Driven by the growth of the power electronics market in recent years, GaN-on-Si technology has become quite mature, mainly due to the development of technology that was initially intended for power electronics applications. Given the level of maturity, digging into the physics behind device operation provides an additional tool to improve the device characteristics. imec complements technology development with modeling activities that will ultimately help achieve better performance and reliability. The insights gained will not only benefit the development of GaN HEMT devices for mmWave applications, but they will also enable performance improvements in other application domains, including GaN-based power electronics.

DEVICE ISOLATION BY ION IMPLANTATION

Figure 3

Figure 3 (a) Benchmark of sheet resistance versus activation energy magnitude. (b) Benchmark of sheet resistance versus peak heating temperature.

Figure 4

Figure 4 (a) Diagram illustrating surface and bulk leakage paths in transmission line model structures. (b) Energy band diagram of the AlGaN/AlN/GaN heterostructure showing band bending at the GaN surface.

As an example of these modeling activities, this section focuses on device isolation. This is one of the technology building blocks of the GaN-on-Si platform. When integrating GaN HEMTs in a common Si platform, the devices must be electrically isolated from each other, with as few leakage paths as possible between neighboring devices. This electrical isolation reduces power loss and improves the breakdown behavior of active devices. For GaN HEMTs, the ion implantation technique has already proven to be an attractive isolation approach over other isolation techniques, such as mesa etching, providing lower leakage and higher breakdown voltage of the isolation regions. The technique was initially developed for GaN-based power electronics applications, where it is still one of the isolation techniques actively being used today.

Ion implantation introduces several defects into the GaN heterostructure that act as trapping centers for the charge carriers. In terms of physics, these defects pin the Fermi level away from the conduction or valence band of GaN. Implanting ions, such as nitride (N) ions, in the region surrounding the devices will reduce the number of conductive free carriers, creating an electrically insulating region. In experiments, researchers have also observed that the ion implantation-induced damages disappear after annealing at high temperatures, typically above 600°C, thereby compromising isolation quality. Featuring a low post-epitaxy thermal budget, imec’s GaN-on-Si manufacturing flow guarantees high-quality isolation of HEMT devices. imec has already demonstrated a GaN HEMT ion implantation isolation technique that contributes to the highest reported sheet resistance, with values in the range of 1013 to 1015 Ω/sq. This is an essential metric for quantifying isolation. Figure 3a and Figure 3b illustrates benchmarks of the sheet resistance (Rsh) of AlGaN/(AlN)/GaN heterostructures subjected to ion implantation isolation with varying activation energy magnitudes and peak heating temperatures. The benchmark in Figure 3a suggests a common physical mechanism behind isolation, while the benchmark in Figure 3b indicates the dominant impact of processing temperature on isolation quality.

THE MECHANISM BEHIND ION IMPLANTATION ISOLATION: A FUNDAMENTAL INSIGHT

Why this technique works so well and precisely where the remaining current leakage path is formed has remained a mystery. A fundamental understanding and modeling of the leakage mechanism in ion-implanted regions is needed. This could help improve the process conditions such as thermal budget, implantation dose and energy for various applications, including mmWave communication.

There is a reason why it is so difficult to understand the exact mechanism behind the insulation. The ion-implanted region is full of defects of various natures. There are point defects, such as vacancies or interstitial atoms, defect complexes, foreign ion impurities and lattice disorder to name a few. In addition, polarization charges reside at the interface between AlGaN and GaN. This complex cocktail of defects and charges makes it highly challenging to simulate the behavior of the charges within the isolated heterostructure and to locate the leakage path.

By combining experimental and modeling work, imec researchers have unveiled the leakage mechanism in isolated GaN-based heterostructures for the first time. The details of this work have recently been published in the Journal of Applied Physics.9 By setting up dedicated experiments with varying AlGaN and AlN thicknesses, researchers extracted and analyzed the sheet resistances of the isolated regions and the corresponding activation energies. The conclusion from these experiments was that the dominant leakage occurs via an ohmic path of electrons at the GaN surface. Revert to the terms of physics, this translates into a downward bending of the GaN conduction band near the GaN surface. These insights laid the foundation for more detailed modeling of the isolated heterostructure and for reconstructing its energy band diagrams. The theory helped extract the net defect densities in these isolated implanted regions, which amounted to ∼2×1019cm-3 and ∼2×1018cm-3 for GaN and AlGaN, respectively for these experiments. The majority of those defects are found as point defects. The point defects were created by ion implantation techniques and preserved from recombination with imec’s low thermal budget HEMT fabrication. The high densities of point defects are essential to limit the GaN surface energy band bending and thus limit the leakage. Figure 4a and Figure 4b illustrate the leakage mechanism in GaN heterostructures. Figure 4a shows the surface path leakage path versus bulk leakage path in transmission line model structures. Figure 4b illustrates the energy band diagram of the AlGaN/AlN/GaN heterostructure showing band bending at the GaN surface.

CONCLUSION

For the first time, imec researchers have unveiled the exact mechanism behind ion implantation as a technique for electrically isolating GaN HEMT devices. These insights help improve the process conditions to obtain good isolation quality when targeting RF/mmWave communication. The findings can be extended to power electronics applications as well. Moreover, the study led to a novel method to estimate the net defect density in isolated GaN-based heterostructures. These activities fit into the broader framework of GaN device optimization for RF applications through both technology and modeling. The efforts and the results illustrate how uncovering the physics secrets behind the technology building blocks can help take these GaN-based devices to the next level of maturity.

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