Non-metallic crystalline materials conduct heat by the transport of quantized atomic lattice vibrations called phonons. Thermal conductivity depends on how far phonons travel between scattering events-their mean free paths. Due to the breadth of the phonon mean free path spectrum, nanostructuring materials can reduce thermal conductivity from bulk by scattering long mean free path phonons, whereas short mean free path phonons are unaffected. Here we use a breakdown in diffusive phonon transport generated by high-frequency surface temperature modulation to identify the mean free path-dependent contributions of phonons to thermal conductivity in crystalline and amorphous silicon. Our measurements probe a broad range of mean free paths in crystalline silicon spanning 0.3-8.0 mm at a temperature of 311 K and show that 40±5% of its thermal conductivity comes from phonons with mean free path 41 mm. In a 500 nm thick amorphous silicon film, despite atomic disorder, we identify propagating phonon-like modes that contribute 435±7% to thermal conductivity at a temperature of 306 K.
Thermal conductivities (k) of the individual layers of a GaN-based light emitting diode (LED) were measured along  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.  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.  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 -oriented SiC substrates. Th...
Thickness dependent thermal conductivity measurements were made on aluminum nitride (AlN) thin films grown by two methods on the (0001) surfaces of silicon carbide (SiC) and sapphire substrates with differing surface roughness. We find that the AlN itself makes a small contribution to the overall thermal resistance. Instead, the thermal boundary resistance (TBR) of 5.1 6 2.8 m 2 K/GW between the AlN and substrate is equivalent to 240 nm of highly dislocated AlN or 1450 nm of single crystal AlN. An order-of-magnitude larger TBR was measured between AlN films and SiC substrates with increased surface roughness (1.2 nm vs. 0.2 nm RMS). Atomic resolution TEM images reveal near-interface planar defects in the AlN films grown on the rough SiC that we hypothesize are the source of increased TBR. Nitride semiconductors are essential for blue/green light emitting diodes (LEDs) and high electron mobility transistors (HEMTs). LEDs are now being pushed to high power for lighting applications causing considerable heat generation due to Joule heating and inefficiencies in light production. 1 HEMTs are essential to high-power and high-speed switching operations that generate heat as a byproduct. 2,3 While thermal packaging is essential to minimize operating temperatures, nearly half of the total thermal resistance comes from the nitride device itself. 2,3 Experiments on bulk nitrides show that thermal transport is phonon-dominated.  Internal thermal resistance of nitride devices is complicated by the presence of interfaces between films and defects within films that scatter phonons and suppress thermal conductivity below bulk values.Non-native silicon carbide (SiC) and sapphire substrates are used for the commercial growth of nitride films because gallium nitride (GaN) and aluminum nitride (AlN) substrates are not yet economically viable. Growth is initiated with an AlN nucleation layer because direct growth of GaN on these substrates is not favorable at high temperatures. Due to a mismatch in the lattice parameters (a) of AlN (a AlN ¼ 3.07 Å ) with SiC (a SiC ¼ 3.11 Å ) and sapphire (a Sapp ¼ 4.785 Å ), dislocations and surface defects form at the interface and impact the quality of subsequent growth. Though SiC and AlN have a small mismatch (1%), their high elastic moduli require stress relief through such defect formation. 8,9 Prior studies agree that the AlN nucleation layer is a dominant thermal resistance in both LED 10 and HEMT architectures. 2,3,11,12 The source of this thermal resistance, however, is as yet experimentally unresolved as the total thermal resistance R T consists of three components: (1) the AlN/substrate TBR (TBR sub ), (2) the AlN intrinsic resistance L AlN /k AlN (where L is film thickness and k is thermal conductivity), and (3) the GaN/AlN TBR. Our prior study suggests that TBR sub is largest for AlN films grown on mechanically polished (MP) SiC substrates. 5 Nonetheless, it is unclear whether this conclusion holds for SiC vs. sapphire substrates, different growth techniques, or varied su...
