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The purpose of this paper is to investigate the N-face InGaN semi-bulk as a solution to achieve a high-efficiency solar cell based on III-nitride materials, which could mitigate the issues of both InGaN crystalline quality and charge polarization. Simulation of both InGaN bulk and semi-bulk with both N and Ga faces, using an analytical model, shows that this approach leads to a significant increase of the power conversion efficiency. It is found that the conversion efficiency of the new solar cell structure with 25% indium content is about three times larger than the one based on the fully relaxed bulk InGaN absorber (no detrimental effect of the polarization effect) with the same indium concentration. This approach could thus be beneficial to the realization of efficient InGaN-based solar cells. [ABSTRACT FROM AUTHOR]

The effect of indium (In) in the barrier of InGaN/GaN multiple quantum well (MQW) has been studied for MQWs with and without semi-bulk InGaN buffer. From simulation, the optimum In content in the barrier with 3-5 nm width is 5-7% to get the optimal material quality and internal quantum efficiency (IQE) of ∼65% for 450-480 nm emission range. Simulation shows a reduction of the potential barrier due to band flattening, a more homogeneous distribution of electrons and holes in the active region and subsequently, a more radiative recombination rate with InGaN as barrier layer. Both cathodoluminescence (CL) and photoluminescence (PL) experimental results show a blue-shift of emission wavelength along with an enhancement in the emission intensity when GaN barrier is replaced with InGaN barrier, for a MQW structure both with and without the semi-bulk InGaN buffer. We attribute this blue shift to the reduced polarization mismatch and increased effective bandgap. This InGaN barrier-related improvement in IQE and efficiency droop could be useful for the realization of longer wavelength 'green-gap' range LEDs where poor IQE and efficiency droop are more prominent due to high indium (In) in the active region. [ABSTRACT FROM AUTHOR]

We report on the optimization of p-GaN layers for high indium (In) content InGaN applications by optimizing temperature, precursor CP2Mg flow rate and III/V ratio and report also on the optimization of p-contact performance. Using MOVPE, a 150 nm thick p-type GaN with moderate Mg doping and a 50 nm contact layer with high Mg doping concentration were grown on standard GaN templates at growth temperatures in the range of 850-1000 °C. Hall measurement yields hole concentration of 4.8 × 1017 cm−3 for the optimized sample. SIMS shows Mg concentration of 1.7 × 1020 cm−3 on average in the heavily doped and 4 × 1019 cm−3 on average in the moderately doped p-GaN layer for the optimized p-GaN. A multilayer Pd/Ag/Ni/Au metal contact has been deposited on this p-GaN and studied using the CTLM technique. Optimization of both p-GaN layers and p-contact processing led to a low resistance contact with specific contact resistivity of 6 × 10−4 Ω cm2. We believe, the very high Mg concentration of the surface layer in intimate contact with the contact metal reduces the Schottky barrier height and band bending. This optimization of p-GaN is an important step towards high efficiency green LEDs and solar cells. [ABSTRACT FROM AUTHOR]

In this work, we present a double-junction solar cell with a crystalline silicon solar cell as a bottom junction and an indium gallium nitride-based semibulk-structured solar cell as a top junction. Using SILVACO Atlas and taking into account polarization effects in III-N materials, we have shown that 50% of indium is needed to ensure the current matching between the top cell and the bottom cell in 2-terminal configuration. Such high indium composition is technologically challenging to grow. Thus, we have also modeled a 4-terminal configuration showing the same performance (i.e. conversion efficiency close to 29%) where only 25% of indium content is needed. Copyright © 2016 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

We study the influence of V-pits on the overall conversion efficiency of bulk In0.12Ga0.88N 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 2.56 mA/cm-2 under AM 1.5G conditions. [ABSTRACT FROM AUTHOR]

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 5 nm 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 1.14 nm on the separated surface. Structural characterizations performed before and after the lift-off confirm the conservation of structural properties after lift-off. Cathodoluminescence at 454 nm was present before lift-off and 458 nm was present after. Electroluminescence near 450 nm from the lifted-off structure has also been observed. These results show that the high crystalline quality ultrathin h-BN serves as an effective sacrificial layer—it maintains performance, while also reducing the GaN buffer thickness and temperature ramps as compared to a conventional two-step growth method. These results support the use of h-BN as a low-tack sacrificial underlying layer for GaN-based device structures and demonstrate the feasibility of large area lift-off and transfer to any template, which is important for industrial scale production. [ABSTRACT FROM AUTHOR]

