Youssef El Gmili

Model of Ni-63 battery with realistic PIN structure

GaN, with its wide bandgap of 3.4 eV, has emerged as an efficient material for designing high-efficiency betavoltaic batteries. An important part of designing efficient betavoltaic batteries involves a good understanding of the full process, from the behavior of the nuclear material and the creation of electron-hole pairs all the way through the collection of photo-generated carriers. This paper presents a detailed model based on Monte Carlo and Silvaco for a GaN-based betavoltaic battery device, modeled after Ni-63 as an energy source. The accuracy of the model is verified by comparing it with experimental values obtained for a GaN-based p-i-n structure under scanning electron microscope illumination.

Wafer-scale controlled exfoliation of metal organic vapor phase epitaxy grown InGaN/GaN multi quantum well structures using low-tack two-dimensional layered h-BN

Recent advances in epitaxial growth have led to the growth of III-nitride devices on 2D layered h-BN. This advance has the potential for wafer-scale transfer to arbitrary substrates, which could improve the thermal management and would allow III-N devices to be used more flexibly in a broader range of applications. We report wafer scale exfoliation of a metal organic vapor phase epitaxy grown InGaN/GaN Multi Quantum Well (MQW) structure from a 5u2009nm thick h-BN layer that was grown on a 2-inch sapphire substrate. The weak van der Waals bonds between h-BN atomic layers break easily, allowing the MQW structure to be mechanically lifted off from the sapphire substrate using a commercial adhesive tape. This results in the surface roughness of only 1.14u2009nm on the separated surface. Structural characterizations performed before and after the lift-off confirm the conservation of structural properties after lift-off. Cathodoluminescence at 454u2009nm was present before lift-off and 458u2009nm was present after. Electrolumi...

Flexible metal-semiconductor-metal device prototype on wafer-scale thick boron nitride layers grown by MOVPE

Practical boron nitride (BN) detector applications will require uniform materials over large surface area and thick BN layers. To report important progress toward these technological requirements, 1~2.5u2009µm-thick BN layers were grown on 2-inch sapphire substrates by metal-organic vapor phase epitaxy (MOVPE). The structural and optical properties were carefully characterized and discussed. The thick layers exhibited strong band-edge absorption near 215u2009nm. A highly oriented two-dimensional h-BN structure was formed at the film/sapphire interface, which permitted an effective exfoliation of the thick BN film onto other adhesive supports. And this structure resulted in a metal-semiconductor-metal (MSM) device prototype fabricated on BN membrane delaminating from the substrate. MSM photodiode prototype showed low dark current of 2u2009nA under 100u2009V, and 100u2009±u200920% photoconductivity yield for deep UV light illumination. These wafer-scale MOVPE-grown thick BN layers present great potential for the development of deep UV photodetection applications, and even for flexible (opto-) electronics in the future.

Nanoselective area growth of GaN by metalorganic vapor phase epitaxy on 4H-SiC using epitaxial graphene as a mask

We report the growth of high-quality triangular GaN nanomesas, 30-nm thick, on the C-face of 4H-SiC using nano selective area growth (NSAG) with patterned epitaxial graphene grown on SiC as an embedded mask. NSAG alleviates the problems of defective crystals in the heteroepitaxial growth of nitrides, and the high mobility graphene film can readily provide the back low-dissipative electrode in GaN-based optoelectronic devices. The process consists in first growing a 5-8 graphene layers film on the C-face of 4H- SiC by confinement-controlled sublimation of silicon carbide. The graphene film is then patterned and arrays of 75-nanometer-wide openings are etched in graphene revealing the SiC substrate. 30-nanometer-thick GaN is subsequently grown by metal organic vapor phase epitaxy. GaN nanomesas grow epitaxially with perfect selectivity on SiC, in openings patterned through graphene, with no nucleation on graphene. The up-or-down orientation of the mesas on SiC, their triangular faceting, and cross-sectional scanning transmission electron microscopy show that they are biphasic. The core is a zinc blende monocrystal surrounded with single-crystal hexagonal wurtzite. The GaN crystalline nanomesas have no threading dislocations, and do not show any V-pit. This NSAG process potentially leads to integration of high-quality III-nitrides on the wafer scalable epitaxial graphene / silicon carbide platform.

