For the growth of low-defect crack-free GaN heterostructures on large-area silicon substrates, compositional grading of AlGaN is a widely adapted buffer technique to restrict the propagation of lattice-mismatch induced defects and balance the thermal expansion mismatch-induced tensile stress. So far, a consolidation of the design strategy of such step-graded buffers has been impaired by the incomplete understanding of the effect of individual buffer design parameters on the mechanical and microstructural properties of the epilayers. Herein, we have analyzed a series of metal-organic chemical vapor deposition grown GaN/graded-AlGaN/AlN/Si heterostructures through in situ curvature measurements and post-growth x-ray diffraction (XRD). Our results reveal that in such epi structures, the GaN layer itself induces more compressive stress than the AlGaN buffer, but the underlying AlGaN layers dictate the magnitude of this stress. Furthermore, for a fixed AlGaN buffer thickness, the mean-stress accumulated during the GaN growth is found to be correlated with its structural properties. Specifically, one µ m thick GaN layers that acquire 1.50 GPa or higher compressive mean-stress are seen to possess 20 2 ˉ 1 XRD ω -FWHM values less than 650 arc-sec. Also, the evolution of instantaneous stresses during the growth of the AlGaN layers is found to be a valuable indicator for buffer optimization, and composition difference between successive layers is established as a crucial criterion. The results also show that increasing the total buffer thickness (for a fixed number of steps) or increasing the number of steps (for a fixed total buffer thickness) may not always be beneficial. Irrespective of the buffer thickness, optimized high electron mobility transistor structures show similarly low sheet-resistance (∼350 Ω □) −1 and high mobility (∼2000 cm 2 V −1 s −1 ) at room temperature.

The performance of transistors designed specifically for high-frequency applications is critically reliant upon the semi-insulating electrical properties of the substrate. The suspected formation of a conductive path for radio frequency (RF) signals in the highly resistive (HR) silicon substrate itself has been long held responsible for the suboptimal efficiency of as-grown GaN high electron mobility transistors (HEMTs) at higher operating frequencies. Here, we reveal that not one but two discrete channels distinguishable by their carrier type, spatial extent, and origin within the metal-organic vapor phase epitaxy (MOVPE) growth process participate in such parasitic substrate conduction. An n-type layer that forms first is uniformly distributed in the substrate, and it has a purely thermal origin. Alongside this, a p-type layer is localized on the substrate side of the AlN/Si interface and is induced by diffusion of group-III element of the metal-organic precursor. Fortunately, maintaining the sheet resistance of this p-type layer to high values (∼2000 Ω/□) seems feasible with particular durations of either organometallic precursor or ammonia gas predose of the Si surface, i.e., the intentional introduction of one chemical precursor just before nucleation. It is proposed that the mechanism behind the control actually relies on the formation of disordered AlSiN between the crystalline AlN nucleation layer and the crystalline silicon substrate.

We report wafer-scale growth and mechanical exfoliation of 20 periodAl(0.58)Ga(0.42)N/Al0.37Ga0.63N multiple quantum wells (MQWs) on h-BN buffered templates and compare them to control samples of the same structure on AlN templates. X-ray diffraction measurements of the MQW structure on h-BN clearly featured satellite peaks up to third order in the 2 theta-omega scans indicating good MQW periodicity. Detailed transmission electron microscope (TEM) analysis show good heterointerface quality in the structure and large V-pits on the surface. Depth resolved cathodoluminescence of the MQWs on h-BN revealed a UV emission peak at 299 nm and a sharp shoulder at 292 nm. We also report lift-off and transfer of the MQW on the h-BN structure and have investigated post-transfer optical emission, which demonstrates good preservation of optical emission characteristics. Together these results show the suitability of h-BN buffers for the realization of free-standing or flexible optical devices emitting in the deep UV region.

