Apr 18, 2019

Thick (>20 µm) and high-resistivity carbon-doped GaN-buffer layers grown by metalorganic vapor phase epitaxy on n-type GaN substrates

To improve the performance of GaN power devices, we have investigated the crystalline quality of thick (>20 µm) carbon-doped GaN layers on n-type GaN substrates and templates. The surface morphologies and X-ray rocking curves of carbon-doped GaN layers were improved by using GaN substrates. However, the crystalline quality degraded when the carbon concentration was too high (1 × 1020 cm−3), even in the case of GaN substrates. High breakdown voltages (approximately 7 kV under a lateral configuration) were obtained for the carbon-doped GaN layers on n-type GaN substrates when the carbon concentration was 5 × 1019 cm−3. These results indicate that lateral power devices with high breakdown voltage can be fabricated by using thick carbon-doped GaN buffer layers, even on n-type GaN substrates.


Source:IOPscience

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Apr 9, 2019

Vertical GaN-based trench metal oxide semiconductor field-effect transistors on a free-standing GaN substrate with blocking voltage of 1.6 kV

This paper reports the characteristics of vertical GaN-based trench metal–oxide–semiconductor field-effect transistors on a free-standing GaN substrate with a blocking voltage of 1.6 kV. The high blocking voltage was obtained by using field plate edge termination around the isolation mesa of the transistor. To our knowledge, the blocking voltage is the highest ever reported for vertical GaN-based transistors on free-standing GaN substrates. Normally off operation with a threshold voltage of 7 V is also demonstrated.



Source:IOPscience

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Apr 3, 2019

Homoepitaxial growth of GaN crystals by Na-flux dipping method

The realization of low-dislocation-density bulk GaN crystals is necessary for use in the fabrication of future high-power devices with low power consumption. In this study, we attempted the regrowth of low-dislocation-density (104–105 cm−2) GaN substrates to fabricate thick and low-dislocation-density GaN crystals using the dipping technique with the Na-flux method. In the growth using this dipping technique, the generation of dislocations at the interface between the GaN substrate and the regrowth layer was prevented, and we succeeded in fabricating thick and low-dislocation-density GaN crystals. In the growth without the dipping technique, the surface of the GaN substrate demonstrated meltback immediately before the growth, and dislocations were newly generated. These results indicate that the Na-flux dipping technique has potential use for the fabrication of low-dislocation-density bulk GaN crystals.



Source:IOPscience


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Mar 25, 2019

GaN substrate and GaN homo-epitaxy for LEDs: Progress and challenges

After a brief review on the progresses in GaN substrates by ammonothermal method and Na-flux method and hydride vapor phase epitaxy (HVPE) technology, our research results of growing GaN thick layer by a gas flow-modulated HVPE, removing the GaN layer through an efficient self-separation process from sapphire substrate, and modifying the uniformity of multiple wafer growth are presented. The effects of surface morphology and defect behaviors on the GaN homo-epitaxial growth on free standing substrate are also discussed, and followed by the advances of LEDs on GaN substrates and prospects of their applications in solid state lighting.


Source:IOPscience

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Mar 18, 2019

Selective-area growth of GaN on non- and semi-polar bulk GaN substrates

We carried out the selective-area growth of GaN and fabricated InGaN/GaN MQWs on non- and semi-polar bulk GaN substrates by MOVPE. The differences in the GaN structures and the In incorporation of InGaN/GaN MQWs grown on non- and semi-polar GaN substrates were investigated. In the case of selective-area growth, different GaN structures were obtained on GaN,  GaN, and  GaN substrates. A repeating pattern of  and  facets appeared on  GaN. Then, we fabricated InGaN/GaN MQWs on the facet structures on  GaN. The emission properties characterized by cathodoluminescence were different for  and  facets. On the other hand, for InGaN/GaN MQWs on non- and semi-polar GaN substrates, steps along the a-axis were observed by AFM. In particular on  GaN, undulations and undulation bunching appeared. Photoluminescence characterization indicated that In incorporation increased with the off-angle from the m-plane and also depended on the polarity.



Source:IOPscience

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Mar 12, 2019

GaN-based light-emitting diodes on various substrates: a critical review

GaN and related III-nitrides have attracted considerable attention as promising materials for application in optoelectronic devices, in particular, light-emitting diodes (LEDs). At present, sapphire is still the most popular commercial substrate for epitaxial growth of GaN-based LEDs. However, due to its relatively large lattice mismatch with GaN and low thermal conductivity, sapphire is not the most ideal substrate for GaN-based LEDs. Therefore, in order to obtain high-performance and high-power LEDs with relatively low cost, unconventional substrates, which are of low lattice mismatch with GaN, high thermal conductivity and low cost, have been tried as substitutes for sapphire. As a matter of fact, it is not easy to obtain high-quality III-nitride films on those substrates for various reasons. However, by developing a variety of techniques, distincts progress has been made during the past decade, with high-performance LEDs being successfully achieved on these unconventional substrates. This review focuses on state-of-the-art high-performance GaN-based LED materials and devices on unconventional substrates. The issues involved in the growth of GaN-based LED structures on each type of unconventional substrate are outlined, and the fundamental physics behind these issues is detailed. The corresponding solutions for III-nitride growth, defect control, and chip processing for each type of unconventional substrate are discussed in depth, together with a brief introduction to some newly developed techniques in order to realize LED structures on unconventional substrates. This is very useful for understanding the progress in this field of physics. In this review, we also speculate on the prospects for LEDs on unconventional substrates.


Source:IOPscience

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Mar 5, 2019

Enhancement in wafer bow of free-standing GaN substrates due to high-dose hydrogen implantation: implications for GaN layer transfer applications

Two-inch free-standing GaN wafers were implanted by 100 keV H+2 ions with a dose of 1.3 × 1017 cm−2 at room temperature. The hydrogen implantation induced damage in GaN extends between 230 to 500 nm from the surface as measured by cross-sectional transmission electron microscopy (XTEM). The wafer bow of the free-standing GaN wafers was measured using a Tencor long range profilometer on a scan length of 48 mm before and after the hydrogen implantation. Before implantation the bow of two different free-standing GaN wafers (named A and B) with different thicknesses was 1.5 µm and 6 µm, respectively. Initially, both wafers were concave in shape. After implantation the bow changed to convex with a value of 36 µm for wafer A and a value of 32 µm for wafer B. High dose hydrogen implantation leads to an in-plane compressive stress in the top damaged layer of the GaN, which is responsible for the enhancement of wafer bow and change of bow direction. The high value of bow after implantation hinders the direct wafer bonding of the free-standing GaN wafers to sapphire or any other handle wafers. Tight bonding between hydrogen implanted GaN wafers and the handle wafers is a necessary requirement for the successful layer transfer of thin GaN layers onto other substrates based on wafer bonding and layer splitting (Smart-cut).



Source:IOPscience

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