Single-event effects (SEEs) refer to phenomena that arise from the interaction of single energetic particles with microelectronic devices, as is experienced in harsh radiation environments. Carrier generation induced by two-photon absorption (TPA) has become a valuable tool for SEE investigations of microelectronic structures owing to its unique ability to inject carriers through the wafer, directly into well-defined locations in complex circuits. Recent effort has focused on putting the TPA SEE technique on a more quantitative basis. This paper addresses the recent successes in achieving this goal, as well as the challenges that are faced moving forward.
Two numerical approaches for determining the charge generated in semiconductors via two-photon absorption (2PA) under conditions relevant for laser-based single-event effects (SEE) experiments are presented. The first approach uses a simple analytical expression incorporating a small number of experimental/material parameters while the second approach employs a comprehensive beam propagation method that accounts for all the complex nonlinear optical (NLO) interactions present. The impact of the excitation conditions, device geometry, and specific NLO interactions on the resulting collected charge in silicon devices is also discussed. These approaches can provide value to the radiation-effects community by predicting the impacts that varying experimental parameters will have on 2PA SEE measurements.
A predictive computational approach that limits use of DLTS experiments is presented, developed using the experimental data and proposed physics based models. Three-dimensional NanoTCAD simulations are used for physicsbased prediction of space radiation effects in III-V solar cells, and validated with experimentally measured characteristics of a p+n GaAs solar cell with AlGaAs window. The computed dark and illuminated I-V curves as well as corresponding performance parameters matched very well experimental data for 2 MeV proton irradiation at various fluences. We analyze the role of majority vs. minority and deep vs. shallow carrier traps in the solar cell performance degradation. The traps/defects parameters used in the simulations were derived from Deep Level Transient Spectroscopy (DLTS) data obtained at NRL. It was noticed that the degradation caused by deep traps observed in single-trap numerical tests exhibit a very similar trend to the degradation caused by a full spectrum of defect traps, but to a lesser degree. This led to the development of a method to accurately simulate the degradation of a solar cell by using only a single deep level defect whose density is calculated by the Stopping and Range of Ions in Matter (SRIM) code. Using SRIM, we calculated the number of vacancies produced by 2 MeV proton irradiation for fluences ranging from 6x1010 cm-2 to 5x1012 cm-2. Based on the SRIM results, we applied trap models in NanoTCAD and performed full I-V simulations from which the amount of degradation of performance parameters (Isc, Voc, Pmax) was calculated. The physics-based models using SRIM allowed obtaining good match with experimental data.
We present a predictive computational approach that may reduce the need for extensive inputs from Deep Level Transient Spectroscopy (DLTS) experiments. Three-dimensional NanoTCAD simulations are used for physics-based prediction of space radiation effects in a p<sup>+</sup>n GaAs solar cell with AlGaAs window, and validated with experimental data. The computed dark and illuminated I-V curves, as well as corresponding performance parameters, matched experimental data very well for 2 MeV proton irradiation at various fluence levels. We analyze the role of majority vs. minority and deep vs. shallow carrier traps in the solar cell performance degradation. The defects level parameters used in the simulations were taken from DLTS data obtained at NRL. It was determined from numerical simulations that the degradation of the photovoltaic parameters could be modeled and showed similar trends when a only a single deep level defect was considered compared to a spectrum of defect levels. This led to the development of an alternate method to simulate the degradation of a solar cell by using only a single deep level defect whose density is calculated by the Stopping and Range of Ions in Matter (SRIM) code. Using SRIM, we calculated the number of vacancies produced by 2 MeV proton irradiation for fluence levels ranging from 6x10<sup>10</sup> cm<sup>-2</sup> to 5x10<sup>12</sup> cm<sup>-2</sup>. Based on the SRIM results, we applied trap models in NanoTCAD and performed I-V simulations from which the degradation of the photovoltaic parameters (I<sub>sc</sub>, V<sub>oc</sub>, FF, P<sub>max</sub>) was calculated. The simulations using SRIM-derived defect concentrations showed reasonable agreement with simulations using parameters determined from DLTS.
The radiation response mechanisms operative in space solar cells are described. The effects of electron and proton
radiation-induced defects on the cell performance are identified and methods for modeling the radiation response are
presented. The space radiation environment is described, and a methodology for modeling the response of a solace cell
to exposure to the space radiation environment is presented. It is shown how this model an be used to predict on orbit
performance, and examples from space experiments are shown.
