Modulation of material and electrical properties of AlGaN/GaN heterostructure and Schottky barriers in relation to GaN substrates offcut angle

Recently, there has been significant interest in GaN based electrical devices as a highly efficient power switching solution. This is due to their outstanding properties such as high current density, fast recovery time, and high breakdown field. GaN and AlN stand out for their high polar bonding character, which enables the creation of an electrically highly conductive layer known as two-dimensional electron gas (2DEG) at the interface of an AlGaN/GaN heterostructure without the need for extrinsic doping. This not only leads to a high charge carrier density but also to a high charge carrier mobility, which makes such a heterostructure an ideal candidate for High Electron Mobility Transistors (HEMTs). To fabricate AlGaN/GaN heterostructures, it is imperative to utilize GaN native substrates which provides low dislocation density. Low density of dislocations is essential in AlGaN/GaN heterostructure devices, as dislocations can act as trapping centres for charge carriers. This can lead to increased leakage currents, and charge scattering in the two-dimensional electron gas (2DEG), which in turn reduces the electron mobility in the channel. Utilizing homoepitaxial growth on GaN often yields superior epitaxial crystal quality, characterized by more than two orders of magnitude threading dislocations. Consequently, this improvement may facilitate a reduction in gate contact size due to enhanced efficiency in the active area, thereby potentially shrinking the overall device dimensions, particularly in lateral HEMT configurations [1], [2]. Such optimizations aim to maximize critical electric field strength and minimize leakage currents, as both are influenced by threading dislocations. In the past, the high cost and limited size of GaN freestanding substrates were major hindrances to the advancement of epitaxial growth and technological progress in this field. However, recent developments in seeding techniques on foreign substrates in Hydride Vapor Phase Epitaxy (HVPE) reactor have led to more affordable solutions [3]. As a result of this latest breakthrough, the availability of 100 mm GaN substrates has increased, thereby making further progress in this area. Moreover, in large-scale production, the price becomes less significant when reliability and highest performance is a concern. GaN substrates have a vicinal surface featuring a surface offcut angle. This offcut angle promotes lateral step flow growth, which could result in a smooth epitaxial surface morphology avoiding undesired surface disturbances [4], [5]. However, variations in the commercially available GaN substrate surface offcut angle across the wafer arise due to the internal lattice bow present in GaN bulk crystals. Understanding the epitaxial growth mechanism of GaN and AlGaN films in relation to GaN substrate surface morphology is critical for achieving structural and compositional uniformity while minimizing growth disturbances like macrosteps and hillocks. These disturbances can significantly impact epitaxial film quality and device performance. Despite its importance, the lack of systematic methods for micro- and nanoscale assessment of GaN and epitaxial layer surfaces remains a challenge, hindering comprehensive understanding and optimization of GaN substrate surface morphology and epitaxial growth processes. This dissertation aims to conduct a comprehensive investigation into the offcut angle uniformity of vicinal GaN substrate surfaces and their impact on epitaxial growth and the electrical characteristics of heteroepitaxial structures not only in the nanoscale but also conducted in macroscopic Schottky barrier diode as the easiest test devices. The role of GaN substrate surface properties such as roughness and defects are analysed, as they act as local surface energy barriers that can hinder the diffusion and incorporation of Al and Ga adatoms differently on the surface. The study specifically concentrates on clarifying the growth mechanism of GaN and AlGaN epitaxial films in relation to the offcut angle of the GaN substrate surface. It investigates the impact of AlGaN layer composition inhomogeneities cross-correlated to atomic step density and height on the electrical characteristics of test Schottky barrier diodes (SBDs), highlighting the Schottky barrier height modulation. Additionally, the dissertation investigates the AlGaN composition inhomogeneity at macrosteps and hillocks and its electrical influence on current conduction behavior. The manuscript is divided into three main sections, providing a comprehensive exploration of the topic. The initial section focuses on optimizing the epitaxial growth conditions of AlGaN/GaN on sapphire substrates as a reference system. This optimization involves adjusting parameters such as barrier thickness and composition to achieve a high-density two-dimensional electron gas (2DEG) at the heterostructure interface, crucial for developing high-performance HEMTs. Additionally, enhancing the crystal quality of the GaN/AlGaN heterostructure is pursued through optimizing the deposition of a GaN 3D nucleation layer, specifically focusing on the density and size of nucleation islands. Subsequently, the refined growth parameters are applied to GaN native substrates, driven by the relatively high cost of GaN substrates for initial optimization of epitaxial growth parameters. In the second part, attention turns towards GaN substrate surface preparation and the fine-tuning of homoepitaxial growth parameters, building upon the results gained from the previous section. This optimization process specifically takes into account the surface offcut angle of the GaN substrates. During the in-situ back etching process in the MOCVD reactor, crystal defects in the GaN substrates are recognized as the primary sites where desorbed Ga atoms nucleate. Threading screw dislocations are identified as one of the primary causes of GaN nanograins formation. The study then investigates the surface morphology of GaN/AlGaN epitaxial layers in relation to substrate surface offcut angle and supersaturation conditions during epitaxial growth. Optimized supersaturation conditions based on the average offcut angle of different GaN substrates are presented. Substrates with higher offcut angles required lower growth rate to minimize atomic step bunching. Additionally, the modulation of the chemical composition of AlGaN layers, influenced by atomic terrace width and density, as well as step bunching of the underlying epilayer and GaN substrate, is explored. This involves examining the relative current flow over bunched steps, where the edges of the bunched step exhibit conductive stripes. Additionally, the study investigates the impact of offcut angle variation across three distinct GaN substrates, spanning from 0.05⁰ to 0.56⁰. A state-of-the-art combined thermodynamic kinetics model is developed based on experimental findings to explore the relative diffusion length of Ga and Al atoms during AlGaN growth. It has been discovered that the Ga diffusion length is approximately half that of Al adatoms on the surface under specific epitaxial growth conditions in the MOCVD reactor. In the third part, the study investigates the influence of offcut angle variation on the electrical properties of Ni/Au Schottky barrier diodes. Through analysis of devices from different regions of a sample with varied offcut angles, it was observed that the Schottky barrier height changes by 13 meV when the offcut angle shifts from 0.29⁰ to 0.42⁰, suggesting a broader change of 50 meV when the offcut angle ranges from 0.05⁰ to 0.56⁰. Moreover, the study delves into the impact of hillocks and AlGaN composition inhomogeneities on the I-V characteristics of Schottky Barrier Diodes (SBDs), assessing their criticality on devices. The criteria for grouping were determined by the high current density at low on-voltage and excessive reverse leakage current, which were explicitly defined. It was determined that threading screw dislocations, which lead to spiral hillocks formation, did not result in complete device failures, but caused leakage current.