Extending the wavelength response of short-wavelength infrared detectors (SWIR) to 2 μm and beyond offers a key enabling technology for Lidar, spectroscopy, and imaging applications. Lattice-matched InGaAs/GaAsSb type-II superlattices (T2SL) have demonstrated SWIR detection around 2 μm on a highly producible InP platform but are limited in further extending wavelength response out to 2.5 μm. Introducing strain between alternating periodic layers of the superlattice provide a means to further extend wavelength while maintaining a strain balance (net zero strain). Numerical modeling utilizing 8 band k.p simulations were studied to examine the impact of bandgap energy, quantum confinement, and strain on the effective bandgap, wavefunction overlap, and corresponding absorption strength. T2SL with 4.73 nm In0.52Ga0.48As and 4.5 nm GaAs0.48Sb0.52 demonstrate extended wavelength response with absorption coefficient ~1000 cm-1 around the cutoff wavelength and over most of the 2-2.5 μm range indicating efficient overlap between the wavefunctions and high quantum efficiency values. Effect of period thickness, and strain on absorption coefficient and effective bandgap is analyzed and compared with the lattice matched case. Devices were fabricated with the superlattice structure as the absorber layer (3 μm thick) in a p-i-n photodiode configuration. Dark current voltage characteristics were measured and compared with simulated results. Measured dark current values were comparable to those of lattice-matched InGaAs/GaAsSb type II superlattices without extensive device optimization. Simulated quantum efficiency results showed clear extended cutoff of 2.5 μm with high quantum efficiency values in the 1.4-2.1 μm spectral region and features that closely follow absorption spectra validating the suitability of the strainbalanced approach in reducing the effective bandgap values while maintain absorption strength.
A shortwave infrared camera system with extended bandwidth of greater than 2 m and high resolution of 1.3 megapixels is demonstrated. The imager has a conventional p-i-n structure with type-II superlattice (T2SL) multi quantum well as the absorption region, and is backside illuminated to allow response to 400nm. The focal plane array is shown on both 8- and 12-µm pitches, with a wideband responsivity in a 0.4-2 µm range. New processing methodologies were developed and executed in a fabless model, achieving scalability, cost-efficiency, and high-performance metrics.
Ultrafast (femtosecond pulse) laser irradiation provides unique laser-material interactions associated with high instantaneous electric fields and short timescales that lead to a nonequilibrium state between excited electrons and phonons. These interactions result in material modifications that differ from conventional chemical, physical, and thermal processing at much longer timescales, and an opportunity to address material and device processing challenges associated with wide-bandgap materials such as Ga2O3. In this work, we explore ultrafast laser irradiation (Ti:sapphire, 150 fs pulse width) of bulk (010) Sn-doped β-Ga2O3 under two different wavelengths, fundamental (780 nm) and frequency-doubled (390 nm), and a range of laser fluences. We identified a regime for laser-induced damage threshold resulting in material ablation, thermally-induced straight crack formation, and recrystallization. Rastering on a β-Ga2O3 substrate created surface nanostructures including laser-induced periodic surface structures at a high spatial frequency (period ~250 nm). These highly aligned periodic structures can be controlled by laser polarization and wavelength, presenting a means for direct writing of surface nanostructures. Enhanced atomic movement associated with a transient metallic state can provide a means for intentional generation of point defects via the laser irradiation. These point defects may offer a means of electrical modification, which was demonstrated as more than five orders of the magnitude enhancement of lateral conductance on rastered β-Ga2O3. Moreover, a hydrophobic surface of β-Ga2O3was achieved by ultrafast laser irradiation.
