GaN-based devices are ubiquitous in everyday life, ranging from InGaN/GaN light-emitting diodes (LEDs) for illumination to AlGaN/GaN high-electron-mobility transistors for compact power supplies. Nevertheless, despite the tremendous development in the past thirty years, GaN technology is far from reaching maturity, and many challenges are yet to be solved. Among them, the realization in a practical way of fully-vertical GaN-on-Si power devices and the demonstration of high-efficiency red InGaN microLEDs are crucial from an industry perspective.
The growth and quality of a III-nitride film critically depend on the choice of the substrate and on the design of the intermediate buffer layer. This suggests that a novel approach in selecting the substrate and buffer may be the key to solve the open challenges.
In this thesis, we propose an innovative and original technique to directly grow GaN at high temperature (HT) on a foreign substrate by metalorganic chemical vapor deposition (MOCVD), without the use of any low-temperature (LT) buffer, by simply employing an in-situ preflow with trimethylaluminum (TMAl).
We investigated for the first time this method to directly growth GaN on ScAlMgO4 (SAM), a novel substrate with a small lattice mismatch with respect to GaN. Our proposed technique resulted in mirror-like Ga-polar GaN layers on SAM with morphology and crystal quality comparable to the use of conventional LT GaN buffers. At the same time, our method enabled the growth of arbitrarily thin GaN layers directly on SAM, thanks to the absence of the initial 3D growth step. The effect of the TMAl preflow parameters (duration, flow, and carrier gas) was studied in detail, and a model for the growth mechanism was proposed.
This technique was applied to grow an ultra-thin GaN layer on SAM as a buffer for the subsequent growth of a thick In0.17Ga0.83N layer, with the same lattice constant of SAM, for high-indium-content InGaN quantum wells, which presented a higher crystal quality and photoluminescence efficiency compared to reference samples grown with a conventional LT InGaN buffer. Therefore, our growth technique may contribute to the achievement of red InGaN microLEDs.
Moreover, as an alternative to the in-situ TMAl preflow, we demonstrated that GaN could be grown on SAM at HT also using an ex-situ Al deposition, resulting in smooth films with an even better crystal quality compared to a TMAl preflow. We clarified the growth mechanism, revealing the role of the thermally-dewetted Al layer on the improvement in GaN quality.
Finally, we demonstrated that the direct HT GaN growth with TMAl preflow was successful also in the case of sapphire and Si substrates. In particular, we found that n-GaN layers directly grown on n-Si with a TMAl preflow not only present a better crystalline quality compared to the use of thin AlN buffers, but also exhibit orders-of-magnitude improvement in vertical current conduction between GaN and Si, thanks to the absence of highly resistive AlN layers. Therefore, our proposed technique opens a new pathway for the effective realization of fully-vertical GaN-on-Si devices.
In conclusion, in this thesis we proposed a new technique to directly grow GaN on foreign substrates (SAM, sapphire and Si) by simply using a TMAl preflow, without any intentional buffer layer. The obtained results confirmed that our proposed growth method could contribute to solve the open challenges holding back the next generation of GaN devices