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This paper reviews materials challenges and recent progress for selective area regrowth and doping for vertical gallium nitride (GaN) power devices. The purpose is to realize randomly placed, reliable, contactable, and generally useable laterally patterned p-n junctions, which are the building blocks for various advanced power rectifiers and transistors. The general regrowth process and regrowth dynamics in trenches were discussed, where the effects of trench geometries, growth methods, and bulk substrates were elucidated. Comprehensive materials characterization techniques were utilized to analyze the regrown structures, including scanning electron microscopy, transmission electron microscopy, atom probe tomography, scanning probe microscopy, and secondary-ion mass spectrometry. Cathodoluminescence and secondary electrons in scanning electron microscopy and atom probe tomography were used to achieve lateral and vertical dopant profiling at a sub-micron scale. The regrowth interface after dry etching accumulated a high density of impurities and charges, contributing to the formation of a p+-n+ tunneling junction. This hypothesis was further confirmed by the electrostatic potential profile at the regrowth interface using electron holography. Novel etching technologies were investigated to improve the regrowth interface. It was found that low-power dry etching significantly reduced the interfacial charges and the reverse leakage currents of regrown p-n junctions. Photoelectrochemical wet etching was found to be effective in reducing deep-level defects near the regrowth interface. Atomic layer etching uses self-limiting chemical processes, thus removing the damaged layers without inducing further etching damage. Tertiarybutylchloride-based in situ etching may serve as an alternative etching method to dry etching with reduced etching damage. In terms of devices, regrown p-n junctions with low leakage currents and vertical junction field-effect transistors were demonstrated. Further improvements in selective area regrowth and associated devices can be expected using regrowth optimization and regrowth interface engineering via surface treatments and low-damage etching. These results represent an important step towards realizing selective area regrowth and doping for high performance GaN power electronics devices and systems.

We report the results of the investigation of bulk and surface acoustic phonons in the undoped and boron-doped single-crystal diamond films using the Brillouin-Mandelstam light scattering spectroscopy. The evolution of the optical phonons in the same set of samples was monitored with Raman spectroscopy. It was found that the frequency and the group velocity of acoustic phonons decrease nonmonotonically with the increasing boron doping concentration, revealing pronounced phonon softening. The change in the velocity of the shear-horizontal and the high-frequency pseudo-longitudinal acoustic phonons in the degenerately doped diamond, as compared to that in the undoped diamond, was as large as ∼15% and ∼12%, respectively. As a result of boron doping, the velocity of the bulk longitudinal and transverse acoustic phonons decreased correspondingly. The frequency of the optical phonons was unaffected at low boron concentration but experienced a strong decrease at the high doping level. The density-functional-theory calculations of the phonon band structure for the pristine and highly doped samples confirm the phonon softening as a result of boron doping in diamond. The obtained results have important implications for thermal transport in heavily doped diamond, which is a promising material for ultra-wide-band-gap electronics.

a-Si:H/c-Si heterojunction solar cells hold the efficiency world record around 27%, yet their market penetration is delayed. One concern is the migration of passivating hydrogen away from the interface, that some suspect may speed up the degradation of their performance. Mitigating the performance degradation necessitates the understanding of the structural evolution of a-Si:H/c-Si structures, with a focus on hydrogen migration. To this end, we have developed the SolDeg structural simulation platform that is capable of capturing extremely slow degradation processes. SolDeg integrates molecular dynamics methods that optimize the Si structure with femtosecond time steps, with the nudged elastic band method that captures the defect generation on time scales extending to gigaseconds. The molecular dynamics layer of SolDeg requires a high quality Si-H interatomic potential. While classical parametric interatomic potentials have been used extensively, the recent development of machine-learning driven interatomic potentials ignited the ambition of achieving DFT-level accuracy with classical molecular dynamics simulations. In this paper we report the development of the first machine-learning driven Gaussian Approximation Potential (GAP) to describe Si-H interactions. This potential will be used in the SolDeg platform to determine the performance degradation of a-Si:H/c-Si heterojunction solar cells.