Advanced Materials InterfacesVolume 6, Issue 16 1970102 FrontispieceFree Access Light-Emitting Diodes: Large-Area van der Waals Epitaxial Growth of Vertical III-Nitride Nanodevice Structures on Layered Boron Nitride (Adv. Mater. Interfaces 16/2019) Suresh Sundaram, Suresh Sundaram Georgia Institute of Technology, School of Electrical and Computer Engineering, GT-Lorraine, 57070 Metz, France CNRS, UMI 2958, GT-CNRS, 2 Rue Marconi, 57070 Metz, France GT Lorraine, 2 Rue Marconi, 57070 Metz, FranceSearch for more papers by this authorXin Li, Xin Li CNRS, UMI 2958, GT-CNRS, 2 Rue Marconi, 57070 Metz, FranceSearch for more papers by this authorYacine Halfaya, Yacine Halfaya CNRS, UMI 2958, GT-CNRS, 2 Rue Marconi, 57070 Metz, FranceSearch for more papers by this authorTaha Ayari, Taha Ayari CNRS, UMI 2958, GT-CNRS, 2 Rue Marconi, 57070 Metz, France GT Lorraine, 2 Rue Marconi, 57070 Metz, FranceSearch for more papers by this authorGilles Patriarche, Gilles Patriarche Centre de Nanosciences et de Nanotechnologies, Université Paris-Saclay, C2N – Site de Marcoussis, Route de Nozay, F-91460 Marcoussis, FranceSearch for more papers by this authorChristopher Bishop, Christopher Bishop Institut Lafayette, 2 Rue Marconi, 57070 Metz, FranceSearch for more papers by this authorSaiful Alam, Saiful Alam CNRS, UMI 2958, GT-CNRS, 2 Rue Marconi, 57070 Metz, FranceSearch for more papers by this authorSimon Gautier, Simon Gautier Institut Lafayette, 2 Rue Marconi, 57070 Metz, FranceSearch for more papers by this authorPaul L. Voss, Paul L. Voss Georgia Institute of Technology, School of Electrical and Computer Engineering, GT-Lorraine, 57070 Metz, France CNRS, UMI 2958, GT-CNRS, 2 Rue Marconi, 57070 Metz, FranceSearch for more papers by this authorJean Paul Salvestrini, Jean Paul Salvestrini Georgia Institute of Technology, School of Electrical and Computer Engineering, GT-Lorraine, 57070 Metz, France CNRS, UMI 2958, GT-CNRS, 2 Rue Marconi, 57070 Metz, France GT Lorraine, 2 Rue Marconi, 57070 Metz, FranceSearch for more papers by this authorAbdallah Ougazzaden, Abdallah Ougazzaden Georgia Institute of Technology, School of Electrical and Computer Engineering, GT-Lorraine, 57070 Metz, France CNRS, UMI 2958, GT-CNRS, 2 Rue Marconi, 57070 Metz, FranceSearch for more papers by this author Suresh Sundaram, Suresh Sundaram Georgia Institute of Technology, School of Electrical and Computer Engineering, GT-Lorraine, 57070 Metz, France CNRS, UMI 2958, GT-CNRS, 2 Rue Marconi, 57070 Metz, France GT Lorraine, 2 Rue Marconi, 57070 Metz, FranceSearch for more papers by this authorXin Li, Xin Li CNRS, UMI 2958, GT-CNRS, 2 Rue Marconi, 57070 Metz, FranceSearch for more papers by this authorYacine Halfaya, Yacine Halfaya CNRS, UMI 2958, GT-CNRS, 2 Rue Marconi, 57070 Metz, FranceSearch for more papers by this authorTaha Ayari, Taha Ayari CNRS, UMI 2958, GT-CNRS, 2 Rue Marconi, 57070 Metz, France GT Lorraine, 2 Rue Marconi, 57070 Metz, FranceSearch for more papers by this authorGilles Patriarche, Gilles Patriarche Centre de Nanosciences et de Nanotechnologies, Université Paris-Saclay, C2N – Site de Marcoussis, Route de Nozay, F-91460 Marcoussis, FranceSearch for more papers by this authorChristopher Bishop, Christopher Bishop Institut Lafayette, 2 Rue Marconi, 57070 Metz, FranceSearch for more papers by this authorSaiful Alam, Saiful Alam CNRS, UMI 2958, GT-CNRS, 2 Rue Marconi, 57070 Metz, FranceSearch for more papers by this authorSimon Gautier, Simon Gautier Institut Lafayette, 2 Rue Marconi, 57070 Metz, FranceSearch for more papers by this authorPaul L. Voss, Paul L. Voss Georgia Institute of Technology, School of Electrical and Computer Engineering, GT-Lorraine, 57070 Metz, France CNRS, UMI 2958, GT-CNRS, 2 Rue Marconi, 57070 Metz, FranceSearch for more papers by this authorJean Paul Salvestrini, Jean Paul Salvestrini Georgia Institute of Technology, School of Electrical and Computer Engineering, GT-Lorraine, 57070 Metz, France CNRS, UMI 2958, GT-CNRS, 2 Rue Marconi, 57070 Metz, France GT Lorraine, 2 Rue Marconi, 57070 Metz, FranceSearch for more papers by this authorAbdallah Ougazzaden, Abdallah Ougazzaden Georgia Institute of Technology, School of Electrical and Computer Engineering, GT-Lorraine, 57070 Metz, France CNRS, UMI 2958, GT-CNRS, 2 Rue Marconi, 57070 Metz, FranceSearch for more papers by this author First published: 23 August 2019 https://doi.org/10.1002/admi.201970102AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Graphical Abstract In article number 1900207, Suresh Sundaram, Abdallah Ougazzaden, and co-workers demonstrate self-organized GaN nanorods formation on layered h-BN templates by van der Waals epitaxial growth. This approach is used to grow III-N nanowire light-emitting diodes. In addition, this approach also mitigates transfer processes and scaling issues seen with other 2D materials, since both the 1D (GaN nanorods) and 2D (h-BN) are grown at the wafer-scale and in one epitaxial growth run. Volume6, Issue16August 23, 20191970102 RelatedInformation