The advent of subwavelength dielectric gratings enables narrowband spectral filtering on a compact, low-loss, and readily fabricable platform. Subwavelength gratings realize narrow spectral features via coupling to laterally propagating leaky modes (guided mode resonance). Given their minimal number of layers and geometrically-tunable pass bands, these structures are particularly useful in infrared hyperspectral applications. We previously demonstrated long-wave IR (8 – 12 μm) infrared filters based on high-index contrast suspended silicon/air gratings. High-contrast gratings placed above a slab of the same index are zero-contrast gratings (ZCGs) and possess several advantages. In this study, we present mid-wave IR (MWIR, 3 – 6 μm) ZCG filters using air/Si/SiO2 gratings fabricated on commercial silicon-on-insulator wafers. Geometric parameters are optimized using a genetic algorithm. We demonstrate ZCG filters with quality factors (Q) as high as 175 at oblique incidence for a 4.4 μm wavelength, and with a background high-reflectivity window from about 4.0 to 5.5 μm. The filters are optimized for coupling to light polarized with the electric field perpendicular to the gratings (transverse magnetic, TM). We also demonstrate coupling to transverse electric (TE) modes under azimuthally oblique incidence. For the same mode order, TE modes are more weakly coupled than TM, and therefore enable narrower spectral linewidths. To obtain an experimental Q of 175, full conical mounting allows strong TM mode coupling for the background reflection, and weak TE mode coupling for a narrow transmission band. Experimental results closely agree with transmittance spectra calculated via rigorous coupled wave analysis. The ZCG approach also offers a means for the design and fabrication of 2D gratings that offer polarization independent operation. We present polarization-independent filter response on square and hexagonal lattice designs.
In past decade, T2SL detectors with promising performance have been reported by various institutions thanks to the extensive modeling efforts, improvement of T2SL material quality, and development of advanced low-dark-current architectures with unipolar barriers (Xbn, CBIRD, pBiBn, M-structure, etc). One of the most demanding challenges of present day T2SL technology is the suppression of surface leakage currents associated with the exposed mesa sidewalls, which appear during the definition of device optical area. Typical FPA pixels have large surface/volume ratio and their performance is strongly dependent on surface effects. In order to overcome the limitation imposed by surface leakage currents, a stable surface passivation layer is needed. In this paper we report on InAs/GaSb T2SL detectors operating in the LWIR spectral region (100% cut-off wavelength of ~10 μm at 77K) passivated with epitaxially grown ZnTe. In order to compensate for the high conductivity of ZnTe passivation it was doped with chlorine to 1 × 1018cm−3 concentration. Dark current measurements reveal the significant reduction of noise current after ZnTe passivation.
We report broadband reflectance in the long-wavelength infrared (LWIR, 8-12 μm) utilizing suspended-Si, high-index-contrast subwavelength gratings (HCGs). Iterative design optimization using finite element analysis software has been performed accounting for silicon’s wavelength-dependent index of refraction and extinction coefficient. Grating arrays were fabricated using commercial silicon-on-insulator (SOI) substrates, photolithography and reactive ion etching; subsequent selective wet etching of SiO2 was used to provide suspended Si/air gratings. Fourier transform infrared (FTIR) spectroscopy demonstrates broadband, polarization–dependent reflectance between 8.5 and 12 μm, which agrees with the simulated response.
We present in this study a theoretical and experimental investigation of the MWIR HgCdTe nBn device concept.
Theoretical work has demonstrated that the HgCdTe nBn device is potentially capable of achieving performance
equivalent to the ideal double layer planar heterostructure (DLPH) detector. Comparable responsivity, low current
denisty Jdark, and high detectivity *D values rival those of the DLPH device without requiring p-type doping. The
theoretical results suggests that the HgCdTe nBn structure may be a promising solution for achieving a simplified MWIR
device structure and addressing problems associated with reducing thermal generation in conventional p-on-n structures
and processing technology limitations such as achieving low, controllable in-situ p-type doping with MBE growth
techniques. Furthermore, the physical mechanisms for selective carrier conduction in the nBn structure may provide a
basis to incorporate into future device structures to suppress intrinsic Auger carrier generation. Likewise, the
experimental demonstration of the MWIR HgCdTe nBn devices introduces a promising potential alternative to
conventional high performance p-n junction HgCdTe photodiodes. The experiments described in this study illustrate the
successful implementation of a HgCdTe barrier-integrated structure. The measured current-voltage characteristics of
planar-mesa and mesa HgCdTe nBn devices exhibit barrier-influenced behavior and follow temperature-dependent
trends as predicted by numerical simulations. Optical measurements of the planar-mesa MWIR HgCdTe nBn device
indicate a bias-dependent spectral response. Further changes to MWIR HgCdTe nBn layer structure has shown an over
105 A/cm2 reduction in Jdark as well as a shift to a lower turn-on operation bias. This experimental investigation highlights
the potential for pursuing similar and related unipolar, type-I barrier devices for high performance infrared detector
applications.
