Substantial developments have been achieved in the synthesis of chemical vapour deposition (CVD) diamond in recent years, providing engineers and designers with access to a large range of new diamond materials. CVD diamond has a number of outstanding material properties that can enable exceptional performance in applications as diverse as medical diagnostics, water treatment, radiation detection, high power electronics, consumer audio, magnetometry and novel lasers. Often the material is synthesized in planar form; however, non-planar geometries are also possible and enable a number of key applications. This paper reviews the material properties and characteristics of single crystal and polycrystalline CVD diamond, and how these can be utilized, focusing particularly on optics, electronics and electrochemistry. It also summarizes how CVD diamond can be tailored for specific applications, on the basis of the ability to synthesize a consistent and engineered high performance product.

https://iopscience.iop.org/article/10.1088/0953-8984/21/36/364221/pdf

Chemical vapour deposition synthetic diamond: materials, technology and applications.

GaN high electron mobility transistors (HEMTs) are well suited for high-frequency operation due to their lower on resistance and device capacitance compared with traditional silicon devices. When grown on silicon carbide, GaN HEMTs can also achieve very high power density due to the enhanced power handling capabilities of the substrate. As a result, GaN-on-SiC HEMTs are increasingly popular in radio-frequency power amplifiers, and applications as switches in high-frequency power electronics are of high interest. This paper explores the use of GaN-on-SiC HEMTs in conventional pulse-width modulated switched-mode power converters targeting switching frequencies in the tens of megahertz range. Device sizing and efficiency limits of this technology are analyzed, and design principles and guidelines are given to exploit the capabilities of the devices. The results are presented for discrete-device and integrated implementations of a synchronous Buck converter, providing more than 10-W output power supplied from up to 40 V with efficiencies greater than 95% when operated at 10 MHz, and greater than 90% at switching frequencies up to 40 MHz. As a practical application of this technology, the converter is used to accurately track a 3-MHz bandwidth communication envelope signal with 92% efficiency.

High-Frequency PWM Buck Converters Using GaN-on-SiC HEMTs

High-power operation of AlGaN/GaN high-electron-mobility transistors (HEMTs) requires efficient heat removal through the substrate. GaN composite substrates, including the high-thermal-conductivity diamond, are promising, but high thermal resistances at the interfaces between the GaN and diamond can offset the benefit of a diamond substrate. We report on measurements of thermal resistances at GaN-diamond interfaces for two generations (first and second) of GaN-on-diamond substrates, using a combination of picosecond time-domain thermoreflectance (TDTR) and nanosecond transient thermoreflectance techniques. Two flipped-epitaxial samples are presented to determine the thermal resistances of the AlGaN/AlN transition layer. For the second generation samples, electrical heating and thermometry in nanopatterned metal bridges confirms the TDTR results. This paper demonstrates that the latter generation samples, which reduce the AlGaN/AlN transition layer thickness, result in a strongly reduced thermal resistance between the GaN and diamond. Further optimization of the GaN-diamond interfaces should provide an opportunity for improved cooling of HEMT devices.

Improved Thermal Interfaces of GaN–Diamond Composite Substrates for HEMT Applications

https://research-information.bris.ac.uk/files/72057588/Control_of_the_inplane_thermal_conductivity_of_ultra_thin_nanocrystalline_diamond_films_through_the_grain_boundary_properties.pdf

Control of the in-plane thermal conductivity of ultra-thin nanocrystalline diamond films through the grain and grain boundary properties

We demonstrate the nucleation of self-assembled, epitaxial GaN nanowires (NWs) on (111) single-crystalline diamond without using a catalyst or buffer layer. The NWs show an excellent crystalline quality of the wurtzite crystal structure with m-plane faceting, a low defect density, and axial growth along the c-axis with N-face polarity, as shown by aberration corrected annular bright-field scanning transmission electron microscopy. X-ray diffraction confirms single domain growth with an in-plane epitaxial relationship of (1010)GaN ∥ (011)Diamond as well as some biaxial tensile strain induced by thermal expansion mismatch. In photoluminescence, a strong and sharp excitonic emission reveals excellent optical properties superior to state-of-the-art GaN NWs on silicon substrates. In combination with the high-quality diamond/NW interface, confirmed by high-resolution transmission electron microscopy measurements, these results underline the potential of p-type diamond/n-type nitride heterojunctions for effici...

Self-assembled GaN nanowires on diamond.

The performance of AlGaN/GaN high-electron-mobility transistors (HEMTs) on diamond and SiC substrates is examined. We demonstrate GaN-on-diamond transistors with periphery WG = 250 mum, exhibiting ft = 27.4 GHz and yielding a power density of 2.79 W/mm at 10 GHz. Additionally, the temperature rise in similar devices on diamond and SiC substrates is reported. To the best of our knowledge, these represent the highest frequency of operation and first-reported thermal and X -band power measurements of GaN-on-diamond HEMTs.

Comparison of GaN HEMTs on Diamond and SiC Substrates

A significant improvement of contact transfer resistance on undoped GaN/AlGaN/AlN (10 A)/GaN high electron mobility transistor (HEMT) structure was demonstrated using a Ta/Ti/Al/Mo/Au metallization scheme compared to a Ti/Al/Mo/Au metallization scheme. A contact resistance as low as 0.16 ± 0.03 ohm-mm was achieved by rapid thermal annealing of evaporated Ta (125 A)/Ti (150 A)/Al (900 A)/Mo(400 A)/Au(500 A) metal contact at 700 °C for 1 min followed by 800 °C for 30 sec in a N2 ambient. An excellent edge acuity was also demonstrated for the annealed Ta/Ti/Al/Mo/Au ohmic contacts.

Yunju Sun

Papers

Comparison of GaN HEMTs on Diamond and SiC Substrates

Comparison of GaN HEMTs on Diamond and SiC Substrates

Electrical Comparison of Ta/Ti/Al/Mo/Au and Ti/Al/Mo/Au Ohmic Contacts on Undoped GaN HEMTs Structure with AlN Interlayer