Over 20% of electricity in US is used by lighting. Solid state lighting (SSL) efficiency can surpass that of incandescent and fluorescent lighting techniques. Nonetheless SSL efficiency is greatly reduced at high temperatures that result from inadequate heat dissipation. SSL requires blue and green light emitting diodes (LEDs) made from Gallium Nitride (GaN) and Indium Gallium Nitride (InGaN) to eventually generate white light. Conduction within the LED is a major thermal resistance for heat dissipation, and motivates study of thermal properties of LED materials, including GaN and InGaN. Bulk thermal properties are poor estimates of thin film properties due to increased boundary and defect scattering of phonons in the films. By examining real nitride based LED architectures with the 3-omega technique, thin film thermal conductivities of nucleation, buffer, contact, and active regions were measured from 100–400K. We find that the AlN nucleation layer is a bottleneck to heat transfer, having a thermal conductivity (κ) two orders of magnitude less than bulk crystalline AlN. Further, the temperature dependent behavior is characteristic of an amorphous solid. TEM images of the AlN layer show a very high dislocation density (4×1010 cm−2). We hypothesize that scattering from these dislocations as well as the film boundaries, causes the observed behavior.
Gallium Nitride (GaN) possesses superior electronic properties for RF power electronics that play critical roles in various wireless communication technologies and military applications . Heat generated as a byproduct of operation in these devices, increases their operating temperature and degrades their performance and lifetime. While bulk GaN has a high thermal conductivity (k) approaching 250 W/m-K, [2, 3] GaN thin films and devices experience a much lower k due to the presence of additional phonon scattering mechanisms and departures from Fourier transport [4, 5]. We will review thermal transport in GaN based devices, broadly addressing the impact of heat source dimensions, film thicknesses, interfaces, and defects.
Over 20% of electricity in US is used by lighting. Solid state lighting (SSL) efficiency can theoretically surpass that of incandescent and fluorescent lighting techniques. Nonetheless SSL efficiency is greatly reduced at high temperatures that result from inadequate heat dissipation. SSL requires blue and green light emitting diodes (LEDs) made from Gallium Nitride (GaN) and Indium Gallium Nitride (InGaN) to eventually generate white light. Using an infrared thermal imaging camera, temperatures of working blue and green LEDs with different efficiencies were measured. The results show that higher efficiency LEDs have lower active region temperatures when driven with the same power. Further, they motivate our study of thermal properties of the individual thin films that compose the LEDs, since earlier studies show that conduction is the primary dissipative mechanism for heat in LEDs. Bulk thermal properties are poor estimates of thin film properties due to increased boundary and defect scattering of phonons in the films. By examining real LED structures with the 3-omega technique, thin film thermal conductivities can be measured. For this technique, a thin metal line was fabricated onto a smooth dielectric sample surface. This thin metal line works as both a heater and a thermometer. Benchmark studies on Pyrex 7740 were used to validate our 3-omega setup. Data from real GaN/InGaN LED structures show that the effective thermal conductivities of the AlN buffer layer and multi-quantum-well active region are substantially suppressed relative to their anticipated values based on bulk properties.
Thickness dependent thermal conductivity measurements were made on aluminum nitride (AlN) thin films grown by two methods on the (0001) surfaces of silicon carbide (SiC) and sapphire substrates with differing surface roughness. We find that the AlN layer itself makes a small contribution to the overall thermal resistance. Instead, the thermal boundary resistance (TBR) of 5.1±2.8 m2-K/GW between the AlN and substrate is equivalent to 240 nm of highly dislocated AlN, or 1450 nm of single crystal AlN. An order-of-magnitude larger TBR was measured between AlN films and SiC substrates with increased surface roughness (1.2 nm vs. 0.2 nm RMS). High resolution TEM images reveal near-interface planar defects in the AlN films grown on the rough SiC that we hypothesize are the source of increased TBR.
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