggests a way to exploit this advantage for the realization of heterostructures containing an M-structured superlattice without bias dependent operation.

Background Limited Performance of Long Wavelength Infrared Focal Plane Arrays Fabricated From M-Structure InAs–GaSb Superlattices

The recent introduction of a M-structure design improved both the dark current and R0 A performances of type-II InAs-GaSb photodiodes. A focal plane array fabricated with this design was characterized at 81 K. The dark current of individual pixels was measured between 1.1 and 1.6 nA, 7 times lower than previous superlattice FPAs. This led to a higher dynamic range and longer integration times. The quantum efficiency of detectors without antireflective coating was 74%. The noise equivalent temperature difference reached 23 mK, limited only by the performance of the testing system and the read out integrated circuit. Background limited performances were demonstrated at 81 K for a 300 K background.

Surface leakage reduction in narrow band gap type-II antimonide-based superlattice photodiodes

Inductively coupled plasma (ICP) dry etching rendered structural and electrical enhancements on type-II antimonide-based superlattices compared to those delineated by electron cyclotron resonance (ECR) with a regenerative chemical wet etch. The surface resistivity of 4×105 Ω cm is evidence of the surface quality achieved with ICP etching and polyimide passivation. By only modifying the etching technique in the fabrication steps, the ICP-etched devices with a 9.3 μm cutoff wavelength revealed a diffusion-limited dark current density of 4.1×10−6 A/cm2 and a maximum differential resistance at zero bias in excess of 5300 Ω cm2 at 77 K, which are an order of magnitude better in comparison to the ECR-etched devices.Inductively coupled plasma (ICP) dry etching rendered structural and electrical enhancements on type-II antimonide-based superlattices compared to those delineated by electron cyclotron resonance (ECR) with a regenerative chemical wet etch. The surface resistivity of 4×105 Ω cm is evidence of the surface quality achieved with ICP etching and polyimide passivation. By only modifying the etching technique in the fabrication steps, the ICP-etched devices with a 9.3 μm cutoff wavelength revealed a diffusion-limited dark current density of 4.1×10−6 A/cm2 and a maximum differential resistance at zero bias in excess of 5300 Ω cm2 at 77 K, which are an order of magnitude better in comparison to the ECR-etched devices.

Demonstration of midinfrared type-II InAs/GaSb superlattice photodiodes grown on GaAs substrate

We report the growth and characterization of type-II InAs/GaSb superlattice photodiodes grown on a GaAs substrate. Through a low nucleation temperature and a reduced growth rate, a smooth GaSb surface was obtained on the GaAs substrate with clear atomic steps and low roughness morphology. On the top of the GaSb buffer, a p+-i-n+ type-II InAs/GaSb superlattice photodiode was grown with a designed cutoff wavelength of 4 μm. The detector exhibited a differential resistance at zero bias (R0A) in excess of 1600 Ω cm2 and a quantum efficiency of 36.4% at 77 K, providing a specific detectivity of 6×1011 cmHz/W and a background limited operating temperature of 100 K with a 300 K background. Uncooled detectors showed similar performance to those grown on GaSb substrates with a carrier lifetime of 110 ns and a detectivity of 6×108 cmHz/W.

Elimination of surface leakage in gate controlled type-II InAs/GaSb mid-infrared photodetectors

The electrical performance of mid-infrared type-II superlattice M-barrier photodetectors is shown to be limited by surface leakage. By applying gate bias on the mesa sidewall surface, leakage current is significantly reduced. Qualitatively IV modeling shows diffusion-dominated behavior of dark current at temperatures greater than 120 K. At 110 K, the dark current of gated device is reduced by more than 2 orders of magnitude, reaching the measurement system noise floor. With a quantum efficiency of 48% in front side illumination configuration, a 4.7μm cut-off gated device attains a specific detectivity of 2.5 × 1014 cmHz1/2/W at 110 K, which is 3.6 times higher than in ungated devices.

The effect of doping the M-barrier in very long-wave type-II InAs∕GaSb heterodiodes

A variation on the standard homodiode type-II superlattice with an M-barrier between the π-region and the n-region is shown to suppress the dark currents. By determining the optimal doping level of the M-superlattice, dark current densities of 4.95mA∕cm2 and quantum efficiencies in excess of 20% have been demonstrated at the moderate reverse bias of 50mV; allowing for near background-limted performance with a Johnson-noise detectivity of 3.11×1010cmHz∕W at 77K for 14.58μm cutoff wavelength for large area diodes without passivation. This is comparable to values for the state-of-the-art HgCdTe photodiodes.

High quantum efficiency two color type-II InAs∕GaSb n-i-p-p-i-n photodiodes

A n-i-p-p-i-n photodiode based on type-II InAs∕GaSb superlattice was grown on a GaSb substrate. The two channels, with respective 50% of responsivity cutoff wavelengths at 7.7 and 10μm, presented quantum efficiencies (QEs) of 47% and 39% at 77K. The devices can be operated as two diodes for simultaneous detection or as a single n-i-p-p-i-n detector for sequential detection. In the latter configuration, the QEs at 5.3 and 8.5μm were measured as high as 40% and 39% at 77K. The optical cross-talk between the two channels could be reduced from 0.36 to 0.08 by applying a 50mV bias.

Type-II superlattice dual-band LWIR imager with M-barrier and Fabry-Perot resonance.

We report a high performance long-wavelength IR dual-band imager based on type-II superlattices with 100% cutoff wavelengths at 9.5 μm (blue channel) and 13 μm (red channel). Test pixels reveal background-limited behavior with specific detectivities as high as ~5×10¹¹ Jones at 7.9 μm in the blue channel and ~1×10¹¹ Jones at 10.2 μm in the red channel at 77 K. These performances were attributed to low dark currents thanks to the M-barrier and Fabry-Perot enhanced quantum efficiencies despite using thin 2 μm absorbing regions. In the imager, the high signal-to-noise ratio contributed to median noise equivalent temperature differences of ~20 milli-Kelvin for both channels with integration times on the order of 0.5 ms, making it suitable for high speed applications.