Thermodynamics

Stringfellow, G. B.

doi:10.1016/b978-012673842-1/50005-3

Cited by 83 publications

(127 citation statements)

References 183 publications

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“…GaN cannot be directly grown on SiC (Refs. [15][16][17][18] at the normal deposition temperatures used to produce nitridebased LED structures. Instead, a nucleation layer must be grown that is sufficiently closely matched in chemistry, crystallography, lattice parameters, and the coefficients of thermal expansion with the substrate and the subsequently grown layer.…”

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confidence: 99%

Layer-by-layer thermal conductivities of the Group III nitride films in blue/green light emitting diodes

Huang

Liu

et al. 2012

Applied Physics Letters

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Thermal conductivities (k) of the individual layers of a GaN-based light emitting diode (LED) were measured along [0001] using the 3-omega method from 100-400 K. Base layers of AlN, GaN, and InGaN, grown by organometallic vapor phase epitaxy on SiC, have effective k much lower than bulk values. The 100 nm thick AlN layer has k ¼ 0.93 6 0.16 W/mK at 300 K, which is suppressed >100 times relative to bulk AlN. Transmission electron microscope images revealed high dislocation densities (4 Â 10 10 cm À2 ) within AlN and a severely defective AlN-SiC interface that cause additional phonon scattering. Resultant thermal resistances degrade LED performance and lifetime making layer-by-layer k, a critical design metric for LEDs. More than 20% of electricity in the United States is consumed by lighting. Solid-state light emitting diodes (LEDs) hold considerable potential to be more efficient and effective sources of artificial light than incandescent and fluorescent technologies. 1 Group III nitride-based blue and green LEDs are currently being developed for this purpose. Device architectures consist of light-emitting multiple quantum wells (MQWs) supported by base layers of aluminum nitride (AlN), gallium nitride (GaN), and indium gallium nitride (InGaN). The MQW itself is a periodic structure composed of alternating InGaN wells and GaN barriers.Heat generation and removal in the LED are complicated by multiple interfaces, causing high operating temperatures that degrade efficiency, shift the emission spectrum, and reduce the lifetime of LEDs. 2 Prior experimental investigations of bulk GaN and AlN have shown that thermal transport is phonon-dominated. [3][4][5][6] Previous thin film studies have been (i) based on ideal films with low defect concentrations that are unrepresentative of LED grade films 3,7 or (ii) focused on high electron mobility transistors (HEMTs), rather than LED architectures. [8][9][10] Neither submicron GaN films nor real nitride based LED devices have been investigated.Studies of Si and other thin films have demonstrated that thermal conductivity is greatly reduced when the thickness of the film or the defect spacing is less than the intrinsic mean free path of the phonons. 5,11,12 Nitride based LED structures are fabricated from multiple layers of thin films ranging from 3 nm to 100 nm in thickness; whereas, the spectrum-average phonon mean free path in GaN at 300 K is $100 nm, 13 and the longest phonon mean free paths in the spectrum are much greater. 14 To achieve economic viability, the nitride films are grown by organometallic vapor phase epitaxy (OMVPE) on foreign substrates, including SiC, sapphire, and Si. Mismatched lattice constants at the filmsubstrate interface create high densities of atomic and line defects, the latter of which that extend throughout the device as a source of phonon scattering. Given these considerations, this paper reports layer-by-layer experimental measurements of thermal conductivity in real nitride based LED architectures grown on [0001]-oriented SiC substrates. Th...

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confidence: 99%

Layer-by-layer thermal conductivities of the Group III nitride films in blue/green light emitting diodes

Huang

Liu

et al. 2012

Applied Physics Letters

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“…H and Sb as dual surfactants were reported to enhance Zn incorporation in GaP [39]. During the standard OMVPE growth processes, hydrogen is a common unavoidable impurity [48,49], often decomposed from precursors or carrier gases [50]. As a result, hydrogen-involved surface reconstructions have also been reported on the polar surfaces of ZnO [51][52][53][54][55] and GaN [56,57] both in experiments and theories.…”