The performance of leading HgCdTe p-n junction infrared (IR) device technology is limited by thermal generationrecombination
(G-R) mechanisms and material processing challenges associated with achieving low, controllable in-situ
p-type doping using molecular beam epitaxy (MBE) growth techniques. These aspects are addressed in the proposed
hybrid HgCdTe NBνN structure which relies on band gap engineered layers to suppress Shockley-Read-Hall (SRH) and
Auger G-R processes contributing to performance degradation. The unipolar NBνN architecture provides the desired
advantages of a simplified fabrication process, eliminating p-type doping requirements. Physics-based numerical device
simulations incorporating established HgCdTe material parameters and G-R mechanisms are used to study the
performance characteristics of a long wavelength infrared (LWIR) NBνN device with a 12 μm cut-off wavelength. The
calculated results are compared to those values obtained for an LWIR HgCdTe nBn device. Theoretical dark current
density (Jdark) values of the NBνN device are lower by an order of magnitude or more for temperatures between 50 K and
245 K. Calculated detectivity (D*) values of 2.367 x 1014 - 2.273 x 1011 cm Hz0.5/W for temperatures ranging from 50 K
and 95 K, respectively, are observed in the NBνN structure.
A nearly universal goal for infrared photon detection systems is to increase their operating temperature without
sacrificing performance. For high quality HgCdTe photovoltaic infrared detectors at elevated temperatures, the lowdoped
absorber layer becomes intrinsic, carrier concentrations are high and Auger processes typically dominate the
dark current. Device designs have been proposed to suppress Auger processes in the absorber by placing it between
exclusion and extraction junctions under reverse bias. In this work, we analyze the non-equilibrium operation of
very long wavelength infrared (VLWIR) HgCdTe devices and identify the performance improvements (operation
temperature, responsivity, detectivity) expected when Auger suppression occurs. We identify critical structure
design requirements that must be satisfied for optimal performance characteristics from VLWIR non-equilibrium
devices and compare these devices with current state of the art double layer planar heterostructure (DLPH) devices.
High sensitivity HgCdTe infrared detector arrays operating at 77 K can be tailored for response across the infrared
spectrum (1 to 14 μm and beyond), and are commonly utilized for high performance infrared imaging applications.
However, the cooling system required to achieve the desired sensitivity makes them costly, heavy and limits their
applicability. Reducing cooling requirements and eventually operating at temperatures that could be reached with
thermoelectric coolers can lead to lighter and more compact systems. However, at these elevated temperatures, the
absorber layer becomes intrinsic, carrier concentrations are high and Auger processes typically dominate the dark current
and noise characteristics. Auger processes can be suppressed by placing the absorber layer between an exclusion junction
and an extraction junction at reverse bias. This reduces the minority carrier concentration in the absorber by several
orders of magnitude below thermal equilibrium. The majority carrier concentration also drops significantly below
thermal equilibrium to maintain charge neutrality, eventually reaching the extrinsic doping level. This device architecture
produces a lower dark current density and lower noise at non-cryogenic temperatures than standard p-n junction
photodiodes. Due to the precise control of the layer's thicknesses and compositions that could be achieved with
molecular beam epitaxy (MBE), this technique is the method of choice for implementing these novel non-equilibrium
devices. In this work, we analyze Auger suppression in HgCdTe alloy-based device structures and determine the
operation temperature improvements expected when Auger suppression occurs. We identified critical material (absorber
dopant concentration and minority carrier lifetime) requirements that must be satisfied for optimal performance
characteristics. Experimental dark current-voltage characteristics between 120 and 300 K are fitted using numerical
simulations. Based on this, the negative differential resistance (NDR) observed in experimental data is attributed to the
full suppression of Auger-1 processes and the partial suppression of Auger-7 processes. We will also present an analysis
and comparison of our theoretical and experimental device results in structures where Auger suppression was realized.
