Encyclopedia
Gallium nitride (GaN) is a wide-bandgap semiconductor material with excellent electrical and optical properties, making it a promising candidate for various electronic and optoelectronic devices. In particular, the unique characteristics of GaN make it a popular choice for high-power and high-frequency applications, such as power electronics, RF amplifiers, and light-emitting diodes (LEDs).
- gallium nitride
- high electron mobility transistor
- GaN on Si/SiC/QST
1. Introduction:
Gallium Nitride (GaN) is a wide-bandgap semiconductor material with excellent electrical and optical properties, making it a promising candidate for various electronic and optoelectronic devices. In particular, the unique characteristics of GaN make it a popular choice for high-power and high-frequency applications, such as power electronics, RF amplifiers, and light-emitting diodes (LEDs). One of the key challenges to realizing the full potential of GaN devices is the quality of the epitaxial layer. The epitaxial layer is a thin layer of GaN grown on a substrate, and it plays a crucial role in determining the performance and reliability of GaN devices. Therefore, a thorough understanding of the epitaxial growth process and the structural characteristics of the epitaxial layer is necessary.
In recent years, significant progress has been made in improving the quality of GaN epitaxial layers by developing advanced growth techniques and optimizing process parameters. The issues in GaN epitaxial layers are attributed to various factors, including defects, traps, and dislocations in the material. These defects serve as sites for electron capture and recombination, leading to local heating and thermal runaway. In addition, the high electric fields in GaN devices lead to collision ionization and the generation of electron-hole pairs, further exacerbating the breakdown problem. To address the breakdown problem in GaN devices, scientists have developed various solutions. One approach is to optimize the growth conditions of the GaN epitaxial layer to reduce the density of defects and dislocations. This is achieved through the use of advanced growth techniques (such as MOCVD and molecular beam epitaxy (MBE)) and the combination of buffer layers and strain engineering. Another approach is to develop new device structures and architectures that can mitigate the issue of defects. For example, the use of field plates, edge termination structures, and deep trench isolation helps to reduce the electric field concentration and improve the breakdown voltage of GaN devices. Additionally, the use of advanced gate dielectrics, such as Al2O3 and HfO2, helps to reduce gate leakage and improve the reliability of the devices.
2. GaN Epitaxy
GaN epitaxy on Si (111) substrate faces issues related to lattice mismatch between the substrate and the epitaxial layer. This mismatch leads to high-density threading dislocations and other defects, which impact the performance of the final device. To mitigate this problem, various techniques have been developed, such as strain engineering, defect reduction, and the use of graded buffer layers, to reduce the number of dislocations and improve the quality of the epitaxial layer. Additionally, thermal management of GaN on Si substrate is another critical factor to consider during epitaxial growth. The high thermal conductivity of Si creates significant thermal stress on the GaN epitaxial layer, leading to cracking and other defects. Therefore, proper thermal management techniques, such as the use of thermal interface materials and optimized structural design, are necessary. The Qromis substrate technology (QST) is designed specifically for epitaxial growth, and its thermal expansion coefficient (CTE) closely matches that of the epitaxial layer grown on top of it. The substrate is composed of several layers, including a polycrystalline ceramic core that provides structural support, a first adhesive layer coupled to the core, a conductive layer coupled to the first adhesive layer, a second adhesive layer coupled to the conductive layer, and a barrier layer coupled to the second adhesive layer. In addition to these layers, the substrate also includes an oxide silicon layer coupled to the support structure, followed by a essentially single crystal silicon layer. Finally, the epitaxial III-V layer is coupled to the single crystal silicon layer. A comprehensive review can be found in. One example in this regard is growing thicker buffer layers on QST substrates, which requires introducing tensile stress to limit the increase in in-situ curvature.
To overcome the challenges brought about by the CTE mismatch between Si and (Al)GaN, which affects the mechanical strength of the chip during buffer layer epitaxial growth, additional measures have been taken. High-quality (Al)GaN buffers can be grown on Si substrates, but the CTE mismatch limits the quality, thickness, and substrate size of the buffer. One method that has been attempted is using non-standard thickness substrates, such as the 1150 µm used in theour GaN-on-Si technology, for growing GaN on 200 mm Si substrates. However, this is not a scalable solution. A more practical solution is to use commercially available SEMI standard thickness engineering substrates, such as QST®, which have a polycrystalline aluminum core with a CTE that matches that of the (Al)GaN buffer and good thermal conductivity. These substrates exhibit exceptional properties, including high crystal quality, thick buffer layers (>15 µm), high thermal conductivity, and the potential for scalability up to 12 inches in diameter. By using such substrates, the CTE mismatch and mechanical strength issues during buffer layer epitaxial growth have been overcome.
After epitaxial growth, the quality of the epitaxial layer is evaluated by measurements such as PL, XRD, AFM, and surface scanning (surfscan). Information obtained from these measurements is used to optimize the epitaxial structure and improve the MOCVD recipe for subsequent epitaxial runs. If any surface defects or cracks are found on the wafer, further investigation is carried out to identify the cause and improve the quality of the epitaxial layer.
To improve the issue of melting erosion caused by GaN contacting Si substrates, GaN or AlN nucleation layers can be used as interface layers. However, due to growth process limitations, in practice, only AlN nucleation layers (NL) can be selected. The surface morphology and unexpected oxygen impurities determine the vertical leakage of AlN NL/Si. Interestingly, AlN NL affects the subsequent growth of the epitaxial layer and its vertical breakdown voltage. Furthermore, it has been found that growing AlGaN intermediate layers and multi-pairs of AlGaN/AlN strain layer superlattices on AlN NL with better surface characteristics increases the vertical breakdown voltage. Before using the AlN nucleation layer, surface treatment, such as spraying aluminum or NH3, can be used to create a rough SiNx surface. This helps to mitigate the stress caused by lattice mismatch between the Si substrate and GaN lattice, which can otherwise cause cracks and warping in the epitaxial layer.
To further alleviate this problem, a gradient buffer layer with decreasing aluminum concentration is used, and a stair-step gradient layer of AlxGa1-xN is typically used instead of a linear gradient layer. In addition, superlattices or interrupted AlN layers can be inserted to improve the crystal quality of the epitaxial layer. The carbon-GaN layer is also critical for device performance due to its high insulating properties, as it determines breakdown voltage and leakage. Therefore, achieving uniform carbon doping is crucial. There are several options for carbon doping, including CH4, C2H4, C3H8, and CBr4. Some literature also suggests using Fe doping, which can improve the conductivity of the epitaxial layer.
In 2020, IMEC planned to grow GaN on silicon using 200 mm 8-inch QST substrate technology, which is a promising development for the electronics industry. Compared to traditional substrates, this technology has many advantages, including reduced parasitic effects, a matching substrate AlN thermal expansion coefficient, high thermal conductivity, high mechanical yield, and the ability to grow thick GaN buffer layers. These advantages make it possible to achieve a high breakdown voltage of 650 V, which is essential for high-power devices.
In the field of GaN epitaxial growth, ensuring surface quality is crucial because it affects the final properties of the subsequent processes and devices. Therefore, surface inspection is critical for ensuring the quality of the epitaxial layer. OM is typically used to check if there are visible defects on the substrate and epitaxial layer surface, while SEM and TEM are used for more detailed analysis. XPS and SIMS can also be used to analyze impurities on the surface of the epitaxial layer.