Hexagonal boron nitride (h‐BN) is a promising 2D template that decouples substrate effects from the layer above it by van der Waals epitaxy, because there are only weak forces out of the h‐BN. It also permits convenient mechanical transfer of devices grown on their surface to an appropriate substrate. Here, van der Waals epitaxial growth resulting in the formation of self‐organized GaN nanorods on h‐BN templates is demonstrated. This approach to the growth of III‐N nanostructures avoids transfer processes and scaling issues seen with other 2D materials, since both the 1D (GaN nanorods) and 2D (h‐BN) are grown at the wafer‐scale and in one growth run. Further, this process is used to grow vertical core–shell p‐GaN/InGaN/n‐GaN nano‐PIN device structures on wafer‐scale 2D h‐BN on sapphire and silicon substrates. The high quality of the core–shell nanostructures is confirmed by detailed electron microscopy study, which gives more insight into the nanorod formation mechanism on 2D material. Mechanical transfer of nanostructures on sapphire substrates to copper tape is then demonstrated, with no resulting damage of nanorods. Use of MOVPE grown large‐area h‐BN to realize nanostructures is a significant advancement and can lead to new nanodevice architectures needed for next‐generation optoelectronic devices.

A practical III-nitride photovoltaic (PV) application consists of transfer to a foreign substrate with enhanced functionality and integration with mature group W or III-V based solar cell technologies. This requires a lift-off technique compatible with good quality III-N materials, with photovoltaic device fabrication and with subsequent bonding on a heterogeneous substrate. Here, we demonstrate the first InGaN-based solar cells on a 2 in. h-BN/sapphire wafer and their transfer to glass with a backside reflector. III-N solar cells with various sizes and designs are processed and characterized on a full 2 in. wafer giving performances comparable to similar devices on sapphire. The subsequent crack-free transfer of the solar cells, enabled by Van der Waal bonded 2D layered h-BN, to a substrate with a backside reflector yields an increase in the short circuit current density of up to 20%. This demonstration of transferred InGaN-based solar cells on foreign substrates while increasing performance represents a major advance toward lightweight, low cost, and high efficiency photovoltaic applications.