A model is developed to explain the hysteretic electric field dependence of the electrooptic coefficient in ferroelectric thin films. The reversible electric polarization and the tunable dielectric susceptibility of the ferroelectric thin film are proposed to explain the hysteretic ρ-E (electrooptic coefficient- applied electric field) loop. An empirical model used in ferroelectric capacitors to predict the high frequency C-V curve is utilized here to find the field dependence of the nonlinear susceptibility. The tunable susceptibility can also explain the peaked characteristics of the ρ-E loop. We also show that the linear electrooptic effect in ferroelectric thin films could produce the pseudo-quadratic electrooptic effect on field-induced birefringence as a result of the switchable spontaneous polarization of ferroelectrics. Thus, a careful interpretation of the field-induced birefringence is required to avoid misleading conclusions. This model provides a fundamental understanding to the tunability of the electrooptic coefficient and is useful for the electrooptic characterization of the ferroelectric thin films.
The intensity-voltage output characteristics of thin-film linear electro-optic Mach-Zehnder interferometer modulators can be nonperiodic for configurations where the optic axis is perpendicular to the applied electric field. As a result, the electro-optic coefficient for the material can not be determined assuming a periodic half-wave voltage. From the Jones matrix calculation, an analytic expression of the output intensity is derived in terms of the phase retardation. A method of determining the linear electro-optic coefficient is proposed based on the determination of the first intensity minimum in intensity-voltage characteristics. This method provides a simple expression for determining the electro-optic coefficient given values for the ordinary and extraordinary refractive indices, the modulator geometry, and the first half-period voltage. The first half-period voltage is found to be approximately inversely proportional to the square root of the electrode length. The method shows close agreement to the exact Jones matrix method for the case where the sum of the principal refractive indices in one arm of Mach-Zehnder interferometer is close to that of the other arm under an electric field.
State-of-the-art large area photovoltaic detectors fabricated in HgCdTe grown by Molecular Beam Epitaxy have been demonstrated for the Crosstrack Infrared Sounder instrument. Large area devices (1 mm in diameter) yielded excellent electrical and optical performance operating at 81K for LWIR band and at 98K for MW and SWIR bands. LWIR and MWIR detectors have near-theoretical electrical performance, and AR-coated quantum efficiency is greater than 0.70. Measured average RoA at 98K is 2.0E7 W-cm2 and near-theoretical quantum efficiencies greater than 0.90 were obtained on SWIR detectors. These state-of-the-art large area photovoltaic detector results reflect high quality HgCdTe grown by Molecular Beam Epitaxy on CdZnTe substrates in all three spectral bands of interest.
Carrier dynamics in self-assembled quantum dots, grown by molecular beam epitaxy, have been studied. The temperature dependence of the relaxation times, measured by room temperature high frequency impedance response of quantum dot lasers and by low temperature (T=4K) differential transmission spectroscopy. strongly suggests that electronhole scattering is the dominant scattering mechanism in quantum dots. The favorable relaxation times can be exploited to realize far infrared emission and detection based on intersubband transitions in the dots.
The characteristics of high-speed quantum well and quantum dot lasers are described. It is seen that substantial improvements in small-signal modulation bandwidth are obtained in both 1 micrometers (48 GHz) and 1.55 micrometers (26 GHz) by tunneling electrons directly into the lasing subband. In quantum dots the small-signal modulation bandwidth is limited by electron-hole scattering to approximately 7 GHz at room temperature and 23 GHz at 80 K.
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