Diamond on GaN integration for power semiconductor devices with efficient thermal management

The international actions against global warming demands reductions in carbon emission and more efficient use of energy. Energy efficiency in the conversion and use of electricity, as an important form of energy in the modern life, has strong environmental and economical impacts. Power electronic devices determining roles in the overall system efficiency in many applications such as power converters and inverters. Gallium Nitride (GaN) devices have emerged as an superior alternative to the silicon devices and enabled unprecedented efficiencies. GaN-based high electron-mobility transistors (HEMTs) offer excellent characteristics such as high switching speed, low switching and conduction losses, small device size and high power density capabilities. However, such significant increases in the power density and the reduction of device size comes at the price of dramatic increases in the heat fluxes and appearance of hot spots in the device footprint that cannot be addressed using conventional methods. The integration of diamond and GaN has been highly pursued for thermal management purposes as well as combining their exceptional complementary properties for power electronics applications and novel semiconductor heterostructures. In this thesis, we discuss the key hindrances towards realisation of diamond on GaN substrates and devices, and we propose novel solutions for improving the feasibility of diamond-on-GaN integration for efficient thermal management of hot spots in GaN devices and development of diamond devices integrated with GaN devices in the same chip.The growth of diamond on GaN is challenging due to the high lattice and thermal expansion mismatches. The weak adhesion of diamond to GaN and high residual stresses after the deposition often result in the diamond film delamination or development of cracks, which prevents the subsequent device fabrication. In this thesis, a new seed dibbling method is presented for seeding and growing high-quality diamond films on cost-effective GaN-on-Si substrates, which result in significantly larger diamond grains and lower residual stresses (as low as 0.2 GPa) compared to conventional methods. Moreover, the excellent adhesion obtained by this method enabled a reliable polishing of the as-grown diamond films on GaN-on-Si without any delamination, resulting in smooth diamond-on-GaN substrates with sub-nanometer roughness. The high temperature hot spots in GaN devices are addressed by the near-junction heat spreaders using diamond films deposited on GaN. An analytical model for the heat spreading is presented to analyse the performance and optimize the heat spreaders, considering the geometry and thermal conductivity of the heat spreader and the substrate. Experimental demonstration of diamond heat spreaders on vertical GaN diodes together with the thermal characterization of their behaviour shows significant reduction of thermal spreading resistances, hot spot temperatures and temperature gradients from the device footprint, which highlights the impact of such heat spreaders for the thermal management of high power density GaN devices.The diamond films deposited on GaN were also used to demonstrate high-performance diamond transistors integrated on AlGaN/GaN-on-Si substrates with characteristics such as high breakdown voltage, high current density, and high on/off ratio, which opens many new possibilities for the development of power ICs with complementary logics, gate drivers and power

Leakage mechanisms and contact technologies in InAlN/GaN high electron mobility transistors

Lorenzo Lugani

GaN based electronic devices have progressed rapidly over the past decades and are nowadays starting to replace Si and classical III-V semiconductors in power electronics systems and high power RF amplifiers. AlGaN/GaN heterostructures have been, until recently, the materials system of choice for nitride based electronics. The limits of AlGaN/GaN technologies are now known and alternative routes able to overcome them are actively investigated. Nearly lattice-matched InAlN/GaN heterostructures have emerged as viable solutions for extending the frequency range achievable by GaN based devices. These heterostructures have also proven a very high thermal stability, becoming of high interest for extreme environment applications. However, despite those great potentials, InAlN/GaN based devices, in particular high electron mobility transistors, have suffered from high leakage currents, strong short channel effects and low breakdown voltage that made them not competitive with respect to AlGaN/GaN technologies. The goal of this thesis is to investigate, understand and control the mechanisms that limit the performance of InAlN/GaN based transistors. Particular attention in this thesis will be given to leakage currents, having gate or buffer origin. Concerning gate leakage currents, an accurate model for InAlN/GaN heterostructures will be established and the expected presence of deep levels will be confirmed by means of photocapacitance spectroscopy. Based on these results, it will be shown that two conduction mechanisms contribute to gate leakage currents. A first mechanism dominates in heterostructures with thin (

