PCSI2025 Session PCSI-SuE2: Wide Bandgap Materials I

Beta phase gallium oxide (β-Ga₂O₃) is an emerging ultra-wide bandgap semiconductor that has attracted attention for its potential to outperform existing materials operating at high breakdown voltages and high temperature. Alloying of In and Al in β-Ga₂O₃ provides the ability to individually engineer the bandgap and lattice parameters of the material, providing a useful toolbox for heterostructure engineering. However, the tendency of (Al,In,Ga)₂O₃ alloys to form competing phases, along with the complex suboxide chemistry of Ga and In, results in a growth window that is difficult to map and an alloy which is difficult to control.

We report on a high-throughput molecular beam epitaxy (MBE) technique to screen the growth conditions for the ternary alloy (In_yGa_1-y)₂O₃, and the application of these findings to the successful synthesis of monoclinic (Al_xGa_1-x-yIn_y)₂O₃. By leveraging the sub-oxide chemistry of Ga₂O₃ and in-situ monitoring by reflection high-energy electron diffraction (RHEED), a cyclical growth and etch-back method is developed to rapidly characterize the (In_yGa_1-y)₂O₃ growth space. This cyclical method provides approximately 10x increase in experimental throughput and 46x improvement in Ga₂O₃ substrate utilization. Growth conditions for monoclinic (In_yGa_1-y)₂O₃ are identified and targeted growths are characterized ex-situ to confirm improved In incorporation. These conditions are then used to grow quaternary (Al_xGa_1-x-yIn_y)₂O₃ with Al cation composition x ranging from 1% – 24% and In cation composition y ranging from 3% to 16%. The structural, chemical and optical properties of the alloys are investigated. An (Al_0.17Ga_0.76In_0.07)₂O₃ alloy lattice-matched to Ga₂O₃ is examined by high resolution microscopy, highlighting the correlation between surface facets and composition. Such lattice-matched material can be grown arbitrarily thick without elastic strain and relaxation, making it suitable for high voltage diodes, transistor barriers, and epitaxial dielectrics.

Evolution of the structural and thermal interfacial properties of direct wafer bonded (0001) GaN to (001) Si during annealing is investigated. Direct wafer bonding provides a pathway to fabricate and engineer heterointerfaces free of lattice mismatch restrictions. Here, an EVG^® ComBond^® wafer bonder was used to bond the GaN and Si under high vacuum at room temperature by first removing native oxide with an Ar⁺ beam prior to bonding. We have demonstrated as-bonded GaN on Si with high thermal boundary conductance of 143 MW/(m²·K) prior to annealing. High resolution transmission electron microscopy of the as-bonded structure revealed abrupt bonded interfaces with a ~1.3 nm amorphous interface due to the Ar⁺ surface treatment. After annealing at 450 °C up to 7 hours, a 1 nm Ga-rich layer is observed across the interface near the surface of the Si in addition to SiN_x formation at the original bonded interface. Further annealing at 700 °C up to 24 hours led to the formation of Ga-rich pyramidal features that form across the bonded interface in the Si along the (111) planes. While recrystallization was observed to have a beneficial impact in other bonded systems, the chemical and structural reconfiguration of these GaN-Si interfaces resulted in poorer interfacial thermal transport by a factor of two (71 MW/(m²·K)). This reduction is attributed to the degradation of thermodynamically less stable phases as the GaN breaks down into SiNx and Ga in the presence of silicon.We show that high TBC can be achieved through wafer bonding of GaN and Si and that interfacial properties that are stable at typical device operating temperatures (250 °C), but higher temperature annealing processing steps are deleterious to thermal transport across GaN-Si interfaces.

Gallium nitride is attracting increasing attention in the semiconductor industry, especially for high-power and high frequency electronic applications, owing to its unique features, i.e. direct band gap of 3.4 eV, high electron mobility, good thermal stability, and elevated mechanical hardness. MOCVD and MBE are the most employed methods to produce high quality GaN layers, yet they have their own drawbacks. On the one hand, MOCVD uses toxic gases as precursors and operates at very high temperatures (~1000 °C) to enable the pyrolysis of precursors. On the other hand, MBE faces issues of (i) high cost associated with the use of ultra-high vacuum pumping, and (ii) scalability. Additionally, it also operates at high temperature. This latter point induces thermal mismatch strain due to large thermal expansion coefficient difference between GaN and Si that may produce interface defects, film cracking and wafer bowing upon cooling. A potential solution to avoid those issues is to resort to a lower temperature method such as low-pressure plasma deposition. In this work, we demonstrate the direct growth of GaN thin films on silicon substrate using reactive sputtering of a liquid Ga target by an Ar/N₂ plasma at room temperature [1]. The morphology, microstructure, and composition profile of the GaN thin films have been analyzed using a set of ex-situ solid-state characterization techniques while the plasma has been investigated using in-situ technics, including OES, MW interferometry and TALIF to measure electron density, gas temperature and N-atoms density, respectively. In the presentation, we will discuss the resulting properties of the films as a function of plasma characteristics.

[1] L. Srinivasan et al. J. Vac. Sci. Technol. A. Vol.41, Issue 5 (2023)