Current collapse phenomenon that occurs in GaN HEMTs under a moderately large DC bias stress, poses serious problems for usage of GaN technology in high-power high-frequency applications from a reliability standpoint. Additional problem in these devices operated at high biases is the appearance of selfheating effects that degrade device characteristics and, as shown in this work further amplify the problem of current collapse by changing the device electrostatics. Index t erms: GaN HEMTs, self-heating, current collapse

Diamond Schottky p-i-n diodes have been grown by plasma-enhanced chemical vapor deposition (PECVD) and incorporated as a shunt element within coplanar striplines for RF characterization. The p-i-n diodes have a thin, lightly doped n-type layer that is fully depleted by the top metal contact, and they operate as high-speed Schottky rectifiers. Measurements from dc to 25 GHz confirm that the diodes can be modeled by a voltage-dependent resistor in parallel with a fixed-value capacitor. In the OFF state with a dc bias of 0 V, the diode insertion loss is less than 0.3 dB at 1 GHz and increases to 14 dB when forward biased to 7.6 V. With a contact resistance, $R_{C}$ , of 0.25 $\text{m}\Omega \cdot $ cm2 and an OFF capacitance, $C_{\mathrm{\scriptscriptstyle OFF}}$ , of 17.5 nF/cm2, the diodes have an RF figure of merit $F_{\mathrm {oc}} =$ ( $2\pi R_{C}\,\,C_{\mathrm{\scriptscriptstyle OFF}})^{-1}$ of 36.5 GHz. The RF model suggests that reducing $R_{C}$ to less than $5\times 10^{-5} \Omega \cdot $ cm2 will enable input power rejection exceeding 30 dB. Compared to conventional silicon or compound semiconductor based power limiters, the superior thermal conductivity of the diamond Schottky p-i-n diodes makes them ideally suitable for RF receiver protectors (RPs) that require high power handing capability.

This letter demonstrates novel hydrogen-plasma based guard rings (GRs) for high voltage vertical GaN p-n diodes grown on bulk GaN substrates by metalorganic chemical vapor deposition (MOCVD). The GR structure can significantly improve breakdown voltages ( BV ) and critical electric fields ( ${E}_{c}$ ) of the devices. Not having field plates or passivation, the p-n diodes with a $9~\mu \text{m}$ drift layer and 10 GRs showed BV /on-resistance ( ${R}_{\textit {on}}$ ) of 1.70 kV/0.65 $\text{m}\Omega \cdot $ cm2, which are close to the GaN theoretical limit. Moreover, the device also exhibited good rectifying behaviors with an on-current of ~ 2.6 kA/cm2, an on/off ratio of ~ 1010, and a turn-on voltage of 3.56 V. This work represents one of the first effective GR techniques for high performance kV-class GaN p-n diodes.

Unique properties can be achieved in engineered nanoparticle films, including those related to quantum confinement, surface chemistry, and light propagation, which can be exploited for energy appli...

Electric utility rate plans are becoming more complex, making it increasingly difficult to calculate the cost of electricity for residential users. This masks the costs and benefits of residential solar and solar + batteries and sends incorrect price signals to consumers. To our knowledge, the impacts of the costs of complex rate plans on the LCOE of solar and batteries for residential consumers has not been calculated and presented. This paper calculates the LCOE for combinations of solar and batteries, including no solar and/or batteries, examining the impact of effects such as time of use and demand charges.The key results are that more complex rate plans show little economic benefit for solar and battery users and continue to exacerbate the problems (commonly referred to as the "duck curve") which is often given as the main roadblock to renewable expansion. These results are primarily due to electricity charges not directly related to overall energy use. There is an increasing component of fixed charges (either solar-related or applicable to all users), such that reducing the electricity usage from the grid with lower-cost solar electricity does not substantially reduce overall LCOE. These fixed charges also substantially increase the LCOE for smaller electrical loads, in some cases nearly doubling it. The second type of electricity charge giving high rates is a demand charge. For demand charges, eliminating every peak is not feasible, such that the charges from the utility are essentially constant. In addition to discouraging the use of solar, these plans negatively impact the utility, as batteries in residential systems have a beneficial impact on both consumers and utilities, enabling reductions in electricity use during early evening by more than 80%.