EPFL2015

III-Nitride Semiconductor Photonic Nanocavities on Silicon

Ian Michael Rousseau

Optical nanocavities enhance light-matter interaction due to their high quality factors (Q) and small modal volumes (V). The control of light-matter interaction lies at the heart of potential applications for integrated optical circuits, including optical communication, quantum computation, biophotonics, and optical projection. Many of these applications could benefit from wide band gap III-nitride semiconductor material, which already forms the basis of industrial lighting technology based on blue light-emitting diodes. Cleanroom fabrication and optical spectroscopy comprise the laboratory work in this thesis. Air-suspended III-nitride nanocavities are fabricated using electron beam lithography, reactive ion etching, and vapor phase etching from thin (< 300 nm) III-nitride epilayers grown on silicon (111) substrates by metal-organic vapor phase epitaxy. An embedded single InGaN/GaN quantum well serves as an internal light source during micro-photoluminescence experiments. Strain and heating effects are analyzed by micro-Raman spectroscopy. A dedicated quantum optics laboratory is constructed in order to study nanocavities and quantum emitters in III-nitrides at short wavelengths (lambda < 500 nm). Photonic crystal nanocavities exhibit the highest figure-of-merit for light-matter interaction enhancement, Q/V. However, in contrast to silicon and gallium arsenide nanocavities working at telecommunication wavelengths, III-nitride based nanocavities underperform theoretical Q estimates by nearly three orders of magnitude at short wavelengths (lambda = 430 - 500 nm). This thesis accounts for this discrepancy by combined experimental and theoretical studies of nanobeam fabrication statistics. Surface state absorption is found to be the primary factor limiting Q at short wavelengths. Surface effects are studied in the microdisk resonator geometry. UV photoinduced desorption of oxygen gas from the III-nitride surface redshifts and broadens whispering gallery mode resonances over minute timescales. Oxygen desorption incurs additional optical absorption losses up to 100 cm^{-1}. The redshift and broadening is nearly reversible upon the introduction of oxygen into the measurement environment, implicating oxygen in passivation of the III-nitride surface. Finally, optimized surface passivation techniques using rapid thermal processing and conformal chemical vapor deposition more than double state-of-the-art Q of GaN-based microdisk resonators to beyond Q=10,000 in the blue spectral range. The results demonstrate the capabilities and limitations of III-nitride materials for optical nanocavities and photonic integrated circuits at short wavelengths.

EPFL2018

A perspective on multi-channel technology for the next-generation of GaN power devices

Elison de Nazareth Matioli, Luca Nela, Yuhao Zhang

The outstanding properties of Gallium Nitride (GaN) have enabled considerable improvements in the performance of power devices compared to traditional silicon technology, resulting in more efficient and highly compact power converters. GaN power technology has rapidly developed and is expected to gain a significant market share in an increasing number of applications in the coming years. However, despite the great progress, the performance of current GaN devices is still far from what the GaN material could potentially offer, and a significant reduction of the device on-resistance for a certain blocking voltage is needed. Conventional GaN high-electron-mobility-transistors are based on a single two-dimensional electron gas (2DEG) channel, whose trade-off between electron mobility and carrier density limits the minimum achievable sheet resistance. To overcome such limitations, GaN power devices including multiple, vertically stacked 2DEG channels have recently been proposed, showing much-reduced resistances and excellent voltage blocking capabilities for a wide range of voltage classes from 1 to 10 kV. Such devices resulted in unprecedented high-power figures of merit and exceeded the SiC material limit, unveiling the full potential of lateral GaN power devices. This Letter reviews the recent progress of GaN multi-channel power devices and explores the promising perspective of the multi-channel platform for future power devices. Published under an exclusive license by AIP Publishing.

AIP Publishing2022

III-Nitride Semiconductor Photonic Nanocavities on Silicon

Ian Michael Rousseau

EPFL2018