Commercially available read out integrated circuits (ROICs) require the FPA to have high dynamic resistance area product at zero bias (R0A) which is directly related to dark current of the detector. Dark current arises from bulk and surface contributions. Recent band structure engineering studies significantly suppressed the bulk contribution of the type-II superlattice infrared photodetectors (N structure, M structure, W structure). In this letter, we will present improved dark current results for unipolar barrier complex supercell superlattice system which is called as “N structure”. The unique electronic band structure of the N structure increases electron-hole overlap under bias, significantly. N structure aims to improve absorption by manipulating electron and hole wavefunctions that are spatially separated in T2SLs, increasing the absorption while decreasing the dark current. In order to engineer the wavefunctions, we introduce a thin AlSb layer between InAs and GaSb layers in the growth direction which also acts as a unipolar electron barrier. Despite the difficulty of perfect lattice matching of InAs and AlSb, such a design is expected to reduce dark current. Experiments were carried out on Single pixel with mesa sizes of 100 × 100 – 700 × 700 μm photodiodes. Temperature dependent dark current with corresponding R0A resistance values are reported.
We describe a relationship between the noise characterization and activation energy of InAs/GaSb superlattice Mid-Wavelength-Infrared photodiodes for different passivation materials applied to the device. The noise measurements exhibited a frequency dependent plateau (i.e. 1/f-noise characteristic) for unpassivated as well as Si3N4 passivated samples whereas 1/f-type low noise suppression (i.e. frequency independent plateau) with a noise current reduction of more than one order of magnitude was observed for SiO2 passivation. For reverse bias values below -0.15V, the classical Schottky-noise calculation alone did not appear to describe the noise mechanism in a SL noise behavior, which shows a divergence between theoretically and experimentally determined noise values. We identify that, the additional noise appears, with and without passivation, at the surface activation energy of < 60 meV and is inversely proportional to the reverse bias. This is believed to be caused by the surface dangling-bonds (as well as surface states) whose response is controlled by the applied reverse bias. The calculated noise characteristics showed a good agreement with the experimental data.
Poor passivation on photodetectors can result in catastrophic failure of the device. Abrupt termination of mesa
side walls during pixel definition generates dangling bonds that lead to inversion layers and surface traps leading
to surface leakage currents that short circuit diode action. Good passivation, therefore, is critical in the
fabrication of high performance devices. Silicondioxide has been the main stay of passivation for commercial
photodetectors, deposited at high temperatures and high RF powers using plasma deposition techniques. In
photodetectors based on III-V compounds, sulphur passivation has been shown to replace oxygen and saturate
the dangling bonds. Despite its effectiveness, it degrades over time. More effort is required to create passivation
layers which eliminate surface leakage current. In this work, we propose the use of sulphur based
octadecanethiol (ODT), CH3(CH2)17SH, as a passivation layer for the InAs/GaSb superlattice photodetectors that
acts as a self assembled monolayer (SAM). ODT SAMs consist of a chain of 18 carbon atoms with a sulphur
atom at its head. ODT Thiol coating is a simple process that consist of dipping the sample into the solution for a
prescribed time. Excellent electrical performance of diodes tested confirm the effectiveness of the sulphur head
stabilized by the intermolecular interaction due to van der Walls forces between the long chains of ODT SAM
which results in highly stable ultrathin hydrocarbon layers without long term degradation.
We report on the development of InAs/AlSb/GaSb based N-structure superlattice pin photodiode. In this new design, AlSb layer in between InAs and GaSb layers acts as an electron barrier that pushes electron and hole wave functions towards the GaSb/InAs interface to perform strong overlap under reverse bias. Experimental results show that, with only 20 periods of intrinsic layers, dark current density and dynamic resistance at -50 mV bias are measured as 6x10-3 A/cm2 and 148 Ωcm2 at 77K, respectively. Under zero bias, high spectral response of 1.2A/W is obtained at 5 μm with 50% cut-off wavelengths (λc) of 6 μm. With this new design, devices with only 146 nm thick i-regions exhibit a quantum efficiency of 42% at 3 μm with front-side illimunation and no anti-reflection coatings.
We have achieved significant improvement in the electrical performance of the InAs/GaSb midwave infrared
photodetector (MWIR) by using atomic layer deposited (ALD) aluminium oxide (Al2O3) as a passivation layer. Plasma
free and low operation temperature with uniform coating of ALD technique leads to a conformal and defect free
coverage on the side walls. This conformal coverage of rough surfaces also satisfies dangling bonds more efficiently
while eliminating metal oxides in a self cleaning process of the Al2O3 layer. Al2O3 passivated and unpassivated diodes
were compared for their electrical and optical performances. For passivated diodes the dark current density was
improved by an order of magnitude at 77 K. The zero bias responsivity and detectivity was 1.33 A/W and 1.9 x 1013
Jones, respectively at 4 μm and 77 K. Quantum efficiency (QE) was determined as %41 for these detectors.