A critical step in developing type-II superlattice (T2SL) based LWIR focal plane array (FPA) technology is to achieve high performance levels in FPA pixel-sized devices having 20-40 μm pitch. At this scale, device performance tends to be limited by surface effects along mesa sidewalls which are etched to provide pixel isolation. While control of surface leakage has been achieved for MWIR T2SLs, as evidenced by the availability of commercially produced FPAs, the same cannot be said for LWIR T2SLs. Several groups have approached this problem as strictly a matter of surface treatment, including cleaning, chemical treatment, and dielectric coating or epitaxial overgrowth, but with limited success. Here we describe an approach based on shallow-etch mesa isolation (SEMI), which takes advantage of bandgap grading to isolate devices without exposing narrow-gap LWIR regions on diode mesas sidewalls. The SEMI process consists of defining mesa diodes with a shallow etch that passes only 20-100 nm past the junction of a graded-gap "W"-structured type-II superlattice p-i-n structure, where the bandgap remains large (>200 meV). A second, deeper etch is then used to define a trench along the chip border for access to the <i>p</i>-contact. As a result, SEMI diodes have only MWIR layers exposed along sidewalls, while the LWIR regions remain buried and unexposed. We also discuss an investigation of surface passivation of GaSb with sulfur using thioacetamide.
Recently we have achieved significant improvements in the performance of LWIR type-II superlattice photodiodes,
with discrete devices beginning to demonstrate dynamic impedance-area product (R0A) levels approaching the
MCT trend line and quantum efficiency exceeding 30% in devices without anti-reflection coatings. We discuss the
key innovations that have led to these improvements, including modified W-structures, band-gap grading, and
Significant recent advances in the high-temperature, high-power performance of type-II antimonide interband cascade lasers (ICLs) operating in the mid-infrared are reported. A 5-stage ICL with a 12μm ridge width and Au electroplating for improved epitaxial-side-up heat sinking operates cw to a maximum temperature of 257 K, where the emission wavelength is 3.7 μm. A similar device with a ridge width of 22 μm emits > 260 mW per facet for cw operation at 80 K (λ = 3.4 μm) and 100 mW at 200 K (λ = 3.6 μm). Beam qualities for the narrowest ridges approach the diffraction limit. The recent development of type-II "W" photodiodes for the long-wave infrared is also reviewed. A "W" photodiode with an 11.3 μm cutoff displayed a 34% external quantum efficiency (at 8.6 μm) operating at 80 K. A graded-gap design of the depletion region is shown to strongly suppress dark currents due to tunneling and generation-recombination processes. The median dynamic impedance-area product of 216 Ω-cm<sup>2</sup> for 33 devices with 10.5 μm cutoff at 78 K is comparable to that for state-of-the-art HgCdTe-based photodiodes. The sidewall resistivity of ≈70 kΩ-cm for untreated mesas is also considerably higher than previous reports for passivated or unpassivated type-II LWIR photodiodes, apparently indicating self-passivation by the graded bandgap.
The development of a photovoltaically (PV) powered laser communication system that constitutes a miniature, highly energy-efficient wireless communication technology is described. The technology is based on the direct integration of a multiquantum well (MQW) modulating retroreflector (MRR) optical communication node and a monolithically integrated module (MIM) PV power source. The MQW MRR optical data link exploits the shift in the MQW absorption peak under an applied reverse bias to modulate incident laser light, enabling binary encoding of data for transfer. A MIM consists of many individual solar cells monolithically integrated on a single substrate and offers the design versatility necessary to enable efficient electrical conversion of both incident sunlight and the system laser light and the ability to match the voltage output to the MRR requirements. A description of the development of the MRR and MIM components of the system is given. Results of bench-top demonstrations of the operational system are presented.
Recent improvements in material quality and design have led to large improvements in the quantum efficiency (QE) of long-wave infrared (LWIR) photodiodes based on W-structured type-II superlattices (WSL), which now have achieved external QE of up to 35% on an 11.3 μm cutoff photodiode operating at 80K. While single band and dual band WSLs have been demonstrated with cutoff wavelengths out to 17 μm, the initial devices also showed significant losses of photo-excited carriers resulting in QE levels of ≤ 10%. Here we describe recent results in which these losses have been dramatically reduced by modifying the WSL barrier layers to increase the mini-band width and improve the material properties. An additional 35-55% increase in QE also resulted from the use of semitransparent Te doped n-GaSb substrates that allowed for IR reflections off the backside from the Au plated chip carrier. A series of PIN photodiodes using the improved WSL, with intrinsic regions from 1 to 4 μm thick, were used to study minority carrier transport characteristics in the new structure. As a result of the improved design and material properties, the electron diffusion length in the undoped i-region, as determined from a theoretical fit to the thickness-dependent data, was 3.5 μm, allowing for much higher collection efficiency in PIN photodiodes with intrinsic regions up to 4 μm thick.
