J. B. Rodriguez

nBn structure based on InAs/GaSb type-II strained layer superlattices

The authors report on a type-II InAs∕GaSb strained layer superlattice (SLS) photodetector using an nBn design that can be used to eliminate both Shockley-Read-Hall generation currents and surface recombination currents, leading to a higher operating temperature. We present such a SLS based structure with a cutoff wavelength of 5.2μm at room temperature. Processed devices exhibited a quantum efficiency around 18%, and a shot-noise-limited specific detectivity ∼109Jones at 4.5μm and 300K, which are comparable to the state of the art values reported for p-i-n photodiodes based on strained layer superlattices.

Mid-IR focal plane array based on type-II InAs/GaSb strain layer superlattice detector with nBn design

A midwave infrared camera (λc=4.2μm) with a 320×256 focal plane array (FPA) based on type-II InAs∕GaSb strain layer superlattice (SLs) has been demonstrated. The detectors consist of an nBn heterostructure, wherein the SL absorber and contact layers are separated by a Al0.2Ga0.8Sb barrier layer, which is designed to have a minimum valence band offset. Unlike a PN junction, the size of the device is not defined by a mesa etch but confined by the lateral diffusion length of minority carriers. At 77K, the FPA demonstrates a temporal noise equivalent temperature difference (NETD) of 23.8mK (Tint=16.3ms and Vb=0.7V) with a peak quantum efficiency and detectivity at 3.8μm equal to 52% and 6.7×1011 Jones, respectively.

Bias dependent dual band response from InAs∕Ga(In)Sb type II strain layer superlattice detectors

We report on the multispectral properties of infrared photodetectors based on type II InAs∕Ga(In)Sb strain layer superlattices using an nBn heterostructure design. The optical and electrical properties of the midwave and long wave infrared (MWIR-LWIR) absorbing layers are characterized using spectral response and current-voltage measurements, respectively. The dual band response is achieved by changing the polarity of applied bias. The spectral response shows a significant change in the LWIR to MWIR ratio within a very small bias range (∼100mV), making it compatible with commercially available readout integrated circuits.

Type II InAs∕GaSb strain layer superlattice detectors with p-on-n polarity

We report on high operating temperature midwave infrared detectors based on type II InAs∕GaSb superlattices (SLs) with a p-on-n polarity. All InAs∕GaSb SLs photodiodes reported so far have a n-on-p polarity with a thin InAs n-type top contact, that is incompatible with most present day readout integrated circuits. Current-voltage measurements reveal dark current densities of ∼5×10−7A∕cm2 (82K) and 0.18A∕cm2 (240K) at −0.1V. R0A products were equal to ∼1×105Ωcm2 (82K) and 0.24Ωcm2 (240K). Zero-biases D* were estimated to be 2×1012 and 2×109 Jones at 82 and 240K, respectively.

Ultralow noise midwave infrared InAs-GaSb strain layer superlattice avalanche photodiode

Eye-safe midwavelength infrared InAs–GaSb strain layer superlattice p+-n−-n homojunction avalanche photodiodes (APDs) grown by solid source molecular beam epitaxy were fabricated and characterized. Maximum multiplication gain of 1800 was measured at −20V at 77K. Excess noise factors between 0.8 and 1.2 were measured up to gain of 300. Gain of 200 was measured at 120K. Exponential nature of the gain as a function of reverse bias along with low excess noise factor at higher gain confirms single carrier electron-only impact ionization in the avalanche regime. Decrease in the multiplication gain at higher temperatures correlates with standard APD characteristics.

Characterization of carriers in GaSb∕InAs superlattice grown on conductive GaSb substrate

We report on mobility spectrum analysis of electrical transport in a GaSb∕InAs superlattice (SL) grown on GaSb substrate. Despite domineering contribution to conduction from the substrate, it was possible to discern and characterize carriers from SL. A single electron specie with an ambient temperature mobility of ∼104cm2∕Vs was found to emanate from SL. We show that this carrier has an activation energy of 0.27eV and is associated with the SL band gap.

Type-II InAs/GaSb strain layer superlattice detectors for higher operating temperatures

Type-II InAs/GaSb superlattice photodiodes for mid-IR (3-5μm) region grown by solid-source molecular beam epitaxy are reported. Different approaches for realization of high quality interfaces between compositionally abrupt GaSb and InAs layers during the growth of the SLs are discussed. Mid wave infrared (&lgr;c~ 4.5 µm at T=300K) P-on-N designs of SLs detectors were developed to ensure compatibility with most present day readout integrated circuits (ROICs). Variable size diode arrays were fabricated using standard photolithography technique and hybridized to silicon fanout chip. The sizes of the detector mesas were varied from 29μm x 29μm to 804μm x 804μm. The single pixel characterization was undertaken at Santa Barbara Focal Plane. Temperature-dependent IV measurements revealed dark current density below 1 x 10-8 A/cm2 at 82K and below 2 x 10-5 A/cm2 at 240K. (Vbias = 0V). Dynamic resistance-area product at zero bias was found to be ~ 1 x 105 Ωcm2 at 82K and 0.24 Ωcm2 at 240K. Influence of protective silicon nitride coating on reduction surface leakage currents of detectors was investigated. We found that rsurface was equal to ~ 3 x 106 Ωcm indicating the proper surface preparation followed by room temperature Si3N4 deposition is effective in reduction of leakage currents in type-II MWIR InAs/GaSb superlattice photodiodes.

Type II Strain Layer Superlattices (SLS's) grown on GaAs Substrates

We report on type-II SLSs photodiodes grown on GaAs. The GaSb buffer was grown using a novel interfacial- misfit-approach, which relieves the strain though 90 misfit dislocations. The dark-current, quantum-efficiency and detectivity were measured.

Suppressed Surface Leakage Current Using nBn Infrared Detector Based on Type II InAs/GaSb Strain Layer Superlattices

We present a SLS-based nBn infrared detector which suppresses the surface leakage current without using any passivation technique. The dark current density is reduced by an order of magnitude at 77 K compared with a conventional mesa etched device.

Structural and Optical Characterization of InAs/GaSb nanoscale superlattices for mid-infrared detection

Optimization of various growth parameters for Type-II GaSb(10MLs)/InAs(10MLs) nanoscale superlattices and GaSb layers, grown by solid molecular beam epitaxy, has been undertaken. These include the As/Sb soak times and substrate temperature during the growth. We present optical and structural characterization for these heterostructures, using high resolution X-ray diffraction (HRXRD), photoluminescence (PL) and atomic force microscopy (AFM). Optimized parameters were then used to grow a thick structure suitable for mid-infrared detection.