Nanoselective area growth (NSAG) of thick InGaN nanopyramid and nanostripe arrays was demonstrated using metal organic chemical vapor deposition. SiO2 nanopattern masks were fabricated using a simple and industry friendly e-beam lithography process. One hundred nanometer thick InGaN is grown with perfect selectivity over patterned GaN templates. InGaN nanostructures are homogenously pyramidal in shape, are mostly free from intrinsic material defects, and show six smooth semipolar facets. Catholuminescence emission peaks from the nanostructures are stronger than those from planar InGaN, which is due to improvement in crystal quality. The emission peaks from nanostructures are considerably redshifted from 397 to 425 nm, confirming increase in In incorporation in the nanostructures from 7 to 9% indium in InGaN. The ability to incorporate more indium depending on the geometry of the patterns and to grow selectively defect-free thick InGaN nanostructures via a simple patterning process offers a new route to develop monolithic InGaN-based high efficiency solar cells. [ABSTRACT FROM AUTHOR]

We report an elongation of emission wavelength by inserting a ∼70 nm thick high quality semi-bulk (SB) In y Ga 1-y N buffer layer underneath the In x Ga 1-x N/In y Ga 1-y N (x > y) multi-quantum-well (MQW).While the MQW structure without the InGaN SB buffer is fully strained on the n-GaN template, the MQW structure with the buffer has ∼15% relaxation. This small relaxation along with slight compositional pulling induced well thickness increase of MQW is believed to be the reason for the red-shift of emission wavelength. In addition, the SB InGaN buffer acts as an electron reservoir and also helps to reduce the Quantum Confined Stark Effect (QCSE) and thus increase the emission intensity. In this way, by avoiding fully relaxed buffer induced material degradation, a longer emission wavelength can be achieved by just using InGaN SB buffer while keeping all other growth conditions the same as the reference structure. Thus, a reasonably thick fully strained or very little relaxed InGaN buffer, which is realized by “semi-bulk” approach to maintain good InGaN material quality, can be beneficial for realizing LEDs, grown on top of this buffer, emitting in the blue to cyan to green regime without using excess indium (In). [ABSTRACT FROM AUTHOR]

Due to its high absorption coefficient and variable bandgap, InGaN is being intensively studied for photovoltaic applications. Growth of thick homogenous InGaN absorbers is challenging due to relaxation, clustering, and transition from 2D to 3D growth. These issues can be avoided by a semibulk multilayer structure. In this work, we analyze InGaN-based semibulk-structured solar cells in detail. We show that for indium content lower than 15%, GaN interlayers’ thickness has no influence on carrier transport due to the low barrier height. A conversion efficiency of 2.4% can be expected for this indium content. However, for higher indium content (15–30%), we show that the thinner the GaN interlayers, the better the conversion efficiency. Beyond 30% of indium, the conversion efficiency is hindered by the barriers’ important height even for very thin thicknesses of GaN interlayers. We show also that, for semibulk structure, both growth direction (N-face and metal-face) have similar impact on efficiency. This theoretical study gives the guidelines for the fabrication of InGaN-based solar cells that can be used as a wide-bandgap top cell in multijunction solar cells. [ABSTRACT FROM AUTHOR]

In 0.16 Ga 0.84 N/In 0.05 Ga 0.95 N Multiple Quantum Well (MQW) structure grown on a 70 nm thick high quality semi-bulk InGaN buffer layer is reported. Temperature dependent photoluminescence (PL) reveals 67.5% of room temperature Internal Quantum Efficiency (IQE) at an emission peak of ∼455 nm with FWHM of 20 nm. Low temperature PL study shows clear improvement in emission intensity when conventional GaN buffer and GaN barrier are replaced by semi-bulk InGaN buffer in addition with InGaN barrier. Simulation confirms improved IQE and reduced efficiency droop when using semi-bulk as buffer which is attributed to the improved overlapping of electron-hole wave functions due to the reduced internal electric field from counteraction by surface polarization field. This efficiency improvement is very beneficial for high In content green LEDs where the efficiency is limited by polarization induced Quantum Confined Stark Effect (QCSE) for excess indium content. [ABSTRACT FROM AUTHOR]