Multiband detection capability is a critical attribute of practical infrared (IR) sensing systems for use in missile defense detect-and-track applications. This capability, already demonstrated in mercury-cadmium telluride (MCT) photodiodes and quantum well infrared photodetectors (QWIPs), has not previously been explored in type II-superlattices (T2SLs), a newer system which is under consideration to meet next-generation sensor needs. Like QWIPs, T2SLs are composed of layers of III-V compound semiconductors grown by molecular beam epitaxy (MBE), and have an infrared gap that is determined primarily by the layer thicknesses. With the exceptional control of MBE over layer thicknesses and the ability to grow multiple bandgap structures under compatible growth conditions, T2SL-based multiband IR focal plane arrays (FPAs) are expected to have advantages in spectral control and pixel-to-pixel uniformity over MCT. Additionally, T2SLs have intrinsically higher quantum efficiency than QWIPs, in which the optical selection rules for intersubband transitions forbid the absorption of normally incident light.
Here we describe the first results for a T2SL dual band detector with independent long-wave and very-long-wave infrared responsivity bands, with cutoffs of 11.4 and 17 μm respectively. The p-n-p device contains "W"-structured T2SL (WSL) active regions for enhanced band selectivity, owing to the quasi-two-dimensional density of states for WSLs. Photodetector results are demonstrated using a maskset designed to fabricate single-band diodes, 3-terminal dual band devices, and 2-terminal band selectable devices to comply with different dual band FPA read-out architectures.
W-structured type-II superlattices (W-SLs) were initially developed to increase the gain of mid-wave infrared (MWIR) lasers. The design addressed the reduced optical transition matrix elements due to the spatial displacement between valence and conduction band wavefunctions in the type-II superlattice (T2SL), and further improved the differential optical gain by providing a mostly two-dimensional density of states. As a result, W-SL and W interband cascade lasers have lower thresholds and higher pulsed and cw operating temperatures than any other III-V interband MWIR lasers. These same features give W-SLs desirable properties for IR detectors, and here we report for the first time on characteristics of W-SLs used for long-wave and very long-wave IR photodiodes. IR transmission measurements of W and conventional T2SL photodiodes revealed absorption characteristics that are well described by theory, including line shape and peak absorption coefficient values which are about a factor of 2 greater in the W-SLs. Similarly, the low temperature photoluminescence shows much higher and sharper emission intensity in the W-SLs. While the W-SLs have demonstrated superior optical properties, as predicted, additional work is needed to achieve higher detector quantum efficiency. Results suggest that the excess carrier collection in the W-structures is reduced with respect to similar T2SL structures, especially for the lowest energy state. Possible mechanisms of excess carrier loss, as well as new designs to improve charge collection, in the W-SL, will be discussed.
The photovoltaic characterization of triple-junction InGaP<sub>2</sub>/GaAs/Ge solar cells is presented. Measurements made using a single light source solar simulator are compared with other measurements made using a multi-light source solar simulator that provides a close match to the air mass zero (AM0) solar spectrum. The output spectrum of the solar simulators has been measured, and two methods for calibrating the simulator output intensity haven been employed. The spectral response of the solar cells has been characterized through quantum efficiency measurements. These data are analyzed to determine the effect of the simulator spectrum on the measured photovoltaic response, and in particular, areas where spectral mismatch between the simulator and AM0 can lead to inaccurate performance predictions are highlighted. In addition, the effects of the different calibration techniques on the measured data are studied. Exploiting the capabilities of the multi-source, close matched simulator, the response of each of the three sub-junctions are studied individually, and the interplay between the spectral response of the sub-junctions and the incident spectrum is investigated.
The development of a photovoltaically (PV) powered, laser communication system that constitutes a miniature, highly energy-efficient wireless communication technology is described. The technology is based on the direct integration of a multi-quantum well (MQW) modulating retroreflector (MRR) optical communication node and a monolithically integrated module (MIM) PV power source. The MQW MRR optical data link exploits the shift in the MQW absorption peak under an applied reverse bias to modulate incident laser light enabling binary encoding of data for transfer. A MIM consists of many individual solar cells monolithically integrated on a single substrate and offers the design versatility necessary to allow efficient electrical conversion of both incident sunlight and the system laser-light and the ability to match the voltage output to the MRR requirements. A description of the development of the MRR and MIM components of the system along with the power management and distribution circuitry is given. Results of bench-top demonstrations of the operational system are presented.