a-Si/c-Si heterojunction solar cells hold the efficiency world record around 27%, yet their market penetration is delayed. One concern is the presence of an amorphous Si layer that some suspect may speed up the degradation of their performance. To address this concern, we developed the SolDeg structural simulation platform that is capable of capturing extremely slow degradation processes in a-Si. SolDeg integrates molecular dynamics methods that optimize the Si structure with femtosecond time steps, with the nudged elastic band method that captures the defect generation on time scales extending to gigaseconds. In this paper we report SolDeg simulations for Si-only heterojunctions. The SolDeg platform enabled us to determine the defect generation rate to be in the 15-20%/year range, translating into a 1-1.5%/year Voc degradation rate. These results establish that SolDeg can be a uniquely useful platform to describe degradation processes with an eye on finding strategies to mitigate the performance degradation of these promising heterojunction cells.

In this work we use machine learning to extract actual Cu doping profiles that result from the process of diffusion annealing and cool-down in the fabrication sequence of CdTe solar cells. We use two deep learning neural network models (Artificial Neural Network (ANN) model using a Keras API with TensorFlow backend and a Radial Basis Function Network (RBFN) model) to predict the Cu doping profiles for different temperatures and duration of the annealing process. We find excellent agreement between the simulated results obtained with the PVRD-FASP Solver and predicted values. It takes significant amount of time to generate with the PVRD-FASP Solver the Cu doping profiles given the initial conditions. The generation of the same with machine learning is almost instantaneous and can serve as an excellent simulation tool to guide future fabrication of optimal doping profiles in CdTe solar cells.

Agrivoltaics provides benefits for both photovoltaic systems and agriculture, enabling dual "harvests" from the sun. An issue with agrivoltaic systems may be the competition between PV modules and crops, leading to agrivoltaic modules that have fewer solar cells and produce less electricity. We present an agrivoltaic PV module design that is angularly and wavelength selective, passing light to the crops in the morning when they are able to utilize it, and to the solar cells during periods of high insolation when they cannot. The agrivoltaic module embeds a holographic film into the PV module, and the cells operate at low concentration, allowing both low costs and high electricity production.

The ever increasing demand for faster microprocessors and the continuous trend to pack more transistors on a single chip has resulted in an unprecedented level of power dissipation, and therefore higher temperatures at the chip level. Thermal phenomena are not directly responsible for the electrical functionality and performance of semiconductor devices, but adversely affect their reliability. Four major thermally-induced reliability concerns for transistors are: (1) degradation of device thermal characteristics due to heating effects, (2) failure due to the electrostatic discharge phenomenon, (3) stresses due to different rates of thermal expansion of transistor constituents, and (4) failure of metallic interconnects due to diffusion or flow of atoms along a metal interconnect in the presence of a bias current, known as the electromigration phenomenon. Self-heating of the device and interconnects reduces electron mobility and results in a poor or, at best, non-optimal, performance of these devices and structures. Fig. 1 shows the trend of the average power density for high-performance microprocessors according to the ITRS. No flattening or slower decelerated increase will occur after the introduction of the Silicon on Insulator (SOI) technology. It should be noted that the power density shown in Fig. 1 is the average power density, i.e. the total chip power divided by the chip area. In logic circuits, such as microprocessors, the power is non-uniformly distributed. There are portions of the chip of quite low power dissipation (memory blocks) and, on the other hand, portions running at full speed with high activity factors where the power density can easily be more than a magnitude higher than the average chip power density from Fig. 1. The latter portions will create hot spots with quite high local temperature. The power density in the active transistor region (essentially the channel region underneath the gate) is again much higher than the average power density in a hot spot when the transistor is in the on-state. Thus, the treatment of self-heating and the realistic estimation of the power density is quite a complex problem. Sometime within the next five years, traditional CMOS technology is expected to reach limits of scaling. As channel lengths shrink below 22 nm, complex channel profiles are required to achieve desired threshold voltages and to alleviate the short-channel effects. To fabricate devices beyond current scaling limits, Integrated Circuits (IC) companies are simultaneously pushing planar, bulk silicon CMOS design while exploring alternative gate stack materials (high-k dielectrics and metal gates), band engineering methods (using 3

Metal nanoparticles have unique chemical, physical, electrical, and optical properties that make them attractive for a wide range of applications in sensing, anti-fouling surfaces, medicine, and conductive inks. Pulsed Laser Ablation in Liquid (PLAL) is a green method of nanoparticle colloid production, capable of producing ligand-free nanoparticles in solution without the need for hazardous, environmentally unfriendly chemicals. Control of the process parameters can give control over the resulting colloid properties such as particle size distribution. In this work, silver (Ag) nanoparticles (NPs) with average particle size from 2.04 to 19.3 nm and copper (Cu) NPs with average particle size from 40 to 85.9 nm were produced by PLAL) technique.

