Understanding of semiconductor breakdown under high electric fields is an important aspect of materials’ properties, particularly for the design of power devices. For decades, a power-law has been used to describe the dependence of material-specific critical electrical field ($${\mathcal{E}}_{\text{crit}}$$ E crit ) at which the material breaks down and bandgap (Eg). The relationship is often used to gauge tradeoffs of emerging materials whose properties haven’t yet been determined. Unfortunately, the reported dependencies of $${\mathcal{E}}_{\text{crit}}$$ E crit on Eg cover a surprisingly wide range in the literature. Moreover, $${\mathcal{E}}_{\text{crit}}$$ E crit is a function of material doping. Further, discrepancies arise in $${\mathcal{E}}_{\text{crit}}$$ E crit values owing to differences between punch-through and non-punch-through device structures. We report a new normalization procedure that enables comparison of critical electric field values across materials, doping, and different device types. An extensive examination of numerous references reveals that the dependence $${\mathcal{E}}_{\text{crit}}$$ E crit ∝ Eg1.83 best fits the most reliable and newest data for both direct and indirect semiconductors. Graphical abstract

Ab initio calculations of the electronic structure of the Al3Ni compound have been performed, by use of the actual orthorhombic structure and of two different model structures having cubic symmetry. The self-consistent extended linear augmented plane-wave method is used. The details of the calculations are presented and the results are discussed and compared with experimental data. The calculated density of electron states (DOS) turns out to represent well the experimental ultraviolet photoelectron spectrum. We conclude that model structures should be used with some caution, because they can yield DOS curves that differ substantially from the real energy distribution of the valence electrons.

We report on the first demonstration of fluorescence detection using single-photon avalanche photodiodes (SPADs) monolithically integrated with a microfabricated surface ion trap. The SPADs are positioned below the trapping positions of the ions, and designed to detect 370 nm photons emitted from single 174 Yb + and 171 Yb + ions. We achieve an ion/no-ion detection fidelity for 174 Yb + of 0.99 with an average detection window of 7.7(1) ms. We report a dark count rate as low as 1.2 kHz at room temperature operation. The fidelity is limited by laser scatter, dark counts, and heating that prevents holding the ion directly above the SPAD. We measure count rates from each of the contributing sources and fluorescence as a function of ion position. Based on the active detector area and using the ion as a calibrated light source we estimate a SPAD quantum efficiency of 24±1%.

Recently there has been much interest in wide- and ultra-wide-bandgap (WBG/UWBG) semiconductors for power conversion, radio-frequency, and other applications. The benefits of these materials for high-power devices ultimately stems from the increase in critical electric field (EC) with increasing bandgap (EG). Most researchers cite the work of Hudgins et al. [IEEE Trans Power Elec. 18, 907 (2003)] which reported that EC ~ EG 2.0 for indirect-gap materials and EC ~ EG 2.5 for direct-gap materials. These dependencies are based on empirical power-law fits to reported experimental data. The exponent in the power-law is critical when predicting the performance of power devices composed of new UWBG materials, since the Unipolar Figure-of-Merit (UFOM) scales as EC 3. This work has re-analyzed old data and also analyzed new data, and has found EC ~ EG 1.8 with no difference between direct- and indirect-gap materials. A theory that explains this dependence has also been derived and shows good agreement with the experimental data. A key point is that the critical electric field is not a constant for a given material. Rather, EC depends on temperature and doping. Further, it is defined for a triangular field distribution, i.e. a non-punch-through (NPT) drift region, not a punch-through (PT) drift region with a trapezoidal field distribution. Careful examination of the literature revealed that critical electric field values have been reported for various doping levels, and that in general no distinction between values derived from PT and NPT structures has been made. Fortunately, in most cases values have been reported at room temperature, which is taken to be standard. As such, we have derived a procedure that allows EC values to be normalized to a fixed doping (1×1016 cm-3) and a NPT field distribution. The method equates the ionization integrals for the non-standard and standard configurations and backs out the values of EC normalized to the standard configuration. Using this algorithm, normalized values of EC for a variety of semiconductors were tabulated and plotted against bandgap. These data are shown in the attached Figure, where a power-law fit yielding an exponent of 1.8 for both direct- and indirect-gap semiconductors is also shown. The conventional semiconductors Ge, Si, InP, GaAs, and GaP are all included. The narrow-gap semiconductors InSb and InAs are not included, as it is believed that breakdown in these materials may have substantial contributions from tunneling, in addition to impact ionization. GaSb is also excluded due to experimental uncertainty. The wide-bandgap semiconductors SiC and GaN are included in the analysis, although considerable debate still exists concerning the critical field of GaN. For the UWBG semiconductors such as Ga2O3, AlN, and diamond, even more uncertainty exists, and only the critical electric field of diamond has been included in the analysis, which is nevertheless subject to large uncertainty. To explain the observed experimental data, a model based on the ionization integral has been derived. In this model, the ionization integral is transformed from an integral over space into an integral over electric field. The ionization rate as a function of field is then inserted into the integral. For this, the lucky-drift model of Ridley is used [J. Phys. C 16, 3373 (1983)]. This model is an extension of the more-commonly-cited lucky electron model first proposed by Shockley. In the extended model of Ridley, elastic momentum relaxation is considered in addition to energy relaxation. The model depends on three input parameters, which are the threshold energy required for ionization (Eth), the mean carrier relaxation length (l), and the ratio of the average energy lost per collision to the threshold energy (r). These parameters all depend on bandgap, with Eth ~ EG, l ~ EG -3/2, and r ~ EG -1. Inserting these dependencies into the ionization integral and numerically extracting the electric field at which avalanche occurs for a given bandgap yields a curve of EC versus EG, which closely matches the experimental EC ~ EG dependence. This work was partially supported by the Laboratory Directed Research and Development program at Sandia National Laboratories, a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia LLC, a wholly-owned subsidiary of Honeywell International Inc, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-NA-0003525. This material is also based upon work supported by the Assistant Secretary of Defense for Research and Engineering under Air Force Contract No. FA8702-15-D-0001. Any opinions, findings, conclusions, or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the Assistant Secretary of Defense for Research and Engineering. Figure 1

A B S T R A C TLateral bulk heterojunctions (LBHJ) provide a tool to directly probe the active area of photovoltaic devices using microscopy techniques. Here, we use scanning photocurrent microscopy (SPCM) to probe an organic photovoltaic (OPV) device with poly [{4,8-bis[(2-ethylhexyl)(PTB7):[6,6] phenyl-C61-butyric acid methyl ester (PC 61 BM). The effects of the additive 1,8-diiodooctane (DIO) on the recombination dynamics and morphology are probed in real space in a LBHJ structure. By using SPCM, we can see a larger increase in the space charge region for samples with DIO when compared to those without DIO. This indicates that the additive improves film morphology leading to increased charge extraction efficiency and decreased recombination.