Dec 11, 2018

FEM thermal analysis of high power GaN-on-diamond HEMTs


A three-dimensional thermal analysis of GaN HEMTs on diamond substrate is investigated using the finite element method. The diamond substrate thickness, area and shape, transition layer thickness and thermal conductivity of the transition layer are considered and treated appropriately in the numerical simulation. The temperature distribution and heat spreading paths are investigated under different conditions and the results indicate that the existence of the transition layer causes an increase in the channel temperature and the thickness, area and shape of the diamond substrate have certain impacts on the channel temperature too. Channel temperature reduces with increasing diamond substrate thickness and area but with a decreasing trend, which can be explained by the saturation effects of the diamond substrate. The shape of diamond substrate also affects the temperature performance of GaN HEMTs, therefore, to achieve a favorable heat dissipation effect with the settled diamond substrate area, the shape should contain as many isothermal curves as possible when the isothermal gradient is constant. The study of the thermal properties of GaN on diamond substrate is useful for the prediction of heating of high power GaN HEMTs devices and optimal designs of an efficient heat spreader for GaN HEMTs.



Source:IOPscience

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Nov 27, 2018

Theoretical considerations on efficiency degradation due to thermal effect in a planar GaN-based LED with a GaN substrate


In this paper, using a fully-coupled, three-dimensional electro-thermal device simulator, we study the mechanism of efficiency degradation at high current operation in planar GaN-based light emitting diodes (LED). In particular, the improvement of the efficiency degradation using thicker conductive GaN substrates has been demonstrated. First, it is found that local Joule heating inside thin conductive GaN substrates degrades internal quantum efficiency (IQE) and increases the series resistance. Then, we introduced thicker conductive GaN substrates and simulated distributions of the current density and temperature inside the substrate. It is found that the maximum current density inside the GaN substrate decreases by about six times for a 100-µm-thick substrate compared to that for a 5-µm-thick substrate. Therefore, the maximum junction temperature decreases, and then IQE and the driving voltage are improved. The present study proves that thick GaN substrates are effective to improve the properties of planar LEDs at high current operation.




Source:iopscience

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Nov 12, 2018

GaN substrate materials


Why are native GaN wafers impractical? Recall that nitrogen is a gas at room temperature, while gallium is a solid... so how could the two both exist in the liquid state and be forced to solidify into a uniform crystal?

Substrates for GaN are either silicon carbide, sapphire, or silicon. Expensive alchemy is needed to align the GaN crystal onto these mismatched substrates, using molecular-beam epitaxy (MBE) or metal-organic chemical vapor deposition (MOCVD). Four-inch (100mm) SiC substrates are just becoming available for GaN-on-SiC, four inch GaN on silicon wafers are also available with a growth path toward six inch (150mm) and larger. Most MMIC processing lines can handle either 100 mm or 150mm wafers or both, there just isn't a market that will drive toward 200 mm any time soon. Silicon wafers are dirt cheap ($10 for 200mm diameter) while silicon carbide wafers currently cost 100X more for only 100mm. Sapphire seems to have fallen by the wayside in the past few years.



Source:Ieee

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Aug 16, 2018

Non-destructive method for strain imaging in an individual GaN nanorod by confocal Raman technique

GaN based layer structures on highly lattice mismatched substrates are widely used for electronic and optoelectronic devices. Top down etched, GaN based nanorod structures are mainly studied due to their more effective strain relaxation. The previous measurements on the strain state of these structures have been performed either on single detached nanorods or on ensembles of nanorods still on the substrate. Here we demonstrate a technique based on confocal Raman scattering spectroscopy to probe the strain state of a single GaN nanorod still on the original substrate non-destructively. Both lateral and depth resolved imaging is achieved close to the diffraction limit of light. We observe that a GaN nanorod on the substrate is compressively strained throughout. The strain decreases from the base of the nanorod towards the top surface, but the top surface is still compressively strained. The detached GaN nanorod is less compressively strained overall, and the strain relaxes from the center towards all the edges.

Source:IOPscience

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Aug 3, 2018

Using the kinetic Wulff plot to design and control nonpolar and semipolar GaN heteroepitaxy

For nonpolar and semipolar orientations of GaN heteroepitaxially grown on sapphire substrates, the development of growth procedures to improve surface morphology and microstructure has been driven in a largely empirical way. This work attempts to comprehensively link the intrinsic properties of GaN faceted growth, across orientations, in order to understand, design and control growth methods for nonpolar (1 1 2 0) GaN and semipolar (1 1 2 2) GaN on foreign substrates. This is done by constructing a comprehensive series of kinetic Wulff plots (or v-plots) by monitoring the advances of convex and concave facets in selective area growth. A methodology is developed to apply the experimentally determined v-plots to the interpretation and design of evolution dynamics in nucleation and island coalescence. This methodology offers a cohesive and rational model for GaN heteroepitaxy along polar, nonpolar and semipolar orientations, and is broadly extensible to the heteroepitaxy of other materials. We demonstrate furthermore that the control of morphological evolution, based on invoking a detailed knowledge of the v-plots, holds a key to the reduction of microstructural defects through effective bending of dislocations and blocking of stacking faults. The status and outlook of semipolar and nonpolar GaN growth on sapphire substrates will be presented.


Source:IOPscience

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Jul 24, 2018

The management of stress in MOCVD-grown InGaN/GaN LED multilayer structures on Si(1 1 1) substrates

The tensile stress in light-emitting diode (LED)-on-Si(1 1 1) multilayer structures must be reduced so that it does not compromise the multiple quantum well emission wavelength uniformity and structural stability. In this paper it is shown for non-optimized LED structures grown on Si(1 1 1) substrates that both emission wavelength uniformity and structural stability can be achieved within the same growth process. In order to gain a deeper understanding of the stress distribution within such a structure, cross-sectional Raman and photo-luminescence spectroscopy techniques were developed. It is observed that for a Si:GaN layer grown on a low-temperature (LT) AlN intermediate layer there is a decrease in compressive stress with increasing Si:GaN layer thickness during MOCVD growth which leads to a high level of tensile stress in the upper part of the layer. This may lead to the development of cracks during cooling to room temperature. Such a phenomenon may be associated with annihilation of defects such as dislocations. Therefore, a reduction of dislocation intensity should take place at the early stage of GaN growth on an AlN or AlGaN layer in order to reduce a build up of tensile stress with thickness. Furthermore, it is also shown that a prolonged three dimensional GaN island growth on a LT AlN interlayer for the reduction of dislocations may result in a reduction in the compressive stress in the resulting GaN layer.


Source:IOPscience

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Jul 1, 2018

Growth and characterization of free-standing zinc-blende (cubic) GaN layers and substrates

In this paper, we describe bulk, free-standing, zinc-blende (cubic) GaN wafers grown by plasma-assisted molecular beam epitaxy. We have grown GaN layers of up to 60 µm in thickness. We present the data from characterization measurements that confirm the cubic nature of the GaN crystals and show that the fraction of the material that is hexagonal in nature is not more than about 10% in the best thick samples. Cubic (0 0 1) GaN does not exhibit the spontaneous and piezoelectric polarization effects associated with (0 0 0 1) c-axis wurtzite GaN. Therefore, the free-standing GaN wafers we have grown would make ideal lattice-matched substrates for the growth of cubic GaN-based structures for blue and ultraviolet optoelectronic devices, and high-power and high-frequency electronic applications.


Source:IOPscience

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