Role of V-pits in the performance improvement of InGaN solar cells

We study the influence of V-pits on the overall conversion efficiency of bulk In0.12 Ga0.88N based heterojunction solar cells grown by MOVPE. We show that V-pits significantly enhances the extraction of the photogenerated carriers in the InGaN absorber, resulting in a peak external quantum efficiency of 79% and a short circuit current density (twice the state of the art) of 2.56u2009mA/cm−2 under AM 1.5G conditions.

Experimental Study and Device Design of NO, NO 2 , and NH 3 Gas Detection for a Wide Dynamic and Large Temperature Range Using Pt/AlGaN/GaN HEMT

We report an AlGaN/GaN HEMT gas sensor designed to enable NO, NO2, and NH3 detection from 100 °C–400 °C over a large concentration range. Device modeling is performed to optimize several HEMT device parameters for sensing, and the experimental results show that the optimized sensor has improved performance compared with the previously reported HEMT sensors. The device shows significant no sensitivity for the first time in an HEMT device, with sensitivity up to 7% at 400 °C. In addition, high sensitivities of up to 17% are reported for NO2, and NH3 is detected at concentrations as low as 150 ppb.

Nanoselective area growth of defect-free thick indium-rich InGaN nanostructures on sacrificial ZnO templates

Nanoselective area growth (NSAG) by metal organic vapor phase epitaxy of high-quality InGaN nanopyramids on GaN-coated ZnO/c-sapphire is reported. Nanopyramids grown on epitaxial low-temperature GaN-on-ZnO are uniform and appear to be single crystalline, as well as free of dislocations and V-pits. They are also indium-rich (with homogeneous 22% indium incorporation) and relatively thick (100 nm). These properties make them comparable to nanostructures grown on GaN and AlN/Si templates, in terms of crystallinity, quality, morphology, chemical composition and thickness. Moreover, the ability to selectively etch away the ZnO allows for the potential lift-off and transfer of the InGaN/GaN nanopyramids onto alternative substrates, e.g. cheaper and/or flexible. This technology offers an attractive alternative to NSAG on AlN/Si as a platform for the fabrication of high quality, thick and indium-rich InGaN monocrystals suitable for cheap, flexible and tunable light-emitting diodes.

Single-crystal nanopyramidal BGaN by nanoselective area growth on AlN/Si(111) and GaN templates

We report nano-selective area growth (NSAG) of BGaN by MOCVD on AlN/Si(111) and GaN templates resulting in 150 nm single crystalline nanopyramids. This is in contrast to unmasked or micro-selective area growth, which results in a multi-crystalline structure on both substrates. Various characterization techniques were used to evaluate NSAG as a viable technique to improve BGaN material quality on AlN/Si(111) using results of GaN NSAG and unmasked BGaN growth for comparison. Evaluation of BGaN nanopyramid quality, shape and size uniformity revealed that the growth mechanism is the same on both the templates. Further STEM analysis of BGaN nanopyramids on AlN/Si (111) templates confirmed that these are single-crystalline structures without any dislocations, likely due to single nucleation occurring in the 80 nm mask opening. CL results correspond to boron content between 1.7% and 2.0% in the nanopyramids. We conclude that NSAG is promising for growth of high-quality BGaN nanostructures and complex nano-heterostructures, especially for low-cost silicon substrates.

Investigation of new approaches for InGaN growth with high indium content for CPV application

We propose to use two new approaches that may overcome the issues of phase separation and high dislocation density in InGaN-based PIN solar cells. The first approach consists in the growth of a thick multi-layered InGaN/GaN absorber. The periodical insertion of the thin GaN interlayers should absorb the In excess and relieve compressive strain. The InGaN layers need to be thin enough to remain fully strained and without phase separation. The second approach consists in the growth of InGaN nano-structures for the achievement of high In content thick InGaN layers. It allows the elimination of the preexisting dislocations in the underlying template. It also allows strain relaxation of InGaN layers without any dislocations, leading to higher In incorporation and reduced piezo-electric effect. The two approaches lead to structural, morphological, and luminescence properties that are significantly improved when compared to those of thick InGaN layers. Corresponding full PIN structures have been realized by growing a p-type GaN layer on the top the half PIN structures. External quantum efficiency, electro-luminescence, and photo-current characterizations have been carried out on the different structures and reveal an enhancement of the performances of the InGaN PIN PV cells when the thick InGaN layer is replaced by either InGaN/GaN multi-layered or InGaN nanorod layer.