E. M. Jackson

Nonionizing energy loss (NIEL) for heavy ions

The concept of nonionizing energy loss (NIEL) has been found useful for characterizing displacement damage effects in materials and devices. Published tabulations, however, are limited with respect to target materials, particle types and energies. In this paper we show how the NIEL database can be significantly expanded to include heavy ions in the coulombic limit by using the Monte Carlo code SRIM. The methodology used to extract NIEL from SRIM is described. This greatly adds to the number of materials and incident particles for which the NIEL concept can be applied. To show that values so derived are consistent with previous calculations, we compare alpha particle NIEL for GaAs derived from SRIM with a direct analytical calculation. The SRIM code is limited in that only coulombic interactions are considered. General rules of thumb are also described which permit prediction of NIEL for any target material over a large energy range. Tabulated values of NIEL for alpha particles incident on Si, GaAs and InP are presented.

Graded band gap for dark-current suppression in long- wave infrared W-structured type-II superlattice photodiodes

A new W-structured type-II superlattice photodiode design, with graded band gap in the depletion region, is shown to strongly suppress dark currents due to tunneling and generation-recombination processes. The long-wave infrared (LWIR) devices display 19%–29% quantum efficiency and substantially reduced dark currents. The median dynamic impedance-area product of 216Ωcm2 for 33 devices with 10.5μm cutoff at 78K is comparable to that for state-of-the-art HgCdTe-based photodiodes. The sidewall resistivity of ≈70kΩcm 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 band gap.

W-structured type-II superlattice long-wave infrared photodiodes with high quantum efficiency

Results are presented for an enhanced type-II W-structured superlattice (WSL) photodiode with an 11.3μm cutoff and 34% external quantum efficiency (at 8.6μm) operating at 80K. The new WSL design employs quaternary Al0.4Ga0.49In0.11Sb barrier layers to improve collection efficiency by increasing minority-carrier mobility. By fitting the quantum efficiencies of a series of p-i-n WSL photodiodes with background-doped i-region thicknesses varying from 1to4μm, the authors determine that the minority-carrier electron diffusion length is 3.5μm. The structures were grown on semitransparent n-GaSb substrates that contributed a 35%–55% gain in quantum efficiency from multiple internal reflections.

Very-long wave ternary antimonide superlattice photodiode with 21 μm cutoff

We describe a ternary antimonide superlattice photodiode with a 21 μm cutoff wavelength. The active region consists of 150 periods of 10 monolayers (MLs) of In0.07Ga0.93Sb and 19 MLs of InAs with InSb-like interfacial bonds. The device has a detectivity of 3×109 cm√Hz/W, dynamic impedance-area product of 0.18 Ω cm2, and peak external quantum efficiency of 3% at 40 K. X-ray diffraction and cross-sectional scanning tunneling microscopy show the structure to have a high degree of order with abrupt interfaces. A simulation of the absorption spectrum effectively reproduces the observed spectrum.

W-structured type-II superlattice-based long- and very long wavelength infrared photodiodes

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.

Analysis and performance of type-II superlattice infrared detectors

We discuss the current performance of long-wavelength infrared photodetectors based on type-II superlattices, and the projected characteristics for diffusion-limited operation. For optimized architectures such as graded-gap and abrupt-heterojunction designs, the dark currents are strongly dominated by Shockley-Read (SR) rather than Auger processes. A factor of 10 improvement over the demonstrated SR lifetimes would lead to a factor of 4 lower dark current than state-of-the-art HgCdTe devices.

Scream: A new code for solar cell degradation prediction using the displacement damage dose approach

based executable code has been developed called SCREAM (Solar Cell Radiation Environment Analysis Models) which implements the displacement damage dose (DDD) approach to solar cell degradation prediction in a space radiation environment.

Minority carrier diffusion length and lifetime for electrons in a type-II InAs∕GaSb superlattice photodiode

We use the electron-beam-induced current (EBIC) technique to investigate carrier transport characteristics in a type-II InAs∕GaSb superlattice photodiode with cutoff wavelength at 7.7μm. We use a theoretical model that includes an extended generation source and depletion region to simulate the EBIC current on both sides of the p–n junction. The electron minority diffusion length in the p-superlattice, Le, is extracted from the simulation, from which the electron lifetime τe is obtained. Le increases from 0.275μm at 5.3Kto0.355μm at 100K. τe drops from 1.5ns at 5.3Kto0.13ns at 100K.

Enhanced total ionizing dose tolerance of bulk CMOS transistors fabricated for ultra-low power applications

The first radiation tests of transistors fabricated in a commercial bulk CMOS process designed for ultra-low power applications in space are presented and analyzed. The predominant failure mode of bulk CMOS, i.e., radiation-induced parasitic leakage currents in n-channel transistors, is greatly suppressed by the use of low threshold voltage devices and by the application of backbias used to optimize their performance. With 2 volts of backbias applied, the transistors tested here show no degradation up to a dose of 200 krad(Si).

Correlation of Telemetered Solar Array Data With Particle Detector Data On GPS Spacecraft

BDD-IIR radiation environment detector data are used to calculate the expected solar array degradation for over 9 years of GPS orbit and compared with telemetered solar array data. The large discrepancy between the predicted and measured solar array output remains, thereby eliminating displacement damage as the sole damage mechanism to the solar arrays.