Layered h-BN were grown on sub-micron size dome patterned sapphire substrates (PSS) by metal organic vapor phase epitaxy (MOVPE) to study the effects of substrate orientations, strain and subsequent thermal stress evolution. Electron micrograph images on the surface of the c-plane sapphire in-between the dome structures show typical wrinkled morphology of layered h-BN, similar to the reference sample surface confirming the high quality h-BN growth. However, the h-BN morphology on the domes exhibited pattern like folding which were drastically different from the h-BN on the c-plane sapphire. Detailed TEM studies show that the h -BN is layered and covers the dome patterns of the sapphire substrates like a "blanket of snow" continuously with localized strain variations induced turbostratic phase insertions. In addition, the local variation in optical emission properties of layered materials, which are dramatically affected by the rippled morphology were also investigated using depth resolved cathodoluminescence. The intensity variation of the 320 nm peak confirms that the BN on dome structures have more strain related defects than the c-plane h-BN. Thus, these results give better insight on the origin of turbostratic BN grains and local control over the structural and optical emission characteristics.

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, 1~2.5 µm-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 215 nm. 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 2 nA under 100 V, and 100 ± 20% 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.

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 3-5nm width is 5-7% to get the optimal material quality and internal quantum efficiency (IQE) of approximate to 65% for 450-480nm 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.

In0.16Ga0.84N/In0.05Ga0.95N 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 similar to 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. (C) 2017 Elsevier Ltd. All rights reserved.

The transfer of GaN based gas sensors to foreign substrates provides a pathway to enhance sensor performance, lower the cost and extend the applications to wearable, mobile or disposable systems. The main keys to unlocking this pathway is to grow and fabricate the sensors on large h-BN surface and to transfer them to the flexible substrate without any degradation of the performances. In this work, we develop a new generation of AlGaN/GaN gas sensors with boosted performances on a low cost flexible substrate. We fabricate 2-inch wafer scale AlGaN/GaN gas sensors on sacrificial two-dimensional (2D) nano-layered h-BN without any delamination or cracks and subsequently transfer sensors to an acrylic surface on metallic foil. This technique results in a modification of relevant device properties, leading to a doubling of the sensitivity to NO 2 gas and a response time that is more than 6 times faster than before transfer. This new approach for GaN-based sensor design opens new avenues for sensor improvement via transfer to more suitable substrates, and is promising for next-generation wearable and portable opto-electronic devices.

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 degrees C. Hall measurement yields hole concentration of 4.8 x 10(17) cm(-3) for the optimized sample. SIMS shows Mg concentration of 1.7 x 10(20) cm(-3) on average in the heavily doped and 4 x 10(19) 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 x 10(-4) Omega cm(2). 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. (C) 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

We report an elongation of emission wavelength by inserting a similar to 70 nm thick high quality semi-bulk (SB) InyGa1-yN buffer layer underneath the InxGa1-xN/InyGa1-yN (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 similar to 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). (c) 2017 Elsevier Ltd. All rights reserved.

The effects of different Mg doping concentrations in the main p-GaN layer and the p-GaN capping layer on the electroluminescence (EL) properties of three-dimensional semipolar InGaN/GaN light emitting diode structures grown on GaN stripes with triangular cross-section were investigated. Secondary ion mass spectrometry analysis revealed the Mg concentration of the 3D semipolar p-GaN, indicating a higher Mg incorporation efficiency on the {10 (1) over bar1} facet as compared to the {11 (2) over bar2)over bar2} facet. The EL output power is low with a too low Mg concentration of 3x10(19)cm(-3), probably due to the inferior hole injection efficiency and stays almost constant with the Mg concentration ranging from 4x10(19)cm(-3) until 1.3x10(20)cm(-3) for the 3D LEDs with the {10 (1) over bar1} facet. Heavy Mg doping in the p-GaN capping layer is required to achieve good ohmic contact performance.