Section: Introductionmentioning

confidence: 91%

Hydrogen-surfactant-assisted coherent growth of GaN on ZnO substrate

Zhang

Tse

et al. 2018

Phys. Rev. Materials

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Heterostructures of wurtzite based devices have attracted great research interests since the tremendous success of GaN in light emitting diodes (LED) industry. Among the possible heterostructure material candidates, high quality GaN thin films on inexpensive and lattice matched ZnO substrate are both commercially and technologically desirable. However, the energy of ZnO polar surfaces is much lower than that of GaN polar surfaces. Therefore, the intrinsic wetting condition forbids such heterostructures.As a result, poor crystal quality and 3D growth mode were obtained. To dramatically change the growth mode of the heterostructure, we propose to use hydrogen as a surfactant, confirmed by our first principles calculations. Stable H involved surface configurations and interfaces are investigated, with the help of newly developed algorithms. By applying the experimental Gibbs free energy of H2, we also predict the temperature and chemical potential of H, which is critical in experimental realizations of our strategy. This novel approach will for the first time make the growth of high quality GaN thin films on ZnO substrates possible. We believe that our new strategy may reduce the manufactory cost and improve the crystal quality and the efficiency of GaN based devices.

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“…32 Carbon also has the advantage of being an intrinsic dopant and therefore can be controlled using the epitaxial growth conditions in metal-organic vapor-phase epitaxy. 33 However, this use of traps to make the material more insulating must be carefully controlled, as carbon has been identifi ed as a potential source of current collapse. 27 A commonly discussed aspect in the development of GaN heteroepitaxy is the impact of the high density of dislocations, typically around 1 × 10 9 cm -2 .…”

Section: The Materials Challengementioning

confidence: 99%

Power electronics with wide bandgap materials: Toward greener, more efficient technologies

et al. 2015

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IntroductionPower electronics is the branch of electronics specifi cally dealing with collecting, delivering, and storing energy, including general and local/commodity energy supplies, by conversion and control of electrical power.1 Specifi c applications range from power supply systems to motor vehicle drives, 2 photovoltaic and fuel cell converters, inverters, and high-frequency heating, 3 among many others. The impact of power electronics has already been signifi cant and is anticipated to be even more so in the future for effi cient energy production and management.As in most other electronics areas, silicon also has been the predominant semiconductor in power electronics to date. However, with wide bandgap materials power electronic components are faster, smaller, more effi cient, and more reliable than their Si-based counterparts. Moreover, they permit the operation of devices at higher voltages, temperatures, and frequencies, 4 making it possible to reduce volume and weight in a wide range of applications. This could lead to large energy savings in industrial and consumer appliances, accelerate widespread use of electric vehicles and fuel cells, and integrate renewable energy into the electric grid.The power electronics market as a whole was about $20 billion in 2012, and power electronics will remain one of the most attractive branches of the semiconductor industry over the next decade. 5 The latest power electronics market forecasts 6 predict an increase of 60% for low-voltage technologies (below 900 V) by the year 2020, accompanied by an approximately 100% market increase for medium (1.2-1.7 kV) and high voltage (2 kV and above) technologies. This implies that the largest market increase in this sector over the next decade (medium and high voltage) will rely on semiconductor technologies beyond silicon (also see article by Okumura in this issue). This can be likened to a second revolution, equivalent to the introduction of large scale fabrication of silicon complementary metal oxide semiconductor (CMOS) technology, which led to the commoditization of electronics and formed the basis of our modern technological era. The power electronics revolution could be setting the basis for a world centered on greener technologies: improved energy conversion effi ciencies, faster switching, and more compact, lighter systems with better thermal management and tolerance will have tremendous impact on industrialization and energy systems, encompassing energy savings, renewable energy systems, and Greener technologies for more effi cient power generation, distribution, and delivery in sectors ranging from transportation and generic energy supply to telecommunications are quickly expanding in response to the challenge of climate change. Power electronics is at the center of this fast development. As the effi ciency and resiliency requirements for such technologies can no longer be met by silicon, the research, development, and industrial implementation of wide bandgap semiconductors such as gallium nitride (GaN) and sil...

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Thermodynamics

Cited by 83 publications

References 183 publications

Layer-by-layer thermal conductivities of the Group III nitride films in blue/green light emitting diodes

Layer-by-layer thermal conductivities of the Group III nitride films in blue/green light emitting diodes

Hydrogen-surfactant-assisted coherent growth of GaN on ZnO substrate

Power electronics with wide bandgap materials: Toward greener, more efficient technologies

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