High Thermal Conductivity of Gallium Nitride (GaN) Crystals Grown by HVPE Process

Shibata, Hiroyuki; Waseda, Yoshio; Ohta, Hiromichi; Kiyomi, Kazumasa; Shimoyama, K.; Fujito, Kenji; Nagaoka, Hirobumi; Kagamitani, Yuji; Simura, Rayko; Fukuda, Tsuguo

doi:10.2320/matertrans.mrp2007109

Cited by 128 publications

(67 citation statements)

References 16 publications

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“…This value is consistent with the previously reported data for free-standing HVPE grown GaN. [7][8][9] With increasing Si concentration, the thermal conductivity gradually decreased and for the sample with the highest doping (sample F), k = 210±6 W/m.K was found. Such a behavior is quite reasonable and can be simply explained by an increased contribution of phonon-point-defect scattering.…”

Section: Resultssupporting

confidence: 81%

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

et al. 2017

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The effect of Si doping on the thermal conductivity of bulk GaN was studied both theoretically and experimentally. The thermal conductivity of samples grown by Hydride Phase Vapor Epitaxy (HVPE) with Si concentration ranging from 1.6×1016 to 7×1018 cm-3 was measured at room temperature and above using the 3ω method. The room temperature thermal conductivity was found to decrease with increasing Si concentration. The highest value of 245±5 W/m.K measured for the undoped sample was consistent with the previously reported data for free-standing HVPE grown GaN. In all samples, the thermal conductivity decreased with increasing temperature. In our previous study, we found that the slope of the temperature dependence of the thermal conductivity gradually decreased with increasing Si doping. Additionally, at temperatures above 350 K the thermal conductivity in the highest doped sample (7×1018 cm-3) was higher than that of lower doped samples. In this work, a modified Callaway model adopted for n-type GaN at high temperatures was developed in order to explain such unusual behavior. The experimental data was analyzed with examination of the contributions of all relevant phonon scattering processes. A reasonable match between the measured and theoretically predicted thermal conductivity was obtained. It was found that in n-type GaN with low dislocation densities the phonon-free-electron scattering becomes an important resistive process at higher temperatures. At the highest free electron concentrations, the electronic thermal conductivity was suggested to play a role in addition to the lattice thermal conductivity and compete with the effect of the phonon-point-defect and phonon-free-electron scattering.

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Section: Resultssupporting

confidence: 81%

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

et al. 2017

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“…This value exceeds the proposed upper limit of the thermal conductivity for GaN of 230 W/mK [23]. However, our result fits well with other recently published data [24,25] above this limit of 230 W/mK (Fig. 6).…”

Section: Methodssupporting

confidence: 55%

GaN boules grown by high rate HVPE

Richter

Gründer

Schineller

et al. 2011

Phys. Status Solidi C

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The most promising approach for GaN substrate fabrication on the base of HVPE consists in the growth of thick GaN crystals and subsequent cutting of these boules into slices followed by conventional surface preparation. This work focusses on a HVPE GaN boule growth process optimized to highest growth rate and the use of a commercially available vertical HVPE reactor from AIXTRON in order to develop a technology of high productivity to enable low‐cost substrates. To this end, GaN crystals up to thicknesses of 6.3 mm have been grown on 2 inch GaN‐on‐sapphire templates produced by MOVPE. The rate in HVPE growth was 450 µm/h and the material properties of the crystals have been evaluated. No degradation of the material quality was found by comparison of structural, optical and transport properties with published data of the state of the art of bulk GaN. Dislocation densities as low as 6x105 cm‐2, high electron mobility of 960 cm2/Vs for a carrier density of 1x1016 cm‐3, low background impurity incorporation of oxygen and silicon, luminescence with well resolved excitonic emission, and a high value of thermal conductivity of 294±44 W/mK were determined. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

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“…We have modeled the temperature dependence of thermal conductivity using a power law relation of the form: Using reported experimental data, 16 the optimal fitting coefficients are: α k = 2.31 W K −1 m −1 and β k = 1.56. The assumption of an ideal contact assumes that a thermal resistance, R T H , is zero, which means that the expected results should give qualitative insights in the behavior of the device while still can be easily adapted to a particular device packaging for which the thermal resistance can be determined.…”

Section: Self-heating Effects In Gan Hemtsmentioning

confidence: 99%

Buffer Trap Related Knee Walkout and the Effects of Self-Heating in AlGaN/GaN HEMTs

Ubochi

Ahmeda

Kálna

2017

ECS J. Solid State Sci. Technol.

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Mixed-mode simulations of a class A amplifier is used to study the DC/RF dispersion commonly observed in AlGaN/GaN based HEMTs. We show that the observed knee walkout at frequencies greater than the emission rates of buffer traps (time constants t a e > 1 week) is related to the steady state trap density and spatial location due to the DC operational bias. An increase in the drain bias point and an initial distortion of the RF signal, that is expected to disappear as the device global temperature reduces, is observed when a self-heating model is included. Finally, we propose that a reduction in the DC/RF dispersion is possible with a suitable location and concentration of an acceptor doping in the buffer. Direct current (DC) to radio frequency (RF) dispersion highlighted in experimental studies 1,2 of Gallium Nitride (GaN) based High Electron Mobility Transistors (HEMTs) in the amplifier operation has been attributed to trapping in the device structure. The effect of this dispersion is a reduction in the available RF output power, and effectively results in the reduced performance of GaN HEMTs in RF and power applications in spite of its very promising material properties. The reduction in RF output power can be observed in a so-called knee walkout. The knee walkout describes a situation where the minimum drain voltage under RF drive is smaller than that obtained from DC as opposed to the current collapse (CC) which refers to the reduction in the RF maximum drain current compared to that observed under DC. 3Although experiments have revealed a dependence of the knee walkout on the DC operating point, it remains not understood to what extent degradation processes due to either the surface or the buffer traps affect the device's RF performance. Pulsed measurements of the device have been employed to study the collapse of the current and the dispersion at the knee of the output characteristics, 2 mirroring thus the expected behavior of the device in amplifier operation. This approach is useful in gaining important insights to possible trapping behavior but loses its accuracy in simulations due to the inaccurate determination of the pulsed knee voltage. The simultaneous application of voltages on the drain and the gate contacts in pulsed simulations is in reality achieved with one bias ensuing the other. Here, we have employed a mixed mode simulation of a GaN HEMT in class A operation, which ensures that the instantaneous applied biases depend only on the amplifier operation. The behavior of the device is thus accurately modeled using a physically based device simulator while the remaining part of the circuit is modeled using conventional circuit simulation techniques 4 in a real spirit of TCAD. In addition to the much more realistic simulation approach resulting in increased accuracy, the internal device conditions can be examined at any given point.The investigations are carried out using the drift-diffusion transport model. The study is based on a careful calibration of device I-V characteristics to experimental ...

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High Thermal Conductivity of Gallium Nitride (GaN) Crystals Grown by HVPE Process

Cited by 128 publications

References 16 publications

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

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

GaN boules grown by high rate HVPE

Buffer Trap Related Knee Walkout and the Effects of Self-Heating in AlGaN/GaN HEMTs

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