Behrang H. Hamadani

Wide bandgap semiconductors, including gallium nitride (GaN), are crucial for next-generation power electronics due to their superior properties compared to silicon. However, challenges in their fabrication, particularly with epitaxial regrowth for vertical diode device structures, can introduce defects that impact performance. In this study, we report on vertical GaN pn diodes that exhibit an exceptionally large hysteresis loop with on/off ratios exceeding nine orders of magnitude, a behavior absent in devices grown in situ without changing growth reactors. We demonstrate that these devices exhibit robust endurance, maintaining a stable memory window for over 60 000 switching cycles at room temperature (20 °C), and remain partially viable up to at least 275 °C. Through analytical characterization, including secondary ion mass spectrometry and electroluminescence, we provide evidence that the hysteresis behavior is linked to a defect-rich interface region containing silicon and carbon impurities. To further investigate the underlying mechanism, we developed a technology computer aided design model that successfully replicates the observed hysteresis and identifies the likely mechanisms involved, key attributes of which include interfacial barriers, fixed negative charge, and positive charge trapping. These findings highlight a critical challenge for traditional GaN power devices while also demonstrating their potential for novel high-temperature memory applications.

This study presents a comparative analysis of the open circuit voltage behavior of perovskite vs gallium arsenide (GaAs) solar cells at low temperatures relevant to space applications. While GaAs solar cells demonstrate high power conversion efficiencies and superior environmental stability, their widespread adoption is limited by high manufacturing costs. In contrast, perovskite solar cells have emerged as a promising alternative due to their high efficiency, excellent optoelectronic properties, and low fabrication costs. To evaluate and compare their optoelectronic characteristics and voltage losses at low temperatures, we have characterized both devices using temperature-dependent current–voltage (J–V) and photoluminescence-based radiative efficiency measurements in the range of 300–160 K. GaAs devices exhibited a linear increase in open-circuit voltage (Voc) with decreasing temperature, consistent with theory. In contrast, perovskite devices showed suppressed Voc enhancements and an S-shaped J–V response below 240 K. Using temperature-dependent external radiative efficiency measurements, we show that temperature-dependent entropic losses related to nonradiative recombination are higher in perovskite devices compared to GaAs at high temperatures, but the gap between the two shrinks substantially at lower temperatures. These findings provide valuable insight into temperature-dependent Voc in perovskite solar cells, particularly relevant for space deployment applications.

Halide perovskites have emerged as promising candidates for high-performance solar cells. This study investigates the temperature-dependent optoelectronic properties of mixed-cation mixed-halide perovskite solar cells using electroluminescence (EL) and photoluminescence (PL) hyperspectral imaging, along with current-voltage analysis. Luminescence images, which were converted to EL and PL external radiative efficiency (ERE) maps, revealed significant changes in the optoelectronic behavior of these devices at low temperatures. Specifically, we found that a significant source of heterogeneity in the low-temperature EL ERE maps below 240 K is related to local charge injection and extraction bottlenecks, whereas PL ERE maps show suppressed non-radiative recombination and significant improvements in efficiency throughout the investigated temperature range. The spatial distribution of ERE and its variation with applied current were analyzed, offering insights into charge-carrier dynamics and defect behavior. Our results reveal that while the perovskite layer exhibits enhanced ERE at low temperatures, charge injection barriers at the interfaces of the perovskite solar cells significantly suppress EL and degrade the fill factor below 240 K. These findings reveal that a deeper understanding of the performance of perovskite solar cells under low-temperature conditions is an essential step toward their potential application in space power systems and advanced semiconductor devices.