High-temperature photovoltaics (PV) for terrestrial and extraterrestrial applications have presented demanding challenges for current solar cell materials, such as Si, III–V AlGaInP, and II–VI. Wid...

A new technique for neutralizing the polarization effect of diamond detectors has been demonstrated. The technique, which relies on the diamond detector to be configured as a diode, is to periodically pulse the diamond diode with forward bias. A 210Po alpha source was used to induce the polarization effect for a thin PIN diamond radiation detector. Results show that for a single forward bias pulse, a forward current of about 60 nA applied for 1 s removes the polarization buildup. The benefits of using this technique add significant value to the diamond diode configuration.

Here we investigate recombination in polytype gallium arsenide (GaAs) nanowires (NWs) for photovoltaic applications, through photoluminescence studies coupled with rate equation analysis. Polytype NWs exhibit switching between zinc-blende (ZB) and wurtzite (WZ) crystal phases along the wire due to rotational twinning during self-catalyzed growth. When photons are absorbed in polytype NWs, electrons and holes separate: in the simplest case, electrons quickly thermalize to the band-edge of the ZB phase, while holes thermalize to the band-edge of the WZ phase, recombining indirectly in space across the type-II offset. The recombination mechanisms of this system are investigated experimentally through time-resolved photoluminescence (TRPL) at liquid helium temperature, and time-integrated photoluminescence (TIPL) at various temperatures, for the baseline case of AlGaAs capped GaAs NWs. The effects of the surface recombination on sub-bandgap transitions are also investigated using Al2O3 and no capping at the surface. We infer that carriers quickly thermalize to the spatially closest, lowest energy level, where they radiatively recombine across a sub-bandgap energy gap at a slower radiative rate than band-to-band. We use a rate equation model to investigate different configurations of polytype defects along the wire, including the effects of the surface and temperature, which compares well with experiment considering spatially indirect recombination between different polytypes, and defect-related recombination due to twin planes and other defects.

THE 2018 IEEE Educational Activities Board (EAB), chaired by IEEE Educational Activities Vice President Witold Kinsner, upon the recommendation of the EAB Awards and Recognition Committee (ARC) has named the recipients of the 2018 IEEE EAB Awards. EAB Awards recognize and honor individuals and companies for major contributions to engineering and technical education. Awards are given for meritorious activities in accreditation, continuing education, educational innovation, pre-university education, service to the IEEE EAB, standards education, employee professional development, and informal education systems, as well as related achievements that advance the practice of engineering and engineering education.

An interpretation of the unipolar figure of merit is formulated for wide bandgap (WBG) semiconductors based on the on-state specific resistance ([Formula: see text]) derived from the space charge limited current–voltage relationship (Mott–Gurney square law). The limitations of the traditional Ohmic [Formula: see text] for WBG semiconductors are discussed, particularly at low doping, while the accuracy of the Mott–Gurney based [Formula: see text] is confirmed by Silvaco ATLAS drift–diffusion simulations of diamond Schottky pin diodes. The effects of incomplete ionization are considered as well.

In order to achieve a hot carrier solar cell, one must: (a) prevent significant energy loss to phonons; and (b) extract primarily the hot carriers. We have recently suggested a new approach to these problems by using metastable upper energy levels to store the photogenerated carriers. A proof of concept cell based upon an InGaAs/InAlAs structure has yielded encouraging results that suggest that valley storage is occurring and yielding a $V_{OC} > E_{G}$ . Current output, however, is restricted apparently due to a barrier between the L valleys in the two materials. New materials may provide the solution to this problem and lead to a new HCSC with greatly enhanced efficiency.