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

Abstract

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...

Effect of Si doping on the thermal conductivity of bulk GaN at elevated temperatures – theory and experiment

“…Secondary ion mass spectroscopy (SIMS) measurements were used to determine the total Si concentrations, as well as some of the typical residual impurities in HVPE grown GaN -oxygen, hydrogen and carbon. The Si concentration was found to range from 1×10 16 15 cm -3 were unaltered by the varying the Si doping. The free electron concentration, n e , as determined by conventional Hall effect measurements at room temperature showed a nearly complete donor ionization, i. e. the n e scales closely with the Si concentration in an agreement with previous studies.…”

The impact of film thickness and substrate surface roughness on the thermal resistance of aluminum nitride nucleation layers

“…Accordingly, the value of k AlN-eff ¼ 0.93 6 0.16 W/mK reported in our prior study should be used for with MPSiC, 10 2 Our study suggests that differences result primarily from the microstructure of the AlN near its interface with SiC. The diffuse mismatch model predicts that the TBR of a perfect AlN-SiC interface can be as low as 0.6 m 2 K/ GW.…”

“…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.…”

“…Further details can be found in the supplementary online material. 10,15,18 Representative temperature amplitude vs. frequency data and analytical fits are shown in Figure 1(b) for AlN films grown by OMVPE on CMP SiC. The temperature amplitude can be broken down as DTðxÞ ¼ DT sub ðxÞ þ DT AlN , where DT sub ðxÞ is the portion due to thermal resistance in the substrate.…”

“…Substrate RMS roughness was characterized by tapping mode AFM and film thicknesses were measured by spectral reflectance with an accuracy of 61 nm. Transmission electron microscopy (TEM) imaging of the AlN/SiC interface and the (0001) AlN surface for determination of dislocation density followed previously reported procedures 10 summarized in the supplementary online material. 18 To measure thermal conductivity via the 3-omega method one uses a resistive heater/thermometer that is microfabricated on the sample surface to induce and measure a periodic temperature change that has a sample-dependent amplitude.…”