All-perovskite tandem solar cells with narrow and wide bandgap perovskite absorbers are promising candidates for low-cost and high efficiency photovoltaic applications. However, the open circuit voltage of typical tandem structures is generally smaller than the sum of the individual voltages in the single-junction form; a quantity we call the voltage gap. Subcell optimization can only begin once nonradiative losses associated with each absorber layer can be properly identified. To address this, we used absolute electroluminescence hyperspectral imaging to construct external radiative efficiency maps of each subcell within the tandem stack and compare these measurements with single junction devices. These measurements were then combined with additional electro-optical characterization and modeling to construct subcell current vs voltage curves. We find that the narrow band gap subcell contributes the most towards the voltage gap and therefore fabrication and processing efforts should focus on reducing nonradiative recombination losses within the narrow band gap absorber.

GaN on GaN vertical power devices are promising for high voltage applications beyond Si and SiC. Developing GaN substrates with low defect densities for high-quality epitaxial growth has been an ongoing challenge. Consequently, extended defects are a concern for high power device performance and reliability and must be mitigated. In particular, newer ammonothermally grown GaN substrates with lower extended defect densities have not yet been evaluated as extensively as hydride vapor phase epitaxy GaN. Here, we identify different electrically active defects on epitaxial p-GaN layers grown on strain-patterned (dot-core) and ammonothermal GaN substrates. On dot-core substrates, “star” defects were observed that consist of pits and cracks, have an increased hole carrier density, display occupied bandgap states, and show reduced and variable forward bias currents. In contrast, on ammonothermal substrates, triangular patches were found on certain rough epitaxy ridges and show increased gap states along with a lower carrier density and reduced conductivity. The star and triangular defects likely correspond to threading dislocations and stacking faults, respectively, and may act as killer defects. As a result, p–i–n diode devices on ammonothermal substrates frequently exhibited poor reverse and forward bias behavior due to the random extended defect distribution that cannot be intentionally avoided as in patterned dot-core substrates, as well as different impurity defects in the epitaxy. Therefore, we have found that ammonothermal GaN substrates require tighter control over both point and extended defect distributions in order to achieve reliable high power electronic devices.

Current Germicidal Ultraviolet (GUV) devices are designed to inactivate pathogens in air at either 222 nm or 254 nm wavelengths. Previous research has demonstrated both wavelengths can produce oxidants in air (222 nm: ozone, 254 nm: hydroxyl radicals) and potentially directly photolyze some chemicals. This study sought to determine the impacts of GUV devices on indoor air chemistry in both laboratory chamber and field settings. To ensure the devices were operating using wavelengths and intensities of interest, spatial spectral irradiance measurements of one 222 nm (GUV222) and one 254 nm (GUV254) device were performed. Chamber testing to determine air quality impacts consisted of operating the above devices for four hours in a sealed chamber containing six challenge chemicals. Field testing consisted of operating the devices in an unoccupied restroom on the campus of the National Institute of Standards and Technology (NIST). Ozone, formaldehyde, other volatile organic compound (VOC) oxidation products and ultrafine particles were measured for each device in both the chamber and restroom. In chamber experiments GUV254 generated formaldehyde and likely directly photolyzed an equivalent amount of acetone, contributing to a minimal net change in VOC oxidation products. In addition, GUV254 generated measurable ultrafine particles in the chamber experiments, albeit less than GUV222. For GUV254, formaldehyde, VOC oxidation products, ultrafine particles and ozone generation were not measurable in the restroom. In both the chamber and restroom installation GUV222 generated ozone, VOC oxidation products and ultrafine particles. GUV222 generated quantifiable formaldehyde in chamber testing only. GUV222 was demonstrated to directly photolyze tetrachloroethylene in the chamber. Lastly, to better understand the irradiance spectrum in indoor spaces where GUV devices may be deployed, spatial and temporal changes to the 375 nm to 850 nm spectrum were examined at the NIST Net-Zero Energy Residential Test Facility (NZERTF). A measurement location near a window was compared to one close to an interior ceiling. Indoor diurnal total irradiance and spectrums collected near the spring equinox and summer solstice were compared. This work was funded by the U. S. Environmental Protection Agency via contract DW1392571901.