Feb 11, 2019

The 2018 GaN power electronics roadmap

Gallium nitride (GaN) is a compound semiconductor that has tremendous potential to facilitate economic growth in a semiconductor industry that is silicon-based and currently faced with diminishing returns of performance versus cost of investment. At a material level, its high electric field strength and electron mobility have already shown tremendous potential for high frequency communications and photonic applications. Advances in growth on commercially viable large area substrates are now at the point where power conversion applications of GaN are at the cusp of commercialisation. The future for building on the work described here in ways driven by specific challenges emerging from entirely new markets and applications is very exciting. This collection of GaN technology developments is therefore not itself a road map but a valuable collection of global state-of-the-art GaN research that will inform the next phase of the technology as market driven requirements evolve. First generation production devices are igniting large new markets and applications that can only be achieved using the advantages of higher speed, low specific resistivity and low saturation switching transistors. Major investments are being made by industrial companies in a wide variety of markets exploring the use of the technology in new circuit topologies, packaging solutions and system architectures that are required to achieve and optimise the system advantages offered by GaN transistors. It is this momentum that will drive priorities for the next stages of device research gathered here.



Source:IOPscience


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Jan 29, 2019

MOVPE growth of GaN on 6-inch SOI-substrates: effect of substrate parameters on layer quality and strain

We demonstrate that higher crystalline quality, lower strain and improved electrical characteristics can be achieved in gallium nitride (GaN) epitaxy by using a silicon-on-insulator substrate compared to a bulk silicon (Si) substrate. GaN layers were grown by metal–organic vapor phase epitaxy on 6-inch bulk Si and SOI wafers using the standard step graded AlGaN and AlN approach. The GaN layers grown on SOI exhibited lower strain according to x-ray diffraction analysis. Defect selective etching measurements suggested that the use of SOI substrate for GaN epitaxy reduces the dislocation density approximately by a factor of two. Furthermore, growth on SOI substrate allows one to use a significantly thinner AlGaN buffer compared to bulk Si. Synchrotron radiation x-ray topography analysis confirmed that the stress relief mechanism in GaN on SOI epitaxy is the formation of a dislocation network to the SOI device Si layer. In addition, the buried oxide layer significantly improves the vertical leakage characteristics as the onset of the breakdown is delayed by approximately 400 V. These results show that the GaN on the SOI platform is promising for power electronics applications.


Source:IOPscience

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Jan 21, 2019

Lateral epitaxial overgrowth of GaN on a patterned GaN-on-silicon substrate by molecular beam epitaxy

We report here the lateral epitaxial overgrowth (LEO) of GaN on a patterned GaN-on-silicon substrate by molecular beam epitaxy (MBE) growth with radio frequency nitrogen plasma as a gas source. Two kinds of GaN nanostructures are defined by electron beam lithography and realized on a GaN substrate by fast atom beam etching. The epitaxial growth of GaN by MBE is performed on the prepared GaN template, and the selective growth of GaN takes place with the assistance of GaN nanostructures. The LEO of GaN produces novel GaN epitaxial structures which are dependent on the shape and the size of the processed GaN nanostructures. Periodic GaN hexagonal pyramids are generated inside the air holes, and GaN epitaxial strips with triangular section are formed in the grating region. This work provides a promising way for producing novel GaN-based devices by the LEO of GaN using the MBE technique.



Source:IOPscience

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Jan 14, 2019

Electrical properties of Au/Bi0.5Na0.5TiO3-BaTiO3/n-GaN metal–insulator–semiconductor (MIS) structure


We investigated the electrical properties of solution processed high-k Bi0.5Na0.5TiO3(BNT)-BaTiO3(BT) on n-GaN with Au electrode. Higher barrier height is obtained for Au/BNT-BT/n-GaN structure compared to Au/n-GaN structure. Thin interfacial layer is formed in between BNT-BT and n-GaN confirmed by TEM results. The interface state density of Au/BNT-BT/n-GaN structure is lower than that of Au/n-GaN structure due to the existence of interfacial layer (Ga-O) at the interface. It is observed that the frequency dispersion is decreased in the Au/BNT-BT/n-GaN structure. Poole–Frenkel mechanism is found to dominate the reverse leakage current in both Au/n-GaN and Au/BNT-BT/n-GaN structures.



Source:IOPscience

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Assessment of transistors based on GaN on silicon substrate in view of integration with silicon technology

In this work, AlGaN/GaN high electron mobility transistors on (1 0 0) and (1 1 0) oriented silicon substrates are investigated in view of monolithic integration with silicon MOSFETs for making more compact microwave power electronics. Epilayers are grown by molecular beam epitaxy on highly resistive substrates. It was shown that a better crystal quality as well as higher low-field electron mobility are obtained on the (1 1 0) orientation. Sub-micron gate length devices are then processed to estimate the millimeter-wave and microwave power performances of this new generation of devices. Load-pull measurements are performed from 4 GHz up to 40 GHz. Optimizations for the best power-added efficiency or for maximum output power density show the great potential of the Si(1 1 0) substrate for GaN-based power devices. At 18 GHz, these two different optimizations lead to a saturated output power density and an associated power-added efficiency of 2.4 W mm−1–40% and 3.76 W mm−1–33%, respectively. At 40 GHz, a record saturated output power density of 3.3 W mm−1 is achieved with an associated power-added efficiency of 20.1% and a linear power gain of 10.6 dB. In comparison, devices on Si(1 0 0) show less attractive performance due to a lower material quality with an output power density of 2.9 W mm−1, an associated power-added efficiency of 20.4% and a linear power gain of 7.5 dB at 10 GHz.



Source:IOPscience

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Jan 8, 2019

Non-polar and semi-polar ammonothermal GaN substrates


In this paper we review the developments of producing non-polar (i.e. m-plane and a-plane) and semi-polar (i.e. (20.1)-plane) wafers by ammonothermal method. The growth method and polishing results are described. We succeeded in producing 26 mm × 26 mm non- and semi-polar wafers. These wafers possess outstanding structural and optical properties, with threading dislocation density of the order of 104 cm−3. Detailed studies of homoepitaxial layers, as well as AlGaN heterostructures are also presented, showing the potential of studied ammonothermal substrates in the fabrication of optoelectronic devices.



Source:iopscience

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Dec 24, 2018

Wafer-scale epitaxial lift-off of optoelectronic grade GaN from a GaN substrate using a sacrificial ZnO interlayer


Full 2 inch GaN epilayers were lifted off GaN and c-sapphire substrates by preferential chemical dissolution of sacrificial ZnO underlayers. Modification of the standard epitaxial lift-off (ELO) process by supporting the wax host with a glass substrate proved key in enabling full wafer scale-up. Scanning electron microscopy and x-ray diffraction confirmed that intact epitaxial GaN had been transferred to the glass host. Depth-resolved cathodoluminescence (CL) analysis of the bottom surface of the lifted-off GaN layer revealed strong near-band-edge (3.33 eV) emission indicating a superior optical quality for the GaN which was lifted off the GaN substrate. This modified ELO approach demonstrates that previous theories proposing that wax host curling was necessary to keep the ELO etch channel open do not apply to the GaN/ZnO system. The unprecedented full wafer transfer of epitaxial GaN to an alternative support by ELO offers the perspective of accelerating industrial adoption of the expensive GaN substrate through cost-reducing recycling.



Source:IOPscience

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