A new approach to the fabrication of efficient heat sinks for GaN-based transistors is demonstrated. A key feature of this work is the growth of polycrystalline diamond coating on the functional silicon layer of SOI wafers followed by etching of a thick silicon substrate and a thin thermal oxide. As a result, composite epi-ready substrates consisting of a thin (410 nm) monocrystalline silicon functional layer on top of the 150 mu m-thick polycrystalline diamond heat sink were fabricated. GaN heterostructures were grown on top of the silicon layer, which resulted in an effective thermal contact between CVD diamond and GaN structure. The packaged ungated transistors were made to analyze the efficiency of the developed heat sink. Improved heat removal structures showed the decrease in surface temperature by more than 50 degrees C at base temperature of T-b =85 degrees C and dissipation power of P-d(i)ss=6.9 W/mm compared to conventional GaN-on-SiC technology and by more than 20 degrees C at T-b=25 degrees C, P-diss=6.9 W/mm compared to up-to-date GaN-on-Diamond equivalent transistors reported by other groups. New substrate fabrication technology positively impacts GaN-based device output characteristics and reliability, which is important in improving communication systems, radars, and secondary power supply systems. (C) 2021 The Authors. Published by Elsevier Ltd.

In this work, thermometric measurements of gallium nitride-based ungated transistors on silicon-on-diamond composite substrates are performed. Their heat sink efficiency is compared with transistors made by standard technology on a silicon carbide substrates. Reducing of the surface temperature by more than 50oC using new type of silicon-on-diamond composite substrates at dissipation power above 7 W is shown. The proposed approach is promising for increasing the output power and reliability of gallium nitride-based devices. Keywords: gallium nitride, heat sink, diamond, dissipation power.

The formation of β-Si3N4 for subsequent growth of AlGaN and GaN heterostructures of silicon wafers has been studied. It is established that the native oxide layer protects the silicon surface from the formation of amorphous silicon nitride when heating in an ammonia flow. Controlled formation of β-Si3N4 at partial ammonia pressures up to 3 × 10–5 Torr is demonstrated. This circumstance makes it possible to perform epitaxy of nitride films without cleaning the growth chamber from ammonia, which is usually required to remove the native silicon oxide by high-vacuum annealing.

A new type of substrates for the growth of nitride heterostructures is presented that consists of a 125-nm thick silicon layer and 290-μm thick polycrystalline diamond. The possibility of epitaxial growth of nitride heterostructures on silicon–polycrystalline diamond substrates with characteristics at the level of heterostructures on silicon substrates is shown. The test transistors demonstrated the following: saturation current density of more than 1 A/mm and breakdown voltage of more than 90 V. The achieved results open up opportunities for the emergence of a new class of silicon-polycrystalline diamond substrates and the creation of powerful gallium nitride transistors with previously unattainable characteristics.

The effect of trimethylaluminum preflow time on the crystalline quality of AlN films grown by metalorganic chemical vapor deposition on Si(111) substrates has been investigated. It is found that semipolar (10$$\bar {1}$$1) layers are formed in the AlN film with long preflow times. It is shown that the AlN nucleation can be controlled by choosing an optimal preflow time, which provides a desired film quality. At the optimum preflow time, the FWHM of the rocking curve for the 0002 reflection amounts to 0.59°.

GaN heterostructures on silicon substrates have been grown by metalorganic chemical vapor deposition. Transistors with the gate periphery of 1.32 mm are designed. The saturation power of the package die at a frequency of 1 GHz was 4 and 6.3 W at supply voltages of 30 and 60 V, respectively. The maximum drain efficiency is 57%.

In this study, AlN films on Si(1 1 1) using LT-AlN nucleation layer with various conditions at TMAl preflow were grown and investigated. It was shown that the main factor influencing the quality of AlN films is the degree of the substrate coating by aluminium at preflow. The qualitative model of AlN growth using the LT-AlN nucleation layer for three different coverage by Al (high, optimal and low) was suggested. For the film grown under optimal conditions, the rocking curve FWHM for the AlN (0 0 0 2) reflection was 0.59 degrees. The demonstrated possibility of the high-quality growth of AlN films below 1000 degrees C would be useful for high power electronics.

The growth kinetics of AlGaN in NH3 MBE under significant Ga desorption was studied. It was found that the addition of gallium stimulates 2D growth and provides better morphology of films compared to pure AlN. The effect was experimentally observed at up to 98% desorption of the impinging gallium. We found that under the conditions of significant thermal desorption, larger amounts of gallium were retained at lateral boundaries of 3D surface features than at flat terraces because of the higher binding energy of Ga atoms at specific surface defects. The selective accumulation of gallium resulted in an increase in the lateral growth component through the formation of the Ga-enriched AlGaN phase at boundaries of 3D surface features. We studied the temperature dependence of AlGaN growth rate and developed a kinetic model analytically describing this dependence. As the model was in good agreement with the experimental data, we used it to estimate the increase in the binding energy of Ga atoms at surface defects compared to terrace surface sites using data on the Ga content in different AlGaN phases. We also applied first-principles calculations to the thermodynamic analysis of stable configurations on the AlN surface and then used these surface configurations to compare the binding energy of Ga atoms at terraces and steps. Both first-principles calculations and analytical estimations of the experimental results gave similar values of difference in binding energies; this value is 0.3 eV. Finally, it was studied experimentally whether gallium can act as a surfactant in AlN growth by NH3 MBE at elevated temperatures. Gallium application has allowed us to grow a 300 nm thick AlN film with a RMS surface roughness of 2.2 angstrom over an area of 10 x 10 mu m and a reduced density of screw dislocations. Published by AIP Publishing.

The growth of high-temperature AlN and AlGaN layers on (0001) sapphire substrates by ammonia based molecular-beam epitaxy is studied. Factors affecting the formation of inverted domains in high-temperature AlN films are examined. The density of inverted domains is found to correlate with the density of nucleation islands during the initial stages of growth. The denser coverage of a surface by nucleation islands suppresses the formation of inverted domains. It is possible to increase the density of surface coating at the nucleation growth stage by increasing the degree of substrate nitriding, reducing the deposition temperature, and using intense ammonia fluxes during deposition of the initial layers. The kinetic model in the mean field approximation is developed to explain the observed effects of growth parameters on the density of nucleation islands. The growth features of AlN and its structure are taken into account. The obtained results are used to grow AlN/AlGaN layers with improved structural quality. The grown films have a root-mean-square surface roughness of 2 Å and 120 arc s FWHM of X-ray diffraction peaks for the AlN 0002 reflection. The density of inverted domains is decreased to below 10 5 cm -2 . Improvement in the quality of the AlN films is achieved by using two-step growth and by the application of gallium as a surfactant.